Off-flavor metabolites

It has been suggested that microbial cata holism of off-flavor metabolites may be enhanced to provide an effective method ) control off-flavors. Indeed, microbial cultures have been reported to degrade relatively high levels of off-flavor metabolites (53, 54, 55), but whether microbial activity can be used practically to improve the flavor quality of aquaculture systems remains to be determined. 

In addition to biological processes, off-flavor metabolites may be adsorbed onto substrates and lost by volatilization (56), These physical factors must be considered in the evaluation of the effectiveness of control strategies, and may account for some of the reductions in metabolite concentration attributed to bacterial action (56),... 

Fish can rapidly accumulate off-flavors from water containing even very low concentrations of off-flavor metabolites (46), Therefore, to be totally effective, removal systems must continually biodegrade or dissipate even trace amounts of metabolites from very large volumes of water which reduces the potential effectiveness of this approach. An increased understanding of the flux rate of metabolites within a system will enhance channel cattish production, but research efforts must also focus upon preventing the occurrence of off-flavor metabolite synthesis to be fully effective. 

If microbes cause off-flavors, it might seem logical that the destruction of microbial populations with chemical control agents (biocides) would prevent the occurrence of off-flavor metabolites. Unfortunately, nonselective biocides can initiate a series of events that may be ultimately more problematic than off-flavors (49, 58, 59). 

Even if fish populations could be maintained by aeration, biocides may not be effective in controlling off-flavors. Most of the geosmin present (ca. 90-99%) is associated with cells rather than the media (16,43, 60,12), Biocide induced cell lysis may release off-flavor metabolites from cells (67) and render them available for adsorption by fish (8,9,13), Off-flavor metabolites require long periods of time to clear from fish (57). Off-flavor-producing populations may become reestablished before off-flavor metabolite clearance is complete. In addition, the biocides used may be carried off-site or taken up by fish. 

Although many terrestrial herbicides may also control phototrophic bacteria (algae 67 - 70), several species of non-phototrophic bacteria, such as Streptomyces, are prolific producers of off-flavor metabolites (Table I). The growdi of non-phototrophic populations may be fostered by nutrients released fiom phototrophic populations exposed to herbicides (77), thereby limiting the potential effectiveness of adapting terrestrial herbicides for the control of off-flavor metabolite synthesis. 

The process of registration and marketing is extremely costly and can only be justified for markets that represent a significant profit potential. The current aquaculture market is not sufficient to justify an effort to synthesize and develop specific agents to selectively inhibit off-flavor metabolite pi uction. 

Biocides, use for elimination of off-flavor metabolites, 326-327 Biocompetitive agents, use for control of aflatoxin contamination, 280-282 Biosynthesis of geosmin adaptive advantages of production,... 

Cooling, sensation based on generalized membrane response, 15-18 Copper sulfate, use for control of off-flavor metabolites, 327 Cysticercus species, ionizing radiation, 296,298f... 

Recent advances in sample preparation techniques now make analysis of off-flavor metabolites simple, fast, accurate, and sensitive. With the SPME-MS-MVA technique, a mass intensity list representing all the volatiles in the milk sample is the basis for shelf-life prediction—not GC peak area data. SPME-MS-MVA prediction of processed milk shelf life is based on measurement of volatiles and semivolatiles present in milk after a pre-incubation period. 

Humans may be programmed to be more sensitive to natural food contaminants. For instance, (-)geosmin (rrans-l,10-dimethyl-rran5-9-decalol) occurs in earth, natural surface water, and in foods in contact with soil or water, such as beets, clams, or fish. Geosmin is a microbial, fungal, or algal metabolite. It is a water pollutant and off-flavor compound. The naturally occurring (-)enantiomer has a threshold 11 times lower than the (-I-) enantiomer (Polak and Provasi, 1992). 

Figure 1. A schematic representation of the terpene pathway showing the possible derivation of the musty/earthy "off-flavor" microbial metabolites 2-methylisobomeol and geosmin. Abbreviations PP, pyrophosphate IPP, isopentyl pyrophosphate DMAPP, dimethylally pyrophosphate Pi, inorganic phosphate (Adapted from ref. 42),...

Second, off-flavors can be acquired by some fish when grown in earthen ponds and this presents marketing problems. Off-flavors are mainly caused by the rapid absorption of compounds produced by cyanobacteria. The most common off-flavors in aquaculture are two metabolites of Oscillatoria spp. and Anabaena spp.—geosmin and 2-methylisobomeol. The sensory properties and kinetics of uptake and depuration of these chemicals in fish have been reviewed elsewhere (Howgate, 2004). The off-flavors produced by cyanobacteria have no adverse affect on the health of the fish or the consumer however, they do have a substantial impact on marketability. The elimination of these compounds is much slower than the rate of uptake, and the depuration rate is reduced as water temperature decreases. 

Christoph N, Gessner M, Simat TJ, Hoenicke K (1999) Off-flavor compounds in wine and other food products formed by enzymatic, physical, and chtanical degradatitni of tryptophan and its metabolites. Adv Exp Med Biol 467 659-669... 

Microbial activity can produce off-flavors in several ways the production of undesirable primary metabolites, incidental conversion of food constituents of otherwise little flavor significance, or through residual enzyme activity after cell death [62],... 

Presence of aroma components and deterioration reaction during handling and storage may generate undesirable odor and cause loss of flavor of the food products (Rubio et al., 2004). In food packaging, flavor, and odor absorbers are used to remove undesirable volatile gaseous molecules, chemical metabolites of foods, respiration products, or off-flavors in raw food (Rooney, 2005). On the other hand, flavor and odor releaser are added to either make the food more desirable to consumers or improve the flavor of food (Pereira de Abreu et al., 2012). 

Furthermore, volatiles produced as bacterial metabolites or from active microbial enzymes are not the only causes of off-flavors in milk and shortened shelf life. For example, chemical contaminants can generate significant off-flavor formation. Contamination of milk with relatively low levels of copper, for example, can result in formation of oxidation off-flavors during shelf life. Another nonmicrobiological cause of off-flavors that sometimes occurs in milk samples that first come off the processing line is contamination by sanitizer from pro-... 

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