Effect of lactulose and the extracted native inulin on the viability of some probiotic bacteria and their uses in the manufacture of set type yoghurt

The present study aimed to investigate the effect of prebiotics (inulin and lactulose) on the growth and activity of three probiotic bacteria (L. plantarum B -531, L. casei B- 444 and L. rhamnosus B -445) as well as yoghurt culture inoculated sparetly or mixed in skim milk. Also to study the effect of incorporating the three tested cultures and the selected prebiotics on the properties and overall acceptability of probiotic and symbiotic yoghurts as well as lactobaclli viability during cold storage. The ratios of probiotic inoculums and prebiotics (0.5, I and 1.5 for probiotic and 1,3 and 5 % for prebiotics) were used. Samples were incubated for 24h at 37 ºC and 42 ºC, the cell density (OD 640mm), lactobacilli count and pH were measured after 4 and 24h of incubation. Some kinetic growth parameters such as specific growth rate (µ/h), mean doubling time (Td/ min) and generation number (gn) were calculated. The results indicated that the highest cell density, lactobacilli count and the development of acidity were obtained with 1.5% size of inoculum and with 5% prebiotics at 37°C and 42°C. While the highest kinetie growth parameters (µ/h, Td/min, g.n) were associated with 0.5% of inoculum size of single or mixed probiotic cultures with yoghurt culture in presence of the highest dose of prebiotic (5%) at 37°C and 42°C at the end of fermentation. L.casei exhibited the highest growth and activity among the other tested cultures either in single culture or mixed with yogurt culture. Using the three probiotic cultures in the manufacture of probiotic yoghurt, resulted yoghurts having living population levels satisfying the international recommendation. The incorporation of probiotic cultures in yoghurt did not affect neither the viability of the starter cultures during cold storage nor the organolaptic properties of yoghurt. Above all probiotic yoghurt containing (Y + Lc) emerged as the best candidate for probiotic yoghurt production mainly because of the highest viability (24.5) and very good sensory properties of the manufactured yoghurt The results of symbiotic yoghurt (probiotic -prebiotic) using two different fermentation temperatures 37°C and 42°C showed that using L.c as probiotics beside yoghurt culture and the supplementation with inulin or lactulose as prebiotic produced symbiotic yoghurt with highest overall acceptability and viability among the other symbiotic yoghurts. Also the fermentation temperature affects the bacterial growth and there by the structure and flavor of product. Moreover the addition of prebioties enhanced the viscosity, syneresis properties as well as the count of lactobacilli and overall acceptability when compared with these of the control yoghurts. It can be concluded that the symbiotic yoghurts obtained in the present study showed a good properties of symbiotic yoghurt