Yeasts are a growth form of eukaryotic microorganisms classified in the kingdom Fungi, with approximately 1,500 species described.[1] Most reproduce asexually by budding, although a few do by binary fission. Yeasts are unicellular, although some species with yeast forms may become multicellular
through the formation of a string of connected budding cells known as pseudohyphae, or
true hyphae as seen in most molds.[2] Yeast size can vary greatly depending on the
species, typically measuring 3–4 µm in diameter, although some yeasts can reach over 40 µm.[3]
The yeast species Saccharomyces cerevisiae has been used in
baking and fermenting alcoholic beverages for thousands of years. It is also extremely important as a model organism in modern cell biology research,
and is the most thoroughly researched eukaryotic microorganism. Researchers can use it to gather information into the biology of
the eukaryotic cell and ultimately human biology.[4] Other species of yeast, such as Candida
albicans, are opportunistic pathogens and can cause infection in humans. Yeasts have recently been used to generate electricity in microbial fuel cells,[5] and produce ethanol for the biofuel industry.
Yeasts do not form a specific taxonomic or phylogenetic grouping. At present it is estimated that only 1% of all yeast species have been
described.[6] The term "yeast" is
often taken as a synonym for S. cerevisiae,[7] however the phylogenetic diversity of yeasts is shown by their placement
in both divisions Ascomycota and Basidiomycota. The budding yeasts ("true yeasts") are classified in the order Saccharomycetales.[8]
History
- See also: History of wine and
History of beer
The word "yeast" comes from the Old English language "gist",
"gyst", ultimately from the Indo-European root "yes-",
meaning boil, foam, or bubble.[9]
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.[10] 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.[11] In 1857 French microbiologist
Louis Pasteur proved in the paper "Mémoire sur la fermentation alcoolique" that
alcoholic fermentation was conducted by living yeasts and not by a chemical catalyst.[10][12] Pasteur showed that by bubbling oxygen into the yeast broth, cell growth could be increased,
but the fermentation inhibited - an observation later called the Pasteur
effect.
Growth and nutrition
Yeasts are chemoorganotrophs as they use organic
compounds as a source of energy and do not require light to grow. The main source of carbon is obtained by
hexose sugars such as glucose, or disaccharides such as
sucrose and maltose. Some species can metabolize
pentose sugars such as fructose, alcohols, and organic acids. Yeast species either
require oxygen for aerobic cellular respiration (obligate aerobes), or are anaerobic but also have aerobic methods of energy production
(facultative anaerobes). Unlike bacteria, there are no known yeast species that grow only anaerobically (obligate anaerobes). Also, because they are adapted to them, yeasts grow best in a neutral pH
environment.
Yeasts are ubiquitous in the environment, but are most frequently isolated from sugar-rich samples. Some good examples include
fruits and berries (such as grapes, apples or peaches), and exudates from plants (such as plant saps or cacti). Some yeasts are found in association
with soil and insects.[13][14] Yeast are generally grown in the laboratory on solid growth media or liquid broths. Common media used for the cultivation of
yeasts include; potato dextrose agar (PDA) or potato dextrose broth, Wallerstien
Laboratories Nutrient agar (WLN), Yeast Peptone Dextrose agar
(YPD), and Yeast Mould agar or broth (YM). The antibiotic cycloheximide is sometimes added to yeast growth media to inhibit the growth of Saccharomyces
yeasts and select for wild/indigenous yeast species.
Reproduction
The yeast life cycle.
1. Budding
2. Conjugation
3. Spore
- See also: Mating of yeast
Yeasts have asexual and sexual
reproductive cycles; however the most common mode of vegetative growth in yeast is asexual
reproduction by budding or fission.[15] Here a small bud, or daughter
cell, is formed on the parent cell. The nucleus of the parent cell splits into a daughter
nucleus and migrates into the daughter cell. The bud continues to grow until it separates from the parent cell, forming a new
cell.[16] The bud can develop on different parts of the
parent cell depending on the genus of the yeast.
Under high stress conditions 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.[17]
Yeast of the species Schizosaccharomyces pombe reproduce by
binary fission instead of budding.[15]
Uses
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[18] production. Yeasts are also one of the most widely used
model organisms for genetics and cell biology.
Alcoholic beverages
Alcoholic beverages are loosely defined as a beverage that contains ethanol (CH3CH2OH). This ethanol is
almost always produced by fermentation - the metabolism of carbohydrates by certain species of yeast. Beverages such
as wine, beer, or distilled spirits all use yeast at some stage of their production.
Beer
Beer brewers classify yeasts as top-fermenting and
bottom-fermenting. This distinction was introduced by the Dane Emil Christian Hansen. Top-fermenting yeasts are so-called because they form a foam at the
top of the wort during fermentation. They can produce higher alcohol concentrations and prefer higher temperatures, producing fruitier ale-type
beers. An example of a top-fermenting yeast is Saccharomyces cerevisiae,
known to brewers as ale yeast. Bottom-fermenting yeasts are used to produce lager-type beers. These yeasts ferment more sugars, leaving a crisper taste, and grow well at low
temperatures. An example of a bottom-fermenting yeast is Saccharomyces
pastorianus.
For both types, yeast is fully distributed through the beer while it is fermenting, and both equally flocculate (clump together and precipitate to the bottom of the vessel) when it is finished. By no means do
all top-fermenting yeasts demonstrate this behaviour, but it features strongly in many English ale yeasts which may also exhibit
chain forming (the failure of budded cells to break from the mother cell) which is technically different from true
flocculation.
Lambic, a style of Belgian beer, is fermented spontaneously
by wild yeasts primarily of the genus Brettanomyces.
Fermenting tanks with yeast being used to
brew beer.
In industrial brewing, to ensure purity of strain, a 'clean' sample of the yeast is stored refrigerated in a laboratory. After
a certain number of fermentation cycles, a full scale propagation is produced from this laboratory sample. Typically, it is grown up in about three or four
stages using sterile brewing wort and oxygen.
Root Beer and Sodas
Root Beer and Sodas can be produced using the same methods as Beer only the carbonation process created by the active yeast is
stopped sooner producing only trace amounts of alcohol (consumable by all ages) and a significant amount of sugar is left in the
drink.
Distilled beverages
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 flavor of
the beverage.
Wine
Grapes covered in yeast growth observable as a white film, also known as the "
blush".
Yeast is used in winemaking where it converts the sugars present in grape juice or must into alcohol. Yeast is
normally already present on the grapes, often visible as a powdery film (also known as the bloom or blush) on their
exterior. The fermentation can be done with this indigenous (or wild) yeast;[19] however, this may give unpredictable results depending on the exact types of
yeast species that are present. For this reason a pure yeast culture is generally added to the must, which rapidly predominates
the fermentation as it proceeds. This represses the wild yeasts and ensures a reliable and predictable fermentation.[20] Most added wine yeasts are strains of
Saccharomyces cerevisiae, however not all strains of the species are
suitable.[20] 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.[21] Significant research has been undertaken into the develoment of novel wine yeast strains
that produce atypical flavour profiles or increased complexity in wines.[22][23]
The growth of some yeasts such as Zygosaccharomyces and Brettanomyces in wine can result in wine faults and subsequent
spoilage.[24] 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.[25]
Baking
Bread showing pockets left by carbon dioxide.
Yeast, specifically Saccharomyces cerevisiae, is used in
baking as a leavening agent, where it converts the
fermentable sugars present in the dough into carbon dioxide. This causes the dough to expand or rise as the
carbon dioxide forms pockets or bubbles. When the dough is baked it "sets" and the pockets
remain, 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. Salt and fats such as
butter slow down yeast growth. The majority of the yeast used in baking is of the same species
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
It is not known when yeast was first used to bake bread. The first records that show this use came from Ancient Egypt.[26] 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 more tasty than the normal flat, hard cake.
Active dried yeast, a granulated form in which yeast is commercially sold.
Today there are several retailers of baker's yeast; one of the best-known 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, cutting down
on baking time. Baker's yeast is also sold as a fresh yeast compressed into a square "cake". This form perishes quickly, and must
be used soon after production in order to maintain viability. A weak solution of
water and sugar can be used to determine if yeast is expired. When dissolved in the
solution, active yeast will foam and bubble as it ferments the sugar into ethanol and carbon dioxide.
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 always used for bread made in a bread machine.
Bioremediation
Some yeasts can find potential application in the field of bioremediation. One such
yeast Yarrowia lipolytica is known to degrade palm oil
mill effluent,[27] TNT (an explosive material),[28] and other hydrocarbons such as
alkanes, fatty acids, fats and
oils.[29]
Industrial ethanol production
The ability of yeast to convert sugar into ethanol has been harnessed by the biotechnology industry, which has various uses including ethanol
fuel. The process starts by milling a feedstock, such as sugar cane, sweetcorn, or cheap cereal
grains, and then adding dilute sulfuric 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. 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.[30]
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.[31] 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.[32]
Kombucha
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.[33]
Nutritional supplements
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 from bacteria. Nutritional
yeast, though it has a similar appearance to brewer's yeast, is very different and has a very different taste.
Nutritional yeast has a nutty, cheesy, creamy flavor which makes it popular as an ingredient in cheese substitutes. It is often used by vegans in place of parmesan
cheese. Another popular use is as a topping for popcorn. Some movie theaters are
beginning to offer it along with salt or cayenne pepper as a popcorn condiment. It comes in the form of flakes, or as a yellow
powder similar in texture to cornmeal, and can be found in the bulk aisle of most natural food stores. In Australia it is
sometimes sold as "savory yeast flakes". Though "nutritional yeast" usually refers to commercial products, inadequately fed
prisoners have used "home-grown" yeast to prevent vitamin deficiency.[34]
Probiotics
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 diarrhea in children,[35][36] prevent reinfection of Clostridium difficile,[37] reduce bowel movements in diarrhea predominant IBS
patients,[38] and reduce the incidence of
antibiotic,[39] traveler's,[40] and HIV/AIDS[41] associated diarrheas.
Science
Diagram showing a yeast cell
Several yeasts, particularly Saccharomyces cerevisiae, have been
widely used in genetics and cell
biology. This is largely because the cell cycle in a yeast cell is very similar to the
cell cycle in humans, and therefore the basic cellular mechanics of DNA replication, recombination, cell division and metabolism are comparable.[8] Also many proteins important in human biology
were first discovered by studying their homologs 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.[42] 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.[43] The second yeast species to have its
genome sequenced was Schizosaccharomyces pombe, which was completed in
2002.[44] It was the 6th
eukaryotic genome sequenced and consists of 13.8 million base pairs.
Yeast extract
-
Marmite and Vegemite have a distinctive dark colour
|
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 acids. 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 and
Bovril (the United Kingdom, Republic of Ireland and South Africa), Oxo (South Africa, United
Kingdom, and Republic of Ireland), and Cenovis (Switzerland).
Pathogenic yeasts
