Biomedical Engineering Reference
In-Depth Information
emulsions, the aqueous phase of which contained the polysaccharides, were measured.
Because gum arabic and soluble soybean polysaccharide have an emulsifier ability [71, 72],
the oil droplets in the emulsions prepared with these wall materials were small. Especially,
the soluble soybean polysaccharide was the best wall material among the tested
polysaccharides to prepare the emulsion with small oil droplets. The addition of ascorbic acid
to the aqueous phase had no significant effect on the median diameter for every wall material.
Because acyl ascorbates were surface active and acted as emulsifiers, the median diameters of
the oil droplets in the emulsions, in which the mixture of arachidonoyl ascorbate and methyl
oleate was used as the oil phase, were smaller than those of the oil droplets in the emulsions
which included arachidonic acid as the oil phase. Figure 21(a) shows the oxidation of the
nonencapsulated arachidonic acid and arachidonoyl ascorbate at 37
o
C and 12% relative
humidity. The oxidation of arachidonic acid was observed in the presence and absence of
ascorbic acid. Arachidonic acid was very rapidly oxidized in the absence of ascorbic acid.
The addition of ascorbic acid to arachidonic acid retarded the oxidation of arachidonic acid.
The arachidonoyl moiety of the ascorbate was very slowly oxidized during the long-term
storage.
We reported that the oxidation of an n-6 polyunsaturated fatty acid could be
expressed by the kinetic equation of the autocatalytic type [63, 64]. Although the equation
was applicable for the oxidation of arachidonic acid in the absence and presence of ascorbic
acid, and for that of arachidonoyl ascorbate, it was not easy to apply the equation to the
oxidation of the microencapsulated arachidonic acid and arachidonoyl ascorbate because of
complicated phenomena, such as the diffusion of oxygen and interaction of the fatty acid with
the wall material that occurred in the microcapsules [73]. Therefore, the oxidation kinetics
was empirically expressed by the Weibull equation (Eq. 3).
The
k
values for arachidonic acid
in the presence of ascorbic acid and for arachidonoyl ascorbate were about 1/10 and 1/100,
respectively, of that for arachidonic acid in the absence of ascorbic acid. The shape constant,
n
, was greater than unity for every substrate, reflecting the slow progress of the oxidation
during the early stage of storage. Figures 21(b), (c) and (d) show the oxidation of arachidonic
acid and the arachidonoyl moiety of the ascorbate encapsulated with maltodextrin, gum arabic
and soluble soybean polysaccharide, respectively. The
k
values for arachidonic acid and
arachidonoyl ascorbate encapsulated with the soluble soybean polysaccharide were very
small, indicating that the polysaccharide had a high antioxidative ability to both arachidonic
acid and arachidonoyl ascorbate. The oxidation of the microencapsulated unsaturated fatty
acid often has two characteristics: one is the rapid oxidation progress during the early stage of
storage and the other is the leveling off of the oxidation during the late stage [74]. The
n
values smaller than unity, which were obtained for some microcapsules, reflected the former
characteristic. The oxidation of arachidonic acid, in both the presence and absence of ascorbic
acid, and arachidonoyl ascorbate was slightly suppressed by the microencapsulation with
maltodextrin. Gum arabic and soluble soybean polysaccharide were effective wall materials
for the suppression of the oxidation of arachdonic acid in the absence of ascorbic acid. The
addition of ascorbic acid to the water phase during the preparation of the O/W emulsions
could suppress the oxidation of arachidonic acid encapsulated with any polysaccharide. The
encapsulated arachidonoyl ascorbate exhibited the highest resistance to the oxidation for
every wall material. The resistance would be ascribed to the antioxidative ability of the
ascorbate moiety of the ester. Fatty acid molecules interact with the wall material within the
microcapsule, and a molecule interacting with the material is oxidized more slowly than that
not interacting with it [73]. The oxidative stability of linoleic acid in the microcapsule
