Abstract: Fresh milk with added hyaluronic acid (HA) was fermented by inoculating Streptococcus thermophilus and Lactobacillus bulgaricus. By measuring the pH, water holding capacity, viable bacteria count, rheological properties, volatile flavor substances, stability of yoghurt, the effects of HA with various molecular masses and various amounts of HA on the quality of yoghurt were evaluated. The results showed that various molecular masses of HA had different effects on the quality of yoghurt, but all could significantly improve its water holding capacity, smoothness and fineness (P < 0.05). The addition of high molecular mass (1 280 kDa) HA improved the elastic modulus (G’) and viscosity modulus (G”) of yoghurt, enhanced its stability index, promoted the growth and reproduction of L. bulgaricus and S. thermophilus, but caused obvious whey syneresis at the end of fermentation. Medium molecular mass (350 kDa) HA inhibited the growth and reproduction of L. bulgaricus, increased the pH, reduced the titratable acidity, and optimized the sensory quality of yoghurt. Low molecular mass (180 kDa) HA reduced the stability of yoghurt. To sum up, medium molecular mass HA can improve the quality of fermented milk.

Key words: hyaluronic acid; yogurt; volatile flavor substances; stability analysis; rheological properties; texture