Soy protein nano-aggregates with improved functional properties prepared by sequential pH treatment and ultrasonication

•A method treating soy protein with pH shifting in combination with ultrasound is proposed.•pH shifting at pH12 followed by ultrasound produced nano-sized protein aggregates (<100 nm).•Protein solubility and selected functional properties were improved.•The protein nano-structures provided protection of vitamin D against UV irradiation.

The functional properties of soluble nano-sized soy protein aggregates produced by pH treatment followed by ultrasonication (US) were investigated. Commercial soy protein isolate (SPI) was subjected to pH treatment under acidic (pH 2–4) or alkaline (pH 9–12) conditions followed by US before neutralizing to pH 7. The pH treatment at pH 12 followed by sonication for 5 min (pH12 + US5) was the most effective in reducing the sizes and turbidity of soluble protein aggregates, enhancing protein solubility and surface hydrophobicity, and modifying protein subunits. The pH12 + US5 treated SPI increased protein solubility from 1.49% for the control to 82.73%, producing soluble protein aggregates with average size of 22 nm. SPI nanoemulsions prepared using pH12 + US5 treated protein and canola oil had particle sizes in the range of 70–117 nm. Nanoemulsions and nanocomplexes (no oil) prepared with the SPI nano-aggregates provided good protection of vitamin D3 against UV exposure (180 min), with retention of 73.5 and 70.7%, respectively, compared to 5.2% in the control. This new treatment may offer an effective method to modify the functional properties of commercial SPI for use as building blocks for preparing nanoparticles and nano-structures to protect and deliver bioactive compounds.

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