Microbial utilization of mono- and di-saccharide residues
The by-products (molasses, sulphite liquor, whey) listed in group 1, Table 1,
are rich in fermentable sugars, and they serve as a major source of carbon for a
great variety of micro-organisms. At least 5,000 microbial metabolic products
have been isolated from solutions in which the simpler sugars have served as the
main source of carbon for metabolism by micro-organisms. These metabolites
include not only simple alcohols, organic acids, gases, antibiotics, vitamins,
enzymes, toxins, etc. but also some unique compounds whose use or function
remains unknown. Great opportunities exist for finding uses for some of these
substances, or for developing technologies that may be applicable to rural
processing of such materials.
Large quantities of molasses are produced in countries where sugar-cane is
grown and processed. Rolz, for example, estimates that over 6.3 million tons are
available annually in the major sugar-cane-growing countries of Latin America
The sugar in molasses can be metabolized by many micro-organisms and by
several known pathways. The particular pathway followed, and the end-products
produced, depend not only on the particular microbe, but also on a variety of
Special strains of Saccharomyces cerevisiae, S. fragilis, and Candida utils
are used in the baking industry, as feed and food supplements, and for other
purposes. World production of such yeast is over 300,000 tons per year. The raw
materials for cultivation of such yeasts are generally a mixture of molasses,
ammonium salts, and other essential inorganic salts.
In recent years the production of filamentous fungi as a source of protein
has been emphasized. Espinosa et al., for example, have shown that the growth of
Verticillium sp. on cane blackstrap molasses and coffee-waste water is
technically feasible (6).
Mushroom mycelium has also been grown in molasses, as well as in vinasse, a
waste product from the distillation of fermented sugar-cane juice.
Perhaps the greatest potential use of molasses, other than as a sweetener in
foods for human consumption, and as a livestock feed supplement, is for the
production of ethanol by fermentation, or as a feedstock for the manufacture of
other useful products. The fermentation of molasses to ethanol by yeast is not
an especially complex process, and it can be easily adapted on a small scale to
rural areas. In Brazil, however, the production of ethyl alcohol from
sugar-cane, manioc, and other tropical plants has become a major project of the
government to reduce petroleum imports (Figure 1). Approval was given by
Brazil's National Alcohol Commission for government financing in the amount of
US$800 million in 1977 for over 30 of the 170 proposed distilleries. The plan
calls for increasing alcohol production to over 3,800 million I by 1982. As
fossil fuels become scarcer, many nations may need to turn to the ethanol
fermentation of waste saccharide materials as a source of energy (7).
Figure. 1. Fermentation of Biomass to Ethanol or
Other Organic Chemicals, and Other Organic Chemicals (From Altepohl )
Sulphite Waste Liquor
Several million tons of sugar occur in the sulphite liquor that results from
the production of paper products; most is discarded in the United States (Table
3), and similar amounts are probably considered waste in other countries. Apart
from the fact that sulphite liquor from the paper mills causes a disposal
problem, it is also an economic loss because it can be converted into
single-cell protein (SCP), ethanol, or D-lactic acid.
Candida utilis has been used for alcohol and feed yeast production from paper
mill waste because it has a high tolerance for sulphite and can convert both
hexoses and pentoses into yeast protein. A commercial operation called the
Pekilo Process has been developed in Finland for the production of single-cell
animal feed. Spent liquor from sulphite pulp mills is used as the substrate, and
the fungus Paecilomyces variotil, which consists of 55 to 60 per cent protein,
is used in the fermentation process. The first Pekilo plant built produces about
10,000 tons of single-cell protein annually.
Lactobacillus pentosus seems superior to other bacteria for producing
D-lactic acid from sulphite waste liquors. Estimates for a mill producing 100
tons of pulp daily are that over 3 million kg of lactic acid can be manufactured
Mushroom mycelium has been grown in sulphite waste liquor, and the process
has been granted a patent.
In countries where cheese-making is important, large volumes of whey
accumulate and must be disposed of as a waste, as profitable uses have not been
found for the material. Development of new uses for whey would do much to reduce
the waste and avoid the loss of milk nutrients. The possibilities for such
developments offer some of the most interesting challenges in applied science.
Whey has some limitations as a substrate for attack by micro-organisms
because fewer microbes utilize lactose than other sugars such as glucose. The
best suited organisms for fermentation of whey are lactobacilli and certain
Lactobacillus bulgaricus is capable of converting over 90 per cent of the
lactose in whey to DL-lactic acid, and the organism is now used commercially for
this purpose. Various lactosefermenting yeasts (Saccharomyces fragilis, Candida
pseudo-tropicalis, or Torula cremoris) can convert the sugar to various products
without altering the other nutrients in whey; this has become a commercial
process for producing lactose-free whey and ethanol (80 to 90 per cent
conversion of the lactose).
Several hundred-thousand tons of yeast for baking, feed, and food supplements
have been manufactured for many years, utilizing low-grade sugars as a
substrate; the demand for such protein is increasing. Recently a new,
continuous-flow, closed-system plant has been put into operation to produce the
lactose-fermenting yeast Candida utilis from whey. The plant is capable of
manufacturing 7,500 tons of yeast annually.
Juices from various fruits, leaves, and stalks of plants contain sugars that
can be grouped in categories I and II (Table 1). Many of the materials are
abundant and cheap, and could be readily converted by microbial processes to
useful substances. One example may be mentioned.
Agave juice from plants growing on arid lands has been used experimentally as
a substrate for SCP production (8). Both pure cultures of yeast (Saccharomyces
carbajali, Candida utilis, etc.), and mixed cultures of yeast, fungi (Ustilago
maydis), and bacteria (Corynebacterium glutamicum, Brevibacterium flavum) were
used to produce the SCP biomass. The yields of high-quality microbial protein
obtained were good (20 g/l) from a 24hour semi-continuous operation. Indications
are that a plant would have considerable socioeconomic impact on production in
Mexico, where protein feed and food are badly