Yeast is cheap labour, but yeast does not work for free. The main "wages" that yeast collects is from the sugar that it “eats”, in other words, some of the sugar is not converted into alcohol. Although this process serves as its main energy source, yeast still needs additional nutrients, known as “nutritional supplements”. The main additional components needed are phosphate and one or more nitrogen sources. The latter is so important that it has drawn much scientific and commercial attention, to the extent that the term Yeast Assimilable Nitrogen (YAN) has been coined. YAN measurement has been deemed so important, that performing it prior to fermentation has become standard practice in wineries.
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