Sedimentation in UHT milk

Sediment from ultra-high temperature (UHT) processed milk was composed of κ-casein depleted casein micelles, with low levels of denatured whey protein. Direct UHT was more prone to sediment formation than indirect UHT. In direct UHT milk samples, at low pH (<∼pH 6.65) and/or high ionic calcium levels (>∼1.5 mm), the unstable κ-casein-depleted casein micelles were prone to calcium-mediated aggregation and high sedimentation levels. Sedimentation in direct UHT milk samples could be reduced by increasing the pH with alkali or reducing the ionic calcium with chelators. Tri-sodium citrate was effective as it reduced ionic calcium levels and increased pH. Decreasing the pH by adding acid or increasing the ionic calcium level by adding of calcium salts markedly increased sedimentation. In indirect UHT milk samples, higher levels of denatured whey proteins were associated with the casein micelles and this may stabilise these micelles, reducing their propensity to aggregate and sediment.