Microbes in Fermented Foods

Do you know that fermented foods are manufactured by exploiting the metabolic activities of microorganisms? As a matter of fact, they are able to transform raw materials (e.g milk) in final products (e.g. cheese) that have completely different taste, smell and consistency. Fermented foods are an excellent example of how man was able to exploit microorganisms to prolong the shelf life of certain very perishable raw materials (e.g fresh meat) through their fermentation (e.g fermented sausage). As you have learnt, fermentation is a very ancient process, and its combination with certain technological procedures, like drying and salting, allowed for the production of safer and more stable foods.

Fermentation can be defined as the microbial transformation of sugars to organic acids, mainly lactic acid, or ethanol. In the first case we talk about lactic fermentation, while in the second case we consider alcoholic fermentation. The first one is extremely important in a number of well-known foods we usually consume, such as dairy products (yoghurt, kefir, cheese), fermented vegetables (olives, sauerkrauts, pickles), fermented meats (sausages), sourdoughs and even cocoa and coffee. The second one is responsible for the production of alcoholic beverages, such as wine and beer, but it is essential also in the production of bread and other bakery products.

Fermentative microorganisms belong mainly to two distinct groups: lactic acid bacteria (LAB) and yeasts. The first ones are bacteria able to transform sugars into lactic acid, and they are differentiated in homofermentative and heterofermentative, based on their ability to produce only lactic acid, or also acetic acid, ethanol and carbon dioxide, from the fermentation of the sugars. Different genera of LAB are important in food fermentation: the most known are Lactococcus, Streptococcus, Pediococcus, Enterococcus, Leuconostoc and Lactobacillus. Yeasts are a more heterogenous group of microorganisms, but Saccharomyces cerevisiae is the one involved in alcoholic fermentation for the production of wine, beer and bread. In this case, sugars are fermented and together with alcohol, carbon dioxide is produced, which is very important for the leavening of bread, the presence of bubbles in sparkling wines and the foam formation in beer.

Apart from their fermentative abilities, LAB and yeasts are also able to contribute to the final taste, smell and flavour of the fermented food through the production of hydrolytic enzymes such as proteases and lipases. In the first case, proteases degrade proteins (such as caseins in milk and myoglobin in meat) releasing peptides and amino acids, which are precursors of aromas; lipases are able to release fatty acids from triglycerides, being then oxidized to compounds contributing to smell and taste.

In the last decade, fermented foods have attracted a lot of attention being considered healthy due to the fact that they contain high numbers of bacteria, known to be beneficial for the human health. It is important to underline, though, that fermented foods should not be considered probiotic foods, because, usually, they do not contain probiotic bacteria. As you will learn later this week, the term probiotic can be used only in case of scientifically demonstrated benefits for the human health, and in the case of fermented foods, those benefits are not demonstrated yet.