Microstructure of acid–induced skim milk–locust bean gum–xanthan gels

Abstract

Abstract The microstructure of acid skim milk gels (14% w/w milk protein low heat powder) with or without addition of locust bean gum (LBG), xanthan gum (XG) and LBG/XG blends was determined by transmission electron microscopy (TEM), phase-contrast light microscopy (PCLM) and scanning electron microscopy (SEM). Three polysaccharide concentrations (0.001%, 0.02% and 0.1%, w/w) were used for binary mixtures. In the case of ternary mixtures, three LBG/XG weight ratios were used (4/16, 11/9 and 16/4) at 0.02% total polysaccharide concentration. Control acid skim milk gels were structured by a homogeneous network of casein particles (0.1–0.7 μm in diameter) and clusters immobilizing whey in small pores (1–5 μm in diameter). Filamentous structures and small aggregates were observed at the surface of casein particles. Low concentration of LBG or XG (0.001% w/w) did not affect markedly the microstructure of acid skim milk gels. Conversely, LBG or XG at 0.02 or 0.1% concentration and LBG/XG blends at the three ratios selected had a great influence on the gel microstructure. Although the size and surface structure of the casein particles were not modified by the presence of polysaccharides, the primary casein network appeared very compact with a decrease of pore size and a large increase in the porosity of the network at the supramolecular level (sponge-like morphology). The effect is stronger for gels containing LBG and XG used at higher concentration and less apparent for gels containing LBG/XG blends. Skim milk/XG gels were highly organized into fibrous structures whereas skim milk/LBG gels were more heterogeneous. These structures were discussed in the light of volume-exclusion effects (demixing) and specific interactions between casein micelles and polysaccharides. At the three weight ratios, skim milk/LBG/XG gels displayed both jagged “coral-like”, “veil-like” and filamentous structures. These structures could originate from a secondary network constituted by the known LBG/XG synergistic interactions.