Biomedical Engineering Reference
In-Depth Information
superficial residence time in the immobilized-lipase column, τ, and the acyl ascorbate
concentration in the effluent. For every acyl ascorbate, the maximum product concentration
was given at τ = 5 min or longer. Arachidonoyl ascorbate was synthesized for 1 day using the
reactor system. The fatty acid to be fed was changed to oleic acid, and the synthesis of oleoyl
ascorbate was continued for 2 days. Then, linoleic, decanoic, lauric and myristic acids were,
in this order, fed to the system to produce the corresponding ascorbates every two days
(Figure 7). For the synthesis of each acyl ascorbate, the τ value was fixed at 5 min. The
system could be stably operated for 11 days, and no loss of the enzyme activity was observed.
The product concentrations in the effluent were in the range of 14 to 17 mmol/L. The lower
concentration of unsaturated acyl ascorbates than those of saturated acyl ascorbates would be
ascribed to the lower purity of the unsaturated fatty acids. These product concentrations
corresponded to the productivity of 1.6 to 1.9 kg/L-reactor
·
day, depending on the molecular
mass of the product. The productivity was much higher than that for the synthesis of acyl
erythritol and mannose using a packed-bed reactor [41, 42].
3. Solubility of Acyl Ascorbate in Water and Oil
The solubilities of the saturated acyl ascorbates in water or soybean oil were measured at
various temperatures [29]. The purified acyl ascorbate (10 mg - 1 g) was weighed in an amber
vial, and 1 - 4 mL of distilled water or soybean oil was added. After the vial was immersed in
a water-bath at 50
o
C for 1 h, the vial was transferred to a chamber or water-bath kept at 5, 25,
37 or 50
o
C for 24 h. The solution was filtered using a membrane filter as quickly as possible,
and the filtrate was analyzed using an HPLC.
Although ascorbic acid is insoluble in soybean oil [27] and highly soluble in water, the
acylation of ascorbic acid significantly improved its solubility in soybean oil but decreased
the solubility in water. The temperature dependence of the solubilities,
S
, of the decanoyl,
lauroyl, myristoyl, palmitoyl and stearoyl ascorbates in water or soybean oil (Figure 8) could
be expressed by the following van't Hoff equation:
dS
Δ
H
=
−
(2)
()
R
d
1
T
where
Δ
H
is the dissolution enthalpy,
R
is the gas constant, and
T
is the absolute temperature.
The HPLC peaks of the palmitoyl and stearoyl ascorbates in aqueous solution were not
detected at every temperature because of their very low solubilities in water. The solubilities
of the acyl ascorbates in both soybean oil and water were higher for those with a shorter acyl
chain. The dependence of the solubility in water on the acyl chain length of the acyl ascorbate
was much stronger than that of the solubility in soybean oil. The plots in Figure 8 produced a
straight line for each acyl ascorbate. The dissolution enthalpy,
Δ
H
, was evaluated from the
slope. The relationship between the acyl chain length and
Δ
H
for the solubilization of the
saturated acyl ascorbates in soybean oil or water is elucidated. The
Δ
H
values were about 20
kJ/mol for soybean oil and about 90 kJ/mol for water.
