All the above changes in spectra implied that during the heating process, some chemical reaction/reactions occurred and certain substance/substances formed or disappeared. These changes are useful for preliminarily identifying the degree of heating, although the chemical reaction or reactions are complicated and the ascription of the peaks needs further study.
3.1.2. Effect of heating on AV of oils
The AV of unmixed oils for each kind of oil was showed in Fig. 3. It can be seen that the initial AV of CO, SBO and SFO were almost the same and obviously lower than those of others, while the initial AV of SO was the highest. The AV of the six kinds of oils increased slowly within 3 h of heating time, afterwards the AV increased significantly, especially for the RO samples. The acid values of RO sample heated for about 5 h and SO sample heated for about 7 h became higher than 3 mg/g, i.e. the specified value of Chinese GB. While for the other four kinds of oils, the acid values of samples heated for 10 h were still below 3 mg/g. These indicated that the quality of oil, with respect to the AV and resistance to oxidation, relates greatly to the type of oil. Obviously, CO had the best quality and oxidative stability among the six kinds of oils.