Effect of enzymatic crosslinking of milk proteins on the resulting properties of yoghurt products

In the frame of a research project (AiF-project-No. 11247 N) the crosslinki ng of milk proteins with microbial transglutaminase (EC 2.3.2.13) was inves tigated in the yoghurt manufacture. Heated milk (92 degrees C, 5min) was en zymatically crosslinked (S=3.4%; E/S=1/2000; T=40 degrees C; t=120min; pH=6 .6-6.7). A further heating (80 degrees C, 1 min) stopped the reaction with transglutaminase. A culture from Streptococcus thermophilus and Lactobacill us delbrueckii subsp. bulgaricus was used for fermentation at pH 4.68. The effects of the enzymatic modification were analyzed by evaluating the ferme ntation curve and by measuring quality parameters as acidity, gel strength and products' syneresis. Most of the investigations showed longer fermentat ion times for the yoghurt manufacture (pH 4.68) from enzyme treated milk th an from untreated milk. Similarly, the acidification in yoghurt samples fro m enzyme treated milk during a three-week storage at 6 degrees C was lower than the one of samples from untreated milk. The gel strength of yoghurt sa mples from crosslinked milk was generally higher than the one from non-cros slinked milk. The whey syneresis was significantly reduced by the enzymatic modification. Furthermore, the yoghurt samples from enzyme treated milk sh owed a dry, smooth and white shining surface in most of the investigations and were of a milder taste than samples from untreated milk. The gel streng th increased with increasing dry matter whereas the strength in the enzyme treated samples was twice as high as in untreated yoghurt products. The enh ancement with milk proteins led to an exponential increase of the gel stren gth and to a corresponding, reciprocal decrease of the whey syneresis. Up t o a fat content of 3.5 % enzyme treated samples showed a higher stability t han untreated samples. inverted results were obtained for cream yoghurt.