Nonvolatile, flavor generation

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). 

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.

Alliin and deoxyalliin, two important nonvolatile flavor precursors of garlic, were reacted separately with inosine-5 -monophosphate (IMP) in an aqueous solution at pH 7.5 in a closed sample cylinder at 180 °C for one hour. The volatile compounds generated were isolated by using a modified Likens-Nickerson (L-N) distillation-solvent extraction apparatus, and analyzed by GC and GC-MS. 

These compositional changes usually negatively influence the odor and flavor of citrus peel oils by generating off-flavor products. It has been shown that nonvolatile residues of citrus peel oil contain some compounds that exhibit antioxidative activities, among which permethoxylated flavones, dehydroabietic acid (46), coumarins, and psoralens have been identified (33). In this respect, cold-pressed citrus peel oil is more stable than distilled oil and essence oil, in which most of the natural antioxidants present are left behind when the oil is distilled (1). 

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... 

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