Yeast Characteristics and Reproduction

Yeast Overview

• Yeast are unicellular and non-filamentous fungi
• Some species of yeast can be both useful and harmful
• Yeast are eukaryotic organisms
• There are at least 1,500 recognized species

Morphological Characteristics

• Yeast cells are 3-4um in diameter
• Yeast can be spherical to ovoid in shape
• Cells can be lemon to pear-shaped
• Yeast cells can be triangular to cylindrical
• Yeast cells can be rounded to elongated
• Some species develop pseudomycelium, such as Candida albicans
• Parts seen under a microscope include cell wall, cytoplasm, water vacuoles, fat globules, and granules that are either albuminous or starchy

Asexual Reproduction

• Budding involves a parent/mother cell producing a bud/small cell on its surface
• Once the bud matures, it separates from the mother cell, leaving behind a scar
• Cell division occurs at one particular site
• The new cell is a clone, genetically identical to the parent cell
• Budding takes place in favorable conditions and when growing in sugar solutions
• Multilateral budding involves forming on multiple sites on the surface of the parent cell (spherical or ellipsoidal yeast)
• Polar budding forms on one specific pole
• Splitting fission involves the parent cell elongating then dividing into two equal daughter cells
• A septum (cell wall) forms at the center of the cell
• Some species of yeast use both budding and fission

Sexual Reproduction

• Common reproduction method of "True Yeast"
• Formation of ascospore follow conjugation of two cells
• Conjugation involves a temporary cytoplasmic union with the exchange of nucleic material and is considered the sexual process
• It can be fusion of similar gamete, with union of their nuclei followed by fertilization
• Fungi Imperfecti or "False yeast” are yeasts that don't produce ascospore or other sexual spore(Candida)
• Ascospores are formed in an ascus, which is the sexual spore-bearing cell produced in ascomycete fungi
• Each ascus typically contains 8 ascospores, produced by meiosis followed in most species by mitotic cell division

Generation Time and Growth Rates

• The time required for a cell division to occur is referred to as generation time
• Fission requires 4 hours and 48 minutes
• Budding requires 2 hours and 48 minutes
• Change in number of cell per unit time is referred to as growth rate
• Fission results in 13 doublings per minute
• Budding results in 21 doublings per minute
• Differences in yeast species, media type, and growth conditions (temperature and pH) affects variations in generation times

Examples based on reproduction style

• Lemon-shaped Budding: Schizosaccharomyces pombe
• Fission: Saccharomycodes, Nadsonia, Hanseniaspora, Kloeckera

Cultural Characteristics

• Young yeast appears moist, shiny, and mealy
• Mature yeast appears dry and wrinkled
• Most yeasts are whitish and cream-colored, like Saccharomyces cerevisiae
• Few species produce pigment, such as Rhodotorula spp.
• Film yeast or oxidative yeast develops a pellicle or film at the surface of liquid medium and causes problems in wine production
• Fermentative yeast grows throughout the liquid medium, making it blurred and cloudy
• The addition of gas is formed during fermentation, where glucose is converted to ethanol and carbon dioxide: C6H12O6 -> CH3CH2OH+CO2

Physiological Characteristics

• Most yeasts grow best in a plentiful supply of moisture

• Halophiles: thrive in high concentrations of salt

• Osmophiles: thrive in high concentrations of sugar

• In general, yeast require more moisture than molds

• Ordinary Yeast: do not do well in high concentration of solute and can't grow low water activity food materials (Aw: 0.88-0.94)

• Osmophilic Yeast: grow in high concentration solutes and grow low water activity food materials (Aw: 0.62-0.65 in syrups)

• Temperature Requirement: generally mesophilic

• Most yeast prefer to grow at 25-47°C

• Few yeast species can grow at 0°C

• pH Requirement: yeast prefer acidic conditions

• 4-4.5 is the favored pH range

• Generally, yeast don't grow at alkaline pH until adopted

• Food Requirement: sugar is the most important food

• Oxidative yeast oxidize organic acid to alcohol

• Other species use nitrogenous material

• Oxygen Requirement: most yeast prefer to grow in presence of oxygen

• Fermentative type can grow anaerobic, though slowly