What are yeasts

موضوع: What are yeasts الجمعة نوفمبر 19, 2010 10:05 am

]Yeast are unicellular fungi. The precise classification is a field that uses the

characteristics of the cell, ascospore and colony.

Physiological characteristics are also used to identify species.

One of the more well known characteristics is the ability to ferment sugars for

the production of ethanol. Budding yeasts are true fungi of the phylum Ascomycetes,

class Saccharomycetes (also called Hemiascomycetes).

The true yeasts are separated into one main order Saccharomycetales.

Yeasts are characterized by a wide dispersion of natural habitats.

Common on plant leaves and flowers, soil and salt water.

Yeasts are also found on the skin surfaces and in the intestinal tracts

of warm-blooded animals, where they may live symbiotically or as parasites.

The common "yeast infection" is typically Candidiasis is caused by

the yeast-like fungus Candida albicans. In addition to being the causative agent in

vaginal yeast infections Candida is also a cause of diaper rash and thrush

of the mouth and throat.

Yeasts multiply as single cells that divide by budding (eg Saccharomyces) or direct division (fission,

eg. Schizosaccharomyces), or they may grow as simple irregular filaments (mycelium).

In sexual reproduction most yeasts form asci, which contain up to eight haploid ascospores.

These ascospores may fuse with adjoining nuclei and multiply through vegetative

division or, as with certain yeasts, fuse with other ascospores.

The awsome power of yeast genetics is partially due to the ability to quickly

map a phenotype producing gene to a region of the S. cerevisiae genome.

For the past two decades S. cerevisiae has been the model system for much of

molecular genetic research because the basic cellular mechanics of replication,

recombination, cell division and metabolism are generally conserved between

yeast and larger eukaryotes, including mammals.

The most well-known and commercially significant yeasts are the related species

and strains of Saccharomyces cerevisiae. These organisms have long been

utilized to ferment the sugars of rice, wheat, barley, and corn to produce

alcoholic beverages and in the baking industry to expand, or raise, dough.

Saccharomyces cerevisiae is commonly used as baker's yeast and for some types of fermentation.

Yeast is often taken as a vitamin supplement because it is 50 percent protein

and is a rich source of B vitamins such as niacin, folic acid, riboflavin, and biotin.

In brewing, Saccharomyces carlsbergensis, named after the Carlsberg Brewery in Copenhagen,

where it was first isolated in pure culture by Dr. Emil Christian Hansen (1842-1909) in 1883,

is used in the production of several types of beers including lagers. S. carlsbergensis

is used for bottom fermentation. S. cerevisiae used for the production of

ales and conducts top fermentation, in which the yeast rise to the

surface of the brewing vessel. In modern brewing many of the original top fermentation strains

have been modified to be bottom fermenters. Currently the S. carlsbergensis designation is not

used, the S. cerevisiae classification is used instead.

The yeast's function in baking is to ferment sugars present in the flour or added to the dough.

This fermentation gives off carbon dioxide and ethanol. The carbon dioxide is trapped within tiny

bubbles and results in the dough expanding, or rising. Sourdough bread,

is not produced with baker's yeast, rather a combination of wild yeast (often Candida milleri) and an

acid-generating bacteria (Lactobacillus sanfrancisco sp. nov).

It has been reported that the ratio of wild yeast to bacteria in San Francisco sourdough

cultures is about 1:100. The C. milleri strengthens the gluten and the L. sanfrancisco

ferments the maltose. For more information about sourdough see rec.food.sourdough FAQ.

The fermentation of wine is initiated by naturally occurring yeasts present in the vineyards.

Many wineries still use natural strains, however many use modern methods of strain maintenance

and isolation. The bubbles in sparkling wines is trapped carbon dioxide, the result of yeast

fermenting sugars in the grape juice. One yeast cell can ferment approximately its own weight

of glucose per hour. Under optimal conditions S. cerevisiae can produce up to 18 percent,

by volume, ethanol with 15 to 16 percent being the norm.

The sulfur dioxide present in commercially produced wine is actually added just after

the grapes are crushed to kill the naturally present bacteria, molds, and yeasts.

The yeastlike fungus, Candida albicans, is commonly found in the mouth, vagina, and intestinal

tract. Candida is a normal inhabitant of humans and normally causes no ill effects.

However, among infants and individuals with other illness a variety of conditions can occur.

Candidiasis of the mucous membranes of the mouth is known as thrush.

Candidiasis of the vagina is called vaginitis. Candida also causes severe disease in persons with

AIDS and chemotherapy patients].

موضوع: رد: What are yeasts الخميس ديسمبر 02, 2010 1:30 pm

افادكم الله 11111111111

موضوع: رد: What are yeasts الخميس ديسمبر 02, 2010 3:03 pm

الحمد لله مفهمتش ولا كلمه
ما اجمل الرياضة والفيزيا حقيقى الواحد فى نعمة الحمد لله

موضوع: رد: What are yeasts الجمعة ديسمبر 03, 2010 1:17 am

شكرا لمروركم الغالى

قوليلى يارغده ده بتكلم عن الخماير
يعنى ميكرو هتفهميه ازاى
6632462346