Evaluation On The Analysis Methods Of The Oxidative Stability Of Powdered Oil

Oxidative stability is an important quality indicator for powdered oil, howerer, there is few researches on the oxidative stability of powdered oil. The oxidative stabilities of 14 representative commercial powdered oils including 7 non-dairy creamers and 7 polyunsaturated fatty acid(PUFA) powdered oils(4DHA and 3ARA powdered oils), were analyzed by the traditional accelerated oxidation test in oven at the beginning of this research to establish the key quality indicator of powdered oil in storage. The powdered oils liable to be oxidated were evaluated systematically by the traditional accelerated oxidation test and three new methods including headspace-GC, Rancimat, Oxitest to establish the effectiveness of the methods.First of all, the oxidative stabilities of 14 samples were evaluated by traditional method.The result showed that the appearance is the key quality indicator of non-dairy creamers and the oxidative stability is of PUFA powdered oil. During the accelerated oxidation, the appearance of all non-dairy creamer varied obviously. The powder agglomerations were so serious that the test was terminated after 30 days. The color of powder changed from white to tawny and the whiteness value of the most serious sample decreased from 95 to 70. However, the oxidative stabilities of non-dairy creamers were very good because of the invariable peroxide values and anisidine values during the accelerated oxidation. The surface oils of non-dairy creamers were also invariant excepet one sample. The peroxide values and anisidine values of PUFA samples increased evidently. A DHA sample and an ARA sample were oxidized seriously during 50 days 60°C accelerated oxidatinon and produced off-flavors. The peroxide value and anisidine value of the DHA sample reached 6.08 mmol/kg、20.50 and the ARA sample reached 5.96 mmol/kg、50.02. The surface oils of all PUFA samples were stable. The further room temperature storage experiments of PUFA samples showed that the result of oxidative stability of samples at 60°C in accordance with result in room temperature. The traditional method could precisely characterize oxidative stability of powdered oils.The oxidative stabilities of the most oxidized DHA and ARA powdered oil were evaluated by headspace GC-MS to analyze the volatile compounds after the two samples were oxidized at 60°C for 50 days. The main compounds of the two samples were the same including propanal, pentanal and hexanal. The three aldehydes contents of the 7 PUFA samples during the 60°C accelerated oxidation were determined by headspace GC and the content variations were in accordance with the peroxide values and anisidine values. The further correlation analysis showed that the correlation coefficients between propanal contents and peroxide values(r=0.9286), anisidine values(r=0.8268) were better in DHA samples, the result of hexanal content and peroxide values(r=0.9570), anisidine values(r=0.8890) in ARA samples were better. The correlation coefficients of samples stored in the room temperature also verified the conclusion. In addition, the obtained values, r=0.6760 and r=0.8660, denote a statistical correspondence bewteen the total contents of pentanal and hexanal and the peroxide values, anisidine values So, propanal and hexanal determined by headspace technique could be the characteristic oxidation index of DHA/ARA samples instead of the peroxide values and anisidine values.The oxidative stabilities of 7 PUFA samples were also evaluated by Rancimat and Oxitest. The optimum accelerated oxidation conditions were established, accelerated temperature 110℃, air flow 20 L/h, sample weight 1.5 g and accelerated temperature 90℃, oxygen pressure 6 bar, oil content 4.5 g for Rancimat and Oxitest, respectively. The feasibility study of evaluating oxidative stability by Rancimat was accessed and the result showed that the correlation coefficients between the induction time and peroxide values, anisidine values was r=0.7850 and r=0.7582, respectively. The oxidative stabilities of samples stored in room temperature could be evaluated by Rancimat. The antioxidant effects of ascorbyl palmitate(AP), tertiary butylhydroquinone(TBHQ), vitamin e(VE), epigallocatechin gallate(EGCG) were also evaluated and the result showed that the order of antioxidant effect(TBHQ> AP>EGCG>VE) was in accordance with literatures. However, the oxidative stabilities of seriously oxidized samples and the shelf-life prediction of PUFA samples could not be evaluated. The oxidative stabilities of non-dairy creamers could also not be evaluated because of the existence of essences. The feasibility study of evaluating oxidative stability by Oxitest was also accessed and the result showed that antioxidant effect could be evaluated by Oxitest and the Oxitest could characterize antioxidant effects of thermo sensitivity antioxidants compared to Rancimat. The Oxitest could characterize accurately the oxidative stabilities of PUFA samples according to the consistent result between the predictive oxidative stabilities of PUFA samples by Oxitest and the oxidative stabilities during the room temperature storage. However, the oxidative stability of oxidative samples could not be evaluated by Oxitest. In addition, The induction times of 12 powdered oils determined by two instruments was significant correlation with correlation coefficient r=0.9721. The result showed that both instruments could evaluate the oxidative stability of fresh PUFA samples and the Oxitest was better than Rancimat in application aspect.