25 Jun 2010 --- In response to consumer demand for more natural food, the food industry has reduced the amount of preservatives in food over recent years. A common preservative is acetic acid, which is used to stop bacterial growth in dressings, sauces, cheese and pickles.

However, new research shows that a small amount of acetic acid does not have the intended effect, but rather the opposite -- it increases the amount of toxin from the harmful bacteria in the food.

"In my studies I saw that a small amount of acetic acid caused the bacteria to become stressed, which meant they reacted by producing more toxin. However, if a large amount of acetic acid is added, as was done in the past, the acidity is greatly increased and the bacteria do not survive," explains Nina Wallin Carlquist, Doctor of Philosophy in Engineering at the Division of Applied Microbiology, Lund University.

She recently defended a thesis on the subject, in which she studied two of the most common food poisoning bacteria, Staphylococcus aureus and Campylobacter jejuni.

The Staphylococcus were used in the acetic acid study. A common vehicle for staphylococcal food poisoning is pork meat. Therefore Nina Wallin Carlquist also chose to study how these bacteria behave in different types of pork meat at room temperature: boiled and smoked ham, Serrano ham and salami. The bacteria could get into the food in the first place from an infected cut on the finger of the person who has handled the meat, for example.

Her results show that it only took a few hours for the bacteria to multiply in the boiled and smoked ham. In the Serrano ham, it took a week before the number of bacteria increased and on the salami they did not survive at all.

"A possible explanation is that the bacteria could not survive the salami's combination of acidity, salt, fat and dryness. However, there are other bacteria that thrive on salami. The Serrano ham is manufactured and stored at room temperature over long periods, which means it is important that the staff have good hygiene so that the Staphylococcus cannot get a foothold," comments Nina Wallin Carlquist.

A starting point was to study how the bacteria behave in food. This type of research is otherwise usually carried out in a controlled environment in laboratories where a pure culture of a certain type of bacteria is studied.

According to Nina Wallin Carlquist this provides far from the whole picture because the bacteria are affected by other micro-organisms in the food and also by how much fat, acid and salt the food contains.

"If we know more about what it is in the food that enables the bacteria to thrive, we can then adapt the composition of the food product and thereby improve food safety. This is a new way to approach food safety," explains Nina Wallin Carlquist.

The other bacterium, Campylobacter jejuni, is becoming the next big problem after salmonella. Like salmonella, the bacteria occur naturally in chicken, without harming the host animal.

However, if the contents of the intestines come into contact with the meat during slaughter, the meat can become infected. If the chicken is then not properly cooked the consumer may suffer food poisoning.

"It would be best if the chickens did not get infected with these bacteria to begin with. In my studies I have therefore found out how the bacteria become established in the intestines. In the long term, these results could help in the drawing up of guidelines for hygiene procedures on poultry farms or in developing a vaccine for the animals," says Nina Wallin Carlquist.