Biomedical Engineering Reference
In-Depth Information
Mixed microbial populations are commonly used in cheese making, a good example of
using mixed cultures in food production. Cheeses of various types are produced by inocu-
lating pasteurized fresh milk with appropriate lactic acid organisms. The bacteria used for
lactic acid production are various species of Streptococcus and Lactobacillus in a mixed culture.
Other organisms are used to develop flavor and aroma. Among these are Brevibacterium
linens, Propionibacterium shermanii, Leuconostoc sp., and Streptococcus diacetilactis. After inocu-
lation of pasteurized milk, a protein-rich curd is precipitated by the acidity of the medium,
and the liquid is drained off. The precipitated curd is allowed to age by action of bacteria or
mold. Some molds used in cheese making are Penicillum camemberti and Penicillum roqueforti.
Lactic acid bacteria are also used in whiskey manufacture. Lactobacillus added to the yeast
reduces pH and, therefore, the chance of contamination. Lactobacillus also contributes to the
flavor and aroma of whiskey. Lactic acid and ethanol react forming ethyl lactate which gives
an aroma. A favorable interaction between yeast and lactic acid bacteria exists in ginger-beer
fermentation.
The types of mixed cultures vary among applications. Food processing involves largely
defined mixed culture and with batch operations, which represents one extreme of the mixed
culture applications. Another extreme is the wastewater treatment process, where contin-
uous operation is warranted for mass processing needs and undefined (may even be
unknown) microbial cultures is typical and unavoidable. The low value (or zero value) of
the product and a desire of minimum cost to destroy the substrate(s), both contributed to
complex mixer of microbial cultures applied in wastewater treatment.
Wastewater treatment constitutes one of the largest scale uses of bioprocesses. Mixed
cultures are also utilized in the anaerobic digestion of waste materials. Cellulase producers,
acid formers, and methane producers are typical organisms involved in the anaerobic diges-
tion of cellulosic wastes. However, attempts to encourage the growth of a particular species
on waste materials are commonly made to maximize waste treatment rate and/or particular
final products (valoration of wastes).
The Symba process was developed in Sweden for treating starchy wastes, particularly
those from potato processing. This process utilizes Endomycopsis fibuligera for amylase
production and a yeast, Candida utilis, for the utilization of sugar molecules (glucose, maltose,
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/
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FIGURE 16.24
Phase trajectories for prey
e
predator interactions: D
¼
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bmax
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3 X
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.
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