Flavor compound formation

During extrusion, general browning typified by carameli-zation, Maillard, and oxidative decomposition reactions are paramount in flavor compound formation. Temperature and shear conditions occurring during extrusion can provide the chemical and physical means whereby complex starch and protein can be partially degraded to provide reactants that can then participate in browning. 

Published studies on extrusion formation/retention of flavor are rather limited (2J3). None have reported the effects of protein sources and concentrations on flavor compound formation in extrudates, the major objective of this study. 

The type of amino acids and sugars were found as the main factors in flavor compound formation [184], For example meat-related flavor compounds are mainly sulfur containing compounds, derived from cysteine and ribose (coming from the nucleotides), while the amino acid proline gives rise to typical bread, rice, and popcorn flavors. 

CHAN REINECCIUS Kinetics of Flavor Compound Formation... 

The catabolism of amino acids has quality (flavor compound formation) and safety (biogenic amine synthesis) implications for fermented foods. Amino acid catabolism is also thought to have an important role in the abilities of LAB to obtain energy in nutrient-limited conditions. Furthermore, amino acid catabolism has also been identified as a mechanism of pH control. 

Biochemical Processes of Flavor Compound Formation in Food and Potential of LAB... 

In plant tissues, various enzymes convert the hydroperoxides produced by LOX to other products, some of which are important as flavor compounds. These enzymes include hydroperoxide lyase, which catalyzes the formation of aldehydes and oxo acids hydroperoxide-dependent peroxygenase and epoxygenase, which catalyze the formation of epoxy and hydroxy fatty acids, and hydroperoxide isomerase, which catalyzes the formation of epoxyhydroxy fatty acids and trihydroxy fatty acids. LOX produces flavor volatiles similar to those produced during autoxidation, although the relative proportions of the products may vary widely, depending on the specificity of the enzyme and the reaction conditions. 

The flavor quality of food is a primary factor involved in a consumer s decision to purchase a food item. Therefore, food technologists require a thorough understanding of how flavor deteriorates if they are to prepare products that consumers will purchase repeatedly. This knowledge is particularly important in meat and meat products, since the deterioration of meat flavor is a serious and continual process (1-4) that involves both the loss of desirable flavor components 4,5) and the formation of off-flavor compounds (6-9) many of which are associated with lipid oxidation (10). 

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). 

Law, B. A. 1979. Reviews of the progress of dairy science Enzymes of psychrotrophic bacteria and their effects on milk and milk products. J. Dairy Res. 46, 573-588. Law, B. A. 1981. The formation of aroma and flavor compounds in fermented dairy products. Dairy Sci. Abstr. 43, 143-154. 

Law, B. A. 1981. The formation of aroma and flavor compounds in fermented dairy products. Dairy Sci. Abstr. 43, 143. 

Lipolysis is considered to be an important biochemical event during cheese ripening and the current knowledge have been discussed in detail (Collins et al., 2003, 2004 McSweeney and Sousa, 2000). The formation of short-chain FFAs by the lipolysis of milk fat by lipases is a desirable reaction in many cheese types (e.g., mold-ripened cheeses). The catabolism of FFAs, which is a secondary event in the ripening process, leads to the formation of volatile flavor compounds such as lactones, thioesters, ethyl esters, alkanols, and hydroxyl fatty acids. The contributions of lipolysis to the flavor of bacterially ripened cheeses are limited. 

Free amino acids are further catabolized into several volatile flavor compounds. However, the pathways involved are not fully known. A detailed summary of the various studies on the role of the catabolism of amino acids in cheese flavor development was published by Curtin and McSweeney (2004). Two major pathways have been suggested (1) aminotransferase or lyase activity and (2) deamination or decarboxylation. Aminotransferase activity results in the formation of a-ketoacids and glutamic acid. The a-ketoacids are further degraded to flavor compounds such as hydroxy acids, aldehydes, and carboxylic acids. a-Ketoacids from methionine, branched-chain amino acids (leucine, isoleucine, and valine), or aromatic amino acids (phenylalanine, tyrosine, and tryptophan) serve as the precursors to volatile flavor compounds (Yvon and Rijnen, 2001). Volatile sulfur compounds are primarily formed from methionine. Methanethiol, which at low concentrations, contributes to the characteristic flavor of Cheddar cheese, is formed from the catabolism of methionine (Curtin and McSweeney, 2004 Weimer et al., 1999). Furthermore, bacterial lyases also metabolize methionine to a-ketobutyrate, methanethiol, and ammonia (Tanaka et al., 1985). On catabolism by aminotransferase, aromatic amino acids yield volatile flavor compounds such as benzalde-hyde, phenylacetate, phenylethanol, phenyllactate, etc. Deamination reactions also result in a-ketoacids and ammonia, which add to the flavor of... 

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. 

Dehydration results in the formation of important flavor compounds such as 2-furaldehydes (furfural), a formylpyrrole, and similar compounds. Another pathway possibly beginning with the 2 3 enediol involves the enolic form of a l-deoxy-2,3-dicarbonyl intermediate or an equilibrium enolic form which leads to formation of flavor compounds such as methylfuranones, isomaltol, and maltol... 

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. 

This important flavor compound was identified in the head-space volatiles of beef broth by Brinkman, et al. (43) and although it has the odor of fresh onions, it is believed to contribute to the flavor of meat. This compound can be formed quite easily from Strecker degradation products. Schutte and Koenders (49) concluded that the most probable precursors for its formation were etha-nal, methanethiol and hydrogen sulfide. As shown in Figure 5, these immediate precursors are generated from alanine, methionine and cysteine in the presence of a Strecker degradation dicarbonyl compound such as pyruvaldehyde. These same precursors could also interact under similar conditions to give dimethyl disulfide and 3,5-dimethyl-l,2,4-trithiolane previously discussed. 

The volatile compounds formed by the Maillard reaction are only one group of flavor compounds in foods. Schutte (1) presents a brief summary of the major classes and their modes of formation from precursors. Some of them can be formed by different pathways. An example is the furans, which can be formed by non-enzymatic browning reaction but also by biotransformation. 

Formation of 2-Acetyl-l-pyrroline and Other Important Flavor Compounds in Wheat Bread... 

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