GENERAL MICROBIOLOGY - T.P. Pyroh - 2004

9. YEAST

9.6. PRACTICAL APPLICATIONS OF YEAST

Yeast, specifically Saccharomyces cerevisiae, has served humanity since ancient times. Today, various strains of this yeast species are used in brewing, winemaking, baking, and alcohol production. The lactose-fermenting yeast Kluyveromyces fragilis is used to produce alcohol from whey. Candida lipolytica metabolizes Hydrocarbons and serves as an industrial source of citric acid.

Yeasts of the genus Candida are capable of growing on a wide range of substrates: hydrocarbons (Candida tropicalis), methanol (Candida boidinii), peat and wood hydrolysates (Candida utilis), and ethanol (Candida utilis). Their biomass is used as a feed Supplement in livestock diets. Detailed information on protein products derived from yeast can be found in Chapter 22, "Microorganisms as Biotechnology Objects."

Pichia guilliermondii is used to produce riboflavin. Yeasts of the genus Rhodotorula are a source of Lipids and β-carotene. Trichosporon cutaneum plays a crucial role in aerobic wastewater Treatment systems due to its ability to oxidize Organic compounds, even those toxic to Fungi (such as Phenolic Compounds). The yeast Paffia rhodozyma synthesizes an unusual carotenoid, astaxanthin, which can serve as a pigment source for farmed salmon and trout. The flesh of artificially raised fish is naturally white, but it acquires a normal appearance when the pigment is added to their feed. Astaxanthin imparts the characteristic orange-pink color to the meat of salmon and trout.

Saccharomyces Yeast in Industry

The primary property of saccharomycetes widely utilized in industry is their ability to efficiently convert high concentrations of sugars into alcohol. Alcoholic Fermentation products include not only ethanol, but also carbon dioxide and yeast biomass. Important fermentation products also include organoleptic compounds produced by saccharomycetes that give alcoholic beverages their distinct taste and aroma (higher alcohols, acids, aldehydes along with ketones, and sulfur-containing substances).

Production of alcoholic beverages. Yeasts are used to produce the following beverages (main raw Materials are indicated in parentheses): beer, whisky (barley), vodka (corn, potato, wheat), gin (corn, potato), sake (rice), rum (molasses), wine (grapes), brandy (grapes), cider (apples), and calvados (apples).

Preparation of raw materials. Since yeasts easily assimilate the sugars present in grapes, this raw material requires no preliminary Processing—it is sufficient to crush the berries and press the juice. For red wine production, grape skins and seeds are left in the juice throughout fermentation, whereas for white wine production, they are removed immediately after crushing.

Unlike grape sugars, the primary carbohydrate in barley (starch) cannot be assimilated by yeast. Therefore, an essential technological step in brewing is barley malting, during which a complex of amylase Enzymes is synthesized. The sprouted barley is kiln-dried, causing sugar caramelization, and the resulting dark-colored compounds give beer its characteristic color. Next, the malt is crushed and mixed with Water; this process forms The basis of wort preparation. During this stage, amylolytic enzymes break down starch into maltose and other sugars that yeast can assimilate. The resulting extract (wort) is filtered to remove barley residues and used as a growth medium for yeast.

Fermentation (yeast cultivation). In beer production, a single strain—Saccharomyces cerevisiae—is added to the wort. In winemaking, the natural microflora of the grape Skin is utilized, meaning multiple yeast species participate in the fermentation. Along with Saccharomyces, these include Brettanomyces sp., Candida sp., Debaryomyces sp., and Hansenula. Fermentation lasts for several days (for beer) or several weeks (for wine).

Maturation. All beers and certain wines undergo maturation for several weeks, whereas some wines and distillates used to produce whisky and brandy can be aged for several years. In brewing, the maturation stage is necessary to remove undesirable volatile impurities and precipitate polyphenolic compounds that could cause beer cloudiness. During wine maturation, Bacteria proliferate and perform malolactic fermentation, which removes malic acid from the wine and improves its flavor profile.

Production of fuel ethanol. Ethanol is an eco-friendly fuel that Burns to form carbon dioxide and water. It is used in internal combustion engines either in pure form or as a 10–20% blend with gasoline (gasohol). In the 1980s in Brazil, 75% of automobiles operated on 95% ethanol, while the rest ran on gasohol.

Substantial arable land areas are projected to be dedicated to growing crops specifically for conversion into ethanol. Unfortunately, this creates a food-versus-energy dilemma. Ethanol production from plant biomass is not waste-free: every liter of alcohol generates 12–14 liters of wastewater with high concentrations of pollutants hazardous to natural ecosystems. Structure/149.html">The problem of their rational treatment remains unresolved.

It should be noted that saccharomycetes used in ethanol production have several drawbacks:

1. Competition between fermentation and Respiration. The substrate (glucose) is only partially fermented to ethanol. The remaining substrate is lost as it is converted into carbon dioxide

and water via respiration. The process must be conducted under anaerobic conditions or by using respiration-deficient mutants (lacking functional Cell/35.html">Mitochondria).

2. Ethanol sensitivity, which reduces the target product yield per unit of bioreactor volume. Ethanol-tolerant mutants characterized by altered Cell Membrane Structures have been successfully isolated.

3. Lack of enzymes that catalyze The breakdown of starch, Cellulose, and Xylan. Preprocessing via substrate Hydrolysis or The Use of mixed cultures of saccharomycetes and hydrolytically active microorganisms is required.

Theoretically, ethanol can be produced from any carbohydrate source that can be converted into a yeast-accessible form. In practice, economic viability requires that the substrate be converted into an assimilable form simply and cheaply. In Brazil, sucrose (in the form of sugarcane extract) and starch (from cassava) are used. Starch is hydrolyzed using amylases. Other countries utilize corn starch and sucrose (in the form of molasses). Since sucrose and starch are food commodities, alternative carbohydrate sources for ethanol production are actively being sought. Utilizing cellulose for this purpose is highly attractive; however, currently, all tested chemical and Enzymatic hydrolysis Methods for this feedstock remain too expensive.

Bacterial cultures—such as Zymomonas mobilis, thermophilic bacteria of the genus Clostridium, and Thermoanaerobacter ethanolicus—show greater promise for ethanol production than yeasts. They ferment sugars more efficiently and tolerate elevated ethanol concentrations (sometimes up to 15%). Thermophiles are also capable of the direct bioconversion of polysaccharide substrates into ethanol.

Autolysates and hydrolysates. Yeast hydrolysates can impart a savory, meat-like flavor to food products, making them widely used in the food industry for soups and seasonings. Hydrolysates are produced by heating yeast Cells (100 °C) in the presence of Hydrochloric acid until the majority of Proteins are hydrolyzed into Amino Acids. The resulting preparation is then neutralized with sodium hydroxide, filtered, and concentrated into a thick paste.

Autolysis differs from hydrolysis in that the breakdown of cellular components is driven by enzymes synthesized by the yeast cell itself. This process is accelerated by heating to 50 °C and adding sodium chloride. Yeast autolysates (as well as yeast extracts) are widely used in microbiological practice as a source of growth factors for numerous microorganisms.

Enzymes. The enzyme invertase is widely used in the confectionery industry to manufacture chocolates with soft fondant fillings. To give the flavored filling firmness and a specific shape, it is prepared using crystalline sucrose, a small amount of flavorings, and invertase. This solid core is then coated with chocolate. During storage, invertase acts on the sucrose, converting it into a mixture of glucose and fructose, thereby turning the crystalline filling into a smooth, syrup-like consistency.

Invertase is present in large quantities in yeast cells grown on molasses. This enzyme is released during cell autolysis, then purified, and supplied to confectionery plants as a standard solution with a known enzymatic activity.

Vitamins. Yeast is a rich source of vitamins (thiamine, riboflavin, pyridoxine, Folic acid, calcium pantothenate, Para-aminobenzoic Acid).