pH-induced demineralization of casein micelles modifies their physico-chemical and foaming properties

The organization and functional properties of caseins are influenced by several factors like temperature, pH, ionic environment, ionic strength, casein type and organization. The objective of this work was to prepare suspensions of casein micelles with different mineralization states to study their new physico-chemical and foaming properties. The different suspensions were obtained after milk acidification at various pH values (6.7 as control and 6.4, 6.1, 5.8 and 5.5) and then dialyzed against milk ultrafiltrate to have similar pH values and ionic environment. Each suspension was characterized in terms of i. composition and distribution of minerals and casein molecules between continuous and colloidal phases, ii. physico-chemical characteristics (percentage of sedimentable material, turbidity, particle size distribution, state of aggregation of casein molecules, shape, charge and hydration) and iii. foaming properties (foaming capacity and foam stability). The obtained suspensions were progressively demineralized and partial dissociations of casein micelles were observed. The dissociated part corresponded to non-sedimentable particles by ultracentrifugation. These particles have average diameter of about 20–35 nm and molecular weight between 106 and 107 g mol−1. The proportion of this population smaller in size increased when the pH used to obtain the different casein suspensions was acid (6.7 < 6.4<6.1 < 5.8<5.5). In terms of size, hydration, shape and zeta potential, the non-dissociated casein micelles seemed similar to the native ones. The foaming capacity of each suspension was similar but the stabilities of foams were reduced. These decreases of stability were related to the presence of small casein particles and the quantitative reduction of casein micelles.