Effect of alkalinization and ultra-high-pressure homogenization on casein micelles in raw and pasteurized skim milk

Abstract

Mechanical and physicochemical treatments of milk induce structural modifications of the casein (CN) micelles, affecting their techno-functional properties in dairy processing. Here, we studied the effect of alkalinization and ultra-high-pressure homogenization (UHPH) on CN micelles in raw skim milk (rSM) and pasteurized skim milk (pSM). The pH of both skim milks (approximately 6.7) was adjusted to 8.5 and 10.5 before UHPH at 100, 200, and 300 MPa. The structural changes of the CN micelles during the treatments were assessed using laser diffraction, transmission electron microscopy, and turbidity measurements. Finally, ultracentrifugation (70,000 × g for 1 h at 20°C) was carried out to evaluate the protein's distribution between the supernatant (serum phase) and the pellet (colloidal phase) by gel electrophoresis and protein concentration measurement. Alkalinization of both skim milks induced a significant reduction in turbidity, whereas an increase of the average particle size was observed, the effect being more severe in pSM than rSM. At alkaline pH, more proteins were recovered in the serum phase, which suggested that the CN underwent major rearrangements into nonsedimentable CN forms of various sizes, as confirmed by transmission electron microscopy. The amount of CN found in the serum phase at pH 8.5 also increased with the UHPH pressure. Although UHPH did not influence the average CN micelle size at pH 6.7 and 8.5, a pressure-dependent decrease was observed at pH 10.5 for both skim milks. The structural changes of the CN micelles observed in this study throughout the combination of alkalinization and UHPH could be of interest for developing new dairy ingredients with improved functionality.

The effects of full‐stream and partial high‐pressure homogenisation on the properties of milk emulsion

“…Homogenisation inhibits creaming during storage of liquid dairy products by a decrease in the size of milk fat globules. High‐pressure homogenisation (HPH) inhibits microbial growth (Hayes et al., 2005; Pereda et al., 2007), but affects protein adsorption on the surface of homogenised fat globules (Ye et al., 2008; Zamora et al., 2012), whey protein denaturation (Hayes et al., 2005), and it modifies the structural properties of casein micelles (Sandra & Dalgleish, 2005; Touhami et al., 2022), which alters the functional properties of proteins and their coagulation (Sandra & Dalgleish, 2007). Research studies examining the influence of HPH on the properties of dairy products were carried out on drinking milk (Hayes et al., 2005), milk concentrates (Mercan et al., 2018a), milk powder (Mercan et al., 2018b) and butter (Sert & Mercan, 2020).…”