Nonvolatile precursors, flavor

The first five chapters of this book focus on the grape derived and varietal flavors of wines. Many of these compounds occur as nonvolatile glycosidic flavor precursors and the separation and analysis of these precursors have been a challenging and active field of research. The isolation and quantification of trace volatiles represent examples of the difficulties faced by flavor chemists as they attempt to characterize varietal flavors with sensory thresholds in the parts per trillion range and lower. 

Unlike enzymatic reactions, microorganisms have the ability to perform multiple reactions, and they do not require cofacors for regeneration (albeit they require nutrients). They may be used to generate a flavor compound from a nonvolatile precursor (e.g., produce a lactone from castor oil), to effect the bioconversion of one volatile to another (e.g., valencene to nootkatone), or effect a chiral resolution (a racemic mixture of menthol). The primary limitation of using microoganisms for... 

Nonvolatile Conjugates of Secondary Metabolites as Precursors of Varietal Grape Flavor Components... 

One pragmatic approach to the problem is the flavorese concept (i). A crude enzyme preparation from the fresh food or a closely related species is added to the processed food in the hope that the nonvolatile flavor precursors are still present and will yield the full range of fresh flavor components upon enzymatic treatment. This approach has been applied with some success to watercress (1,2), cabbage 1,2, 3, 4), horseradish 2, 3), onions 2, 3), carrots 2, 3, 5), peas, beans 2,3,6), citrus juice (2, 7), raspberries (8), tomato juice (2, 3, 9), bananas (JO), and various flower fragrances 11). This would seem to be a desirable approach since the products of different enzymatic reactions are probably necessary for full flavor. However the flavor-forming activity of the enzyme preparations was variable (5, 6, 7, 9), and the flavors were not always like that of the fresh vegetable (3). The several enzymes and flavor precursors may not have been present in the normal ratios. Also, to be practical, the enzyme preparation must be inexpensive to prepare and store, the substrates must persist in the processed food, and they must be available to enzyme action. One also wonders how processed foods can be enzymatically treated under sterile conditions. 

Monohydroperoxides are the primary products of lipid oxidation. A variety of hydroperoxides with positional and geometrical isomers are formed depending on the position and number of double bonds of the unsaturated fatty acids and the oxidation mechanism. A number of reviews have been published on the composition of isomeric hydroperoxides formed from oxidation of oleate, linoleate, and linolenate (286, 287-291). The hydroperoxides formed are odorless, but they are relatively unstable and are the precursors of a variety of volatile and nonvolatile scission products that are important to the oxidized flavor. 

Williams, P.J., Sefton, M.A. and Wilson, B. (1989) Nonvolatile conjugates of secondary metabolites as precursors of varietal grape flavor components, in R. Teranishi, R.G. Buttery, and F. Shahidi, (Eds), Flavor chemistry trends and developments, ACS Symp. Series 388, Am. Chem. Soc., Washington DC. 

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). 

These results were interpreted as an indication that the relatively high molecular weight, nonvolatile tobacco wax components were the major precursors in tobacco of the PAHs in smoke, whereas the moderate to low molecular weight and volatile flavorful components in tobacco did not contribute significantly to the PAH levels in smoke. 

Compounds Probably Geiieraled from Thermal Inlernclions of Sugars and Nonvolatile Flavor precursors ... 

Table II lists the major volatile compounds from baked blanched and fried blanched shallot slices generated from thermal degradation of nonvolatile flavor precursors of shallot were methyl propyl trisulfide, dimeihylthiophencs, methyf propyl disulfide, and dipropy trisulfide. The major volatile compounds that were probably generated from ihemia imeractiuns of nonvolatile flavor precursors of shallot and sugars were pyrazines, especially ethyl dimethyl pyrazines, dimethyl-pyrazines, ethyl methyl pyrazines, and trimethylpyrazine.

Natural antioxidants, phenols, 219 Nitrogen-containing compounds, source of pungent flavors in spices, 98 Nonvolatile sulfur-containing flavor precursor of Allium, contribution to flavor of thermally processed Allium vegetables... 

Saffron, production, 66 Saffron flavor characterization using aroma extract dilution analysis aroma-active components, 74-78 detection of aroma-active component using OC-olfactometry, 67 experimental procedure, 67-68 volatile components, 68-74 Safranal, role in flavor, 66-78 Scmivolatile components in powdered turmeric, characterization using direct thermal extraction GC-MS, 80-96 Shallot, contribution of nonvolatile sulfur-containing flavor precursors to flavor, 53-63... 

Contribution of Nonvolatile Sulfur>Containiiig Flavor Precursors of the Genus Allium to the Flavor of Thermally Processed Allium Vegetables... 

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