Bactofugation is widely used as a means to remove sporeformers from milk; well-functioning bactofugation removes up to 98% of the spores (14). During the autumn and winter seasons, when cows are fed with silage, the spore content of Clostridium tyrobutyr-icum may be as high as 4000 per liter of milk; as few as 10-20 spores per liter may cause late blowing in cheese. With a removal efficincy of 98% by bactofugation, the number of sporeformers remaining is about 80 per liter; thus, bactofugation is not completely effective in preventing late blowing. A relatively new process involving double bactofugation is usually enough to prevent late blowing. Normally, bactofugation is able to remove about 70% of the non-sporeforming flora, but this is far from sufficient removal of the non-sporeforming microorganisms.
A better, but also more expensive, way to remove bacteria in general is microfiltration. Microfiltration will remove about 99.9% or more of the microbial flora present, including sporeformers. By this, the quality of the cheese milk is improved and the risk of the presence of microorganisms will decrease considerably. In cases where it is crucial that special spoilage microorganisms are absent, it is appropriate to perform microfiltration prior to pasteurization.
Pasteurization is the ultimate step for removal of pathogens. Low pasteurization is defined as the combination of time and temperature and is sufficient to kill all vegetative pathogens.
Still, it should be noted that not all microorganisms are killed by low pasteurization. Spores of Bacillus and Clostridium sp. will survive along with a few spoilage microorganisms such as heterofermentative Lactobacillus.
In the pasteurization process, it is important to control the temperature. During pasteurization the temperature will oscillate from the set point, and it is crucial that the lower temperature be above 71.8 °C. Controlling this requires an accurate temperature detection system. Such a system should be able to register the temperature rapidly and with high frequency. Another issue in pasteurization is the temperature differences between the components in the pasteurization unit. For example, if the differences in the regenerative system are too big, fouling may occur, leading to lower efficiency in the pasteurization unit. The operation time for the pasteurization unit is also of importance. With the demands for high production efficiency, running times tend to increase, but this is often compromised due to biofilm formation. Finally, of course, the cleaning of the pasteurization unit is important; one must consider the concentration of the cleaning agents and the temperature used.
The cheese process is normally conducted at 30 °C, with a cooking temperature range from 35 to 55°C. These temperatures are the normal interval in which pathogens or spoilage microorganisms are able to grow or survive. It is, therefore, necessary that the cheese vats be maintained in a highly hygienic condition. There should not be any dead ends in the vats, and the interfaces between the cheese vat and pumps, stirring systems, and so on should be secured properly. Finally, it is important that cleaning is easy to perform either as a ''cleaning in place'' (CIP) system or manually.
In most cases, cheeses are subjected to brine with a NaCl content of about 21%. Direct salting may also be used—for example, in Cheddar and cream cheeses. Due to the high salt content, only a few microorganisms pose a risk; yeast (as spoilage microorganisms), S. aureus, and Listeria monocytogenes are the only microorganisms of concern. It is also important to note that an infection in the brine leads only to surface contamination, as the cheese at this stage is already formed and the surface has been closed during pressing.
There are three methods for curing cheeses: packaged in bags or foil; unprotected on shelves, with surface ripening; and unprotected on shelves, without surface ripening. If the cheeses are packaged, the risk of contamination and/or growth is small. In general, packed and subsequently cured cheeses keep their characteristic low pH, which along with the packaging protects against contamination.
Cheeses that are not packed, and without surface ripening, have a higher risk of contamination. For surface-ripened cheeses the risk of contamination is higher than if they are also packed. The microorganism used for surface ripening will develop into a thick layer and, hence, protect against contamination partly by producing antagonistic compounds such as methanthiol and bacteriocins, and by substrate competition.
Obviously the risk of contamination is very low when cheeses are distributed packaged and will only be contaminated if the packaging is damaged. The risk is if pathogenic or spoiling microorganisms are already present in low amounts. They may grow if the cooling chain is broken. This factor is often seen in the cooling desks at the supermarkets, especially, where the temperature often is as high as 15°C, 10°C above the required 5°C; precautions should be taken to keep the temperature at 5°C or below.
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