Solubilized micellar calcium induced low methoxyl-pectin aggregation during milk acidification

Low methoxyl (LM) pectin was combined with 3-kDa molecular weight cut-off permeates from milk subjected to pH 6.7 to 5 and 7°C or 40°C with the objective of studying the effect of solubilized micellar calcium on viscoelastic properties of LM-pectin-milk mixes. Lowering the pH of skim milk with hydrochloric acid during ultrafiltration gradually promoted permeates to exhibit gel-like behavior when combined with LM-pectin. The onset of the gel-like behavior (G′ > 1) occurred at a higher pH when permeates were obtained from milk filtered at 7°C compared with 40°C. As pH value during ultrafiltration approached 5 and regardless of temperature, G′ for permeate-pectin mixes approached the same values (∼70 Pa) as G′ for skim milk-pectin mixes. In all cases G′ was highly correlated with free calcium concentration (r > 0.95). The gradual acidification of skim milk-LM-pectin using glucono-δ-lactone, promoted a sharp increase in storage modulus as pH approached 5.2 and a maximum G′ increment (ΔG′) at pH ∼4.9. From pH 4.9 to 4, G′ continued to increase but at smaller increments. It was concluded that LM-pectin-casein micelle interaction in milk is a 2-step process: 1) solubilized micellar calcium dependent pectin-pectin interaction as pH approaches 5.0 to 4.9, and 2) pectin-casein micelle interaction in the 5.0-4.9 to 4.0 pH range.