Reducing the risk of spoilage caused by Bacillus cereus in cow's and goat's milk yogurt with berries puree

Abstract

The objective of this study was to investigate the impact of adding aronia (Aronia melenocarpa), raspberry (Rubus idaeus), and strawberry (Fragaria xananassa) in the form of 10% puree to a mixture of cow's and goat's milk in order to reduce the risk of spoilage caused by activity of Bacillus cereus in yoghurt made from a mixture of cow's and goat's milk. During the fermentation process (360 min.), changes in water activity, pH, moisture content, bacterial growth curve were monitored. The results showed a decrease in water activity, pH, and moisture content. Adding aronia puree to yoghurt yielded the most significant results for water activity modification (0.971-0.868), pH (5.31-4.28), moisture content (85.12-81.55 %), and optical density (0.19-0.34). Also, the investigation of the kinetics of the Bacillus cereus grows indicated that the stationary phase was reached at 360 minutes, resulting in a total population of 5.5·1013 c.f.u./g. Microscopic examination revealed Gram-positive strains of lactic acid bacteria, arranged in chains of varying lengths, in an amount of 139·102 c.f.u./g. Antimicrobial activity showed a zone of inhibition with a diameter of 18.5±0.1 mm in aronia yoghurt, a zone of inhibition with a diameter of 16.2±0.2 mm in raspberry yoghurt and a zone of inhibition with a diameter of 15.2±0.1 mm in strawberry yoghurt compared to classic yoghurt which showed a zone of inhibition with a diameter of 12.1±0.2 mm.