Kris, Seattle, US
In all cultures, fermented products are a very important part of the diet. These products are obtained through the modification of different substrates, both of animal and vegetal origin, by diverse microorganisms, mainly lactic acid bacteria and yeasts.
Even though they are both present in multiple fermented foods and beverages of different origin, lactic acid bacteria are mostly known for their use in the manufacture of dairy products such as yoghurt and cheese, while yeasts are worldly known for their contribution to the production of alcoholic beverages, mainly wine and beer.
Yeasts constitute a big group of eukaryotic microorganisms that are divided into two main phyla, Ascomycetes and Basidiomycetes. They have very heterogeneous properties that vary among genera, and are widely distributed in the environment.
Yeasts are commonly present in numerous food products, both as contaminants that can cause food spoilage and as part of the starter culture used specifically to obtain the product in which they are present.
As the latter, they have been employed by mankind for centuries in the production of diverse fermented foods and beverages, giving place to the fact that almost all civilizations have developed traditional fermented products based on the ingredients typical from their regions. For example, wine, beer and bread are worldwide consumed fermented products known to be produced already in Assyria in 3500 BC, Sumeria before 7000 BC, and Rome in 100 BC, respectively.
Yeasts In Fermented Food Products And Beverages
Nowadays, yeasts are still widely used in the production of these traditional fermented products, where they usually appear in combination with other microorganisms, like lactic acid bacteria or mycelial fungi. For example in Europe, yeasts have been traditionally used in the production of sour milk, cream, certain varieties of cheeses, mead, cider, beer and wine.
In the Asian region, many yeast-based traditional food products can also be found, such as ragi in Indonesia, murcha/ marcha in India and Nepal, loong-pang in Thailand, men in Vietnam and soy sauces in Japan and China.
Also in this region, many beverages are traditionally produced using yeasts as a starter culture such as kao-mark in Thailand, sake in Japan, tapuy in the Philippines and palm wines in India, Bangladesh, Sri Lanka, Thailand, Malaysia, Indonesia and the Philippines.
These fermented foods and beverages have been traditionally produced because of the desirable properties they have, especially regarding food quality parameters such as flavour, aroma, texture and nutritive value. Moreover, during the fermentation process pre-digested nutrients are liberated into the product, increasing the quality of the product and making the original substrate easier to digest.
Fermentation has been used as a preservation method as well because the microorganisms responsible for the fermentation usually produce antimicrobial compounds such as bacteriocins by lactic acid bacteria and mycocins by yeasts, which inhibit the growth of pathogens and spoilage microorganisms, thus increasing the product shelf life.
Consequently, as some of these traditional products are currently manufactured on an industrial scale, yeasts are also widely used nowadays in the food industry for applications such as alcoholic fermentation for the production of beverages like wine, beer, kefir or koumiss and the production of foods like bread, table olives and some cheeses ripened with certain yeast species.
Furthermore, yeasts are also used by the biotechnology industry for production of recombinant proteins, synthesis of vitamins and additives, as well as a source of colorants and antioxidants.
Fermented Beverages, The Way To Health
Extensive research has been done to prove that fermented beverages provide health benefits to the host through different mechanisms, such as the intake of more digestible nutrients and bioactive metabolites not present in the nonfermented product, as a result of a partial degradation of the initial components and the direct administration of beneficial or probiotic microorganisms.
These effects have been associated especially to fermented milks, but also to non-dairy fermented foods and beverages, such as wine, which have been proved to improve health. Different anti-pathological specific activities have been associated with fermented beverages, the most important being an enhancement of the gastrointestinal health and an improvement of the immune system response.
Some studies have demonstrated that fermented beverages have also anti-hypertensive, anti-carcinogenic and anti-inflammatory activities. In the particular case of red wine, cardioprotective effects have been reported as well.
These healthy properties in many occasions have been associated to the antioxidant effects that some of the metabolites isolated from yeast-based fermented beverages and yeast extracts have shown during both in vitro and in vivo assays.
Many studies have pointed that many pathological processes (e.g. rheumatoid arthritis, osteoarthritis, cardiovascular disease and some tumours) are caused by an oxidative damage, which is usually produced by free radicals, especially reactive oxygen species (ROS).
Hence, the reduction of the causes of this oxidative damage by adding antioxidants to the diet can help prevent the development of these diseases. The mechanism of antioxidant activity is unknown, but it is believed to be due to the reduction, i.e. donation of a proton (H+), to the free radical, thus neutralising its effect.
The antioxidant activity of yeasts and yeast extracts has been recognised for some time, as they synthesise many bioactive compounds which can act as antioxidants such as torulahoidin, citric acid, coenzyme Q, glutathione, various forms of tocopherols, riboflavin (vitamin B2), yeast-derived cell wall β-glucans, cytochrome C, etc.
These substances help food preservation by retarding oxidative degeneration, and many can be found in yeastfermented products as they are secreted by the cell, or liberated after cell death. Of these compounds found in yeast-based fermented products, special attention has been paid to the study of the antioxidant capacity of cell wall β-glucans and proteins.
Antioxidant Activity In Wine-Producing Yeasts
The use of multifunctional microorganisms in food production, like starter cultures with probiotic activity, has been increasing in the last years. Therefore, the evaluation of additional functional properties in these microorganisms, like antioxidant activity, is gaining interest.
But regardless of their multiple applications in food industry, the probiotic or health beneficial potential of yeasts is usually overlooked, as probiotic research is frequently focused on lactic acid bacteria isolated from dairy products.
In a study by the Institute of Research in Food Science and Technology CIAL, researchers measured and compared were the antioxidant activities of 39 preselected strains of Saccharomyces cerevisiae, the main yeast species responsible for alcoholic grape and apple must fermentation during production of wine and cider, as well as other alcoholic beverages.
As antioxidant activity is an interesting additional feature of probiotic microorganisms, the strains were preselected by their capacity of surviving the gastrointestinal transit before being analysed, as they are administered orally and probiotics need to be viable to exert their beneficial effects.
Briefly, to measure the antioxidant activity of the yeast strains, a suspension of viable cells is mixed with a reagent consistent of a free radical (1,1-diphenyl-2-picrylhydrazyl; [DPPH]) dissolved in methanol. After reduction by yeast cells, the colour changes from purple to yellow. This colour change is proportional to the amount of radical that is reduced and is measured by absorbance.
This quantification was then utilised to obtain a percentage as well as Trolox equivalents, by comparison with the activity of a solution of Trolox at a known concentration. Trolox is a chemical compound analogue to vitamin E, with a high antioxidant activity.
The study revealed that all the strains tested reduced DPPH in different proportions, and in general, the levels registered were high—61.5 percent of the yeasts analysed rendered antioxidant activities values between 30-40 percent. Furthermore, one of the strains (S. cerevisiae IFI-279) rendered an excellent antioxidant activity, with a percentage of free radical scavenging above 50 percent.
The analysis of the antioxidant activity of the different yeast strains both in percentage and as Trolox equivalents made possible the comparison of and conclusion of a similar consensus for these results with those obtained in other studies for yeasts and lactic acid bacteria.
As discussed before, the antioxidant activity of yeast is believed to be mainly due to the high content of β-glucans found in their cell wall, as well as proteins and enzymes, but it has been reported that viable cells exhibit higher antioxidant activities than cell extracts, thereby suggesting that other mechanisms, dependent on the viability of the cells, may be involved.
The study showed promising results as it demonstrated the high capacity of viable yeasts to act as natural antioxidants. Traditionally, fermented products were obtained by the action of the yeasts naturally occurring in the substrate. Nowadays fermentation is initiated by the inoculation of the must or other substrate with a controlled starter culture.
Taking this into account, these results can be used to select the most adequate yeast strain as starter culture for each industrial process, in order to enhance the antioxidant activity of the final product and obtain beverages with improved quality and functionality.