Soybean oil is the major ingredient and easily oxidized in salad dressing, which makes the products less acceptable or unacceptable to consumers. Therefore, the addition of healthful antioxidant ingredients have been utilized to enhance the nutritional qualities while maintaining extended shelf-life of the salad dressing. This study was to evaluate the effects of riboflavin and its decomposed product (lumichrome) on soybean oils in salad dressing based on the changes in values from differential scanning calorimetry (DSC), headspace oxygen and solid phase microextraction (SPME), gas chromatography (GC)-mass spectrometry (MS), high-performance liquid chromatography (HPLC) and peroxide analysis (PV). To determine the proposed mechanism of riboflavin activity, the data from antioxidant property analyses were also evaluated.
Salad dressings containing 0, 5, 20, 50 and 100 ppm added riboflavin were prepared and stored under light (3,000 lux) or in the dark at 25°C for 5 days. The crystallization peak of DSC thermogram of the oil sample shifted to lower temperature and its enthalpy decreased as the storage time increased under the light. As the riboflavin concentration increased from 0 to 100 ppm, (1) the enthalpy of the DSC peak increased from 27.3 to 32.2 J/g, (2) the maximum temperature of the DSC peak increased from -63.2 to -61.2°C, (3) the on-set temperature of the DSC peak increased from 59.3 to 57.5°C, and (4) peroxide value decreased after 5 day storage under light. Riboflavin in aqueous portion of salad dressing was analyzed by HPLC. The steady state kinetic study showed that the reaction rate of riboflavin with singlet oxygen in salad dressing was 3.2 x 108 M-1s-1. This reaction rate is three times lower than that in water system. Overall results showed that the decrease in lipid oxidation of salad dressing was accompanied with the increase in the concentration of added riboflavin.
In order to evaluate the antioxidant property of riboflavin, the radical scavenging activity of riboflavin was determined by DPPH and ABTS reaction, reducing power, and ion-chelating effect of riboflavin. The results showed the antioxidant capacity of riboflavin is very low when compared with that of other known antioxidants such as Trolox, butylated hydroxyanisole (BHA), and sodium acetate. The result may suggest that the mode of action in antioxidant effect of riboflavin might be different from that of other antioxidants.
The effect of lumichrome, the major photo-degradation products from riboflavin at low pH, on the oxidation of salad dressing was studied. DSC, headspace oxygen and peroxide value were subsequently analyzed in salad dressing samples that had been supplemented with lumichrome. The decrease in the maximum temperature and the enthalpy of the crystallization peak in the DSC thermogram of lumichrome added sample was higher than that of the un-added sample in storage time up to 5 days under light. At the same time, the increase in headspace oxygen depletion rate and peroxide value of lumichrome added sample was higher than that of the un-added sample. In general, the addition of lumichrome accelerated the light-induced lipid oxidation of salad dressing.