Microbiological data


Pathogenic micro-organisms

A particularly wide variety of micro-organisms are found on and in food. The WHO have counted more than 250 types of food poisoning. Two types of bacteria often cited in recent cases are salmonella and listeria.

a) Salmonella

Salmonella is widespread in nature and, even though it is present throughout the animal kingdom, it affects mainly the bird population. It is found in the digestive tracts of humans and animals, and contaminates the outside world through excrement. These specific pathogenic bacteria cause illness when contaminated food or water is absorbed (fecal-oral type contamination). Salmonella is a major cause of infant mortality in developing countries and still a risk in industrialized nations.
Salmonella usually causes food poisoning as a result of eating infected food such as raw or inadequately cooked eggs, mayonnaise, cream cakes, pâté or meat. Eggs and egg products are most often found to be the culprits. The ingestion of salmonella-infested food can lead to a colonization of the intestinal mucous when the inoculum overcomes the defensive capacities of the digestive tract, i.e. enough for a minimum infectious dose generally in the order of 105. Signs of infection are frequent vomiting and diarrhoea that usually recede in two to five days. The infection could however be more severe in infants, the immune deficient or the elderly.

 

b) Listeria monocytogenes

Telluric bacteria are very common throughout the environment, in soil, vegetation, fresh and salt water, etc. and are particularly resistant to outdoor conditions (several years at +4°C). In food, dairy products are often highly contaminated with listeria monocytogenes (45% of unpasteurized milk contamination), as well as cheese made from unpasteurized or pasteurized milk.
 

Adequate pasteurization destroys listeria. Contamination after pasteurization is attributable to poor hygiene during maturing. Listeria monocytogenes can also be found in meat products (41% of frozen minced meat, 31% of raw meat and 60% of smoked fish). These bacteria grow at temperatures as low as 4°C, resulting in problems for long-term food preservation. Occurrences of infection from listeria depend on a combination of factors : particularly virulent strains, strong inoculum, host immunity level. Infection occurs as a result of ingestion.

 

Listeriosis is common to both humans and animals, mainly affecting :
- Pregnant women. It is benign for the mother but can bring on miscarriages and premature births of infected children with precocious septicaemia.
- The elderly or immune deficient, where cases of listeriosis are 300 times more likely than normal.

 

Using micro-organisms in food

Some bacteria are beneficial in the preparation of food. Before Louis Pasteur no one knew why grape juice could become wine or water and flour could produce bread, but man has known how to make them for thousands of years. Pasteur, in the course of his work on fermentation, showed that micro-organisms were responsible. Observing fermented substances under a microscope, he saw how cells grow and multiply on the surface of sour milk. He added samples to fresh milk, triggering lactic fermentation. He gradually came to demonstrate that milk, grape juice and moist flour only ferment if they are in contact with airborne micro-organisms. Without oxygen to feed them, they decompose or partially degrade organic molecules, the most common of which is glucose. Fermentation is therefore partial degradation, and the remaining product, e.g. ethyl alcohol or lactic acid, changes the taste and the texture of the food.

a) Yoghurt

Since 1908, microbiologists have known about the benefits of lactic bacteria. The Spanish pharmacist Daniel Carasso (hence the brand name Danone) wondered if our daily food might not be improved with this fermented milk that helps digestion. In 1929 he developed the first industrially prepared yoghurt. Of all fermented foods, this is the simplest: boiled or sterilized milk and bacteria are mixed in a pot at the correct temperature.

b) Cheese making

Roquefort, for instance, is made from ewe's milk heated to 32°C and curdled using rennet, a mixture of two enzymes (one being pepsin), which occurs naturally in the stomach of young calves. This changes the milk on which it feeds in order to digest it. During this stage, microscopic fungi called penicillium roqueforti, from the mould family, are added to the milk.

The cheese is then strained and matured in cellars which have a microclimate ideally suited to the development of the fungi, i.e. a constant temperature of 8°C and high levels of humidity. The maturing Roquefort is spiked with holes through which carbon dioxide resulting from fermentation can escape.
The spread of this micro-organism is so rapid that it has to be checked in order to preserve some of the cheese... The wheels of Roquefort are wrapped in tinfoil to stem the flow of oxygen. Deprived of oxygen, the germs multiply at a much slower pace.

c) Yeast

The properties of yeast are due to natural fermentation. Discovered and used since ancient times, it was only between 1857 and 1863 that Louis Pasteur demonstrated the role of yeast as being the micro-organism responsible for fermentation. Consumers are mainly familiar with two types of yeast:

 
- Baker's yeast, a microscopic fungus (saccharomyces cerevisiae), is used mainly in bread making. In the presence of oxygen, yeast transforms sugars into water and carbon dioxide. When yeast is added to gluten-rich flour, the gas is trapped in the gluten and causes the dough to rise.
- Baking powder, a white powder usually based on a sodium carbonate derivative, reacts on contact with water and heat to release carbon dioxide, which raises the dough. But this "yeast" has nothing to do with fermentation and does not contain micro-organisms. Baking powder is not suitable for bread making because it generates too little gas to obtain a light, bread-like texture.