knowelge check to yeast ageing

Yeast Cell Division and Reproduction

  • Sexual Reproduction in Yeast

    • False: Sexual reproduction via spores is not common in brewing yeast.

  • Cell Division (Mitosis)

    • True: A mother cell can typically divide and produce a new daughter cell approximately 20 times.
    • After the daughter bud appears, the mother cell ceases to grow.

  • Yeast Cell Cycle

    • Growth of daughter bud is linked to the yeast cell cycle.
    • Separation occurs when the daughter bud reaches about the same size as the mother cell.
    • In some cases, if not fully separated, a phenomenon known as chain formation occurs, which is observed in some ale strains of brewing yeast.

  • Budding Process

    • Live microscope images show mother cells with emerging daughter buds:
    • Chromosomes duplicate and segregate to ensure genetic complement post-division.
    • Time for budding: approximately 1.5 to 2 hours under optimal laboratory conditions (rich growth medium, correct temperature pH).
    • In brewery conditions, this process takes longer, generally resulting in 3 to 4 cell division cycles during fermentation.

Scanning Electron Microscope (SEM) Observations

  • Cell Division Images

    • SEM images show:
    • Right: Mother cell
    • Left: Daughter bud with visible birth scar (remnants of previous division) and bud scar (previous daughter buds).
    • It is possible to count the bud scars to estimate the age of yeast cells.

  • Composition of Bud Scars

    • Bud scars are rich in chitin (a polymer of N-acetylglucosamine), which is also found in crustaceans and human nails.
    • Chitin's toughness is crucial for successful cell division.
    • Fluorescent Dyes: Chitin can be stained with dyes like calcafluor, helping in the visualization of bud scars under a fluorescent microscope.

Knowledge Checks

  • Optimal Conditions for Cell Budding
    • False: It does not take 12 hours; it takes about 1.5 to 2 hours for budding cycles.
  • Location of New Daughter Buds
    • False: New daughter buds do not always emerge from the same location as previous ones. They can originate from different sites on the mother cell's surface.

Multilateral Budding and Yeast Aging

  • Multilateral Budding

    • Daughter buds can develop from various locations on the mother cell, resulting in multiple differing bud scars.
    • In Saccharomyces cerevisiae, a mother cell typically undergoes about 20 cell divisions before programmed cell death occurs.

  • Impact on Fermentation:

    • Older mother cells with multiple bud scars may compromise fermentation efficiency as they are less capable than younger cells.
    • The demographic of yeast during fermentation is analyzed:
    • 50% are virgin cells (newly divided daughter buds).
    • 25% are single budded mothers.
    • 25% are multi-budded mothers.

  • Impact of Cell Weight on Fermentation:

    • Older mother cells are larger/heavier and settle faster in fermentation tanks.
    • This necessitates care in harvesting yeast post-fermentation to avoid higher concentrations of older cells.
    • Ideal harvesting targets: single or double budded mother cells, ensuring a higher likelihood of healthy yeast for future fermentations.