Abstract
The bacterium Zymomonas mobilis is a promising alternative to the
traditional yeast in ethanol production processes when glucose is used as
carbon source.In this study the reasons for decreased ethanol yields on
sucrose and fructose based media were investigated.In addition to the
previously recognized high-molecular weight fructopo Iymer, levan, two types
of new by-products of sucrose fermentations by Z. mobilis were detected:
sorbitol and oligosaccharides.When, fructose was used as carbon source, for
mation of mannitol, dihydroxyacetone and glycerol decreased the ethanol yield.
In the utilization of sucrose, transfructosylation reactions lead to the
formation of glucose and fructose, oligosaccharides or levan.The amounts of
oligomers and levan increased with the number of fructosyl transfers in the
cell, i.e. with increasing sucrose concentration or dilution rate in
continuous fermentation.Levan formation was inhibited by glucose and a number
of other sugars, whereas no inhibition by ethanol was observed.
Oligosaccharides consisted of trimers, tetramers and pentamers, containing
respectively two, three or four fructose molecules per molecule of glucose.Two
trimers and two tetramers were isolated, representing structurally different
series in which the fructosyl unit is transferred to either the 1-or the
6-position of the fructose moiety of sucrose or oligomers.Reduction of
fructose to form sorbitol is coupled to the oxidation of glucose to glu conate
by a dehydrogenase having a non-dissociable coenzyme.Thus, sorbitol is formed
in high quantities when both fructose and glucose are present in the medium.
However, small amounts of sorbitol were also formed from glucose or fructose
alone, as a consequence of the non-specificity of the enzyme system.The
production of sorbi tol provides the cell with an alternative route for the
formation of 6-phosphogluconate and thus a means of overcoming the rate
limiting step in the metabolic pathway.Mannitol is formed from fructose by an
NADPH-specific mannitol dehydrogenase, which is apparently coupled to the
oxidation of glucose-6- phosphate.This may also serve as a way of increasing
the rate of 6-phosphogluconate production in the cell.Accumulation of
dihydroxyacetone decreased the ATP yield of the carbohydrate meta bolism, thus
together with its inhibitory action decreasing the cell yield on fructose
containing media.Acetaldehyde, which was formed particularly in aerated
cultures of Zymomonas, was also shown to be toxic for growth.Different
proportions of by-products were formed from sucrose in batch and con tinuous
fermentations, mainly due to the different glucose concentrations in the media
under different fermentation conditions.High ethanol yields were obtained
only at very low dilution rates.Different Z. mobilis strains were compared
with respect to their by-product patterns, but in spite of some variations in
the by product composition, the ethanol yields were always within the same
rather narrow limits.Thus, the develop ment of new strains is necessary in
order to obtain improved ethanol yields on sucrose based media.
Original language | English |
---|---|
Qualification | Doctor Degree |
Awarding Institution |
|
Award date | 10 Apr 1986 |
Place of Publication | Espoo |
Publisher | |
Print ISBNs | 951-38-2531-0 |
Publication status | Published - 1986 |
MoE publication type | G4 Doctoral dissertation (monograph) |
Keywords
- bacteria
- ethanols
- fermentation
- pentoses
- bioengineering