Effect Of Storage Temperature On The Crystallization Behavior And Physical Properties Of Quick-frozen Special Oil

The demand for quick-frozen food is increasing with the quickening pace of life and improvement of people’s living standard. As a major ingredient, quick-frozen special oil plays a vital role on the flavor and quality of quick-frozen food. In addition to manufacturing process, storage process also has influence on the quality of quick-frozen special oil. However, there is no research report about the effect of storage temperature on the crystallization behavior and physical properties of quick-frozen special oil. In our previous study, the quick-frozen special oil was prepared with enzymatic interesterified oil from palm stearin and soybean oil as base oil, which had ideal crystal form and showed good performance in its application for quick-frozen food production. It has important guidance effect in preparing high quality quick-frozen special oil by enzymatic interesterification to understand its quality variation at different storage temperature. Hence, in this study, the changes in the peroxide value, solid fat content(SFC), hardness, thermal property, crystal form and crystalline morphology of two quick-frozen special oils from interesterified oil and physically mixed oil were studied respectively when they stored at different temperature. The special oils before and after storage were applied in the preparation of quick-frozen rice dumplings to further investigate the effect of storage temperature on which quality. The major research work is as follows:(1) Preparation of quick-frozen special oils and its physicochemical properties and crystalline behaviorTwo quick-frozen special oils were prepared with enzymatically interesterified oil(soybean oil : palm stearin=3:7) : soybean oil : palm stearin = 83 : 13 : 3 and soybean oil : palm stearin = 38.2 : 62.8 as base oils, and the slip melting point, iodine value, acid value, peroxide value, SFC, hardness, thermal property, crystal form and crystalline morphology of which were examined. The results indicate that the slip melting point of quick-frozen special oil from enzymatically interesterified fat(EIF) was approximate 6℃ lower than that from physical mixing fat(PMF). For both samples, the acid value and iodine value were accord with the national standards of oil and fat, and there was no obvious difference in the iodine value and fatty acid compositon. The contents of SUS and SUU TAGs of EIF were approximate 5% and 16.5% higher than that of PMF. However, the SFC of EIF was lower than that of PMF at 0~45℃. Except for 30℃, the PMF was harder than the EIF at 0℃, 10℃and 20℃. The content of β′ forms in the EIF was higher than that of PMF. Also, the crystal particle size of the EIF was smaller than that of PMF. Generally, properties of the EIF were better than that of PMF.(2) Effect of storage temperature on the physicochemical properties and crystalline behavior of quick-frozen special oilsThe peroxide value of both quick-frozen special oils stored at different temperature(low temperature 4℃, ambient temperature 20℃ and high temperature 30℃) was examined. It was found that peroxide values of both samples stored at 4℃ and 20℃ were firstly increased during storage, and then kept stable, but they were accord with the national standards of oil and fat. However, the peroxide values of two samples stored at 30℃ after 3 weeks were greater than those stored at 4℃ and 20℃, and the oil exudation was found, indicating that high storage temperature would accelerate the oxidation of oils. The SFC of two samples did not change significantly with the extension of storage time at any given temperatures. The final hardness of both samples stored at 4℃ and 20℃ was greater than that of original samples before storage. The possible reason was that the polymerized network structure was formed for the crystals of oils. After stored at 30℃ for 3 weeks, both samples’ hardness decreased due to the oil exudation. The thermal properties of the PMF were more stable and the endothermic peaks slightly moved to right with the increase of storage temperature. For the EIF, the number of endothermic peaks in the melting thermograms decreased with the increase of storage temperature. This phenomenon was partly due to the melting of some low melting point TAGs with the increase of temperature. On the other hand, it was related to the transition of β′ to β crystal. For PMF, there was no obvious change in the relative contents of β′ and β crystal during storage. It may be due to high content of β′ crystal in the PMF. However, when stored at 20℃ and 30℃, the crystals aggregated and formed the plate-like crystal cluster which size became larger with the increase of storage temperature and extention of storage time. However, for EIF, the content of β′ and β crystals kept stable when stored at 4℃. The content of β crystal increased while the content of β′ crystal decreased with the extension of storage time when stored at 20℃ and 30℃. Moreover, the crystals aggregated and formed the crystal cluster which size became larger. In conclusion, the physicochemical properties of quick-frozen special oils stored at 4℃ were relatively stable, however, its quality decreased when stored at 20℃ and 30℃.(3) Application of quick-frozen special oils before and after storageTwo samples stored at 4℃ and 20℃ for 8weeks, two fresh prepared quick-frozen special oils and the commercially avaibale one were applied in the preparation of quick-frozen rice dumplings, respectively. And the frost crack rate and sensory evaluation of quick-frozen rice dumplings were examined. The results showed that the frost crack rate of quick-frozen rice dumplings prepared by EIF stored at 4℃ for 8 weeks was comparable to that of quick-frozen rice dumplings prepared by fresh EIF, which was significantly lower than that of the quick-frozen rice dumplings prepared by other oil samples. Additionally, the sensory evaluation results of the above two quick-frozen rice dumplings were comparatable and higher than other dumplings, indicating that the quality of quick-frozen special oil prepared by enzymatic interesterification is better than that prepard by physical mixing method, which can be well maintained when stored at 4℃.This study will provide the theoretical and practical basis for solving the problem of quality deterioration of quick-frozen special oil during storage. It also can promote the development of producing high quality quick-frozen special oil by enzymatic interesterificationn.