The role of Bifidobacterium lactis and the effect of the addition of inulin and lactulose on the viscosity of a functional fermented milk product have been investigated. The effect of chosen starter and prebiotics on the properties of the functional fermented milk was significant for the apparent viscosity. Generally the viscosity of analyzed products was lower than the viscosity of classical fermented milk products, such as yoghurt, kefir or others. The viscosity was strongly affected by the content of total solids of the analyzed products: with an increase in the total solids there was increase in the viscosity. Among analyzed fermented milk samples, sample with 5% of lactulose had the highest viscosity. In general, inulin did not seem to have an effect on the viscosity of the analyzed product samples compared with the control sample. Bifidobacterium lactis has a weak proteolytic activity; therefore the structure and consistency of functional fermented milk product were characterized as weak, too. It is known that inulin is not only dietary fibre or prebiotic, it has the functions of food additives, too. Inulin is added to food formulations to modify products' texture or viscosity and sweetness of products. Comparison of the obtained results shows that the viscosity of the functional fermented milk product with various concentrations of lactulose or inulin is different. This suggests that the role of inulin in a food matrix is bi-functional. Inulin does not increase the viscosity of a milk product but gives a richer texture to liquid products and spreads.

Lactulose is recognized world-wide as one of the most clinically reliable bifidogenic factor. Lactulose is used in various types of food products (infant formula, baby food, confectionary, soft drink, milk products) and also pharmaceutically to improve hepatic encephalopathy and constipation (Mizota, 1996; Strohmaier, 1998). The object of this study was to investigate the influence of concentration of lactulose on growing Bifidobacterium lactis in milk. A bifidogenic lactor, lactulose was added (1, 2, 3, 4 and 5%) into 100g of milk. The milk samples were inoculated with Bifidobacterium lactis (BB-12, Chr. Hansen, Denmark) and incubated at 38 deg C for 16 hours. Lactulose as a prebiotic influences the growth of bifidobacteria in milk. Trends of development of acidity in the milk samples with or with out lactulose were not similar. Laboratory studies have generally shown that growth of Bifidobacterium lactis in milk depends on the concentration of lactulose. Data on the final cell count in fermented milk indicates that increasing the lactulose concentration from 2% to 3% enhanced the growth of the Bifidobacterium lactis, whereas no significant difference between lactulose concentration 4-5% and control sample was observed. Present result furthermore indicate that finding combination of prebiotic and probiotic pairs where the prebiotic would benefit the specific probiotic strain, e.g. during production and formulation into foods, is not a simple task.

Fermentation of milk with probiotics and prebiotics has been shown to increase the concentration of B group vitamins in fermented milk products. The task of the study was to investigate the influence of prebiotics on the increasing of thiamin and riboflavin concentrations in milk during fermentation by Bifidobacterium lactis. Lactulose (Duphilac, Netherlands) and inulin (Raftiline, Belgium) were added (1, 2, 3, 4 and 5%) into 100 g of milk. The milk samples were inoculated with Bifidobacterium lactis (BB-12, Chr. Hansen, Denmark) and incubated at 38 deg C for 16 hours. The amount of thiamine and riboflavin was determined by fluorometric method. Laboratory studies have shown that the increasing of thiamin and riboflavin concentrations in milk depends on the concentration of lactulose and inulin. Data on the final thiamin and riboflavin concentrations in fermented milk indicates that increasing the lactulose concentration from 1% to 3% enhanced the production of thiamin and riboflavin. The similar data was obtained with inulin. Increasing of inulin concentration from 1% to 4% enhanced the production of vitamins thiamin and riboflavin. The present results furthermore indicate that finding optimal combination of prebiotics and probiotic pairs where the prebiotic would benefit the specific probiotic strain, e.g. during production and formulation into foods, is one of the research priority for functional foods.