Useful Microorganism - Yeast

              what is meant by yeast ?

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Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized.

Yeast growth :

Yeasts are non-pathogenic and are very easy to work with4.  Yeasts can be grown in chemical-defined media both in liquid culture and on solid agar.  Both Saccharomyces cerevisiae and Schizosaccharomyces pombe can grow optimally at 30°C with an average doubling time of 90 – 120 minutes.  However, they also can grow at temperatures as low as 16°C and as high as 42°C.  In between usage, yeast can be stored in 10% glycerol at -80°C freezer for years.

The Structure of a Yeast Cell

The Role Of Yeast In Fermentation Process

2.1. Alcoholic Fermentations

The production of alcoholic beverages from fermentable carbon sources by yeast is the oldest and most economically important of all biotechnologies. Yeast plays a vital role in the production of all alcoholic beverages. Yeast plays a vital role in the production of all alcoholic beverages and the selection of suitable yeast strains is essential not only to maximise alcohol yield, but also to maintain beverage sensory quality [2].

2.1.1. Wine Fermentation 

In wine fermentation, strains with specific characteristics are needed, for instance, highly producers of ethanol to reach values of 11–13% v/v, typically found in this beverage. On the other hand, beers and ciders contain less amounts of ethanol with a balanced and distinctive sensory profile characteristic of each one. In recent years, new consuming trends and requirements for new and innovative products have emerged. This situation led to rethink about the existing fermented beverages and to meet the demands of consumers. Yeasts are largely responsible for the complexity and sensory quality of fermented beverages. Based on this, current studies are mainly focused on the search of new type of yeasts with technological application. Non-Saccharomyces yeasts have always been considered contaminants in the manufacture of wine and beer. Therefore, procedures for eliminating them are routinely utilized such as must pasteurization, addition of sulfite and sanitization of equipment and processing halls. In recent years, the negative perception about non-Saccharomyces yeasts has been changing due to the fact that several studies have shown that during spontaneous fermentations of wine, these yeasts play an important role in the definition of the sensory quality of the final product. Based on this evidence, the fermentative behavior of some non-Saccharomyces yeasts is being studied in deep with the purpose of finding the most adequate conditions and the most suitable strain to be utilized in the production of fermented beverages.

2.1.2 Beer Fermentation :

Beer is the most consumed alcoholic beverage worldwide. It is traditionally made from four key ingredients: malted cereals (barley or other), water, hops, and yeast. Each of these ingredients contributes to the final taste and aroma of beer. During fermentation, yeast cells convert cereal-derived sugars into ethanol and CO2. At the same time, hundreds of secondary metabolites that influence the aroma and taste of beer are produced. Variation in these metabolites across different yeast strains is what allows yeast to so uniquely influence beer flavor [9]. Although most breweries use pure yeast cultures for fermentation, spontaneous or mixed fermentation is nowadays used for some specialty beers. These fermentation procedures involve a mix of different yeast species (and bacteria as well) that contribute to the final product sequentially, giving the beer a high degree of complexity. Commonly, breweries have their own stock of selected yeasts for their specific beers. As it is well-known, two types of yeast are used in brewing: S. cerevisiae as the top-fermenting yeast to make ales while S. pastorianus is a bottom-fermenting yeast used in lager brewing processes [10].

2.2. Non-Alcoholic Fermentations

Moreover, yeast can act in the fermentation of global non-alcoholic products (bread, chocolate or coffee, beverages .

2.2.1. Bread Fermentation 

The fermentation of the dough made by the yeasts is the most critical phase in the making of bread. The fermentative yield of yeast cells during this fermentation is crucial and determines the final quality of the bread. Yeasts not only produce co2 and

other metabolites that influence the final appearance of the dough, volume, and texture, and of course, the taste of the bread. The yeast strain, pregrowth conditions, its activity during the dough fermentation process, the fermentation conditions, as well as the dough ingredients are basic to control the process. The fermentation rate is also conditioned by the ingredients of the dough, including the amounts of sugar and salt used in its preparation. Commercial bread producers currently produce various types of dough such as lean, sweet or frozen dough. Depending on the type of dough, and to obtain optimal fermentation rates, it is recommended to use suitable yeast strains with specific phenotype straits. 2

2.2.2 Coffee fermentation

Yeasts play an important role in coffee production, in the post-harvest phase. Its performance can be done in two phases. On the one hand, aerobically, in which the berries just collected are deposited in a tank and the yeasts are allowed to act. This process is carried out under control of basic parameters, such as time and temperature. Alternatively, coffee berries are deposited in a container mixed with water and microorganisms are allowed to act anaerobically (in the absence of oxygen). This second process is more homogeneous and easy to control than the aerobic. Sometimes, coffee beans are even fermented in a mixed process, first in an aerobic and finally anaerobic manner [16]. To develop these processes in a satisfactory manner, and to preserve/improve the organoleptic properties of coffee, refine its sweetness, control acidity, give them body or add sensory notes (chocolate, caramel, fruits) mucilage should be removed. The process is naturally carried out by the yeasts present in the mixture, although the process can be improved by the addition of appropriate enzymes (polygalacturonase, pectin lyase, pectin methylesterase) .

2.3. Not Only Food: Biofuels and Other Chemicals

The fermentation processes of substrates such as xylose are also of high interest on an industrial level. In addition to expanding the range of substrates that can be used for this purpose, they allow the environmental cost of efficient production of biofuels and other advanced chemicals to be reduced. Some interesting approaches have been made in biorefinery to reprogram yeast for use in these bioprocesses .