Wine proteins are derived primarily from grapes and autolyzed yeast. They consist of several (protein) fractions which appear to be the subunits of denatured grape enzymes. Their molecular weight varies from 20,000 to 40,000 Daltons. The polypeptides with molecular weights of less than 10,000 are mostly derived from yeast autolysis. The isoelectric point (PI) of wine protein fractions have been reported to be in the range of 2.5 - 8.7.
Turbidity or cloudiness in wine is one of the important problems known to occur in commercial table wines. Often the cloudiness in wine results from microbial activity. However, sometimes it is caused by proteins which are heat sensitive and unstable.