The good, the bad and the bacteria

The lactic acid bacteria in cheese made from unpasteurised milk can reduce the growth of the harmful bacteria that may occur in these cheeses.

Norsk Gardsost and Nofima Mat are working on a project to find out more about the microorganisms in cheeses made from unpasteurised milk. They have now found bacterial cultures that can inhibit the growth of the pathogenic bacteria that can occur in these cheeses.

"We have collected cheeses from many parts of Europe and studied their ability to inhibit the growth of pathogenic bacteria," explains Hans Blom, senior consultant at Nofima Mat at Ås. The bacteria in question are Staphylococcus aureus, Listeria monocytogenes and E. coli O157. The cheeses were curdled and then the harmful bacteria were added.

"In 53 out of 170 samples, growth of the pathogenic bacteria was inhibited," concludes Blom with satisfaction.

Further studies on the most promising cheeses revealed two bacterial cultures with an inhibiting effect in milk. The bacterial cultures were tested in pilot production of cheese at the Norwegian University of Life Sciences (UMB) at Ås. During the cheese production, the three different pathogenic bacteria were also added. "The bacterial cultures we had identified reduced the growth of harmful bacteria in the cheese."

Cheeses were made with and without the addition of pathogenic bacteria and with and without the addition of the inhibiting culture. The mature cheeses were tasted by Nofima Mat's tasting panel. "There was no change in taste, aroma and texture, which is absolutely vital of course. The inhibiting culture cannot affect these parameters," says Blom. Now cheese producers around Europe are making cheese with the inhibiting culture to see if it has the same effect on their own cheeses.