| Lipid oxidation is an common phenomenon. How to detect the oil oxidation and predictthe shelf life of oils is the crucial work during the production process in factories. Based onthe relationship of the impedances decreasing with time and the peroxide values (POVs)increasing with time during the process of oil oxidation, this dissertation is to establish amodel to predict the shelf life of oil from the induction time and the activation energy of oils.The main research contents and results were as follows.1. Impedance analysis of oil oxidation processThe effect of solvent, the concentrations of supporting electrolyte, the amounts of oil,and the frequencies of measurement on the detection of the oil impedances and theirvariations were throughly investigated. The POV-time and impedance-time curves werepolted by simultaneously measuring the oil’s POV and impedance.Because some smallmolecular substances began to produce in the oil at the first few hours, slowly POV increasingand slowly impedance decreasing were observed. However, the variation trends of the POVand the impedance were accelerated during the next period of time. Moreover, the POV andthe impedance would speed up with the temperature increasing. It was found that there werecertain corresponding relations between the impedances decreasing and the POVs increasingwith time during the oil oxidation process.2. Dynamic analysis of oil oxidation processAccording to the POV-time curves and impedance-time curves of the4oils (olive oil,safflower oil, almond oil and wheatgerm oil), some dynamic parameters, such as the oxidationrate constants and the activation energy of oil, were obtained. The data of rate constants ofoils indicated that the oxidation rates of oils were accelerated with temperature increasing.The rate constants of the4oils at different tempetures obtained by POV and impedance werehad the same series, ie, olive oil <almond oil <wheatgerm oil <safflower oil. The activationenergy calculated by AC impedance and POV of the4oils were also had the same series, thatis olive oil> almond oil> wheatgerm oil> safflower oil. Experimental results showed thatthere were certain corresponding relations between the oxidation stability of oil and the oilsactivation energy or the rate constant.The stability of4oils was found to be olive oil>almond oil> wheatgerm oil> safflower oil.3. Establishment of a model to predict the shelf life of oilThe induction time is the key parameter which closely related to the oxidation stability ofoils. The longer the induction time is, the more stability of the oil will be. The induction timesof the4studied oils were got from the POV-time curves and the impedance-time curveswith hyperbolic analysis. The t-test (P＞0.1) results revealed that the induction times obtainedfrom POV-time curves and the impedance-time curves had no significant differences.Basedon the Arrhenius model, analog calculation and data optimization, a new model,lnt SLt=Kln(T Tn10), was established to predict the shelf life of oils. The predicted shelfilives of olive oil, safflower oil, almond oil and wheatgerm oil were176~192days,16~18 days,28~29days and18~20days, respectively. The analysis of deviation factor andaccurate factor revealed that the predicted data of shelf lives were much close to the real dataof shelf lives.4. Impedance analysis of emulsion oxidation processThe oxidation of oil phase in emulsion is closely related to the stability of theemulsion.The variation characteristics of impedances and pH values of practical emulsionsamples were studied. The effects of the supporting electrolyte, the amounts of emulsion andthe frequency of measurement on the detection results were investigated. It was found that theimpedances of emulsion were firstly decreased and then increased with time. However, thepH values remained nearly constant at first and sharply decreased after a certain time. Acertain corresponding relationship between the impedance and pH was found. In addition, thetime corresponding to the inflection point of impedance and pH was the same and it could beused to evaluate the stability of emulsions. The shelf lives of the practical emulsion samples,shower gel and shampoo, were respectively predicted to be4.65~4.86years and4.80~4.83years with the proposed model. |
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