MICROBIOLOGY Study Guide - 2012

CHAPTER 15. FOOD MICROBIOLOGY

15.9. MICROBIOLOGY OF BEER

15.9.1. Brewers' Yeast

Brewed beverages have been produced since ancient times. Documents from the Sumerian civilization dating back to 3000 BC mention the preparation of a beverage that can be considered an early precursor of modern beer. Ancient Egyptians, Greeks, Romans, and Germanic peoples brewed unhopped beer primarily from barley, wheat, and other cereal grains, using pine shoots and other botanicals for flavor and aroma. Hops were introduced into brewing only in the 9th century. Hopped beer was brewed around the same time in Siberia, followed by Kievan and Novgorodian Rus'.

Brewers' Yeast plays the primary role in beer production. The Fermentation process is driven by yeast Enzymes that catalyze The breakdown of wort CARBOHYDRATES, producing ethanol, carbon dioxide, and secondary metabolites that shape the flavor and aroma of the finished beer. These secondary metabolic products include higher alcohols, acetaldehyde, acetoin, diacetyl, ethyl acetate, n-propanol, isobutanol, and others.

Both bottom-fermenting and top-fermenting strains of the yeast species Saccharomyces cerevisiae are used in brewing. Bottom-fermenting yeast Cells, as they develop in the fermenting wort, aggregate into flocks and settle to the bottom of the fermentation vessel, forming a dense sediment. These Yeasts ferment efficiently at low temperatures ranging from 5–10 °C, with fermentation coming to a halt near 0 °C.

Top-fermenting yeast cells rise to The surface of the wort and accumulate there as a foamy layer. This fermentation is typically conducted at 12–25 °C and stops at temperatures below 10 °C.

A distinctive feature of bottom-fermenting yeast is its ability to ferment not only glucose, fructose, maltose, and sucrose, but also the trisaccharide raffinose. The enzyme system of bottom-fermenting yeast contains both invertase and melibiase, whereas top-fermenting yeast possesses only invertase. As a result, top-fermenting yeast ferments raffinose by only one-third.

Yeasts used in brewing must possess specific properties: they should ferment wort rapidly and thoroughly, provide excellent clarification at low temperatures, and impart a distinct aroma and smooth taste to the beer. Today, pure cultures of brewers' yeast comprise numerous varieties known as races or strains. When selecting a yeast strain for beer production, morphological and physiological characteristics are of paramount importance. Domestic brewing facilities utilize the following bottom-fermenting strains of Saccharomyces cerevisiae: 129, 148, 338, 34/70, and Rh, as well as top-fermenting strains: W 210 for ale and W 68 for wheat beer.

Morphological properties. The shape and size of yeast cells are closely tied to their physiological state, age, and environmental composition. When cultivation conditions are kept constant, Cell dimensions and Morphology become characteristic of a given strain. The size of most brewers' yeast cells ranges from (9–11) × (5–8) µm. Variations in the shape of individual cells stem from shifting environmental conditions, such as medium composition, Temperature, and the presence of impurities. Healthy yeast cultures always contain a mixture of large and small cells. Small cells are typically round, while large cells are oval or elongated.

The cytoplasmic Structure depends on cell age and oxygen availability. In "respiring" cells under aerobic conditions, the Cytoplasm is homogeneous and devoid of inclusions. In "fermenting" cells under anaerobic conditions, vacuoles are readily observable.

Physiological properties.

1. The rate of yeast multiplication is critical, as fermentation begins only after a sufficient cell concentration has accumulated in the wort. In standard brewing practice, a 3- to 4-fold increase in yeast biomass during primary fermentation is considered normal. If the initial pitching rate is (7–10) × 10⁶ cells per 1 cm³, the population reaches (30–70) × 10⁶ cells during peak fermentation.

2. Flocculation capacity is the ability of yeast cells to clump together into small aggregates (flocs) and settle out toward the end of primary fermentation. This trait depends on genetic factors, wort composition, fermentation temperature, and aeration levels. Flocculation tends to decrease in worts with high concentrations of fermentable sugars, as well as under intensive aeration, low pitching rates, or elevated fermentation temperatures.

Based on their flocculation behavior, yeasts are categorized as either flocculent or powdery. Flocculent bottom-fermenting yeasts clump into aggregates at the end of primary fermentation and settle to the bottom as a dense sediment. Top-fermenting yeasts rise to the surface to form a "HEAD." This upward movement occurs because daughter cells fail to detach from mother cells after budding, forming clusters that are carried to the surface of the fermenting medium by CO₂ bubbles. Flocculent yeasts form flocs while the wort is still partially unfermented, which facilitates efficient clarification of the green beer.

Powdery yeasts remain suspended throughout the entire fermentation process and settle as a thin, soup-like sludge. They yield lower biomass growth and are more susceptible to autolysis, yet they exhibit higher fermentation activity and ferment the wort more completely.

3. Fermentation activity is a key biological property of brewers' yeast.

The primary indicator of yeast fermentation activity is the degree of wort attenuation. Based on this parameter, all yeast strains can be roughly divided into three groups:

✵ low-attenuating — attenuation degree below 80 %;

✵ medium-attenuating — 80–90 %;

✵ high-attenuating — 90–100 %.

Fermentation activity is evaluated using various Methods: by final attenuation degree, volume of carbon dioxide released per unit of time, quantity of ethanol produced, or The amount of sugar fermented per unit of time (fermentation kinetics).

The final attenuation degree is determined based on the initial and final dry matter content of the wort using the formula:

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where E is the extract content of the original wort; e is the extract content of the fermented wort.

The final attenuation limit is determined twice a month, especially when switching to new raw Materials.

The physiological state of yeast—such as age, previous storage conditions, nutrient reserves (Glycogen content), and viability—is of great importance for its fermentative activity.

4. Oxygen demand varies among different strains of brewing yeast, typically ranging from 2 to 30 mg/L or more. It depends on their preliminary propagation conditions. If the yeast has been in contact with oxygen for an extended period prior to fermentation, it can grow and induce fermentation regardless of the oxygen concentration in the wort. When using harvested (sedimented) yeast from previous fermentations, it is essential to oxygenate the fresh wort pumped into the fermentation vessels.

To initiate primary fermentation, the prepared wort is pumped into a fermentation vessel and inoculated with a pure yeast culture (PYC) or pitching yeast. The pure yeast culture is propagated to increase biomass, first in the laboratory and subsequently in the plant's propagation vessel, to yield the required quantity of yeast pitch for the fermenter.

Pitching yeast refers to the yeast settled in fermentation vessels, which is collected and reused for several subsequent production cycles. The yeast from each fermentation cycle is designated as a specific generation. Prior to use, the yeast is purified by passing it through a vibrating sieve to separate the cells from large protein flocs and hop trub residues. After purification, the yeast is transferred to special tanks, mixed with 2 to 3 times its volume of Water chilled to 0–2 °C, and thoroughly stirred. After settling for 2–3 hours, the turbid supernatant water is drained off. The yeast is then stored under a layer of chilled water at the specified temperature for no more than 4–5 days, with the surface water being replaced with fresh water 1–2 times a day.

If the pitching yeast contains a significant amount of contaminating microorganisms, it undergoes purification. Yeast that has been stored for several days is activated by mixing it with aerated wort at 15–17 °C in a 1:1 ratio, followed by The addition of 1/3 to 1/4 of cold wort. Reused yeast generations must meet the following quality criteria: dead cell count must not exceed 5 %; glycogen storage (cell plumpness) must be at least 70 %; bacterial contamination must not exceed 0.5 %, and the presence of wild yeast cells is strictly prohibited.