Ice-cream is a frozen mixture of combination of components, such as milk, sweeteners, stabilizers, emulsifiers and flavoring agents. Ice-creams are food products which show excellent potential for delivering probiotics to consumer. Probiotics are basically health promoting gut friendly bacteria. Minimum viable quantity of probiotics which is beneficial for human beings is 106 or 107 cfu/g. Benefits are strain specific and cannot be extrapolated. Keeping in view the importance of probiotics the present research study was planned to determine the influence of various levels of probiotics (Lactobacillus acidophilus and Bifidobacterium bifidum) on physicochemical and sensory characteristics of ice cream, assessing the viability of probiotics at different storage periods and to give innovative and value added product to commercial ice cream manufacturers. In order to carry out the study, Freeze dried cultures of two probiotic bacteria i.e., Lactobacilus acidophilus (DVS LA-5 Probio-Tec®) and Bifidobacterium bifidum (DVS BB-12® Probio-Tec®) were obtained from Chr. Hansen (Hørsholm Denmark). The probiotic cultures were stored at - 18? C in freezer.

These two probiotic strains were inoculated alone and in combination in ice cream mix at three different inclusion levels. Hence there were total nine experimental treatments. A regular or control ice cream i.e., without any probiotic culture was also used along with treatments. Probiotic ice cream preparation and physical tests were performed at walls Ice Cream factory, Lahore, Pakistan. Chemical analysis were performed at department of Food and Nutrition laboratory, whereas, Microbiological tests were conducted at department of Microbiology, University of Veterinary and Animal Sciences, Lahore.

Probiotic ice cream was manufactured in the pilot plant of the research and development department, Unilever Walls Ice Cream factory Lahore. Ice cream mix was prepared by following standard procedure. After mixing, homogenization and pasteurization of ice cream mix, it was cooled to 41°C and was divided into ten equal parts. Probiotics cultures alone and in combination, according to experimental design, were added in ice cream mix. Then it was fermented for two hours at 41°C. Then mix was aged at 4°C for a period of 2 hours followed by freezing and hardening. Ice cream samples were packed in one liter plastic tubs and were stored at - 18°C in freezer.

Ice cream samples were analyzed after every fifteen days interval during storage period of three months. Physicochemical tests including overrun, viscosity, melting resistance, pH, acidity, fat, protein, total solids were performed by following the methods of AOAC (2000). As far as results of physicochemical tests are concerned, it was observed that both Lactobacillus acidophilus and Bifidobacterium bifidum are able to produce acid in ice cream. Acidity tended to increase as a result of increase in the number of bacteria. Highest pH was observed in control (R) sample, whereas, lowest pH was observed for samples having mixed culture. pH tended to decrease throughout storage. Lactobacillus acidophilus found more acid producing than Bifidobacterium bifidum. Ice cream samples containing high levels of probiotic bacteria showed decreased value of overrun. Whereas no effect was found in case of viscosity. Samples with increased level of bacteria showed significantly less melting resistance. Also, melting resistance tended to decrease significantly with increase in storage. Total solids and fat contents remained constant throughout storage and effect of treatments was statistically non significant. In case of protein contents, a positive correlation was observed .Increase in level of bacteria , increased the protein contents of ice cream, but it remained constant throughout the storage.

Viable probiotic bacteria were enumerated by using the technique of spread plate method by using RCA media. In present study, cell count of viable bacteria, after fermentation, showed increase number of colonies. Samples inoculated with B3 were excellent in regard of exhibiting probiotic property followed by treatments B2 and A3. Treatments in combination i.e., C1, C2 and C3 showed results close to treatments having single cultur , which might be due to nutrients competency with each other. Furthermore, it was quite difficult to count colonies in mixed culture. Ice cream samples prepared from mixed culture were more acidic in taste and flavour and were least liked by sensory evaluators.

Probiotic ice cream was subjected for sensory evaluation by five panel of judges. They rated ice cream by using nine point hedonic scale. Color of ice cream did not show any change throughout storage and was not affected by various levels of probiotics. No surface spots were found in any treatment at any storage period. Due to acid producing nature of probiotics, slight acidic changes were observed in thickness, flavour and taste of ice cream. However, overall acceptability for all probiotic ice cream was good. The data were analyzed according to analysis of variance technique under factorial arrangement. Significance of means was compared by using Duncan's multiple range test.