Yeast

Yeast and Fermentations

General Information

  • Date: October 15, 2025
  • Instructor: Dr. Ahmed Abdelhamid
  • Reference: Matthews KR, Kniel KE, Montville TJ (2017): Food Microbiology, an Introduction, pp. 327-341

Overview of Yeasts

  • Yeasts are eukaryotic microorganisms classified under the kingdom Fungi.
  • They are unicellular organisms, commonly referred to as single cells.
  • The most famous yeast utilized in various fermentations is Saccharomyces cerevisiae, also known as baker’s yeast.
  • Primary Applications: Yeasts are instrumental in the production of bread, beer, and wine.

Yeast in Bread Production

  • The carbon dioxide produced by yeast during fermentation acts as a leavening agent, causing bread to rise.
  • The process of beer production involves several complex steps:
    • Enzymes break down starches from barley, rye, or sorghum into fermentable carbohydrates.
    • Yeast then ferments these carbohydrates into alcohol.
  • Wine production begins with the fermentation of crushed grapes.

Historical Context of Bread Making

  • Bread production dates back to ancient Egypt where yeast was used in the fermentation process.
  • Figure 20.1: Illustrates scenes from ancient Egyptian bread-making, including kneading techniques and the use of conical molds for shaping bread.

Bread Making Process

  1. **Ingredients Preparations:
    • Flour, sugar (fermentable carbohydrate), fat (for texture), salt, and other additional ingredients are combined.**
  2. Yeast Addition:
    • Baker’s yeast (S. cerevisiae) is incorporated, either as dried powder, in blocks, or as cream, at a concentration of 1-6% relative to the weight of the flour.
  3. **Fermentation & CO₂ Production:
    • The primary role of yeast in bread-making is to generate carbon dioxide, aiding in the rise of the dough.**
  4. Amylase Activity:
    • Yeast produces amylases that convert starch into more fermentable forms, specifically glucose.
  5. Dough Formation:
    • Water is added to the mixture, leading to the formation of dough.
  6. First Fermentation:
    • Dough ferments at a temperature of 28-32°C for several hours, subsequently portioned into loaves.
  7. Second Fermentation:
    • Loaves are allowed to ferment again until they double in volume.
  8. Baking:
    • Baked at 200°C for approximately 30 minutes.
  9. Sourdough Variation:
    • Sourdough bread incorporates both lactic acid bacteria (to enhance texture and prevent spoilage) and yeast.

Beer Production Process

  • Essential Ingredients:
    • Hops (for flavor and antimicrobial properties)
    • Yeast (to produce alcohol and carbon dioxide)
    • Water (essential component)
    • Malt (provides fermentable carbohydrates)
  • Key Steps in Brewing:
    1. Malting:
    • Germination of barley degrades cell walls, releases starch granules, and produces enzymes for fermentable sugars.
    1. Mashing:
    • Hot water is introduced to solubilize nutrients and enzymes, forming a sweet wort.
    1. Wort Boiling:
    • Boiling the wort with hops extracts flavor compounds.
    1. Fermentation:
    • Yeast (such as S. cerevisiae or S. carlsbergensis) converts fermentable sugars into alcohol and carbon dioxide.
    1. Post-Fermentation Processing:
    • Optional aging, clarification, pasteurization, and bottling of beer.

Wine Production Process

  • Different grape varieties yield distinct wines (white and red).
  • The flavor profile of the wine is intrinsically linked to the type of grape used.
  • White Wine Production Method:
    • Juice is immediately separated from grape skins post-crushing, clarified, then sent for fermentation.
  • Red Wine Production Method:
    • Juice and skins are processed together in fermentation, allowing for the extraction of anthocyanins and phenolics responsible for the wine’s color.
    • Fermentation can take place in barrels or large stainless-steel tanks.

Fermentation Techniques in Winemaking

  • Inoculation:
    • Grape juice may be inoculated with yeast; alternatively, native yeast can also facilitate fermentation.
    • Native yeasts typically consist of species such as Hanseniaspora, Candida, Pichia, and Hansenula that are either derived from the grapes or winery equipment.
  • Fermentation Conditions:
    • White wines ferment over 1 to 2 weeks at temperatures ranging from 10 to 18°C, preserving volatile flavors.
    • Red wines ferment for approximately 7 days at higher temperatures (20 to 30°C) to extract color from skins.
  • By-Products of Fermentation:
    • Fermentation yields not only ethanol and carbon dioxide but also glycerol (which contributes to smoothness and viscosity) and various flavor compounds (higher alcohols, esters, aldehydes).
  • Malolactic Fermentation:
    • A secondary fermentation that lowers acidity in wines, initiated by the bacterium Oenococcus oeni, converting malic acid into lactic acid and carbon dioxide. The equation is represented as:
      ext{Malic acid}
      ightarrow ext{Lactic acid} + ext{Carbon dioxide}
  • Summary of Wine Production Steps:
    • For red wine, the process includes crushing grapes, optional yeast inoculation, maceration (for color and tannin extraction), pressing to eliminate skins, and completing fermentation.
    • For white wine, crushing occurs followed by pressing to remove skins before fermentation.
  • Optional aging, fining, clarification, and bottling processes also follow.

Key Question on Fermentation

  • A critical inquiry posed: Why do different fermentation processes (bread, beer, and wine) necessitate the use of distinct yeast species or strains?