Biomedical Engineering Reference
In-Depth Information
Figure 8. van't Hoff plots for the solubilities,
S
, of (
{
,
z
) decanoyl, (
,
) lauroyl, (
U
,
S
) myristoyl,
(
V
) palmitoyl and (
) stearoyl ascorbates in soybean oil (open symbols) and water (closed symbols).
The straight lines were empirically drawn.
4. Stability of Acyl Ascorbate in an Aqueous Solution and Air
The decomposition of saturated acyl ascorbate in an aqueous solution was measured at
various pHs and temperatures [34]. An appropriate amount of an acyl ascorbate, which was
determined to give a final concentration of 5 x 10
-5
to 1 x 10
-2
mol/L, was weighed into an
amber vial with a screw-cap. Five milliliters of a buffer solution was added to the vial to
dissolve the ascorbate. The buffer solutions used were 0.1 mol/L sodium citrate-HCl for pHs
2, 3 and 4, 0.1 mol/L sodium citrate-NaOH for pHs 5 and 6, and 0.1 mol/L Tris-HCl for pHs
7, 8 and 9. The vial was immersed in a water-bath controlled at a specified temperature of 30
to 60
o
C, and then vigorously shaken. At appropriate intervals, a portion of the solution (10
μ
L) was sampled to determine the remaining ascorbate by HPLC. The Weibull model is
flexible and has a potential for describing many degradation kinetics [43]. Therefore, we used
the model to describe the degradation of acyl ascorbate:
[
]
C
()
n
= exp
−
kt
(3)
C
0
where
C
is the acyl ascorbate concentration at time
t
, and
C
0
is the initial ascorbate
concentration.
k
is the rate constant, the reverse of which is called the scale constant, and
n
is
the shape constant. The kinetic parameters,
k
and
n
, were evaluated by fitting the
experimental results by non-linear regression using the Solver of Microsoft Excel
®
for
Windows [44-46] or the Origin
®
Ver. 6.1 (Microcal Software, Inc., USA). The stability of
acyl ascorbates was assessed at 30, 40, 50 and 60
o
C. Each ascorbate was dissolved in the
sodium citrate-NaOH buffer, pH 5, at a concentration of 0.2 mmol/L. The parameters,
k
and
n
, were evaluated at the temperatures, and the temperature dependence of the rate constant,
k
,
for each ascorbate was analyzed based on the Arrhenius equation:
