Biofuels in Perspective

140 Biofuels
Ratio of initial reaction rate (-)
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0
0.0 0.5 1.0
Fatty acid ratio indicated by R f (-)
Figure 8.10 Effect of fatty acid ratio (R f) on development of initial reaction rate and methyl ester content.
Left-hand scale shows ME content after 2.5-h in second batch cycle as percentage of that in first. (From Ref.
50, with permission of Elsevier.)
activity than saturated fatty acid-enriched cells, among which palmitic acid-enriched cells
exhibited significantly greater enzymatic stability than unsaturated fatty acid-enriched
cells. It was assumed that fatty acids significantly affect the permeability and rigidity of the
cell membrane, and that higher permeability and rigidity lead to increases in methanolysis
activity and enzymatic stability, respectively.
Next, to investigate the optimal composition of the mixture of oleic and palmitic acid
added to the culture medium, two cycles of repeated batch operation were carried out using
mixtures with varying ratios of oleic to palmitic acid. Figure 8.10 shows ME content after
2.5 h in the first batch cycle and the initial reaction rate of ME production in the second
batch cycle as a percentage of that in the first. This value reflects the degree of enzymatic
stability: the higher it is the greater the enzymatic activity that can be expected.
Following increase in the fatty acid ratio, indicated by Rf [= oleic acid/(oleic acid +
palmitic acid), w/w], there was no significant acceleration of the decrease in the initial
reaction rate up to Rf value of 0.67, beyond which however it decreased rapidly. ME
content showed its maximum value at Rf of 0.67, and decreased at Rf values greater or less
than this critical level. It was thus concluded that there exists an optimal Rf value of 0.67
which yields both higher methanolysis activity and higher stability.
Figure 8.11 shows the time course of ME content in repeated methanolysis using R.
oryzae cells obtained at various Rf values. The figure shows that, at Rf value of 1.0, both
ME content and the initial rate of ME production decreased sharply with each cycle to give
an ME content of only 5 % at the end of ten batch cycles. In contrast, at Rf value of 0.83
and 0.67, ME production rates decreased gradually with each cycle, and, in the latter case
in particular, ME content was maintained at a high value of around 55 % even after ten
batch cycles. These results are consistent with the data in Figure 8.10 and indicate that the
optimal value of Rf in terms of both ME content and ME production rate is around 0.67.
Several studies have shown that free fatty acids affect the properties of the plasma
membrane. Many have reported that membrane permeability is significantly affected by
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Methyl ester content (wt.%)