Wine Chemistry and Biochemistry

(Steven Felgate) #1

8D Yeasts and Wine Flavour 335


8D.4.2.3 Modulating Factors


Many factors affect formation of higher alcohols during fermentation, includ-


ing yeast species and strain, initial sugar, fermentation temperature, the pH and


composition of grape juice, assimilable nitrogen, aeration, level of solids, grape


variety and skin contact time (Fleet and Heard 1993; Houtman and Du Plessis 1981;


Houtman et al. 1980). Choice of yeast has a considerable impact on higher alcohol


content of wine due to wide variability amongst strains ofSaccharomycesspecies


withSaccharomyces cerevisiaegenerally producing lesser concentrations of higher


alcohols than cryotolerantSaccharomyces bayanus/uvarumstrains (Antonelli et al.


1999; Massoutier et al. 1998). Most non-Saccharomycesspecies are reported to


produce lesser concentrations of higher alcohols thanSaccharomyces cerevisiae


although in fermentation with mixed yeast species higher concentrations of higher


alcohols can be produced (Heard 1999).Metschnikowia pulcherrimahas a notably


high production of 2-phenyl ethanol (Clemente-Jimenez et al. 2004).


The assimilable nitrogen composition of grape juice has a strong influence on


the production of higher alcohols during fermentation. When initial nitrogen con-


centrations are very low, increasing nitrogen results in greater concentrations of


higher alcohols (Ayr ̈ ̈ ap ̈a ̈a 1971; Carrau et al. 2008; Garde-Cerd ́an and Anc ́ın-


Azpilicueta 2008; Vilanova et al. 2007). According to Oshita et al. (1995), under


these conditions, surplus keto acids cannot be converted into amino acids, due to the


lack of available nitrogen, and are therefore excreted as higher alcohols. Conversely,


at high initial juice nitrogen concentrations, increased availability of nitrogen causes
a reduction in higher alcohol production because most of the keto acids produced


are directly converted into the corresponding amino acids.


The degree of juice turbidity increases the production of higher alcohols, which is


linked to biomass production (Houtman and Du Plessis 1981; Klingshirn et al. 1987);


red wines, produced by fermentation on grape solids typically contain more higher


alcohols. Similarly, chemically inert solids stimulate higher alcohols production,


although the mechanism of action is not clear. Aeration and increased fermentation


temperature are believed to stimulate the production of higher alcohols by stimulat-


ing nutrient (nitrogen) uptake and growth.


8D.4.3 Volatile Fatty Acids


8D.4.3.1 Significance


Wine contains a mixture of straight chain fatty acids, usually referred to as short


chain (C 2 –C 4 ), medium chain (C 6 –C 10 ), long chain (C 12 –C 18 ), and a group of


branched-chain fatty acids that include 2-methyl propanoic, 2-methyl butanoic,


and 3-methyl butanoic acids. Acetic acid is quantitatively and sensorially the most


important volatile fatty acid produced during alcoholic fermentation. Accounting for


more that 90% of the total wine volatile acidity, acetic acid plays the most important

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