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Document Type

Thesis - University Access Only

Award Date


Degree Name

Master of Science (MS)

Department / School

Dairy Science

First Advisor

Sanjeev K. Anand

Second Advisor

K. Rajinder Nauth


This study was performed to isolate nisin–producing Lactcoccus lactis subsp. lactis from raw milk, to evaluate their antimicrobial activity against food spoilage organisms and selected food pathogens, and to develop the conditions for improved antimicrobial production. The component of the antimicrobial activity was characterized. Some lactic acid bacteria producing antimicrobial activity were found in raw milk samples from the dairy farm. Forty-seven different colonies of lactococci were screened for bacteriocin production under conditions excluding the inhibitory effect of organic acids. Five strains appeared to secrete antibacterial compounds which were active against Micrococcus luteus and Lactococcus lactis subsp. cremoris. One of the isolates, N40615, was selected for further studies, because it produced the largest zones of inhibition against the indicator strain Lactococcus lactis subsp. cremoris. The inhibitor was not hydrogen peroxide or bacteriophage. The effects of the proteolytic enzymes, α- chymotrypsin, proteinase K, trypsin, and pepsin on both N40615 produced antimicrobial and the nisin standard, (Nisaplin™) were similar. Thus, N40615 produced bacteriocin was assumed to be nisin. The antimicrobial activity of N40615 in fermented skim milk was higher than that of reconstituted skim milk. Addition of 0.50% peptone, 0.25% yeast extract, and 0.3% dipotassium phosphate (W/W) to the media increased the antimicrobial activity. Fermentation of supplemented skim milk under pH-controlled conditions improved the antimicrobial activity significantly to over 3000 IU mL-1 against spoilage organisms such as Lactobacillus brevis, L. buchneri, and L. plantarum.

Library of Congress Subject Headings

Anti-infective agents
Lactic acid bacteria
Raw milk
Lactococcus lactis


Includes bibliographical references (pages 39-50)



Number of Pages



South Dakota State University


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