Effect of Various Humectants and Aw on Proteolysis, Yeast and Mold Growth and Shelf-Life During Cold Storage of Yogurt

Abstract

Twenty-seven experimental samples were prepared by adding humectants to plain yogurt according to a complete 33factorial design with 3 factors at 3 levels: NaCl (0, 4, 8% addition level), sucrose (0, 7.5, 15%) and sorbitol (0, 7.5, 15%). Samples were studied for a maximum of 12 weeks storage at 6°C (4 to 12 weeks depending on the sample) for proteolysis and yeast and mold counts. Water activity (Aw) of the experimental yogurts ranged from 0.974 to 0.908. Proteolysis for all samples remained limited during the storage period, lower than the control (except for one). Lag time for yeast and mold growth was largely increased by humectant addition, from 4 weeks for the control to over 12 weeks for various combinations of humectants, which resulted in Aw values lower than 0.932. The results showed that for most samples, yeast and mold growth determined product shelf-life, which ranged from 3 to more than 12 weeks. Previous data collected in this laboratory on the viability of lactic bacteria and post-acidification during cold storage of yogurt confirm the previous finding. Significant linear relationships were established between Aw, humectant content and shelf-life of yogurt during cold storage (R2> 0.90). A 1% increase in salt, sucrose and sorbitol content of yogurt resulted in about 7.7, 1.5 and 1.3 days increase in shelf-life, respectively. Similarly, a 0.01 decrease in Aw increased shelf-life by 11.5 days. The impact of these results on optimization of the shelf-life of commercial yogurt and on the development of more stable yogurt products for industrial use are underlined.