Acidification of grape marc for alcoholic beverage production: effects on indigenous microflora and aroma profile after distillation

Grappa is an Italian alcoholic beverage obtained from distillation of grape marc, the raw material derived from separation of must during the winemaking process. Marc is stored for a period lasting from few days to several weeks, when fermentation of residual sugars occurs mainly by yeast activity. Many distilleries have adopted different solutions to manage this critical phase in order to avoid spoilage microorganisms: marc acidification is the most widely diffused. In this work, Prosecco grape pomace was acidified with sulphuric acid (to pH 2.9) and stored, whereas non-acidified grape marc was used as control (pH 3.9). Samples for microbiological analysis were collected at the beginning of the storage period, after 15 and 43 days. At the beginning of the ensilage (time T0) the indigenous microflora was represented both by yeasts and bacteria at a concentration of about 10⁶ cfu/g. During the first 15 days, when the fermentation generally takes place, yeast population grew considerably (up to 10⁷ cfu/g) in acidified grape marc, where bacterial population was maintained at low levels. Moreover, yeast populations recovered at the three sampling times in both treated and untreated marc were genetically characterised. This analysis showed that the species succession lead to non-Saccharomyces species dominance (in particular Issatchenkia and Pichia genera) in both conditions although acidified marc showed a lower percentage of Saccharomyces at any sampling time analysed, this meaning that non-Saccharomyces species were favoured in this environment. Gas chromatographic analysis showed a remarkable change in the aromatic profile of distilled grape marcs at the end of the storage, thus evidencing that concentration of monitored volatile compounds usually produced by microflora was generally lowered by the acidification treatment.This work demonstrates for the first time the strong effect of a persistent acidification treatment both on the microbiota of grape pomace and on the aromatic profile of the distillate. Indeed, the lowering of the pH caused significant changes in yeast-bacteria populations ratio and in yeast species turnover. These microbiological changes determine an improvement of the aromatic profile of the distillate, due to the reduction of the main volatile products associated with potential off-flavours.