Amino acids, aroma generation

The origin of many of the components of black tea aroma has been studied. Aldehydes are produced by catechin quinone oxidation of amino acids. Enzymic oxidation of carotenoids during manufacture generates ionones and their secondary oxidation products such as theaspirone and dihydroactinidolide. Oxidation of linoleic acid is responsible for the formation of trans-2-hexenal.82... 

Finally, heating of amino acids can produce volatiles Including aldehydes, amines and hydrogen sulfide. One minor, but Important, flavor generating pathway Involves the Strecker degradation of an amino acid as shown in Figure 2. In this reaction, an alpha amino acid reacts with an alpha dicarbonyl at an elevated temperature to produce an aldehyde (one carbon less than the amino acid) as well as an alpha amino ketone. These products can react further to yield Important heterocyclic aroma chemicals such as pyrazines, thlazoles, and dihydrofuranones. 

Lipid decomposition volatiles. Reactions of sugar and amino acids give rise to odor profiles that are, at best, common to all cooked or roasted meats. The water soluble materials extracted from chicken, pork, or beef give reasonably similar meat flavor. To develop a species specific aroma one needs to study the lipid fraction and the volatiles produced from those lipids. The work of Hornstein and Crowe (10) reported that the free fatty acids and carbonyls generated by heating will establish the specific species flavor profiles. 

One o-f the world s most popular -flavors, chocolate, is a product o-f both -fermentation and roasting. Early studies by Rohan (44-47) pointed out the importance o-f liberating the precusor materials (amino acid) so that roasting will generate chocolate aroma. 

It is demonstrated that a great many flavor compounds are formed in both model systems. On the other hand, phenylalanine formed by aldol condensations some special aroma products. Furthermore, the generation of thermal aroma compounds depend on the pH, the sugar/amino acid ratio and the temperature. 

Formation of Amino Acid Specific Maillard Products and Their Contribution to Thermally Generated Aromas... 

Table II. Generation of "Meaty, Beefy" Aromas in Heated Mixtures of Amino Acids vith Glucose...

The first two sections of this book provide the reader with a background on the thermal generation of aromas. Included in these sections are perspectives on the regulatory aspects and the analytical methodologies at the forefront of aroma research. Subsequent sections present original research on aromas derived from various food sources. In addition, we have included a section on mechanistic studies to provide insights into aroma formation through thermal decomposition of lipid, carbohydrate, and amino acid precursors. The final section is entirely... 

Hemandez-Orte, P., Ibarz, M.J., Cacho, J., Ferreira, V. (2006) Addition of amino acids to grape juice of the Merlot variety Effect on amino acid uptake and aroma generation during alcoholic fermentation. Food Chem., 98, 300-310. 

As well as the amino and carboxy groups, the amino-acid side-chain is also involved early in the Maillard reaction, so the side-chain functional groups become incorporated into the products from Maillard reactions. Cysteine, in particular, reacts with glucose to generate numerous reaction products, some of which introduce attractive tastes and aromas to foods associated with sulphur functional groups. 

Saccharides and polysaccharides — starch and cellulose (Bqczkowicz et al., 1991), pectins (Sikora etal., 1998), and hemicelluloses (Tomasik and Zawadzki, 1998) — heated with amino acids develop scents specific for polysaccharide, amino acid, and reaction conditions. Thus, supplementation of saccharides and polysaccharides with amino acids and proteins, as well as supplementation of protein-containing products with saccharide, can be useful in generation, modification, and enrichment of flavor and aroma of foodstuffs and tobacco. 

Traditional fermentation using microbial activity is commonly used for the production of nonvolatile flavor compounds such as acidulants, amino acids, and nucleotides. The formation of volatile flavor compounds via microbial fermentation on an industrial scale is still in its infancy. Although more than 100 aroma compounds may be generated microbially, only a few of them are produced on an industrial scale. The reason is probably due to the transformation efficiency, cost of the processes used, and our ignorance to their biosynthetic pathways. Nevertheless, the exploitation of microbial production of food flavors has proved to be successful in some cases. For example, the production of y-decalactone by microbial biosynthetic pathways lead to a price decrease from 20,000/kg to l,200/kg U.S. Generally, the production of lactone could be performed from a precursor of hydroxy fatty acids, followed by p-oxidation from yeast bioconversion (Benedetti et al., 2001). Most of the hydroxy fatty acids are found in very small amounts in natural sources, and the only inexpensive natural precursor is ricinoleic acid, the major fatty acid of castor oil. Due to the few natural sources of these fatty acid precursors, the most common processes have been developed from fatty acids by microbial biotransformation (Hou, 1995). Another way to obtain hydroxy fatty acid is from the action of LOX. However, there has been only limited research on using LOX to produce lactone (Gill and Valivety, 1997). 

The mutant which was blocked in the synthesis of branched chain amino acids produced very low levels of methoxy pyrazines. Cultures of this mutant did generate a new N peak and produced a strong butter-like aroma. TVo compounds were identified in these cultures as 2,3,5,6-tetramethy1 pyrazine and diacetyl. The synthesis of tetramethylpyrazine by a Corynebacterium glutamicum that was also metabolically blocked in the branched chain amino acid pathway has previously been reported (24). 

The major precursors in meat flavors are die water-soluble components such as carbohydrates, nucleotides, thiamine, peptides, amino acids, and the lipids, and Maillard reaction and lipid oxidation are the main reactions that convert these precursors in aroma volatiles. The thermal decomposition of amino acids and peptides, and the caramelization of sugars normally require temperatures over 150C for aroma generation. Such temperatures are higher than those normally encountered in meat cooking. During cooking of meat, thermal oxidation of lipids results in the formation of many volatile compounds. The oxidative breakdown of acyl lipids involve a free radical mechanism and the formation of... 

Figure 5 shows the influence of harvest time and storage on total volatiles production. Since the production of many but not all of the aroma volatiles is linked to amino acid precursors it may be expected that the total volatiles behavior may reflect that of the amino acids especially those which supply many of the carbon skeletons for the esters found in melons. The pattern of behavior for the total volatiles is generally in accord with that of these amino acids and does very clearly illustrate the profound influence of harvest time on the generation of the aroma profile. Fruit harvested only two days before folly ripe develops only about one quarter of the total volatiles concentration shown a few days postharvest, by a folly ripe sample. This rather dramatic difference may reflect the inability of prematurely harvested fiuit to accumulate sufficient concentrations of required volatiles substrates because certain metabolic responses have not been activated. 

Meat flavor is due to a great number of volatiles from different chemical classes. However, most of the odorants described as meaty aroma contain sulfur. The two most important reactions which generate meaty aroma compounds are the reactions between sulfur containing amino acids and reducing sugars (Maillard reaction) and the thermal degradation of thiamin [35], Sulfur-containing furans are the basic chemicals responsible for the aroma of thermally treated meat. 

---