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Stopping the spread of Listeria

22 July 2019

Contaminated food processing machinery is one of several key factors behind rising numbers of product alerts and recalls. Although machines may be hygienically designed with smooth and easily cleaned surfaces, the crevices in components such as mounted bearings and their housings can still harbour bacteria, says David Oliver. 

A particular concern is the bacterium listeria monocytogenes – otherwise known as Listeria – which causes the disease Listeriosis. This highly resistant organism may be present, along with other bacteria, in residues washed from food processing lines onto bearing units and other parts during cleaning.

Pressure washing can drive these food particles and bacteria through the bearing shaft gap and into spaces around the bearing’s housing which never dry. By the same token, bacteria sheltering and breeding within the unit can be subsequently washed out, together with contaminated lubricating grease, and spread elsewhere. 

Relubrication practices often compound the problem. Regular flushing out and replacement of grease to remove its contaminants, and restore its lubricant properties can result in over-greasing, creating another source of contamination. Contaminated grease may be washed onto floors, and then trodden more widely, or into drainage holes, where it makes removal of listeria very difficult. 

All cleaning methods tend to extend the distribution of bacteria, particulate matter and grease. Most alarmingly, research shows that a high-pressure, low-volume spray lance can raise them to a height of 3m in an aerosol of small water droplets. They can remain airborne for several hours before falling onto everything within a 7m radius. 

In a test conducted by SKF and Research Institute of Sciences and Engineering (RISE) in Sweden, five sample food production plants were examined for bacteria. Swabs taken before and after thorough hygienic cleaning revealed that high bacterial concentrations persisted in all cases. In two plants the bacterial densities were actually higher after cleaning, and in one case listeria was still present.

SKF surveys have also identified concerns over use of non-food-grade lubricants and lack of information on lubricant-borne allergens. In addition, there were worries that bearing construction materials were not inherently food-safe and were hard to detect in the event of a malfunction.

Food-safe bearings
The designers at SKF have responded with the SKF Food Line ball bearing unit - Blue Range. Its seals are designed to prevent penetration by microbes and leakage of grease, so it is relubrication-free and the grease enclosed is allergen-free. 

Following the rules of hygienic design, the units feature smoothly shaped surfaces and a self-draining geometry. Polypropylene has been selected as the material for their moulded housing, because of its highly inert and chemically resistant properties. For extra mechanical strength, its composition includes 40% long glass fibres. Components have been coloured blue for easy detection by eye.

Co-injection of the housing with a rubber seal effectively fills any space between the face of the housing and the frame of the machine. The back seal uses static sealing against the housing and dynamic sealing against the shaft, to avoid gaps that might allow contaminants to enter the unit. 

Total exclusion of detergents is almost impossible, as their formulations are purpose-designed to pass through barriers. The new units have another sealing innovation which deals with this: the gutter concept. Its structure blocks most of the detergent, but the small amounts which do enter the seal’s top lip are directed around and out of the other side.

Cost savings
These bearing units do come at a higher price but the investment is quickly repaid through operational savings. Their service life is longer, and there is less need for cleaning, while the labour, downtime and material costs of re-greasing are eliminated. Moreover, they could spare you the considerable expense and reputational damage of bearing-related product recalls.

David Oliver is the food and beverage industry manager at SKF. 


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