The complexity of protein haze formation in wines

The mechanism responsible for protein haze formation in wines remains essentially to be elucidated. Current knowledge suggests the absolute requirement of one or more as yet unknown non-proteinaceous wine components (termed the X factor) for protein precipitation in wines. Using the single grape variety Arinto wine, naturally containing 280 mg protein/l, a series of heat stability tests were performed over a range of wine-relevant pH values (from 2.8 to 3.8). The results obtained indicate the existence of at least two different mechanisms responsible for the heat-induced precipitation of the Arinto wine proteins: one occurring only at the higher pH values, that appears to result from isoelectric precipitation of the proteins; another prevailing at the lower pH values, but possibly operating also at other pH values, that depends on the presence of the X factor. Therefore, conclusive evidence is provided for the existence of the X factor, here defined as one or more low molecular mass wine components that sensitise proteins for heat-induced denaturation at low wine pH values and whose presence is a pre-requisite for the precipitation of proteins in wines under these circumstances. The chemical nature of protein aggregation was further analysed as a function of pH. Neither of the two proposed mechanisms responsible for the heat-induced precipitation of the wine proteins is electrostatic in nature, lectin-mediated or divalent cation-dependent. Both mechanisms show minimum turbidity at pH 7, but increased turbidity towards lower and higher pH values.