Effect of Added Conjugated Linoleic Acid and Iodine Concentrations on Conjugated Linoleic Acid-Rich Soy Oil Oxidative Stability

Jain and Proctor [1] reported pilot scale production of 20 % conjugated linoleic acid (CLA) in soy oil by photoisomerization of soy oil linoleic acid (LA) in oil triacylglycerides to CLA using an iodine catalyst. Recently, we reported in a food oil oxidation study of freshly produced CLARSO that the pro-oxidant effect of CLA and iodine was independent of concentration [2]. However, the study did not distinguish between the effect of CLA and iodine on CLA oxidation. Removal of iodine may be important if iodine is identified as prooxidant. However, if CLA is the major pro-oxidant, storage conditions and added antioxidants may be very important after iodine removal to protect against oxidation. The objective of the following study was to distinguish between the effect of added CLA and added iodine concentration on soy oil oxidative stability. Commercial RBD (Refined, bleached, and deodorized) soy oil was obtained from Riceland Foods (Stuttgart, AR) containing 52 % linoleic and 6 % linolenic acid. Resublimed iodine crystals were used as a catalyst (Alfa Aeser, Ward Hill, MA). Commercial CLA triacylglyceride (Tonalin TG80) consisting of a 50:50 mixture of cis-9, trans11 and trans-10, cis-12 was obtained from Cognis Nutrition and Health (Cincinnati, OH). Nine RBD soy oil treatments containing combinations of 0, 0.175, and 0.35 % iodine; 0, 10 and 20 % commercial CLA triacylglyceride (Tonalin TG80) were prepared. CLARSO oxidative stability was studied by gravimetric analysis [3] and headspace oxygen determination. The 500-mg samples of soybean oil treated with combinations of CLA and iodine were weighed into aluminum pans. Samples were stored in an oven in the dark at 65 C for 10 days and weighed daily. Results were expressed as change in weight (mg per 500 mg of oil). To measure headspace oxygen, 3 g of oil was weighed into 10-mL amber colored glass vials and the vials were stored in an oven in the dark at 65 C for 10 days. The headspace oxygen content in each vial was determined daily by using Oxygen Analyzer-Quantek Model 905 (Quantek Instruments, Grafton, MA). Data were analyzed as a full-factorial design by analysis of variance (ANOVA) and mean comparison of triplicate runs was done by a student’s t test (p \ 0.05) using JMP 9.0 (SAS Institute Inc., Cary, NC). RBD soybean oil was used in this study in place of CLARSO because the CLA in CLARSO may not allow for the effects of iodine to be determined independently. Figure 1 shows the effects of iodine and CLA concentration on weight change of RBD soy oil. The presence of iodine in the oils resulted in shorter induction time, more rapid weight increase, and therefore greater oxidation. However, there was no increase in induction time with increasing iodine levels. At 0 % CLA, both 0.175 and 0.35 % iodine had an induction period of 1 day. However, at 10 % CLA, 0.175 % iodine had an induction period of 0 days, and 0.35 % iodine had a 1-day induction period. At 20 % CLA, the two iodine concentrations again showed similar behavior with a 1 day induction period. The main general observation was that the presence of iodine increased oxidation but increasing the concentration did not. This agreed with our previous finding in CLARSO [2]. In the absence of iodine in the oil, there was increasing oxidation with an increasing CLA concentration with the 20 % CLA oil weight increase being most rapid followed by 10 % CLA oil. The change in weight was affected by iodine concentration, CLA concentration, time (day), and all interactions (p \ 0.05). The % iodine 9 % R. R. Yettella C. Castrodale A. Proctor (&) Department of Food Science, University of Arkansas, 2650 N. Young Avenue, Fayetteville, AR 72704, USA e-mail: aproctor@uark.edu