Evaluation of the antioxidant activity and oxidative stability in ginger- and turmeric-enriched soybean oils

The food industry is currently moving towards replacing synthetic additives such as TBHQ, BHA, and BHT with natural antioxidants to prevent the lipid oxidation in oils and fats. In this study, the commercial and freeze-dried powders of ginger and turmeric rhizomes and peels were added to the soybean oil at the concentration of 10% (w/w). After that, the exhaustive powders were subjected to other two recovery cycles. The antioxidant activity and oxidative stability of ginger- and turmeric-enriched soybean oils were evaluated respectively by spectrophotometric and RANCIMAT methods. The phenolic characterization has been also carried out by detecting total polyphenols through Folin-Ciocalteu assay and HPLC analysing 6-gingerol, 6-shogaol and curcumin. Based on the correlation studies, the antioxidant activity as well as oxidative stability, which non-linearly decreased over cycles, linearly increased with increasing phenolic contents. The soybean oils containing freeze-dried peels achieved the best antioxidant results as a consequence of their enrichment in 6-gingerol and curcumin, the main bioactive compounds of respectively ginger and turmeric. Hence, ginger and turmeric can be proposed as a valid source of natural antioxidants for extending the shelf-life of oils and fats. Moreover, the valorisation of peels is useful to convert these agricultural wastes into value-added products.