Starch removal treatment, rather than thermal processing, improved the protein digestibility of corn Zea mays gluten meal

Abstract: Corn gluten meal (CGM) is a major by-product of wet milling, which consists of 62–71% of protein, 12%-15% of starch, and 9%-11% of non-starch carbohydrates. Currently, CGM is mainly used as animal forage, but not human food, mainly due to its low protein digestibility. Therefore, this work aimed to evaluate the impact of starch removal and different thermal processing methods (microwave, extrusion, boiling, and baking) on in vitro protein digestibility of CGM, and reveal the underlying possible mechanisms by examining the changes of the physicochemical and structural properties of treated CGM. In vitro protein digestibility was evaluated based on the degree of hydrolysis by the o-phthaldialdehyde (OPA) method using a simulated gastrointestinal digestion model. The results suggested that starch removal has significantly increased the protein digestibility by 26.3% to 37.8% compared to the samples with starch (P < 0.05). The possible factors to result in the increased protein digestibility after removing starch are associated with significantly improved water absorption capacity and zeta potential and significantly reduced disulfide bond content and particle size (P < 0.05). In addition, the disrupted ordered β-sheet secondary structures and increased disordered random coils after starch removal as assessed in Fourier-transform infrared spectroscopy (FTIR) suggest a reduction in protein aggregation, which positively contributed to higher digestibility. Moreover, scanning electron microscopy and confocal laser scanning microscopy data revealed the altered surface morphology: More pores, gaps, loosened structures, and elevated surface area which leads to the increased digestibility by providing more enzymatic cleavage sites. In conclusion, starch removal is a promising approach to increase CGM digestibility and allow its utilization as a sustainable protein alternative which provides new avenues for addressing UN sustainable development goals and meeting the protein needs of the growing population.