Thermal Stability of Starch-Proanthocyanidin Complexes

Abstract

Proanthocyanidins (PA) form poorly digestible complexes with starch via non-covalent linkages, showing promising potential to reduce the caloric density of the starch-rich products. The present study examined amylase degradation mechanism and hydrothermal stability of starch-PA complexes. Additionally, this study aimed to demonstrate the effect of Ebeam irradiation on enhancing the stability of starch-PA complex physiochemical properties and in vitro digestibility. Methylating PA as a means to enhance their hydrophobic interactions with starch was also investigated. Sorghum-derived PA was complexed with wheat starch, reconstituted into flour (10% gluten added) and processed into crackers and pancakes. In vitro digestion profile of the complexes and products were characterized. The molecular weight (MW) distribution after 20 min and 120 min of digestion revealed higher proportions of medium and high MW for PA treatments compared to controls. Debranching amylopectin further revealed higher retention of DP 11 – 30 chains in the digested starch-PA complexes than controls, suggesting amylopectin complexation contributed to reduced starch digestion. Starch-PA complexes retained reduced digestibility (50-56% higher resistant starch vs. controls) in the cracker, but not pancake model. However, removing gluten from the pancake formulation restored the reduced digestibility of the starch-PA complexes. This indicates the starch-PA complexes are stable to hydrothermal processing, but can be disrupted by hydrophobic gluten proteins under excess moisture conditions. Subjecting starch-PA treatments to Ebeam (2-50 kGy) increased MW compared to their controls (P<0.05) suggesting covalent crosslinking of starch polymers by PA. Amylose fraction of starch-PA treatments had the highest MW increase vs. controls (15-102%) after Ebeam treatment. Ebeam starch-PA treatments had significantly (P<0.01) lower starch digestibility compared to their controls (resistant starch of 2 kGy PA treatment was 85.9% vs. 44.4%, in the control). Our findings indicates that Ebeam likely facilitated PA-starch crosslinking via redox mediated reaction. Methylation conditions led to PA-PA crosslinking leading to polymers that were sterically bulky and unable to form V-complexed with starch. This work demonstrates that starch-PA complexes are stable under hydrothermal food processing, and that Ebeam can covalently crosslinking them and further expand their functionality.