338 M. Ugliano and P.A. Henschke
low amounts of acetic acid, typically<150 mg/L (Antonelli et al. 1999; Eglin-
ton et al. 2000; Giudici et al. 1995) and might be useful when low concentrations
are required, such as when musts already contain excessive concentrations, and for
malolactic fermentation and long term barrel maturation during which acetic acid
content often increases.
Acetic acid production amongst non-Saccharomycesyeasts varies widely.To r u -
laspora delbrueckii (Candida colliculosa), Metschnikowia pulcherrima and
Issatchenkia orientalis (Candida krusei)produce low to moderate amounts whereas
apiculate yeasts,Candida stellataandPichia anomalaproduce moderate to high
amounts and Brettanomycesspecies and Zygosaccharomyces bailiivery high
amounts (Giudici and Zambonelli 1992; Heard 1999; Shimazu and Watanabe 1981).
As is the case forSaccharomycesspecies, intraspecific strain variability is high,
meaning that strain selection is essential for winemaking purposes. Laboratory
cofermentation studies with mixed cultures of yeasts, which produce different con-
centrations of acetic acid in monoculture, typically show lower levels in the final
wine than expected and are usually similar to that of the lowest producer yeast in the
cofermentation (for examples see Ciani and Comitini 2006; Grossmann et al. 1996;
Soden et al. 2000). As has been observed with acetaldehyde and redox balance
in cofermentation, diffusion of various metabolites between yeasts with different
‘metabolic tuning’ can result in metabolite concentrations different from those that
would be achieved by blending wines (Cheraiti et al. 2005; Howell et al. 2006).
The assimilable nitrogen concentration of must, across the range 50–500 mg/L
yeast assimilable nitrogen, has a relatively strong effect on acetic acid accumulation
during fermentation. Lowest acetic acid concentrations occur around 200–250 mg/L
yeast assimilable nitrogen with increases of up to twofold at nitrogen concentrations
well outside this range (Bely et al. 2003; Vilanova et al. 2007), apparently irre-
spective of initial sugar concentration or osmotic stress. Nitrogen additions made
at inoculation rather than later during fermentation are more effective in preventing
acetic acid accumulation.
Some strains of yeast have partial requirements for the vitamins, nicotinic acid,
inositol and panthothenic acid, which can affect acetate metabolism (Nordstr ̈om
1964). Nicotinic acid addition to musts can markedly increase acetic acid production
by yeast with the extent of production depending on yeast strain, fermentation tem-
perature, sugar concentration and mustcomposition (Eglinton and Henschke 1993;
Monk and Cowley 1984). Many strains ofSaccharomyces cerevisiaecannot synthe-
sise pyridine nucleotides under anaerobic conditions and are therefore dependent
on the nicotinic acid content of must. Increasing nicotinic acid content of must is
presumed to act by optimising the pyridine nucleotide content of yeast, stimulating
NADH production for growth, and consequently increasing acetic acid production.
Fermentation temperature affects acetic acid production, which is maximal around
the optimum temperature for biomass production (≈ 25 ◦C; Monk and Cowley 1984).
As yeast growth becomes progressively inhibited by growth at suboptimal tem-
peratures biomass production decreases asdoes acetate accumulation; nevertheless
acetate yield remains constant for each gram of yeast protein formed. At very low
temperatures, however, yeast growth becomes uncoupled from sugar metabolism
resulting in low biomass yield but high acetate production.