Abstract
This study examined the storage stability of tocochromanols and carotenoids in the oils prepared from microwave pre-treated (MV) rapeseeds (2-10 min, 800W) during storage at 20 °C for 12 months. In line with lipophilic antioxidant degradation throughout the storage period, changes in the antioxidant capacity of the oil were monitored. Microwaving significantly affected the concentration of lipophilic antioxidants in the oil. After 10 min of MV pre-treatment the highest content of total tocochromanols (76.64 mg/100g) was achieved, whereas a maximum carotenoid concentration (861.28 μg/100g) was obtained following 6 min seed MV pre-treatment. The degradation kinetics for the tocochromanols and carotenoids followed a zero-order kinetic model. From the kinetic analysis, it was shown that the degradation rate constant (k) of both tocochromanols and carotenoids decreased with longer seed exposure to MV radiation. The kinetics of antioxidant capacity degradation during the storage of oils followed a zero-order reaction. The rate of antioxidant capacity degradation in the control oil was higher (k=9.1 x 10-2 mmol TEAC/l/month) compared with oils prepared from MV pre-treated seeds (k=6.8-8.0 x 10-2 mmol TEAC/l/month).
Highlights
Natural antioxidants, such as tocopherols, carotenoids, phospholipids, and flavonoids, delay or inhibit lipid oxidation at low concentrations
The degradation rate of antioxidant capacity of both hydrophilic and lipophilic oil fractions was similar in all the oil variants studied, the rate of antioxidant capacity of the oil (TF) degradation was lower in the oils pressed from microwave pre-treated (MV) pre-treated seeds than in the control oil
During microwaving, the thermal degradation of β-carotene took place, which led to the formation of 9-Z- and 13-Z-β-carotene
Summary
Natural antioxidants, such as tocopherols, carotenoids, phospholipids, and flavonoids, delay or inhibit lipid oxidation at low concentrations. Antioxidants prevent the auto-oxidation of oils by giving their hydrogen to free radicals formed in the initiation and propagation stages of autoxidation. Antioxidants prevent free radical-induced cell and biological target damage by preventing the formation of radicals, scavenging them, or by promoting their decomposition. Examples of antioxidants to scavenge free radicals are tocopherols, lignans, flavonoids, and phenolic acids, ubiquinone (coenzyme Q), carotenoids, ascorbic acids and amino acid (Choe and Min, 2009). The roasting of sesame seeds before oil extraction increases their sesamol content - the thermal degradation product of sesamolin, which is a more potent antioxidant than sesamolin (Lee et al, 2010). Thermal pre-treatment of rapeseed leads to the formation of canolol (4-vinylphenol) - a decarboxylation product of sinapic acid (Shrestha et al, 2013)
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