Yeasts are eukaryotic microorganisms classified belonging to the kingdom fungi, with over 1,000 species currently known described. It is believed that the species described so far represent only about 1% of all 1.5 million yeast species believed to exist on earth (Prescott et al., 2002: Hutkins, 2006). Yeasts are unicellular fungus that reproduces either asexually by budding and transverse division (binary fission) or sexually through spore formation (Prescott et al., 2002). Although most yeast are unicellular some species with yeast forms may become multicellular through the formation of a string of connected budding cells known as pseudohyphae, or false hyphae, as seen in most moulds. Yeast size can vary greatly depending on the species, typically measuring 3–4 µm in diametre, although some yeast can reach over 40 µm.
The yeast species Saccharomyces cerevisiae has been used in baking and fermenting alcoholic beverages for thousands of years (Prescott et al., 2002; Legras et al., 2007). It is also extremely important as a model organism in modern cell biology research, and is one of the most thoroughly researched eukaryotic microorganisms. Researchers have used it to gather information about the biology of the eukaryotic cell and ultimately human biology. Other species of yeast, such as Candida albicans, are opportunistic pathogens and can cause infections in humans (Prescott et al., 2002). Yeasts have recently been used to generate electricity in microbial fuel cells, and produce ethanol for the biofuel industry().
The word "yeast" is derived from the Old English gist, gyst, and from the Indo-European root yes-, meaning boil, foam, or bubble. Yeast microbes are probably one of the earliest domesticated organisms. People have used yeast for fermentation and baking throughout history. Archaeologists digging in Egyptian ruins found early grinding stones and baking chambers for yeasted bread, as well as drawings of 4,000-year-old bakeries and breweries. In 1680 the Dutch naturalist Anton van Leeuwenhoek first microscopically observed yeast, but at the time did not consider them to be living organisms, but rather globular structures (Prescott et al., 2002).
By the late 1700s, two yeast strains used in brewing had been identified: Saccharomyces cerevisiae, so called "high" or top yeast, and Saccharomyces carlsbergensis, "low" or bottom yeast. High yeast was sold commercially by the Dutch for bread making starting in 1780, while around 1800; the Germans started producing S. cerevisiae in the form of cream. In 1825 a method was developed to remove the liquid so the yeast could be prepared as solid blocks. The industrial production of yeast blocks was enhanced by the introduction of the filter press in 1867. In latter part of 1800, a manufacturing process was developed to create granulated yeast, a technique that was used until the First World War In the United States, naturally occurring airborne yeasts were used almost exclusively until the middle of the 1800’s.
GROWTH AND NUTRITION
Yeasts are chemoorganotrophs as they use organic compounds as a source of energy and do not require sunlight to grow. Carbon is obtained mostly from hexose sugars such as glucose and fructose, or disaccharides such as sucrose and maltose. Some species can metabolize pentose sugars like ribose, alcohols, and organic acids. A relatively unique trait of yeasts that distinguishes them from most other life forms is that they are capable of both aerobic (obligate aerobes) and anaerobic (facultative anaerobes) cellular respiration. Unlike bacteria, there are no known yeast species that grow only anaerobically (obligate anaerobes). Yeasts grow best in a neutral or slightly acidic pH environment.
Yeasts vary in what temperature range they grow best. For example, Leucosporidium frigidum grows at -2 to 20 °C (28 to 68 °F), Saccharomyces telluris at 5 to 35 °C (41 to 95 °F) and Candida slooffi at 28 to 45 °C (82 to 113 °F). The cells can survive freezing under certain conditions, with viability decreasing over time.
Yeasts are generally grown in the laboratory on solid growth media or in liquid broths. Common media used for the cultivation of yeasts include potato dextrose agar (PDA) or potato dextrose broth. The antibiotic cycloheximide is sometimes added to yeast growth media to inhibit the growth of Saccharomyces yeasts and select for wild/indigenous yeast species.
Yeasts are very common in the environment, but are usually isolated from sugar-rich material. Examples include naturally occurring yeasts on the skins of fruits and berries (such as grapes, apples or peaches), and exudates from plants (such as plant saps or cacti and oil palm) (Chavan et al., 2009; Ocón et al., 2010). Some yeasts are found in association with soil and insects. The ecological function and biodiversity of yeast are relatively unknown compared to those of other microorganisms. Yeasts including Candida albicans, Rhodotorula rubra, Torulopsis and Trichosporon cutaneum have been found living in between people's toes as part of their skin flora. Yeasts are also present in the gut flora of mammals and some insects and even deep-sea environments host an array of yeasts.
An Indian study of seven bee species and 9 plant species found that 45 species from 16 genera colonise the nectaries of flowers and honey stomachs of bees. Most were members of the Candida genus; the most common species in honey stomachs was Dekkera intermedia and in flower nectaries, Candida blankii.
Yeasts have asexual and sexual reproductive cycles. The most common mode of vegetative growth in yeast is asexual reproduction by budding (Prescott et al., 2002). Yeast budding is initiated when the mother cells attain a certain critical cell size at a time coinciding with the onset of DNA synthesis. This is followed by a weakening of the cell wall and this, together with tension exerted by turgor pressure, allows extrusion of cytoplasm into an area bounded by new cell wall material. This then begins the process of forming a new or daughter cell. The new bud continues to grow until it separates from the parent cell, forming a new cell. After cell separation, the mother cell develops a bud scar at its surface (Prescott et al., 2002). Also, the new or daughter cell bears a birth scar at its surface. The number of bud scars left on the surface of a yeast cell is a useful determinant of cellular age. Hence, the yeast cell cycle can be defined as the period between division of a mother cell and subsequent division of its daughter progeny.
Though vegetative growth is the major way of yeast reproduction, sexual reproduction is an alternative when nutrient supplies fall short. Some yeasts, including Schizosaccharomyces pombe, reproduce by binary fission. Under conditions of stress haploid cells will generally die, however under the same conditions diploid cells can undergo sporulation, entering sexual reproduction (meiosis) and producing a variety of haploid spores, which can go on to mate (conjugate), reforming the diploid.
Fruits, vegetables, drinks and other agricultural products are very important microhabitats for a number of yeast species in nature. A succession of yeast populations is involved in a variety of biochemical and ecological processes due to the ability of yeast to quickly use the simple sugars present in these agricultural products (Stringini et al., 2008).
The preparation of many indigenous or traditional fermented beverages is still a traditional art in homes, villages and small-scale industries. These fermentations are carried out by yeasts, lactic acid bacteria and fungi, sometimes forming a complex microbiota acting in cooperation. Wild yeasts found in these fermentations mainly degrade carbohydrates, while bacteria possess a proteolytic activity (Osorio-Cadavid et al., 2008). Species of lactic acid bacteria (LAB) and yeasts have also been found in the fermentation of Kenyan Busa, Sout African Kaffir beer, Nigerian Ogi, Pito, Sekete, Colombian Champús and West African palmwine (Osorio-Cadavid et al., 2008: Omemu et al. 2007).
Apart from degraging sugars, wild yeasts have potential for producing extracellular enzymes of interest in some food fermentations. These enzymes are able to alter certain components of the subtrates and thus enhance the sensory attributes of such foods (Fernandez et al., 2000). For instance, pectinases, proteases, and glycosidases are some of the enzymes secreted by yeasts that are of interest in winemaking because of their technological effects and their contribution to aroma formation (Fleet et al., 2007).
Wild Yeasts in Brewery
Yeasts that are not deliberately used in the brewery and not under full control are designated as wild yeasts (Kuhle and Jespersen, 1998). Growth of wild yeasts during processing and in the packaged product may lead to defects, including the formation of phenolic, acidic, fatty acid and estery off-flavours, as well as hazes and turbidity (Kuhle and Jespersen, 1998). It is generally accepted that noticeable spoilage can occur at infection levels as low as one wild yeast per 105 –106 culture yeast (Kuhle and Jespersen, 1998). Detection of wild yeast infections often implies difficulties, because of biochemical and physiological similarities between the wild yeasts and the added yeast culture (Kuhle and Jespersen, 1998). Furthermore, the numbers of contaminant cells are normally vastly outnumbered by brewing yeast cells, thus requiring any diction system requires a high degree of sensitivit (Kuhle and Jespersen, 1998).
The diversity of wild yeasts in terms of beer spoilage means that no general description can be given, however, wild yeasts are traditionally divided into non-Saccharomyces and Saccharomyces wild yeasts (Kuhle and Jespersen, 1998). The non-Saccharomyces wild yeast have been reported to include such different genera as Brettanomyces, Candida, Debaryomyces, Filobasidium, Hanseniaspora, Kluyveromyces, Pichia, Torulaspora and Zygosaccharomyces (Kuhle and Jespersen, 1998). The majority of the Saccharomyces wild yeasts detected belong to Saccharromyces cerevisiae, but other Saccharomyces spp. have been reported also The Saccharomyces wild yeasts are often regarded as the most hazardous wild yeasts.
Kuhle and Jespersen (1998) found that the majority of the wild yeast isolates they investigated were capable of growth in wort and beer, indicating their possible role as spoilage organisms. The Saccharromyces cerevisiae isolates were found to be the most hazardous, with some isolates being capable of extensive growth in bottled beer within seventeen days at ambient temperature.
Producers of alcoholic beverages (especially wines and beers) worldwide are facing intensifying competition brought about by a widening gap between production and consumption trends. There is a shift of consumer preferences away from the basic product or commodity to a focus on the quality attributes, as well as the putative health benefits of such products. This has led to a revolution of sorts in the brewing and winemaking industries (Pretorius and Bauer, 2002). The gradual process of transforming the alcoholic beverage industries from a production-centred to a market-orientated industry is resulting in an increased dependence on, amongst others, biotechnological innovation (Cardona et al., 2007; Valles et al., 2007).
Specialised yeasts are being used in winemaking with excellent results in many countries. Usually, the final product is of better quality than those produced via traditional spontaneous fermentation (Heard and fleet, 1985; Constantí et. al., 1997; Regodon et al., 1997: Pretorius and Bauer, 2002). In the last few years, there has been an increasing use of new local selected yeasts for controlled must fermentation in countries with a wine making tradition (such as Spain, France and Italy). Though there are commercial yeasts to accomplish must fermentation, the use of local selected yeasts is believed to be much more effective (Regodon et al., 1997: Pretorius and Bauer, 2002: Orlic et al., 2007: Lee et al., 2010). Local yeasts are presumed to be more competitive because they are better acclimated to the environmental conditions (Regodon et al., 1997: Orlic et al., 2007). Therefore, they would be better able to dominate the fermentation and become the most important biological agent responsible for the vinification. However, spontaneous fermentations with indigenous yeasts are usually protracted and the outcome is highly unpredictable (Pretorius and Bauer, 2002). Also, Selection of the appropriate local yeasts assures the maintenance of the typical sensory properties of the wines produced in any given region.
There has been no simple and effective procedure for selecting yeasts for industrial use (Regodon et al., 1997). Selected yeasts should have certain technological characteristics that make them suitable for industrial production of beverages like wine and beer. The importance of each of these characteristics varies not only according to the kind of beverage but also according to the different opinions of the experts consulted (Regodon et al., 1997). Nevertheless, there is general agreement among experts that selected yeasts should show low production of volatile acidity, high tolerance to alcohol, ethanol production according to the quantity of sugar in the must, total fermentation of sugars, good fermentation speed, growth at high temperature, resistance to sulphur dioxide, low sulphur dioxide production, low hydrogen sulphide production, facilitate settling after fermentation, low foaming, killer phenotype, low acetaldehyde production, good glycerol production, invertase activities and limited production of higher alcohols (Regodon et al., 1997: Osho, 2005: Okunowo et al., 2005; Orlic et al., 2007). Unfortunately, measuring all these properties to select a yeast would be very time consuming and require sophisticated equipment and specially-skilled personnel (Regodon et al., 1997: Valles et al., 2007).
YEASTS IN ALCOHOLIC FERMENTATION
In the production of most alcoholic beverages fermentation is carried out by a succession of different yeast populations. Several authors report that the early stages of the alcoholic fermentation are characterized by the activity of mostly apiculate, non-Saccharomyces yeasts from Kloeckera and Hanseniaspora genera (Constantí et. al., 1997; Cocolin et al., 2000; Pina et al., 2004; Clemente-Jimenez et al., 2004; Ciani et al., 2007; Valles et al., 2007). The growth of these yeasts is generally limited to the first two or three days of fermentation, after which they die off, giving way to the more stress tolerant strains of Saccharomyces cerevisiae (Cocolin et al., 2000; Pina et al., 2004; Clemente-Jimenez et al., 2004; Cardona et al., 2007;Valles et al., 2007). Many physiological parameters allow Saccharomyces to dominate such fermentations, but it is the tolerance for high concentrations of ethanol that is the principal feature of this yeast that allows its survival in this environment (Pina et al., 2004; Valles et al., 2007).
The progressive disappearance of non-Saccharomyces fermentation species is generally attributed to their inability to survive the increasing concentrations of ethanol produced in the fermentation. The degree of persistence of these non-Saccharomyces fermentation species during fermentation may depend, however, upon many factors, such as the temperature of fermentation, nutrient availability, inoculum strength of Saccharomyces, use and levels of sulphur dioxide, the numbers and kinds of organisms present on the grapes and the vinification technology (Pina et al., 2004; Valles et al., 2007).
Fungistatic activity of ethanol initially affects the cell membrane, and is associated with its solubility in membrane lipids (Medawar et al., 2003). Anaerobic yeast cultures show cell walls deficient in sterols, phospolipids and polyunsaturated fatty acids. This dramatically reduced their resistance to ethanol (Medawar et al., 2003; Pina et al., 2004). On the one hand, the key function of sterols is regulation of the physical (gel/ liquid) state of the cell membrane. In fact, this state affects the activity of the transport enzymes. For instance, glucose as well as some amino acids (lysine, arginine) uptake can be reduced. Ethanol was also found to inhibit in vitro hexokinase and some amino-acid synthesis enzymes (Medawar et al., 2003). The resulting metabolic deficiencies can explain the mitosis inactivation (Medawar et al., 2003). On the other hand, the asymmetric distribution of unsaturated membrane lipids into cell walls can improve ethanol excretion. Thus, the most ethanol resistant strains are the richest in unsaturated lipids (Medawar et al., 2003).
Wild Yeasts in Some Fermented Foods
Palm Wine: - Palm wine is made from the fermented sap of tropical plants of the palmae family, such as the oil palm (Elaeis guineensis) (Onwuka and Awam, 2001). This beverage is produced and consumed in very large quantities in West Africa, and it is known throughout the major parts of Africa under various names, such as ‘mimbo’ in Cameroon, ‘nsafufuo’ in Ghana, an ‘emu’ in Nigeria. The sap of the palm tree is tapped and allowed to undergo spontaneous fermentation, which promotes the proliferation of yeast species for the conversion of the sweet substrate into an alcoholic beverage containing important nutritional components, including amino acids, proteins, vitamins and sugars. The palm sap is obtained from either the immature male inflorescence (inflorescence tapping) or from the stem (stem tapping). This is commonly practised in Nigeria, Benin and La Cote d’ Ivoire. However, in Cameroon and Ghana, the process of tapping palm wine involves first felling or cutting down the tree, leaving the felled tree for a period of about 2 weeks for the sap to concentrate, followed by tapping for up to 8 weeks (Stringini et al., 2009).
In this indigenous fermentation system, S. cerevisiae is often found co-existing with other microorganisms. Lactic acid bacteria (LAB) may play an important role in the fermentation and preservation of this beverage, while acetic acid bacteria (AAB) plays a role in the development of the vinegary in palmwine that has been stored for too long. LAB and AAB have been found at high levels, while the yeast species S. cerevisiae and Schizosaccharomyces pombe have been reported to be the dominant yeast species here. Other yeast species, however, have also been found, such as Kloeckera apiculata, Candida krusei, Hanseniaspora uvarum Candida parapsilopsis, Candida fermentati and Pichia fermentans (Stringini et al., 2009).
Ogi: - Ogi is produced by fermenting maize grains. It is a popular breakfast gruel and a complementary food for young children in several West African countries. It is made from maize, sorghum or millet (Omemu et al. 2007). The stages of traditional ogi production include steeping (washing the grains, soaking in water for 24–72 h, wet milling and wet sieving) and souring (sedimentation of the filterate for 12–48 h) to obtain sour ogi (Omemu et al. 2007). The microorganisms responsible for the fermentation of maize for ogi production play important role for aroma, microbial stability and flavour. Previous studies have concentrated mainly on the role of the lactic acid bacteria and their use as starter cultures in ogi production (Omemu et al. 2007). However, Omemu et al., (2007) isolated a number of yeast species in fermentation of maize for ogi production. The species isolated include: Saccharomyces cerevisiae, Candida krusei, C. tropicalis, Geotrichum candidum, G. fermentans and Rhodotorula graminis. All the isolates showed lipase and esterase activities, some were able to degrade phytate and a few showed amylase activities. A study of microbial interactions occurring showed that the growth of the yeast strains were enhanced during fermentation by the presence of the lactic acid bacteria, but the growth of the L. plantarum strain was significantly enhanced especially by the C. krusei.
Cider: - The fermentation of apple must is a complex microbial reaction involving the sequential development of various species of yeasts and bacteria (Valles et al., 2007). Among these microorganisms, yeasts are primarily responsible for alcoholic fermentation. Thus, the different yeast species developed during fermentation and their dynamics and frequency of appearance determine the taste and flavour characteristics of products (Valles et al., 2007).
In Asturias (Spain), natural cider is produced by the spontaneous fermentation of apple juice by yeasts originated from fruit and cider making equipment. This type of fermentation is of particular interest to ascertain the yeast species associated with the fermentation processes. Preliminary studies about the population dynamic have shown that the genus Saccharomyces is usually predominant during alcoholic fermentation, while the non-Saccharomyces genera, such as Kloeckera, Candida, Pichia, Hansenula, Hanseniaspora and Metschnikowia mainly grow during the first stages of the process (Valles et al., 2007). On the other hand, several factors such as geographic location, climatic conditions, apple varieties and the cider making technology can influence the diversity of yeasts present in must.
Ogol: - Honey wine, an indigenous fermented beverage, was invented thousands of years ago (Teramoto et al. 2005). Many kinds of honey wine, such as mead, metheglin, hydromel, aguamiel, and medovukha, have been made in various parts of Europe (Teramoto et al. 2005). In Africa, honey wine was also brewed traditionally in many areas. Honey wine generally containing 6 to 17% (v/v) ethanol is well known all over the world (Teramoto et al. 2005). Teramoto et al. (2005), in their work on ogol, isolated a species of Saccharomyces, closely resembling S. cerevisiae var meyen and the pellicle-forming yeast Pichia membranifaciens from ogol.
Cheese: - Pecorino Crotonese is a traditional cheese produced in a well-defined area of Southern Italy and particularly in Calabria Region (Gardini et al., 2006). According to the traditional protocol, it is produced from pasteurized ewe milk by using caprine rennet and natural whey starter cultures. It is ripened at least for 2 months, and can be commercialized after 1 year.
Cheese ripening is a complex phenomenon involving a wide range of biochemical reactions. High microbial counts are present in cheese throughout ripening and the composition of the microbial population has a significant role in the maturation process (Gardini et al., 2006).
Although the starter lactic acid bacteria (LAB) are responsible for acid production and contribute to the ripening process, an important contribution to cheese maturation is recognized to the secondary microbiota, mainly constituted by enterococci, micrococci, non-starter LAB and yeasts (Gardini et al., 2006). While Kluyveromyces lactis and Saccharomyces cerevisiae were isolated prevalently in the first stages of Pecorino Crotonese production, Yarrowia lipolytica and Debaryomyces hansenii dominated during the later stages of maturation.
Tempeh: - Tempeh is a traditional Indonesian food prepared by fermentation of dehulled and cooked soybeans with moulds of the genus Rhizopus (mainly Rhizopus oligosporus) to a compact and sliceable cake (Feng et al., 2007). Tempeh can also be produced from cereal grains, e.g. barley, through fermentation with R. oligosporus (Feng et al., 2007).
Yeasts can grow spontaneously in soybean tempeh (Feng et al., 2007). However, their role in the tempeh fermentation is not well understood. Many yeast species, e.g. Trichosporon beigelii, Clavispora (Candida) lusitaniae, Candida maltosa, Candida intermedia, Yarrowia lipolytica, Lodderomyces elongisporus, Rhodotorula mucilaginosa, Candida sake, Hansenula fabiani, Candida tropicalis, Candida parapsilosis, Pichia membranaefaciens, Rhodotorula rubra, Candida rugosa, Candida curvata, Hansenula anomola) are found in soybean tempeh (Feng et al., 2007). Among them, T. beigelii was the most frequently detected yeast species.
Fermented foods are food substrates that are invaded or overgrown by edible microorganisms whose enzymes, particularly amylases, proteases and lipases hydrolyze the polysaccharides, proteins and lipids to non-toxic products with flavours, aromas and textures pleasant and attractive to the human consumer. If the products of enzyme activities have unpleasant odours or undesirable, unattractive flavours or the products are toxic or disease producing, the foods are described as spoiled. Food fermentations can be classified in a number of ways: by categories (e.g. alcoholic beverages fermented by yeasts), by classes (e.g. dairy products), by commodity (e.g. fermented starchy roots or legumes), by functional basis (e.g. sauces and relishes for the staples) or by type of fermentation (e.g. Alkaline fermentations or Alcoholic fermentations) (Steinkraus, 1997).
Food fermentations become an effective means of food processing for a number of reasons. The first is that, food substrates overgrown with desirable, edible microorganisms become resistant to invasion by spoilage or toxic or food poisoning microorganisms. Other, less desirable, possibly disease-producing organisms find it difficult to compete.
In most societies, fermented beverages and foods have a unique place because of their economical and cultural importance and the development of fermentation technologies is deeply rooted in their history. Archaeologists have found evidence for the production of a fermented beverage in China at 7000 BC (Legras et al., 2007), and of wine in Iran and Egypt at 6000 BC and 3000 BC, respectively (Legras et al., 2007). Since that time, it is believed that these fermentation technologies expanded from Mesopotamia through the world. For example, the cultivation of grapevine and the production of wine has spread all over the Mediterranean Sea towards Greece (2000 BC ), Italy (1000 BC ), Northern Europe (100 AD ) and America (1500 AD ) (Legras et al., 2007). Beer technology is supposed to be almost as ancient as wine and was acquired from the Middle East by Germanic and Celtic tribes around 1st century AD, whereas lager beer technology appeared more recently in the 16th century (Legras et al., 2007).
In addition, the question of the natural environment for Saccharomyces cerevisiae is has been controversial for some time now (Ocón et al., 2010). Because strain isolation from nature or plants is rare (Legras et al., 2007), it is currently agreed that the yeasts participating in fermentation may come from two possible sources: the grapes or winery surfaces and equipment (Heard and fleet, 1985; Constantí et. al., 1997; Ocón et al., 2010).
Bread strains displayed a combination of alleles intermediate between beer and wine strains, and strains used for rice wine and sake were most closely related to beer and bread strains (Legras et al., 2007).
BENEFITS OF FERMENTATION
Enrichment with protein
In the Indonesian tape ketan fermentation, rice starch is hydrolyzed to maltose and glucose and fermented to ethyl alcohol. The loss of starch solids results in a doubling of the protein content (Steinkraus, 1997). Thus, this process provides a means by which the protein content of high starch substrates can be increased for the benefit of consumers needing higher protein intakes. This is particularly important for people consuming principally cassava which has a protein content of about 1% (wet basis). In addition, the flavour of the cassava becomes sweet/sour and alcoholic, flavours consumers may prefer to the bland starting substrate.
Bio-enrichment with essential amino acids
The Indonesian tape ketan/tape ketella fermentation not only enriches the substrate with protein, the microorganisms also selectively enrich the rice substrate with lysine, the first essential limiting amino acid in rice. This improves the protein quality. An increase in methionine, a limiting essential amino acid in legumes, greatly improves the protein value.
Bio-enrichment with vitamins
In the wealthy, Western world, nutrients, particularly vitamins, are added to selected, formulated, manufactured foods as a public health measure. Examples are addition of vitamin D to milk, vitamin A and D to milk and butter and riboflavin to bread. Fruit juices may be fortified with ascorbic acid (vitamin C). While fortification is within the means of the Western world, it is far beyond the means of the developing world. Thus, much of the world must depend upon biological enrichment via fermentation to enrich their foods (Steinkraus, 1997).
Mexican pulque is the oldest alcoholic beverage on the American continent. It is produced by fermentation of juices of the cactus plant (Agave) (Steinkraus, 1997). Pulque is rich in thiamine, riboflavin, niacin, pantothenic acid, p-amino benzoic acid, pyridoxine and biotin.
Kaftir beer is an alcoholic beverage with a pleasantly sour taste and the consistency of a thin gruel (Steinkraus, 1997). It is a traditional beverage of the Bantu people of South Africa. Alcohol content ranges from 1 to 8% v/v. Kaffir beer is generally made from kaffircorn (Sorghum cafiorum) malt and unmalted kaffircorn meal. During the fermentation, thiamine remains about the same, but riboflavin more than doubles and niacin/nicotinic acid nearly doubles, which is very significant in people consuming principally maize (Steinkraus, 1997). Consumers of usual amounts of kaffir beer are not in danger of developing pellagra.
Palm sap is a sweet, clear, colourless liquid containing about l0-12% fermentable sugar and is neutral in reaction (Steinkraus, 1997). Palm wine is a heavy, milk-white opalescent suspension of live yeasts and bacteria with a sweet taste and vigorous effervescence. Palm wines are consumed throughout the tropics. Palm wine contains as much as 83 mg ascorbic acid/l (Steinkraus, 1997). Thiamine increases from 25 to 150 µg/l, riboflavin increases from 35 to 50 µg/l and pyridoxine increases from 4 to18 µg/l during fermentation. Surprisingly, palm wine contains considerable amounts of vitamin B-12. Palm toddies play an important role in nutrition among the economically disadvantaged in the tropics. They are the cheapest sources of B vitamins.
METABOLIC BY-PRODUCTS OF YEAST FERMENTATION
Glycerol is quantitatively the most important fermentation product after ethanol and carbon dioxide. Although this compound has no direct impact on aromatic characteristics, glycerol has a favourable effect on quality of alcoholic beverages like wine. Sweetness is the main contribution of glycerol to sensory characteristics at levels commonly found in wines Michnick et al., 1997: Remize et al., 1999). Saccharomyces cerevisiae yeast strains producing large amounts of glycerol would therefore be of considerable value in improving wine quality (Michnick et al., 1997). Moreover, the overproduction of glycerol at the expense of ethanol might present an opportunity for developing beverages with low ethanol contents. This would be an alternative to physical techniques for alcohol removal that do not always conserve the organoleptic characteristics of the product (Michnick et al., 1997).
Glycerol is involved in osmotic cell regulation (Michnick e al., 1997: Remize et al., 1999). During alcoholic fermentation, the main role of glycerol formation is to equilibrate the intracellular redox balance (Remize et al., 1999) by converting the excess NADH generated during biomass formation to NAD+. Its formation requires the reduction of dihydroxyacetone phosphate to glycerol-3-phosphate (G-3-P), a reaction catalyzed by G-3-P dehydrogenase (GPDH) and followed by the dephosphorylation of G-3-P to glycerol by glycerol-3-phosphatase (Michnick et al., 1997: Remize et al., 1999).
Many growth and environmental factors have been reported to influence the amount of glycerol produced by yeast in alcoholic beverages, some of which include, sulphite concentration, pH, fermentation temperature, aeration, inoculation level, grape variety and ripeness, and nitrogen composition (Remize et al., 1999).
The dominant and major compounds contributing to aroma are formed during yeast fermentation (Estévez et. al., 2004). These compounds are higher alcohols, fatty acids, acetates, ethyl esters, ketones and aldehydes (Estévez et. al., 2004). Several studies have demonstrated that fermentation conditions (skin contact time, temperature, yeast, etc.) affect the final aromatic composition (Estévez et. al., 2004). For this reason, those yeasts that have the best fermentative aroma and that produce the most desirable taste (such as esters or acetates, which produce fruity nuances) as well as reducing production of others (such as the higher alcohols or volatile phenols) that contribute negatively to the wine’s final aroma, are being actively sought and selected.
Several studies have shown that yeast strains have a great impact on the chemical complexity of wines (Kotseridis and Baumes, 2000;Murat et. al., 2001 Rojas et al., 2001: Ooi et al., 2008) and that the volatile composition of wines could be an alternative method for characterising yeast strains used to produce the wine (Ooi et al., 2008). Some authors used differences in the wine levels of acetoin, 2, 3-butanediol, or acetic acid (Ooi et al., 2008) as the basis for studying genetic variability among Saccharomyces cerevisiae strains and attempt to improve wine qualities or fermentation properties in the yeast by crossing different strains. In their work, Ooi et al., (2008) reached the conclusion that, the flavour and aroma composition of the wines produced by the different S. cerevisiae strains using the same substrate and fermentation conditions was shown to be highly dependent upon the yeast strains. They further concluded that, the wine composition rather than the genetic characteristics of yeasts may be useful for distinguishing the different strains within a yeast population if all other fermentation conditions are kept constant.
It is also known that yeasts have the ability to amplify grape aromas, during fermentation, in grape varieties known as “simple-flavored,” whose musts do not have very intense aromas. This amplification effect of the grape aroma by yeast has been clearly demonstrated for Sauvignon blanc (Murat et. al., 2001: Swiegers et al., 2009).
The production of secondary aroma or spoilage compounds by yeast can be significantly affected by winemaking practices such as clarification of grape musts, aeration, yeast strain, addition of yeast nutrients to fermenting grape musts, and fermentation temperature (Marks et al., 2003). The nitrogen composition of grape musts affects fermentation kinetics, the production of aroma and spoilage compounds, and the amount of urea, the major precursor of the carcinogen ethyl carbamate, present in wine.
During fermentation the yeast Saccharomyces cerevisiae produces a broad range of aroma-active substances, which are vital for the complex flavour of fermented beverages such as beer, wine and sake (Fleet et al., 2007). Flavour-active substances produced by fermenting yeast cells can be divided into six main groups: organic acids, higher alcohols, carbonyl compounds, sulfur-containing molecules, phenolic compounds and volatile esters (Saerens et al., 2010). Although volatile esters are only trace compounds in fermented beverages, they comprise the most important set of yeast-derived aroma-active compounds (Saerens et al., 2010). Volatile esters are of major industrial interest because they are responsible for the highly desired fruity, candy and perfume-like aroma character of beer, wine and sake (Saerens et al., 2010).
Flavour is the most important distinguishing characteristic of most fermented foods. Flavour is usually classified according to the sources of the different compounds contributing to it. This includes flavour contributed by the substrate, pre-fermentative flavour (compounds formed during extraction and conditioning of substrate), fermentative flavour (produced by yeast and bacteria during alcoholic and malolactic fermentation) and post-fermentative flavour (compounds that appear during the ageing or storage period) (Kotseridis and Baumes, 2000; Orlic et al., 2007; Fleet et al., 2003Marks et al., 2003). Post-fermentative flavour is usually a result of enzymatic or physicochemical changes during storage. The formation of volatile compounds during the fermentation is a complex phenomenon involving a number of factors. In particular, it depends on the nature and concentration of the compounds initially present in the substrate, the capacity of the yeast to transform them and the fermentation conditions employed.
In the sensory evaluation of dry and semidry wines Orlic et al., (2007) demonstrated the influence of different indigenous strains of Saccharomyces paradoxus on final wine quality.
Also, non-Saccharomyces yeasts species such as Hanseniaspora, Candida, Pichia and Metschnikowia present in the initial stages of the fermentation are reported to have an influence on the final organoleptic properties of the wine, as they are the main producers of some fermentation compounds, such as acetic acid, glycerol and esters (Lee et al., 2010: Rojas et al., 2001; Ciani et al., 2007). Several studies have also shown their capabilities to contribute positively to wine flavour (Lee et al., 2010). Strain biodiversity exists in the non-Saccharomyces wine yeasts with regard to their production levels of enzymatic activities (Lee et al., 2010) and fermentation metabolites (Lee et al., 2010) that give rise to the unique oenological characteristics of each wine-producing zone.
Amines have an important metabolic role in living cells. Polyamines are essential for growth; other amines like histamine and tyramine are involved in nervous system functions and the control of blood pressure. Biogenic amines are undesirable in all foods and beverages because if absorbed at too high a concentration, they may induce headaches, respiratory distress, heart palpitation, hyper or hypotension, and several allergenic disorders (Lonvaud-Funel, 2001). Histamine is the most toxic and its effect can be potentiated by other amines. But human sensitivity varies with the individual detoxifying activities of human body.
Moreover some enzymes involved in biogenic amine metabolism, such as histamine methyltransferase, are specific; others such as monoaminoxidase and diaminoxidases are less specific. However these enzymes are inhibited by several types of drugs, by ethanol and even by other food amines, lowering the efficiency of detoxification. Therefore when considering the toxic effects of biogenic amines, the quantity of food, the concentration of total biogenic amines, and the consumption of ethanol and drugs must also be taken into account (Lonvaud-Funel, 2001).
Biogenic amines are produced by lactic acid bacteria during the process of fermentation of foods and beverages by amino acid decarboxylation, for example in cheese, sausage, fermented vegetables and wine. Many bacterial genera are able to decarboxylate amino acids. This reaction is thought to favor growth and survival in acidic media, since it induces an increase in pH (Lonvaud-Funel, 2001). In wine, several amino acids can be decarboxylated; as a result histamine, tyramine, putrescine, cadaverin and phenylethylamine are usually found, the first three being the most frequent.
TRADITIONAL USES OF YEASTS
The useful physiological properties of yeast have led to their use in the field of biotechnology. Fermentation of sugars by yeast is the oldest and largest application of this technology. Many types of yeasts are used for making many foods: baker's yeast in bread production; brewer's yeast in beer fermentation; yeast in wine fermentation and for xylitol production. So-called red rice yeast is actually a mould, Monascus purpureus. Yeasts include some of the most widely used model organisms for genetics and cell biology.
Alcoholic beverages are defined as beverages that contain ethanol (C2H5OH). This ethanol is almost always produced by fermentation – the metabolism of carbohydrates by certain species of yeast under anaerobic or low-oxygen conditions. Beverages such as wine, beer, or distilled spirits all use yeast at some stage of their production. A distilled beverage is a beverage that contains ethanol that has been purified by distillation. Carbohydrate-containing plant material is fermented by yeast, producing a dilute solution of ethanol in the process. Spirits such as whiskey and rum are prepared by distilling these dilute solutions of ethanol. Components other than ethanol are collected in the condensate, including water, esters, and other alcohols which account for the flavour of the beverage.
Brewing yeasts may be classed as "top cropping" (or "top fermenting") and "bottom cropping" (or "bottom-fermenting"). Top cropping yeasts are so called because they form a foam at the top of the wort during fermentation. An example of a top cropping yeast is Saccharomyces cerevisiae, sometimes called an "ale yeast". Bottom cropping yeasts are typically used to produce lager-type beers, though they can also produce ale-type beers. These yeasts ferment more sugars, creating a dryer beer, and grow well at low temperatures. An example of bottom cropping yeast is Saccharomyces pastorianus, formerly known as Saccharomyces carlsbergensis.
The most common top cropping brewer's yeast, Saccharomyces cerevisiae, is the same species as the common baking yeast. However, baking and brewing yeasts typically belong to different strains, cultivated to favour different characteristics: baking yeast strains are more aggressive, in order to carbonate dough in the shortest amount of time possible; brewing yeast strains act slower, but tend to produce fewer off-flavours and tolerate higher alcohol concentrations (with some strains, up to 22%).
Brettanomyces is a genus of wild yeast important in brewing lambic, a beer produced not by the deliberate addition of brewer's yeasts, but by spontaneous fermentation by wild yeasts and bacteria. Brettanomyces lambicus, B. bruxellensis and B. claussenii are native to the Senne Valley region of Belgium, where lambic beer is produced.
Wine is a natural product resulting from a number of biochemical reactions, which begin during ripening of the grapes and continue during harvesting, throughout the alcoholic fermentation, clarification and after bottling (Romano et. al., 2003). Many of these reactions are left to nature and microorganisms present on the grapes and in the winery environment (Heard and fleet, 1985; Ocón et al., 2010). Under the generic term of ‘‘wine’’, there is a diversity of quality which makes each type of wine unique and this diversity is determined mainly by interaction between grapes, yeasts and technology. Traditionally, yeasts are employed in winemaking mainly to convert the sugars present in grape juice or must into alcohol and carbon dioxide (Onwuka and Awam, 2001). Apart from this primary role yeasts contribute significantly to the development of other aspects of the wine, among them are: the bouquet or aroma and the flavour (Romano et. al., 2003; Combina et al., 2005).
Historically, wine was a product of only grapes and one species, Vitis vinifera was used exclusively. Tropical fruit wine fermentation is thought to have started somewhere around began in 1951(Lee et al., 2010). Today, wine can be produced from different fruits other than grapes under carefully controlled methods of production. Fruits like mango, papaya (Lee et al., 2010), guava (Srivastava et. al., 1997) and banana (Onwuka and Awam, 2001) have all been employed in making fruit wines. Wines produced from grapes are regarded as wine, while those from other fruits are simply referred to as fruit wines and designated with the name of the fruit.
The production of many fermented fruit based alcoholic beverages, can be regarded as two stage processes. Typically, a Saccharomyces cerevisiae fermentation mainly provides an alcoholic base for the wine, which after attentuation, is racked into a storage vat where an indigenous mixed microflora can play a significant secondary role in dictating the finished product's individual character (Scott and O'Reilly, 1996). In general, maturation of wine, a process of significant commercial impact and value, remains uncontrolled through a reliance on development and activity of a relatively ill-defined and uncoordinated microflora.
Pasteur first proposed that wine yeasts were present on the surface of grapes. Many studies since then have both confirmed and failed to confirm this original observation (Mortimer and Polsinelli, 1999). Most researchers today agree that, wine yeasts (S. cerevisiae) are rarely isolated from natural surfaces, including grapes and vineyard soil’, and that ‘natural fermentation of musts is carried out mainly by winery resident flora’ (Heard and fleet, 1985; Mortimer and Polsinelli, 1999; Ocón et al., 2010). Generally, fermentations carried out with endogenous wild yeast; give unpredictable results depending on the exact types of yeast species present. For this reason a pure yeast culture is generally added to the must, which rapidly comes to dominate the fermentation. This represses wild yeasts and ensures a reliable and predictable fermentation. Most added wine yeasts are strains of Saccharomyces cerevisiae, though not all strains of the species are suitable. Different S. cerevisiae yeast strains have differing physiological and fermentative properties, therefore the actual strain of yeast selected can have a direct impact on the finished wine. Significant research has been undertaken into the development of novel wine yeast strains that produce atypical flavour profiles or increased complexity in wines.
The growth of some yeasts such as Zygosaccharomyces and Brettanomyces in wine can result in wine faults and subsequent spoilage. Brettanomyces produces an array of metabolites when growing in wine, some of which are volatile phenolic compounds. Together these compounds are often referred to as "Brettanomyces character", and are often described as antiseptic or "barnyard" type aromas. Brettanomyces is a significant contributor to wine faults within the wine industry.
Yeast, most commonly Saccharomyces cerevisiae, is used in baking as a leavening agent, where it converts the fermentable sugars present in dough into the gas carbon dioxide. This causes the dough to expand or rise as gas forms pockets or bubbles. When the dough is baked, the yeast dies and the air pockets "set", giving the baked product a soft and spongy texture. The use of potatoes, water from potato boiling, eggs, or sugar in a bread dough accelerates the growth of yeasts. Most yeasts used in baking are of the same species as those common in alcoholic fermentation. Additionally, Saccharomyces exiguus (also known as S. minor) is a wild yeast found on plants, fruits, and grains that is occasionally used for baking. Sugar and vinegar are the best conditions for yeast to ferment. In bread making the yeast initially respires aerobically, producing carbon dioxide and water. When the oxygen is depleted anaerobic respiration begins, producing ethanol as a waste product; however, this evaporates during baking.
It is not known when yeast was first used to bake bread. The first records that show this use came from Ancient Egypt. Researchers speculate that a mixture of flour meal and water was left longer than usual on a warm day and the yeasts that occur in natural contaminants of the flour caused it to ferment before baking. The resulting bread would have been lighter and tastier than the normal flat, hard cake.
Today there are several retailers of baker's yeast; one of the best-known in North America is Fleischmann’s Yeast, which was developed in 1868. During World War II Fleischmann's developed a granulated active dry yeast, which did not require refrigeration and had a longer shelf life than fresh yeast. The company created yeast that would rise twice as fast, reducing baking time. Baker's yeast is also sold as a fresh yeast compressed into a square "cake". This form perishes quickly, and must therefore be used soon after production. A weak solution of water and sugar can be used to determine if yeast is expired. In the solution, active yeast will foam and bubble as it ferments the sugar into ethanol and carbon dioxide. Some recipes refer to this as proofing the yeast as it "proves" [tests] the viability of the yeast before the other ingredients are added. When using a sourdough starter, flour and water are added instead of sugar; this is referred to as proofing the sponge.
When yeast is used for making bread, it is mixed with flour, salt, and warm water or milk. The dough is kneaded until it is smooth, and then left to rise, sometimes until it has doubled in size. Some bread doughs are knocked back after one rising and left to rise again. A longer rising time gives a better flavour, but the yeast can fail to raise the bread in the final stages if it is left for too long initially. The dough is then shaped into loaves, left to rise until it is the correct size, and then baked. Dried yeast is usually specified for use in a bread machine, however a (wet) sourdough starter can also work.
NOVEL APPLICATIONS OF YEAST TECHNOLOGY
Some yeasts can find potential application in the field of bioremediation (Brady et. al., 1994; Simmons et al., 1995; kaszycki et al., 2004; Seki et al., 2005). One such yeast, Yarrowia lipolytica, is known to degrade palm oil mill effluent, TNT (an explosive material), and other hydrocarbons such as alkanes, fatty acids, fats and oils. Some yeasts can also tolerate high concentrations of salt and heavy metals, and are being investigated for their potential to remove heavy metals from industrial effluents (Brady et. al., 1994; Simmons et al., 1995; kaszycki et al., 2004). The mechanisms involved may be metabolism-dependent or independent and both living and dead (denatured) yeast biomass is capable of metal uptake.
Botrytis cinerea is a well-known fungus with a wide host range that causes heavy losses of yield in onion, potato, strawberry, table grapes and the wine industry (Santos et al., 2004). Chemical control of Botrytis has been partially successful. However, the risk of appearance and establishment of resistance is considerable. Biocontrol, non-hazardous alternative to the chemical fungicides, consists of the use of biological processes to reduce crop loss. Killer yeasts secrete proteinaceous killer toxins lethal to susceptible yeast and fungi strains (Santos et al., 2004). Killer yeasts were first found in brewing strains of Saccharomyces cerevisiae and since then have been shown to occur in a large number of yeast isolates of environmental, clinical, industrial and agronomic interest (Santos et al., 2004). Killer yeasts have been studied with a view toward exploitation for potential applications (Santos et al., 2004). Killer yeasts have also been considered useful in biological control of undesirable yeasts in the preservation of foods (Santos et al., 2004). There are few reports in the literature of inhibitory effects of killer yeasts against fungi. Some workers have shown that certain killer yeasts are inhibitory to some wood-decay and plant pathogenic fungi (Santos et al., 2004). This has raised the possibility of using killer toxins as novel biocontrol agents against fungi of environmental and agronomical significance. Several yeast species have already been shown to be effective biological control agents in protecting plants against fungal diseases (Santos et al., 2004; Fleet et al., 2007).
Industrial bioethanol production
The production and utilization of bioethanol has attracted worldwide attention as a strategy for reducing global warming and improving global energy security (Tian et al., 2009). The ability of yeast to convert sugar into ethanol has been harnessed by the biotechnology industry to produce ethanol fuel. The process, traditionally, starts by milling a feedstock, such as sugar cane, field corn, or cheap cereal grains, and then adding dilute sulphuric acid, or fungal alpha amylase enzymes, to break down the starches into complex sugars. A gluco-amylase is then added to break the complex sugars down into simple sugars (Kłosowski et al., 2006). After this, yeasts are added to convert the simple sugars to ethanol, which is then distilled off to obtain ethanol up to 96% in concentration.
Today however, there is a growing call for feed stocks used in bioethanol production to be derived from inedible parts of food crops, among others, in order to avoid direct competition between bioethanol and the population’s food supply (Tian et al., 2009). Bioethanol from lignocellulose, such as wood, straw and switchgrass does not compete with food production, other than through the conversion of land from food production to cellulose production. Also, Lignocellulosic plant residue contains up to 70% carbohydrates (this occurs as cellulose and hemicellulose). However, due to the close association of cellulose and hemicellulose with lignin in the plant cell wall, pretreatment is necessary to make these carbohydrates available for enzymatic hydrolysis and fermentation.
With respect to sugar utilization, Saccharomyces cerevisiae efficiently converts both glucose and mannose into ethanol, but is unable to convert xylose into ethanol (Tian et al., 2009). Saccharomyces yeasts have been genetically engineered to ferment xylose, one of the major fermentable sugars present in cellulosic biomasses, such as agriculture residues, paper wastes, and wood chips (Ho et al., 1998: Brat et al., 2009). Such a development means that ethanol can be efficiently produced from more inexpensive feedstocks, making cellulosic ethanol fuel a more competitively priced alternative to gasoline fuels.
Root beer and other sweet carbonated beverages can be produced using the same methods as beer, except that fermentation is stopped sooner, producing carbon dioxide, but only trace amounts of alcohol, and a significant amount of sugar is left in the drink. Yeast in symbiosis with acetic acid bacteria is used in the preparation of Kombucha, a fermented sweetened tea. Species of yeast found in the tea can vary, and may include: Brettanomyces bruxellensis, Candida stellata, Schizosaccharomyces pombe, Torulaspora delbrueckii and Zygosaccharomyces bailii. Kefir (Lopitz-otsoa, et al., 2006) and kumis are made by fermenting milk with yeast and bacteria.
Single Cell Protein (SCP) Production
Yeast extract is the common name for various forms of processed yeast products that are used as food additives or flavours. They are often used in the same way that monosodium glutamate (MSG) is used, and like MSG, often contain free glutamic acid. The general method for making yeast extract for food products such as Vegemite and Marmite on a commercial scale is to add salt to a suspension of yeast making the solution hypertonic, which leads to the cells shrivelling up. This triggers autolysis, where the yeast's digestive enzymes break their own proteins down into simpler compounds, a process of self-destruction. The dying yeast cells are then heated to complete their breakdown, after which the husks (yeast with thick cell walls which would give poor texture) are separated. Yeast autolysates are used in Vegemite and Promite (Australia); Marmite, Bovril and Oxo (the United Kingdom, Republic of Ireland and South Africa); and Cenovis (Switzerland).
Yeast is used in nutritional supplements popular with vegans and the health conscious, where it is often referred to as "nutritional yeast". It is a deactivated yeast, usually Saccharomyces cerevisiae. It is an excellent source of protein and vitamins, especially the B-complex vitamins, whose functions are related to metabolism as well as other minerals and cofactors required for growth. It is also naturally low in fat and sodium. Some brands of nutritional yeast, though not all, are fortified with vitamin B12, which is produced separately by bacteria. Nutritional yeast, though it has a similar appearance to brewer's yeast, is very different and has a very different taste.
Probiotics are foods that contain microoganisms which are beneficial to health. A probiotic is a live microbial food supplement used in fermented dairy products and cheeses that beneficial affects the host animal by improving the microbial balance. However, based on recent advances of research in this field, the following revised definition has been proposed: “Probiotics are microbial cell preparations or components of microbial cells that have a beneficial effect on the health and well being of the host” (Lopitz-otsoa, et al., 2006). A probiotic improves one of three main functions (colonization resistance, immunomodulation or nutritional contribution) of normal gastrointestinal microbiota when ingested by human or animal hosts (Lopitz-otsoa, et al., 2006; Fleet et al., 2007). Two important criteria are used for the selection of probiotic microorganisms: they must be able to survive in the gastrointestinal environment and they must present at least on beneficial function.
Some probiotic supplements use the yeast Saccharomyces boulardii to maintain and restore the natural flora in the large and small gastrointestinal tract. S. boulardii has been shown to reduce the symptoms of acute diarrhoea in children, prevent reinfection of Clostridium difficile, reduce bowel movements in diarrhoea predominant Irritable Bowel Syndrome (IBS) patients, and reduce the incidence of antibiotic, traveller’s, and HIV/AIDS associated diarrhoeas.
Yeast is often used by aquarium hobbyists to generate carbon dioxide (CO2) to fertilize plants in planted aquariums. A homemade setup is widely used as a cheap and simple alternative to pressurized CO2 systems. While not as effective as these, the homemade setup is considerably cheaper for less demanding hobbyists.
There are several recipes for homemade CO2, but they are variations of the basic recipe: Baking yeast, with sugar, baking soda and water are added to a plastic bottle. A few drops of vegetable oil at the start reduces surface tension and speeds the release of CO2. This will produce CO2 for about 2 or 3 weeks; the use of a bubble counter determines production. The CO2 is injected in the aquarium via a narrow hose and released through a CO2 diffuser that helps dissolve the gas in the water. The CO2 is used by plants in the photosynthesis process. CO2 injection is very important to plant growth in planted aquariums.
THE ECONOMICS OF YEASTS PRODUCTION
In recent years, cell immobilization techniques have become increasingly important and are being successfully applied in industrial processes such as the production of alcohols (ethanol, butanol and isopropanol), organic acids (including malic, citric, lactic and gluconic acids), enzymes (cellulase, amylase, lipase and others) and biotransformation of steroids for hormone production, wastewater treatment, and food applications (beer and wine) (Reddy et al., 2008). Cell immobilization in alcoholic fermentation is a rapidly expanding research area because of its beneficial technical and economic advantages compared to the conventional free cell system
Several yeasts, particularly Saccharomyces cerevisiae, have been widely used in genetics and cell biology. This is largely because S. cerevisiae is a simple eukaryotic cell, serving as a model for all eukaryotes including humans for the study of fundamental cellular processes such as the cell cycle, DNA replication, recombination, cell division and metabolism. Also yeasts are easily manipulated and cultured in the lab which has allowed for the development of powerful standard techniques, such as Yeast two-hybrid, Synthetic genetic array analysis and tetrad analysis. Many proteins important in human biology were first discovered by studying their homologues in yeast; these proteins include cell cycle proteins, signaling proteins, and protein-processing enzymes.
On 24 April 1996 S. cerevisiae was announced to be the first eukaryote to have its genome, consisting of 12 million base pairs, fully sequenced as part of the Genome project. At the time it was the most complex organism to have its full genome sequenced and took 7 years and the involvement of more than 100 laboratories to accomplish. The second yeast species to have its genome sequenced was Schizosaccharomyces pombe, which was completed in 2002. It was the sixth eukaryotic genome sequenced and consists of 13.8 million base pairs.
Cryptococcus neoformans is a significant pathogen of immuno-compromised people causing the disease termed cryptococcosis. This disease occurs in about 7–9% of AIDS patients in the USA, and a slightly smaller percentage (3–6%) in western Europe. The cells of the yeast are surrounded by a rigid polysaccharide capsule, which helps to prevent them from being recognised and engulfed by white blood cells in the human body.
Yeasts of the Candida genus are another group of opportunistic pathogens which causes oral and vaginal infections in humans, known as candidiasis. Candida is commonly found as a commensal yeast in the mucus membranes of humans and other warm-blooded animals. However, sometimes these same strains can become pathogenic. Here the yeast cells sprout a hyphal outgrowth, which locally penetrates the mucosal membrane, causing irritation and shedding of the tissues. The pathogenic yeasts of candidiasis in probable descending order of virulence for humans are: C. albicans, C. tropicalis, C. stellatoidea, C. glabrata, C. krusei, C. parapsilosis, C. guilliermondii, C. viswanathii, C. lusitaniae and Rhodotorula mucilaginosa. Candida glabrata is the second most common Candida pathogen after C. albicans, causing infections of the urogenital tract, and of the bloodstream (candidemia).
Yeasts are able to grow in foods with a low pH, (5.0 or lower) and in the presence of sugars, organic acids and other easily metabolized carbon sources. During their growth, yeasts metabolize some food components and produce metabolic end products. This causes the physical, chemical, and sensible properties of a food to change, and the food is spoiled. The growth of yeast within food products is often seen on their surface, as in cheeses or meats, or by the fermentation of sugars in beverages, such as juices, and semi-liquid products, such as syrups and jams. The yeast of the Zygosaccharomyces genus have had a long history as a spoilage yeast within the food industry. This is mainly due to the fact that these species can grow in the presence of high sucrose, ethanol, acetic acid, sorbic acid, benzoic acid, and sulphur dioxide concentrations, representing some of the commonly used food preservation methods. Methylene blue is used to test for the presence of live yeast cells.
Yeasts play a central role in the spoilage of foods and beverages, mainly those with high acidity and reduced water activity (aw). A few species are capable of spoiling foods produced according to good manufacturing practices (GMPs). These can survive and grow under stress conditions where other microorganisms are not competitive. However, many of the aspects determining yeast spoilage have yet to be clarified (Loureiro and Malfeito-Ferreira, 2003; Cardona et al., 2007).
In many cases, microbial spoilage is not easily defined, particularly in fermented foods and beverages, where the metabolites produced contribute to the flavour, aroma, and taste of the final products. In fact, for cultural or ethnic reasons, there is little difference between what is perceived as spoilage or beneficial activity (Loureiro and Malfeito-Ferreira, 2003). An example of this can be found in the wine industry, where the production of 4-ethylphenol by Brettanomyces/Dekkera spp. in red wines is only regarded as spoilage when this secondary metabolite is present at levels higher than about 620 Ag/ l (Loureiro and Malfeito-Ferreira, 2003). At less than 400 Ag/l, it contributes favourably to the complexity of wine aroma by imparting aromatic notes of spices, leather, smoke, or game, appreciated by most consumers. Above 620 Ag/l, the wines are clearly substandard for some consumers, but remain pleasant for others. One of the most recent handbook of yeast taxonomy describes the characteristics of 761 species (Loureiro and Malfeito-Ferreira, 2003). Of these, about a quarter may be isolated from foods, but only a handful plays a significant role in food spoilage. Those that can survive in foods but are not able to grow and, for that reason, do not affect the sensory appeal of the food may be termed adventitious or innocent; those responsible for undesirable changes are called spoilage yeasts. However, for food technologists, the concept of spoilage yeast has, in general, a stricter sense. It applies only when a particular species is able to spoil foods which have been processed and packaged according to the standards of good manufacturing practices (GMPs) (Loureiro and Malfeito-Ferreira, 2003), in spite of the subjective character of these practices. If this is not achieved, many other adventitious yeast contaminants can develop in a product
Moreover, yeasts play a central role in wine spoilage. Non-Saccharomyces wine yeast species have traditionally been associated with high volatile acidity, off-flavors and wine spoilage. The major spoilage organisms include species and strains of the yeast genera Brettanomyces, Candida, Hanseniaspora, Pichia and Zygosaccharomyces (Ocón et al., 2010)
The microbial spoilage of milk is generally associated with the growth of bacteria (fleet et al., 1996). Very little consideration has been given to the ability of yeasts to grow in milk (fleet et al., 1996). However, a range of observations indicate an ability of yeasts to metabolise milk constituents. These observations include the occurrence and growth of yeasts in many cheeses, especially soft mould-ripened cheeses (fleet et al., 1996), the spoilage of condensed milks and yoghurts by yeasts (fleet et al., 1996), isolated incidences of yeast spoilage of pasteurised milks (fleet et al., 1996) and previous studies showed the potential of yeast species to exhibit significant growth in UHT processed milk.
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