| Malolactic fermentation(MLF)is a necessary secondary fermentation process for quality wine production,initiated and completed by lactic acid bacteria(LAB).The conversion of L-malic acid to Llactic acid reduces the acidity of the wine,increases its microbial stability and improves the flavor quality of the wine.However,the malolactic starter cultures used in the Eastern Foothill of Helan Mountain are mainly imported,and there are no local malolactic starter cultures on the market The long-term use of starter cultures made from several imported strains not only increases the production cost,but also threatens the diversity of local microorganisms.At the same time,it also brings about the homogeneity of wine and other problems.In addition,with the global warming,the sugar content of grapes continues to increase,which brings about the continuous rise of alcohol accuracy.Especially,the rapidly falling temperature during the MLF process in northwest China makes the MLF process of quite a number of wineries with poor temperature control conditions stop or sluggish.In this study,20 strains of Oenococcus oeni(O.oeni),which were isolated from the Eastern Foothill of Helan Mountain and initially screened by 15%(v/v)ethanol stress in the laboratory,were screened for good O.oeni and analyzed for fermentation performance by adversity stress tolerance analysis and esterase activity assay tests.To understand the MLF ability of the good strains and their effects on physicochemical indexes,quality and safety and aroma components of wines.The main conclusions are as follows:1.The excellent strains O4,O5,O6,O14 and O16 with good tolerance to adversity stress were obtained through ethanol,pH,SO2,low temperature single factor and ethanol×SO2×low temperature compound factor screening.In particular,strains O14 and O16,which still had high growth in ethanol concentration of 17%(v/v),30 mg/L SO2 and 10℃ stress environment,had the potential to initiate and complete MLF in wines at low temperature and high ethanol conditions.All five O.oeni strains had high esterase activity,with strains O5,O14 and O16 showing significantly higher esterase activity than commercial strains.2.Fermentation tests were conducted by inoculating strains O5,O14 and O16 with good tolerance to adversity stress and esterase activity into Cabernet Sauvignon wines.Strains O5,O14 and O16 showed good ability of reducing acid and producing aroma substances.The rate of acid reduction of strain O14 was similar to that of commercial strains.It reduced L-malic acid to about 0.018 g/L within 28 days after starting MLF,followed by strain O16,which reduced L-malic acid to about 0.08 g/L within 32 days.The alcohol content,residual sugar content,total acid and volatile acid content of MLF wine sample all meet the requirements of national standard.Compared with commercial strains,the types and contents of aroma volatiles of the three O.oeni strains screened in this study were increased.In a comprehensive comparative analysis,strain O14 had the most variety and total content of aroma substances,followed by strain O16.They showed more prominent performance in fermentation aromas such as esters,higher alcohols and terpenes,which are more beneficial to improve wine aromas.It has important production value application for making wines with regional characteristics.In addition,the total contents of biogenic amines(BAs)and ethyl carbamate(EC)in the wine after MLF of all indigenous O.oeni strains were slightly lower than those of commercial strains,and the contents were all within the maximum limits set by the European Union(EU). |
