Flavor Quality: Objective Measurement - American Chemical Society

4 Structural and Mechanical Indicators of Flavor Quality ZATA VICKERS

Downloaded by CORNELL UNIV on July 28, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0051.ch004

Department of Food Science and Nutrition, University of Minnesota, St. Paul, MN 55108

Is the flavor of a food in any way related to its structural and mechanical properties? Most food scientists would say yes. There is some literature on the subject but l i t t l e , i f any, attempts to explain how and why such relationships exist. Many of the newer foods on the market have their texture and flavor constructed or developed separately, making any general relationships between the two difficult to determine. At times it seems possible to combine almost any type and intensity of flavor with just about any texture. Assuming though, that there is some connection between flavor and texture, how does it occur or why does it exist? Is it due to an interaction between the sensory-systems perceiving flavor and those perceiving mechanical properties? In other words, does stimulating one set of sense organs affect the sensitivity of others? Or do changes in the structural and mechanical properties of a food affect the rate and extent of flavor formation and release? Both suggestions are valid. The first explanation is a phenomenon of an individual's sensory system. The latter is a result of changes occurring in the food. The sense organs involved in perceiving flavor are the taste buds, olfactory epithelium, and nerve endings responding to chemicals. Those involved in perceiving the structural and mechanical properties of foods are those responding to touch, pressure, position and sound. Periferally these senses are quite distinct. However, a l l sensory systems can interact together at higher levels in the brain. If one sense is stimulated it will affect to some degree the sensitivity of other senses. Therefore, if all or part of the sensory system perceiving mechanical properties was stimulated, one would expect some alteration in the perception of flavor. Stimulating one sense may enhance or depress sensations in another sense modality. Generally, a low level of an accessory stimulus will enhance perception, whereas higher levels of an accessory stimulus probably depress sensations. Little information is available on the enhancement or depression of odors or 45

Scanlan; Flavor Quality: Objective Measurement ACS Symposium Series; American Chemical Society: Washington, DC, 1977.


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F L A V O R Q U A L I T Y : OBJECTIVE

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t a s t e s by t e x t u r e s or sounds (1), but expecting such e f f e c t s i s reasonable. A crunch reminiscent of the T i t a n i c h i t t i n g an i c e berg may leave the cruncher with l i t t l e i d e a o f whether the product was cheese or onion f l a v o r e d . The s t r u c t u r a l and mechanical p r o p e r t i e s of foods may a f f e c t f l a v o r by c o n t r o l l i n g both the r a t e o f f l a v o r r e l e a s e and the t o t a l amount of f l a v o r r e l e a s e d . T h i s i s f a i r l y easy to understand. A product that breaks down or melts q u i c k l y i n the mouth w i l l r e l e a s e i t s f l a v o r r e l a t i v e l y r a p i d l y . A product that d i s i n t e g r a t e s slowly gives up i t s f l a v o r l e s s r a p i d l y . I f two such products contained equal amounts of f l a v o r compounds, the one undergoing r a p i d breakdown would be p e r c e i v e d as being more intensely flavored. In many cases v i s c o s i t y modifying agents such as starches and gums appear t o change the t a s t e and odor i n t e n s i t i e s of s o l u t i o n s to which they are added. This e f f e c t does not appear t o be due t o the v i s c o s i t y . An i n t e r a c t i o n between the t a s t e or odor compound and the h y d r o c o l l o i d i s probably r e s p o n s i b l e (2). For example, the f l a v o r compound acetaldehyde t a s t e s more intense i n a s o l u t i o n of sodium a l g i n a t e than i n p l a i n water. This i n t e n s i t y d i f f e r e n c e appears to be due to the i n t e r a c t i o n o f acetaldehyde with the a l g i n a t e . When acetaldehyde was mixed with other gums such as carboxymethyl c e l l u l o s e or hydroxypropyl c e l l u l o s e over e q u i v a l e n t ranges of v i s c o s i t y , there was no s i g n i f i c a n t change i n f l a v o r i n t e n s i t y (2). The same authors found s i m i l a r r e s u l t s with b a s i c t a s t e compounds and h y d r o c o l l o i d s . Enhancement or depression o f compounds, e.g., s a c c h a r i n and c a f f e i n e , a l s o appeared to r e s u l t from i n t e r a c t i o n s between the t a s t e compound and the v i s c o s i t y modifying agent. Some gums produced changes i n t a s t e i n t e n s i t y whereas others at equal v i s c o s i t i e s d i d not (3_) . The r e s u l t s o f these s t u d i e s provide evidence a g a i n s t any i n t e r a c t i o n between the p e r i p h e r a l sense organs f o r f l a v o r and those f o r v i s c o s i t y . From the consumer's p o i n t o f view, the most important way the s t r u c t u r a l and mechanical p r o p e r t i e s o f a food are r e l a t e d to f l a v o r i s through a s s o c i a t i o n . We a s s o c i a t e c e r t a i n t e x t u r e s and sounds with c e r t a i n f l a v o r s because through years o f e a t i n g experience we have learned they always occur together. Given two e q u a l l y red tomatoes, the s o f t e r one w i l l l i k e l y be r i c h e r i n f l a v o r . We expect a s o f t e r l o a f of bread t o be more f l a v o r f u l than a f i r m e r l o a f because the s t a l i n g process which makes i t firmer a l s o makes i t l e s s f l a v o r f u l . A c r i s p , j u i c y apple w i l l l i k e l y have more f l a v o r than a s o f t , mealy one. A curdy, rubbery Cheddar cheese has a m i l d e r , greener f l a v o r compared to the sharp nutty t a s t e of a more waxy aged cheese. Products where such a s s o c i a t i o n s can be made are g e n e r a l l y n a t u r a l or t r a d i t i o n a l l y processed meats, cheese, f r u i t s , e t c . , as opposed to f a b r i c a t e d foods. N a t u r a l or t r a d i t i o n a l foods are dynamic systems. Many r e a c t i o n s are o c c u r r i n g simultaneously and



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are producing changes i n both the s t r u c t u r a l and mechanical prop e r t i e s and the f l a v o r . These r e a c t i o n s are commonly caused by spontaneous chemical changes, enzyme systems, and h e a t i n g . Now, I would l i k e to look at some r e p r e s e n t a t i v e examples of such t e x t u r e - f l a v o r a s s o c i a t i o n s i n a few d i f f e r e n t commodities. R e l a t i o n s h i p s between the s t r u c t u r a l and mechanical propert i e s of meat and the f l a v o r of meat are commonly found. Probably the best known are those o c c u r r i n g during aging. Aging meat produces a more tender, f l a v o r f u l product, whereas imaged meat i s r e l a t i v e l y tough with a bland, m e t a l l i c and a s t r i n g e n t t a s t e . Why i s t h i s increase i n tenderness accompanied by an i n c r e a s e i n flavor? I f we look at one of the changes o c c u r r i n g i n a cut of meat as i t ages, we f i n d the cathepsins or p r o t e o l y t i c enzymes beginning to break up the m y o f i b r i l s . T h i s degradation i n c r e a s e s the f r a g i l i t y or tenderness of the muscle because the aged f i b e r bundles break more e a s i l y when subjected to t e n s i l e or shear stress. I f the m y o f i b r i l s are not e n z y m a t i c a l l y degraded, they s t r e t c h more when s t r e s s e d , producing a tougher muscle (4) (5). The p r o t e o l y t i c breakdown a l s o produces f r e e amino a c i d s . When meat i s heated or cooked, these f r e e amino a c i d s may p a r t i c i p a t e i n the non-enzymatic browning r e a c t i o n s which produce the lean meat f l a v o r (6). T h i s i s an example of an enzymatic process that produces both f l a v o r and texture changes. The p r o t e o l y t i c breakdown of m y o f i b r i l s c o n t r i b u t e s to both meat tenderness and flavor. Browning r e a c t i o n s are a l s o r e s p o n s i b l e f o r the formation of v o l a t i l e s that give f r e s h l y baked bread much of i t s f l a v o r . During s t a l i n g , t h i s f l a v o r p r o g r e s s i v e l y disappears. But the most pronounced change that takes place during s t a l i n g i s an i n c r e a s e i n firmness or hardness of the crumb. The extent of f i r m i n g can be used as an approximate index of f l a v o r l o s s or d e t e r i o r a t i o n duri n g s t a l i n g . T h i s i s why people shopping f o r bread judge i t s freshness and f l a v o r by squeezing the l o a f . Both firmness and f l a v o r l o s s are time and temperature dependent processes. Furthermore, the b a s i c molecular changes producing an increase i n firmness are l i k e l y the same ones producing the change or apparent l o s s of f l a v o r . Most of the increase i n firmness during s t a l i n g i s a t t r i b u t e d to changes i n the s t a r c h f r a c t i o n of the product. The s t a r c h i n bread increases i n c r y s t a l l i n i t y during aging. The exact nature of t h i s c r y s t a l l i n i t y i s not c l e a r (_7) . The l o s s of f l a v o r during s t a l i n g does not appear to be through v o l a t i l i z a t i o n or through chemical r e a c t i o n s (8). On reheating, when the process of s t a r c h rétrogradation or c r y s t a l l i z a t i o n i s temporarily reversed, the f l a v o r compounds are r e leased. T h i s suggests the f l a v o r compounds are probably trapped w i t h i n the c r y s t a l l i n e regions of the s t a r c h molecules. When trapped, they are prevented from v o l a t i l i z i n g or s o l u b i l i z i n g and, t h e r e f o r e , can make no c o n t r i b u t i o n to the t a s t e or aroma of the

American Chemical Society Library 1155 16th St. N. tf. Scanlan; Flavor Quality: Objective Measurement Washington, D. Chemical C. 20031 ACS Symposium Series; American Society: Washington, DC, 1977.


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product. The molecular process r e s p o n s i b l e f o r the increase i n firmness during s t a l i n g i s a l s o the mechanism f o r trapping and r e l e a s i n g f l a v o r compounds. Experienced cheese graders can u s u a l l y guess the f l a v o r of a cheese by observing the t e x t u r e . The s t r u c t u r e and mechanical p r o p e r t i e s of a cheese depend p a r t l y on the b a c t e r i o l o g i c a l and/or enzymatic treatment of the milk, the processing techniques, any added p r o t e o l y t i c c u l t u r e s , and the aging process. These f a c t o r s are a l s o l a r g e l y r e s p o n s i b l e f o r the f l a v o r . For example, a f r e s h , unripe Cheddar cheese has a rubbery, curdy texture and a bland f l a v o r . T h i s texture i s due to the m i c r o s t r u c t u r e of c a s e i n m i c e l l e aggregates. As the cheese ages, the p r o t e i n s are broken down and the rubberiness changes i n t o a smooth, p l a s t i c texture. To a cheese grader t h i s smooth, s i l k y character i n d i c a t e s favorable f l a v o r development. A dry texture i n Cheddar cheese would be a s s o c i a t e d with a l e s s d e s i r a b l e high a c i d f l a v o r , and a pasty, s t i c k y texture would i n d i c a t e a f r u i t y , fermented f l a v o r (9). Calves rennet i s c u r r e n t l y the most widely used p r o t e o l y t i c enzyme i n cheese making. The cheese i n d u s t r y has been engaged i n f i n d i n g a c l o t t i n g agent to s u b s t i t u t e f o r rennet. Problems a r i s e when other p r o t e o l y t i c enzymes are used because the p r o t e i n s break down i n a d i f f e r e n t manner. The way p r o t e o l y s i s occurs appears to a f f e c t how the c a s e i n m i c e l l e s a t t a c h to each other to form a g e l or curd. The m i c r o s t r u c t u r e of cheese made with proteases other than rennet tends to be more open (10). This a l t e r a t i o n i n b a s i c m i c r o s t r u c t u r e produces cheese with a l e s s p l a s t i c s t r u c t u r e . The p r o t e o l y s i s a l s o determines the peptides and amino a c i d s a v a i l a b l e . These not only c o n t r i b u t e to the t a s t e of the cheese but may undergo f u r t h e r enzymatic breakdown i n t o other f l a v o r compounds. In f r u i t s and vegetables, very important changes i n both texture and f l a v o r occur during r i p e n i n g . The texture of most f r u i t s becomes s o f t e r and l e s s c r i s p as i t matures. The f l a v o r becomes sweeter and more intense. The biochemical r e a c t i o n s that produce these changes occur independently. Changes i n f l a v o r are due to an increase i n the synthesis of sugars and an increase i n the r a t e at which v o l a t i l e s r e s p o n s i b l e f o r the aroma are synthesized. Changes i n the texture are l a r g e l y due to r e a c t i o n s taking place i n the p e c t i c substances of the middle l a m e l l a . In green f r u i t , the p e c t i c m a t e r i a l s have a high molecular weight and are i n s o l u b l e . They serve to cement the w a l l s of adjacent c e l l s together, thereby imparting considerable strength to the t i s s u e . During r i p e n i n g and senescence, enzymes i n the plant hydrolyze and otherwise a l t e r these p e c t i c substances, making them more s o l u b l e and l e s s e f f e c t i v e as cement. As a r e s u l t , the f r u i t becomes s o f t e r and e v e n t u a l l y mushy. Heat a l s o promotes the h y d r o l y s i s of p e c t i c m a t e r i a l s . This s o f t e n i n g i s r e a d i l y seen when f r u i t s or vegetables are cooked. The strength or cementing power of the middle l a m e l l a has important i m p l i c a t i o n s f o r both the f l a v o r and the texture of a



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product. I f the middle l a m e l l a i s stronger than the c e l l w a l l , which i s g e n e r a l l y the case with green and s l i g h t l y underripe f r u i t , the t i s s u e w i l l tend to f r a c t u r e or break across the c e l l w a l l s . I f the cementing power of the p e c t i c substances i s weakened, whether through enzymatic degradation or heating, the t i s s u e w i l l tend to f r a c t u r e between the c e l l w a l l s (11). From a sensory p e r s p e c t i v e , t h i s change i n f l u e n c e s both texture and f l a v o r . I f the c e l l s break across the c e l l w a l l s the c e l l contents w i l l run out, c r e a t i n g the sensation of j u i c i n e s s , and a l s o r e l e a s i n g the f l a v o r compounds i n s i d e the c e l l . The c e l l s of most f r e s h f r u i t s are t u r g i d , meaning there i s an i n t r a c e l l u l a r pressure d i r e c t e d outward against the c e l l w a l l . I f the product f r a c t u r e s across the c e l l w a l l , t h i s turgor p r e s sure i s r e l e a s e d r e s u l t i n g i n the r a p i d expansion of the c e l l ' s contents. T h i s sudden expansion produces the sounds that t e l l us the product i s c r i s p . I f the product f r a c t u r e s between the c e l l s , they are not broken open. The product appears l e s s j u i c y , l e s s f l a v o r f u l , has a mushy or mealy texture, and l i t t l e , i f any, c r i s p n e s s . An example of such a system would be apples. A f r e s h , s l i g h t l y underr i p e or j u s t r i p e , c r i s p apple breaks across the c e l l w a l l s producing c r i s p , j u i c y , and f l a v o r f u l sensations. During prolonged storage or senescence, the middle l a m e l l a r p e c t i n s l o s e t h e i r cementing power. The same apple, i f stored f o r s e v e r a l months, would tend to l o s e i t s c r i s p n e s s and become mushy or mealy. The preceding examples i l l u s t r a t e that the processes genera t i n g changes i n f l a v o r are not n e c e s s a r i l y independent of those causing changes i n t e x t u r e . The s t r u c t u r a l and mechanical prop e r t i e s of foods are mainly due to l a r g e polymeric molecules. These molecules, e.g., s t a r c h , p r o t e i n s , c e l l u l o s e and p e c t i n s l i n k together or i n t e r a c t with each other to form the b a s i c s t r u c t u r e of the food. Such molecules themselves do not have any inherent f l a v o r p r o p e r t i e s . Molecules producing f l a v o r sensations are much smaller and g e n e r a l l y make no c o n t r i b u t i o n to t e x t u r e . When the s t r u c t u r a l molecules degrade or are broken down to smaller f l a v o r - p r o d u c i n g compounds, e.g., meat and cheese, or when changes i n the molecular s t r u c t u r e r e s p o n s i b l e f o r texture entrap or r e l e a s e f l a v o r compounds, e.g., s t a r c h systems and f r u i t s , the texture and f l a v o r changes w i l l take place c o n c u r r e n t l y .


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flavor quality: objective measurement Literature Cited

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Stone, H. and Pangborn, R. M. In "Basic Principles of Sensory Evaluation" p. 30-46. American Society for Testing and Materials. Philadelphia. 1968. Pangborn, R. M. and Szczesniak, A. S. J . Texture Studies (1974) 4 (4) 467-482. Pangborn, R. M . , Trabue, I. Μ., and Szczesniak, A. S. J . Texture Studies (1973) 4 (2) 224-241. Eino, M. F. and Stanley, D. W. J . Food S c i . (1973) 38 (1) 45-50. Eino, M. F. and Stanley, D. W. J . Food S c i . (1973) 38 (1) 51-55. Hornstein, I. In Price, J . F. and Schweigert, B. S. "The Science of Meat and Meat Products". W. H. Freeman and Co. San Francisco. 1971. Willhoft, E. M. A. J . Texture Studies (1973) 4 (3) 292-322. Schoch, T. J. Baker's Dig. (1965) 39 40. Nelson, J . A. and Trout, G. M. "Judging Dairy Products" 4th ed. Olsen Publishing Co. Milwaukee. 1965. Stanley, D. W. and Tung, M. A. 1974. "Microstructure of food and its relation to texture." Presented at the symposium Advances in Food Texture, August 29, at Guelph, Ontario. Bourne, M. C. 1974. "Texture of fruits and vegetables." Presented at the symposium Advances in Food Texture, August 29, at Guelph, Ontario.


Flavor Quality: Objective Measurement - American Chemical Society

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