After pre-incubation of skim milk with microbial transglutaminase (10.5 U/100 ml skim milk or 3 U/g milk protein) at 40 degrees C for 150 min and subsequent heat inactivation, yoghurt was prepared. The breaking strength of this yoghurt increased in the given system from 550 cN to 920 cN with increasing pre-incubation time, reaching a plateau after 60 min. This increase in breaking strength correlates with a rather small amount of casein oligomerization, ranging from 10.9% to 25.8% using gel-permeation chromatography under reducing and denaturing conditions (sum of predominantly formed dimers and trimers in the total casein fraction). After enzymatic hydrolysis, N-epsilon-(gamma-glutamyl)-L-lysine was quantified by amino acid analysis. The isopeptide was formed exclusively intermolecularly, indicating that even a very small amount of casein crosslinking is capable of inducing significant changes in the functional properties of milk proteins.