Microbial Quality

The microbial quality of dairy products is related to the number of viable organisms present. A high number of microorganisms in raw milk suggests it was produced under unsanitary conditions or that it was not adequately cooled after removal from the cow. If noncultured dairy products contain excessive numbers of bacteria, in all likelihood postpasteurization contamination occurred, or the product was held at a temperature permitting substantial growth. Raw-milk and pasteurized-milk products are examined for the concentration of microorganisms by the agar plate method or the direct microscopic method.

The agar plate method consists of adding a known quantity of sample (usually <1.0 mL, depending on the concentration of bacteria) to a sterile petri plate and then mixing the sample with a sterile nutrient medium. After the agar medium solidifies, the petri plate is incubated at 32°C for 48 h, after which the bacterial colonies are counted and the number expressed in terms of standard plate count or colony-forming units per 1 mL or g of sample. This procedure measures the number of viable organisms present and able to grow under test conditions. An alternative method to the agar plate method is the Petrifilm® aerobic count (PAC) method. This method uses a cold-water soluble gelling agent instead of agar in the nutrient medium. The methodology is similar to the agar plate count, but the Petrifilm® plates require less labor in preparation and less incubator space.

The direct microscopic count determines the number of viable and dead microorganisms in a milk sample. The small amount (0.01 mL) of milk is spread over a 1.0 cm2 area on a microscope slide and allowed to dry. After staining with an appropriate dye (usually methylene blue), the slide is examined with the aid of a microscope (oil-immersion lens). The number of bacteria cells and clumps of cells per microscopic field is determined and by proper calculations is expressed as the number of organisms per 1 mL of sample.

The keeping quality of milk and milk products is determined by several variations of the agar plate method. Co-liform bacteria are detected by using the agar plate method and a selective culture media (violet-red bile agar). Because coliform bacteria do not survive pasteurization, they are an indication of recontamination or improper processing. Psychrotrophic bacteria are capable of growing at refrigerator temperatures, 2 to 10°C. They are responsible for fruity, putrid off-flavors in milk. These bacteria are measured by the agar plate method and incubation at 7°C for 10 days. Additional tests for milk quality can be found in Ref. 4.

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