Increasing the shelf-life of plain and pekmez added yogurts

In this study four methods; pasteurization, nisin addition, lysozyme treatment and microwave treatment, were applied in order to increase the shelf -lif e of yogurt. Also a new type of fruit yogurt was produced by adding pekmez (concentrated grape juice) into milk, and nisin and microwave treatments were applied in order to increase the shelf -lif e of pekmez yogurt. In the first part, yogurt prepared under laboratory conditions was pasteurized by applying different time/ temperature combinations (60-65 °C, 5-10-15 min). The second part was undertaken to observe the effect of nisin addition (50-100-150 RU/ml yogurt) on set yogurt. The third part was related with the use of lysozyme for preservation of yogurt. 0.3-0.6-1.2 g/1 lysozyme containing yogurt samples were compared with the control samples during storage. The forth part involved microwave treatment at three different power settings and four different times (5- 10-20-40 sec), combinations in a home scale microwave oven. The objective of the fifth part was to develop a new type of fruit yogurt by using pekmez. Influence of pekmez on the overall quality and fermentation process of yogurt was evaluated. Effect of nisin addition (150 RU/ml yogurt) and microwave treatment (power setting P5 for 10 sec) on the quality of pekmez yogurt was followed during storage. Yogurt samples were prepared under laboratory 111conditions, stored at 4 "c for a month. Protein content, pH, titratable acidity, viscosity, whey syneresis, number of starter culture, number of total bacteria, yeast and mold were determined weekly during refrigerated storage. Heat treatment at 60 and 65 °C increased the storage stability of yogurt by reducing the lactic acid production. pH and titratable acidities of pasteurized yogurt stayed within TSE limits (min. 3.8, max. 1.6 %). At the end of one month of storage none of the samples except heat treated ones at 65 °C for 15 min. had bacterial count of less than 20xl06 which is the lower limit of some international standards. Nisin sufficiently diffused through the yogurt. Addition of low level of nisin (50 RU/ml) effectively improved the storage stability of set yogurt. As the concentration of lysozyme increased lethality of starter culture increased. 0.6-1.2 g/1 lysozyme was found to be appropriate for the preservation of yogurt. But nisin diffused faster than the lysozyme in the yogurt. During microwave treatment, high power setting and longer treatment time P5 10 sec, P3 20 sec and PI 40 sec, slowed down the acid production. Higher temperatures were needed in conventional heat treatment comparing the microwave treatment to kill yogurt bacteria. This indicated that non-thermal effects of microwaves were also responsible for microbial inactivation. Additionally shorter come up period to reach the necessary temperature for preventing textural damage is an important advantage for microwave treatment of yogurt. Addition of 10 % pekmez to milk gave the sufficient sweetness to yogurt. Pekmez addition increased the fermentation time of yogurt by slowing down the growth rate of starter bacteria and decreased the viscosity of yogurt significantly. Nisin treatment was more effective on storage stability than microwave treatment of pekmez yogurt. Throughout this work, in none of the samples mould and yeast were detected.Key words: Yogurt, Storage stability, Post heat treatment, Microwave, Nisin, Lysozyme, Pekmez