Biomedical Engineering Reference
In-Depth Information
The transient changes in the fractions of the remaining lauroyl ascorbate and lauric acid
liberated by the hydrolysis of lauroyl ascorbate at 65
o
C and 75% relative humidity were
examined. During the early stage during the storage, the decomposition of lauroyl ascorbate
rapidly proceeded, but free lauric acid was scarcely formed. Lauric acid gradually formed
when the fraction of the remaining lauroyl ascorbate became less than 0.5. The amount of
consumed lauroyl ascorbate was calculated by subtracting the fraction of the remaining
lauroyl ascorbate, which was estimated using the parameters,
k
and
n
, by the Weibull
equation, from unity. No lauric acid was liberated during the early stage of the decomposition
of lauroyl ascorbate. This indicated that the oxidative degradation of the ascorbyl moiety
occurred first, and then the hydrolysis of the ester bond followed to liberate the lauric acid;
that is, the decomposition of acyl ascorbate appears to be a consecutive process.
Ascorbic acid and various polyunsaturated fatty acids, such as α- and γ-linolenic,
dihomo-γ-linolenic, arachidonic, eicosapentaenoic, and docosahexaenoic acids, were
condensed by the immobilized lipase, Chirazyme
®
L-2 C2, in acetone dehydrated with
molecular sieves to produce polyunsaturated acyl ascorbates [25, 27]. Figure 13 shows the
autoxidation processes of the unmodified polyunsaturated fatty acid and the polyunsaturated
acyl moiety of the polyunsaturated acyl ascorbates at 65
o
C and nearly 0% relative humidity.
The unmodified polyunsaturated fatty acids were almost completely autoxidized within 5 h,
whereas all the polyunsaturated acyl ascorbates were significantly resistant to autoxidation.
The polyunsaturated acyl moiety of 90% or more remained in the unoxidized state during the
test period for every ascorbate. Esterification of polyunsaturated acid with ascorbic acid
significantly improved its oxidative stability. The processes of the unmodified
eicosapentaenoic acid mixed simply with equimolar ascorbic acid were also measured at 65
o
C
and 0% relative humidity. The oxidation process of eicosapentaenoyl ascorbate was almost
the same as that of eicosapentaenoic acid mixed with an equimolar amount of ascorbic acid.
Acyl ascorbate was also synthesized using conjugated fatty acid as a substrate for
condensation reaction. The oxidation processes of the unmodified conjugated linoleic acid
isomers, which were
c
9,
t
11- and
t
10,
c
12-conjugated linoleic acid, and the linoleoyl moiety of
the linoleoyl ascorbates at 65
o
C and 75% relative humidity were examined [37]. The
unmodified conjugated linoleic acid isomers were almost completely oxidized within 4 h,
whereas all the conjugated linoleoyl ascorbate were significantly resistant to oxidation. The
oxidation kinetics was empirically expressed by the Weibull equation (Eq. 3). The
k
values
for conjugated linoleoyl ascorbates and linoleoyl ascorbate were about 1/30 and 1/60,
respectively, of that for the corresponding unmodified conjugated linoleic acid isomers.
Although it is reported that
t
10,
c
12-conjugated linoleic acid was more easily oxidized than
c
9,
t
11-conjugated linoleic acid [52], there was no significant difference in the susceptibility to
oxidation between the conjugated linoleic acid isomers esterified with ascorbic acid. The
n
values were greater than unity for all the unmodified conjugated linoleic acid isomers,
whereas the values for conjugated linoleoyl ascorbates were smaller than unity. This fact
indicated that there were induction periods, which reflected the slow progress of the oxidation
during the early stage of storage, for unmodified conjugated linoleic acid isomers, whereas
the oxidation for conjugated linoleoyl ascorbates progressed very slowly without an induction
period. Thus, it was quantitatively demonstrated that the oxidation process for conjugated
linoleoyl moiety of conjugated linoleoyl ascorbate was different from that for unmodified
conjugated linoleic acid.
