Bunch Exposure Alters Fatty Acid Level In 'Cabernet Sauvignon' Must And Unsaturated Fatty Acids Affect Physiological Metabolism Of Saccharomyces Cerevisiae

The ultrasonic-assisted extraction(UAE)of fatty acids in grape must was optimized in this study.The detailed investigation was conducted regarding the influences of leaf removing(LR),half leaf removing(HLR),and leaf moving(LM)at stages of grape berry pea-size(PS),the onset of verasion(V),the end of verasion(AV),and two-week pre-harvest(PH),respectively,on concentration of fatty acids of 'Cabernet Sauvignon' grape must.The results showed that bunch exposure greatly increased photosynthetically active radiation(PAR)around grape cluster.The PAR values were higher for full leaf-removal and leaf-moving treatments than for half leaf-removal treatment.Amongst all treatments,only leaf-removing treatment at the stage of grape berry pea-size significantly decreased fatty acid concentrations while others all increased the production of fatty acids,in particular unsaturated fatty acids(UFAs)in grape berries.Compared with other treatments,bunch exposure at the onset of verasion or the end of verasion promoted the production of fatty acids more significantly.Additionally,bunch exposure at the end of verasion also increased the percentage of UFAs to total fatty acids.To understand what effect high level of UFAs in grape must causes on the growth and metabolism of Saccharomyces cerevisiae and volatile profile in wine,we added linoleic acid,oleic acid and a-linolenic acid to Vitis vinifera L.Merlot must and investigated the growth of yeast and the production of volatiles during alcoholic fermentation.The results showed that high concentration of UFAs accelerated yeast growth,reduced the level of saturated fatty acids(C6:0-C24:0)in yeasr cells,promoted the production of higher alcohols and ethyl esters in the wine,and surpressed the generatiopn of mid-chain fatty acids(MCFAs)-derived volatiles and acetate esters,eventually intensifying the flowery and sweet attributes of fresh wine.But little difference was observed for grape-derived aldehydes,monoterpenes and norisoprenoids amongst the wines made from the must with different concentrations of UFAs.To further explore the mechanism underlying the effect of UFAs on the physiological mechanism and the volatile production of Saccharomyces cerevisiae,we used synthetic grape must MS300 and studied the effects of high and the low UFAs addition(HFM and LFM)on whole-genome expression profile of yeast.The results showed that a total of 140 differentially-expressed genes between HFM and LFM were identified at three phases of yeast growth.The greatest difference in gene expression profile was present in the late-stationary phase of yeast growth when there were 129 differentially-expressed genes.Genes involved in cell cycle and cell wall degration were significantly up-regulated while genes involed in meiosis were down-regulated,which implied that yeast tended to budding proliferation in the high UFAs medium and sporulation in the low UFAs medium.The genes DSE1,CAT8,ADR1,SCH9,MCK1,and USV1 required for yeast growth were differentially expressed and they were also involved in cell stress response to external stimuli.In addition,the up-regulated ATX1 and ATX2,and down-regulated gene SCH9,XBP1,GAC1,CTT1,PSR1,ROX1,CIN5,SFL1 and SIP18 were also related to cell stress responses.These indicated that high UFAs in the medium could accelerate yeast growth and improve yeast cellular resistance.Genes(NDE2,SDH2,NDI1,COX19,CYC3,ATP10 and PUF3)for carbohydrate metabolism were down-regulated,which meant that high UFAs depressed mitochondrial electron transport chain(mETC).Among genes related to lipid metabolism,the expression of CSH1 was up-regulated while the expression of SUT1,MOT3,FAA4,FAT3,MGA2,ADR1,INA1,AGP2,LPX1,K1N82,and GAT2 was down-regulated,suggesting that high UFAs medium could promote yeast sphingolipid biosynthesis,in contrast,low UFAs could cause the depression of ergosterol biosynthesis,the increasment of its uptake,and the enhancement of fatty acid metabolism and phospholipid synthesis.Genes for amino acid metabolism(MUP3 and BAP3)were up-regulated and PUT4,ARG80 and CARI were down-regulated,which meant that high UFAs in the medium increased the transport of branch-chain amino acid and methionine,and decreased proline transport,promoted arginine degradation and depressed its synthesis.With regard to the synthesis of volatile compounds,the expression of AAD4,ALD3 and BAP3 were significantly up-regulated and MGA2 was down-regulated.Among these genes,AAD4 expression was positively correlated with the production of 2-phenylethanol and 2-phenylethyl acetate,and ALD3 with 3-methyl-butanoic acid.Furthermore,the production of most volatile compounds in the wine presented a positive correlation with OD600 modification and sugar consumption.According to these findings,we inferred that the increase in the concentration of volatiles in high UFAs medium was as a comprehensive result of the promotion of yeast growth and sugar consumtion,and the up-regulated expression of genes involved in transportation of substrate and aroma compounds synthesis.