Flavor of Browning Reaction Products - American Chemical Society

9 Flavor of Browning Reaction Products A H M E D FAHMY MABROUK

Downloaded by IOWA STATE UNIV on April 19, 2017 | http://pubs.acs.org Publication Date: December 14, 1979 | doi: 10.1021/bk-1979-0115.ch009

Food Sciences Laboratory, U.S. Army Natick Research and Development Command, Natick, MA 01760

With the exception of fresh vegetables and fruits, most consumed food has to be subjected to processing (boiling, broiling, roasting, canning, baking, concentration, pasteuriza tion,...etc.) to render it edible; to increase its consumer acceptance or to extend its shelf life. Table I lists the consumption of food commodities per capita in the United States (1). About 78% of these comodities are processed, and account for approximately 75% of the American family food budget. Natural food flavors such as terpenes, hydrocarbons, alcohols, aldehydes, ketones, esters, acids, lactones, amines, sulfur compounds are enzymatically produced in fruits and vegetables. On the contrary, processed food develops its characteristic acceptable flavors from chemical reactions within its components at temperatures far below those at which its major components, i.e., lipids, proteins and carbohydrates pyrolyze. Food flavor precursors responsible for the productivity of volatile flavors are given in Table II. Aqueous flavor precursors undergo nonenzymic browning during processing, which is the most important flavor producing reaction. Also, at low temperature pyrolysis, i.e., roasting between 100-270°C, these compounds undergo thermal degradation, producing new compounds some of which have desirable organoleptic notes. Some degradation products r e a c t f u r t h e r . For example, a l l purines are decomposed upon h e a t i n g i n a c i d media a t temperatures above 100°C t o g l y c i n e , formic a c i d , carbon d i o x i d e and ammonia. Methylated p u r i n e s give r i s e to methylamine i n s t e a d o f ammonia. P r o t e i n s and g l y c o p r o t e i n s p a r t i c i p a t e i n the browning r e a c t i o n v i a t h e i r free-NR^ groups. Furthermore, d u r i n g process i n g , compounds o f these two c l a s s e s undergo h y d r o l y s i s and degradation;thus ammonia, hydrogen s u l f i d e , p e p t i d e s , amino a c i d s , amines and sugars are produced. A d e l i c i o u s t a s t e peptide which has the f o l l o w i n g s t r u c t u r e : H-Lys-Glc-Asp-Glu-Ser-Leu-Ala-OH was i s o l a t e d from beef gravy (2). In animal c e l l s , the w i d e l y occurr i n g peptides are c a r n o s i n e , a n s e r i n e , g l u t a t h i o n e , phosphopept i d e s , l i p o p e p t i d e s and nucleopeptides. The l a r g e s t c l a s s o f This chapter not subject to U.S. copyright. Published 1979 American Chemical Society Boudreau; Food Taste Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Boudreau; Food Taste Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Meats : Beef Veal Lamb and mutton Pork (excluding l a r d ) F i s h ( e d i b l y weight) P o u l t r y products: Eggs Chicken (ready-to-cook) Turkey (ready-to-cook) Dairy products: Cheese Condensed and evaporated m i l k F l u i d m i l k and cream (prod.weight) Ice cream (product weight) Fats and o i l s - t o t a l , f a t content B u t t e r ( a c t u a l weight) Margarine ( a c t u a l weight) Lard Shortening Other e d i b l e f a t s and o i l s Fruits: Fresh Citrus Noncitrus

Commodity 151.4 84.1 2.4 2.9 62.0 11.8 39.5 40.5 8.0 11.5 7.1 296.0 17.7 53.0 5.3 11.0 4.7 17.3 18.2 79.1 27.9 51.2

42.4 27.8 6.2 8.3 13.7 321.0 18.3 45.3 7.5 9.4 7.6 12.6 11.5 90.0 32.5 57.5

1970

134.1 64.3 5.2 4.3 60.3 10.3

1960

76.3 26.8 49.5

14.6 5.6 288.0 17.5 53.2 4.6 11.3 3.2 17.0 20.3

36.6 41.1 8.9

152,5 86.4 1.9 2.0 62.2 12.2

1974

81.3 28.7 52.6

14.5 5.0 291.1 18.7 53.3 4.8 11.2 4.0 17.3 20.3

35.4 40.3 8.6

145.4 88.9 3.5 1.8 51.2 12.2

1975

Consumption of Major Food Commodities Per C a p i t a i n USA, l b

Table I


83.6 28.6 55.0

15.9 4.7 292.0 18.1 56.1 4.4 12.2 3.6 18.1 22.0

34.9 43.3 9.2

155.3 95.7 3.3 1.7 56.4 12.9

1976

82,4 25.2 57,2

16.3 4.4 289,4 17,7 54.4 4.4 11.6 3.5 17,5 21.6

34.5 44.3 9.2

154.8 93.2 3.2 1.5 56.9 12.8

1977


Reference (1)

Processed: Canned f r u i t Canned j u i c e Frozen ( i n c l u d i n g j u i c e s ) Chilled citrus juices Dried Vegetables : Fresh Canned, e x c l u d i n g potatoes and sweet potatoes Frozen, e x c l u d i n g potatoes Potatoes, (including fresh e q u i v a l e n t of processed) Sweetpotatoes, ( I n c l u d i n g f r e s h e q u i v a l e n t of processed) Grains : Wheat f l o u r Rice Other: Coffee Tea Cocoa Peanuts ( s h e l l e d ) Dry e d i b l e beans Melons Sugar ( r e f i n e d )

5,0 107 7,8

52.8 10.2 114,7 5.3 111 7.2 9.4 0.8 3.0 6.3 6.3 18.6 94.7

52.1 9.7 120.2 5.3 107 7.7 9.0 0.8 2.6 6.5 6.5 17.5 90.2

53.3 10.2 112.3 5.1 106 7.6 9.5 0.8 3.0 6.4 6.7 17.2 96.6

51.2 9.6 115.3 5.2 110 6.7 10.5 0.7 3.1 5.9 5.9 21.2 101.8

43.4 7.0 105.0 6.5 118 6.1 11.6 0.6 2.9 4.9 7.3 23.2 97.4

6.9 .9 2.7 6.5 6.0 19.0 95.7

120.7

52.8 9.7

93.1

93.9

93.6

91.0

96.0

94.5

19.3 15.3 12.6 5.7 3.0

19.6 14.7 11.3 5.2 2.5

19.5 13.7 12.4 5.8 2,6

1977

23.3 14.6 9.8 4.7 2.7

19.2 15.3 12.2 6.2 2.7

1976

22.6 13.0 9.1 2.1 3.1

Table I (cont'd) Consumption of Major Food Commodities Per C a p i t a i n USA, l b 1974 1975 1970 1960 Commodity


O 5

g

Ci

!»

(Ή

S*

g

ta -* ο

1

S

208

FOOD TASTE CHEMISTRY

Table I I Food F l a v o r Precursors and Their F l a v o r Components Possible Flavor Components Produced Number Per Compound Per C l a s s a- Aqueous F l a v o r Precursors

Precursors


Class 12345678-

Glycoproteins Glycopeptides Proteins Peptides 60 Amino A c i d s ) Amines ) 35 Nucleotides ) Nucleotide S sugars < 35 9- N u c l e o t i d e J sugaramine < 10- N u c l e o t i d e J acetylsugaraminej 11- Peptide bound 12 nucleotide 12- Nucleosides 12 13- Purines and 12 pyrimidines 14- Sugars \ 15- Sugaramine ) 50 16- Sugar phosphate ) 17- Organic a c i d s 25 Subtotal >241

40

2,400

40

1,400

40

1,400

40 40

480 480

25

300

50 >275

b- Non-Auqeous F l a v o r Precursors 1- N e u t r a l l i p i d s 25 50 2- P o l a r L i p i d s 25 50 3- Isoprenoids 150 5 4- Carotenoids 10 25 5- U n s a p o n i f i a b l e compounds Subtotal >210 22

Grand t o t a l

>451

2,500 >8,960 1,250 1,250 750 250

>3,500 >12,460



9.

MABROUK

Browning Reaction Products

209

peptides i n p l a n t s c o n s i s t of γ-glutamyl d i p e p t i d e s along w i t h some t r i p e p t i d e s , one of which i s homoglutathione ( y - g l u t a m y l c y s t e i n y l - g - a l a n i n e ) which r e p l a c e s g l u t a t h i o n e i n some enzymic r e a c t i o n s Q ) . About t h i r t y - f i v e amino a c i d s and amines occur n a t u r a l l y i n foods. D-ribose i s found i n nucleo t i d e s , D - r i b u l o s e , D-lyxose, D - x y l u l o s e , glucose, f r u c t o s e , sedoheptulose, and glyceraldehyde are present as phosphoric a c i d e s t e r s i n the products of carbohydrate metabolism. Fucose i s a c o n s t i t u t e n t of blood group substances which c o n t a i n g a l a c t o s e and mannose. The presence of carbohydrate a l c o h o l s i n vegetables, f r u i t s and animals may be accounted f o r by a r e d u c t i o n of the reducing sugars, or through d e c a r b o x y l a t i o n of the corresponding higher a l d o n i c a c i d s . Sorbose, glucose, sucrose, i n o s i t o l , m a n n i t o l , p e n t i t o l and two u n i d e n t i f i e d sugars were reported i n cocoa beans (4). Sugar a c i d s , mono-, d i - , and t r i - c a r b o x y l i c a c i d s , k e t o - , hydroxy-acids as w e l l as unsaturated organic a c i d s are present i n foods. While u r o n i c a c i d s p a r t i c i p a t e i n the browning r e a c t i o n , others might a c c e l e r a t e or r e t a r d the r e a c t i o n . Non-aqueous f l a v o r p r e c u r s o r s c o n t r i b u t e d i r e c t l y and i n d i r e c t l y to food f l a v o r s . Boar meat l i p i d s c o n t a i n 5-androst16-ene-3-one which i s r e s p o n s i b l e f o r n o t i c e a b l e u n d e s i r a b l e notes (5). The c h a r a c t e r i s t i c f l a v o r s of mutton and goat meat are a t t r i b u t e d to the presence of odd-numbered η-fatty a c i d s , and abnormal p r o p o r t i o n of branched chain f a t t y a c i d s (6). During p r o c e s s i n g of foods, l i p i d s undergo a u t o x i d a t i o n , h y d r o l y s i s , dehydration, d e c a r b o x y l t i o n , and degradation. Thermal degradation of n e u t r a l l i p i d s , p o l a r l i p i d s and f r e e f a t t y a c i d s produce the f o l l o w i n g c l a s s e s o f compounds (7-13) : - ( l ) n - and i s o - a l k a n a l s , (2) a l k a d i a n a l s , (3) o x o - a l k a n a l s , (4) a l k e n a l s , (5) a l k a d i e n a l s , (6) aromatic aldehydes, (7) methyl ketones, (8) s a t u r a t e d and unsaturated a l c o h o l s , (9) γ- and 6- l a c t o n e s , (10) hydrocarbons, (11) keto- and hydroxy-acids, (12) d i c a r b o x y l i c a c i d s , and (13) s a t u r a t e d and unsaturated f a t t y a c i d s , c o n t a i n i n g l e s s carbon atoms than the parent ones. L i p i d s i n foods vary from t r a c e s as i n c e r e a l s to 30-50% as i n nuts. The p h y s i c a l s t a t e and d i s t r i b u t i o n of l i p i d s vary c o n s i d e r a b l y among food items. In each item l i p i d d i s t r i b u t i o n s a f f e c t i t s f l a v o r as i t undergoes chemical r e a c t i o n s and a c t as a f l a v o r components v e h i c l e or p a r t i t i o n i n g medium. Furthermore, l i p i d s have a pronounced e f f e c t upon the s t r u c t u r e of food items. F a t t y a c i d s of n e u t r a l ( t r i g l y c e r i d e s ) and p o l a r l i p i d s of beef and pork are t a b u l a t e d i n Table I I I . Pork f a t contains more unsaturated f a t t y a c i d s than beef and i t s l i n o l e i c a c i d content i s double t h a t i n beef. Heat produced v o l a t i l e s i n red meat f a t s are l i s t e d i n Table IV. The presence of p y r a z i n e s i n the v o l a t i l e s of beef f a t i s due to the presence of nitrogenous compounds i n the f a t amounting to 0.1-0.2%N, ( K j e l d h a l method). These compounds might be p r o t e i n s , p e p t i d e s , amino a c i d s , and amine moitiés i n p o l a r l i p i d s . Such compounds



210

Table I I I F a t t y Acids Composition of Beef and Pork L i p i d s


F a t t y Acids

Triglcerides Polar L i p i d s Beef Pork Beef Pork % of T o t a l F a t t y A c i d Content

a.- Saturated Capric Laurie Myristic Palmitic Stearic

0.1 0.1 2.2 27.5 16.9

0.1 0.2 1.2 23.9 11.6

-

-

2.6 13.2 15.6

2.0 20.0 11.0

Total

46.8

37.0

31.4

33.0

1.0 4.7 41.3

7.4 45.3

0.9 2.2 21.2

0.2 2.3 16.2

47.0

52.6

24.3

18.8

0.6 4.4 -

-

1.3 20.2

-

0.6 27.9 0.9

5.0

8.7

21.5

29.3

b. - Monounsaturated Tetradecenoic Palmitoleic Oleic Total - Dienoic Tetradecenoic Linoleic Docasadienoic Total d. - T r i e n o i c Linolenic Eicosatrienoic Total Tetraenoic Arachidonic Total

8.7

-

1.1

1.6

-

-

1.8 1.9

1.0 1.6

1.1

1.6

3.7

2.6

0.1

0.1

19.1

16.3

0.1

0.1

19.1

16.3

Reference (14)


9.

MABROUK


Table IV Heat Produced Carbonyls I n Red Meat Fats


V o l a t i l e Compounds ALKANALS Ethanal(Acetaldehyde) n-Propanal n-Pentanal n-Hexanal η-Hep t a n a l n-Octanal n-Nonanal n-Decanal n-Undecanal n-Dodecanal METHYL-ALKANALS Methylpropanal Me t h y l b u t a n a l

Pork

Beef

10 10 7 7,10 7 7,10 7,10 7 7 7

9a, 10 9a, 10 8 8,9,10 8,9 8,9 8,9 8,9

7

9a

OXOALKANAL 2-0xopropanal(Pyruvaldehyde)

9a

2-ALKENAL 2t-Butenal(Crotonal) 2t-Heptenal 2t-0ctenal 2t-Nonenal 2t-Decenal 2t-Undecenal

7 7,10 7,10 7,10 10

9a 8,9 8,9 8,9 8,9 8

2,4-Alkadienal 2,4-Heptadienal 2,4-Nonadienal 2,4-Decadienal

10 10 10

8,10

7

10

10

9a 8,9a

ALKANEDIAL Ethanedial(Glyoxal)

2-ALKAN0NE Acetone 2-Butanone 2-Decanone 2-Undecanone 2-Tridecanone 2-Pentadecanone 2-Hep tadecanone

Lamb

10 10 10

10 9a 7 7 8 8 8


212


Table IV (cont'd) Heat Produced Carbonyls


V o l a t i l e Compounds

i n Red Meat Fats Pork

Beef

HYDROXY-2-ALKANONE 3-Hydroxy-2-butanone(acetoin)

9a

PYRAZINE 2,5-Dimethyl2-Ethyl2-Ethyl-3,6-dimethyl2-Ethyl-5-methyl2,3,5-Trimethyl-

8 8 8 8 8

Lamb

Reference (7,8,9,10) "a designates h e a t i n g i n n i t r o g e n atmosphere, otherwise i n a i r . 11

degrade d u r i n g h e a t i n g and produce ammmonia and b a s i c compounds. When ammonia, amino a c i d s and degraded nitrogenous compounds r e a c t w i t h α-dicarbonyls produced from l i p i d o x i d a t i o n , p y r a z i n e s a r e formed. Cooked chicken f l a v o r concentrate (13) contained the f o l l o w i n g aldehydes; 3c-nonenal; 4c-decenal; 2 t , 4 c - d e c a t r i e n a l ; 2t,5c-undecadienal; 2t-dodecenal; 2 t , 4 c - d o c a d i e n a l ; 2t,6c- and 2t,6t-dodecadienal 2 t - t r i d e c e n a l ; 2 t , 4 c - t r i d e c a n d i e n a l ; 2 t , 4 c , 7 c - t r i d e c a t r i e n a l ; and 2 t , 4 c - t e t r a d e c a d i e n a l . Three of these aldehydes: 4c-decenal; 2t,6c-dodecadienal; and 2 t , 4 c , 7 c - t r i d e c a t r i e n a l a r e t y p i c a l breakdown o f a r a c h i d o n i c a c i d , and to a major extent a l s o 2t,5c-undecadienal. These aldehydes p l a y an important r o l e i n cooked chicken f l a v o r v i a the browning r e a c t i o n . 2,4-Decadienal which i s considered t o be a key compound of chicken aroma, was found i n the v o l a t i l e s of cooked chicken (15,16). 2,4-Heptadienal; 2,4-nonadienal and 2,4-decadienal were i d e n t i f i e d i n heated pork f a t (10) and heated beef f a t (8,10). D i c a r b o n y l s , d i e n a l s , t r i e n a l s undergo amino-carbonyl r e a c t i o n s , r e s u l t i n g probably i n the formation of meaty f l a v o r notes i n s p e c i f i c p r o p o r t i o n s c h a r a c t e r i s t i c of each s p e c i e s . Although t h e i r y i e l d i s s m a l l i n comparison w i t h monocarbonyls, t h e i r h i g h r e a c t i v i t i e s a r e r e s p o n s i b l e f o r t h i e r important r o l e i n f l a v o r p r o d u c t i o n . The complexity of food f l a v o r p r e c u r s o r s i s manifested by the number of compounds estimated i n each c l a s s of compounds and by the p o s s i b l e t o t a l number i n a s i n g l e raw food. The p o s s i b l e number o f f l a v o r components produced p e r one compound of f l a v o r p r e c u r s o r i s estimated i n the range 10-150, but i s reduced t o 5-40 t o e l i m i n a t e compounds that might be produced


9.

MABROUK


by more than one precursor and to account f o r v a r i a t i o n s i n p r o c e s s i n g methods. One hundred compounds were i d e n t i f i e d i n thermal degradation products o f glucose (17,18,19): aldehydes ketones, aromatics, f u r a n s , oxygenated f u r a n s , n o n - v o l a t i l e s . Sugars r e a c t w i t h ammonia at temperatures ranging from 20 t o 260°C., to produce h e t e r o c y c l i c compounds : s u b s t i t u t e d i m i d a z o l e s , s u b s t i t u t e d p y r a z i n e s , s u b s t i t u t e d p i p e r a z i n e s , p y r i d i n e and s u b s t i t u t e d p y r i d i n e s ( 2 0 ) . A t 0-120°C., a-dicarbonyls (2,3-butadione, e t h a n e d i a l " g l y o x a l " , 2-oxopropanal "pyruvaldehyde , and α-hydroxycarbonyl " g l y c o l a l d e h y d e " r e a c t w i t h ammonia i n presence o f formaldehyde to g i v e s u b s t i t u t e d i m i d a z o l e s . Dipeptides r e a c t w i t h sugars e i t h e r as an e n t i t y or as amino a c i d s produced by h y d r o l y s i s during p r o c e s s i n g . I n the f i r s t case, one compound i s formed, pyrazinone (21), i n the second case numerous f l a v o r compounds are produced from the r e a c t i o n of the two amino a c i d s w i t h sugar. The r e a c t i v i t y of d i p e p t i d e s towards r e a c t i o n w i t h carbonyl compounds i s much higher than t h a t of amino a c i d s . The t a s t e o f g l y c y l - L - l e u c i n e i s very b i t t e r , the product o f i t s r e a c t i o n w i t h g l y o x a l has an a s t r i n g e n t , a l i t t l e sour and l a t e r a m i l d t a s t e . Some products from the browning r e a c t i o n and p y r o l y s i s o f f l a v o r precursors r e a c t w i t h ammonia and hydrogen s u l f i d e , thus i n c r e a s i n g the number o f f l a v o r components produced from food p r e c u r s o r s . The molecular s t r u c t u r e o f the amino a c i d i n f l u e n c e s the f l a v o r notes i n the food. For example, thiophenes are reported i n the v o l a t i l e s produced from the p y r o l y s i s of c y s t e i n e o r from i t s r e a c t i o n w i t h glucose or pyruvaldehyde (22,24) but were absent i n the case of c y s t i n e (23,24). Hundreds o f compounds have been i d e n t i f i e d i n the v o l a t i l e f l a v o r components of processed foods. Hydrocarbons, a l c o h o l s , e t h e r s , aldehydes, ketones, a c i d s , a c i d anhydrides, e s t e r s , aromatic, l a c t o n e s , pyrones, f u r a n s , p y r i d i n e s , p y r r o l e s , n-alkylpyrrole-2-aldehydes, pyrazines, s u l f i d e s , d i s u l f i d e s , t h i o l s , thiophenes, t h i a z o l e s , t r i t h i o l a n e s , t h i a l d i n e . . . e t c . Each compound has a c h a r a c t e r i s t i c note and a s p e c i f i c t h r e s h o l d which makes i t s c o n t r i b u t i o n to food f l a v o r unique. But none o f the i n d i v i d u a l compounds were reported to completely produce the c h a r a c t e r i s t i c f l a v o r of the processed food. This i n d i c a t e s that the food f l a v o r s are of mixed f l a v o r notes o f numerous compounds, w h i l e others are necessary f o r i t s s y n e r g i s t i c e f f e c t , others exert a background of modifying e f f e c t . Browning r e a c t i o n has become a major t o p i c o f food research because o f i t s relevance t o the d e s i r a b l e and u n d e s i r a b l e changes ( f l a v o r , c o l o r , t e x t u r e , and n u t r i t i v e value) o c c u r r i n g i n foods during p r o c e s s i n g and storage. During the Second World War and i t s post e r a , research on food d e t e r i o r a t i o n due to nonenzymic browning r e c e i v e d c o n s i d e r a b l e a t t e n t i o n due t o the problems r e s u l t i n g from the n e c e s s i t y to prepare dehydrated foods and food concentrates which had to be s t o r e d under t r o p i c a l c o n d i t i o n s without s e r i o u s d e t e r i o r a t i o n . 11


213



214


The purpose o f t h i s paper i s t o review the numerous papers p u b l i s h e d on f l a v o r s , t a s t e s and odors r e s u l t i n g from the browning r e a c t i o n . I n v e s t i g a t i o n s of model systems which have been observed under l a b o r a t o r y c o n d i t i o n s are considered and t h e i r p o s s i b l e s i g n i f i c a n c e i n b a s i c and i n d u s t r i a l processes w i l l be discussed. S p e c u l a t i o n on the p o s s i b l e c o r r e l a t i o n between model system r e s u l t s and s p e c i f i c processed food items w i l l be presented. R e s u l t s o f recent work i n our l a b o r a t o r y on f l a v o r notes developed upon heating r i b o s e w i t h v a r i o u s amino a c i d s w i l l be discussed. The numerous p u r e l y chemical papers are considered t o be o u t s i d e the scope of t h i s paper. The reader i s r e f e r r e d t o s e v e r a l reviews on the s u b j e c t (25-30). Comprehensive reviews on nonenzymic browning i n r e l a t i o n t o s p e c i f i c food problems had been published (31,32,33). As f l a v o r production i n n a t u r a l food i s governed by too complicated r e a c t i o n s due t o i t s complex components (Table I I ) , chemists concentrated t h e i r research e f f o r t s on simpler systems to understand the r e a c t i o n s i n v o l v e d and t h e i r products. Amino Acids-Sugars Model Systems Model system s t u d i e s o f the r e a c t i o n between s i n g l e amino a c i d s and n a t u r a l l y o c c u r r i n g substances capable of r e a c t i n g w i t h them has f u r n i s h e d v a l u a b l e i n f o r m a t i o n l e a d i n g toward an under standing of food f l a v o r s . The w e l l known S t r e c k e r degradation of α-amino a c i d s , Schonberg and Moubacher (25), has been used as the c e n t r a l r e a c t i o n around which other amino compound degradation systems may be o r i e n t e d . I n t h i s r e a c t i o n α-amino a c i d s a r e deaminated and decarboxylated by s p e c i f i c carbonyl compounds or others t o y i e l d aldehydes and ketones c o n t a i n i n g one carbon atom less. 3-amino a c i d s a l s o undergo o x i d a t i v e deamination and d e c a r b o x y l a t i o n t o a ketone w i t h one l e s s carbon atom; e.g., 3-amino-n-butyric a c i d produces 2-propanone, which a l s o r e s u l t s rrom the S t r e c k e r degradation of α-amino-isobutyric a c i d . Organic d i - and t r i c a r b o n y l s are not the only oxidants e f f e c t i v e i n S t r e c k e r degradations. Hydrogen peroxide i n presence o f f e r r o u s s u l f a t e degrade α-amino a c i d s a t room temperature ; f o r example g l y c i n e y i e l d s formaldehyde. The c o - o x i d a t i o n of s u l f u r c o n t a i n i n g amino a c i d s i n an a u t o - o x i d i z i n g l i p i d system i n d i c a t e s that l i p i d peroxides a c t as oxidants (34). The r e s u l t i n g carbonyl compounds from amino a c i d s through S t r e c k e r degradation p a r t i c i p a t e i n the formation o f d e s i r a b l e c h a r a c t e r i s t i c f l a v o r s during p r o c e s s i n g o r u n d e r s i r a b l e ones r e s p o n s i b l e f o r f l a v o r d e t e r i o r a t i o n d u r i n g storage. F l a v o r s and odors given by amino a c i d s and sugars i n d i l u t e aqueous s o l u t i o n s a t d i f f e r e n t temperatures has been the s u b j e c t of i n t e n s i v e s t u d i e s by v a r i o u s researchers. Tables V and VI summarize the d e s c r i p t i v e aroma evolved from r e a c t i n g carbonyl compounds w i t h amino a c i d s a t 100, and 120/180°C., r e s p e c t i v e l y



weak caramelÇ35) strong,yeasty protein

hydrolyzate(35)

α-Alanine Valine

α-Aminobutyr i c Leucine

Phenylglycine Methionine

Threonine

Serine

Isoleucine

baked potato(35)

Glycine

baked potato(35)

moderate c r u s t (35) vaguely breadl i k e (35) v e r y weak(35)

strong,cheesy,

Dihydroxyacetone

Amino Compounds

chocolate(37) maple (38) *" b i t t e r almond(38) overcooked sweet potato(36) potato(37)

objectionable chopped cabbage(36)

overcooked cabbage(36)

obj e c t i o n a b l e weak NH (36)

weak(36)

caramelized sugar unpleasant (36,39), f a i n t carmel beer(40). smell(36). beer aroma(40) rye bread(37) fruity,aromatic (39) maple(38) sweet c h o c o l a t e ( 3 7 ) toast(39),bread(40) rye bread(38) musty(37),fruity, aromatic(39) maple syrup(38)

unpleasant burned wood(36)

3

Sucrose

Maltose

Glucose

Fructose

Aromas Developed by the Reactions of Carbonyl Compounds and V a r i o u s Amino Compounds at 100°C

Table V



Phenylalanine

Glutamine Me thy lamine Ammonia

Histidine

Arginine

Hydroxyproline

Proline

Cystine

Cysteine

Amino Compounds

9

very s t r o n g , hyacinth(35)

v e r y weak(35)

very s t r o n g cracker,crust, toast(35) weak, vaguely l i k e proline(35) very weak(35)

(35)

mer cap tan

Dihydroxyacetone

Fructose

Maltose

popcorn(37) butterscotch(39) buttery note(39), none(37) chocolate(37) empyreumatic t a s t e ( 4 0 ) t a r r y odor, b i t t e r taste(40) r a n c i d caramel, s t i n g i n g smell, pleasant unpleasant(36) very objecsweet v i o l e t s ( 3 7 ) rose tionable(36) caramel(36) perfume(39)

meat(39) sulfide(37) meat, burnt turkey s k i n ( 3 9 ) corn-like(39) burnt p r o t e i n (37) potato(39)

Glucose

Aromas Developed by the Reactions o f Carbonyl Compounds and V a r i o u s Amino Compounds a t 100°C

Table V (cont'd)


unpleasant sweet caramel(36)

Sucrose


References

(35,40)

a-Methylaminobutyric a-Aminoisobutyric

s t r o n g dark corn syrup(35)

very weak(35) c h i c k e n broth(35)

Tyrosine Aspartic Glutamic

Arginine Lysine

Dihydroxyacetone

Amino Compounds

maple(38)

maple(38)

caramel(37) rock candy(37) oldwood pleasant(36) b u t t e r y note(39) baked sweet potato(36)

Glucose

objectionable fried butter (36)

too weak(36)

Fructose

Sucrose

unpleasant wet wood (36)

r o t t e n wet potato(36

too weak(36) p l e a s a n t caramel(36)

Maltose

Aromas Developed by the Reactions of Carbonyl Compounds ans V a r i o u s Amino Compounds a t 100°C

Table V (cont'd)



References

Glutamic Arginine

(37,38,41)

brunt brunt

sugar(37) sugar(37)

violets,lilac (41) caramel(37)

strong flower (41) s t r o n g breadc r u s t (41) moderate(41) no s i g n i f i c a n t aroma(41)

Phenylalanine

Aspartic

burnt(37)

objectionable burnt cheese(37)

penetrating chocolate(37) burnt cheese(37)

weak(41)

moderate breadcrust(41) moderate breadcrust(41) moderate breadcrust(41)

Aroma D e s c r i p t i o n 120°C 180°C

Threonine

Isoleucine

Leucine

Valine

Amino A c i d

Tryptophan

Glutamine

Histidine

strong(41)

s t r o n g baked potato(41) s t r o n g breadcrust cracker (41) no s i g n i f i c a n t aroma(41)

Methionine Proline

weak(41)

cornbread(37) b u t t e r y note(38) butterscotch(37)

p l e a s a n t bakery aroma(37)

potato(37)

bread-like(37)

Aroma D e s c r i p t i o n 120°C 180°C

Lysine

Amino A c i d

Aromas Developed by Heating Glucose w i t h V a r i o u s Amino A c i d s at 120°C and 180°C

Table VI



9.

MABROUK


219

(35-41). The aroma produced by the i n t e r a c t i o n o f sugars and amino a c i d s ranged from p l e a s a n t t o o b j e c t i o n a b l e . The molecular s t r u c t u r e o f sugar i n f l u e n c e s the r e s u l t i n g aroma o f the products of i t s r e a c t i o n w i t h the amino a c i d . While a p l e a s a n t caramel sweet aroma developed upon r e a c t i n g p h e n y l a l a n i n e w i t h maltose, an unpleasant caramel aroma developed w i t h f r u c t o s e , and a h y a c i n t h aroma i n case o f dihydroxyacetone. While the aroma o f methionine-sugar browning product was r e m i n i s c e n t o f a p l e a s a n t baked potato i n presence o f dihydroxyacetone, aroma notes o f overcooked potatoes developed w i t h glucose. I n presence o f a reducing d i s a c c h a r i d e (maltose), overcooked cabbage aroma was n o t i c e d and an unpleasant burnt wood aroma developed i n presence of non-reducing d i s a c c h a r i d e (sucrose). P r o l i n e , v a l i n e and i s o l e u c i n e i n presence o f glucose gave a p l e a s a n t bakery aroma (41). Keeney and Day (42) s t u d i e d the c h a r a c t e r odor o f the products of S t r e c k e r Degradation o f amino a c i d s u s i n g i s a t i n as the oxidant (Table V I I ) . Table V I I Aroma o f S t r e c k e r Degradation Products o f α-Amino A c i d s with I s a t i n (Diketodihydroindole) Sensory D e s c r i p t i o n Amino A c i d s Aldehyde Keeney & Day(42) Others(43.44) α-Alanine acetaldehyde reminiscent of wine, malty coffee(43) Valine 2-methy1apple f r u i t y , b a n a n a - l i k e (4 3) propanal green,pungent sweet(44) Leucine 3- methylmalty fruity,peach-like(43) butanal burnt,green,sickly(44) Isoleucine malty-apple 2-methylburnt,sickly(44) butanal Norleucine pentanal flower slightly fruity,herb aceous , n u t - l i k e (43) burnt-green(44) Phenyl h y a c i n t h odor,resenfole phenylacetviolets alanine benzaldehyde i n aldehyde taste(43) Glutamic b a c t e r i a l agar Cystine methional cheesy-brothy References (42,43,44) There i s some agreement and disagreement between the sensory notes s t a t e d i n Tables V and V I I , which may be a t t r i b u t e d t o v a r i a t i o n s i n the c o n c e n t r a t i o n o f the r e a c t a n t s and l a c k o f agreement on d e s c r i p t i o n o f sensory notes. The d i s c r e p a n c i e s can be r e s o l v e d , by a c a r e f u l c o n s i d e r a t i o n of the s p e c i f i c c o n d i t i o n s of the r e a c t i o n . I t i s p o s s i b l e t h a t both the nature and extent o f the



220


r e a c t i o n vary widely with the p r i n c i p a l v a r i a b l e s ( c o n c e n t r a t i o n , temperature, h e a t i n g p e r i o d , s p e c i f i c r e a c t a n t s ) . U n f o r t u n a t e l y , many of the papers i n s u f f i c i e n t l y d e f i n e the experimental c o n d i t i o n s f o r an adequate e v a l u a t i o n of the r e s u l t s . The d e s c r i p t i o n of odor and t a s t e notes of a product or a compound v a r i e s according to i t s c o n c e n t r a t i o n , the media used f o r e v a l u a t i o n ( water, p a r a f f i n o i l , skim m i l k . . .etc ) > and the sensory e v a l u a t o r . The data i n Table VI i n d i c a t e that the aromas developed when sugars and amino a c i d s were h e a t i n g a t 120 and 180°C, were q u i t e d i f f e r e n t from those produced a t 100°C. N o t i c e a b l e d i f f e r e n c e s e x i s t between aroma notes developed a t 120°C and 180°C, t h i s may be a t t r i b u t e d t o sugar degradation notes which become n o t i c e a b l e i n the system heated a t 135*C and above. A l l a l i p h a t i c compounds c o n t a i n i n g primary o r secondary amine group r e a c t i n browning r e a c t i o n but a t d i f f e r e n t r a t e s depending upon the molecular s t r u c t u r e . The r e a c t i v i t y of the r e a c t a n t s (amino a c i d s , carbonyl compounds) i n the browning r e a c t i o n had been reported i n the l i t e r a t u r e as : a. i n t e n s i t y o f c o l o r developed, b. amount of carbonyl compounds determined by GC o r as dinitrophenylhydrazones. c. percent l o s s of r e a c t a n t s (one r e a c t a n t or both). I t i s very hard t o compare r e s u l t s reported i n the l i t e r a t u r e , as there are v a r i o u s v a r i a b l e s i n f l u e n c i n g the data. Concentrat i o n of r e a c t a n t s , r a t i o of amine compound to sugar o r c a r b o n y l , r e a c t i o n i n b u f f e r o r water, m o l a r i t y and type of b u f f e r used, pH o f b u f f e r , d u r a t i o n o f r e a c t i o n . Rate of carbonyl compounds formation w i l l be the c r i t e r i o n used f o r r e a c t i v i t y except i n few occasions where the percent l o s s of one o f the r e a c t a n t s or both w i l l be r e f e r r e d t o i n the manuscript. Rooney e t a l (£5) reported that the r a t e of carbonyl formation v a r i e d w i t h the molecular s t r u c t u r e of sugar. Xylose was most r e a c t i v e as i t produced the g r e a t e s t q u a n t i t y of c a r b o n y l s , f o l l o w e d by glucose, then maltose. I n the presence of these sugars i s o l e u c i n e was more r e a c t i v e than phenylalanine. In a study on the S t r e c k e r degradation of v a l i n e - c a r b o n y l , d i a c e t y l showed the g r e a t e s t r e a c t i v i t y f o l l o w e d by sorbose> arabinose>xylose>fructose>glucose>sucrose>rhamnose, S e l f ( 4 6 ) . The number of v o l a t i l e carbonyls produced by the r e a c t i o n of glucose and glutamic a t pH 5.0,6.5 and 8.0 a t 100°C was almost equal. When these systems were heated a t 180°C, the number of v o l a t i l e s was g r e a t e r and increased more d r a s t i c a l l y as pH increased to a l k a l i n i t y (36). The amount of aldehydes produced from mixtures of 0.01M amino a c i d and 0.1M glucose i n water was f a r below those produced i n 0.01M phosphate b u f f e r a t pH6.5, which i n t u r n was f a r below the amount obtained a t pH 7.5. The a d d i t i o n of phosphate b u f f e r increased the amount of aldehydes produced s i g n i g i c a n t l y . The amount of aldehyde produced i s a l s o i n f l u e n c e d by the chain l e n g t h of the a c i d . I n g e n e r a l ,



9.

MABROUK


221

s t r a i n g t chain amino acids produced more aldehydes than branched chain amino acids o f the same number o f carbon atoms (k6). Alanine, v a l i n e , s e r i n e , glutamic, glutamine, methionine, t a u r i n e , h i s t i d i n e , c r e a t i n e , c i t r u l l i n e , carnosine, c y s t i n e , s y s t e i n e , a s p a r t i c , asparagine, l e u c i n e , i s o l e u c i n e , t y r o s i n e , p h e n y l a l i n i n e , tryptophan, α-aminobutyric, p r o l i n e , o r n i t h i n e , 2 - p y r r o l i d o n e - 5 - c a r b o x y l i c , threonine, g l y c i n e , l y s i n e , a r g i n i n e , cysteine were i d e n t i f i e d i n aqueous r e d meat f l a v o r precursors ( V f ) . Ribose was a l s o reported as the major sugar. F l a v o r evalu a t i o n o f the browning r e a c t i o n products o f these amino acids and r i b o s e were undertaken. 0.1M s o l u t i o n o f each amino a c i d was pre pared by d i s s o l v i n g the amino a c i d i n 0.05M S^rensen phosphate b u f f e r and r e a d j u s t i n g the pH by the a d d i t i o n o f monopotassium o r dipotassium phosphate. 0.1M r i b o s e s o l u t i o n was prepared i n the same b u f f e r . Ten ml o f amino a c i d and 10ml o f ribose s o l u t i o n s were p i p e t t e d i n 50ml glass ampoule and 50ml round bottom f l a s k with 2k/kO glass j o i n t . 1.0 mMoles o f i n s o l u b l e amino acids were weighed d i r e c t l y i n r e a c t i o n v e s s e l s , and 10ml b u f f e r added. To study the e f f e c t o f heating on f l a v o r o f amino acids i n absence of r i b o s e , 10ml b u f f e r were p i p e t t e d i n glass ampoules and f l a s k s along with the amino a c i d . The ampoules were sealed a f t e r evacu a t i o n , and heated f o r one hour at l80°C. The f l a s k s were heated i n g l y c e r o l bath at 100°C, a f t e r f i t t i n g with condensors. A f t e r the heating p e r i o d , the f l a s k s and the ampoules were removed, cooled i n wet i c e , and the contents t r a n s f e r r e d t o v i a l s and kept at -10°C. A l l experiments were run i n t r i p l i c a t e s . In absence of r i b o s e , while s u l f u r containing amino acids s o l u t i o n s produced s u l f u r y notes upon heating, none o f the others developed any f l a v o r notes. Table VIII l i s t s the s t r i k i n g f l a v o r notes produced by heating various amino a c i d s - r i b o s e s o l u t i o n s at 100° and l80°C. Table VIII F l a v o r Notes Produced by Heating Amino Acid-Ribose AT 100°C and l80°C Amino A c i d

Alanine Valine Serine Glutamic

Methionine Cysteine Taurine

Flavor 100°C

Description

very m i l d caramel s i c k l y sweet sweet b o u i l l i o n brothy, s l i g h t l y sweet, l i n g e r i n g i n mouth s u l f u r y , savory s u l f u r y , r o t t e n egg pleasant t o f f e e

180°C sweet burnt caramelized sugar penetrating burnt chocolate burnt sugar roasted meat

c r u s t o f roast meat s u l f u r y , s p i c y meat s i c k l y , sweet, burnt caramel


222


Table V I I I (cont'd) F l a v o r Notes Produced by Heating Amino Acid-Ribose At 100°C and 180°C Amino A c i d

Tryptophan


Phenylalanine Histidine

Creatine Tyrosine Leucine Aspartic Cystine Glutamine Asparagine a-Aminob u t y r i c γ-Aminobuturic 3-Aminobutyric Arginine Ornithine Proline Cysteic

Flavor Description 100°C 180°C

o i l y aromatic, naphththalene flowery w i t h aromatic and caramel notes, u n d e r s i r a b l e pleasant s l i g h t l y burnt salty, slightly b i t t e r caramelized caramel toffee s l i g h t l y sweet caramel slightly salty custard s l i g h t l y burnt sugar s l i g h t caramel b i t t e r almond toasted bread caramelized bread c r u s t bread crumb meaty w i t h H2S note hard b o i l e d egg yolk butterscotch caramel w i t h burnt sugar note creamy b u t t e r s c o t c h d e s i r a b l e burnt sugar sugar maple undersirable burnt sugar maple burnt sugar maple custard b u t t e r y , burnt sugar burnt sugar bread-like bread crumb cracker, toast bread crumb meaty, s u l f u r y undersirable, sulfury meaty, p l e a s a n t brothy o i l y aromatic, sugar sweet sharp f l o w e r

2-Pyrrolidone -5-carboxylic Isoleucine aromatic, undesirable Glycine caramel, f a i n t Lysine custard Homocystine canned m i l k Threonine c u s t a r d weak Citrulline toffee-like Carnosine buttery-toffee

burnt cheese burnt sugar bread scorched b o i l e d m i l k burnt c u s t a r d meaty meaty



9.

MABROUK


223

The previous d i s c u s s i o n covered S t r e c k e r degradation o f amino a c i d s sugar aqueous s o l u t i o n s . A l i m i t e d number o f i n v e s t i gations were c a r r i e d out on the nature of the browning r e a c t i o n a t h i g h temperature, i . e . low temperature p y r o l y s i s ana at low moisture content. Rohan and coworkers had provided an e x t e n s i v e research on model systems s i m u l a t i n g cocoa n i b r o a s t i n g i n absence of l i p i d . Amino a c i d s and sugars were r e s p o n s i b l e f o r chocolate f l a v o r development (48,49,50,51). While the degradation o f amino a c i d s i n cocoa beans d u r i n g r o a s t i n g was incomplete, the degrading agent, reducing sugar was completely destroyed. In a study on model systems prepared from s i n g l e amino a c i d and glucose i n molar r a t i o 2:1, approximating the composition of s h e l l ~ f r e e cocoa beans, Rohan and Stewart (52) reported t h a t amino a c i d d e s t r u c t i o n from h e a t i n g was temperature dependent and p r a c t i c a l l y ceased a f t e r one hour. Sugar d e s t r u c t i o n c o n t i n t u e d at a r a t e dependent on r e a c t i o n temperature u n t i l the end o f the experiment (Table I X ) . Table IX D e s t r u c t i o n of Amino A c i d s and Ρeducing Sugars on Heating Chocolate Aroma P r e c u r s o r E x t r a c t Heating D e s t r u c t i o n Percent °C Min. Amino A c i d s Red. Sugars T o t a l Sugars Aroma 50 50 50 100 100 100 120 120 120 150 150 150 150

6.2 8.9 8.9 18.8 29.6 30.9 43.0 54.4 54.4 58.8 81.7 85.4 85.1

30 60 120 30 60 120 30 60 120 10 30 60 120

Reference

(52)

27.0 41.1 27.5 56.6 81.4 84.1 100 93.5

35.8 79.6 22.8 38.9 79.2 91.2

Cocoa Cocoa Cocoa Cocoa Cocoa Cocoa Cocoa Cocoa

journal of Food Science

The f a i l u r e of amino a c i d degradation to go to completion and the r e l a t i o n between maximal d e s t r u c t i o n and temperature might be a t t r i b u t e d to the f a c t that i n d i v i d u a l amino a c i d s reacted a t d i f f e r e n t r a t e s at d i f f e r e n t temperatures. For example c e r t a i n amino a c i d s , l e u c i n e , a r g i n i n e , methionine, and l y s i n e showed l i t t l e or no d e t e c t a b l e r e a c t i o n a t 100°C a f t e r one hour, w h i l e others were r e a c t i v e a t t h i s temperature(Table X). At higher temperatures, r a t e o f amino a c i d degradation was measurable and p r o p o r t i o n a l t o the temperature o f the r e a c t i o n .


224


The e f f e c t of i n c r e a s i n g temperature on the amount of amino a c i d destroyed i n one hour was n o t i c e a b l y greater a t temperatures above 140°C than below 120°C. Between 120° and 140 C,,increase i n r e a c t i o n temperature had l e s s marked e f f e c t on the r e a c t i o n due to changes i n the p h y s i c a l c o n d i t i o n of the r e a c t i o n mixture. At about 150°C. glucose melts and , as i t might be expected, i t s mixture w i t h amino a c i d s would begin to melt a t lower temperature. e

Table X


D e s t r u c t i o n of Amino A c i d s A f t e r Heating With Glucose a t D i f f e r e n t Temperatures f o r One Hour Temp. °C 80 90 100 110 120 130 135 140 150

D e s t r u c t i o n Percent Threo G l u Meth Leu V a l A r g L y s

A l PhAl

11.3 24.3 33.4 44.8 76.9

13.9 21.0 22.2 23.2 32.0

2.0 22.9 41.2 42.2 59.4 90.0

59.8

0.9 11.1 0.9 23.1 28.8 28.5 26.2 40.5 39.0 44.3

10.5 19.5 27.2 39.9

7.0 14.7 23.1 26.2 28.2 30.2 42.5

50.9 51.9 62.1

1.5 1.0 14.5 29.0 33.8 40.0

4.5 19.5 28.0 30.2 44.5

51.5

Reference (52)

Journal of Food Science

Table XI i n d i c a t e s that the degree of degradation was i n f l u e n c e d by the molecular s t r u c t u r e of the amino a c i d . A t 120°C the r a t e of d e s t r u c t i o n of amino a c i d s by c l a s s was i n the f o l l o w i n g order: s u l f u r - c o n t a i n i n g , hydroxy>acidic>neutral, basic,aromatic. Table XI Degradation of Amino Compounds A f t e r Heating f o r One Hour i n Presence of Glucose Class of Amino Compounds Percent Degradation a t ^ 120 135 150 Neutral a- n - A l k y l b*- i s o - a l k y l Hydroxy Aromatic Acidic Sulfur-containing Basic

20-28 42 23 36 41 20-29

40 42.5 90 — — —

40-45

45 — —

60 51 52 —

Reference (,52)



9.

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225

Moisture content o f the r e a c t i o n mixture i n f l u e n c e d the degradation o f amino a c i d s , Rohan and Stewart (52) reported that chocolate precursor aroma e x t r a c t s when heated i n the dry s t a t e f o r one hour a t 100°C, l o s t 30% o f the amino a c i d s by degradation. When very l i g h t l y moistened the amino a c i d s degradation dropped t o 9% under the same r e a c t i o n c o n d i t i o n s . When the r e a c t i o n mixture was wetted w i t h an equal weight o f water, no amino a c i d degradation was n o t i c e d . Cocoa beans moisture content averaged 5 t o 6 % which might be s u f f i c i e n t to i n h i b i t browning r e a c t i o n a t lower temperatures, thus i n the e a r l y stages o f r o a s t i n g the removal of moisture i s important. Roasting cocoa beans r e s u l t s i n the p r o d u c t i o n of v o l a t i l e and n o n - v o l a t i l e compounds which c o n t r i b u t e t o the t o t a l f l a v o r complex. 5-Methyl-2-phenyl-2-hexenal, which e x h i b i t e d a deep b i t t e r p e r s i s t a n t cocoa note, was reported i n the v o l a t i l e f r a c t i o n (53). I t was p o s t u l a t e d t o be the r e s u l t o f a l d o l condensation of phenylacetaldehyde and i s o v a l e r a l d e h y d e w i t h the subsequent l o s s of water. The two aldehydes were the p r i n c i p a l products o f S t r e c k e r degradation products of phenylal a n i n e and l e u c i n e , r e s p e c t i v e l y . N o n - v o l a t i l e s contained d i k e t o p i p e r a z i n e s ( d i p e p t i d e anhydride) which i n t e r a c t w i t h theobromine and develop the t y p i c a l b i t t e r n e s s o f cocoa (54). Theobromine has a r e l a t i v e l y s t a b l e m e t a l l i c b i t t e r n e s s , b u t cocoa b i t t e r n e s s i s r a p i d l y n o t i c e d and disappears q u i c k l y . While cocoa b i t t e r n e s s i s f e l t i n the whole mouth, theobromine i s recognized by the h i n d p a r t o f the tongue. Furthermore, cocoa b i t t e r n e s s i s more intense than t h a t o f concentrated aqueous s o l u t i o n of theobromine. The f o l l o w i n g d i k e t o p i p e r a z i n e s had been i d e n t i f i e d as the components r e s p o n s i b l e f o r cocoa b i t t e r n e s s : - cyclo(-Pro-Leu-), cyclo(-Val-Phe-), cyclo(-Pro-Phe-), cyclo(-Pro-Gly-) , c y c l o ( - A l a - V a l - ) , c y c l o ( - A l a - G l y - ) , c y c l o (Ala-Phe-), c y c l o ( - P h e - G l y - ) , c y c l o ( - P r o - A s n - ) , and c y c l o (-Asn-Phe-). Those c o n t a i n i n g phenylalanine e x h i b i t e d b i t t e r n e s s resembling that of theobromine. D i k e t o p i p e r a z i n e s have stronger b i t t e r n e s s than the corresponding d i p e p t i d e s o r i t s two i n d i v i d u a l amino a c i d s . They develop upon h e a t i n g p r o t e i n s t o temperature above 100°C Kato e t a l (55) reported that r o a s t i n g s e r i n e a t 280°C f o r 30 min under slow n i t r o g e n stream, r e s u l t e d i n the p r o d u c t i o n o f 2,5-diketo-3,6-dimethylpiperazine, which upon a l k a l i n e h y d r o l y s i s produced the d i p e p t i d e a l a n y l a l a n i n e . A b i t t e r p e p t i d e , cyclo(-L-Leu-L-Trp-), was i s o l a t e d from c a s e i n enzymic d i g e s t (J>6). The formation o f c y c l o (-Pro-Leu-) i n aged sake i s r e s p o n s i b l e f o r i t s b i t t e r n e s s (57)· Recently, f i v e b i t t e r L - p r o l i n e - c o n t a i n i n g d i k e t o p i p e r a z i n e s were reported i n roasted malt (210°C) used i n dark beer brewing. These compounds and t h e i r approximate " b i t t e r t h r e s h o l d % v a l u e s " i n aqueous s o l u t i o n s were: cyclo(-L-Phe-L-Pro-),0.1; cyclo((-L-Leu-L-Pro) , 0.05; cyclo(-L-Pro-L-Pro-),0.1; c y c l o ( - L - V a l - L - P r o - ) , 0 . 1 ; and c y c l o ( - L - I l e - L - P r o - ) , 0 . 0 5 . The authors concluded that these f


FOOD T A S T E


226

CHEMISTRY

d i k e t o p i p e r a z i n e s d i d not c o n t r i b u t e d i r e c t l y to beer b i t t e r n e s s and questioned t h e i r r o l e i n i n f l u e n c i n g the t a s t e of the product (58). More than 300 compounds had been i d e n t i f i e d i n cocoa v o l a t i l e s , 10% of which were c a r b o n y l compounds (59,60). Acetaldehyde, 2-methylpropanal, 3-methylbutanal, 2-methylbutanal, phenylacetaldhyde and propanal were products of S t r e c k e r degradation of a l a n i n e , v a l i n e , l e u c i n e , i s o l e u c i n e , phenylacetaldehyde, and α-aminobutyric a c i d , r e s p e c t i v e l y . Eckey (61) reported that raw cocoa beans c o n t a i n about 50-55% f a t s , which c o n s i s t e d of p a l m i t i c (26.2%), s t e a r i c (34.4%), o l e i c (37.3%), and l i n o l e i c (2.1%) a c i d s . During r o a s t i n g cocoa beans these a c i d s were o x i d i z e d and the f o l l o w i n g c a r b o n y l compounds might be produced:- o l e i c : 2-propenal, b u t a n a l , v a l e r a l d e h y d e , hexanal, h e p t a n a l , o c t a n a l , nonanal, decanal, and 2-alkenals of C3 to C - Q . L i n o l e i c : e t h a n a l , p r o p a n a l , p e n t a n a l , hexanal, 2-alkenals of C3 to C , 2 , 4 - a l k a d i e n a l s of Cg to C - Q , methyl e t h y l ketone and h e x e n - l , 6 - d i a l . Carbonyl compounds p l a y a major r o l e i n the formation of cocoa f l a v o r components. Another example of food i n which the browning r e a c t i o n occurs at near anhydrous c o n d i t i o n i s c o f f e e . Coffee beans are u s u a l l y r o a s t e d a t temperatures ranging from 180° to 260°C. f o r s p e c i f i c p e r i o d to o b t a i n the d e s i r e d degree of r o a s t i n g ( l i g h t , medium, and d a r k ) . P r o t e i n , sucrose, and c h l o r o g e n i c a c i d were the compounds d r a s t i c a l l y destroyed i n c o f f e e beans upon r o a s t i n g , Table X I I "the data i n t h i s t a b l e were not c o r r e c t e d f o r dry weight l o s s which v a r i e d from 2 to 5%"(62). 1Q

Table X I I

Composition of Green and Roasted Coffee Percent, Dry B a s i s Roasted Green

Constituent Hemicelluloses Cellulose Lignin Fat Caffeine Sucrose Chlorogenic a c i d Protein(Based on n o n a l k a l o i d nitrogen) Trigonelline Reducing sugars Unknown Total

23.0 12.7 5.6 11.4 1.2 7.3 7.6

24.0 13.2 5.8 11.9 1.3 0.3 3.5

11.6 1.1 0.7 14.0

3.1 0.7 0.5 31.7

100.0 Reference

100.00

(62) Journal of Agricultural and Food Chemistry


9.

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227


The degree of r o a s t i n g i n f l u e n c e d the magnitude of degradation of sucrose, the major sugar present i n c o f f e e , Table X I I I . Table X I I I E f f e c t of Roasting On Sucrose Content I n Coffee (Percent Dry B a s i s )


Colombian Sucrose % Loss Content Green 4.59 L i g h t Roast 0.45 Medium Roast 0.17 Dark Roast 0.06 Reference (62)

90.20 96.30 98.69

Santos Sucrose % Loss Content 5.47 0.68 0.27 0.10

87.57 95.06 98.17

Feldman et a l (62) found no n o t i c e a b l e changes i n the contents of the f o l l o w i n g amino a c i d s i n roasted c o f f e e beans proteins: alanine, glutamic, g l y c i n e , i s o l e u c i n e , l e u c i n e , p h e n y l a l a n i n e , p r o l i n e , t y r o s i n e , and v a l i n e . These f i n d i n g s are i n agreement w i t h those reported by Underwood and Deatherage (63). Table XIV showed the degree of d e s t r u c t i o n of amino a c i d s i n c o f f e e bean a f t e r r o a s t i n g . I t i s q u i t e obvious that the magnitude of nonenzymic browning of amino a c i d s was i n f l u e n c e d by i t s molecular s t r u c t u r e and degree of r o a s t i n g . Coffee o i l contains about 47% l i n o l e i c , 8% o l e i c , 1% hexadecenoic, 32% p a l m i t i c , 8% s t e a r i c , and 5% behenic and longer c h a i n f a t t y a c i d s (64). As l i n o l e i c a c i d i s the major unsaturated f a t t y a c i d i n c o f f e e o i l , i t s major o x i d a t i o n products 2,4a l k a d i e n a l s and hexen-1,6-dial would p l a y a major r o l e i n v o l a t i l e p r o d u c t i o n . Green c o f f e e beans c o n t a i n 50 to 60% carbohydrates: 18% nonhydrolyzable c e l l u l o s e , 13% h y d r o l y z a b l e c e l l u l o s e , 13% starches and p e c t i n s e a s i l y s o l u b l i z e d , and 9-12% s o l u b l e carbohydrates of which sucrose i s the major component. R a f f i n o s e and stachyose are the t r i - and t e t r a saccharides reported i n robusta c o f f e e beans. Arabinogalactan and galactomannon are the w a t e r - s o l u b l e p o l y s a c c h a r i d e s reported i n c o f f e e beans (62). The above mentioned carbohydrates, amino a c i d s , l i p i d s along w i t h other f l a v o r p r e c u r s o r s produce s e v e r a l hundreds of v o l a t i l e and n o n v o l a t i l e compounds through d i f f e r e n t r e a c t i o n s during r o a s t i n g . Molecular s t r u c t u r e s , q u a n t i t i e s , and r a t i o s of these compounds i n f l u e n c e c o f f e e f l a v o r . The v a r i e t y of c o f f e e as w e l l as the degree of r o a s t i n g e x e r t c h a r a c t e r i s t i c f l a v o r s . The molecular s t r u c t u r e of carbonyls i n f l u e n c e the type of p r y a z i n e formed. While r e f l u x i n g rhamnose (100g) and ammonia (28%; 40ml) i n water (100g) produced methyl- and e t h y l - s u b s t i t u t e d p y r a z i n e s , glucose and ammonia r e a c t i o n r e s u l t e d i n formation of m e t h y l - s u b s t i t u t e d



5.60 3.73

Serine Threonine

Reference (62)

Loss %

39.09

2.85 6.19 2.06

Histidine Lysine Methionine

Total

4.72 10.50 3.44

Green

Arginine Asparagine Cysteine

Amino A c i d

47.56

20.50

1.77 2.43

1.99 2.54 2.32

0.00 9.07 0.38

Roast I

Haita

51.80

18.84

1.26 1.83

2.17 2.74 1.48

0.00 9.02 0.34

Roast I I

37.85

5.88 3.82

2.79 6.81 1.44

3.61 10.61 2.89

Green

40.79

22.41

2.60 2.71

2.27 3.46 1.08

0.00 9.53 0.76

Roast I

Columbia

(After Acid Hydrolysis),%

60.02

15.63

0.80 1.38

1.61 2.76 1.26

0.00 7.13 0.69

Roast I I

17.73

32.48

55.26

14.53

0.00 1.08

0.85 2.56 1.71

0.00 8.19 0.14

Roast I I

Journal of Agricultural and Food Chemistry

45.41

0.14 2.37

2.23 2.23 1.68

0.00 8.94 0.14

4.97 3.48

1.79 5.36 1.29

2.28 9.44 3.87

Roast I

Angola Robusta Green

Composition of Amino A c i d s i n Green and Roasted Coffee

Table XIV


9.

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229

p y r a z i n e s o n l y (53). The f l a v o r s o f the products o f c y s t e i n e glucose and cysteine-pyruvaldehyde i n anhydrous c o n d i t i o n at d i f f e r e n t temperatures 80-190°C. are l i s t e d i n Table XV (24). Five-mM cysteine+5-mM glucose or pyruvaldehyde r e a c t e d a t d i f f e r e n t temperatures f o r 5 minutes and then evaluated by f l a v o r panel.


Table XV F l a v o r s Produced by Cysteine-glucose And Cysteine-pyruvaldehyde a t D i f f e r e n t Temperatures F l a v o r D e s c r i p t i o n , a t °C. Reactants 80° 100° 130° 160° Japanese rice cracker with sesamelike.

Cysteine & Pyruvaldehyde

Japanese rice cracker weak.

Sesame, weak

Sesame,

Cysteine & Glucose

no odor

no odor

Japanese Japanese rice rice cracker cracker with with seasame- sesamelike. like, sweet.

Reference (24)

190° Sesameburnt,

Sweet sesame, burnt,

Agricultural and Biological Chemistry

Pyruvaldehyde i s a l i q u i d a t room temperature and b o i l s a t 72°C, thus when cysteine-pyruvaldehyde mixture was heated a t 80°C, the components are i n s o l u t i o n and f l a v o r notes r e m i n i s c e n t o f Japanese r i c e c r a c k e r developed. As r e a c t i o n temperatures i n c r e a s e d g r a d u a l l y other f l a v o r notes developed. I n the case o f c y s t e i n e - g l u c o s e system, no r e a c t i o n took p l a c e u n t i l the r e a c t i o n temperature reached 130°C. The f l a v o r o f c y s t e i n e - g l u c o s e was comparable t o that o f cysteine-pyruvaldehde a t 160°C, w i t h one e x c e p t i o n , the glucose system had a sweet note. As temperature increased the f l a v o r impression of both systems increased i n s i m i l a r i t y . The v o l a t i l e compounds produced a t 160°C i n the presence of pyruvaldehyde were d i f f e r e n t from those i n presence of glucose. While t h i a z o l e and t h a z o l i n e s were absent i n the v o l a t i l e s o f c y s t e i n e - g l u c o s e , cysteine-pyruvaldehyde v o l a t i l e s were devoid o f p y r i d i n e s , p i c o l i n e s and furans (24).


230


Food f l a v o r s c o n s i s t of numerous compounds, none of which alone i s c h a r a c t e r i s t i c o f s p e c i f i c food. Classes of compounds which emcompass food f l a v o r s are:- hydrocarbons ( a l i p h a t i c , a l i c y c l i c , aromatic); carbonyls (aldehydes, ketones); c a r b o x y l i c a c i d s , e s t e r s , imides, anhydrides; a l c o h o l s , phenols, e t h e r s ; alkylamines, a l k y l i m i n e s ; a l i p h a t i c s u l f u r compounds ( t h i o l s , mono-, d i - and t r i - s u l f i d e s ) ; n i t r o g e n h e t e r o c y c l i c s ( p y r r o l e s , p y r a z i n e s , p y r i d i n e s ) ; s u l f u r h e t e r o c y l i c s (thiophenes, t h i a z o l e s , t r i t h i o l a n e , t h i a l i d i n e ) ; and oxygen-heterocyclics (lactone, pyrone, furan). D i s c u s s i o n w i l l be l i m i t e d to s t r i k i n g developments i n h e t e r o c y c l i c s .


H e t e r o c y c l i c Compounds During browning r e a c t i o n i n foods, h e t e r o c y c l i c compounds of known f i v e - and six-membered r i n g systems c o n t a i n i n g one or more atoms than carbon as r i n g members, are produced. These compounds encompass s e v e r a l c l a s s e s o f compounds that e x h i b i t d e s i r a b l e c h a r a c t e r i s t i c o r g a n o l e p t i c notes, i . e . t o a s t e d , b r e a d - l i k e , roasted, brothy, mushroom-like, nutty, ... e t c . , or undesirable notes, e.g. peppery ammoniacal, obnoxious, ... e t c . The nomenc l a t u r e s and the molecular formulas of h e t e r o c y c l i c compounds that most f r e q u e n t l y encountered i n food v o l a t i l e s are given i n Figures 1 and 2. Nitrogen-heterocyclic P y r r o l e and p y r r o l e d e r i v a t i v e s . The chemical and b i o l o g i c a l value o f p y r r o l e and i t s d e r i v a t i v e s cannot be overemphasized, n a t u r a l pigments, heme, c h l o r o p h y l l , b i l e pigments and enzymes l i k e cytochromes, contain p y r r o l e nucleus. A l s o , many a l k a l o i d s , p r o l i n e and hydroxyproline contain the reduced p y r r o l e r i n g (pyrrolidine). P y r r o l e and i t s d e r i v a t i v e s are found among the products of the browning r e a c t i o n products i n processed foods. A l k y l p y r r o l e s have intense petroleum-like odor, but they give sweet, s l i g h t l y burnt aroma on extreme d i l u t i o n . On the other hand a c y l p y r r o l e s have c h a r a c t e r i s t i c sweet smoky, and a l i t t l e m e d i c i n e - l i k e odor (65). Although a l k y l - and a c y l - p y r r o l e s do not e x h i b i t favorable aroma l i k e tne d e s i r a b l e roasty aroma o f p y r a z i n e s , they may p l a y an important r o l e i n the c h a r a c t e r i s t i c r o a s t y f l a v o r o f processed foods. In anhydrous c o n d i t i o n , Na c e t o n y l p y r r o l e , which had a b r e a d - l i k e aroma was i s o l a t e d from the r o a s t i n g products of p r o l i n e - g l u c o s e , hydroxyproline-glucose, and pyrrolidine-pyruvaldehyde (66). 1 - P y r r o l i n e which e x h i b i t e d an amine l i k e or c o r n - l i k e odor was the product of Strecker degradation of p r o l i n e - , and ornithione-sugar or- polycarbonyl reagents i n aqueous s o l u t i o n s ( 6 j ) . 2-Pyrrolealdehyde, 2 - a c e t y l p y r r o l e , 2 - p r o p i o n y l p y r r o l e , N-methyl-2-pyrrolealdehyde, N-methyl-2-acetyl p y r r o l e , 5-methyl-2-pyrrolealdehyde, N-methyl-5-methyl-2-pyrrole-


9.


MABROUK

ο ο

Η pyrrole (azole)

o

Η pyrrolidine (azolidine)

Η 2,3-dihydropyrrole (2,3-dihydroazole)

ο Ο


thiophene (thiole)

/r-N

ΟΗ

imidazole (1,3-diazole)

tetrahydrothiophene (thiolane)

trithiolane

tetrahydrofuran (oxolone)

Figure 1.

Q

2,5-dihydrofuran 2,5-dihydroxole

o

0

t-pyran

S

S

piperidine perhydroazine

pyridazine (1,2-diazine)

1,3,5oxadithiane

S

1,3,5dithiazine

1,3,5trithione

0

pyrimidine (1,3-diazine)

ΗΝ

0

pyrazine (1,4-diazine)

NH

1,3,5thiadiazine

0 0

y-pyran

9

isoxazole

oxazole (1,3-oxazole)

Fwe-membered ring heterocyclics

S 1,2,6trithiane

Η 3H-pyrazole (3H-I,2-diazole)

thiazole (1,3-thiazole)

O O G O

pyridine (azine)

f

0

S-S

Q Q

furan (oxole)

231

a-pyrone

y-pyrone

9

1,3,5dioxathiane Figure 2.

Six-membered ring heterocyclics



232


aldehyde were i d e n t i f i e d i n the p y r r o l i c f r a c t i o n o f coffee v o l a t i l e c o n s t i t u e n t s (68, 69). N- ethylpyrrole-2-aldehyde and 5-methylpyrrole-2-aldehyde are two o f the eighteen compounds r e ported i n s t o r e d dehydrated orange j u i c e c r y s t a l s due to nonenzymic browning (JO). The type o f p y r r o l e d e r i v a t i v e s produced de pended on r e a c t i o n temperature, i t s d u r a t i o n , i t s c o n d i t i o n , i . e . , anhydrous, aqueous, or a l c o h o l i c ; pH o f the media, and molecular s t r u c t u r e o f r e a c t a n t s . While, N - a l k y l p y r r o l e - 2 - a l d e h y d e s were produced upon h e a t i n g D-xylose and alkylamine or amino a c i d i n n e u t r a l aqueous or methanolic s o l u t i o n s at 55 100°C; the c o r r e sponding 5-methyl d e r i v a t i v e s o f N - s u b s t i t u t e d pyrrole-2-aldehydes were formed i n the presence o f L-rhamnose (71, 72). Odor o f the products were: N-methylpyrrole-2-aldehyde (formed from D-xylose and methylamine) cinnamonaldehyde-like, N-n b u t y l p y r r o l e - 2 - a l d e hyde (a product o f butylamine-xylose); and l - n - b u t y l - 5 - m e t h y l pyrrole-2-aldehyde (rhamnose and butylamine i n t e r a c t i o n s product) x y l e n e - l i k e . 2 - F o r m y l p y r r o l - l - y l - a l k y l a c i d s were i s o l a t e d from the products o f the browning r e a c t i o n o f xylose and alkylamines or amino acids i n aqueous s o l u t i o n s (73). N-substituted-5-(hydroxymethyl)-pyrrole-2-aldenydes were formed from the r e a c t i o n o f the aldohexose "glucose" and alkylamines or amino a c i d s c o n t a i n i n g primary amino group at 70 t o 100°C, i n n e u t r a l i z e d aqueous, methanolic or e t h a n o l i c s o l u t i o n s . The r e s u l t i n g compounds were considerably unstable and had no odor i n the pure s t a t e but d e v e l oped r o a s t e d aroma with browning (jh). Upon r o a s t i n g glucose and s e v e r a l alkyl-n-amino a c i d ( g l y c i n e , α-alanine, α-amino-n-butyric, v a l i n e , l e u c i n e , α-amino-n-caproic) at 200-250°C i n two components systems; 2 - 5 - h y d r o x y - m e t h y l - 2 ' - f o r m y l p y r r o l - l ^ y l ) a l k a n o i c a c i d lactones were formed as the main v o l a t i l e products (75)· The aroma d e s c r i p t i o n o f the prepared l a c t o n e d e r i v a t i v e s were: pro p i o n i c a c i d l a c t o n e (from α-alanine and glucose) caramel and a l i t t l e s c o r c h i n g ; i s o b u t y r i c a c i d l a c t o n e (α-amino-n-butyric and glucose) maple and strong sweet; i s o v a l e r i c a c i d lactone ( v a l i n e and glucose) and i s o c a r p r o i c a c i d l a c t o n e (leucine and glucose) miso, soy sauce and a c h o c o l a t e - l i k e . The y i e l d o f p r o p i o n i c a c i d l a c t o n e a f t e r h e a t i n g an equimolar mixture o f 0.01 mole o f glucose and α-alanine at 150, 200, and 250°C were: 1

Temperature, °C

Heating p e r i o d , min.

Y i e l d , unmoles h2

1 5 3 5 5

250 250 200 200 150

3 - h 50 6 16*

I t i s q u i t e obvious from the above data that the p r o p i o n i c a c i d lactone disappeared during the r e a c t i o n f o r longer periods and at higher temperatures. 2-(5 -hydroxymmethyl-2 -formylpyrrol-1 -yl) -3-methylbutanoate, and 2 -(5 -hydroxymethyl-2 -formyl1

,

f

f

1

f


9.


MABROUK

233

p y r r o l - l - y l ) - 3 - m e t h y l b u t a n o i c a c i d lactone were i d e n t i f i e d i n the products o f the browning r e a c t i o n o f glucose and v a l i n e i n aqueous s o l u t i o n at 65°C f o r three weeks (76). Reducing d i s a c c h a r i d e s ( l a c t o s e , maltose and melibiose) r e a c t e d with alkylamine i n aqueous s o l u t i o n s o f pH 6 . 5 , to form l-alkyl-5-hydroxymethylpyrrole-2-aldehyde (77)· N - A l k y l - 2 - a c y l p y r r o l e s and a l i p h a t i c a l d i mines were the products o f the r e a c t i o n between f u r f u r a l and i t s homologs with α-amino acids (78). Reactions o f f u r f u r a l and 2a c e t y l f u r a n with g l y c i n e and v a l i n e produced a small amount of the corresponding a c y l a l k y l p y r r o l e s which had pleasant aromas r e m i n i scent of benzaldehyde. The r e s u l t i n g considerable amount o f a l i phatic aldimines from the r e a c t i o n o f f u r f u r a l with v a l i n e and l e u c i n e possessed a strong odor from b i t i n g and unpleasant t o m i l d and f o o d - l i k e . Nine a l k y l p y r r o l e s , three a c y l p y r r o l e s , three a l k y l p y r r o l e - 2 - a l d e h y d e s , three f u r f u r y l p y r r o l e , eight a l k y l p y r a z i n e s , and one oxazoline were i d e n t i f i e d i n the v o l a t i l e flavorous products o f r o a s t i n g DL-alanine and glucose at 250°C f o r one hour i n n i t r o g e n atmosphere (79). Many o f the compounds i d e n t i f i e d i n the r e a c t i o n products of model systems had been i s o l a t e d from roasted and cooked foods. For example, p y r r o l e , 1-methylpyrrole, 2-methylpyrrole, 1-formylpyrrole, 2 - f o r i L y l p y r r o l e , l - f o r m y l - 2 methylpyrrole, 2-fοrmyl-1-methylpyrrole, 2-fοrmyl-1-methylpyrrole, 2-formyl-5-methylpyrrole, 2 - f o r m y l - l - e t h y l p y r r o l e , 1-acetylpyrr o l e , 2 - a c e t y l p y r r o l e , 1 - f u r f u r y l p y r r o l e , 2 - p r o p i o n y l p y r r o l e , 2f o r m y l - 1 - f u r f u r y l p y r r o l e were reported i n roasted peanut (8θ). Roasted f i l b e r t s v o l a t i l e s contained 1-methylpyrrole, 2-pentylpyrr o l e , 2 - i s o b u t y l p y r r o l e , and 2 - p e n r y l p y r r i d i n e ( 8 l ) .


f

P y r i d i n e and i t s d e r i v a t i v e s . The most unique p y r i d i n e d e r i v a t i v e i s o l a t e d from processed food i s l , ^ , 5 6 - t e t r a h y d r o - 2 - a c e t o p y r i d i n e . This compound was prepared by r o a s t i n g p r o l i n e and dihydroxyacetone at 92°C i n presence o f sodium b i s u l f a t e , and ex h i b i t e d a strong odor reminiscent o f f r e s h l y backed soda crackers (J32). 2 - E t h y l p y r i d i n e and 2 - p e n t y l p y r i d i n e were reported i n v o l a t i l e f l a v o r components of shallow f r i e d (83). P y r i d i n e , 2methylpyridine, 3-methylpyridine, 2 - e t h y l p y r i d i n e , 3 - e t h y l p y r i dine, 5-ethyl-2-methylpyridine, 2 - b u t y l p y r i d i n e , 2 - a c e t y l p y r i d i n e , 2- p e n t y l p y r i d i n e , 2 - h e x y l p y r i d i n e , 3 - p e n t y l p y r i d i n e , 5-methyl-2p e n t y l p y r i d i n e , and 5 - e t h y l - 2 - p e n t y l p y r i d i n e were i d e n t i f i e d i n the v o l a t i l e s of roasted lamb f a t (Qh). 2,5-Dimethylpyridine and 3,5-dimethylpyridine were t e n t a t i v e l y i d e n t i f i e d i n roasted lamb f a t v o l a t i l e s . The odor t h r e s h o l d o f 2 - p e n t y l p y r i d i n e was 0 . 5 - 0 . 7 parts per 1 0 p a r t s o f water. The d i l u t e s o l u t i o n o f 2-pentylpy r i d i n e has a f a t t y or t a l l o w - l i k e odor. The authors a t t r i b u t e the unacceptance of lamb by some consumers to the high content o f a l k y l p y r i d i n e s i n roasted lamb. While p y r i d i n e d e r i v a t i v e s have burnt, heavy f r u i t y odors (JB5 ) ; p y r i d i n e has a disagreeable char a c t e r i s t i c odor and sharp t a s t e ; and p i p e r i d i n e has a peppery ammoniacal odor (06). P y r i d i n e , α-picoline (2-methylpyridine), 3- p i c o l i n e (^-methylpyridine), and U-ethylpyridine were i d e n t i f i e d 5

9


234


i n c o f f e e aroma

(87).

P y r a z i n e s . In the t h i r t i e s , the a t t e n t i o n on p y r a z i n e s was focused on i t s i n d u s t r i a l r o l e i n dyes, photographic emulsions and chemotherapy. I t s importance i n l i f e processes was i n d i c a t e d i n i t s derivative, vitamin B ( r i b o f l a v i n , 6,7-dimethyl-9-(l -Dr i b i t y l i s o a l l o x a z i n e ) . L a t e r , i n the m i d s i x t i e s , i t was i d e n t i f i e d i n foods and i t s c o n t r i b u t i o n s t o the unique f l a v o r and aroma o f raw and processed foods a t t r a c t e d the a t t e n t i o n o f f l a v o r chemista Pyrazine d e r i v a t i v e s c o n t r i b u t e to the r o a s t i n g , t o a s t i n g , n u t t y , chocolaty, c o f f e e , earthy, caramel, m a p l e - l i k e , b r e a d - l i k e , and b e l l pepper notes i n foods. The reader i s r e f e r r e d t o the reviews on Krems and S p o e r r i (88) on the chemistry o f p y r a z i n e s , and the review o f pyrazines i n foods by Maga and S i z e r (89, 90.)· Table XVI summaries sensory d e s c r i p t i o n and t h r e s h o l d o f s e l e c t e d pyrazines. f


2

Oxygen H e t e r o c y c l i c s . During heat p r o c e s s i n g , sugars degrade to aldehydes and ketones which might r e a c t with amino compounds forming caramelized sugar f l a v o r s . C y c l i c diketones, pyrones, and furan d e r i v a t i v e s are examples o f the products o f t h i s r e a c t i o n . Table XVII gives the o r g a n o l e p t i c d e s c r i p t i o n o f s e l e c t e d compounds. U-Hydroxy-2,5-dimethyl(2H)-furanone has intense f r a g r a n t caramel note described as burnt sweet t a s t e (TO ), burnt pineapple ( l O l ) , beef b r o t h (lOO), strawberry preserve (103), nutty sweet aroma o f almonds (lOU), and major c o n t r i b u t o r to sponge cake f l a v o r (105). Seven terms were used to describe the o r g a n o l e p t i c note o f one compound. The question a r i s e s , what was the concent r a t i o n and media used f o r e v a l u a t i o n ? Change i n c o n c e n t r a t i o n alone can be quite s u f f i c i e n t t o a l t e r the c h a r a c t e r o f the f l a v o r or odor note. For example, t r i m e t h y l a m i n e - a i r mixtures only smell f i s h y over a narrow range o f d i l u t i o n s (1:1,500-1:8,000), with a maximum at about ( 1 : 6 , 0 0 0 ) , a l s o ammonia at a d i l u t i o n o f about (1:2,000) smells f i s h y (106). Hodge and Moser (107) r e p o r t e d that i n s p i t e o f the f a c t that the aromas o f pure m a l t o l , e t h y l m a l t o l , i s o m a l t o l , and h-hydroxy-2,5-dimethyl-3(2H)-furanone vary s i g n i f i c a n t l y from each o t h e r , p a n e l i s t s d e s c r i p t i o n was caramel or burnt sugar. The f r u i t y caramel aroma o f i s o m a l t o l i s weaker than that o f 4-hydroxy-2,5-dimethyl-3(2H)-furanone (108). Our sensory vocabulary should be adequate t o express the impact o f f l a v o r , odor, t a s t e notes components. This could be achieved by the p a r t i c i p a t i o n o f food chemist, organic chemist, sensory a n a l y s t and f l a v o r ist. Soy sauce (Shoyu) contains tautomers U-hydroxy-2-ethyl-5ethyl-3(2H)-furanone and U-hydroxy-5-ethyl-2-methyl-3(2H)-furanone (about 3:2 r a t i o ) which has sweet odor s i m i l a r t o that o f short cake (109). The same tautomers were s y n t h e s i z e d and described as possessing the f l a v o r o f cooked f r u i t s (103). M a l t o l , i s o m a l t o l and 2-methyl-5-hydroxy-6-ethyl-a-pyrone are c o n t r i b u t o r s to the c h a r a c t e r i s t i c aroma of molasses (110).



sweet f r i e d odor

2 . 6 - Dimethyl

2-Ethyl-3methyl2 - E t h y l -5methyl 2-Ethyl-3,6dimethyl-

2,3,5-Trimethyl 2,3,5,6-Tetramethyl 2-Ethyl

earthy raw potato (2)

(3)

deep b i t t e r p e r s i s t e n t notes (5)

Sensory D e s c r i p t i o n

2,3-Dimethyl2 . 5 - Dimethyl-

2-Methyl-

Pyrazine

cocoa

17 (91)

60 (92) 22 (91) 0.13 (92)

O.OOOU (92)

0.10 (92)

38 (91)

10 (91)

8 (91)

7 (91)

27 (91)

27 (91)

(92)

(91)

0.02U (92)

0.32

2.6

Odor Threshold ppm Mineral O i l Vegetable O i l

(91) (92) (92) (91) (92) (91) (92) (91)

105 60 2.5 3.5 1.8 5^ 1.5 9

Water

P y r a z i n e s , Sensory D e s c r i p t i o n and Odor Threshold

Table XVI


8 ox

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2-methylamino-3methyl 2-dimethylamino-3methyl 2-dimethylamino-6methyl

2,6-dimethoxy-3isopropyl-5-methyl 2,5-dimethoxy-3i sopropyl-6-methyl 5-methyl 5H-6,7dihydrocyclopenta 5-methyl-2H-3,U,5,7retrahydrocyclopenta 2-acetyl2-methoxy-3-ac e t y l -

Pyrazine

(53)

breadcrust, nutty (96) weak breadcrust, nutty notes (96) r o a s t e d peanuts, green cocoa notes (96) P e a n u t - l i k e more green and l e s s cocoa notes (96) p e a n u t - l i k e , no cocoa notes reminiscent o f burnt c o f f e e (96)

r o a s t e d nuts, burnt

nutty with g r e e n - l i k e b e l l pepper, woody notes (96) nutty notes, green ( b e l l p e p p e r - l i k e ) (96) peanuts (53)


Water

(Cont.)

Odor Threshold ppm Mineral O i l Vegetable O i l

P y r a z i n e s , Sensory D e s c r i p t i o n and Odor Threshold

Table XVI


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238


Table XVII Oxygen H e t e r o c y c l i c Compounds

Compound


Spicy, smoky, s l i g h t l y cinnamonl i k e odor (98).


Fur an

Sweet b r e a d - l i k e , caramel-like t a s t e (98_). (99)»

Furfury1 a l c o h o l

Coconut

5-methyl-2-furfural

Sharp grape (98) ·

M a l t o l (3-hydroxy-2methyl-pyrone)

P l e a s i n g strong f r u i t s y f r e s h bread (99)»

Isomaltol

F r u i t y caramel, f r e s h bread odor, overtone m e d i c i n a l grassy (99)»

N-Furfuryl pyrrole

Green h a y - l i k e aroma

^-hydroxy-5-methy13(2H)-Furanone

Beef broth (lOO) burning sweet t a s t e (JO), burnt pineapple (101).

2,5-Dimethyl-U-hydroxy3(2H)-Furanone

Odor of roasted c h i c o r y roots (102).

References (98

-

(98).

102).

S u l f u r H e t e r o c y c l i c s . S u l f u r c o n t a i n i n g compounds ( t h i o l s , thiophenes, t h i a z o l e s , ... etc.) p l a y a major r o l e i n the f l a v o r o f raw and processed foods. These compounds have c h a r a c t e r i s t i c f l a v o r notes and the f l a v o r thresholds are mostly low. Several reviews ( i l l , 112, 113) demonstrate the important r o l e o f s u l f u r compounds i n food f l a v o r s . Organoleptic p r o p e r t i e s of these compounds may be pleasant, strong n u t - l i k e odor of i+-methyl-5v i n y l t h i a z o l e which i s present i n cocoa (ilk); objectionable p y r i d i n e - l i k e odor o f t h i a z o l e (115.); q u i n o l i n e - l i k e odor of benzothiazole ( l l 6 ) ; strong tomato l e a f - l i k e odor of i s o b u t y l t h i a z o l e (ill); and bread c r u s t f l a v o r o f a c e t y l - 2 - t h i a z o l i n e ( l l 8 ) . A mixture o f oxazoles, t h i a z o l e s , t h i a z o l i n e s , imidazoles, t r i t h i o l a n e s and


9.

MABROUK


239


d i t h i a n e s which had a meaty f l a v o r was obtained from a model sys tem c o n s i s t i n g o f α-dicarbonyls, e t h a n a l , hydrogen s u l f i d e and ammonia (119)« Unsaturated aldehydes r e a c t with hydrogen s u l f i d e and t h i o l s t o give mainly a d d i t i o n products t o the carbon-carbon double bond (120). The nomenclature o f the r e s u l t i n g compounds and t h e i r o r g a n o l e p t i c d e s c r i p t i o n s are given i n Table XVIII. T h i o l s , thiophenes, t h i a z o l e s , s u l f i d e s and furans were i d e n t i f i e d i n the v o l a t i l e s o f heating glucose, hydrogen s u l f i d e , and ammonia at 100°C f o r two hours (121). These v o l a t i l e s gave a roast beef l i k e aroma. Complex mixtures o f mercapto-substituted furans and thiophene d e r i v a t i v e s , which were reminiscent o f r o a s t meat were produced upon h e a t i n g t-hydroxy-5 methyl-3(2H)-furanone and i t s t h i o analog with hydrogen s u l f i d e (122). L i p i d Browning. L i p i d browning r e a c t i o n s o f the M a i l l a r d type between carbonyl groups (provided by sugars o r sugar degradation products and those r e s u l t i n g from unsaturated f a t t y a c i d s o x i d a t i o n ) and the f r e e amino groups present i n phospholipids have been recognized as potent causes o f undesirable f l a v o r , c o l o r and t e x t u r e changes i n dehydrated foods (123). Phosphatidyl ethanolamine, p h o s h a t i d y l s e r i n e , ethanolamine and s e r i n e plasmalogens c o n t a i n f r e e amino groups which can undergo l i p i d browning r e a c t i o n s . Phospolipids are u s u a l l y r i c h i n h i g h l y unsaturated f a t t y acids i n comparison with n e u t r a l l i p i d s , thus they are good sources o f carbonyls. A l s o , the primary amine moitiés o f p o l a r l i p i d s c a t a l y z e the a l d o l condensation o f C i ^ - C i e aldehydes r e s u l t i n g from plasmalogen h y d r o l y s i s , thus forming a,3-unsaturated aldehydes (12k). Phosp h a t i d y l ethanolamine r e a c t e d with propanal and n-hexanal forming phosphatidyl l - ( 2 - h y d r o x y e t h y l ) - 2 - e t h y l - 3 , 5 - d i m e t h y l p y r i d i n i u m , and p h o s p h a t i d y l - l - ( 2 - h y d r o x y e t h y l ) - 2 - h e x y l - 3 , 5 - d i p e n t y l p y r i d i nium, r e s p e c t i v e l y (125). The p e r i d i n i u m r i n g i s formed by the r e a c t i o n between one mole o f amino-N o f p h o s p h a t i d y l ethanolamine and three moles o f n-alkanals. The same r e a c t i o n took p l a c e i n the s y n t h e s i s o f s u b s t i t u t e d p y r i d i n e s by condensation o f carbonyl compounds with ammonia (126, 1 2 7 ) ·

Abstract In processed foods, non-enzymic browning reaction is the major source of its desirable flavors. Flavors of the products of this reaction depend upon: the molecular structure of nitrogenous com pounds (amines, amino acids, peptides, glycopeptides, proteins, . . . etc.); aldoses, ketoses, non-reducing, deoxy sugars, sugar acids, ... etc.); heating temperature; duration of the reaction; initial hydrogen ion concentration, moisture content, and the media of the reaction (alcoholic, aqueous, or anhydrous). The ratio of the nitrogenous compound to sugar or carbonyl compound has great effect on the flavor notes. Comparison betwen browning



Reference

(120)

1-Methyl thiooctan-3-one

l-Octene-3-one

thiobutanal thiohexanal thioheptanal thiononal

3-methyl 3-Methyl 3-Methyl 3-Methyl

A d d i t i o n Product

2-Butenal 2-Hexenal 2-Hebtenal 2-Nonenal

Carbonyl Added Impression

Cheese-like ( B r i t e ) Cabbage (Rubbery) Unripe tomato Bast-like, slightly floral Radish-like

Flavor

0.5 5 5 3

Water

0.5 50 80 20

Paraffin O i l

F l a v o r o f Products o f the A d d i t i o n Between Unsaturated Carbonyls and Methanethiol

Table XVIII


9.

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241

reaction products and rate of degradation of reactants under anhy drous condition, aqueous and alcoholic media was discussed. Flavor notes of products of amino compounds-sugars reaction were reviewed with emphasis on amino-ribose system. Meat-like flavor was imparted by browning reaction products of carnosine-, citrul line-, histidine-, glutamic-, 2-pyrrolidone-5-carboxylic-, methionine-, cysteine-, cysteic-, and taurine-ribose. Recent advancements in nitrogen-, oxygen- and sulfur hetercyclics and lipid browning were presented. Literature Cited.


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Flavor of Browning Reaction Products - American Chemical Society

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