Microstructure and rheology of acid milk gels and stirred yoghurts –quantification of process-induced changes by auto- and cross correlation image analysis

The aim of the study was to evaluate whether emerging multimodal microscopy techniques combined with quantitative image analysis were capable of detecting differences in the microstructure of model dairy systems, and to evaluate whether hydrodynamic cavitation led to differences in microstructure compared to pressure homogenisation. Hydrodynamic cavitation and pressure homogenisation were combined with severe heat treatment to obtain milk samples of different degrees of fat homogenisation and different degrees of whey protein denaturation. Glucono-δ-lactone (GDL) induced acid milk gels and stirred yoghurts were produced from the treated milks, followed by rheological analysis and evaluation of the microstructure by multimodal coherent anti-stokes raman scattering and two-photon excitation microscopy combined with auto and cross-correlation image analysis. Autocorrelation analysis of the protein network of GDL-gels could quantitatively describe differences in acid gel strength, and results indicated that differences in the degree of homogeneity and branching of the protein network were linked to differences in size and shape of the protein domains formed during acidification. However, for the stirred yoghurts, the autocorrelation analysis was limited by the heterogeneous structure of the protein network. By cross-correlation analysis, quantification of the relative distribution of fat and protein was possible for both GDL-gels and yoghurts. Results indicated that severe heat treatment of the milk base translated into differences in the interaction between fat globules and the protein network, which could be explained by a relatively higher degree of whey protein association with the milk fat globule membrane.