Yeast Morphology and Reproduction

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

Morphology

• Yeast cells typically range from 3-4um in diameter
• Yeast exhibit various forms and shapes, including spherical, ovoid, lemon, pear-shaped, triangular, cylindrical, and elongated.
• Some species can develop pseudomycelium. An example is Candida albicans
• Parts visible under a microscope include: cell wall, cytoplasm, water vacuoles, fat globules, and granules (albuminous or starchy).

Reproduction - Asexual

• Asexual reproduction occurs mainly through budding.
• Budding involves the parent cell producing a small bud on its surface.
• Once the bud matures, it separates from the mother cell, leaving behind a scar.
• Division happens at one site
• The new cell produced through budding is a clone, genetically identical to the parent cell.
• Budding occurs under favorable conditions, such as in sugar solutions.
• Multilateral budding occurs at multiple sites on the parent cell surface in spherical or ellipsoidal yeast.
• Polar budding forms on one specific pole.
• Another asexual method is splitting fission, where the parent cell elongates and divides into two equal daughter cells, forming a septum.
• Some species exhibit both budding and fission.

Reproduction - Sexual

• Sexual reproduction is a common method in "True Yeast".
• Ascospore formation follows the conjugation of two cells.
• Conjugation- Temporary cytoplasmic fusion with nucleic material exchange for sexual process, leading to ascospore formation either by fusion of similar gametes or by nuclei fertilization.
• "False yeast" or Fungi Imperfecti- Don't produce ascospores or other sexual spores, Candida is such an example.
• Ascospores are sexual spore-bearing cells produced in ascomycete fungi.
• Asci usually contain 8 ascospores, produced by meiosis, followed by mitotic cell division in most species.

Generation Time & Growth Rates

• Generation Time- Time required for a cell division to occur
  • Fission- 4 hours and 48 mins
  • Budding- 2 hours and 48 mins
• Growth Rate- Change in number of cell per unit time
  • Fission- 13 doublings per minute
  • Budding- 21 doublings per minute
• Variations in generation times depend on yeast species, media type, and growth conditions (temperature and pH).
• Schizosaccharomyces pombe undergo lemon-shaped budding.
• Fission occurs in Nadsonia, Hanseniaspora, and Kloeckera.

Cultural Characteristics

• Young yeast colonies appear moist, shiny, and mealy
• Mature yeast colonies appear dry and wrinkled.
• Most yeast are whitish and cream-colored, like Saccharomyces cerevisiae
• Few species produce pigment, for example: Rhodotorula spp.
• Film yeast or Oxidative yeast- Develops pellicle or film at the surface of liquid medium, causes problems in wine production
• Fermentative yeast- Grow throughout the liquid medium, making the liquid blurred and cloudy, adding gas
  • Glucose turns into ethanol and carbon dioxide C6H12O6-> CH3CH2OH+CO2

Physiological Characteristics

• Moisture Requirement- Yeast grow best in plentiful supply of moisture
  • Halophiles- High concentration of salt
  • Osmophiles- High concentration of Sugar
  • In general, yeast require more moisture than molds
• Types of Yeast Based on Water Activity:
  • Ordinary Yeast- Do not grow in high concentration of solute. Therefore are unable to grow in low water activity food materials, Aw: 0.88-0.94
  • Osmophilic Yeast- Grow high concentration solutes, so grow in low water activity food materials, Aw: 0.62-0.65 in syrups
• Temperature Requirement
  • Generally, Mesophilic
  • Most yeast like to grow at 25-47*C
  • Few yeast species can grow at 0*C
• pH Requirement
  • Yeast favor acidic conditions, with a favored pH range of 4-4.5
  • Generally, yeast will not grow at alkaline mediums until adopted
• Food Requirement
  • Sugar is the most important food source
  • Oxidative yeast oxidize organic acid to alcohol
  • Other species use nitrogenous materials
• Oxygen Requirement
  • Most yeast prefer to grow in the presence of oxygen
  • Fermentative types can grow anaerobic, although slowly