Yeast

Kind

The term running of yeast generally indicates the kind Saccharomyces (yeast of Bière or yeast of Boulangerie). There exists much of other kinds of yeasts. In particular Candida having a pathogenic capacity (responsible for the Mycosis S known by the name of " Candidoses "). The term Candidose S indicates the whole of the human pathological demonstrations having for factors of the mushrooms levuriformes kind candida

The majority are connected in Ascomycètes (standard truffle, pézize), some with the other great group of higher mushrooms, the Basidiomycetes (amanitas type, boletus) and others finally are nonattachable imperfect forms clearly with any definite group.

Modes of multiplication

The yeasts are able to multiply according to two different modes: sexué mode and the asexué.
mode The Ascomycète S which reproduce by a process sexué in a asque resulting from the transformation of a cell after Méiose.
The Basidiomycète S which carry out a reproduction sexuée with formation of Basidiospores on a Baside.
The Deutéromycète S gather the whole of yeasts not presenting known mode of reproduction sexuée.

For the majority of yeasts the asexual Multiplication (mitotic) is the major form of multiplication. There exist 2 types of mitotic division at yeasts: by Budding (case of the Saccharomyces ), or by Scission (case of the Schizosaccharomyces ).

Characteristics and cycle of reproduction

Structural characteristics

The yeasts are micro-organisms eucaryotes, thus they have the specific structural characteristics to this cellular type and others more specific to yeasts they-even:

Constant characteristics

A cellular Wall surrounding the plasmic membrane and protecting yeast from the physicochemical aggressions of the external medium. It consists of an external layer of Mannoprotéine S, associated with Glucane S and an internal layer of glucanes associated with an minor amount of Chitine.

A cytoplasmic membrane made up mainly of Phospholipide S double-layer (left Absorbent outside and left Lipophilic inside). it contains also many proteinic complexes intrinsic and extrinsic whose roles are varied, p. e.g. of the enzymes called proteases carry out transport of substances of the medium external about the intracellular middle and/or conversely with or not transformation of the substrate during the passage.

A core containing the genetic information of the chromosomal Genome of yeast. (see the chapter on the genetic characteristics more)

Of Mitochondrie S which plays a big role in the aerobic breathing of yeast and the production of ATP.

Variable characteristics

One or more Vacuole S , Organite S with the homogeneous aspect, which is used as storage spaces for various substances.

Genetic characteristics

Chromosomes: the yeasts are organizations Eucaryote S and have a core with linear chromosomes. At the Saccharomyces , the chromosomes are 16 simple or 16 pairs according to the form Haploïde or Diploïde of the cell. There exist genes of structure to continuous information as at the Bactérie S, and of the Gène S with discontinuous information (Intron S and Exon S) as at the higher organizations. In addition, the genes of regulation are specific yeasts.
Plasmide S: beside the chromosomes, there exists in the core of the small molecules of circular DNA of approximately 6 000 Even of bases, plasmides, present between 50 and 100 specimens per cell. These plasmides is autoréplicables and autotransférables without affecting the viability of the cell. They carry the genetic information of some natures nonessential with the viability of yeast. They have a considerable role in all the operations of genetic Engineering.
DNA mitochondrial: each Mitochondrie contains several circular molecules of DNA which carry the information of certain enzymes of the respiratory Chaîne.
Yeasts “killer”: certain stocks of Saccharomyces contain in their Cytoplasme two Virus with ARN. The genetic material of the “small virus” codes a toxin exocellulaire able to kill out of other yeasts and a protein of resistance to this same toxin to prevent yeasts “killer” from killing itself between them. The “great virus” is necessary for the multiplication and the maintenance of the “small virus” in the cytoplasm.

Transgenic yeasts

The yeasts belong to the first organizations to have been genetically modified. FAO regards them as substantially equivalent (but this concept of equivalence in substance is still discussed) to a natural yeast, and thus " as sure as the traditional product " and not requiring " thus not of other considerations of public health that those applied to existing food ". In 1998, a yeast genetically modified with Gène S of the same stock was already used in Great Britain for the Panification. Hansenula polymorpha is one of natural yeasts of the cider, naturally present on apples. Genetically modified stocks produce phytases, a vaccine anti-hepatitis B, anticoagulants saratine or hirudine or others Protéine S Enzyme S.

Life cycle

The yeast of laboratory Saccharomyces cerevisiae has a particular biological cycle. It is able to multiply in two forms: a form Diploïde (2n = 32 chromosomes) and a Haploid form (1n = 16 chromosomes).

The haploid cells multiply while budding: the cell mother buds a cell smaller girl (Mitose), but having same genetic information. There exist cells haploid " has " and of the cells haploid " α " who correspond to distinct sexual signs; it is fusion between a cell " has " and a " α " who gives rise to a cell diploïde " a/α ". As long as the environment is favorable, the diploïde multiplies by budding. If the Nutriment S have suddenly missed, the cell passes by again in haploid phase by a process of Méiose. One obtains finally four haploid cores which are included in the Spore S (Ascospore S) contained in a bag called Asque. The envelope of asque breaks with maturity and releases two cells " then; has " and two cells " α " who can start again the cycle.

The industrial stocks are often Polyploïde S (3, 4,5n Chromosomes) and thus have several genes for the same character. They are thus more stable genetically because difficult to make transfer. The majority of these stocks are unable of sporuler under the conditions of industrial crop and reproduce by budding.

Energy assessment and conditions of growth

Energy assessment

The two principal energy processes known at the Hétérotrophe S are the Respiration and the Fermentation S; For their development these yeasts need:

Of made up carbonaceous source of carbon and energy.
Of made up nitrogenized reduced in the form of Ammonium; some yeasts can however use compounds oxidized (like the Nitrate S) or organics for the Synthèse of nucleic acid proteins and .
Of varied biogenic salts, Vitamin S and growth factors which varies according to yeasts.

All the yeasts are able to degrade the Glucose, the Fructose and the Mannose in the presence of oxygen, by a oxydative Métabolisme, leading to the formation of CO₂ and H₂O.

aerobic Breathing : C₆H₁₂O₆ (glucose) + 6O₂ → 6CO₂ + 6:00 ₂O + energy usable

This metabolic way is very energy and allows the cells an important multiplication. In addition to the simple sugars, certain yeasts can use others Glucide S (mono, di or trisaccharides, to see polysaccharides like the Amidon) but as of the alcohol S, the Acide S or the Alcane S. In a more general way, they have a hydrolytic capacity good less as the Moisissure S.

In addition to the oxydative metabolism, certain yeasts can privilege a degradation of glucids by a fermentative metabolism which leads to the formation of ethanol and of CO₂ following the reaction: alcoholic Fermentation : C₆H₁₂O₆ (glucose) → 2CO₂ + 2CH ₃CH₂OH (ethanol) + energy usable

In addition to these majority compounds, higher alcohols, Aldehyde S, Ester S, acids… are formed in more minor amounts and take part qualitatively in an important and complex way in the formation of the Flaveur S of fermented drinks. This metabolism is energy than the oxydative metabolism, thus the cellular multiplication is assigned, but with the advantage of functioning even in Anaérobiose.

Conditions of growth

the temperature: the optimal temperature of culture of yeasts is located in general between 25 and 30°C, but like the other micro-organisms, the yeasts can be classified out of yeasts Psychrophile S, Mésophile S and Thermophile S. Generally, the yeasts are not heat-resisting. The cellular destruction starts as of 52°C (against 120°C for the thermophilous Bactérie S except Archéobactérie S). The yeasts are also sensitive to the Congélation and the Lyophilization with a great variability according to the kinds and species, and the phase of growth (the cells in exponential phase resist less than the cells in stationary phase).
Activity of the Water: the majority of the stocks cannot develop for a Activité of water lower than 0,90; but some tolerate higher pressures osmotic S, corresponding to an activity of about 0.60, by slowing down to them Métabolisme ; these yeasts are known as xérotolérante S.
oxygen: all the yeasts are able to develop in the presence of Oxygène: there is no yeast anaerobic strict E. Certain yeasts are strict Aérobie are (like the Rhodotorula ). The others are aéro-anaerobe optional with among them: yeasts preferring a metabolism either fermentaire or respiratory even in the presence of oxygen.
pH: the cellular envelopes are Imperméable S with the Ion S H₃O⁺ and OH^-. The yeasts thus tolerate very broad ranges of pH, theoretically from 2,4 to 8,6.
the sensitivity to the chemical agents:

organic acids: they have an inhibiting effect in their dissociated form because they can penetrate in the cell and the sensitivity of yeast depends on its capacity to metabolize them. For this reason the sorbic acids and propionic are more inhibiting than the acids acetic, citric and lactic.
ethanol: most resistant is the Saccharomyces which one uses in the processes of alcoholic Fermentation for the development of drinks or of industrial ethanol.
the sulphite: SO₂ has an inhibiting effect more pronounced on the Bactérie S than on yeasts, even if among yeasts of the sensitivities exist.
Antibiotics: the sensitivity to the Cycloheximide (Actidione) is variable and one can distinguish 3 groups from yeasts:

Yeasts inhibited as of 1µg/mL (ex: Saccharomyces)
Yeasts inhibited with 25µg/mL (ex: Schizosaccharomyces)
tolerant Yeasts with 1mg/mL (ex: Zygosaccharomyces)

The Chloramphénicol inhibits the synthesis of Protéine S mitochondriales but not that of the cytoplasmic proteins . Only the yeasts able to ferment can then cultivate in the presence of chloramphenicol.

Culture media

Any culture medium glucose is appropriate. However one uses in a preferential way certain mediums and under particular conditions (incubation with 28°C during 24 to 48 hours):

not-selective Mediums:

ordinary Medium
Medium BCP
Medium with the malt extract (Extracted from malt, agar-agar and water)

selective Mediums:

Gélose Sabouraud (selective by acid pH, to which one can add Chloramphénicol or Gentamycine)

Uses

The use of yeasts for the Panification and the Vinification is known since the prehistoric time. However, the comprehension of the microbiological mechanisms implemented dates from work of Louis Pasteur at the 19th century. The scientific knowledge and techniques thus acquired made it possible to cultivate and use great quantities of yeasts in the processes of industrial Fermentation, but also for the production of vitamins B, of Thiamine, the Antibiotique S and the Hormone S Stéroïde S. As a by-product of manufactoring processes, the yeasts are used like animal food. Another major transformation of yeasts is their autolysis and concentration by various processes to produce yeast extracts which are used as nutritional elements or agents of sapidity in human consumption. These extracts are rich in glutamates, glucanes, nucleotides, vitamins of the group B etc

By extension, the term of yeast is the generic name given to all the living organisms which cause the Fermentation.

The Brewers' yeast ( Saccharomyces cerevisiae ) is a by-product washed, filtered, then pressed and desiccated manufacture of the Bière.

The Levure of baker ( Saccharomyces cerevisiae ) is used to make raise the Pain, thanks to the production of Carbonic gas by Fermentation.

The yeast of paraphine also is very much used in the manufacture of textile.

The term of “Brewers' yeast” is employed in Cuisine to indicate a powder, made up mainly of Bicarbonate of sodium, which one is useful oneself in Pâtisserie and at the time of the Panification to make raise the paste quickly and make it very light.

Role in medical biology

See the article Candida .

See too

Biology | Cell | Unicellular
Yeast of baker | Brewers' yeast

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