Physical and rheological properties of commercial low‐fat yogurt and of skim milk acidified with single strains of Streptococcus thermophilus were evaluated. Concentration of exopolysaccharides (EPS) varied from 40 to 168 mg glucose equivalents per liter, and EPS showed different degrees of ropiness. Further characterization of nonropy and ropy EPS from three S. thermophilus strains revealed intrinsic viscosity of 0.13–0.66 mL/mg, respectively, which had a significant impact on apparent viscosity and the ability of regaining structure after shearing. Compared with regular milk acidification, batch fermentation with supplemented skim milk at constant pH resulted in a strain‐dependent increase of EPS yield by a factor of 4–7.

The knowledge of thermal inactivation kinetics, usually expressed in terms of D‐ and z‐values, is of crucial importance for the design of sanitation and sterilization processes. In this study, we designed a simple, fast‐responding, and mechanically stable aluminum tube for inactivation measurements and compared these experiments with the successive‐sampling method at different temperatures. Up to 65°C, we determined a come‐up time of approximately 15 s for the tubes, which is lower than the corresponding values of other devices, presumably because of lower wall thickness, material properties, and a higher surface to volume ratio. D‐values of Escherichia coli calculated from tube inactivation experiments by first‐order kinetics were 370 s (56°C), 126 s (58°C), 53.2 s (60°C), 33.8 s (62°C), and 3.22 s (65°C), and the corresponding values determined with the successive‐sampling flask method were insignificantly different (417, 138, 48.6, and 29.1 s for 56, 58, 60, and 62°C, respectively). These data as well as those measured for Enterobacter cloacae, Pseudomonas putida, Serratia odorifera, and Yersinia rhodei were in close accordance with literature values.

For a survey of the numbers of lactobacilli present in milk used for cheese-making in creameries, counts were made on an acetate-agar medium (1) selective for lactobacilli. It was observed that the numbers of lactobacilli in the milk in the cheese vat sometimes greatly increased after the addition of the starter to the milk. When these starters were plated on acetate agar growth of lactobacilli, sometimes in large numbers, suggested contamination of the starters with these organisms. The starters in use at sixteen different creameries were then examined on acetate agar, and seven were found to contain lactobacilli. Representative colonies of these organisms were picked from each starter and these strains identified physiologically using the methods of Briggs(2) and Wheater(3,4), and serologically using the group sera of Sharpe(5) and Sharpe & Wheater(6). Table 1 shows the numbers and species of lactobacilli present in the starters. Each starter was sampled several times, usually at monthly intervals.