Kinetics of Flavor Formation During Grape Juice Fermentations

Using solid phase microextraction coupled with gas chromatotgraphic analysis, we are able to “continuously” monitor ester production throughout grape juice fermentations. In previous studies we used this technique to monitor differences in production of acetate and fatty acid ethyl esters that could be related to the progression of the fermentation. In addition a multi-peak pattern of ester production was observed which had not previously been reported. During the past year (2001-2002) our studies showed that:

• Ethanol concentrations did not have a significant effect on measured concentrations of most esters studied using the SPME technique. However, at ethanol levels greater than 5%, measured concentrations of ethyl decanoate were significantly decreased. This may indicate that SPME analysis underestimates concentrations of this ester as fermentations proceed and ethanol concentrations increase.
• Carbon dioxide flow rates at levels approximating those occurring at the height of fermentation had only a minimal effect on measured ester concentrations. These results suggest that SPME sampling provides an accurate picture of total ester concentrations throughout fermentation, even when volatilization rates are expected to be high (i.e,. Logarithmic yeast growth).
• Yeast inoculation level did not significantly impact the concentrations or production profiles of the ethyl esters and acetate esters studied, except for ethyl acetate. The reason why ethyl acetate production responds differentially to yeast inoculum levels is unknown.

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