Naturally occurring precursors occurrence

For the measurement of the hydrocarbon precursors of photochemical oxidants, the naturally occurring methane must be separated from the other so-called nonmethane hydrocarbons. Only one procedure, gas chromatography coupled with flame ionization detection, is available for this separation and measurement. Although instrumentation for routinely accomplishing this process is commercially available, its maintenance (continued operation) requires a degree of operational know-how that may be too costly for most public agencies in the United States to support. Consequently, the data currently are insufficient to relate the occurrence of photochemical oxidants and ozone accurately to some of their most important precursors, the nonmethane hydrocarbons. 

Naturally occurring organoleptically important lactones are mainly saturated and unsaturated 7- and -lactones, and to a lesser extent macrocyclic lactones. The occurrence of these types of lactones reflects their ready formation from natural acyclic precursors. 

Kawabata, T., Ohshima, H., Vibu, J., Nakamura, M., Matsui, M., and Hamano, M., 1979, Occurrence, formation and precursors of N-nitroso compounds in Japanese diet, in "Naturally Occurring Carcinogens-Mutagens and Modulators of Carcinogenesis", E.C. Miller, J.A. Miller, T. Sugimura, S. Takayama and I. Hirono, eds, Jap. Sci. Soc. Press, Tokyo/Univ. Park Press, Baltimore. 

The significance of flavan-3,4-diols in plants rests primarily on their probable role as precursors of the polymeric proanthocyanidins. Co-occurrence of the 5-deoxy compounds - i.e., quibourtacacidins, mollisacacidins, and robinetinidins - with the related proanthocyanidins in Acacia species and the ready synthesis of naturally occurring proanthocyanidins from reactions of these flavan-3,4-diols with catechin under mild acidic conditions constitutes heavy but not definitive evidence for this thesis (31, 315-317). 

Although originally named after the tetraodontidae fish, it is now very apparent that the occurrence of tetrodotoxin is not restricted to this family. In fact it is not even the unique preserve of one phylum, as it has been identified in a series of taxonomically unrelated animals representing at least five different phyla, as noted above. The toxin has no obvious similarity to any known animal or plant natural product, so its synthesis does not merely result from the simple modification of a related (non-toxic) precursor. Whilst it was thought that tetrodotoxin only occurred in fish and newts, it was just possible that the toxin had been "invented" twice... 

Extrusion cooking is a high temperature-short time process which is ideal for the production of flavor volatiles. However, due to the nature of this process there are many opportunities for these volatiles to be lost. This has led many researchers to study the formation and retention of volatiles during extrusion. This review will focus on the flavor related reactions which may occur during extrusion and their occurrence in various flours and due to the addition of reactive precursors. 

Fatty primary alcohols with similar aliphatic chains to fatty acids tend to occur in the free state in tissues at low concentrations only, but they may be of some metabolic importance as precursors of alkyl lipids, as plant growth regulators and as insect pheromones, for example. In addition, they are found in esterified form in wax esters, which are substantial components of many natural materials. Secondary alcohols may be present in plant surface waxes, together with aliphatic diols which are common constituents of skin lipids. In mammalian tissues, the primary alcohols are saturated or monoenoic, but never di- or polyunsaturated in wax esters of marine origin, the alcohol constituents are often closely related in structure to the fatty acids from which they may derive biosynthetically. The occurrence, chemistry and metabolism of fatty alcohols [577] and their chromatographic properties [577,579] have been reviewed. 

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