Aroma flavour precursors

Francis, I.L., Sefton, M.A., Williams. P.J. (1992). Sensory descriptive analysis of the aroma of hydrolysed flavour precursor fractions from Semilion, Chardonnay, and Sauvignon Wane grape juices. J. Sci Food Agric., 59, 511-520. 

Guth, H., Grosch, W. (1993) Furanoid fatty acids as precursors of a key aroma compound of green tea. In Schreier, P., Winterhalter, P. (eds.) Progress in Flavour Precursor Studies. Allured Publishing, Carol Stream, pp. 401-407... 

Owing to very low thresholds, volatile sulfur compounds (VSCs) usually have prime impact on food aromas they are found in lots of natural sources, including fermented foods (e.g. wine, beer, cheese), and act as both flavours and off-flavours [249, 250]. Although their biogenetic formation has been elucidated in detail, only few biotechnological processes with potential for commercial application have been reported. The sulfur-containing amino acids L-methionine and L-cysteine are the natural precursors of a wide variety of VSCs. Methanethiol is the most frequently found VSC in cheese and can be readily oxidised to other VSCs, such as dimethyl suMde and dimethyl disulfide, or... 

Francis, I.L., Kassara, S., Noble, A.C. Williams, P.J. (1999).The contribution of glycoside precursors to Cabernet Sauvignon and Merlot aroma sensory and compositional studies. In A.L. Waterhouse S.E Ebeler (Eds.), Chemistry of wine flavour (pp. 13-30) American Chemical Society Washington, DC. 

The occurrence of many of these reactions depends on the presence of appropriate precursor in the grape must and together with the concentration of precursor and the ability of the yeast to (i) produce an appropriate extracellular enzyme for non-transportable precursors, (ii) take up the precursor, and (iii) transform the precursor different combinations of yeast and grape variety can lead to small or large differences in the aroma and flavour profile of the wine. 

Yeast cells can also influence wine flavour by binding aroma precursors such as glycoconjugated terpenes. Moio et al. (2004) have shown that this binding by yeast cells during fermentation could have important consequences on wine varietal aroma. 

Model reaction trials and modem analytical methods (gas chromatography/mass spectrometry (GC/MS), gas chromatography/olfactometry (GC/0)) permitted the identification of key mechanisms responsible for flavour generation in process flavourings and some of the most important ones are detailed below. Often chemically complex precursor raw materials (vegetables such as onions, spices, yeast extracts, animal products) are used. Research work on these complex reactions is rare but necessary and allows the discovery of new key odorants and formation pathways. For example, Widder and co-workers [13] discovered a new powerful aroma compound, 3-mer-capto-2-methylpentan-l-ol in a complex process flavour based on onion. 

The most important precursors for lipid oxidation are unsaturated fats and fatty acids like oleic (18 1), linoleic (18 2), linolenic (18 3) and arachidonic acid (20 4). The more unsaturated ones are more prone to oxidation. Lipid peroxidation and the subsequent reactions generate a variety of volatile compounds, many of which are odour-active, especially the aldehydes. That is why lipid oxidation is also a major mechanism for thermal aroma generation and contributes in a great measure to the flavour of fat-containing food. Lipid oxidation also takes place under storage conditions and excessive peroxidation is responsible for negative aroma changes of food like rancidity, warmed-over flavour, cardboard odour and metallic off-notes. 

Thiamine degradation has a good share in meat aroma formation [17,64]. Neutral and acidic conditions favour the formation of 13 [65], which is a key component in boiled meat [66, 67[. It has already earlier been identified in a meat-like process flavouring [68[, prepared from cysteine, thiamine, hydrolysed vegetable protein and water [69]. Bolton and co-workers [70] showed that in model experiments with thiamine, [ S]-cysteine, glucose and xylose, only 8% of 13 contained sulphur from cysteine. They concluded that thiamine (43) was the primary precursor for the generation of 13 in this system. 

Besides the Maillard reaction, fat oxidation and thiamine degradation, more aroma compounds in process flavours are formed by the interaction of these different reactions, as well as from other precursors (e.g. phenol compounds, terpenes) present in the raw materials used. Three examples are given below but are far from being exhaustive. 

Animal phospholipids, especially egg phospholipids or egg yolk, are the key precursors for a patent application, which claims the reaction with a sulphur-containing compound like cysteine to produce meaty flavours ]102], Phospholipids and monoglycerides can play a fundamental role in the physical structure of a process flavour. Vauthey and co-workers ]103] use these emulsifiers in water to generate nanostmctured mesophases or microemulsions at the reaction temperature. Better aroma yields and different aroma compounds were obtained in these nanostmctured reaction media compared to aqueous solutions. 

Process flavourings for application in sweet and bakery goods (e.g. flavourings with chocolate, malt, caramel, egg, coffee or biscuit tonality) have been known for an even longer time than their savoury counterparts. Sulphur-containing aroma precursors such a cysteine are not the main concern of sweet process flavourings, and in many cases they even have to be avoided in order to prevent the formation of off-flavours. 

PAN is used in packaging (industrial, medical, and food) and as fibre (Orion), mainly because of its chemical and UV resistance, as well as for the retention of flavour and aroma and excellent durability under sterilisation procedures especially in packaging. PAN fibres have wool-like properties and are chemical precursors of high-quality carbon fibres. Almost all forms of PAN are copolymers with acrylonitrile as the main component. As copolymers of acrylonitrile, there is SAN to mention which will be discussed next. 

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