Eunok Choe

Chemistry and reactions of reactive oxygen species in foods

Reactive oxygen species (ROS) are formed enzymatically, chemically, photochemically, and by irradiation of food. They are also formed by the decomposition and the inter-reactions of ROS. Hydroxy radical is the most reactive ROS, followed by singlet oxygen. Reactions of ROS with food components produce undesirable volatile compounds and carcinogens, destroy essential nutrients, and change the functionalities of proteins, lipids, and carbohydrates. Lipid oxidation by ROS produces low molecular volatile aldehydes, alcohols, and hydrocarbons. ROS causes crosslink or cleavage of proteins and produces low molecular carbonyls from carbohydrates. Vitamins are easily oxidized by ROS, especially singlet oxygen. The singlet oxygen reaction rate was the highest in β-carotene, followed by tocopherol, riboflavin, vitamin D, and ascorbic acid.

Effects of phosphatidylcholine and phosphatidylethanolamine on the photooxidation of canola oil.

Effects of chlorophyll b, phosphatidylcholine (PC), and phosphatidylethanolamine (PE) on the oxidation of canola oil under 1700 lux light at 10 degrees C were studied by determining the headspace oxygen depletion and peroxide value. Chlorophyll b increased the oil oxidation under light via production of singlet oxygen. PC and PE did not affect the headspace oxygen depletion and peroxide formation in the oil in the absence of chlorophyll b at 10 degrees C; however, they increased in the presence of chlorophyll, with higher effect of PC than PE. Chlorophyll was degraded during the oil photooxidation and PC and PE decreased its photodecomposition with higher protection by PC than PE. There was no effect of light or singlet oxygen on PC and PE during the oil photooxidation. The results clearly showed that PC and PE were prooxidants in the photooxidation of canola oil containing chlorophyll b by protecting it from photodecomposition, and thus PC and PE should be removed from the oil containing chlorophyll to improve the photooxidative stability.

Temperature Dependence of Autoxidation of Perilla Oil and Tocopherol Degradation

Temperature dependence of the autoxidation of perilla oil and tocopherol degradation was studied with corn oil as a reference. The oils were oxidized in the dark at 20, 40, 60, and 80 degrees C. Oil oxidation was determined by peroxide and conjugated dienoic acid values. Tocopherols in the oils were quantified by HPLC. The oxidation of both oils increased with oxidation time and temperature. Induction periods for oil autoxidation decreased with temperature, and were longer in corn oil than in perilla oil, indicating higher sensitivity of perilla oil to oxidation. However, time lag for tocopherol degradation was longer in perilla oil, indicating higher stability of tocopherols in perilla oil than in corn oil. Activation energies for oil autoxidation and tocopherol degradation were higher in perilla oil (23.9 to 24.2, 9.8 kcal/mol, respectively) than in corn oil (12.5 to 15.8, 8.8 kcal/mol, respectively) indicating higher temperature-dependence in perilla oil. Higher stability of tocopherols in perilla oil was highly related with polyphenols. The study suggests that more careful temperature control is required to decrease the autoxidation of perilla oil than that of corn oil, and polyphenols contributed to the oxidative stability of perilla oil by protecting tocopherols from degradation, especially at the early stage of oil autoxidation.

Interaction of phosphatidylcholine and α-tocopherol on the oxidation of sunflower oil and content changes of phosphatidylcholine and tocopherol in the emulsion under singlet oxygen.

Interaction of phosphatidylcholine (PC) and α-tocopherol (α-Toc) on the oxidation of oil in the emulsion consisting of sunflower oil and water under singlet oxygen at 25 °C was studied by determining peroxide value (PV) and conjugated dienoic acid (CDA) contents. Singlet oxygen was produced by chlorophyll b under 1700 lux. Single addition of PC or α-Toc decreased the values of peroxides and CDAs of oil in the emulsion via singlet oxygen quenching. PC and α-Toc showed simply additive interaction in decreasing the singlet oxygen oxidation of oil in the emulsion. α-Toc was a physical quencher of singlet oxygen in the emulsion, but PC involved chemical quenching in the antioxidant action. Chlorophyll and PC contents were decreased in the emulsion under singlet oxygen, while α-Toc was not. α-Toc protected chlorophyll and PC from degradation, and was a more important component than PC in the oil oxidation under singlet oxygen in the emulsion.

Stability of meoru (Vitis coignetiea) anthocyanins under photochemically produced singlet oxygen by riboflavin.

This study investigated the stability of meoru (wild vine grape) anthocyanins in the aqueous solution under singlet oxygen. Freeze-dried meoru (1 kg) contained 179.98 mg anthocyanins including delphinidin-3-glucoside, malvidin-3,5-diglucoside, cyanidin-3,5-diglucoside, malvidin-3-glucoside, and cyanidin-3-glucoside. Malvidin-3,5-diglucoside and cyanidin-3-glucoside were the meoru anthocyanins at the highest and the lowest concentration, respectively. Little decrease in total anthocyanins in the aqueous solution was observed in the dark with or without riboflavin, or with light without riboflavin. Singlet oxygen degraded the meoru anthocyanins in the aqueous solution, which suggested chemical quenching of singlet oxygen by the anthocyanins. Degradation of the meoru anthocyanins was structure-dependent; diglucoside anthocyanins were more stable than monoglucoside. And malvidin glucoside was more stable than delphinidin or cyanidin glucoside, which suggested the number of hydroxy groups in the structure was partly related with the anthocyanin stability under singlet oxygen. This is the first report on anthocyanins stability affected by its structure under singlet oxygen.

Effects of solubility characteristics of sensitiser and pH on the photooxidation of oil in tuna oil-added acidic O/W emulsions.

The effects of sensitisers and pH on the oil oxidation of acidic O/W emulsions were studied under light by measuring hydroperoxide content and headspace oxygen consumption in the emulsions. The emulsions consisted of canola and tuna oil (2:1w/w, 32%), diluted acetic acid (64%), egg yolk powder (4%), chlorophyll b or erythrosine (5μM), and/or diazabicyclooctane (DABCO) or sodium azide (0.5M). The emulsion pH values were 2.67, 3.68, and 6.27. Chlorophyll increased oil oxidation in the emulsion under light via singlet oxygen production while erythrosine did not. DABCO significantly decreased photooxidation of the oil containing chlorophyll, suggesting singlet oxygen involvement. However, sodium azide increased photooxidation of the oil containing chlorophyll possibly via azide radical production under acidic conditions. The oil photooxidation was higher in the emulsion containing chlorophyll at pH 6.27 than at pH 2.67 or 3.68, primarily by singlet oxygen and secondarily by free radicals produced from hydroperoxide decomposition.

Contribution of minor compounds to the singlet oxygen-related photooxidation of olive and perilla oil blend

This study evaluated contribution of natural minor compounds including carotenoids, tocopherols, polyphenols, and phosphatidylcholine (PC) to the stability of chlorophyll sensitized oxidation of olive and perilla oil blend under 2,500 lx light at 25°C for 48 h. The oxidation of the oil blend was evaluated by headspace oxygen consumption and peroxide values (POV). Minor compounds were also monitored. The headspace oxygen consumption and POV of the oil blend were increased with oxidation time; however, fatty acid composition was not significantly changed. Carotenoids, tocopherols, polyphenols, and PC were degraded during the oil oxidation, which was closely related with their singlet oxygen quenching. Correlations of the oil oxidation were higher with contents of carotenoids and PC (r2>0.93) than with those of tocopherols or polyphenols, suggesting high contribution of carotenoids and PC to decreasing singlet oxygen-related photooxidation of olive and perilla oil blend.

Effects of water activity on pigments in dried laver (Porphyra) during storage

Degradation of chlorophyll, carotenoids, and phycobilins in dried laver (Porphyra) was studied during storage at water activities (Aw) of 0.112, 0.316, 0.484, 0.747, or 0.890 in the dark at 40°C for 15 days. The chlorophyll, carotenoid, and phycobilin contents were determined using HPLC and spectrophotometry. The chorophyll a, carotenoid, and phycobilin contents in dried laver decreased with storage time in the dark, and degradation was increased and accelerated as the Aw value increased. Among pigments, chlorophyll a was degraded at the highest rate, and differences in degradation rates among pigments became greater as the Aw value increased. Phycoerythrin was more stable than phycocyanin. Changes in the Aw value affected degradation of phycocyanin and chlorophyll more than phycoerythrin or carotenoids. Control of the Aw value can improve the color stability of dried laver in the dark.

Physicochemical properties of mung bean starches in different Korean varieties and their gel textures

To compare the mung bean starches (MBS) and starch gel properties, Korean varieties, ‘Geumsung‘, ‘Dahyeon‘, ‘Sohyeon’ and ‘Eohul‘, were purified using water and alkaline steeping methods. The physicochemical properties of starches, the texture and structure of starch gels were investigated. The apparent amylose and protein contents were significantly different depending on varieties and steeping mediums. Water binding capacities were higher in ‘Geunsung’ and ‘Eohul‘. The granule sizes were ranged 7.8–23.3 μm and the shapes were oval and jelly bean, and ‘Dahyeon’ granule had a distinct hilum. The trough, final, and setback viscosities were significantly different and the viscosities of ‘Sohyeon’ showed lowest values. MBS gels formed a regular shaped 3-dimensional network. The gel structure of ‘Sohyeon’ changed irregularly during storage, but the structures of ‘Geumsung‘, ‘Dahyeon‘, and ‘Eohul’ retained regular shaped networks with decreasing inner cells. Resilience of MBS gels was higher in ‘Dahyeon’ and ‘Eohul’ than in ‘Geumsung’ and ‘Sohyeon‘.

Effects of Monoacylglycerols on the Oil Oxidation of Acidic Water/ Perilla Oil Emulsion under Light in the Presence of Chlorophyll

Monoacylglycerol (MAG) effects on the oil photooxidation of an emulsion containing chlorophyll were studied. The emulsion consisted of equal weights of hexaneextracted perilla oil and 0.5% acetic acid, and 4 ppm chlorophyll b and MAG at 0, 1, or 1.5% were added. The oxidation was performed under 1,700 lx light at 25°C for 48 h. Singlet oxygen was involved in the oil oxidation of the emulsion containing chlorophyll under light. MAG protected chlorophyll and polyphenol compounds from degradation during the oxidation of the emulsion under light. MAG significantly decreased and decelerated headspace oxygen consumption and hydroperoxide production in the emulsion, and thus acted as antioxidant in photooxidation of the acidic water/perilla oil emulsion containing chlorophyll. Antioxidant activity of MAG in the photooxidation of the emulsion could be due to combined results of increased retention of polyphenols and decreased oxygen diffusion by forming a physical barrier.