The Role Lipids Play in the Positive and Negative Flavors of Foods

1 The Role Lipids Play in the Positive and Negative Flavors of Foods

Downloaded by 46.183.221.231 on March 25, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0075.ch001

IRA LITMAN and SCHELLY NUMRYCH Stepan Chemical Company, Flavor/Fragrance Division, 500 Academy Drive, Northbrook, IL 60062 As members of a group of flavor chemists, we have long had a special interest in those flavors that are derived from l i p i d s , for lipids are generally associated with flavor defects in foods. It w i l l not be a surprise to many of you to learn that lipids are also the source of many of nature's finest flavor creations. Lipids, proteins, and carbohydrates, the chief structural components of living cells are also the major sources of flavor in foods. Of the three, lipids may be the most important for the following reasons: 1) They are precursors for many flavorful compounds, with representatives in the aliphatic aldehyde, ketone, lactone, fatty acid, alcohol and ester groups. 2) The intact glyceride tends to modify the flavor of fat soluble compounds by restraining their escape into the air space above. 3) They may also interfere with gustatory ingredients such as salts, sweetening agents, bitterants and acidulants from reaching saliva, a prerequesite for the sense of taste to occur. 4) As a cooking medium, triglycerides produce in foods special flavor effects as a result of, for example, deep fat frying. Their role here is to transfer heat and flavor to the cooking product. 5) It is frequently overlooked that lipids provide a range of polar and non-polar food grade solvents that are used by the food industry. From this range of solvents, one can select a suitable carrier for most any volatile material. Among these solvents are glycerol, mono- and diglycerides, triacetin, tributyrin and vegetable o i l s . 6) It is also important to note that glycerides, which act as reservoirs of volatile flavor, are themselves non-volatile and in this form they contribute to flavor largely through mouth stimulation. For example, they 0-8412-0418-7/78/47-075-001$05.00/0 © 1978 American Chemical Society Supran; Lipids as a Source of Flavor ACS Symposium Series; American Chemical Society: Washington, DC, 1978.


2

LIPIDS AS A SOURCE O F FLAVOR

impart to melting butter a c h a r a c t e r i s t i c "cooling" e f f e c t . When emulsified i n milk, they impart a r i c h ness that i s sorely missed i n the f l a v o r of skim milk. In i c e cream, the s o l i d i f i e d f a t globules give the product i t s c h a r a c t e r i s t i c "creamy-dryness" associated with good q u a l i t y . Generally, the negative q u a l i t i e s i n food f l a v o r are associated more c l o s e l y with l i p i d s than with carbohydrates or proteins. L i p i d s are responsible for r a n c i d i t y and oxidized f l a v o r s i n beverage milk, butter (!L) and vegetable o i l s (2) and f o r s p o i l i n g wet f i s h (3J · They are involved i n the s t a l e f l a v o r s of potato flakes (£) and baked goods ( 1 ) . L i p i d s are thought to be responsible f o r soybean reversion f l a v o r (5), f o r warmed over meat f l a v o r (6), for o l d heated cooking o i l f l a v o r (7), f o r the rancid f l a v o r s i n peanuts, coconut, coffee and chocolate (j3) , and f o r many others. On the other hand, l i p i d s are also responsible for much of the desireable f l a v o r of tangy cheeses such as Cheddar and roquefort (!9) , f o r the f l a v o r of fresh milk (1), for the "creamy" f l a v o r of cream (1) , for the " r i c h " f l a v o r i n heated butter (10) and f o r the c h a r a c t e r i s t i c f l a v o r s of mushrooms "(Tl) , green beans (12) , peas (13) , tomatoes (14J and cucumbers (15) and for much of the r i p e f l a v o r i n f r u i t s and b e r r i e s . The s i g n i f i c a n c e of l i p i d s to odor may well begin at the s i t e of o l f a c t i o n , at the two and a h a l f square centimeter patch of highly enervated t i s s u e located at the roof of the nasal c l e f t . Here, v o l a t i l e molecules are thought to be adsorbed and polar oriented between the l i p i d membrane portion of the nerve and the surrounding aqueous l a y e r . I t has been theorized that the adsorption and desorption of these molecules t r i g g e r s an e l e c t r i c impulse which the brain i n t e r prets (16) . How important are l i p i d s i n a r t i f i c i a l f l a v o r s ? This question was answered i n a recent survey of our f l a v o r formulations. I t was obvious that l i p i d s are the most common ingredients used both i n quantity and i n v a r i e t y . Only t e r p i n o i d compounds derived from the e s s e n t i a l o i l s of spices, woods, c i t r u s , and others compete i n t h i s regard. In the united States, there are presently 1150 v o l a t i l e compounds permitted i n a r t i f i c i a l f l a v o r s . Of these, nearly one fourth (275) are, when found i n nature, presumed to be derived from l i p i d s . Forty f i v e percent of these are esters, sixteen percent are aldehydes, t h i r t e e n percent are alcohols, nine percent are acids, nine percent are ketones and seven percent are lactones. Several

Supran; Lipids as a Source of Flavor ACS Symposium Series; American Chemical Society: Washington, DC, 1978.


1.

LTTMAN AND N U M R Y C H

Positive

and

Negative

Food

Flavor

3

a r t i f i c i a l f l a v o r s are composed e n t i r e l y of l i p i d compounds while most other compositions depend heavily on them. In nature, how do these compounds a r i s e ? We know that oxidation of l i p i d s i s the f i r s t step. Autoxidation of polyunsaturated l i p i d s i s one of these mechanisms and i s of concern to the o i l chemist and the food technologists because i t i s a nonenzymatic and s e l f - s u s t a i n i n g reaction that can cause o f f - f l a v o r development, t o x i c i t y , and destruction of some o i l soluble vitamins. During the storage of processed foods and o i l s , t h e formation of hydroperoxides and t h e i r decomposition products proceeds by way of free r a d i c a l mechanisms. The production of free r a d i c a l s i s i n turn promoted by external energy sources such as heat, l i g h t , high energy i r r a d i a t i o n , metal ions, metallo-proteins such as heme, and others. These l i p i d hydroperoxides, the i n i t i a l products of autoxidation, w i l l , i f l e f t unchecked, decompose non-enzymatically to a v a r i e t y of strongly flavored primary and secondary compounds (Figure 1). This mechanism i s d i f f e r e n t from that which occurs i n animal and plant t i s s u e s . In animal t i s s u e , oxidation of l i p i d s occurs non-enzymatically, being i n i t i a t e d l a r g e l y by hemo-proteins which then are decomposed enzymatically. In plants, l i p i d hydroperoxides are both enzymatically formed and enzymatically decomposed (17). This brings us to the subject of l i p i d oxidation and soybean reversion f l a v o r . As soybean becomes incorporated to a greater and greater degree i n the human d i e t , the reversion f l a v o r of soy assumes more s i g n i f i c a n c e . Many of the compounds a t t r i b u t e d to the reversion f l a v o r are products of l i p i d oxidation and are characterized as "beany, buttery, painty, f i s h y , grassy, or hay l i k e " (_5) . The l i n o l e n i c a c i d component of soybean o i l has been most frequently implicated i n the formation of reversion f l a v o r where i t i s present at about nine percent (18). Soybean o i l also contains substantial amounts of o l e i c and l i n o l e i c acids as do cottonseed, corn and several other o i l s . These o i l s are not, however, subject to f l a v o r reversion but then, they only contain l e s s than one percent l i n o l e n i c acid(18). It would seem that l i n o l e n i c a c i d must occur i n subs t a n t i a l amounts together with l i n o l e i c and possibly o l e i c acids i n order for reversion products to occur. A s a t i s f a c t o r y explanation of t h i s has not yet been developed- and yet more than seventy 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 r a c t i o n s of reverted



Free f a t t y acids

Mono-,Di-,Tri~ Glycerides

LIPID PRECURSOR -

other r i n g structures Light Heat

Light

Heat

General scheme for lipid degradation

lactones Irradiation

Irradiation

Figure 1.

esters

hydrocarbons

ketones

alcohols C a t a l y s i s by Metallo-compds. (free metals; heme compds.)

aldehydes

acids

C a t a l y s i s by Metallo-compds, (free metals; heme compds.)

Autoxidation Enzymolysis (reductase)

Radicals

Sat. & Unsat:

-^DERIVED LIPIDS

Enzymolysis (lipoxigenase)

Autoxidation

Hydroperoxides

INTERMEDIATES



1.

L i T M A N AND N U M R Y C H

Positive

and

Negative

Food

Flavor

5

soybean. These include methyl ketones, esters, saturated and unsaturated aldehydes, and acids. These compounds can be formed i n d i f f e r e n t ways including autoxidation or by way of lipoxygenases and other enzymes to form hydroperoxides. The breakdown of hydroperoxides r e s u l t s i n the formation of hexanal, hexanol, 2-hexenal, ethyl v i n y l ketone, and 2-pentyl furan, a l l of which are characterized as "beany", or "grassy" (.5) . The 2-pentyl furan i s p a r t i c u l a r l y noteworthy because at l e v e l s as low as 1 - 1 0 ppm i t produces a beany f l a v o r while at higher concentration, i t assumes a l i c o r i c e - l i k e character ( 1 9 ) . The biogenesis of f l a v o r s i n plants also involves l i p i d oxidation. F r u i t s and vegetables have v o l a t i l e compounds that are g e n e t i c a l l y c o n t r o l l e d which are responsible f o r t h e i r f l a v o r . These compounds are formed during the maturation and post-harvest storage through s p e c i f i c enzymatic changes i n the mono- and disaccharides, i n the amino acids, and i n c e r t a i n unsaturated l i p i d s that contain the 1,4-pentadiene structure. These l i p i d s , mainly the l i n o l e i c and l i n o l e n i c acids, are oxidized to t h e i r hydroperoxides by action of s p e c i f i c lipoxygenases which i n turn undergo other enzymatic transformations y e i l d i n g spec i f i c aldehydes and other secondary compounds. The presence of a l i p h a t i c aldehydes i s considered an important occurance because they are not only aromat i c a l l y potent but are generally unstable. These aldehydes, together with t h e i r corresponding alcohols, are responsible f o r many of the c h a r a c t e r i s t i c f l a v o r s i n food plants including banana, apple, peas, plums and graces. Their fresh green character i s l a r g e l y due to hexanal, and 2-hexenal which derive from l i n o l e i c and l i n o l e n i c acids. The character of cucumber also derived from these l i p i d s i s mainly due to 2 nonenal and to 2,6-nonadienal and t h e i r corresponding alcohols (1!5) . In tomatoes, the fresh aroma i s due l a r g e l y to c i s - 3 - h e x e n a l , c i s - 3 - h e x e n o l , and t r a n s - 2 hexenal which derive from l i n o l e n i c acid (20i) . The p r i n c i p l e flavorant of mushroom i s l - o c t e n - 3 - o l which i s derived from l i n o l e i c acid ( 2 1 ) . The character of green beans i s i n part due to hexanal, 2-hexenal, and 1- o c t e n - 3 - o l derived from l i n o l e n i c together with l i n o l e i c acid ( 1 2 J . C h a r a c t e r i s t i c pea f l a v o r i s due to a combination of C 3 , 5 , 6 saturated alcohols, C 7 , 8 , 9 2 - enals, C 9 , 1 0 2 , 4 - d i e n a l s as well as 2-pentyl furan which are a l l derived from l i n o l e i c a c i d ( 1 3 ) . Beverage milk, an excellent v e h i c l e f o r demons t r a t i n g the nature of l i p i d f l a v o r s i s an o i l i n water emulsion. I t has a d e l i c a t e yet complex f l a v o r .



6

LIPIDS AS A SOURCE OF

FLAVOR

When f r e s h , i t i s acceptable to most people who might otherwise r e j e c t i t out-of-hand i f rancid or tainted f l a v o r s were detected. The normal compliment of free f a t t y acids i n milk originated through action of native l i p a s e which hydrolyzes milk f a t . These acids are located mainly i n the f a t globules from which they came and where t h e i r f l a v o r s are l a r g e l y masked, and together with short chain free f a t t y acids found i n the aqueous portion, provide milk with i t s normal f l a v o r . If the f a t t y acids located i n the f a t glob ules could be s h i f t e d to the serum, the r e s u l t i n g milk would be considered unacceptably rancid. Small additions of f a t t y acids to the aqueous phase are usually t o l e r a t e d but once past the threshold of r a n c i d i t y , the r e s u l t i n g milk may no longer be accept able. To be sure, i n some areas of the world where r e f r i g e r a t i o n i s generally unavailable, a higher de gree of r a n c i d i t y i s t o l e r a t e d or even preferred. In contrast, i t takes only trace q u a n t i t i e s of c e r t a i n weeds such as wild onions i n the cow's feed supply to t a i n t the f l a v o r of milk. I t was reported that as l i t t l e as two milligrams of 2,β-nonadienal s p o i l s the f l a v o r of one ton of f a t (1). Another product which depends on l i p i d s for most of i t s f l a v o r i s blue mold cheese. Here, the f l a v o r i s developed through s e l e c t i v e l i p o l y s i s of milk f a t y e i l d i n g mainly small chain saturated f a t t y acids. During the ripening period, some of these acids under go enzymatic ^ - o x i d a t i o n , decarboxylation, and r e duction to y e i l d a mixture of f a t t y acids, methyl ketones and methyl c a r b i n o l s . Depending upon t h e i r p o l a r i t y , these compounds are p a r t i t i o n e d between the f a t and the aqueous phases of the cheese and, as i n the case of milk, the p a r t i t i o n i n g r a t i o was found to be c r i t i c a l to the normal f l a v o r ( 2 2 ) . Some of the most appetizing aromas recognized throughout the world are associated with c e r t a i n food products that have something i n common, that being that each had received heat at some point during processing. Examples of these are the aromas eminating from hot, baked bread, roasted coffee and nuts, butterscotch, barbeque, roast beef, pork, and poultry, and others. Compounds responsible for these d e l i c i o u s aromas are derived from non-enzymatic browning of sugar, aided by amino acids, and by products of l i p i d oxidation. For several years, f l a v o r manufacturers have recognized the commercial value of those f l a v o r s that evolve when mixtures of reducing sugars, amines and l i p i d s are heated together. They are searching for i d e a l conditions that recreate s y n t h e t i c a l l y



4

6

c

n

12

C

ClO

C9

C8

°7

C

C5

C

C3

fatty

V

fresh,citrus

fresh,green

ν

fresh,milky

Homologous Series: •Sat. fresh, pungent C2

2

2

0

1

2-Enals

4

5

)

8

Flavor, (+)(-)

+

c

8

xi

&

+

beef

2,4r-Dienals Flavor,(+)(-) 2,6-nonadienal: (-)linseed o i l (+)cis-4-heptenal: (+)cucurriber butter,cream (1) (-fJO^gpork (23) (32)(15) (+)C _i6beef (24) sweet,pungent (+)C 6_i2ham (27) (-)C7,10° dized jrçilk sweet,green (-)C4_nskim milk (+)C _5Coffee, (1) (28) cocoa (-)°7,10^ oxidized (+)Cgfruit (-)C _ soy & veg. veg.oil oils (25) sweet,oily (-)Cgveg.oils, (30) ( - ) Ct^QOxidized skim milk (25) milk (1) ( )Cr,9-12 ham (23) (+)C3_9gr.veg. (26) sweet,fatty, green (+)C

The Role Lipids Play in the Positive and Negative Flavors of Foods

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