Wine proteins isoelectric point

Dawes, H., Boyes, S., Keene, J., Heatherbell, D. (1994). Protein instability of wines Influence of protein isoelectric point. Am. J. Enol. Vitic., 45, 319-326 Derckel, J.P., Legendre, L., Audran, J.C., Haye, B., Lambert, B. (1996). Chitinases of the grapevine (Vitis vinifera L.) five isoforms induced in leaves by salicylic acid are constitutively expressed in other tissues. Plant Sci., 119, 31-37... 

Protein clouding in white wines seems to be a greater problem when the wine pH is close to the isoelectric point of the various protein fractions. This is due to the fact that bentonite will remove, preferentially, the most positively charged proteins. The electrostatic charge of various protein fractions explains the observable phenomena of not being able to stabilize certain wines with the use of bentonite alone, or only with excessive amounts that can strip the wine character. But the pi of proteins only partially explains wine haze formation. It is also important to note that other factors, as yet not clearly identified, can intervene. 

Research into the isoelectric point of wine proteins has often been concurrent with smdies of wine protein size. Proteins with low isoelectric points (pi) were found to be significant contributors to total wine protein (Moretti and Berg 1965) and to wine haze (Bayly and Berg 1967). Hsu and Heatherbell (1987a) confirmed this observation and suggested that the majority of wine proteins had a pi of 4.1-5.8, whilst Lee (1986) suggested the major protein fractions of wine had a pi of 4.8-5.7. Dawes et al. (1994) fractionated wine proteins on the basis of their pi and found that the five different fractions all produced haze after heat treatment. Haze particle formation was found to differ between the fractions however, leading to a statement that other wine components, such as phenolic compounds, need to be considered to understand fully protein haze. 

It would seem, therefore, that the proteins responsible for instability in white wines come exclusively from grapes and have relatively low molecular weights, between 12000 and 35 000 Da. However, the specific types of proteins, as well as their isoelectric points, degree of glycosylation and heat sensitivity, differ according to the grape variety. 

Wine is usually very acidic and below the isoelectric point of the suspended proteins. At a pH of 3.2, 100% of the proteins are positively charged and are capable of being removed with a Bentonite fining agent. 

Isoelectric points for proteins in wine are between 3.6 and 7.1 (Zoecklin, 2000). In some years, wine pH is at or above 3.6. For those proteins above their isoelectric points, they become negatively charged and Bentonite has no effect. In that case, it becomes much more difficult to clarify the wine. 

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