Bacteria enhance safety of artisanal cheese

Brazilian researchers have identified strains of lactic acid bacteria that inhibit the growth of listeria in soft Minas cheese and inactivate it in cured cheese as well as shortening maturation time.

By Elton Alisson  |  Agência FAPESP – Artisanal cheese produced from raw or pasteurized milk in Brazil, such as soft or cured Minas cheese, may be contaminated by disease-causing bacteria such as Listeria monocytogenes if it is not made under proper sanitary conditions.

Postdoctoral research conducted by Fernanda Bovo Campagnollo, a researcher at the University of Campinas’s Food Engineering School (FEA-UNICAMP) in Brazil, with a scholarship from FAPESP and supervision by Professor Anderson de Souza Sant’Ana, has resulted in a solution that can help cheesemakers guarantee the safety of their produce and comply with federal legislation on cheese maturation. 

The researchers selected strains of lactic acid bacteria that inhibited the growth of L. monocytogenes in soft Minas cheese. The strains also inactivated listeria in cured cheese and reduced ripening time. The results of the study were published in the journal Food Microbiology.

“The use of these lactic acid bacteria could help cheesemakers market safer produce and shorten the maturation period of cured cheese,” Campagnollo told Agência FAPESP.

L. monocytogenes can cause listeriosis, an infection with a high mortality rate. This bacterium easily multiplies in soft Minas cheese (known as Frescal) owing to the physicochemical characteristics of the cheese. 

“Frescal cheese is highly susceptible to contamination owing to its relatively high moisture content and low salt content,” Campagnolo said. “It isn’t immune to the proliferation of listeria even when kept refrigerated.” 

Cured Minas cheese contains less water and more salt, but the pathogen can nonetheless proliferate during the ripening period.

A federal law designed to assure the microbiological safety of cured cheese products sold in Brazil requires maturation for at least 60 days, but artisanal Minas cheese is traditionally matured for 17-22 days to maintain flavor and texture.

“This can lead to problems for cheesemakers who sell produce outside Minas Gerais and to a health hazard for consumers,” Campagnollo said.

In an endeavor to circumvent this difficulty, a new law allows the sale of cured artisanal cheese matured for less than 60 days provided that the producer proves by means of technical and scientific studies that the shorter ripening time does not jeopardize the product’s quality and safety. 

“Small cheesemakers, the main source of this type of produce, can’t afford to pay for these technical tests,” Campagnollo said.

Solution from cheese itself

The solution developed by the researchers at UNICAMP to address the risk of cured cheese contamination and reduce maturation time originated in artisanal cheese itself.

Studies published in Brazil in recent years have shown that lactic acid bacteria isolated from cheese produce bacteriocins – proteins or complexes of proteins, lactic acid and hydrogen peroxide that prevent the growth of L. monocytogenes and other disease-causing microorganisms.

However, there were no reports of the use of antilisterial bacteria in artisanal dairy produce such as soft and cured Minas made with raw or pasteurized milk. Although the legislation requires the use of pasteurized milk to make Frescal, many cheesemakers ignore this rule, preferring raw milk.

To test this hypothesis, the researchers collected samples of 244 artisanal cheeses made in five areas of Minas Gerais (Araxá, Campo das Vertentes, Canastra, Cerrado and Serro). From these samples, they isolated 891 strains of lactic acid bacteria derived from the starter culture, a fermented whey inoculum from previous cheese production that aids in coagulation and gives artisanal cheese its flavor, texture and aroma.

Laboratory experiments were performed to evaluate the antilisterial activity of these bacteria, and six strains of the species Lactobacillus brevis, L. plantarum and Enterococcus faecalis were selected.

These strains were added to cheese products made with raw or pasteurized milk and inoculated with L. monocytogenes. Soft cheese was stored for 15 days to ripen, and cured cheese was stored for 22 days. The researchers then investigated the extent to which the selected bacteria influenced the development of listeria and the maturation of cured cheese compared with those in samples of cheese without added bacteria.

The results of their microbiological analysis showed that the mixture of selected lactic acid bacteria inhibited the growth of L. monocytogenes in soft cheese and inactivated it in cured cheese.

The growth of L. monocytogenes in cured cheese without added bacteria during the 22-day ripening period was significant, whereas in samples with the added bacteria, no listeria cells were detected after 15 days in the case of cheese made with raw milk and after 21 days in the case of cheese made with pasteurized milk.

“The secret is the mixture of bacteria,” Sant’Ana said. “This particular combination stops the growth of the pathogen in soft cheese and inactivates it in cured cheese, as well as shortening maturation time, a key requirement for cheesemakers.”

The researchers plan to patent the mixture of bacteria and later license it to companies interested in producing it on an industrial scale for use as an ingredient in artisanal cheese.

The Food Microbiology article “Selection of indigenous lactic acid bacteria presenting anti-listerial activity, and their role in reducing the maturation period and assuring the safety of traditional Brazilian cheeses” (doi: 10.1016/j.fm.2018.02.006) by Campagnollo et al. can be retrieved from sciencedirect.com/science/article/pii/S0740002017310717.