The European regulations requiring the use of heat treatment when preserving food to control the life-threatening bacteria Listeria monocytogenes are effective. This was the conclusion of a study conducted by PhD student Stijn van der Veen of Wageningen University during which he subjected the resistance genes of the bacteria to a series of tests. But he also discovered that a combination of salt and acid can kill Listeria. The primary risk for consumers results from contamination after the preservation process.
Listeria strains do not survive a properly conducted minimum pasteurisation – 15 seconds at 72 degrees Celsius. But you do not always have to use heat to kill Listeria monocytogenes. The bacteria are also killed during food preservation with acid. Van der Veen also ascertained that sodium lactate, a preservative salt, can kill 138 different strains of the bacteria. He therefore recommends a supplementary EU criterion which states that food preserved with a combination of 2% sodium lactate and a pH of 5.2 or lower is also safe.
However, Van der Veen found that the bacteria can survive mild preservation techniques. Refrigerated foods that are heated only slightly or not at all or before consumption can be especially risky, because the pathogenic bacterium can multiply at refrigerated temperatures.
Listeria is therefore still claiming victims among consumers. Last summer, more than 20 Canadians died after eating processed meats contaminated with Listeria, and in 2006 more than 60 people became ill in the Netherlands due to Listeria, and nearly one-third of them died. Pregnant women, infants and the elderly are especially at risk.
‘The safety margins maintained by the industry in food processing are sufficient. The biggest problem is contamination after processing, therefore between the factory and the consumer. Listeria has many mechanisms to survive heat stress and also survives in the refrigerator’, explains Van der Veen. ‘For fresh products, good storage and complying with the 'use by' date are important. A properly cooked steak is almost never a source of Listeria infection. A much greater risk is posed by refrigerated products such as bread toppings, ready-to-eat sausages, soft cheeses and salads.’
Ir. Stijn van der Veen defended his PhD dissertation on 9 December. His supervisors were Prof. Tjakko Abee, Food Hygiene and Microbiology group, and Prof. Willem de Vos, Laboratory of Microbiology. / Albert Sikkema
The above article was written by the editorial staff of Resource, the weekly newspaper for Wageningen University and Research Centre. For more information, contact the press and science information officer of Wageningen UR, e-mail: pers.communicatie@wur or the editorial staff of Resource, e-mail: resource@wur.nl. See the archived articles at http://www.resource-online.nl