Evaluation of Raw Milk Quality Tests

Influence of bovine mastitis on lipolysis ond proteolysis in milk

Lipolysis and proteolysis in milk were determined before, during, and after experimentally induced mastitis. Streptococcus agalactiae was infused into one quarter of five cows to elicit an infection. Milk protease activity was higher during infection, but milk lipase activity was unchanged. Lipolytic damage to milk fat and proteolytic damage to milk casein occurred in the udder prior to milking during an infection. Lipolysis increased due to increased susceptibility of the milk fat to lipase action during infection. The mechanism of the increased susceptibility of the fat to lipolysis was not determined. After infections were eliminated, SCC, initial and stored FFA concentrations, and initial tryosine values returned to preinfection levels. However, after infections were eliminated, milk protease activity as determined by an increase in tryosine values remained elevated as milk SCC returned to preinfection levels. Protease activity returned to preinfection levels within 10 d after SCC returned to preinfection levels.

Selection of tests for monitoring the bacteriological quality of refrigerated raw milk supplies

Raw milk samples were collected from 10 producer bulk tanks. Samples were then subdivided so that milks were subsequently stored at 1.7, 4.4, 7.2, and 10.0 degrees C for 24 and 48 h. After storage, samples were analyzed by seven plating methods: standard plate count, psychrotrophic bacterial count, rapid psychrotrophic count, preliminary incubation count, mesophilic plate count, laboratory pasteurized count, and coliform count by violet red bile agar technique. Impedance protocols on a Bactometer Model 123 for total count, psychrotrophic count, mesophilic count, and coliform count were also used to evaluate the bacteriological quality of the milks. Bacterial counts and impedance detection times were analyzed using nonparametric statistics. Impedance protocols for total count and psychrotrophic count were the best indicators of bacteriological quality. Preliminary incubation count was the best of the plating methods. The laboratory pasteurized count performed poorly. Impedance measurements provided information in the shortest time.

[Proteolytic activities of Ps. fluorescens in milk: determination with azocasein in comparison with HPA]

Proteolytic activity of psychotrophie bacteria in milk can be measured in a practical and comparatively sensitive manner using Azocasein as a substrate. The necessary parameters were exemplarily developed and demonstrated with a typical, strongly proteolytic strain of Pseudomonas fluorescens (No. 112). Comparison of the improved Azocasein method with a modified version of the HPA method of Cliffe and Law--they had proposed it for predicting the stability of UHT milk--showed nearly the same sensitivity with six test strains of Pseudomonas fluorescens. The first significant detection of proteolytic activity could be made approximately at the same time except with one strain of the biotype II (miscellaneous strains). It is discussed, whether the demonstrated methods can be judged (by the bacterial counts of detection) as sufficiently sensitive for to prediction of term spoilage during storage of UHT products.