Biomedical Engineering Reference
In-Depth Information
higher because of high probability in interaction of the acid with the wall material within the
microcapsule [62, 70]. The high oxidative stability of linoleic acid with the high molar ratio
would be attributed to the synergistic effect of the small oil droplet in the emulsion and the
antioxidative ability of palmitoyl ascorbate. It has been reported, in a non-encapsulated
system, that the addition of a large amount of a saturated fatty acid to a polyunsaturated fatty
acid decreased the polyunsaturated fatty acid fraction in the mixture and delayed the
oxidation of the polyunsaturated fatty acid [64]. Thus, it is possible that the addition of
palmitoyl ascorbate to linoleic acid resulted in improved oxidative stability of the
encapsulated linoleic acid by decreasing the fraction of linoleic acid in the mixture. To
examine this possibility, methyl palmitate was added to linoleic acid at various molar ratios,
and the mixture was then microencapsulated with maltodextrin by spray-drying.
The
encapsulation efficiency of linoleic acid, which was defined as a ratio of the actual amount of
the acid in the microcapsules to the one calculated from the composition of emulsion used for
preparing the microcapsules, was 0.4 to 0.6. The efficiency was almost the same as that of the
microcapsules prepared with no additive. At the ratio of methyl palmitate to linoleic acid of
0.1, the oxidation of linoleic acid was suppressed, but the extent was very small. Therefore,
the suppressive effect of palmitoyl ascorbate on the oxidation of the encapsulated linoleic
acid is not due to the decrease in the fraction of linoleic acid by the addition of the ascorbate.
To compare the effects of unmodified ascorbic acid and acyl ascorbate on the oxidation of
encapsulated linoleic acid, the oxidation processes of linoleic acid encapsulated with
maltodextrin to which unmodified ascorbic acid was added to produce specified molar ratios
of ascorbic acid to linoleic acid were examined. The addition of unmodified ascorbic acid at
any ratio had no effect on the encapsulation efficiency of linoleic acid. Unmodified ascorbic
acid slightly suppressed the oxidation of linoleic acid at a molar ratio of 1. Because this ratio
was 10 times that for palmitoyl ascorbate, the antioxidative effect of unmodified ascorbic acid
for encapsulated linoleic acid was very weak. Elongation of the induction period, which was
observed for palmitoyl ascorbate, did not occur. It is expected that saturated acyl ascorbates
with different acyl chain lengths also possess antioxidative ability for the encapsulated
linoleic acid. Figure 19 shows the oxidation processes of linoleic acid mixed with various
saturated acyl ascorbates at a molar ratio of 0.1 and encapsulated with maltodextrin. Mixing
of any ascorbate significantly increased the encapsulation efficiency of linoleic acid and the
efficiency was greater than 0.9. The median diameters of the oil droplets in the O/W
emulsions before microencapsulation are also shown in Figure 20. Ascorbates with acyl chain
lengths of 10 or more gave emulsions with small median diameters and significantly
suppressed the oxidation of the encapsulated linoleic acid. Especially, decanoyl ascorbate was
superior both in emulsification ability (small diameter of emulsion) and in suppression of the
oxidation. The oil droplet size of the emulsion prepared with octanoyl ascorbate was large,
and the ascorbate was less effective than other ascorbates for suppression of the oxidation of
the encapsulated linoleic acid. The solubility of decanoyl ascorbate in water was almost the
same as that in soybean oil. The saturated acyl ascorbates with acyl chain lengths of more
than 10 are apt to dissolve in the oil, while the ascorbates with the lengths of less than 10
preferentially dissolve in water rather than in the oil. Therefore, when octanoyl ascorbate was
used for microencapsulation of linoleic acid, most of the molecules would exist in the
dehydrated wall material layer and could not scavenge the polyunsaturated fatty acid radicals
generated in the oil phase. Because ascorbates which produced emulsions with small oil
droplets were surface-active, they were apt to locate in the interface of linoleic acid and the
