Chapter 2
Progress in the Science of Thermal Generation of Aromas A Review
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Charles H. Manley Takasago International Corporation (USA), 100 Green Street, Teterboro, NJ 07806 For Many thousands of years man has developed desirable flavors by the use of the art of cooking. Only in the last hundred years has man used heat to create aromas which have been found to he useful specifically as food flavorings. At first the practice related to the hydrolysis of proteins to yield products which contained both flavor and taste enhancing properties. The first person to approach the problem scientifically was Dr. Louis Camille M a i l l a r d . The discoveries he made on the non-enzymatic browning reactions of proteins/amino acids with polysaccharides/reducing sugars s t i l l hears his name and have been the focus of many research papers and symposia. The Maillard Reaction was, indeed, the chemical platform for producing many of the aromas currently used as meat flavors. During the 1950's and 1960's significant discoveries by natural product chemists contributed further to establishing the science of themally created aromas. The oxidation of fat, the reactions of sulfur and the degradation of sugars are only a few examples of the chemical mechanisims which are used today in directing precursor material to produce usable aromas. Written h i s t o r y d i d not record the advent of t h e - f i r s t thermal developed aroma used as a -food product. An e a r l y cave man probably determined from some a c c i d e n t that -food which f e l l i n t o a -fire was more d e s i r a b l e either -from an o r g a n o l e p t i c aspect, ease o-f d i g e s t i o n or a higher degree of p r e s e r v a t i o n of the food. Whatever prompted t h e observation that food materials exposed t o heat became more d e s i r a b l e has long been f o r g o t t e n but the use of cooking has become a b a s i c human experience. The skillful use of i n g r e d i e n t s exposed t o t h e proper amount of heat gave r i s e t o s c h o o l s of the f i n e culinary arts. 0097-6156/89/0409-0012$06.00/0 c 1989 American Chemical Society Parliment et al.; Thermal Generation of Aromas ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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MANLEY
Progress in Thermal Generation ofAromas
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The aromas t h a t are developed by the heat induced r e a c t i o n s between components found i n the -food s t u f f i s , indeed, one of man's unsung major accomplishments. Over $120 b i l l i o n d o l l a r s of food s t u f f s which are t h e r m a l l y t r e a t e d i n one way or another was produced i n the USA i n 1987. S c i e n t i f i c efforts are commensurate t o the economic s i g n i f i c a n c e of the products. In the l a s t 15 years over 800 citations covering the M a i l l a r d Reaction as well as over 100 c i t a t i o n s c o v e r i n g meat and c o f f e e aroma have been recorded by Chemical A b s t r a c t s .
Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: October 3, 1989 | doi: 10.1021/bk-1989-0409.ch002
Early History. As we know, the f i r s t e m p i r i c a l use of heat t o develop aromas came from the great c h e f s and their skillful use of i n g r e d i e n t s and heat. A refinement of the empirical approach no doubt occurred during the war e f f o r t s of the 1800's Napoleonic epoch. Meat e x t r a c t s s a t i s f i e d the immediate needs but they became i n s h o r t supply. A Swiss chemist by the name of J u l i u s Maggi developed a meat type flavoring product based on a c i d h y d r o l y s i s of p l a n t p r o t e i n . When such m a t e r i a l s are n e u t r a l i z e d and reduced to paste or powder by heat they a c q u i r e a f l a v o r profile useful as a meat e x t r a c t s u b s t i t u t e . Today the market for t h a t product, c a l l e d Hydrolyzed Vegetable P r o t e i n or HVP, i s more than $300 m i l l i o n world wide (JL> HVP r e p r e s e n t s the f i r s t modern commercial example of the use of heat t o develop a u s e f u l m a t e r i a l f o r i t s use as a f l a v o r i n g . The e a r l y s c i e n t i f i c d i s c o v e r i e s r e l a t i n g t o heat induced aroma development can be t r a c e d t o the work of Louis-Cami11e M a i l l a r d at the U n i v e r s i t y of Nancy during the period of 1912 to 1936 (2). He p u b l i s h e d at l e a s t 8 papers on the s u b j e c t of the r e a c t i o n of sugars with amino a c i d s . The Maillard Reaction, or so—called, non enzymatic browning r e a c t i o n chemistry, has become the focus on a great amount of s c i e n t i f i c work (3). The r e a c t i o n pathways f o r the M a i l l a r d Reactions have been s t u d i e d and reviewed by many r e s e a r c h e r s s i n c e Dr. Mai H a r d ' s early work (4-6) . These papers g i v e a c o n c i s e o u t l i n e of the major chemical pathways identified in the Maillard Reaction Mechanism. In heat t r e a t e d meat with n e a r l y 75% of those v o l a t i l e s generated are p y r a z i n e s d e r i v a t i v e s (7). Those p y r a z i n e s have been found t o p l a y an important r o l e i n developing a roasted f l a v o r i n heated products. They w i l l be d i s c u s s e d l a t e r . The i n i t i a l stages of the M a i l l a r d Reaction d e a l s with the condensation of amino a c i d s , p e p t i d e s or p r o t e i n s with reducing sugars. The reaction occurs with the a p p l i c a t i o n of heat with the formation of an "Amadori compound" (See Chapter 1 of t h i s book).
Parliment et al.; Thermal Generation of Aromas ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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THERMAL GENERATION OF AROMAS
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The Amadori compound -from an aldohexose may decompose t o the 5-hydroxy methyl-2—furaldehyde through a 1,2 eneaminol. The amino a c i d may a l s o r e a c t , by the Strecker pathway, with a dehydroreductone -From a reducing sugar t o -from an aldehyde or ketone of one carbon l e s s that the amino a c i d i n v o l v e d i n the i n i t i a l reactionThe S t r e c k e r pathway was -first proposed i n the mid 1800's. The S t r e c k e r aldehydes -formed -from these r e a c t i o n s have been shown t o be important -flavor compounds (8). Other pathways have been -found t o generate aroma compounds a s s o c i a t e d with heat treatmentThese pathways w i l l be reviewed brie-fly i n t h i s paper with i m p l i c a t i o n s t o o v e r a l l aroma noted- It i s c l e a r that many o-f the components -found i n -food play important r o l e s i n the development o-f acceptable -flavor pro-files. The chemistry of thermogensis of food aroma can best be reviewed by f o c u s i n g on those interesting components that have been discovered i n popular prepared food products- Needless t o say the most s i g n i f i c a n t product t o man i s meat. Meat F l a v o r s From the e a r l y works of Bender and Co-workers (9) and that of Hornstein and Crowe (10) ? i t . was recognized that water soluble extracts of meat contained some of the precusor material that when heated gave r i s e t o cooked aroma. Their original f i n d i n g s have lead t o f u r t h e r r e s e a r c h and more than 500 research a r t i c l e s and patents. A few review articles are worthy of note (11-13)The heterocyclic chemicals are some of the most i n t e r e s t i n g of the chemical v o l a t i l e s . Heterocycl i c compounds. The heterocyclics, particularly sulfur containing ones, are extremely important c o n t r i b u t o r s of r o a s t e d meat f l a v o r p r o f i l e s . For example 4-hydroxy-2,5-dimethyl-3
