Heat Haze Characteristics of Fractionated Wine Proteins

Abstract

The proteins of a heat-unstable wine were fractionated by a combination of salting out with (NH₄)₂SO₄ and ultrafiltration. The protein composition of the fractions, as observed by electrophoresis, and to a lesser extent by amino acid content, was distinct. The least soluble fraction [A, precipitated with 60% saturation of (NH₄)₂SO₄] was dominated by a protein band of M_r 32 000, the fraction salted out with 65% to 70% saturation of (NH₄)₂SO₄ [B] had a major protein of M_r 24 000, and the most soluble material [C, supernatant at 70% saturation of (NH₄)₂SO₄] comprised proteins of M_r 26 000 and 24 000 [E] together with a carbohydrate-rich fraction [D]. The Coomassie dye-binding assay was unsuitable for protein quantification of the fractions because it underestimated the true values by 50% to 80%. A micro heat test applied to the fractions showed that the carbohydraterich component [D] was the most thermo-stable and apparently extended this stabilizing effect to the protein components [E] in the original isolate [C]. Fractions B and E gave the most haze with heat.