Microstructural evolution during acid induced gelation of cow, goat, and sheep milk probed by time-resolved (ultra)-small angle neutron scattering

Milks from small ruminant animals, such as goat and sheep, have gained increasing interest from the industry for the manufacture of various dairy products such as yoghurts. Heat treatment of milk is typically applied to improve the yoghurt texture. Here, time-resolved ultra-small angle neutron scattering (USANS) and small-angle neutron scattering (SANS) were employed to probe in situ the microstructural evolution of cow, goat, and sheep milks in D2O during acidification as affected by the heat treatment. Milk gelation can be envisaged as the progressive aggregation of the building blocks (casein micelles) in a fractal manner, which leads to the formation of micron scaled (∼3–10 μm) protein agglomerates. The heat treatment considerably enhanced the gelation of all milks, which is reflected by a faster increase in the fractal dimensions and aggregate sizes. For all three types of milks, similar final plateau fractal dimensions were observed for unheated (∼2.2–2.4) and heated milks (∼2.4–2.5), suggesting similar mass fractal microstructures in the size range probed by USANS. The final storage modulus after 10 h (G′ final) follow the order of heated sheep milk ≈ sheep milk ≈ heated cow milk > cow milk > heated goat milk > goat milk. The colloidal calcium phosphate (CCP) dissolution kinetics tracked by SANS followed a similar trend as the evolution of the sample acidity (pD-value) and are not significantly affected by the types of milk or the heat treatment. This study shows a great potential of using time resolved USANS and SANS to investigate the microstructural evolution of protein aggregation and gelation.