Use of Liquid Nutrient Broth Media for Growing Bacteria
Broth is a nutrient-infused liquid medium used for growing bacteria.
In order for scientists to experiment with, and learn more about microbes, investigators must be able to grow these tiny organisms in a laboratory by providing them with water, food and an environment to grow in. Media are initially sterilized, then inoculated with the bacteria that is of interest and finally incubated for twenty four hours or more, to encourage bacterial growth.
Broth and Agar Bacterial Growth Media
Broth and Agar Bacterial Growth MediaThe two main types of bacterial growth media used are liquid broth and solid, Jell-o-like agar. Each has specific advantages and disadvantages. The growing environment used will depend on what the researcher wants to do with, or learn from, the microbes.
Nutrient Broth Bacterial Growth Medium
Nutrient broth is typically made of a powdered beef extract that contains peptones (broken down proteins). The powder is dissolved in water, put in test tubes, and sterilized.
Nutrient broth is typically made of a powdered beef extract that contains
peptones (broken down proteins).
The powder is dissolved in water, put in test tubes, and sterilized
peptones (broken down proteins).
The powder is dissolved in water, put in test tubes, and sterilized
Different Types of Oxygen Requirements of Bacteria
Unlike animals, bacteria do not all require oxygen. Some types of bacteria are poisoned by oxygen, others can take it or leave it. Liquid broth allows bacteria to grow at varying oxygen levels, which decrease as the depth of the broth increases.
A growth medium or culture medium is a liquid or gel designed to support the growth of microorganisms or cells, or small plants like the moss Physcomitrella patens. There are different types of media for growing different types of cells.
There are two major types of growth media: those used for cell culture, which use specific cell types derived from plants or animals, and microbiological culture, which are used for growing microorganisms, such as bacteria or yeast. The most common growth media for microorganisms are nutrient broths and agar plates; specialized media are sometimes required for microorganism and cell culture growth. Some organisms, termed fastidious organisms, require specialized environments due to complex nutritional requirements. Viruses, for example, are obligate intracellular parasites and require a growth medium containing living cells.
Types of growth media
The most common growth media for microorganisms are nutrient broths (liquid nutrient medium) or LB medium (Lysogeny Broth). Liquid media are often mixed with agar and poured into Petri dishes to solidify. These agar plates provide a solid medium on which microbes may be cultured. They remain solid, as very few bacteria are able to decompose agar. Bacteria grown in liquid cultures often form colloidal suspensions.
The difference between growth media used for cell culture and those used for microbiological culture is that cells derived from whole organisms and grown in culture often cannot grow without the addition of, for instance, hormones or growth factors which usually occur in vivo. In the case of animal cells, this difficulty is often addressed by the addition of blood serum or a synthetic serum replacement to the medium. In the case of microorganisms, there are no such limitations, as they are often unicellular organisms. One other major difference is that animal cells in culture are often grown on a flat surface to which they attach, and the medium is provided in a liquid form, which covers the cells. In contrast, bacteria such as Escherichia coli may be grown on solid media or in liquid media.
An important distinction between growth media types is that of defined versus undefined media. A defined medium will have known quantities of all ingredients. For microorganisms, they consist of providing trace elements and vitamins required by the microbe and especially a defined carbon source and nitrogen source. Glucose or glycerol are often used as carbon sources, and ammonium salts or nitrates as inorganic nitrogen sources. An undefined medium has some complex ingredients, such as yeast extract or casein hydrolysate, which consist of a mixture of many, many chemical species in unknown proportions. Undefined media are sometimes chosen based on price and sometimes by necessity - some microorganisms have never been cultured on defined media.
A good example of a growth medium is the wort used to make beer. The wort contains all the nutrients required for yeast growth, and under anaerobic conditions, alcohol is produced. When the fermentation process is complete, the combination of medium and dormant microbes, now beer, is ready for consumption
It is a source of amino acids and nitrogen (e.g., beef, *yeast extract)
This is an undefined medium because the amino acid source contains a variety of compounds with the exact composition being unknown. Nutrient media contain all the elements that most bacteria need for growth and are non-selective, so they are used for the general cultivation and maintenance of bacteria kept in laboratory culture collections.
An undefined medium (also known as a basal or complex medium) is a medium that contains:
-a carbon source such as glucose for bacterial growth
-various salts needed for bacterial growth
A defined medium (also known as chemically defined medium or synthetic medium) is a medium in which
-all the chemicals used are known
-no yeast, animal or plant tissue is present
A differential medium is a medium that includes
-some sort of added indicator that allows for the differentiation of particular chemical reactions occurring
Example:Nutrient agar medium composition:
-Beef Extract-0.3gm(mineral and carbohydrate)
-Peptone-0.5gm(protein and nitrogen source)
-Distilled water-100ml pH-7
Minimal media are those that contain the minimum nutrients possible for colony growth, generally without the presence of amino acids, and are often used by microbiologists and geneticists to grow "wild type" microorganisms. Minimal media can also be used to select for or against recombinants or exconjugants.
Minimal medium typically contains:
a carbon source for bacterial growth, which may be a sugar such as glucose, or a less energy-rich source like succinate various salts, which may vary among bacteria species and growing conditions; these generally provide essential elements such as magnesium, nitrogen, phosphorus, and sulfur to allow the bacteria to synthesize protein and nucleic acid
Supplementary minimal media are a type of minimal media that also contains a single selected agent, usually an amino acid or a sugar. This supplementation allows for the culturing of specific lines of auxotrophic recombinants.
Selective media are used for the growth of only selected microorganisms. For example, if a microorganism is resistant to a certain antibiotic, such as ampicillin or tetracycline, then that antibiotic can be added to the medium in order to prevent other cells, which do not possess the resistance, from growing. Media lacking an amino acid such as proline in conjunction with E. coli unable to synthesize it were commonly used by geneticists before the emergence of genomics to map bacterial chromosomes.
Selective growth media are also used in cell culture to ensure the survival or proliferation of cells with certain properties, such as antibiotic resistance or the ability to synthesize a certain metabolite. Normally, the presence of a specific gene or an allele of a gene confers upon the cell the ability to grow in the selective medium. In such cases, the gene is termed a marker.
Selective growth media for eukaryotic cells commonly contain neomycin to select cells that have been successfully transfected with a plasmid carrying the neomycin resistance gene as a marker. Gancyclovir is an exception to the rule as it is used to specifically kill cells that carry its respective marker, the Herpes simplex virus thymidine kinase (HSV TK).
Four types of agar plates demonstrating differential growth depending on bacterial metabolism.
Some examples of selective media include:
eosin methylene blue (EMB) that contains methylene blue – toxic to Gram-positive bacteria, allowing only the growth of Gram negative bacteria
YM (yeast and mold) which has a low pH, deterring bacterial growth
MacConkey agar for Gram-negative bacteria
Hektoen enteric agar (HE) which is selective for Gram-negative bacteria
mannitol salt agar (MSA) which is selective for Gram-positive bacteria and differential for mannitol
Terrific Broth (TB) is used with glycerol in cultivating recombinant strains of Escherichia coli.
xylose lysine desoxyscholate (XLD), which is selective for Gram-negative bacteria
buffered charcoal yeast extract agar, which is selective for certain gram-negative bacteria, especially Legionella pneumophila
Differential media or indicator media distinguish one microorganism type from another growing on the same media. This type of media uses the biochemical characteristics of a microorganism growing in the presence of specific nutrients or indicators (such as neutral red, phenol red, eosin y, or methylene blue) added to the medium to visibly indicate the defining characteristics of a microorganism. This type of media is used for the detection of microorganisms and by molecular biologists to detect recombinant strains of bacteria.
Examples of differential media include:
blood agar (used in strep tests), which contains bovine heart blood that becomes transparent in the presence of hemolytic Streptococcus
eosin methylene blue (EMB), which is differential for lactose and sucrose fermentation
MacConkey (MCK), which is differential for lactose fermentation
mannitol salt agar (MSA), which is differential for mannitol fermentation
X-gal plates, which are differential for lac operon mutants
Transport media should fulfill the following criteria:
temporary storage of specimens being transported to the laboratory for cultivation.
maintain the viability of all organisms in the specimen without altering their concentration.
contain only buffers and salt.
lack of carbon, nitrogen, and organic growth factors so as to prevent microbial multiplication.
transport media used in the isolation of anaerobes must be free of molecular oxygen.
Examples of transport media include:
Thioglycolate broth for strict anaerobes.
Stuart transport medium - a non-nutrient soft agar gel containing a reducing agent to prevent oxidation, and charcoal to neutralise
Certain bacterial inhibitors- for gonococci, and buffered glycerol saline for enteric bacilli.
Venkat-Ramakrishnan(VR) medium for v. cholerae.
Enriched media contain the nutrients required to support the growth of a wide variety of organisms, including some of the more fastidious ones. They are commonly used to harvest as many different types of microbes as are present in the specimen. Blood agar is an enriched medium in which nutritionally rich whole blood supplements the basic nutrients. Chocolate agar is enriched with heat-treated blood (40-45°C), which turns brown and gives the medium the color for which it is named.
Yeasts are eukaryotic microorganisms classified in the kingdom Fungi, with 1,500 species currently described (estimated to be 1% of all fungal species). 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 false hyphae, as seen in most molds. Yeast size can vary greatly depending on the species, typically measuring 3–4 µm in diameter, although some yeasts can reach over 40 µm. Most yeasts reproduce asexually by mitosis, and many do so by an asymmetric division process called budding.
By fermentation, the yeast species Saccharomyces cerevisiae converts carbohydrates to carbon dioxide and alcohols – for thousands of years the carbon dioxide has been used in baking and the alcohol in alcoholic beverages. 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. Yeasts have recently been used to generate electricity in microbial fuel cells, and produce ethanol for the biofuel industry.
Yeasts do not form a single taxonomic or phylogenetic grouping. The term yeast is often taken as a synonym for Saccharomyces cerevisiae, but the phylogenetic diversity of yeasts is shown by their placement in two separate phyla: the Ascomycota and the Basidiomycota. The budding yeasts ("true yeasts") are classified in the order Saccharomycetales.
The word "yeast" comes to us from 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. 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. Pasteur showed that by bubbling oxygen into the yeast broth, cell growth could be increased, but fermentation was inhibited – an observation later called the "Pasteur effect".
By the late 18th century, two yeast strains used in brewing had been identified: Saccharomyces cerevisiae, so-called top-fermenting yeast, and S. carlsbergensis, bottom-fermenting yeast. S. cerevisiae has been sold commercially by the Dutch for bread making since 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 1872, Baron Max de Springer developed a manufacturing process 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 commercial yeast was marketed at the Centennial Exposition in 1876 in Philadelphia, where Charles L. Fleischmann exhibited the product and a process to use it, as well as serving the resultant baked bread.
Nutrition and growth
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. 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). 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.
In general, yeasts are 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, Wallerstein Laboratories nutrient (WLN) agar, yeast peptone dextrose agar (YPD), and yeast mould agar or broth (YM). Home brewers who cultivate yeast frequently use dried malt extract (DME) and agar as a solid growth medium. The antibiotic cycloheximide is sometimes added to yeast growth media to inhibit the growth of Saccharomyces yeasts and select for wild/indigenous yeast species. This will change the yeast process.
The appearance of a white, thready yeast, commonly known as kahm yeast, is often a byproduct of the lactofermentation (or pickling) of certain vegetables, usually the result of exposure to air. Although harmless, it can give pickled vegetables a bad flavor and must be removed regularly during fermentation.
Yeasts are very common in the environment, and are often 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). Some yeasts are found in association with soil and insects. The ecological function and biodiversity of yeasts 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 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. Yeast colonising nectaries of the stinking hellebore have been found to raise the temperature of the flower, which may aid in attracting pollinators by increasing the evaporation of volatile organic compounds. A black yeast has been recorded as a partner in a complex relationship between ants, their mutualistic fungus, a fungal parasite of the fungus and a bacterium that kills the parasite. The yeast have a negative effect on the bacteria that normally produce antibiotics to kill the parasite and so may affect the ants' health by allowing the parasite to spread.
Yeasts, like all fungi, may have asexual and sexual reproductive cycles. The most common mode of vegetative growth in yeast is asexual reproduction by budding. Here, a small bud (also known as a bleb), 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.The daughter cell produced during the budding process is generally smaller than the mother cell. Some yeasts, including Schizosaccharomyces pombe, reproduce by fission instead of budding, thereby creating two identically sized daughter cells.
In general, under high stress conditions like nutrient starvation, haploid cells will die; under the same conditions, however, 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.
Some pucciniomycete yeasts, in particular species of Sporidiobolus and Sporobolomyces produce aerially dispersed, asexual ballistoconidia.
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 mold, 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 mead, wine, beer, or distilled spirits all use yeast at some stage of their production. A distilled beverage is a beverage containing 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 (in addition to that provided by the oak it is aged in) 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 well at low temperatures. An example of bottom-cropping yeast is Saccharomyces pastorianus, formerly known as S. carlsbergensis.
Decades ago, taxonomists reclassified S. carlsbergensis (uvarum) as a member of S. cerevisae, noting that the only distinct difference between the two is metabolic. Lager strains of S. cerevisae secrete an enzyme called melibiase, allowing it to hydrolyse melibiose, a disaccharide, into more fermentable monosaccharides. Top-cropping and bottom-cropping, cold-fermenting and warm-fermenting distinctions are largely generalizations used by the laypersons to communicate to the general public.
The most common top cropping brewer's yeast, S. cerevisiae, is the same species as the common baking yeast. Brewer's yeast is also very rich in essential minerals and the B vitamins (except B12). However, baking and brewing yeasts typically belong to different strains, cultivated to favour different characteristics: baking yeast strains are more aggressive, 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%).
Dekkera/Brettanomyces is a genus of yeast known for their important role in the production of Lambic and specialty sour ales, along with the secondary conditioning of a particular Belgian Trappist beer. The taxonomy of the genus Brettanomyces has been debated since its early discovery and has seen many re-classifications over the years. Early classification was based on a few species that reproduced asexually (anamorph form) through multipolar budding. Shortly after, the formation of ascospores was observed and the genus Dekkera, which reproduces sexually (teleomorph form), was introduced as part of the taxonomy. The current taxonomy includes five species within the genera of Dekkera/Brettanomyces. Those are the anamorphs Brettanomyces bruxellensis, Brettanomyces anomalus, Brettanomyces custersianus, Brettanomyces naardenensis, and Brettanomyces nanus, with teleomorphs existing for the first two species, Dekkera bruxellensis and Dekkera anomala.The distinction between Dekkera and Brettanomyces is arguable with Oelofse et al. (2008) citing Loureiro and Malfeito-Ferreira from 2006 when they affirmed that current molecular DNA detection techniques have uncovered no variance between the anamorph and teleomorph states. Over the past decade, Brettanomyces spp. have seen an increasing use in the craft-brewing sector of the industry with a handful of breweries having produced beers that were primary fermented with pure cultures of Brettanomyces spp. This has occurred out of experimentation as very little information exists regarding pure culture fermentative capabilities and the aromatic compounds produced by various strains. Dekkera/Brettanomyces spp. have been the subjects of numerous studies conducted over the past century, although a majority of the recent research has focused on enhancing the knowledge of the wine industry. Recent research on 8 Brettanomyces strains available in the brewing industry focused on strain specific fermentations and identified the major compounds produced during pure culture anaerobic fermentation in wort.
Yeast is used in winemaking, where it converts the sugars present in grape juice (must) into ethanol. Yeast is normally already present on grape skins. Fermentation can be done with this endogenous "wild yeast," but this procedure gives unpredictable results, which depend upon the exact types of yeast species present. For this reason, a pure yeast culture is usually added to the must; this yeast quickly dominates the fermentation. The wild yeasts are repressed, which ensures a reliable and predictable fermentation.
Most added wine yeasts are strains of S. 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 being 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.
Researchers from University of British Columbia, Canada, have found a new strain of yeast that has reduced amines. The amines in red wine and Chardonnay produce off-flavors and cause headaches and hypertension in some people. About 30 percent of people are sensitive to biogenic amines, such as histamines.
Yeast, the most common one being S. 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 common in alcoholic fermentation. In addition, Saccharomyces exiguus (also known as S. minor), a wild yeast found on plants, fruits, and grains, is occasionally used for baking. In bread making, the yeast initially respires aerobically, producing carbon dioxide and water. When the oxygen is depleted, fermentation 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 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.
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 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, had a longer shelf life than fresh yeast and that rose twice as fast. 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 whether 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. Bread machine recipes usually call for dried yeast; however, a (wet) sourdough starter can also work.
Baker's yeast is the common name for the strains of yeast commonly used as a leavening agent in baking bread and bakery products, where it converts the fermentable sugars present in the dough into carbon dioxide and ethanol. Baker's yeast is of the species Saccharomyces cerevisiae, which is the same species commonly used in alcoholic fermentation, and so is also called brewer's yeast. Baker's yeast is also a one celled microorganism growing around us and on us.
The use of steamed or boiled potatoes, water from potato boiling, or sugar in a bread dough provides food for the growth of yeasts, however, too much sugar will dehydrate them. Yeast growth is inhibited by both salt and sugar, but more so with salt than sugar. Fats such as butter or eggs slow down yeast growth, however others say the effect of fat on dough remains unclear, presenting evidence that small amounts of fat are beneficial for baked bread volume.
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, however, generally used in a pure form, but comes from being propagated in a sourdough starter.
Types of baker's yeast
Baker's yeast is available in a number of different forms, the main differences being the moisture contents. Though each version has certain advantages over the others, the choice of which form to use is largely a question of the requirements of the recipe at hand and the training of the cook preparing it. Dry yeast forms are good choices for longer-term storage, often lasting several months at room temperatures without significant loss of viability. With occasional allowances for liquid content and temperature, the different forms of commercial yeast are generally considered interchangeable.
Cream yeast is the closest form to the yeast slurries of the 19th century, being essentially a suspension of yeast cells in liquid, siphoned off from the growth medium. Its primary use is in industrial bakeries with special high-volume dispensing and mixing equipment, and it is not readily available to small bakeries or home cooks.
Compressed yeast is essentially cream yeast with most of the liquid removed. It is a soft solid, beige in color, and arguably best known in the consumer form as small, foil-wrapped cubes of cake yeast. It is also available in larger-block form for bulk usage. It is highly perishable; though formerly widely available for the consumer market, it has become less common in supermarkets in some countries due to its poor keeping properties, having been superseded in some such markets by active dry and instant yeast. It is still widely available for commercial use, and is somewhat more tolerant of low temperatures than other forms of commercial yeast; however, even there, instant yeast has made significant market inroads.
Active dry yeast is the form of yeast most commonly available to noncommercial bakers in the United States. It consists of coarse oblong granules of yeast, with live yeast cells encapsulated in a thick jacket of dry, dead cells with some growth medium. Under most conditions, active dry yeast must first be proofed or rehydrated. It can be stored at room temperature for a year, or frozen for more than a decade, which means that it has better keeping qualities than other forms, but it is generally considered more sensitive than other forms to thermal shock when actually used in recipes.
Instant yeast appears similar to active dry yeast, but has smaller granules with substantially higher percentages of live cells per comparable unit volumes. It is more perishable than active dry yeast, but also does not require rehydration, and can usually be added directly to all but the driest doughs. Instant yeast generally has a small amount of ascorbic acid added as a preservative. Some producers provide two or more forms of instant yeast in their product portfolio; for example, LeSaffre's "SAF Instant Gold" is designed specifically for doughs with high sugar contents. These are more generally known as osmotolerant yeasts.
Rapid-rise yeast is a variety of dried yeast (usually a form of instant yeast) that is of a smaller granular size, thus it dissolves faster in dough, and it provides greater carbon dioxide output to allow faster rising. There is considerable debate as to the value of such a product; while most baking experts believe it reduces the flavor potential of the finished product, Cook's Illustrated magazine, among others, feels that at least for direct-rise recipes, it makes little difference. Rapid-rise yeast is often marketed specifically for use in bread machines.
For most commercial uses, yeast of any form is packaged in bulk (blocks or freezer bags for fresh yeast; vacuum-packed brick bags for dry or instant); however, yeast for home use is often packaged in pre-measured doses, either small squares for compressed yeast or sealed packets for dry or instant. For active dry and instant yeast, a single dose (reckoned for the average bread recipe of between 500 g and 1000 g of dough) is generally about 2.5 tsp (~12 mL) or about 7 g (1/4 ounce), though comparatively lesser amounts are used when the yeast is used in a pre-ferment. A yeast flavor in the baked bread is generally not noticeable when the bakers' percent of added yeast is less than 2.5.
Some yeasts can find potential application in the field of bioremediation. 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. It can also tolerate high concentrations of salt and heavy metals, and is being investigated for its potential as a heavy metal biosorbent.
Industrial ethanol production
The ability of yeast to convert sugar into ethanol has been harnessed by the biotechnology industry to produce ethanol fuel. The process starts by milling a feedstock, such as sugar cane, field corn, or other cereal grains, and then adding dilute sulfuric acid, or fungal alpha amylase enzymes, to break down the starches into complex sugars. A glucoamylase 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.
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. Such a development means 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 the fermentation is stopped sooner, producing carbon dioxide, but only trace amounts of alcohol, and a significant amount of sugar is left in the drink. Kvass, a fermented drink made from rye, is popular in Eastern Europe; it has a recognizable, but low alcoholic content. 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.Kombucha is a popular beverage in Eastern Europe and some former Soviet republics under the name chajnyj grib , which means "tea mushroom". Kefir and kumis are made by fermenting milk with yeast and bacteria.
Yeast is used in nutritional supplements popular with health conscious individuals and those following vegan diets. It is often referred to as "nutritional yeast" when sold as a dietary supplement. Nutritional yeast is a deactivated yeast, usually S. cerevisiae. It is an excellent source of protein and vitamins, especially the B-complex vitamins, as well as other minerals and cofactors required for growth. It is also naturally low in fat and sodium. Contrary to some claims, it contains little or no vitamin B12. Some brands of nutritional yeast, though not all, are fortified with vitamin B12, which is produced separately by bacteria.
Nutritional yeast has a nutty, cheesy flavor that 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. It can also be used in mashed and fried potatoes, as well as in scrambled eggs. 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.
Some probiotic supplements use the yeast S. boulardii to maintain and restore the natural flora in the gastrointestinal tract. S. boulardii has been shown to reduce the symptoms of acute diarrhea in children, prevent reinfection of Clostridium difficile, reduce bowel movements in diarrhea-predominant IBS patients,[ and reduce the incidence of antibiotic, traveler's, and HIV/AIDS associated diarrheas.
Yeast is often used by aquarium hobbyists to generate carbon dioxide (CO2) to nourish 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.needs citation
There are several recipes for homemade CO2, but they are variations of the basic recipe: Baker's 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 through a narrow hose and released through a diffuser that helps dissolve the gas in the water. The CO2 is used by plants in the photosynthesis process.
Several yeasts, in particular S. 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 laboratory, 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 seven 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. As of 2012, over 30 yeast species have had their genomes sequenced and published.
Yeast extract is the common name for various forms of processed yeast products made by extracting the cell contents (removing the cell walls); they are used as food additives or flavorings, or as nutrients for bacterial culture media. They are often used to create savory flavors and umami taste sensations, and can be found in a large variety of packaged food including frozen meals, crackers, junk food, gravy, stock and more. Yeast extracts in liquid form can be dried to a light paste or a dry powder. Their possible content of Glutamic acid is a major concern to consumers.
Some species of yeast are opportunistic pathogens that can cause infection in people with compromised immune systems.
Cryptococcus neoformans is a significant pathogen of immunocompromised 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 that 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 due mainly 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.
Autolyzed yeast (containing the cell walls) or autolyzed yeast extract consists of concentrations of yeast cells that are allowed to die and break up, so that the yeasts’ endogenous digestive enzymes break their proteins down into simpler compounds (amino acids and peptides).
The general method for making yeast extract for food products such as Vegemite and Marmite on a commercial scale is to add sodium chloride (salt) to a suspension of yeast, making the solution hypertonic, which leads to the cells shrivelling up; this triggers autolysis, in which the yeast self-destructs. The dying yeast cells are then heated to complete their breakdown, after which the husks (yeast with thick cell walls) are separated. Removing the cell walls concentrates the flavors and changes the texture.
One common commercial use of yeast extract is as a flavor enhancer in processed food products, including vegetarian and vegan foods.
Hydrolyzed yeast or hydrolyzed yeast extract is another version used as a food additive for flavoring purposes. Exogenous enzymes or acids are used to hydrolyze the proteins.
Yeast extract contains an amount of naturally occurring glutamic acid or monosodium glutamate; this is produced from an acid-base fermentation cycle, and is only found in some yeasts, typically ones bred for use in baking.
Many food producers have replaced monosodium glutamate with yeast extract, which is cheaper, requires no E-number labeling, and allows food producers to claim their product is 'all natural' or 'with natural flavorings'. Health proponents claim this is a subversive move by the industry to hide a toxic additive since the MSG contained within the yeast extract does not require explicit labeling. Food industry bodies state:
Yeast extract is often mistaken with monosodium glutamate (MSG) – a common flavour enhancer – despite the fact that these ingredients differ strongly, both in composition and function. While yeast extract is made up of a rich mix of proteins, vitamins and amino acids, MSG is composed exclusively of glutamate salt. As a result, monosodium glutamate does not have a taste of its own and is only used to make existing flavours stronger.
The two main types of bacterial growth media used are liquid broth and solid, Jell-o-like agar. Each has specific advantages and disadvantages. The growing environment used will depend on what the researcher wants to do with, or learn from, the microbes.
Nutrient Broth Bacterial Growth Medium
Nutrient broth is typically made of a powdered beef extract that contains peptones (broken down proteins). The powder is dissolved in water, put in test tubes, and sterilized.