Pure coconut oil of the same lot was divided into samples in the stages of crude oil, purified, semi-hardened, and hardened state, and each sample was stored at -20, 5, 15 and 30°C for 6 months. Values of A.V., P.O.V. and Co.V. were measured in every month, and their correlation with crystal growth was examined by electron microscopy observation.
The same experiments were also carried out on a mixure of (a) hardened coconut oil added with hardened beef tallow or cotton seed oil and (b) hardened beef tallow mixed with non-hardened coconut oil or butter fat.
1) The values of A.V. rose in those stored at 15 and 5, in that order, and especially in the hardened oil. Practically no change was seen in those stored at -20 and 30°C.
P.O.V. was zero throughout the test period, while Co.V. showed higher value with increasing storage temperature and less hardening.
2) Hardened coconut oil mixed with hardened beef tallow showed marked rise in A.V. while that mixed with cotton seed oil showed no change in A.V. values.
When hardened beef tallow was mixed with non-hardened coconut oil, rise of A.V. value was about the same as that of hardened coconut oil.
3) Electron microscopic observation of hardened coconut oil showed that the roughness of crystal surface increased in the order of storage temperature of 15, 5, 30 and -20°C, which corresponded to the order of rise in A.V. values. The crystal surface of the oil stored at 5°C showed the presence of whiskers. In the oil stored at 15°C, single crystal had shown great growth and there was a space between that and the next single crystal. If a vapor is present in that space, absorptive reaction might occur in the singular face of the crystal.
4) The surface was smooth in non-hardened coconut oil, and liquid glyceride was found to occupy the spaces between single crystals.
5) In the hardened coconut oil mixed with hardened beef tallow, the crystal surface became rough, while the sample of hardened coconut oil mixed with cotton seed oil was similar to the hardened coconut oil stored at -20°C.
