Physical properties of acid milk gels: Acidification rate significantly interacts with cross-linking and heat treatment of milk

Rheological properties and microstructure of acid milk gels largely depend on the pre-treatment of the milk, including heating and enzymatic modification of the gel-forming proteins. It was the aim of our study to investigate the impact of the acidification rate on gels which were produced from milk reconstituted from powder containing casein cross-linked by microbial transglutaminase (mTGase), and on gels which were produced from cross-linked raw milk. 3–7% Glucono-δ-lactone (GDL) was used as acidulant. Gel stiffness was determined by dynamic small-deformation rheometry and penetration experiments, and forced syneresis and gel permeability served as indicators for gel microstructure. The measurements showed that acidification rate strongly interacts with casein oligomerization and heating. With increasing acidification rate the stiffness of gels from raw milk increased, but decreased if the casein was cross-linked with mTGase (oligomerization degree was approximately 25%, and the action of mTGase was stopped using N-Ethylmaleimide). Gels from heated milk became generally weaker if acidification was accelerated. However, casein cross-linking by mTGase enhanced gel stiffness at low acidification rate but decreased stiffness at high acidification rate. Rheological, syneresis and permeability results indicate that the aggregates formed through heating or cross-linking are responsible for the crucial alteration of gel properties and suggest that these act as structurally relevant nuclei which, depending on the time span which is given for gelation, assist in network formation or act as steric hindrances.