Influence of wine polysaccharides and polyphenols on the crystallization of potassium hydrogen tartrate

Potassium hydrogen tartrate (KHT) is a natural compound of wine which crystallizes spontaneously. Whereas crystal occurrence can be considered as a sign of goodness in old and famous vintage wines, it is usually thought of as a serious failure for most consumers, even though it does not alter wine quality. An efficient and cheap process of wine stabilization versus KHT crystallization has to be found yet. An alternate process to physical stabilization of wines may lie in the addition of an inhibitor of KHT crystallization. Bearing this in mind, we have investigated the effect of several polysaccharides and total polyphenols fractions on KHT crystallization through the measurement of crystal appearance time (induction time) with and without any macromolecule. Red wines. white wines and KHT supersaturated hydroalcoholic solution exhibit different behaviours versus KHT crystallization, red wines crystallizing less easily than white wines and far less easily th an hydroalcoholic solution. Those differences can be explained by our results. The innate inhibition of red wines is the sum of the inhibiting effects of rhamnogalacturonans (RG-I and RG-II), yeasts mannoproteins present in wine and of total polyphenols. Arabinogalactans show no effect on KHT crystallization whereas rhamnogalacturonans display a peculiar concentration dependent behaviour : crystal appearance is accelerated at low concentration and slowed at high concentration. More strongly observed for RG-1I2 fractions, this feature is confirmed by a theory of crystallization in the presence of an additive. The theory predicts that RG-I has almost no effect on the nucleation phenomenon whereas RG-1I2 enhances this phenomenon. Both RG-l and RG-1I2 inhibit crystal growth by adsorption on crystal growth sites, as contirmed by single crystal growth experiments. Red wine tendency to be more difficult to stabilize versus KHT crystallization by cooling than white wine is due to the concentration in RG-II and in total polyphenols : low RG-II content in white wine accelerates crystal appearance whereas high RG-Il content in red wine slows crystal appearance. Thus it intensifies the inhibition due to the high total polyphenol content in red wine. Mannoproteins extracted from yeast cell walls inhibit KHT crystallization far more than yeast mannoproteins present in wine. However, their efficiency is reduced as temperature is lowered.