CHARACTERIZATION OF WILD STRAINS OF LACTIC ACID BACTERIA FOR YOGHURT CULTURE AND LACTOCOCCUS PHASGES ISOLATED FROM INDIGENOUS DAHI

Tariq, Masud (1997) CHARACTERIZATION OF WILD STRAINS OF LACTIC ACID BACTERIA FOR YOGHURT CULTURE AND LACTOCOCCUS PHASGES ISOLATED FROM INDIGENOUS DAHI. Doctoral thesis, Quaid-i-Azam University Islamabad.

[img] Text
2227.htm

Download (27kB)

Abstract

The first part of the project deals with the evaluation of wild strains of Lactic Acid Bacteria for the selection of starter culture for yoghurt making. A total of fifty indigenous dahi samples collected randomly from local markets of Rawalpindi and Islamabad were subjected to bacteriological and chemical examinations. Mean lactic count, titratable acidity and pH recorded were log 7.53/ml, 1.16% and 4.12 respectively. The following lactic acid bacteria were identified and their incidence is given in parentheses: L. delbureckii ssp. bulgaricus (86%), S. thermophilus (80%), S. lactis (74%), L. helveticus (34%), S. cremoris (30%), L. casei (20%) and L. acidophilus (14%) respectively. The strains of S. thermophilus and L. delbrueckii ssp. bulgaricus, selected on the basis of culture activity test were further examined for their optimum growth temperature, generation time and titratable acidity. The results demonstrated that the generation time of S. thermophilus was relatively high as compared to L. delbrueckii ssp. bulgaricus strains. It was also noted that there was a variation among each strain for these tested parameters. Later on, two strains each of high (H) and weak (w) lactic acid producer of S. thermophilus and L. delbrueckii ssp. bulgaricus were evaluated for their symbiotic association at the recommended conditions of yoghurt making in various ratio’s. It was observed that mixed culture of S. thermophilus and L. delbrueckii ssp. bulgaricus produced more lactic acid as compared to individual strain. Moreover, it was observed that 1:2 ratio of S. thermophilus and L. delbrueckii ssp. bulgaricus showed maximum acid production in all the tested combinations. Finally these selected combinations of 1:2 ratio of S. thermophilus and L. delbrueckii ssp. bulgaricus were again evaluated both chemically and organoleptically. It was found that chemical properties effect the organoleptic properties of the final product. The combination of S. thermophilus (H) and L. delbrueckii ssp. bulgaricus (w) was liked more as compared to other tested combinations. These strains were further tested for their survival rate during freeze-drying by using different cryoprotective agents viz skim milk, β-glycerophosphate, Na-glutamate, mannitol and sorbitol. There was non significant difference between log viable counts of these indigenous strains before and after freeze-drying for 12 months of storage at 0ºC in all the cryoprotective agents except sorbitol. These data indicate that these wild strains of S. thermophilus and L. delbrueckii ssp. bulgaricus can produce a good quality yoghurt on the basis of these observations recorded. In part II, whey samples of indigenous dahi were also examined for the presence of virulent phages. A lytic bacteriophage Tk1 of Lactococcus lactis was isolated and was then characterized on the basis of their plaque morphology, lytic activity, latent period, burst size and phage morphology. In a subsequent experiment, variation was also detected among other isolated lactococcal phages for these parameters. All these phages were classified into group B of Bradley. The results of further study revealed that the bacteriophages belonging to this geographical region are different from the bacteriophages those reported in other countries i.e. Germany and Canada on molecular characterization. The present study has provided some excellent understanding about the yoghurt starter culture and lactococcal phages in this region. This might help to create a base-line for research and development regarding scientific production of yoghurt, cheese and allied products which lead in acquisition of self sufficiency in the production and use of this vital element of the dairy industry in future.

Item Type: Thesis (Doctoral)
Uncontrolled Keywords: lactic acid bacteria, yoghurt, lactococcus phasges, dahi, lactic count, titratable acidity, lactococcus lactis, lyophilized cultures, bacteriophage infection, s. thermophilus, l. delbrueckii
Subjects: Q Science > QH Natural history > QH301 Biology
Depositing User: Muhammad Khan Khan
Date Deposited: 08 Sep 2016 05:22
Last Modified: 08 Sep 2016 05:22
URI: http://eprints.hec.gov.pk/id/eprint/1102

Actions (login required)

View Item View Item