Dairy product Processing and impact on microbiological safety

Milk and cream
Pasteurisation is sufficient to destroy pathogenic milk-borne vegetative bacteria. Illness resulting from consumption of pasteurised milk is rare. However, where outbreaks have occurred, these were attributed to inadequate pasteurisation, post-pasteurisation contamination and/or temperature abuse.

Cheese
A number of processing factors influence the growth and survival of pathogens in cheese,
including the severity and duration of heat treatment (including curd cooking); pH; salt
concentration; water activity; and maturation/ripening. These outbreaks have resulted from faulty controls in cheese production; use of contaminated starter cultures or contaminated ingredients; post-pasteurisation contamination; or mishandling during transport and/or distribution.

Dried milk powders
Micro-organisms in dried milk powders will not grow due to low water activity, however, they may survive for long periods and resume growth when the powder is reconstituted and stored under favourable conditions. Heat-treatments given prior to spray-drying are severe enough to destroy all vegetative pathogens in raw material. However, there is opportunity for environmental contamination during spray-drying and subsequent storage.The outbreaks were caused poor plant hygiene; contamination and abuse of reconstituted products; and outgrowth of bacterial spores.

Concentrated milk products
Microbial pathogens are generally not associated with concentrated milks due to the low water
activity of these products. Pasteurisation of cream used in butter manufacture results in the destruction of vegetative microorganisms, although preformed toxins and spores may carry over to butter. The preservative properties of butter are based on moisture distribution. In addition salt in moisture droplets also have a preservative effect.

Ice-cream
The heat treatment applied to ice cream mix destroys pathogenic micro-organisms. However,
pathogens may be introduced with the addition of ingredients. Pathogens will not grow in icecream, but may survive freezing. The outbreaks have been linked to the use of raw ingredients or improper heat treatment during preparation of ice-cream in the home, and contamination during commercial icecream manufacture.

Cultured and fermented milk products
The heat treatment of milk is sufficient to destroy vegetative micro-organisms and rapid growth of starter cultures inhibits the outgrowth of spore-formers. Pathogenic micro-organisms are
prevented from growth by the low pH; the presence of lactic acid, and by refrigerated storage.

Dairy desserts
Heat treatment by pasteurisation or UHT results in the destruction of vegetative cells.
Contamination may occur after heat treatment with the addition of further ingredients, or through survival of spores of B. cereus.

Dairy-based dips
Where pasteurisation or other heat treatments are employed, vegetative cells will be destroyed.
However, spore-formers can survive heat treatments and other hazards can be introduced with
the addition of heat labile ingredients after heating. The low pH of these products assists in their
microbial stability.

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