Slow fermentation in French cider processing due to partial biomass reduction

This study looked at the effects of an operation used by some French cider makers in order to optimise the process. The technique examined is one that consists of reducing the yeast population during fermentation, so that the fermentation velocity decreases. At different stages of the fermentation, several decimal biomass reductions were performed and their effects were determined both by counting the two main yeast populations (fermenting yeast and oxidative yeasts) and by analysing the main parameters involved in yeast growth and fermentation (total nitrogen, density). In the present experiments, it was shown that a single decimal biomass reduction was enough to control the fermentation once the density of the untreated raw juice had undergone a 5 to 10 point drop corresponding to the consumption of about 13–26 g sugar per litre. This drop was linked to the growth phase of the yeasts in the untreated assay. When this reduction was performed later, the fermentation velocity became too low to reach the expected density. Nitrogen consumption appears to be one of the key parameters of this operation.