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May 2018 WINES&VINES 69 PRACTICAL WINERY & VINEYARD GRAPEGROWING sulfur dioxide, which will help preserve the aromatic compounds. 12 The earlier the addition of antioxidants after harvest, the better. Studies show that increasing the amount of antioxidants also increased the amount of volatile thiols formed and maximum thiol potential seemed to be reached so long as a moderate (30 – 50 ppm) level of SO 2 was present prior to fermentation. 2 This does have some limits due to fermentation difficulties in the presence of too much SO 2 and inhibition of the enzyme responsible for increasing C6 compounds (lipoxygenase) dur- ing harvesting. However this effect has not been conclu- sively demonstrated. Excessive SO 2 concen- trations (300 mg/L) led to lower 3MHA concentration probably due to the acetyla- tion pathway converting 3MH to 3MHA being interrupted. Timing of SO 2 additions had an influence on 3MH and 3MHA production. A two-hour delay of the SO 2 additions resulted in wines contain- ing thiol concentrations of about half compared to wines where SO 2 was added as early as 10-15 minutes after mechanical harvesting. 2 Finding practical methods for early SO 2 additions can be challenging. New Zealand wine companies have used methods such as drip-feed of a concentrated SO 2 solution, how- ever, care needs to be taken over corrosion issues with equipment. In another New Zealand study, the addition of increasing amounts (0, 60, 120 mg/L) of SO 2 led to increases in volatile thiol production obtaining levels of up to 12,000 ng/L which would be considered a high thiol wine. 2 This wine was not Sauvignon Blanc, but rather Pinot Gris. These results show the necessity to investigate the contribution of volatile thiols to other varieties and highlights the impor- tance of the use of antioxidants. Other antioxidants that have been under investigation are ascorbic acid and glutathione. Ascorbic acid can be added to juice and wine as a supplement to SO 2 in order to increase antioxidant capacity. Glutathione, on the other hand, is not (yet) registered as a permitted additive, however glutathione levels can pos- sibly be increased by adding some commer- cially available inactive dry yeast products. 13 In a study where 30 mg/L SO2 was added to grape must together with either 100 mg/L ascorbic acid or glutathione, the levels of all three important volatile thiols increased with 4MMP increasing significantly with glutathi- one addition. 14 A 30 mg/L SO2 addition to- gether with 100 mg/L glutathione showed 3MH and 3MHA concentrations of at least double the value compared to treatments where glutathione was added alone. It would thus seem that the combined protective ef- fect could be more effective than the indi- vidual antioxidants. After fermentation, the presence of SO 2 is of utmost importance as the instability of vola- tile thiols leads to drastic decreases if not protected. 53 The thiol most susceptible to degradation and oxidation is 3MHA. During oxidation, very reactive quinones are formed which will readily bind to wine constituents including the volatile thiols rendering them odorless. Sulfur dioxide binds these quinones rapidly and thus prevents further oxidation reactions from occurring. The methods used for antioxidant additions need to be controlled for efficiency. Concen- trated SO 2 mixes quickly with the must in the bins as grapes are transferred into trucks or winery receiving hopper. The efficiency of powder should be investigated and it is impor- tant to check the uniformity and distribution of the antioxidant by taking samples from the juice arriving at the winery. Pockets of juice with low free SO 2 are likely to have lower antioxidant potential. Cold soaking/freezing of grapes In a study where Sauvignon Blanc grapes were frozen to -20° C (-4 F) using dry ice and then July 11 & 12