Study On Wine Aroma Enhancement By Mixed Alcoholic Fermentation Of Selected Rhodotorula Mucilaginosa And Saccharomyces Cerevisiae

The typical continental monsoon climate severely affects the quality of grapes for winemaking in most Chinese wine regions,which subsequently declines wine quality,especially wine aroma.In order to improve wine aroma quality,Ecolly white grapes in Yangling district and Merlot grapes in Heyang County were used as materials in this study.A selected strain of Rhodotorula mucilaginosa was applied in mixed fermentation of wine with a Saccharomyces cerevisiae yeast for the evaluation of wine aroma enhancement by analyzing volatile aroma compounds,sensory characteristics,and polyphenols and color indices.The important results are as follows:(1)The kinetics of mixed fermentation of R.mucilaginosa and S.cerevisiae in model grape juice revealed that R.mucilaginosa was significantly inhibited by S.cerevisiae.However,both the amount and the duration of R.mucilaginosa increased when inoculation ratios of R.mucilaginosa were higher or when the inoculation time point of S.cerevisiae was latter.The inhibition experiments of medium-chain fatty acids on the growth kinetics of R.mucilaginosa showed that the higher contents of hexanoic acid,octanoic acid,and decanoic acid,the lower of the maximum specific growth rate,and the longer of the duration of lag phase and log phase of R.mucilaginosa;moreover,the lower of the p H in the model grape juice,the more significant of the inhibitory effect of the medium-chain fatty acids on the growth of R.mucilaginosa.The inhibition effect of hexanoic acid was the most significant and the inhibition effect of decanoic acid was more significant than that of octanoic acid.(2)In order to analyze glycosidase activity characteristics of the R.mucilaginosa,five principle glycosidase activities were measured by p-NPG method after the R.mucilaginosa being cultivated for 72 h in fermentation medium.Results showed that β-D-glucosidase and β-D-galactosidase activities were around 40 mU/mL;α-L-rhamnosidase and β-D-xylosidase activities were around 15 mU/mL;whereas α-L-arabinosidase activity was the lowest(<10 mU/mL),which revealed that the yeast has potential to improve wine aroma quality.Based on the above results,four relatively higher glycosidase activites were measured during the sequential mixed fermentation of Ecolly dry white wine,and results showed that β-Dglucosidase activity showed an increasing tread while β-D-galactosidase activity gradually decreased during fermentation;α-L-rhamnosidase and β-D-xylosidase activities were higher at the initial stage then kept low activites afterwards.In conclusion,mixed fermentation improved α-L-rhamnosidase,β-D-galactosidase,and β-D-xylosidase activities while had little influence on β-D-glucosidase activity.(3)Two vintages of Ecolly grapes and one vintage of Merlot grapes were used as the material and several mixed inocula treatments were tried in mixed fermentation experiments for the evaluation of aroma enhancement.The results of volatile aroma compounds determined by head-space solid phase microextraction gas chromatography-mass spectrometry(HSSPME-GC-MS)showed that mixed fermentation increased the concentration of varietal aroma compounds,especially those of C6-compounds,C13-norisoprenoids,and terpenols.Moreover,the content of fermentative aroma compounds were elevated by the mixed fermentation,particularly those of higher alcohols,acetates,fatty acid ethyl esters,and phenylethyls.It is worthy to note that the selected R.mucilaginosa yeast preferred to improve the level of C6-compounds,fatty acid ethyl esters,fatty acids,and phenylethyls despite of the factors of grape varieties and vintages.Sensory analysis revealed that the mixed fermentation could enhance the intensity of fruity and floral traits,whereas higher ratios of R.mucilaginosa yeast could induce a slight scent of animal note.The result of polyphenols and color indices of Merlot wine samples showed that higher inoculation ratios of R.mucilaginosa or the inoculation protocol of S48 could increase the level of Co-pigmented anthocyanin proportion,tartaric esters,total anthocyanin,and flavonol.(4)Fourteen Partial Least Square Regression(PLSR)models were successfully built for floral,fruity,green,toast,spicy,and animal characteristics.The correlation coefficients of the models showed that wine sensory characteristics were developed by different contributions from multi volatiles or volatile compound categories,and volatiles or volatile compound categories contributed differently to different sensory characteristics.Besides,due to the fact that the number of fermentative aroma compounds was more than that of the varietal aroma compounds,the number of impact fermentative aroma compounds contributing significantly(correlation coefficient ≥ 0.1)to the models was more than that of the impact varietal aroma compounds.Results of the models reconfirmed the complicated multivariate correlations between aroma characteristics and volatiles,and the results revealed that fermentative aroma compounds played a vital role in the formation of wine sensory characteristics while varietal aroma compounds just played a supplementary role.