Effects of preheat treatment and syringic acid modification on the structure, functional properties, and stability of black soybean protein isolate

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Abstract
This study investigated preheated (25–100°C) black soybean protein isolate (BSPI) conjugated with syringic acid (SA) (25 and 50 µmol/g protein) under alkaline conditions, focusing on the structure, functional properties, and storage stability. The results revealed that the SA binding equivalent and binding rate on BSPI increased continuously as the preheat temperature increased. Additionally, preheating positively impacted the surface hydrophobicity (H 0 ) of BSPI, with further enhancement observed upon SA binding. Preheating and SA binding altered the secondary and tertiary structure of BSPI, resulting in protein unfolding and increased molecular flexibility. The improvement in BSPI functional properties was closely associated with both preheating temperature and SA binding. Specifically, preheating decreased the solubility of BSPI but enhanced the emulsifying activity index (EAI) and foaming capacity (FC) of BSPI. Conversely, SA binding increased the solubility of BSPI with an accompanying increase in EAI, FC, foaming stability, and antioxidant activity. Notably, the BSPI100‐SA50 exhibited the most significant improvement in functional properties, particularly in solubility, emulsifying, and foaming attributes. Moreover, the BSPI‐SA conjugates demonstrated good stability of SA during storage, which positively correlated with the preheating temperature. This study proposes a novel BSPI‐SA conjugate with enhanced essential functional properties, underscoring the potential of preheated BSPI‐SA conjugates to improve SA storage stability. Practical Application: Preheated BSPI‐SA conjugates can be used as functional ingredients in food or health products. In addition, preheated BSPI shows potential as a candidate for encapsulating and delivering hydrophobic bioactive compounds.

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