Comparison of dry culture medium and conventional plating techniques for enumeration of bacteria in pasteurized fluid milk.

Standard plate counts, psychrotrophic bacterial counts, and coliforms were determined by conventional plating techniques and by Petrifilm TM plates, a dry culture medium, for 48 commercially processed milk samples (24 whole milk and 24 skim milk). The Petrifilm SM plate counts were compared with counts on standard methods agar for the standard plate count, psychrotrophic bacterial count, and rapid psychrotrophic bacterial count. The Petrifilm violet red bile plate counts were compared with counts on violet red bile agar for coliform test with a solid medium and the preliminary incubation method for detection of coliforms. Standard plate counts were determined within 24 h of packaging and after 7, 10, and 14 d of storage at 6.1 degrees C. Psychrotrophic bacterial counts and coliform counts were determined with 24 h of packaging and after 7 d storage. There was a strong linear relationship between Petrifilm SM and standard methods agar plates (excluding counts on samples plated within 24 h of packaging) and for the psychrotrophic bacterial count method. Petrifilm SM had a weak linear relationship with Standard Methods Agar plates for the rapid psychrotrophic bacterial count. Coliform counts determined on Petrifilm violet red bile plates were generally within the same range as counts on violet red bile agar plates. The positive predictive values for the Petrifilm violet red bile plates and violet red bile agar plates were essentially the same for samples plated within 24 h of packaging.

Rapid Estimation of Total Solids Content of Reverse Osmosis Retentates Using a Cryoscope.

Rapid, simple, accurate and cost-effective analytical methods are always needed for quality control in the dairy industry. Application of reverse osmosis for concentration of milk will create a need for a method to rapidly estimate the total solids content of milk retentates. A milk cryoscope can be used for this purpose. An excellent correlation (.99) exists between observed freezing point of skim milk retentates and their total solids content. As a result, it would be possible to have a production worker actually do this total solids monitoring using a milk cryoscope while operating a reverse osmosis unit.

Subpasteurization Heat Treatment to Inactivate Lipase and Control Bacterial Growth in Raw Milk.

Raw milk was heat-treated under subpasteurization and suprapasteurization conditions, cooled and stored for up to 72 h at 4.4 and 6.7°C. Milk lipase activity and bacteria counts were monitored in both unheated and heated milks. Inhibition of milk lipase activity ranged from 42 to 98% for treatments of 57.2°C for 10 sec to 73.9°C for 10 sec, respectively. The logs of Standard Plate Count after 72 h of storage at 6.7°C were 6.56, 4.86, 4.31, 4.00 and 2.82 for unheated and 10-sec heat treatments at 57.2, 65.6, 73.9 and 82.2°C, respectively. Psychrotrophic Bacteria Counts were also lower in the heated milks than in the unheated milk. The logs of Psychrotrophic Bacteria Counts after 72 h of storage at 6.7°C were 6.21, 2.45, 2.27, 1.33 and 1.00 for unheated and 10-sec heat treatments at 57.2, 65.6, 73.9 and 82.2°C, respectively. Heat treatment of raw milk supplies would result in limiting action of the milk lipase system and growth of bacteria.

Modified fluorometric determination of vitamin A in milk.

A modified fluorometric procedure for determination of vitamin A in milk was developed to provide rapid analysis of large numbers to samples.. Saponification and a single extraction in a reaction vessel without transfer provided simplicity and standardization. Time and temperature of saponification and time of extraction were studied. Fifteen replicates of four different milks gave standard deviations of .71, .82, 1.26, and .71 on samples with 29, 26, 38, and 26 mug retinol per 100 ml. Recovery of added vitamin A in six amounts in two experiments gave ranges of recovery of 96.0 to104.0 and 95.2 to 105.3% with average recoveries of 100.0 and 99.9%.