SHELF LIFE EVALUATION OF NON-FAT YOGHURT AT ACCELERATED STORAGE CONDITIONS

Abstract

Yoghurt is custard like cultured milk made by fermentation, slightly concentrated whole or skimmed milk with a mixed bacterial culture. It is a popular dairy product in the world as well as in Sri Lanka. This study was carried out to detect the quality changes of non-fat yoghurt and the shelf life of the non-fat yoghurt at different storage conditions. Therefore a complete study on the physical, chemical and microbiological properties of non-fat yoghurt was carried out at the above conditions. Firstly, the well-incubated yoghurt samples were kept at four different storage temperatures 3°C, 8°C, 20°C and 30°C for 28 days. The samples that were in different temperatures were tested using standard physical, chemical and biological methods at different time intervals. The chemical analysis conducted on samples showed that quick acidity development were shown and decrease in pH at higher temperature.Coliform test and yeast mould test were carried out to detect microbial deterioration.No coliform colonies were observed after one week of storage period at all temperatures due to the development of low pH condition. The results of yeast and mould test showed that colonies were uncountable on PDA media at 30°C and 20°C.As the growth rate of yeasts and mould was increased with decreasing pH. The samples stored at 3°C showed little microbial growth throughout the storage period.The sensory evaluation was carried out to determine the organoleptic properties (colour, odour, appearance, texture and overall acceptability) of yoghurt. After analyzing the results obtained, statistically, it was found that the organoleptic characteristics were preferable at 3°C temperature conditions. It has a longer shelf life (25) than other storage conditions. However the organoleptic characteristics of yoghurt stored at 8°C also did not changed until 16 days of storage period. According to the results it can be concluded that the physical properties and the sensory characteristics of non-fat yoghurt at 3°C shows good eating quality and further it is microbiologically acceptable.