Influence Of Fermentation Conditions And Aging Process On Volatile Sulfur Compounds Present In Wines

Volatile sulfur compounds?VSCs?are the dominating doner for “reduced” flavor of wine fault,which have a strong impact on wine aroma attributed to their characteristics of high volatility,high activity and low sensory threshold.The formation of VSCs during winemaking is closely related to the use of different fermentation conditions and aging processes.In the first part of this present study,wine accessories?sulfur dioxide,branch chain amino acids?BCAAs?and unsaturated fatty acids?UFAs??and applications of different types of oak barrels?origin,toasting levels and wood grains?during wine maturation were investigated to explore the impact of different processes on the formation of VSCs in wine.And this work was designed to evaluate the potential contributions of these processes to the “reduced” flavor of wine,and provided a theoretical basis for a comprehensive evaluation of their use in wine industry.Moreover,the evolution of VSCs in Cabernet Sauvignon wines from five regions of China during maturation was investigated to obtain the profiles of VSCs in wines,and to provide more information for the suitability between oak barrels and wine regions.The major research process and results were summarized as follows:1.A HS-SPME method coupled to a GC-FPD analysis was optimized to analyze 16 VSCs in wine.The gas chromatography conditions were studied to obtain the optimization resolution,and the factors?fibre,time of equilibrium and adsorption,temperature of the sample and ionic strength?were studied to get the maximum signal for each compound on the FPD detector.The oven temperature was programmed as follows: 40°C?initial hold for 5 min?,ramp at 8°C/min to 150°C,and next ramp at 2°C/min to 180°C,and then ramp at 15°C/min to 220°C.The SPME fibre was CAR/PDMS/DVB?50/30 ?m × 2 cm?,temperature of the sample was 40°C,time of equilibrium was 10 min,time of adsorption was 50 min,ionic strength was 1.0 M MgSO₄·7H₂O.2.The effect of different SO₂ concentrations on the formation of VSCs in wine was studied.Adding of 50 mg/L and 100 mg/L SO₂ both decreased the concentrations of alcohol,pH and lactic acid of the wines after alcoholic fermentation,and it also kept higher levels of aspartic acid and threonine during fermentation.The productions of methionol?MTP?,2-?methylthio?ethanol?MTE?and ethyl-3-methylthiopropionate?EMTP?also significantly increased with the SO₂ addition.However,the difference of the above substances was not significant between the additions of 50 mg/L and 100 mg/L.3.The effect of adding BCAAs?complexes of valine,leucine and isoleucine?and UFAs?complexes of oleic acid,linoleic acid and linolenic acid?in chemically defined medium before fermentation on the formation of VSCs was studied.Under high concentration of BCAAs conditions,yeast preferred to use BCAAs,meanwhile the production of MTP,dimethyl sulfide?DMS?,2-methyltetrahy-drothiophen-3-one?2MTHF?and 3-?methylthio?propyl acetate?PMTE?of the wines after alcoholic fermentation significantly decreased with the BCAAs addition,while the concentration of methyl-3-methylthiopropionate?MMTP?increased.Under high concentration UFAs,yeast increased the absorption of most amino acids,in particular aspartic acid and serine.Simultaneously,high concentration of UFAs significantly decreased the concentration of MTP and 2MTHF,whereas increased the concentration of DMS and benzothiazole.The influence of different concentrations of BCAAs and UFAs on the expressions of genes for sulfur metabolism in yeast both occurred mainly at the end of stationary phase,the expression of GAP1 was upregulated by high concentrations of BCAAs and expression of MUP3 was upregulated by high concentrations of UFAs.4.The influence of origin?American,Hungary and France?and toasting levels?light and medium?of oak barrels on the evolution of VSCs in wine during maturation was investigated.The origin of oak barrels significantly influenced the concentration of S-methyl thioacetate?MTA?and 2MTHF,especially after aging for 3 months,the contents of MTA were significantly lower in wines aged in oak barrels from Hungary than in others,and the contents of 2MTHF in wines aged in different oak barrels showed an order,from low to high of Hungary,American and France.In the case of the toasting level of oak barrels,it significantly influenced the concentration of MTE.Also after 3 months aging,the MTE contents of wines aged in oak barrels with medium toasting level were significantly lower than that with light toasting level.Wine aging in oak barrel from Hungary with poorer contents of VSCs in the short-term?within 6 months?aging.Most VSCs tended to decline or changed little during the aging,while DMS and ethyl thioacetate?ETA?was found to increase.Comparing to stainless steel tanks,oak barrels were more conducive to reducing the VSCs in wine.5.The evolution of VSCs in Cabernet Sauvignon wines from five regions of China during maturation in different oak barrels was investigated,and the correlation between sensory characteristic and the content of VSCs was also explored.Seven compounds?DMS,MTA,MTE,2MTHF,EMTP,MTP,MMTP?were commonly found in Chinese wines.The patterns of VSCs evolution were different among wine regions.Wines from Manasi County in Xinjiang Province and Hexi Corridor in Gansu Province showed poorer contents of VSCs,especially for the concentrations of MTP?EMTP and 2MTHF.Wine regions and aging time showed the strongest significance on all compounds analyzed.Most VSCs tended to decline or change little during the aging,while DMS was found to increase.And there was a positive correlation between the typical odor of DMS and “reduced” flavor according to the sensory evaluation after post-bottling for 18 months.The “reduced” flavor of wines aging in oak barrels was weaker than that of the corresponding wine aging in stainless steel tank,and this was consistent with the change of contents of VSCs in different wines.