Effects Of Different Yeast Strains And Technological Conditions On Fermentation Of Icewine

In this paper,the effect of yeast species,initial sugar concentration and assimilable nitrogen concentration on fermentation of icewine were studied in natural and synthetie ice grape juice by determining the contents of yeast cells,reducing sugar,glycerol,acetic acid,ethanol,higher alcohols,esters and biogenic amines in the fermentation process.Five species of commercial yeast ST,VC,LV,RV and R2 were inoculated into ice grape juice,respectively to conduct icewine fermentation.The experimental results showed that RV and LV could consume more reducing sugar and produce higher yield of glycerol,higher alcohols and ethanol.However,ethanol content in final wine exceeded legal limits.RV grew faster than other species in the early and mid fermentation stage;LV produced much more ethanol than other species(P<0.05);VC produced less ethanol and more esters and acetic acid.However,acetic acid content exceeded the prescribed standards.ST and R2 grew stably and consumed sugar properly.R2 produced more ethanol,higher alcohols and esters than ST.However,ST produced less acetic acid.ST and R2 are identified as the most suitable inocula for icewine brewing.Compared with spontaneous fermentation,the ice grape juice inoculated with ST yeast fermented faster,in which more reducing sugar was consumed and residual sugar was moderate,and acetaldehyde,acetic acid,glycerol,ethanol,esters and higher alcohol were produced moderately.The endogenous yeast in ice grape juice could resist strongly to low temperatiure,ice grape juice could ferment slowly(probably due less cells)spontaneously,in which the contents of residual sugar,glycerol,higher alcohols and esters were higher,and the contents of acetaldehyde,acetic acid and ethanol were lower,significantly than inoculated fermentation(P<0.05).The high residual sugar and low acetic acid in final wine resulted in imbalance of the ratio of sugar and acid,which affected the taste of wine.Therefore,spontaneous fernentation is not suitable for icewine brewing.To explore the effect of initial sugar concentration on icewine fermentation,ice grape juice was adjusted to the initial sugar concentration of 370,450,500 and 550g/L by adding glucose and inoculated with ST.The experimental results showed that content of initial sugar had a significant negative correlation with sugar consumption(r=0.99,P<0.05)and ethanol production(r=0.992,P<0.05),a significant positive correlation with acetic acid production(r=0.998,P<0.05)and acetaldehyde production(0.971,P<0.05),a negative correlation with glycerol production(r=0.979).The number of viable yeast decreased with the increase of initial sugar concentration.There was no significant difference in the content of total higher alcohols during the fermentation.When the initial sugar concentration was more than 500 g/L,the fermentation was inhibited,esters contents were lower,acetic acid content was higher and the content of residual sugar was higher,which affected the taste of wine.The initial sugar concentration should be controlled below 500 g/L,which is beneficial for the fermentation and the quality of icewine.The synthetic ice grape juice was adjusted to the concentration of assimilable nitrogen of 250,300,350,650 and 800 mg/L,and inoculated with ST for icewine fermentation.The experimental results showed that when assimilable nitrogen concentration was lower than 350 mg/L,yeast growth was inhibited,fermentation was delayed,abnormal metabolism appeared in fermentation,the production of higher alcohol and acetic acid was increased.While the concentration of assimilable nitrogen was more than 350 mg/L,yeast fermented normally,the number of viable yeast increased with the increase of assimilable nitrogen concentration,assimilable nitrogen concentration had a significant positive correlation with acetic acid production(r=0.998,P<0.05).No matter higher alcohols contents or esters contents didn’t display significant difference between the assimilable nitrogen concentration of 500 mg/L and 800 mg/L,but acetic acid was overproduced,more biogenic amine was produced in wine.The assimilable nitrogen residues and biogenic ammonia production increased with the increase of assimilable nitrogen concentration.Excess nitrogen was residued in wine,which probably resulted in microbial contamination not being conducive to later storage,and further affected wine quality.Therefore,it is better to control assimilable nitrogen concentration at approximate 350 mg/L for icewine fermentation,which will be beneficial for icewine quality.