Yeasts are eukaryotic, unicellular microorganisms that belong to the kingdom of fungi. The first yeast appeared hundreds of millions of years ago, and there are approximately 1,500 known kinds. Yeasts are unicellular organisms that developed from multicellular predecessors, with certain species able to acquire multicellular traits by generating pseudohyphae, which are strings of connected budding cells. Some yeasts can attain a diameter of 40 m, despite the fact that most yeasts have a diameter of 3 to 4 m. Yeast diameters vary greatly depending on the species and environment. The majority of yeasts reproduce asexually via mitosis, and many do so via budding, an imbalanced division process.

Principles of yeast growth and fermentation

• Yeast is a facultative anaerobe that may survive and thrive with or without oxygen. The oxygen controls the cell’s metabolic fate.
• In the presence of oxygen, survival, proliferation, and metabolism of yeast cells are maximised. In this situation, yeast will grow rapidly and convert glucose into carbon dioxide and water.
• Under anaerobic circumstances, yeast grows significantly more slowly, densities decrease, and glucose is not completely converted to ethanol and carbon dioxide.
• The purpose of a yeast starter culture is not to produce a tasty fermented beverage, but to provide sufficient yeast for subsequent fermentation.
• The propagation circumstances should be such that a maximum quantity of yeast is produced, resulting in optimal fermentation performance once pitched.
• What does fermentation performance entail? The primary criteria for fermentation performance are the rate and duration of fermentation, as well as the development of a beer with a balanced sensory profile and no off-flavors/aromas or inappropriate esters.

Sources of yeast strains

• Numerous fermented goods necessitate a reliable supply of yeast for fermentation. The goal is to use the appropriate yeast strains in the best possible condition and with the lowest possible failure risk. If various strains are supplied within a group, it is crucial that any mix-ups are foreseen and actively averted.
• The supplier of yeast starter culture must be able to demonstrate the origins of your yeast cultures, guarantee their traceability, and certify that your yeast strains are suitable for their intended purpose.
• The sources of all your yeast strains must be traceable and known first and foremost. Therefore, it should be possible to determine if the yeast culture now in use was isolated in-house or acquired from an outside source, whether that was yesterday or decades ago. 
• Typically, it is preferable to select the yeast strain based on the presence of a number of desired characteristics and the lack of bad characteristics.

Yeast strain identity and purity

• It is crucial that plans are drawn up to guarantee that what is planned has been achieved. Genetic testing of identity and purity can be carried out via PCR-based DNA fingerprinting or by karyotyping.
• In addition, phenotypic testing, such as tests for flocculation, fermentation performance, and the presence of certain traits, such as the capacity to grow at different temperatures, the presence of particular enzymes, or the capacity to grow in the presence of particular antimicrobial agents, is useful.
• Additionally, tests for contaminating microorganisms (wild yeasts and bacteria) must be conducted. All laboratory tests must function as intended, and all procedures must be validated at regular intervals to guarantee performance.

Preservation of yeast starter cultures

• The preservation process adopted is basic. The ignored elements are the primary difficulty.
• Preservation in liquid nitrogen is the procedure for deciding. Yeast strains preserved in straws or cryovials will likely remain unchanged for hundreds of years, if not longer, under such conditions. This approach accurately creates an environment in which yeast cells are suspended in suspended animation.
• Importantly, the technique has no negative impact on the genetic or physiological characteristics of the cells. Viability of the cells is extra kept up under these conditions.
• For the preservation procedure there is a demand for master hardware, authority staff, and access to a reliable source of liquid nitrogen are just three.
• Staff safety is of the utmost importance, and safeguards must be made to protect liquid nitrogen users from the suffocating effects of oxygen deprivation and nitrogen gas.
• Generally speaking, it is optimal to have two types of cultures: master cultures and working cultures. Given sufficient storage space, sufficient working cultures must be available to produce all the required yeast well in advance.

Supply of yeast starter culture

• As part of the risk management strategy, slant cultures are shipped to breweries from a single, approved, and inspected production location with a move-down production facility access.
• It makes obvious that your goal should be to render all of these yeast spreads indistinguishable from one another. This must begin with the initial step of procreation.
• Today, the vast majority of lager breweries proliferate their yeast every week or every other week. Typically, this involves inoculating approximately 10 ml of wort with yeast from a slant culture.
• This yeast can either be extracted from the slope using an inoculating loop or washed off the slope and used as the inoculant. In either case, a yeast slope culture should be abandoned after a single use. Cultures on slopes should be considered disposable.
• Utilizing them many times invites irregularity in the early phases of propagation, which will only grow when the culture starts its active life in the brewery.
• The use of ultra-pure dried yeast cultures is a substitute for yeast slope supply. This is the format of choice when cool shipping and storage cannot be ensured or when the brewery lacks laboratory facilities.
• These dry yeast cultures are sold in 50 g quantities. Each is sufficient to inoculate a single Carlsberg flask with sufficient yeast to initiate multiplication in a brewery yeast propagator.
• This method of cultivation eliminates the requirement for laboratory propagation procedures.
• While this can save you time, money, and equipment, perhaps the greatest benefit is the adaptability it affords you in the event your yeast propagation fails.
• When brewery yeast propagations fail quality tests (for example, due to contamination with foreign microorganisms), production delays caused by having to wait for the laboratory to propagate a new yeast culture from slope can be both frustrating and expensive.
• Nevertheless, by adopting ultra-pure active dried yeast cultures and bypassing the laboratory phases of propagation, the time required for yeast propagation can be reduced by several days.

Quality assurance of yeast cultures

• Cultures are examined prior to shipment to ensure the purity, efficacy, and absence of microbiological contamination of all test techniques.
• All yeast cultures used in a brewery should be verified for their authenticity using microbiological and/or genetic techniques, such as DNA fingerprinting.
• In order to ensure that the culture is pure and uniform, it is necessary to conduct checks on its purity. DNA fingerprinting supports the viability of giant colony morphology on WLN agar for this purpose.
• Both bacteria and wild yeasts must be screened for microbiological contamination. In addition, as is customary, the procedures used for these tests must be authorised to ensure their efficacy.