Flavor compounds mechanisms

It is not yet possible to design a molecule with specific odor (or taste) characteristics because the relations between sensory properties of flavor compounds and their molecular properties are not well understood. As a consequence, the development of compounds with desired flavor qualities has had to rely on relatively tedious synthetic approaches. Recent advances, however, in computer-based methods developed by the pharmaceutical industry to study QSAR (quantitative structure-activity relationships) may ultimately be helpful in the rational design of new flavor-structures with predictable sensory attributes. Results from QSAR studies may also provide insight into the mechanism of the molecule-receptor interaction. 

Spray drying is the most widely used, least expensive and favored route among the methods available for encapsulation (2) Various theories of volatile retention in spray drying have been proposed and reviewed (3). In addition to the nature of flavor compounds, flavor retention is governed by type of carriers, infeed composition, solids concentration (4), dryer inlet/exit air temperature, air velocity and humidity, feeding rate and atomization characteristics. In addition to flavor retention,the stability of the encapsulated product, as mentioned earlier, is also of importance and is governed by nearly the same parameters. However,the effect and mechanics of each individual factor are much less understood. 

The highly flavorable compound diacetyl is an important by-product of lactic acid bacterial fermentation. The mechanism of its formation has recently been unraveled (35). Diacetyl (measured as diacetyl rather than as diacetyl plus acetoin) is present in higher concentrations in wines with malo-lactic fermentation (cf. Ref. 36). At approximately threshold levels, this compound might contribute favorably to the flavor of wine (7) since increased complexity has been shown to enhance the quality of wine (37). 

Musty or potato-like flavor and aroma have been observed as a defect in milk (Hammer and Babel 1957) and Gruyere de Comte cheese (Dumont et al. 1975). This off-flavor results from the production of nitrogenous cyclic compounds by Pseudomonas taetrolens and P. perolens (Morgan 1976). Musty-flavored compounds produced by these organisms include 2,5-dimethylpyrazine and 2-methoxy-3-isopropyl-pyrazine. The Gruyere de Comte with potato off-flavor contained 3-methoxy-2-propyl pyridine, as well as alkyl pyrazine compounds (Dumont et al. 1975). Murray and Whitfield (1975) postulated that alkyl pyrazines are formed in vegetables by condensation of amino acids such as valine, isoleucine, and leucine with a 2-carbon compound. Details of the synthetic mechanism in pseudomonads are unknown. 

Cheese is a complex matrix of several components. Isolation of compounds of interest and the analysis of target compounds without interference from the matrix has been a challenge with analytical techniques. With the development of extraction procedures and new sampling techniques for analysis, not only has this challenge been overcome to a certain extent but also the speed, quality, accuracy, and reliability of analysis have improved tremendously. With the mechanisms behind the formation of several flavor compounds in cheese still not clearly understood, these techniques have an increasing role in the efforts to understanding cheese ripening. Often times most of the techniques provide... 

Many desirable meat flavor volatiles are synthesized by heating water-soluble precursors such as amino acids and carbohydrates. These latter constituents interact to form intermediates which are converted to meat flavor compounds by oxidation, decarboxylation, condensation and cyclization. 0-, N-, and S-heterocyclics including furans, furanones, pyrazines, thiophenes, thiazoles, thiazolines and cyclic polysulfides contribute significantly to the overall desirable aroma impression of meat. The Maillard reaction, including formation of Strecker aldehydes, hydrogen sulfide and ammonia, is important in the mechanism of formation of these compounds. 

Probably the most important reactant in the formation of volatile meat flavor compounds is hydrogen sulfide. It can be formed by several pathways during meat cookery, but one mechanism is Strecker degradation of cysteine in the presence of a diketone as established by Kobayashi and Fujimaki (29). The cysteine condenses with the diketone and the product in turn decarboxylates to amino carbonyl compounds that can be degraded to hydrogen sulfide, ammonia and acetaldehyde. These become very reactive volatiles for the formation of many flavor compounds in meat and other foods. 

Reduction of desirable meat aroma remains as a serious impediment to addition of soy protein. When highly purified soy protein is added to ground patties, thermally generated meat aroma intensity is decreased. Adsorption of flavor compounds onto vegetable protein is a primary mechanism for this aroma loss (5-7). 

Understanding the mechanism by which flavor compounds are formed can lead to better methods of food processing for better formation and retention of flavor. Fundamental flavor chemistry information is also essential in genetic engineering of plants and animals to improve flavor in the starting materials of food products. 

To illustrate some trends and developments in flavor chemistry research, chapters have been included on the importance of enantiomers and the biogenesis of some chiral compounds, production and mechanisms of natural and chemically formed flavor compounds, and a few recent examples of chemical investigations of characteristic flavors. 

Review of Ascorbic Acid Mechanisms of Action. Ascorbic acid and AP have antioxidant activity in fats, oils, vitamin A, and carotenoids. In these systems AP is a better antioxidant than are the phenolic antioxidants BHT and BHA, both from these data and others (29,35). Ascorbic acid protects against oxidation of flavor compounds in wine, beer, fruits, artichokes, and cauliflower (29) presumably by oxygen scavenging. The well-known formation of nitric oxide from nitrites by ascorbic acid is used not only for inhibition of nitrosamine fortnation, but also to promote... 

Aromatic substances are important in wine as they make a major contribution to the quality of the final product. Several hundred chemically different flavor compounds such as alcohols, esters, organic acids, aldehydes, ketones, and monoterpenes have been found in wines (Zhou et al., 1996). It is the combined contribution of these compounds that forms the character of a wine. The ability to determine these substances in wine may be used to help understand their role in flavoring. The method described (Zhou et al., 1996) uses a reversed-phase mechanism with XAD-2 to isolate the volatiles followed by Freon 11 extraction of the XAD-2 column. 

Carbonyl compoimds, ammonia and hydrogen sulfide are some very reactive flavor precursors which could be derived from early stage of Maillard reaction and pre-existing in many food systems. Reactions among them could lead to the formation of various heterocyclic flavor compounds (/). However, research work done regarding these reactions were mostly under high temperature conditions. Reaction mechanism under low tenqjerature condition has not been well researched. The purpose of this study was to elucidate formation... 

This paper reports the antioxidative effect of capsanthin on the chlorophyll-photosensitized oxidation of soybean oil and selected flavor compounds including 2-ethylfiiran, 2,4.5-trimethyloxazole. and 2,5-dimethyl-4-hydroxy-3(2H)-fiiranone (DMHF), which are unstable when exposed to light in the presence of sensitizer 3,20). The quenching mechanism and kintics of capsanthin on the photosensitized oxidation of soybean oil are also reported. J3-carotene and lutein were used as controls for comparing the antiphotooxidative activity with capsanthin. 

The basic constituents of milk - protein, lipid, carbohydrate - can serve as precursor substrates for the formation of a wide variety of flavor compounds. Nearly two hundred volatiles have reportedly been found in fresh and processed milk (4). Numerous research efforts have focused on the conditions and mechanisms of off-flavor development in milk. The chemical compounds responsible for these off-flavors have been characterized (3,5-7). Most fluid milk processing is carefiiUy controlled so that the appearance of caramelized and scorched flavor notes rarely occurs. The rich flavor associated with the thermal formation of diacetyl and various lactones is not objectionable to most consumers and, therefore, not a serious concern. Conversely, the sulfurous off-flavor in cooked milk is of concern and is especially prevalent in freshly processed UHT milk. 

Type I mechanism, which does not involve the singlet oxygen reaction, is the another mechanism for the photosensitized oxidation of flavor compounds. Benzophenone-sensitized photooxidation of tetrahydrofuran leads to formation of the a-hydroperoxide (Scheme 27) (Schench et al, 1963), which is the same product formed in the unsensitized photooxidation of tetrahydrofuran as demonstrated in Scheme 11. 

Some of the mechanisms leading to the formation of several classes of flavor compounds via microbial activity are outlined in this chapter. At this point, two additional ideas should be mentioned. First, we have only considered the formation of flavor via metaboUc activity of the living microorganism. Miaoorganisms could... 

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