Microbiological Quality Evaluation of Dahi / Yoghurt of Kathmandu Valley

Abstract

Dahi is similar to yoghurt and is a fermented milk product by the application of lactic acid bacteria like Lactobacillus bulgaricus and Streptococcus thermophilus in pasteurized or boiled milk and is the popular and prominent dairy product all over the world. With an aim to evaluate the microbiological quality of Dahi / yoghurt, this study was carried out from July 2007 to January 2008. Seventy-one (34 plastic cup, 19 open and 18 ceramic cup) Dahi samples were collected from three districts of Kathmandu valley and laboratory processing was done at Central Department of Microbiology Kritipur, Kathmandu. For microbiological quality evaluation starter Streptococcus thermophilus, common milk borne pathogens like Staphylococcus spp., Bacillus cereus, Salmonella spp., Shigella spp. and indicator organisms like coliform were enumerated in selective media and identified after isolation. Fungal study was done by enumeration of yeasts and molds and by identification of isolated molds. Serial dilution and pour and streak plate method were done for enumeration of organisms. Streptococcus thermophilus was detected in 4.27% (3/71) of samples with mean count 2.57×105 cfu/ml and all isolates were found to be urease deficient. Coliforms were detected in 45.08% (32/71) samples and all coliform positive samples have greater than 10 cfu/ml. Staphylococcus spp. were detected in 92.95% (66/71) in which 87.87% (58/66) samples contained coagulase positive Staphylococcus aureus. Bacillus cereus was detected in 88.73% of samples with the highest and the lowest count were 29.9×105 and 0.04×105 cfu/ml respectively with mean count 5.51×105 cfu/ml. Fungi were found in 96.6% (70/71) samples with mean count 250×105 cfu/ml and are the most abundant among the studied organisms. Salmonella typhimurium was present in only one (1.45%) sample. Among coliform, E. coli 96.87% (31/32) Klebsiella pneumoniae 46.87% (15/32), each Citrobacter freundii and Enterobacter aerogenes 6.25% (2/32) were present. Geotricum spp. 49.20% (31/61), Aspergillus spp. 44.44% (28/61), Mucor 42.85% (27/61), Penicillium spp. 31.74% (20/61), Cladosporium spp. 7.93% (5/61), Fusarium spp. 9.52% (6/61), and Rhizopus spp. 6.34% (4/61), was present among the fungi positive samples. Almost all samples were found to be highly contaminated with harmful bacteria, yeasts and molds. Many did not meet the standard or safe guideline criteria given by DDC, PFA, and FAO. This indicates the potential of food borne diseases in Kathmandu. Therefore establishment of national standard guideline for Dahi and continuous monitoring for microbial quality is essential.