Μελέτη δευτερογενούς αρώματος με GC/MS οίνων που παράγονται με ακινητοποιημένα κύτταρα

Abstract

Repeated batch fermentations of grape must at different temperatures were performed using immobilized cells of Saccharomyces cerevisiae strain AXAZ-1 on delignified cellulosic material (DCM), gluten pellets (GP) and grape skins. The aim of this study was the investigation of the volatile components of the final product (qualitatively and quantitatively) as well as the monitoring of the evolution of some volatiles during the alcoholic fermentation. For comparison reasons, fermentations using free cells were performed under the same conditions. The immobilized biocatalyst on DCM and GP resulted in significant increase of the fermentation rate in comparison with free cells. The produced wines had similar physicochemical characteristics, e.g. alcoholic strength, total and volatile acidity, residual sugar and pH. The GC/MS analysis proved that the cell immobilization and the fermentation temperature did not cause serious changes in the qualitative composition of the wine aroma. Esters, higher alcohols and volatile fatty acids were the major compounds identified in all wines. However, the effect of immobilization and temperature caused signigicant changes on the quantitative profile of the wine aroma. Wines produced by DCM contained higher amounts of esters, at every temperature. The relative percentage of esters on total volatiles remained practically constant with temperature decrease, in the case of immobilized cells, whereas it decreased in the case of free cells. Wines produced by GP-supported biocatalyst contained higher amounts of alcohols. DCM-supported biocatalyst and free cells produced wines with similar contents of alcohols. The decrease of temperature resulted in the decrease of their amount. The same was observed for the concentration of volatile fatty acids. GPsupported biocatalyst produced wines with smaller amounts of acids. Free and immobilized cells on DCM produced wines with similar contents of volatile acids. The kinetics of volatiles’ production were similar for free and immobilized cells on DCM. In all cases, immobilized cells showed a higher rate of volatile production. This was directly connected to sugar consumption. The evolution patterns of acetate esters were different from those of ethyl esters. Generally, ethyl esters reached a maximum earlier than acetates. In most cases, a two-peak profile was observed for ethyl esters. For the first time, grape skins were used as a natural support for S. cerevisiae immobilization. Cell immobilization was confirmed by electron microscopy. In repeated batch fermentations of grape must, the immobilized biocatalyst caused an significant increase in the fermentation rate in comparison with free cells, without any loss of the activity, even at very low temperatures (5oC). The alcohol production and the maximum specific rates on glucose and fructose substrate were much better for cells immobilized on grape skins. They increased exponentially with temperature increase. Qualitative characteristics, such as total acidity, volatile acidity and residual sugars were similar with those of commercial dry wines. Wines produced by immobilized cells had lower volatile acidity than those produced by free cells. They were very clear, especially at low fermentation temperatures. GC/MS analysis showed that the qualitative profile of the produced wines was similar in all cases. The majority of the identified volatile compounds was formed during the alcoholic fermentation and consisted of esters, alcohols and acids. Immobilized cells gave wines with a higher content of esters. The amount of ethyl esters decreased with temperature decrease, whereas the amount of acetate esters increased with temperature decrease (25-15oC), reaching a maximum. The content of volatile alcohols was more pronounced in wines produced by free cells. It decreased dramatically at low fermentation temperatures (10oC). Volatile fatty acids were produced in greater quantities by free cells at higher temperatures (25-20oC). The opposite was observed at low fermentation temperatures (15-10oC). Investigation of the volatiles’ evolution during fermentation, showed that esters were produced shortly after the midpoint of fermentation. For immobilized cells, the formation of esters was continued until the end of the process. Similar kinetics were observed for amyl alcohols and 2-phenylethanol, whereas propanol and isobutanol were formed during the whole alcoholic fermentation period with a constant rate. Acetaldehyde and acetoin were synthesized in the early stages. Afterwards, their amount decreased. Acetic acid was formed throughout alcoholic fermentation with a costant rate, which was greater in the case of free cells at every temperature studied.