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Studies on Yogurt Production Using Lactobacillus Bulgaricus and Streptococcus Thermophilus Isolated from Market Yogurt

S R Rahman*
Department of Plant and Environmental Biotechnology, Sylhet Agricultural University, Sylhet

M Z Alam
Department of Genetic Engineering and Biotechnology, Shahjalal University of Science and Technology, Sylhet

S Mukta
Department of Plant and Environmental Biotechnology, Sylhet Agricultural University, Sylhet

Abstract/ Executive Summary:

Yogurt is a fermented milk product obtained from fermentation of Lactobacillus bulgaricus and Streptococcus thermophilus strains. The aim of this study was to produce yogurt using isolated native starters Lactobacillus delbruckii sub sp. Bulgaricus and Streptococcus saliverius sub sp. thermophilus. In this study yogurt samples were collected from the local markets. The native starters were isolated, purified and identified according to Bergey’s manual of determinative bacteriology. Lactobacillus bulgaricus and Streptococcus thermophilus were screened with respect to their acid and flavour production for the preparation of a natural yogurt and compared to a commercial starter cultures. Yogurt was made by inoculating isolated bacteria (starter culture) usually Lactobacillus delbruckii subsp. bulgaricus and Streptococcus saliverius subsp. thermophilus into milk. After inoculation the mixture was incubated at 42°C for 6 to 8 hours and coagulation was observed and yogurt pH was measured. It is well known that yogurt production using native starter cultures instead of commercial ones is beneficial in respect of both economic and organoleptic aspects. The present results indicated that the isolated native yogurt starters can be used in yogurt manufacturing industry at a large scale.

Keywords: Yogurt, Starter culture, Fermented product, Lactobacillus bulgaricus, Streptococcus thermophilus.

Location: Commercial market of Sylhet district

Start Date:

End Date:

Program Area: Postharvest and Agro-processing

Commodity: Yogurt

Objectives:

The present study was undertaken for isolation and identification and antibiotic susceptibility test of Lactobacillus delbruckii sub sp. Bulgaricus and Streptococcus saliverius sub sp. thermophilus and finally, production of yogurt from isolated native starters Lactobacillus delbruckii sub sp. bulgaricus and Streptococcus saliverius sub sp. thermophilus.

Methodology:

Sample collection The samples of fresh yogurt (curd) were collected from commercial market of Sylhet district and aseptically transferred to the Laboratory of the Department of Genetic Engineering and Biotechnology, Shahjalal University of Science and Technology for studies. The samples were stored under refrigeration conditions for subsequent experiments. Sample preparation Lactic acid bacteria (LAB) isolates were selected from (De Man Rogosa Sharpe) MRS and M17-agar plates of the highest dilutions. The isolates were sub cultured in MRS and M17-broth and streaked onto MRS-agar until pure cultures were obtained (Fadela et al., 2008). Diluted samples (0.1 ml) were plated (spread plate technique) onto MRS medium for Lactobacillus isolation and M17 medium for Streptococcus isolaton and incubated at 37ºC for 24 to 72 hrs. Identification and characterization of bacteria Morphological characterization: Morphological appearance of colonies of lactic acid bacteria were observed carefully. Morphological characteristics that were studied after 24 hours were shape, size, colour, texture, surface, opacity, edge etc. Biochemical characterization: Biochemical studies were performed according to the Bergey, 2006. Several biochemical tests were performed for confirmation of Lactobacillus bulgaricus and Streptococcus thermophilus. The biochemical tests were sugar fermentation tests, indole test, Methyl Red (MR) test, Voge’s Proskauer (VP) test, Oxidase test, Catalase test, Oxidative-Fermentative test (O-F test). These species were characterized by their ability to ferment glucose, maltose, lactose and color changed but not gas formed, negative for indole test, positive for MR test and negative for VP, Oxidase test and catalase test, fermentative for O-F test. Assay of antibiotic sensitivity pattern: To assay the antibiotic sensitivity pattern disc diffusion method was followed. In this method Muller Hinton plates were prepared and swabbed with suspension of selected isolates with the help of sterile cotton bud. MRS agar was used since the Lactobacillus sp. failed to thrive on Muller Hinton Agar which is the conventional medium for test (Okafor and Umeh, 2013). After swabbing the antibiotic discs (ampicillin, tertracycline, gentamycine, azithromycin, chrolamphenicol and ciprofloxacin) were placed on the surface of the plates at equidistance. The plates were then kept at 4°C for 1 to 2 hours for proper disc diffusion of antibiotics. The plates were then incubated for 18 to 24 hours at 37°C. The zone of inhibition was observed for antibiotic sensitivity or resistance and zone diameter was measured (Bauer et al., 1966).

Source of Information: J. Sylhet Agril. Univ. 3(2):307-313, 2016 ISSN: 2308-1597

Article Link (Online Journal): www.jsau.com.bd

Funding Source:

Budget:

Total Budget (Tk.) :

Achievement/Findings:

Lactic acid bacteria is a heterogenous group of bacteria found widely in nature. LABs are added to several probiotic products because of their potential health benefits. Yogurt is a potential source of LAB. A total of 17 isolates were isolated from yogurt samples and finally six species were identified as Lactobacillus bulgaricus and eleven species were identified as Streptococcus thermophilus. From the experimental results of tested antibiotics showed that all the Lactobacillus delbruckii subsp. bulgaricus isolates showed resistance to GM (50%), AM (100%), TE (100%), CIP (83.33%), AZ (100%) and all the Streptococcus saliverius subsp. thermophilus isolates showed 100% resistance to Ampicillin (AM), Gentamycine (GM), Ciprofloxacin (CIP), Tetracycline (TE) and Azithromycin (AZ). Yogurts produced by using startercultures of Streptococcus thermophilus and Lactobacillus bulgaricus have higher therapeutic and/or antimicrobial properties besides of their organoleptic characteristics. Yogurt is a rich source of protein and calcium, and the fermentation process makes these nutrients easier to absorb. Ongoing studies are revealing many health benefits to this great food, such as boosting immunity and reducing yeast infections and colon cancer. Therefore, the present study suggested potential therapeutic benefit following the consumption of fermented dairy products containing viable lactic acid bacteria (LAB) count and decreased coli form count in the intestine as observed in the analysis.

Knowledge Management: Journal