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Micro-organisms can metabolise the constituents of milk and thereby modify its taste, as well as impacting the processing and shelf life of milk and dairy products. Milk and dairy products are therefore tested to ensure that limit values are not exceeded. The presence of micro-organisms is of particular importance in foods for infants and small children. Manufacturers must therefore determine the number of micro-organisms present in their products every month.
Methods of detection
DIN EN ISO 4833 describes a method for the enumeration of micro-organisms. decimal dilutions of the sample are pipetted into sterile Petri dishes and mixed with colony-counting medium that is liquefied and cooled to 44 °c to 47 °c. As soon as the mixture has solidified it is incubated. no more than six Petri dishes can be stacked on top of one another during incubation. The dishes are evaluated under diffuse light and a magnifying glass used for counting if necessary to avoid confusion between colonies and other particles.
Importance of quantifying micro-organisms in milk and dairy products
Under dairy product regulations the number of micro-organisms present in milk and dairy products must be determined at regular intervals. The controls are required at intervals ranging from every 12 weeks to every 2 weeks, depending on the susceptibility of the product to spoilage.
Incubation conditions according to DIN EN ISO 4833:
Temperature: 30 ± 1 °C
Duration: 72 ± 3 h