Biomedical Engineering Reference
In-Depth Information
the surface charge of the oil droplets would be affected by the pH of the aqueous phase. The
-potentials of oil droplets in the O/W emulsions prepared using decanoyl and lauroyl
ascorbates at a concentration of 0.5 mmol/kg were measured at various pH values of the
aqueous phase. For the emulsions with decanoyl and lauroyl ascorbates, the potential largely
decreased at a pH higher than 4. These largely negative surface charges at pH 5 and 6 would
be the reason for the high stability of the emulsions at these pH values.
6. Antioxidative Ability of Acyl Ascorbate
against a Lipid in a Bulk System
The oxidation processes of linoleic acid in the presence of ascorbic acid or saturated acyl
ascorbate were analyzed by applying a kinetic equation of the autocatalytic type to the
processes [35]. Linoleic acid and ascorbic acid or octanoyl, lauroyl or palmitoyl ascorbate
were mixed at the molar ratio of 0.025, 0.05, 0.075, 0.1 or 0.2 (ascorbate/ linoleic acid), and
preserved in a plastic container to maintain the relative humidity at 12%. The container was
stored in the dark at a given temperature (37, 50, 65 or 80
o
C). Samples were periodically
taken, and the amount of unoxidized linoleic acid was measured by the GC analysis with an
FID [61, 62]. The oxidative stabilities at 37
o
C and 12% relative humidity of linoleic acid
mixed with ascorbic acid and octanoyl ascorbate at various molar ratios of the ascorbates to
linoleic acid were measured. The higher the molar ratio was, the more strongly the oxidation
of linoleic acid was suppressed in both cases. The induction period for the oxidation of
linoleic acid was elongated by the addition of the ascorbates. The oxidation processes of
linoleic acid mixed with lauroyl and palmitoyl ascorbates showed similar results. Octanoyl
ascorbate had stronger antioxidative ability than ascorbic acid. The oxidation of linoleic acid
mixed with octanoyl ascorbate at the molar ratio of 0.1 was suppressed for at least 130 h.
Figure 17(a) shows the oxidation processes at 80
o
C and 12% relative humidity of linoleic acid
with no additive and that mixed with ascorbic acid or octanoyl, lauroyl, or palmitoyl
ascorbate at the molar ratio (additive/ linoleic acid) = 0.05. Linoleic acid with no additive was
quickly oxidized. When ascorbic acid was added to linoleic acid, the oxidative stability of
linoleic acid was slightly improved. Octanoyl, lauroyl, and palmitoyl ascorbates retarded the
oxidation of linoleic acid more than ascorbic acid. There seemed to be little difference in the
antioxidative ability among the three acyl ascorbates. We showed previously [63, 64] that the
entire oxidation process of linoleic acid could be expressed by the following kinetic equation
of the autocatalytic type:
dY
=
−
kY
(
−
Y
)
(9)
dt
where
Y
is the fraction of the unoxidized substrate, and
k
is the rate constant. Under the
condition of
Y
=
Y
0
at
t
= 0, the integration of Eq. 9 gives:
1
−
Y
1
−
Y
0
ln
=
kt
+
ln
(10)
Y
Y
0