Effects of storage on quality of yoghurt prepared from cows' and goat’s milk and pure strains of lactic acid bacteria

الملخص EN

Pure strains of lactic acid bacteria which included Lactobacillus bulgaricus and Streptococcus thermophilus,were used in preparation of yoghurt from goat’s and cow’s milk.

The chemical components of goats’ milk were approximately similar to those of cow’s milk except protein which was 2.92% and 3.5% in cows’ and goats’ milk, respectively.

The microbiological analyses revealed absence of salmonella and yeast in the two types of milk; however the total count of bacteria of cows’ milk was 4 x 103 c.f.u/ml which was in close agreement to that of goats’ milk (3.7 x 103 c.f.u/ml).

In addition, high counts of (LAB) were found in goats’ milk (5 x 102 c.f.u/ml) compared to cows’ milk which contained 2.8 x 102 c.f.u/ml, and it revealed presence of coliform and faecal bacteria in a few numbers which gradually disappeared during fermentation of yoghurt.

Great changes were noticed in most of the chemical components of yoghurt during 1, 7 and 15 days of storage at 6C.

The microbiological analyses revealed absence of salmonella and yeast in all yoghurt samples analyzed at the first day of storage, however, few numbers of yeast appeared after that especially in traditional yoghurt.

High counts of LAB were detected in yoghurt prepared by using pure LAB which might inhibited the growth of pathogenic bacteria, hence increased the shelf life of yoghurt.

The sensory evaluation indicated higher acceptability of goat milk yoghurt prepared particularly in its texture and viscosity.

KEYWORDS: Milk, yoghurt, chemical composition, sensory evaluation