Effect of Pasteurization, Centrifugation and Additives on Quality of Concentrated Yogurt (Labneh)

Abstract

Pasteurization of yogurt by heat at 60-80°C or by addition of 0.2 to 2% hydrogen peroxide were investigated. Heat-treat-ment caused a very pronounced reduction in the bacterial count of yogurt samples. Addition of hydrogen peroxide was not ef-fective at the low levels studied. Higher concentrations of hy-drogen peroxide that were effective in reducing the bacterial population persisted in the food for more than 4 weeks. Heat-pasteurization in the presence of hydrogen peroxide, was very efficient in destroying bacterial cells and hastened the disap-pearance of hydrogen peroxide residues from the food. "Lab-neh", which is usually prepared from yogurt by filtering out the whey through a cloth, had an inferior texture when prepared from pasteurized yogurt. Milder heat treatment in the presence of hydrogen peroxide and addition of potassium sorbate and thickeners, greatly improved the quality and the shelf-life of labneh as confirmed by sensory evaluation experiments. Further improvements were possible by preparing labneh using centrifu-gation at various speeds for a short time. "Labneh" is defined as a semi-solid food product de-rived from yogurt by straining away part of its water and water soluble compounds (3). It is widely consumed as a sandwich spread, particularly for local breakfast meals. Yogurt, which is usually high in its bacterial count, is strained through cloth bags for several hours before lab-neh is obtained. The resultant product, although concen-trated, usually has a short shelf-life even if kept under refrigeration. The keeping qualities of fermented milk products are usually improved by pasteurization, freezing, dehydration and addition of preservatives (7). Prekoppova and Slottova (9) reported that the survival rate of lactic acid bacteria in yogurt heated at 60° or 70°C for 1 min were 0.12% and 0.025%, respectively. Roundy (10) studied the additive effect of hydrogen peroxide with heat treatment on the survival rate of lactic acid bacteria in yogurt. He reported that heat treatment of yogurt at 130°F for 1 min caused a reduction rate of 32.7% in the popula-tion of Lactobacillus bulgaricus and of 12% for Strep-tococcus thermophilus. He also showed that addition of hydrogen peroxide at 0.06% before the heating of yogurt, increased the destruction rate of the same organisms to