Preparation and evaluation of shelf stability of ambient yoghurt.

Abstract

Yoghurt is a widely consumed milk product obtained through fermentation using a starter culture consisting of Lactobacillus bulgaricus and Streptococcus thermophilus mixed in a 1:1 ratio. Yoghurt however goes bad after a short period if not stored under refrigeration because the starter culture in the yoghurt continuously breaks down the lactose to form lactic acid resulting in further decrease in pH rendering the product inconsumable.

The general objective of this study was to optimize the terminal pasteurization temperature and determine its effect on various properties of home-made ambient yoghurt. The optimization of the terminal pasteurization temperature involved the evaluation of different temperature and time combinations and finding out the effect of the terminal pasteurization temperature on the starter culture and organoleptic properties of yoghurt. Fresh yoghurt was prepared and subjected to a terminal pasteurization step to inactivate/kill the starter culture in order to prevent further fermentation. This was followed by the determination of the effect of terminal pasteurization temperature on yoghurt parameters such as water holding capacity (WHC), susceptibility to syneresis (STS), pH, titratable acidity, shelf stability.

At time lengths of 20 minutes and 30 minutes at 55ºC, the inoculated samples did not yield colonies on the media. The appearance of the yoghurt sample pasteurized for 20 minutes was liked moderately (7.05±1.10) while the sample pasteurized for 30 minutes was liked slightly with a score of 6.00±1.89. The appearance and texture of both the pasteurized samples was significantly different (P≤0.05) from the control. The optimum pasteurization temperature was 55ºC for 20 minutes.

The WHC of the pasteurized yoghurt sample stored at room temperature increased from 42.33±0.58% to 58.00±0.00%. The STS of the pasteurized yoghurt sample at room temperature storage increased from 65.10±1.04% to 72.67±2.31%. The pH and TA of the pasteurized yoghurt sample days under room temperature storage ranged from 4.37-3.87 and 1.07%- 1.40% respectively. The TA and pH of the pasteurized yoghurt sample were generally significantly different (P≤0.05) from the control. Terminal pasteurization temperature enhanced the shelf life of ambient yoghurt by 4 days because it inactivated the starter culture. Pasteurization negatively affected the organoleptic properties, altered the physico- chemical properties and enhanced the shelf life of ambient yoghurt.