Antioxidant properties of tofu whey concentrate by freeze concentration and nanofiltration processes

• The potential of tofu whey, a by-product, as a source of isoflavones.
• Freeze concentration was efficient in the concentration of tofu whey isoflavones.
• The concentrate from nanofiltration process presented greater isoflavone content.
• Tofu whey antioxidant activity increased after both concentration processes.
• Tofu whey concentrate antioxidant activity was correlated to the malonyl gucosides.

The objective of this study was to add value to tofu whey by enhancing its potential as a source of isoflavones which are associated with the antioxidant activity of soybean. The concentration processes used were the block freeze concentration and the nanofiltration. The concentrates obtained from these processes were evaluated for their isoflavone content and antioxidant activity. The concentrated fluid obtained from freeze concentration showed an increase in the isoflavone contents for all stages. It was observed retention of isoflavones in the ice, however the concentration efficiency remained above 80% in all stages. The nanofiltration process promoted the concentration of β-glucosides and malonyl glucosides conjugates, but it was not possible to concentrate the aglycones. The values for the antioxidant activity of the concentrated fluid obtained from each freeze concentration and nanofiltration stage, measured applying FRAP and ABTS assays, were significantly higher than that of the tofu whey. Moreover, the antioxidant activity was significantly correlated with the isoflavone content of the tofu whey samples.