Yeast propagated for the baking industry represents the largest amount of a single type of microbial biomass produced, with an annual production of baker’s yeast netting $1.5 billion globally. The large-scale production of distilling and fuel ethanol yeasts follows the same principles as those employed for the manufacture of baker’s yeast. A multi-stage propagation in a fermentation media usually consisting of sugar cane or beet molasses (as an inexpensive source of sucrose) with additional sources of nutrients added. The cell density progressively increases. The later stages are highly aerobic and the molasses medium is delivered slowly to avoid the Crabtree effect (i.e. suppression of respiration by high glucose levels where the cells continue to ferment despite the availability of oxygen) and to maximize respiratory growth. Yeast can be supplied to the ethanol industry as a slurry (yeast cream), compressed yeast or an active dry yeast.
Most of the yeast used in distilleries and fuel ethanol plants is purchased from manufacturers of specialty yeast appropriate for the purpose. Breweries in contrast tend to have their own proprietary strains and scale-up systems since they have the ability to crop and re-use their yeast.
Manufacturers propagate from proprietary laboratory stock cultures (best practice stores these cultures at ultra low temperatures) through a succession of fermentation vessels of increasing size. A culture medium of molasses (beet or cane) is supplemented with ammonium salts and any required trace nutrients. All cultures are grown with accurate temperature control at
30oC, with vigorous aeration (Campbell,
2003b). For efficient cultivation it is essential to maintain a low concentration of fermentable sugar in the culture medium. This avoids the Crabtree effect. The sugar is added as sterilized concentrated molasses, slowly at first and then increasing rapidly as the yeast biomass increases, so that there is a consistent level of about 0.5% sugar present. The yeast concentration will eventually surpass the ability of the aeration system of the culture vessel to maintain the necessary aerobic conditions for efficient
growth. When the dissolved O2 concentration
approaches zero (after about 30 hrs growth) propagation is halted. The yeast culture can be
Understanding yeast fundamentals 97
harvested using a rotary vacuum filter. The circumference of the filter is a continuous strip of fabric coated with food-grade starch. As the drum rotates slowly through the culture in the trough, the spent culture medium is drawn under vacuum through the hollow spokes. The yeast is retained on the filter, scraped off and packaged.
YEAST SLURRY
Yeast in the slurry form is used primarily by brewers. They can recycle their yeast at a high viability (>90%). The slurry must be kept cool (4°C) and has a 3-7 day shelf life. If the yeast is fresh it can be acid-washed to lower the bacterial count. The slurry usually has a cell count in the
range of ~3 x 109 cells/mL at 14-18% total solids.
COMPRESSED YEAST
Compressed yeast is more stable than slurry and
requires cooling. The yeast is stored at 3–4oC
and has a 30 day shelf life. It loses viability at ~5-10 % a week. It can be rehydrated easily and has an indigenous bacterial count. The compressed yeast usually has a yeast count in
the range of ~6-13 x 109 cells/mL at 33% total
solids.
Compressed yeast is prepared at 4°C by adding salt to the yeast slurry and then passing the yeast through a cloth press or through a rotary drum vacuum filter. The resultant yeast has the consistency of butter and is then extruded into blocks, packaged, and both stored and shipped at 4°C. The yeast is sold as 25 kg bags of compressed moist yeast (24–33% dry weight) or in bulk as cream yeast slurry of about 18% dry weight. Before using compressed yeast it must first be slurried aseptically in a sterilized yeast mixing vessel to provide the required consistency for pitching. Cream yeast, which is delivered, stored and pitched in bulk, is often used for automated large-scale operations.
ACTIVE DRY YEAST (ADY)
Active dry yeast is used by most batch ethanol plants. It is also used to start continuous plants and to supplement fermentors. This yeast is dried
under partial vacuum at 45–55oC in an
atmosphere of inert gas, usually nitrogen. Dried yeast (92–96% dry weight) has a long shelf life and does not require cold storage (but can be chilled for added security). It is very stable at room temperature and even more stable (over a year) if stored cool and under nitrogen gas. It loses ~7% viability/month at room temperature and <10% year at 4°C under nitrogen gas. The active dry yeast usually has a viable cell count
in the range of ~2.2 x 1010 cells/g. It has an
indigenous bacterial count. The process to make active dry yeast involves extruding the compressed yeast into spaghetti-like strands that are then dried in air lift dryers (fluidized bed). Reactivation (conditioning) of dried yeast at pitching is carried out to prevent loss of viability. It is important that rehydration is carried out under the prescribed conditions and once rehydrated the yeast must be used immediately. The availability of a shelf-stable dried yeast
inoculum where each gram contains ~2.2 x 1010
viable yeasts, is a major biotechnological advance. The yeast, which has been grown under extreme aeration for yield, is easier to store, handle and transport. Selected strains for individual processes are available (i.e. beverage versus fuel). The use of active dry yeast has reduced the need for yeast propagation expertise in the plants and increased the predictability of fermentations. Active dry yeasts are used in distilling and fuel alcohol plants to inoculate fermentors to recommended values of 1.0-2.0 million viable cells/mL per degree Plato/Brix of wort/mash.
CONDITIONING (ACTIVATING) THE YEAST
It is usually recommended that the active dry yeast is conditioned (according to the different manufacturer’s instructions) to get optimal growth. (This is different from propagating a starter culture with the aim of growing up large quantities of yeast from a small lab culture.) For example, one manufacturer suggests a conditioning regime for active dry yeast for fuel ethanol plants where a yeast conditioning tank with a 16–18° Plato/Brix mash and a 1-2% glucose level is used. The recommended conditioning tank is 7–10% the size of the fermentor and the goal is 250-300 million cells/g. Temperature and nutrition at this stage are critical to ensure optimal yeast growth.
For distilled beverages, dried yeast can be added directly to the fermentation vessel or pre- mixed to a slurry with water. For direct inoculation, the wort cooler is adjusted to
produce wort at 38oC. The warm wort is
collected to a depth of 10 cm in the washback and the required amount of dried yeast is sprinkled onto the wort, taking care to avoid clumping. After 5 min. the heat exchanger is
adjusted to the normal set temperature, (~20oC)
and wort collection is completed.
For the pre-mixing method, sterile water or weak wort, 10 times the volume of dried yeast,
is brought to 38oC in a rehydration tank and yeast
is added with stirring, to prevent clumping. After mixing for 5 min. the yeast slurry is pumped into the washback at normal set temperature. The pre-mixing method uses the same equipment as the preparation of a slurry from bags of pressed yeast, but for dried yeast the higher temperature of mixing is critical (Campbell, 2003b).
It should be noted that there are many different methods of conditioning the yeast and it will depend on the particular plant, the instructions with the particular type of yeast purchased and the expertise in the plant. The objectives of yeast propagation/conditioning are to deliver to a fermentor a large volume of yeast of high viability, high budding in the log phase of growth and with a very low level of infection.
There are many methods of propagating yeast (batch, continuous) all with pros and cons depending on the final use of the ethanol i.e. beverage or fuel, the unique characteristics of the plant, the technical skill in the plant, etc. The yeast propagator is the heart of a distillery; and it is important to remember the key objectives that are critical to the performance of a fermentation. Hence cost savings in this area are not advised since they are usually minimal when compared to the cost of production per gallon of ethanol. The loss of ethanol production that is possible when an aged yeast is used can be in the range of 1% alcohol, which can lead to a loss of $0.10 per gallon produced. Infection resident in a plant can cause a drop of 3% alcohol and a potential loss of $0.30 per gallon.