Biomedical Engineering Reference
In-Depth Information
with high-pressure steam. After sterilisation, it is diluted with water and held in
tanks until use. Additional nutrients, including nitrogen, potassium, phosphate,
magnesium and calcium, with traces of iron, zinc, copper, manganese and molybde-
num, are added. Usually, nitrogen is supplied by adding ammonium salts, aqueous
ammonia or anhydrous ammonia. Phosphates and magnesium are added in the form
of phosphoric acid or phosphate salts and magnesium salts. Vitamins such as biotin,
inositol, pantothenic acid and thiamine are also required. The latter is added to the
feedstock, while the others are already present in the molasses malt.
6.5.1.2
Fermentation
Yeast cells are grown in a series of fermentations to decrease the synthesis of ethanol
and CO
2
. The fermentation is operated under aerobic conditions. The initial stage of
propagation takes place in the laboratory. A portion of the pure yeast culture is
mixed with molasses malt in a sterilised flask, and the yeast is allowed to grow for
2-4 days. The entire content of the flask is used to inoculate the first fermentation
vessel. Batch fermentations are carried out, where the yeast is allowed to grow for
13-24 h. Usually, one or two vessels are used for this stage of the process. The batch
fermentations are essentially a propagation of the flask fermentation, except for the
use of sterile aeration and aseptic transfer for the subsequent stage. Following pure
culture fermentations, the yeast mixture is transferred into an intermediate vessel
under batch or fed-batch conditions. The following fermentation is a stock fermen-
tation stage (Fig.
6.8
). The content from the intermediate vessel is pumped into the
stock vessel, which is equipped for increasing feeding under aeration. This stage is
termed stock fermentation, since after the fermentation is completed, the yeast biomass
is separated from the bulk of the vessel through centrifugation, which produces a
stock of yeast for the next stage. The next stage, pitch fermentation, also produces a
stock of yeast. Aeration is vigorous, and molasses and other nutrients are increas-
ingly fed into the vessel. The liquor from this vessel is usually shared into several
parts for pitching the final trade fermentation. Alternatively, the yeast is separated
by centrifugation and stored for several days before use. The final trade fermenta-
tion has the highest degree of aeration. A large air supply is required and the vessels
are often started in a staggered fashion to reduce the size of the air compressors. The
initial fermentation lasts 11-15 h. After which the molasses have been fed into the
vessel, the liquid is aerated for 0.5-1.5 h to allow a further maturation of the yeast.
This step has a stabilising effect for the subsequent refrigerated storage. The content
of the yeast biomass increases stage by stage: ca. 120 kg in the intermediate vessel,
420 kg in the stock vessel, 2,500 kg in the pitch vessel, and 15,000-100,000 kg in
the trade vessel. Once the optimum quantity of yeast is grown, the biomass is recov-
ered from the final trade vessel by centrifugation. The centrifuged yeast biomass is
further concentrated by a filter press or rotary vacuum filter. The filter press forms a
filter cake containing 27-32% of solids. The rotary vacuum filter forms a cake with
ca. 33% of solids. The filter cake is then blended in mixers with small amounts of
water, emulsifiers, and cutting oils to form the end-product. The final packaging
steps vary depending on the type of yeast product.
