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The main benefits of this form of yeast are its reliability, ease of use, stability and accessibility. Dry yeasts are a reliable way of achieving batch- to-batch reproducibility, under like- for-like conditions. Reproducibility in the fermentation process increases predictability, a critical factor for any brewery operating at full capacity. What's more, these yeasts come ready-to-pitch. They can be added to the wort directly, or rehydrated first. No propagator or lab equipment needed. Once pitched, the ADY propagates in the wort, doing away with the need for additional aeration or additives. Freezing the stem cells/mother cells used to produce ADY before storing them at -80 °C in a stem cell bank ensures genomic consistency across batches. Moreover, ADY's long-term stability gives its an excellent shelf life meaning consistent performance over many years. Its long shelf life, and the ease with which it can be stored, (temperature, humidity etc.) transported and handled, make the product accessible to all brewers. Finally, contrary to popular belief, unless you have the facilities and expertise to produce liquid yeast in- house, ADY is the freshest form of yeast on the market. The yeast is dried during the growth phase right at the moment when biomass is being produced. The propagation process through which biomass is produced must be stopped at the right time, when the yeast population has reached optimal conditions for fermentation in terms of the integrity of cell walls and the absorption of fermentable sugars. A gentle drying process suspends the yeast in time, guaranteeing its freshness. Manufacturers of ADY produce regular updates to publicise the specific characteristics of each yeast, including fermentation kinetics, apparent attenuation, alcohol tolerance, flocculation, sedimentation, aromatic profile. The number of bottom- fermenting yeasts ( Saccharomyces pastorianus ) and top-fermenting yeasts ( Saccharomyces cerevisiae ) on the market is gradually increasing, although many breweries remain loyal to particular strains for specific styles. THE MANUFACTURING PROCESS The first step in the process involves propagating the yeast in small flasks which offer a sterile, nutrient-rich environment. Having been decanted through a series of containers, each one bigger than the other, the culture is transferred to a propagator and then to large fermentation vessels where budding (asexual reproduction) takes place resulting in significant growth. There the culture is grown on molasses (source of carbon), and exposed to oxygen (introduced through aeration) and a large number of nutrients that are high in nitrogen, phosphorus and growth factors. Throughout the growth stage, the yeast undergoes asexual reproduction, called budding, in which the mother cell forms a bud which gradually forms a clone of the mother-yeast (cytosol, organelles, nucleus etc.). The bud continues to grow until it separates from the parent cell, forming a daughter cell. This cycle repeats over a number of generations. To achieve an optimal physiological state for drying and fermentation, carbon and other nutrients must be added to the culture under controlled conditions. In its natural habitat, yeast has been shown to produce protective compounds such as glycerol, trehalose and glycogen under adverse conditions. The yeast metabolises glycerol as part of its osmotic stress response and produces trehalose to protect the membrane, a crucial factor in the drying process. Trehalose and Glycogen are also reserve carbohydrates and make the yeast naturally resistant to the drying process. Following propagation, a centrifuge is used to separate the culture from its nutrient medium, producing a yeast cream. THE PRODUCTION CYCLE This fresh cream yeast (20-28% dry matter) is stored at a low temperature. It is then drained to produce compressed yeast (min 32% dry matter), which is then extruded and dried to produce ADY (min 95% dry matter). A range of techniques are now available tomanufacturers of driedmicroorganisms for use in the fermentation process. The most commonly used techniques are: - Fluid bed dryer: After propagation the culture is drained to produce a solid filter cake before being fed into a fluid bed dryer. This filter cake is turned into granules which are deposited on the perforated plate of the dryer. Hot air is introduced through a perforated distribution plate at the base, its upward force causing the culture particles to become suspended in the air. Air temperature and humidity are carefully controlled and, the culture dries as it is exposed to the air, before exiting the machine in the form of a powder via an outlet at the top. A filtration process or cyclone is used to extract this powder from the air. The biomass is kept in the drier until the dry matter is produced. This process doesn’t involve freeze-drying. - Freeze-drying: after the fermentation process, the culture is concentrated and mixed with a protective agent. The mixture is then frozen. Ice is then removed from the frozen culture through a process of sublimation, in which water is evaporated without passing through the liquid state. Through the use of these techniques the culture retains its integrity, viability, vitality, genotype and other aspects of its phenotype, including its technical and sensory characteristics. QUALITY The purity of dry yeast has improved significantly since it was first introduced in the brewing industry. When stored in the right conditions, it remains stable for a number of years. Indeed in 2019 Fermentis introduced a guaranteed shelf life of 2 to 3 years. Initially marketed as a single-use product, increasingly it is cropped and reused in the same way as wet varieties. The following specifications apply to the majority of ADYs used in the brewing industry: - viable yeasts > 1.0 x 1010 CFU/g; - purity: > 99.9 %; - lactic acid bacteria: < 1 CFU/107 yeast cells; - acetic acid bacteria: < 1 CFU/107 yeast cells; - total bacteria: < 5 CFU/107 yeast cells; - wild yeasts: < 1 CFU/107 yeast cells; - meets microbiological safety standards. Estimated ADY viability is 85 %. The formation of bud scars, or even the buds themselves, can result in deformations in the cell wall. Hydration and the resulting turgidity can cause the cell wall to collapse (around 15% of cells). To achieve viability, effective propagation protocols must be used, in which the number of budding cells and areas of cell wall fragility are kept to a minimum. PITCHING METHODS AND RATES In its processed state, ADY resembles a compact spongemade up of miscroscopic beads tightly pressed together. For successful fermentation, yeast cells must reconstitute by recovering any liquid lost during the drying process. In its dehydrated state, the yeast membrane is wrinkled but when exposed to water or wort becomes perfectly smooth again. Two different fermentation techniques can be used: rehydration or direct pitching. According to a recent study, these two techniques produce similar results in terms of viability and vitality. This has been investigated extensively by Fermentis and is now subject to the E2U™ certification system. 34 | Winter 2023 | ontapmag.co.za