Isolation and characterization of predominant Lactococci from 'O' type mixed strain starters

摘要

In the present study, Lactococcus sp. from 11 Mesophilic 'O' type mixed strain starters were isolated, characterized and studied for technological characteristics. Amongst the 92 randomly selected isolates, 53 confirmed as L. lactis ssp. lactis through phenotypic characterization and 16S rRNA genus specific PCR and gadB gene targeted subspecies specific PCR. No strain of L. lactis ssp. cremoris could be isolated from the tested cultures. Based on the acidification profile in skim milk at 30°C/12 h,27.1 % of the isolates were found to be fast acidifiers("pH>1.5), 37.5% medium acidifiers ("pH 1-1.5) and 35.4% as slow acidifiers ("pH <1). Measurement of proteolytic activity by OPA method showed that most of the good acidifiers also possessed higher proteolytic activity, showing a maximum of 208.17 u,g of leucine/ml; moderate acidifiers a maximum of 154.91 ug of leucine/ml and slow acidifiers a maximum of 126.00 pg of leucine/ml. Strain-typing by plasmid profiling revealed eight major clusters at a similarity level of 90% of all the isolates along with the standard culture NCDC91 and NCDC86. Seven isolates showing the best technological properties were identified by API 50 CHL as L. lactis ssp. lactis with good acceptability. The 16S rRNA partial gene sequencing of E13 and A9 showed 100% similarity with the genome of L. lactis ssp. lactis retrieved from NCBI GenBank. The fast acidifying strains of Lactococci were freeze dried and deposited in the culture collection center.