Chemistry Reference
In-Depth Information
low-temperature-flow properties are influenced by the amount of FAs and
not by their chain lengths.
36
Therefore a low amount of SFAs with a good
combination of MUFAs and polyunsaturated fatty acids (PUFAs) favors low
PP and CP values.
The crystallization behavior of moringa oil and other vegetable oils was
also studied using Cryo-DSC.
37
The long-chain compounds (FA chains) of
vegetable oils in the solid state exist in more than one crystalline form and
thus have multiple melting points. The melting points of triacylglycerols
depend on the chain length, the nature of the unsaturation (cis- or trans-
olefins), and the number and position of double bonds.
38,39
Monoacid
triacylglycerols show three distinct melting points corresponding to the
three crystalline forms a, b
0
, and b.
40,41
Since most vegetable oils are mixed
triacylglycerols, no b-form is present, and they take the highest melting
b
0
-form.
42
When the oil is cooled quickly it solidifies in the, lowest melting,
a-form. When heated slowly, the monoacid triacylglycerols melt, and,
held just above the a melting point, resolidify into the b
0
crystalline form.
The b-form is the highest melting form, in the case of monoacid triacylgly-
cerols and is produced by crystallization from solvents. Since crystallization
kinetics are very sensitive to temperature fluctuations, cooling rate and
thermal history, the Cryo-DSC method is relatively simple, reproducible and
robust enough to provide good control of temperature fluctuations, cooling
rate etc.
Cryo-DSC has already been utilized for the identification of various crys-
tallization forms of pure single-acid triacylglycerols.
41,43,44
Since vegetable
oils are mixtures, the situation becomes complex. Multiple transitions have
been observed during the cooling and heating cycle of DSC experiments.
Three measurements (of each heating and cooling cycle) were carried out for
each oil. It was found that more variation in results was observed in the
cooling cycle than the heating cycle. In the case of the cooling cycle, 8 out of
16 measurements (50%) showed standard deviations higher than 1 1C runs
and varied from 1.2 to 14.2 1C. For the heating cycle, only 1 out of 20 (5%)
showed a standard deviation of 1.5 1C. A previous study
45
also showed that in
a heating experiment, DSC thermograms were minimally affected by thermal
history and variations in cooling rate, while in cooling experiments, these
factors can influence the shape of the peak and cause more variation in the
results. Therefore, only the results obtained from the heating cycle were used
to analyze the influence of FA composition on the cold-flow properties of
vegetable oils. The wax disappearance temperatures (WDTs) and SM values
of various peaks were determined. Moringa oil shows a single sharp peak at
aSMof
0.7 1C and a WDT of
11.7 1C because of its predominant oleic
acids. Moringa oils have almost the same amounts of SFAs and UFAs, but the
makeup of its UFAs is different. In moringa oil, MUFAs make up 76% and
PUFAs make up 1%. This change in composition of UFAs, i.e., lower
amounts of PUFAs in moringa oil results in higher SM and WDT values
compared to other oils (sunflower, cotton, jatropha and canola). More SFAs
in an oil results in close-packing of the triacylglycerol molecules during
