Brewing ingredients - yeast

Brewer's yeast belongs to the genus Saccharomyces, and in an acidic, sugary aqueous solution (wort), through the outer cell membrane (plasma membrane), they absorb dissolved sugars, simple nitrogenous substances (amino acids and very small molecular peptides), vitamins , ions and other substances, and then they use these substances through metabolic pathways for their own growth and fermentation, while producing rich beer flavor substances. Therefore, yeast is the leader of beer fermentation and the soul of beer brewing; the type and quality of yeast determine the type and flavor of beer.

1. Morphological characteristics of beer yeast: 

Individual shape: The shape of beer yeast is generally round, oval or oval.

Colony morphology: On wort agar solid medium, brewer's yeast is milky white colony with smooth surface and neat edges. With the prolongation of culture time, the colony gradually turns yellowish brown.

2. Brewer's yeast reproduction method: 

Brewer's yeast has asexual reproduction and sexual reproduction in its reproductive methods. Under normal circumstances, the reproductive method of brewer's yeast vegetative cells is mainly budding reproduction of asexual reproduction. When the beer yeast grows to a certain extent, a small protrusion is formed on the surface of the mother cell, and the nucleus of the mother cell splits into two, one of which enters the small protrusion, which is called a daughter cell; when the daughter cell grows to 1/2 to 2/3 of the mother cell When the size is small, the daughter cell automatically separates from the mother cell to form an independent cell.

The reproduction and growth of brewer's yeast can be divided into six distinct stages:

1. Adjustment period: This stage is also called the initial stage, which is the activation process of metabolism. This stage takes a long time

The short-term fluctuations are large, mainly depending on the type of organism, culture algebra, culture conditions and other factors. Label the cells as soon as they begin to detach

marks the end of this phase.

2. Acceleration period: This period is followed by the adjustment period, and the cell separation speed is accelerated.

3. Logarithmic growth stage: In this stage, the cells proliferate logarithmically, and the proliferation rate is the largest and remains constant. At this time, the time required to form a new generation is the shortest (ie, the time for doubling the number of cells). The generation time is 90-120min under the optimal multiplication condition.

4. Deceleration period: Due to various factors, such as the reduction of substrates and the increase of growth-inhibiting metabolites, the logarithmic growth stage has a certain time limit, and then enters the deceleration period in which the proliferation rate gradually decreases.

5. Stable period: In this stage, the number of microorganisms remains constant. The number of new cells formed is equal to the number of dead cells.

6. Death stage: In this stage, the number of dead cells exceeds the number of new cells formed, and the number of cells decreases.

3. Classification of beer yeast: 

Top Yeast : ①It is the original strain of beer yeast. ②Bud clusters can be formed, which rise up and enter the bubble cap of the fermentation broth during vigorous fermentation, and their coagulation and sedimentation properties are poor. ③It can form spores, can ferment 1/3 of raffinose, but cannot ferment melibiose. ④ The fermentation temperature is 15-25°C, usually 20-25°C, and it is very sensitive to low temperature. ⑤ produce a unique ester aroma. ⑥ The degree of fermentation of beer is high.

Below Yeast: ① It is not easy to form bud clusters, and the daughter and mother cells are separated quickly. At the end of the main fermentation, the yeast settles at the bottom of the container, and its aggregation and precipitation are strong. ②It is not easy to form spores, and can completely ferment raffinose and melibiose. ③The fermentation temperature is 5-12°C, and 9-12°C is commonly used, which can better adapt to low temperature. ④Yeast washing can be carried out, but in general large tank fermentation process, yeast washing is rarely carried out after recycling.

4. Metabolic pathways of brewer's yeast 

After yeast inoculation, it begins to restore its physiological activity under the condition of wort oxygenation, with amino acids in wort as the main nitrogen source and fermentable sugar as the main carbon source to perform respiration and obtain energy from it. Breeding, while producing a series of metabolic by-products, after which alcoholic fermentation is carried out under anaerobic conditions.

Formation and breakdown of fermentation by-products:

Raw green flavor substances: diacetyl, aldehydes, sulfides, making beer taste impure, immature, and uncoordinated.

Aromatic substances: higher alcohols and esters determine the aroma of beer.

The quantity and nature of the above main and by-products determine the quality of beer. Aspects in which they affect beer quality are:

(1) Taste: If the content of diacetyl is high, there will be a smell of rancid rice; if the content of acetaldehyde is high, there will be a raw green taste;

(2) Foam: The higher the CO2 content, the more corresponding foam.

(3) Fragrance: High ester content will bring different ester fragrance.

(4) Stability: including flavor stability, biological stability and non-biological stability.