Study On Acetaldehyde Kinetics During Alcoholic Fermentation By Wine Yeast And Its Affecting Factors

The reduction of SO₂ concentrations in wine is a primary objective because of regulatory pressure or consumer concerns with regards to asthma or allergic reactions. Several wine constituents may bind to SO₂, acetaldehyde （ethanal） typically being the most important because of its quantity and binding power. This research evaluated the ability of 26 yeast strains, including Saccharomyces and non-Saccharomyces strains and commercial starters to form and degrade acetaldehyde in a model medium and during grape must vinifications. The effect of fermentation vigor was considered as well as yeast response to SO₂ additions at different fermentation stages. Specifically, the effect of using SO₂ to stop alcoholic fermentation with the aim of achieving wines with residual sugar was compared with cooling. In addition, the ability of production other SO₂ binders by 8 S. cerevisiae was investigated in grape must fermentation with the aim of calculation bound SO₂ concentrations in different wines. The major results were summarized as following.1 Acetaldehyde metabolism of 26 yeast strains was evaluated in a reproducible resting cell model system. Acetaldehyde kinetics and peak values were highly genus, species, and strain dependent. Peak acetaldehyde values varied from 2.2 to 189.4 mg/L and correlated well （r2 = 0.92） with the acetaldehyde production yield coefficients that ranged from 0.4 to 42 mg acetaldehyde per g of glucose. S. pombe showed the highest acetaldehyde production yield coefficients and peak values. All other non-Saccharomyces species produced significantly less acetaldehyde than the S. cerevisiae strains and were less affected by SO₂ additions. All yeast strains could degrade acetaldehyde as sole substrate, but the acetaldehyde degradation rates did not correlate with acetaldehyde peak values or acetaldehyde production yield coefficients in incubations with glucose as sole substrate.2. In addition, acetaldehyde production property of 26 enological yeast strains was investigated in a natural grape must. All yeast strains led to uniform kinetics where acetaldehyde reached an initial peak value at the beginning of fermentation followed by partial reutilization. Peak acetaldehyde values and final concentrations in wine ranged from 9 to 129 mg/L and 2 to 64 mg/L, respectively. Acetaldehyde production yield coefficient at the beginning of alcoholic fermentation was found to be yeast strain specific and can be used as a discriminating factor between yeast strains. Initial addition of SO₂ led to higher acetaldehyde concentrations throughout the fermentation and the extent of increases at final values in wine was highly strain dependent, which ranged from 217-531μg acetaldehyde per mg of SO₂ addition. Compared with previous work reported in resting cell system, acetaldehyde yield coefficients calculated as acetaldehyde produced per sugar degraded of 26 yeast strains were also correlated with the values in grape must fermentation and could be used as a criteria to discriminate among yeast.3. In wine making, slow or sluggish alcoholic fermentations still occur frequently and is difficult to predict. Acetaldehyde kinetics of 6 S. cerevisiae strains at different fermentation speed was studied in Chardonnay wine vinification. By comparison with fast fermentations, sluggish fermentations led to higher acetaldehyde yield coefficient as well as final concentrations in wine. The average value of acetaldehyde yield coefficient was 1.59±0.21 mg acetaldehyde per g of sugar for sluggish fermentations, but 0.95±0.10 mg acetaldehyde per g of sugar for fast fermentations. The significance was evidenced in 2 strains （EC1118 and E1219） in absence of SO₂ and 5 strains （CY3079, DV10, EC1118, A709 and E1219） with 50 mg/L SO₂ addition.4. The effect of using SO₂ to stop alcoholic fermentation on acetaldehyde production was also studied. Sugar metabolism was stopped with over 150 mg/L SO₂ addition and acetaldehyde production was increased with any SO₂ addition during fermentations. Acetaldehyde formation rate after the time-point of SO₂ adding was decreased as the increase of SO₂ dosage. However, final acetaldehyde concentration was below 79 mg/L in all wines, which did not over the acetaldehyde threshold in wines.5. Concentrations of other SO₂ binders as pyruvate,α-ketoglutaric acid and acetoin were studied throughout Chardonnay wine fermentation. Pyruvate and acetoin had similar kinetics with acetaldehyde during alcoholic fermentation. The amount of bound SO₂ in wines was yeast strain dependent. As calculated by three major binders, wine made with yeast strain EC1118 required at least 89 mg/L total SO₂ addition to reach a free SO₂ concentration of 50 mg/L, while wine made with yeast strain E1219 needed at least 120 mg/L total SO₂ addition, which was 1.34 fold higher.