Date of Award:


Document Type:


Degree Name:

Doctor of Philosophy (PhD)



Department name when degree awarded

Bacteriology and Public Health

Committee Chair(s)

F. J. Post, C. A. Ernstrom


F. J. Post


C. A. Ernstrom


G. H. Richardson


V. T. Mendenhall


P. B. Larsen


Proteolytic activity of Streptococcus cremoris HP, P2 and AM2; Streptococcus lactis-C2, ML8 and a commercial mixed culture was determined in skimmilk and whey by measuring the increase in Absorbency at 274.5 nm of a pH 4.6 soluble filtrate, following incubation in a 2.5% casein solution. Proteolytic activity, rate of acid production and growth rate decreased in order of C2, HP, mixed culture, AM2, ML8 and P2 at a regulated pH (6.1 - 6.2) and 32 C. Maximum proteolytic activity for all cultures was found during the stationary phase of growth. Extracellular protease activity was purified 1000 fold from S. lactis-C2. Enzyme activity was stable at 80 C for 20 min and lost 41% of its activity after 80 min. A linear increase in activity was observed between 25 and 50 C after 5 hr incubation at pH 6.2.

Partial culturing of skimmilk, and incubation of a freeze-dried crude enzyme preparation in skimmilk prior to acidification was used for Cottage cheese making. Partial culturing or incubation of freeze-dried crude enzyme preparation in skimmilk prior to manufacture of direct acid Cottage cheese, increased the moisture and firmness of the finished curd, and significantly improved the body and texture of the creamed product. Partial culturing of skimmilk prior to acidification was also successfully used for the continuous process where good Cottage cheese curd was also obtained.