Combined effects of κ-casein and β-casein genetic variants on yoghurt microstructure and texture

The functionality of bovine milk during acid-induced gelation is governed by the interplay between protein genetic variants and the overall protein composition. This study evaluates the influence of bovine milk κ-casein (κ-CN; AA, AB) and β-casein (β-CN; A1A1, A1A2, A2A2) genetic variants on the microstructure and physicochemical properties of set and stirred yoghurts prepared from 5% (w/w) rehydrated skim milk. The κ-CN(AB) milk contained smaller casein micelles compared to κ-CN(AA), attributed to the larger surface area and higher proportion of κ-CN. Smaller micelles allowed more interaction points for whey protein complexation, leading to the formation of larger protein aggregates and a firmer gel network. This was associated with improved gel strength and lower syneresis after storage in set yoghurts, and increased resistance to shear in stirred yoghurts. While the β-CN genetic variants influenced initial acidification rates and kinetics, results suggest its influence was likely dependent on the β-lactoglobulin (β-Lg) to κ-CN ratio and it is hypothesised that the β-(A1) allele may facilitate more efficient early-stage acidification when β-Lg levels are limited. However, these differences in behaviour observed between milks of varying β-CN genetic variants were not sustained after 24 h of refrigerated storage, as the protein network continued to rearrange towards equilibrium. These results suggest the κ-CN genotype primarily governs yoghurt viscoelastic properties, while β-CN genotype may influence early-stage gelation kinetics. Results also indicate that yoghurt quality is best measured under refrigerated conditions and at least 24 h post-production as measurements taken during fermentation may not reflect the final consumer experience.