Thermal Generation of Aromas - ACS Symposium Series (ACS

Chapter 1

Thermal Generation of Aromas Downloaded by UNIV OF NEWCASTLE on March 26, 2017 | http://pubs.acs.org Publication Date: October 3, 1989 | doi: 10.1021/bk-1989-0409.ch001

A n Overview Thomas H. Parliment General Foods USA Technical Center, 555 South Broadway, Tarrytown, NY 10591

Uncooked foods commonly possess only weak aromas and desirable aromas are generated upon cooking. For exam ple, unroasted coffee beans possess a weak green, bean character while raw beef possesses a weak, slightly sweet, and bloody character. Numerous chemical reac tions occur upon roasting and the typical, highly desir able aroma of roasted coffee and roasted beef are gener ated. Large numbers of volatile aroma chemicals are produced during these thermal reactions. For example, as of 1987 there were approximately 750 volatiles iden tified in roasted coffee and 700 identified in cooked beef. Aromas are generated in foods primarily by three processes: 1. Enzymatic and microbial processes which liberate low molecular weight volatile chemicals. Biological reactions are particularly important in the aromas of fruits and vegetables, berries, fermented dairy products, and alcoholic beverages. A recent ACS Symposium reviewed these processes (1). 2. The second route to aromas involves the production of chemical precursors during a fermentation step. Subsequent heating generates aromas from these bio logically-derived precursors. Cocoa and bread aroma are two examples of these types of reactions. 3. The third path to aromas are non-enzymatic processes resulting from thermal treatment such as cooking and roasting. These reactions typically include thermal decomposition of lipids, carbohydrates and proteins, and are responsible for the aroma of foods such as coffee, meat, nuts, cereals, and also contribute to the aromas of heat processed foods and vegetables. Aromas generated by thermal processes are the subject of this book. 0097-6156/89/0409-0002$06.00/0 ο 1989 American Chemical Society Parliment et al.; Thermal Generation of Aromas ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

1.

PARLIMENT

Thermal Generation ofAromas

Downloaded by UNIV OF NEWCASTLE on March 26, 2017 | http://pubs.acs.org Publication Date: October 3, 1989 | doi: 10.1021/bk-1989-0409.ch001

The M a i l l a r d o r n o n - e n z y m a t i c b r o w n i n g r e a c t i o n was f i r s t d e s c r i b e d by L. C. M a i l l a r d i n 1912 (2) i n h i s c l a s s i c s t u d y c o v e r i n g t h e r e a c t i o n o f s u g a r s and amino a c i d s . The w i d e v a r i e t y o f a r o m a s g e n e r a t e d i n t h e M a i l l a r d R e a c t i o n r e s u l t s f r o m t h e l a r g e number o f r e a c t a n t s w h i c h c a n c o m b i n e d u r i n g t h e t h e r m a l r e a c t i o n . Hodge e t a l (3) have c l a s s i f i e d t h e f l a v o r s p r o d u c e d i n t h i s r e a c t i o n i n f o u r groups. 1. C a r a m e l i z e d s u g a r 2. R o a s t e d , t o a s t e d , b a k e d , n u t t y 3. C o r n y and a m i n e - l i k e 4. B u r n t a r o m a s / b i t t e r t a s t e A summary o f t h e M a i l l a r d R e a c t i o n i s g i v e n i n Scheme 1 ( 4 ) . The f i r s t s t e p i n v o l v e s t h e c o n d e n s a t i o n b e t w e e n t h e c a r b o n y l g r o u p o f a r e d u c i n g s u g a r , and t h e f r e e amino g r o u p o f an a m i n o a c i d o r p e p t i d e t o produce a N-glycosylamine or f r u c t o s y l a m i n e . These g l u c o s y l - o r f r u c t o s y l - amines can r e a r r a n g e t o produce amino-deoxya l d o s e s o r k e t o s e s v i a Amadori (from g l u c o s e ) o r Heyns (from f r u c t o s e ) rearrangements. S t r u c t u r e s o f t h e s e two important i n t e r m e d i a t e s a r e shown i n F i g u r e 1. Q u a n t i t a t i v e l y , t h e m a j o r p a t h o f d e g r a d a t i o n o f t h e Amadori o r Heyns i n t e r m e d i a t e i s a d e h y d r a t i o n r e a c t i o n which y i e l d s f u r f u r a l and h y d r o x y m e t h y l f u r f u r a l . Of g r e a t e r f l a v o r s i g n i f i c a n c e a r e t h e minor pathways which can r e s u l t i n both a r o m a t i c p r o d u c t s as w e l l as r e a c t i v e i n t e r m e d i a t e s . These i n t e r m e d i a t e s can undergo a r e t r o a l d o l i z a t i o n r e a c t i o n t o p r o d u c e a l p h a d i c a r b o n y l compounds, s u c h a s p y r u v a l d e h y d e and d i a c e t y l as w e l l as r e a c t i v e m o n o c a r b o n y l s , such a s g l y c o l a l d e h y d e and g l y c e r a l d e t i y d e . The v a r i e t y o f aromas w h i c h c a n a r i s e f r o m t h e M a i l l a r d R e a c t i o n i s l a r g e . Lane and N u r s t e n (5) r e a c t e d more t h a n 400 m i x t u r e s o f amino a c i d s and c a r b o h y d r a t e s a t e l e v a t e d t e m p e r a t u r e s . They c l a s s i f i e d t h e aromas i n 14 g r o u p s as shown i n T a b l e I .

TABLE I . 6roup 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

GROUPING OF AROMA DESCRIPTORS USED

Descriptor(s) Sweet, b o i l e d s u g a r , c a r a m e l , t o f f e e Chocolate, cocoa Bread, c r u s t y , b i s c u i t s , cakes, t o a s t Meaty, beefy Potato, potato skins, potato crisps Fruity, aromatic ester Celery, chicory, leeks, Brussels sprouts, turnips P u f f e d Wheat, S u g a r P u f f s Nutty Floral Ammoniacal U n p l e a s a n t , 'caused coughing' Aldehydic Burnt, charred, scorched, a c r i d , potato crisps, t o a s t , smoky

Parliment et al.; Thermal Generation of Aromas ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

3


4

THERMAL GENERATION OF AROMAS

In a d d i t i o n t o t h e p r o d u c t s g e n e r a t e d b y t h e M a i l l a r d R e a c t i o n o t h e r r o u t e s e x i s t f o r t h e f o r m a t i o n o f aromas v i a t h e r m a l p r o c e s s e s . When l i p i d s a r e h e a t e d a l a r g e v a r i e t y o f compounds a r e f o r m e d . Hydrolysis o f t h e e s t e r linkage occurs, l i b e r a t i n g glycerol and v o l a t i l e f a t t y a c i d s . I t i s w e l l known t h a t t h e r m a l d e c o m p o s i t i o n o f t r i g l y c e r i d e s g e n e r a t e s a s e r i e s o f compounds i n c l u d i n g a l k a n a l s , a l k e n a l s , a l k a d i e n a l s , m e t h y l k e t o n e s , l a c t o n e s , and h y d r o c a r b o n s . H e a t i n g o f c a r b o h y d r a t e s p r o d u c e s a number o f a r o m a t i c compounds i n c l u d i n g aldehydes, ketones, and d i c a r b o n y l s a s w e l l a s oxygen c o n t a i n i n g h e t e r o c y c l i c compounds s u c h a s f u r a n s , d i h y d r o f u r a n o n e s , and p y r a n s t h r o u g h c a r a m e l i z a t i o n and d e h y d r a t i o n r e a c t i o n s . F i n a l l y , h e a t i n g o f amino a c i d s c a n p r o d u c e v o l a t i l e s i n c l u d i n g a l d e h y d e s , amines and hydrogen s u l f i d e . One m i n o r , b u t i m p o r t a n t , f l a v o r g e n e r a t i n g pathway i n v o l v e s the S t r e c k e r d e g r a d a t i o n o f a n a m i n o a c i d a s shown i n F i g u r e 2. I n t h i s r e a c t i o n , a n a l p h a a m i n o a c i d r e a c t s with an alpha dicarbonyl a t an elevated temperature t o p r o d u c e a n a l d e h y d e (one c a r b o n l e s s t h a n t h e amino a c i d ) a s w e l l a s an a l p h a amino k e t o n e . These products can r e a c t f u r t h e r t o y i e l d i m p o r t a n t h e t e r o c y c l i c aroma c h e m i c a l s s u c h a s p y r a z i n e s , t h i a z o l e s , and d i h y d r o f u r a n o n e s . ANALYTICAL METHODOLOGY I s o l a t i o n , c o n c e n t r a t i o n , separation, and i d e n t i f i c a t i o n o f t h e r m a l l y - d e r i v e d aromas i s a n i m p o r t a n t a r e a . T h e s e aromas a r e frequently associated with other non-volatile products i n c l u d i n g pigments, f a t s , carbohydrates, and p r o t e i n s . Techniques a r e r e q u i r e d t o separate the v o l a t i l e s from the n o n - v o l a t i l e s without causing chemical d e t e r i o r a t i o n . V o l a t i l e s i n t h e r m a l l y - d e r i v e d aromas a r e f r e q u e n t l y p r e s e n t a t levels of parts per million. I t i s generally necessary t o c o n c e n t r a t e t h e aroma b y many o r d e r s o f m a g n i t u d e prior t o identification. An i s o l a t i o n and c o n c e n t r a t i o n scheme s h o u l d b e a p p r o p r i a t e f o r t h e sample u n d e r i n v e s t i g a t i o n . As an example o f t h i s , a d i s t i l l a t i o n t e c h n i q u e would not be s a t i s f a c t o r y f o r s e p a r a t i n g c a r amel aroma compounds s u c h a s M a l t o l , C y c l o t e n e , o r F u r a n e o l w h i c h p o s s e s s a n e n o l o n e s t r u c t u r e and d o n o t steam d i s t i l . The g r e a t c o m p l e x i t y o f t h e r m a l l y - d e r i v e d aromas was p r e v i o u s l y mentioned. The p r e s e n c e o f h u n d r e d s o f v o l a t i l e c o m p o n e n t s r e q u i r e s t h a t t h e s e p a r a t i o n scheme, t y p i c a l l y g a s c h r o m a t o g r a p h y , possess h i g h r e s o l v i n g power. Fused s i l i c a c a p i l l a r y g a s c h r o m a t o g r a p h i c c o l u m n s o f 0.25, 0.32, o r 0.53 mm i . d . a r e u s e f u l s i n c e t h e s e c o l u m n s p o s s e s s 3,000-5,000 p l a t e s p e r m e t e r a n d s i n c e c o l u m n l e n g t h s l o n g e r t h a n 100 m e t e r s a r e a v a i l a b l e . F r e q u e n t l y , aroma c h e m i c a l s decompose i n t h e p r e s e n c e o f h o t m e t a l l i c s u r f a c e s . F u s e d s i l i c a c o l u m n s , i n a d d i t i o n t o b e i n g r o b u s t and e a s y t o h a n d l e , a r e inert. F i n a l l y we come t o t h e s u b j e c t o f i d e n t i f i c a t i o n t e c h n i q u e s . H i s t o r i c a l l y GC/MS h a s b e e n u s e f u l i n t h e aroma f i e l d . As s t r u c t u r e s o f i s o l a t e d a r o m a c h e m i c a l s became m o r e complex, a d d i t i o n a l t e c h n i q u e s , s u c h a s GC/FTIR and NMR were r e q u i r e d . T h e f i r s t s e c t i o n o f t h i s book a d d r e s s e s t h e i m p o r t a n t a r e a s o f i s o l a t i o n , c o n c e n t r a t i o n , and i d e n t i f i c a t i o n .


1. PARLIMENT

5


REDUCING SUGARS AND

Or-AM NO

ACIDS

(MAILLARD REACTION, 1912)

t N-GLYCOSYLAMINES

OR

N-FRUCTOSYLAMINES

t 1-AMINO-1-DEOXY-2-KETOSE

OR

-3H 0 2

1-AMINO-2-DEOXY-2-ALDOSES

(AMADORI AND HEYNS

INTERMEDIATES)

FURFURAL

t

(FROM PENTOSES)

HYDROXYMETHYL-5 FURFURAL


I

(FROM HEXOSES)

-REDUCTONES AND DEHYDROREDUCTONES H S

[

2

-

RETRO-ALDOLIZATION

AMINO

STRECKER

ACIDS

DEGRADATION

"1

f 3 - FURANONES

HYDROXYACETONE

4 - PYRANONES

CYCLOTENE

PYRROLES

DIHYDROXYACETONE

THIOPHENES

HYDROXYACETYL ACETOIN I

GLYOXAL

ALDEHYDES + #-AMINOKETONES

PYRUVALDEHYDE

(

GLYCOLALDEHYDE

(+ H£$ AND NHJ FROM CYSTEINE)

+

METHIONAL FROM METHIONINE)

GLYCERALDEHYDE

J HETEROCYCLIZATION PYRIDINES

THIAZOLES

PYRAZINES

PYRROLES

OXAZOLES

IMIDAZOLES

Scheme I. Formation of heterocyclic compounds in food products. (Reprinted with permission from ref. 4. Copyright 1982 Ellis Horwood Limited.)

H-C=0 H-C-NHR

H -C-0H 2

C=0

H-C-OH

H-C-OH

H-C-OH

H-C-OH

HEYNS

AMADORI

Figure 1. Amadori and Heyns intermediates.

NHp

0

H-CH-COOH + *0ICARB0NYL COMPOUND — R - C - H

0 NHp +• C0 + R-C-CH-R 2

Figure 2. Strecker degradation of an amino acid.


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ISOLATION AND I D E N T I F I C A T I O N STUDIES T h e r e a r e two p r o c e d u r e s w h i c h a r e f o l l o w e d 1n aroma r e s e a r c h . One i n v o l v e s i s o l a t i o n and i d e n t i f i c a t i o n o f a f o o d aroma, w h i l e t h e o t h e r i n v o l v e s model s y s t e m s t u d i e s . I n t h e f i r s t m e t h o d , v o l a t i l e compounds a r e removed f r o m a f o o d , concentrated, separated, and i d e n t i f i e d . More t h a n 7 0 0 c o f f e e v o l a t i l e c h e m i c a l s have been i d e n t i f i e d by t h i s t e c h n i q u e , b u t i t c a n b e v e r y l a b o r i o u s and t i m e c o n s u m i n g . A number o f c h a p t e r s i n t h i s book d i s c u s s c u r r e n t r e s e a r c h involving s p e c i f i c food f l a v o r s . S c h i e b e r l e h a s p r e s e n t e d some p r o v o c a t i v e work o n wheat b r e a d f l a v o r s . B r e a d f l a v o r s r e s u l t f r o m a combination o f both biochemical and thermal p r o c e s s e s . Yeast metabolism o f t h e bread ferment produces a s e r i e s o f v o l a t i l e and non-volatile metabolites. T h e s e compounds c a n r e a c t d u r i n g t h e baking o f the bread t o produce s u r f a c e browning, c r u s t f o r m a t i o n , and d e s i r a b l e aroma g e n e r a t i o n . Through t h e use o f aroma e x t r a c t d i l u t i o n a n a l y s i s , S c h i e b e r l e was a b l e t o d e m o n s t r a t e t h e i m p o r t a n c e o f 2 - a c e t y l - l - p y r r o l i n e i n b r e a d c r u s t aroma. He r e l a t e d t h i s chemical t o t h e presence o f y e a s t c e l l s i n t h e dough. In t h e absence o f yeast c e l l s , a s i n c h e m i c a l l y leavened bread, o n l y low levels o f 2 - a c e t y l - l - p y r r o l i n eare generated. Studies such a s t h i s suggest procedures t o develop h i g h e r q u a l i t y aromas i n f o o d products. O t h e r c h a p t e r s i n t h i s book d e a l w i t h meat f l a v o r s a n d demons t r a t e t h a t e x c i t i n g work c o n t i n u e s i n t h i s a r e a . Buckholz and B a i l e y presented two overviews on these t o p i c s . Approximately 680 compounds were i d e n t i f i e d a s o f 1988, i n c l u d i n g a l a r g e number o f h e t e r o c y c l i c compounds p o s s e s s i n g meaty a r o m a s . T h e i m p o r t a n c e o f w a t e r s o l u b l e , low m o l e c u l a r w e i g h t , n o n - v o l a t i l e p r e c u r s o r s i n b e e f f l a v o r was e m p h a s i z e d . I n one e x p e r i m e n t , b e e f was t a k e n , t h e f a t r e m o v e d , and t h e l e a n r e s i d u e was g r o u n d a n d w a t e r e x t r a c t e d . T h e e x t r a c t was d i a l y z e d , and t h e d i f f u s a t e MW < 15,000 was f r e e z e d r i e d t o g i v e a powder. When t h i s m a t e r i a l was h e a t e d , a g o o d meat a r o m a was d e t e c t e d . A number o f h e t e r o c y c l i c compounds w e r e i d e n t i f i e d . An i n t e r e s t i n g comment b y B a i l e y was t h a t t h e s e M a i l l a r d R e a c t i o n s a r e u s e f u l f o r p r e v e n t i n g warmed-over f l a v o r , w h i c h i s a r a n c i d , m e t a l l i c f l a v o r i n beef. New compound i d e n t i f i c a t i o n s a r e a l s o p r e s e n t e d . W e r k h o f f e t a l described studies involving r e a c t i o n mixtures o f c y s t e i n e , thiamine, glutamate, and ascorbate. T h e i r study produced a l a r g e v a r i e t y o f u n i q u e m i x e d h e t e r o c y c l i c mono- a n d d i s u l f i d e s p o s s e s s i n g r o a s t e d and m e a t y c h a r a c t e r . Many o f t h e s e compounds w e r e s u b s e q u e n t l y i d e n t i f i e d i n a c t u a l meat aroma s y s t e m s . MODEL SYSTEM FLAVOR STUDIES T h e o t h e r t e c h n i q u e e m p l o y e d i n aroma r e s e a r c h i n v o l v e s m o d e l s y s t e m studies. I n t h i s c a s e , we I n v e s t i g a t e t h e v o l a t i l e s f o r m e d when precoursors react. Among t h e a d v a n t a g e s o f model s y s t e m s t u d i e s are: 1. T h e r e a c t i o n m i x t u r e s a r e s i m p l e r - t h e number o f v o l a t i l e chemicals produced i s s m a l l e r than found i n a typical roasted food.


1.

PARLIMENT 2. 3. 4.


5.


S i g n i f i c a n t q u a n t i t i e s o f product are generated w h i c h makes i d e n t i f i c a t i o n e a s i e r . One may be a b l e t o p r e d i c t t h e s t r u c t u r e o f t h e products. I t c a n g i v e us i n s i g h t s i n t o r e a c t i o n s which occur i n aroma g e n e r a t i n g r e a c t i o n s . T h e r e a c t i o n c a n be u s e d t o p r o d u c e n a t u r a l aromas as f l a v o r a n t s when p r o p e r i n g r e d i e n t s and c o n d i t i o n s are chosen.

The t o p i c o f M a i l l a r d R e a c t i o n s was t h e s u b j e c t o f a r e c e n t A m e r i c a n C h e m i c a l S o c i e t y Symposium ( 6 ) . T h e w i d e v a r i e t y o f r e a c t i o n s t h a t c a n o c c u r i n s i m p l e model s y s t e m s c a n be e x t r a o r d i n a r y . F o r e x a m p l e , S h i b a m o t o commented i n t h i s symposium t h a t a s i m p l e rhamnose/NH3/H2S r e a c t i o n m i x t u r e g e n e r a t e d 1,000 g a s c h r o m a t o g r a p h i c p e a k s . A number o f p r e s e n t a t i o n s i n t h i s book c o v e r model systems o f various compositions. Some o f t h e m i x t u r e s s t u d i e d i n g r e a t d e t a i l and r e p o r t e d h e r e i n i n c l u d e : G l u c o s e / c y s t e i ne S h i bamoto S e r i ne/g1ucose Baltes Lys i n e / g 1 u c o s e Leahy P r o l i ne/rhamnose Shaw Cysteine/Furaneol Shu Cystei n e / r i bose/phospholi pi d Mottram B a l t e s concluded that temperature i s a major f a c t o r i n h i s r e a c t i o n system; h i g h e r temperatures f a v o r p y r a z i n e f o r m a t i o n and reduce furan production. L e a h y s t r e s s e d t h e e f f e c t o f Aw, a n d c o n c l u d e d t h a t maximum p y r a z i n e f o r m a t i o n o c c u r s a t Aw o n t h e o r d e r o f 0.75-0.84. T h e work o f Shu r e l a t e d i n t e r e s t i n g m e a t y , r o a s t e d o d o r s to r e a c t i o n parameters. The best r e a c t i o n s o c c u r r e d a t 75% water l e v e l and a temperature o f 160°C. Under these c o n d i t i o n s , t r i t h i o l a n e s , t h i o p h e n o n e s , and d i o n e s p r e d o m i n a t e . Mottram s t r e s s e d t h a t the presence o f phospholipids i n a r e a c t i o n system increased t h e amount o f m e a t y c h a r a c t e r . I n t h e f u t u r e , we c a n e x p e c t t o s e e f u r t h e r i n v e s t i g a t i o n s o f model s y s t e m s , f o r b o t h k n o w l e d g e b u i l d i n g a s w e l l a s p r a c t i c a l uses. M o d e l r e a c t i o n f l a v o r s y s t e m s have b e e n u s e d i n f l a v o r s f o r many years. Four decades ago, U n i l e v e r researchers r e c e i v e d a patent d e s c r i b i n g a r e a c t i o n s y s t e m w h i c h g e n e r a t e d a m e a t y aroma a n d f l a v o r ( 7 ) . That i n v e n t i o n d e s c r i b e d t h e r e a c t i o n o f a monosacchari d e w i t h a s u l f u r c o n t a i n i n g amino a c i d i n a q u e o u s m e d i a . A d e c a d e l a t e r , G i a c i n o (8) r e c e i v e d a patent f o r a r e a c t i o n f l a v o r which c o n s i s t e d o f a p r o t e i n h y d r o l y s a t e and a s u l f u r c o n t a i n i n g r e a g e n t . The s u l f u r s p e c i e s was d e s c r i b e d a s e i t h e r o r g a n i c o r i n o r g a n i c , a n d included c y s t e i n e , methionine, a l k y l mercaptans, hydrogen s u l f i d e , s o d i u m s u l f i d e , e t c . T h e p r o d u c t o f t h e s e r e a c t i o n s was d e s c r i b e d as " m e a t - l i k e . " T h e s e a n d many o t h e r p a t e n t s d e m o n s t r a t e t h e i m p o r tance o f r e a c t i o n chemistry i n the generation o f d e s i r a b l e food aromas.


7

8

T H E R M A L G E N E R A T I O N OF

AROMAS

EXTRUSION FLAVORS


Food e x t r u d e r s a r e d e v i c e s w h i c h one a p p a r a t u s : 1. M i x i n g 2. H e a t i n g 3. E n e r g y t r a n s f o r m i n g 4. Conveying 5. S h a p i n g 6. R e a c t i n g

accomplish

several operations in

C o o k i n g i s a c c o m p l i s h e d a t t h e same t i m e t h a t t h e p r o d u c t i s s h a p e d and e x t r u d e d . G e n e r a l l y , moistened s t a r c h and/or p r o t e i n a c e o u s m a t e r i a l s a r e c o n v e r t e d t o a t h i c k dough and c o o k e d i n t h e e x t r u d e r . C o o k i n g i s a c c o m p l i s h e d by steam i n j e c t i o n , by steam j a c k e t s , and by f r i c t i o n a l f o r c e s . In t h e c o u r s e o f c o o k i n g e x t r u s i o n , s t a r c h i s g e l a t i n i z e d and p r o t e i n i s d e n a t u r e d . Once c o o k e d , t h e p r o d u c t i s f o r c e d t h r o u g h a d i e a t t h e d i s c h a r g e end o f t h e b a r r e l . The p r o d u c t e x p a n d s and l o s e s m o i s t u r e due t o r a p i d d e c r e a s e i n pressure. A f t e r d r y i n g , the extruded product develops a r i g i d y e t p o r o u s s t r u c t u r e . The s u b j e c t has b e e n w e l l c o v e r e d by H a r p e r ( 9 ) . A t y p i c a l f o o d e x t r u d e r i s shown i n F i g u r e 3. C u r r e n t l y , many t y p e s o f e x t r u d e d f o o d p r o d u c t s a r e p r o d u c e d ( 9 ) and i n c l u d e r e a d y t o e a t (RTE) c e r e a l s , s n a c k f o o d s , b e v e r a g e b a s e s , as w e l l as s o f t - m o i s t and d r y p e t f o o d s . From an aroma v i e w p o i n t , t h e c o o k e r e x t r u d e r p o s s e s s e s d i s t i n c t d i s a d v a n t a g e s . The r e s i d e n c e t i m e o f p r o d u c t i n t h e b a r r e l i s s h o r t and r a n g e s f r o m 10 t o 120 s e c o n d s , and i s g e n e r a l l y l e s s t h a n 45 seconds. T e m p e r a t u r e s may be as h i g h as 180°C b u t a r e f r e q u e n t l y s i g n i f i c a n t l y lower. The p r e s s u r e may r e a c h 200 p s i . Moisture l e v e l s i n t h e f e e d c a n r a n g e f r o m 12% t o 55% - a t y p i c a l m o i s t u r e l e v e l f o r a RTE c e r e a l i s 16%. As a r e s u l t o f t h e s e p a r a m e t e r s - s h o r t t i m e , m o d e s t t e m p e r a t u r e , and m o d e r a t e m o i s t u r e l e v e l - t h e p r o d u c t has a b l a n d t a s t e s i n c e t h e d e s i r a b l e b r o w n i n g r e a c t i o n s do n o t o c c u r . Two s o l u t i o n s t o t h i s p r o b l e m have b e e n p r o p o s e d : 1. S p r a y i n g o r d u s t i n g a f l a v o r on t h e s u r f a c e o f t h e product after extrusion. 2. A d d i t i o n o f a f l a v o r p r e c u r s o r s y s t e m t o t h e ingredients prior to extrusion. After extrusion, t h e p r o d u c t c a n be d r i e d and t o a s t e d s o t h e desirable flavors to develop. Several d i s c u s s i o n s of extruder generated f l a v o r s are included i n t h i s b o o k . We c a n e x p e c t t o have t h i s t o p i c c o v e r e d i n g r e a t e r depth i n the f u t u r e . MICROWAVE GENERATED FLAVORS M i c r o w a v e f l a v o r g e n e r a t i o n i s an a r e a w h i c h has n o t y e t r e c e i v e d much a t t e n t i o n . I t 1s e s t i m a t e d t h a t m i c r o w a v e o v e n p e n e t r a t i o n i n US h o u s e h o l d s i s now a p p r o x i m a t e l y 75%. A few y e a r s a g o , f o o d p r o c e s s o r s s i m p l y added m i c r o w a v e i n s t r u c t i o n s t o t h e i r r e g u l a r l i n e o f products. More r e c e n t l y , m i c r o w a v e s p e c i f i c p r o d u c t s h a v e a r r i v e d and i n c l u d e p o p c o r n , p i z z a , c a k e and b r o w n i e m i x e s , s t u f f i n g m i x e s , and m a i n m e a l s . Major food p r o c e s s o r s w i l l c o n t i n u e t o d e s i g n



1.

PARLIMENT


9

Figure 3. Cross section of a typical food extruder. (Reprinted with permission from ref. 10. Copyright 1978 Institute of Food Technologists.)



10


m i c r o w a v e - s p e c i f i c p r o d u c t s , i n an attempt t o overcome the p e r c e i v e d l i m i t a t i o n s o f the microwave oven. M i c r o w a v e e n e r g y i s composed o f h i g h - f r e q u e n c y e l e c t r o m a g n e t i c r a d i a t i o n w h i c h c a u s e s h e a t i n g when t h e m i c r o w a v e s i m p i n g e u p o n t h e food. T h e m i c r o w a v e e n e r g y a l t e r n a t e s i n d i r e c t i o n 2,450 m i l l i o n times per second. T h i s r a p i d l y a l t e r n a t i n g f i e l d causes p o l a r molec u l e s , such a s water, t o attempt t o a l i g n with the f i e l d , and f r i c t i o n a l heat i s generated. In a t y p i c a l c o n v e c t i o n o v e n , h i g h t e m p e r a t u r e s d e h y d r a t e t h e surface o f t h e food, producing drying and d e s i r a b l e M a i l l a r d Reactions. I n a microwave oven such i s not the c a s e ; r a t h e r , t h e s u r f a c e o f t h e food remains c o o l , f r e q u e n t l y no h i g h e r than t h e temperature i n the oven c a v i t y . S u r f a c e h e a t i n g a n d d e h y d r a t i o n d o not o c c u r , a n d d e s i r a b l e browning r e a c t i o n s a r e r e t a r d e d . I n a d d i t i o n , s i n c e the microwave energy heats the i n t e r i o r p o r t i o n o f the food r a p i d l y , t h e t o t a l cooking time o f a microwave food i s r e d u c e d compared t o a c o n v e n t i o n a l l y p r e p a r e d f o o d , a n d l e s s t i m e i s a v a i l a b l e f o r h e a t g e n e r a t e d aroma r e a c t i o n s t o o c c u r . The p r o b l e m o f m i c r o w a v e - d e r i v e d aromas i s j u s t b e g i n n i n g t o b e a d d r e s s e d b y t h e f o o d i n d u s t r y . S e v e r a l c o n t r i b u t i o n s t o t h i s book c o v e r t h i s t o p i c and w i l l open the doors t o s o l v i n g the microwave browning problem. CONCLUSION F o o d s w i t h h e a t g e n e r a t e d aromas have b e e n consumed f o r e o n s . T h e M a i l l a r d o r non-enzymatic r e a c t i o n i s important i n d e v e l o p i n g t h e d e s i r a b l e aromas o f r o a s t e d a n d baked f o o d s s u c h a s c o f f e e , m e a t , nuts, chocolate and breads. T h e s e aromas a r i s e p r i m a r i l y f r o m t h e r e a c t i o n o f n o n - v o l a t i l e c a r b o h y d r a t e s a n d ami n o - c o n t a i n i n g g r o u p s . The p r e c u r s o r s may b e p r e s e n t i n t h e f o o d b e f o r e h e a t i n g , may b e g e n e r a t e d b y h e a t i n g , o r may a r i s e f r o m b i o c h e m i c a l p r o c e s s e s . I n a d d i t i o n , thermal processes p l a y a r o l e i n t h e development o f f l a v o r s i n c o o k e d f r u i t s and v e g e t a b l e s . R e c e n t r e s e a r c h h a s i d e n t i f i e d numerous aroma c h e m i c a l s i n r o a s t e d f o o d s , s u c h t h a t h u n d r e d s a r e known. C u r r e n t work i s d e v o t e d l e s s t o i d e n t i f i c a t i o n o f aroma c h e m i c a l s , b u t more t o i d e n t i f i c a t i o n o f s p e c i f i c f l a v o r notes o f s i g n i f i c a n t importance, and to maximizing the production o f these chemicals. Two a r e a s w h e r e we c a n e x p e c t t o s e e a d v a n c e s made a r e i n extruded f l a v o r s and microwave f l a v o r s . These two t e c h n o l o g i e s d o not l e n d themselves t o browning r e a c t i o n s due t o t h e time/temperature/moisture relationships. Thus t h e s e a r e a s remain a n a r e a o f active research. ACKNOWLEDGMENT The a u t h o r w i s h e s t o t h a n k J u d y Wein f o r s e c r e t a r i a l a s s i s t a n c e . LITERATURE CITED 1. Parliment, T. H.; Croteau, R. Biogeneration of Aromas; American Chemical Society Symposium Series 317: Washington, DC, 1986. 2. Maillard, L. C.; Compt. Rend. Acad. Sci. Paris 1912, 66, 154.



1. PARLIMENT


3. Hodge, J . E . ; Mills, F. D.; Fisher, B. E. Cereal Sci. Today 1972, 17, 34. 4. Vernin, G.; Parkanyi, C. In Chemistry of Heterocyclic Compounds in Flavours and Aromas. Vernin, G., Ed.; E l l i s Horwood Ltd.: Chichester, 1982; p 152. 5. Lane, M. J.; Nursten, H. E. In The Maillard Reaction in Foods and Nutrition; Walter, G.; Feather, M., Eds.; ACS Symposium Series 215; American Chemical Society: Washington, DC, 1983; pp 141-158. 6. Waller, G. R.; Feather, M. S. The Maillard Reaction in Foods and Nutrition; American Chemical Society Symposium Series 215: Washington, DC, 1983. 7. Morton, I; Akroyd, P; May; C. U.S. Patent 2934437, 1960. 8. Giacino, C. U.S. Patent 3394015, 1968. 9. Harper, J . M. Extrusion of Foods Vol. I: CRC Press Inc.: Boca Raton, FL, 1982. 10. Harper, J . M. Food Tech.; 1978; 32, 67 Received May 31, 1989


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Thermal Generation of Aromas - ACS Symposium Series (ACS

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