"Yeasts play a crucial role in the beer brewing process as they are responsible for fermentation, which converts the sugars in the wort into alcohol and carbon dioxide. Yeasts are microorganisms classified as fungi and are available in various strains, each contributing unique flavours, aromas, and characteristics to the final beer.

There are two primary types of yeast used in beer brewing: ale yeast and lager yeast.

Ale Yeast: Ale yeast, also known as top-fermenting yeast, belongs to the species Saccharomyces cerevisiae. It ferments at higher temperatures, typically between 15-24°C (59-75°F). Ale yeast ferments quickly and tends to produce fruity and ester flavours, resulting in a wide range of beer styles such as ales, stouts, porters, and wheat beers.

Lager Yeast: Lager yeast, known as bottom-fermenting yeast, belongs to the species Saccharomyces Pastorious (a hybrid of Saccharomyces cerevisiae and another yeast species). Lager yeast ferments at lower temperatures, typically between 7-13°C (45-55°F) and undergoes a longer fermentation and conditioning period. This yeast produces clean and crisp flavours and is commonly used in brewing lagers and pilsners.

During the brewing process, yeast is added to the wort after the initial boiling and cooling. The yeast consumes the fermentable sugars present in the wort and converts them into alcohol and carbon dioxide through the process of fermentation. In addition to alcohol production, yeast also produces various flavour compounds, including esters, phenols, and other aromatic compounds, which contribute to the overall flavour and aroma of the beer.

The fermentation process typically lasts for several days to weeks, depending on the beer style and the desired characteristics. After fermentation, the beer may undergo a period of conditioning, where the yeast continues to work and clean up any unwanted byproducts produced during fermentation. This conditioning phase helps to refine the flavours and enhance the beer's overall quality.

In recent years, there has been a growing interest in using different yeast strains, including wild and non-Saccharomyces yeasts, to impart unique flavours and aromas to craft beers. These experimental yeasts can create a wide array of beer styles, offering beer enthusiasts a diverse range of flavours to explore.

Overall, yeast is a vital component in the brewing process, influencing the taste, aroma, and character of the final beer. Brewers carefully select yeast strains based on the desired beer style and aim to create well-balanced and flavourful brews that cater to various consumer preferences.

Controlling yeast in breweries is essential to ensure consistent fermentation and maintain the desired characteristics of the beer. Here are some key aspects of yeast control in breweries:

Yeast Propagation: Brewers often start with a small amount of yeast, typically obtained from a yeast bank or a previous batch of beer. They then propagate the yeast to obtain a sufficient quantity for fermentation. Yeast propagation involves providing the yeast with a nutrient-rich environment, usually in the form of a yeast starter or a propagation tank, where it can multiply and increase in cell count before being pitched into the main fermentation vessel.

Yeast Health and Viability: To maintain optimal yeast performance, it is crucial to monitor and maintain yeast health and viability. Yeast health refers to the overall condition of the yeast cells, including their vitality and ability to ferment sugars efficiently. Viability refers to the percentage of live and active yeast cells present in a given yeast population. Techniques such as microscopic examination, viability staining, and yeast cell counting are employed to assess yeast health and viability.

Pitching Rate: The pitching rate refers to the amount of yeast cells added to the wort. It is crucial to pitch an adequate amount of healthy yeast to ensure a robust fermentation. Pitching too few yeast cells can result in slow or incomplete fermentation, while pitching too many can lead to off-flavours or stressed yeast. Brewers calculate the pitching rate based on factors such as desired fermentation time, original gravity of the wort, and yeast strain characteristics.

Temperature Control: Yeast is highly sensitive to temperature, and controlling fermentation temperature is vital for achieving desired flavour profiles and preventing off-flavours. Different yeast strains have specific temperature ranges in which they perform optimally. Brewers use various methods such as fermentation chambers, cooling jackets, or temperature-controlled fermentation vessels to maintain consistent and appropriate fermentation temperatures.

Oxygen Control: Oxygen exposure can negatively impact yeast performance and lead to off flavours. Brewers take measures to minimize oxygen contact during the brewing process, as excess oxygen can hinder yeast growth and vitality. Techniques such as closed transfers, purging vessels with carbon dioxide, and using airtight fermentation vessels help prevent oxygen exposure and ensure yeast health.

Yeast Nutrient Management: Yeast requires various nutrients, including nitrogen, vitamins, and minerals, for healthy fermentation. Brewers may supplement the wort with yeast nutrients to ensure an adequate nutrient supply for the yeast. This is particularly important in high-gravity or low-nutrient works, where yeast may require additional support for optimal fermentation.

Yeast Strain Management: Brewers often work with specific yeast strains chosen for their desired flavour and aroma characteristics. Managing yeast strains involves maintaining yeast banks or cultures and ensuring their purity and stability over time. Yeast strains may be stored under controlled conditions and periodically refreshed to maintain their vitality and genetic stability.

During the fermentation phase of brewing, yeasts typically exhibit high ATP levels. Fermentation is an energetically demanding process for yeast cells, and they require significant amounts of ATP to carry out the conversion of sugars into alcohol and carbon dioxide, as well as to support other metabolic activities.

When yeasts are actively fermenting wort, they undergo vigorous metabolic activity to metabolize sugars, produce ethanol, and generate carbon dioxide. This process involves the breakdown of glucose through glycolysis, which yields a limited amount of ATP compared to oxidative phosphorylation. Therefore, yeasts primarily rely on glycolysis for ATP production during fermentation.

While ATP levels in yeasts can vary based on specific conditions and yeast strains, high ATP levels are generally observed during the fermentation process. The increased ATP levels provide the necessary energy for yeast cells to perform various functions, including nutrient uptake, biosynthesis of necessary molecules, maintenance of cellular processes, and cell division during fermentation.

Additionally, the high ATP levels during fermentation also reflect the metabolic activity and vitality of the yeast cells, ensuring efficient conversion of sugars and proper completion of the fermentation process.

It's worth noting that as fermentation progresses and the availability of fermentable sugars decreases, the metabolic activity of yeasts may slow down, leading to a gradual decrease in ATP levels. At the same time, yeast cells may enter a stationary phase or transition to a conditioning phase where they continue to work on cleaning up byproducts and maturing the beer.

In summary, during the active fermentation phase of beer brewing, yeasts generally exhibit ATP levels to support their metabolic activities, energy production, and completion of the fermentation process."