Depolymerization of sorghum procyanidin polymers into oligomers using HCl and epicatechin: Reaction kinetics and optimization

• Sorghum procyanidin polymers were depolymerized into oligomers with epicatechin and HCl.
• Depolymerization followed a pseudo first-order kinetic model.
• Depolymerization rate largely depended on reaction temperature and pH.
• Highest yield of oligomers was achieved at 74 °C, pH 1.24, 61 min, and mass ratio of 0.89.

Sorghum procyanidin polymers were depolymerized under acidic condition in presence of epicatechin to produce oligomers with better bioavailability. The objective of this research was to investigate the kinetics of this reaction and to optimize conditions. It was found that depolymerization followed a pseudo first-order kinetics model. Increasing temperature from 50 to 80 °C significantly enhanced reaction rate from 0.028 min−1 to 0.382 min−1. Moreover, decreasing pH from 2.0 to 0.5 dramatically cut half-time from 65.4 min to 3.0 min. More epicatechin also shortened half-time when monomer was not sufficient in the system. Furthermore, a central composite design model was built to show the influence of temperature, pH, reaction time and epicatechin to polymer mass ratio and their interactions on the yield of oligomers. The analysis of variance revealed significance of these factors on response and a high R2 of 0.98 suggested the model fitted well. The optimized reaction condition was predicted as 74 °C, pH 1.24, 61 min and mass ratio 0.89. The predicted yield was 87.0 ± 4.0% which was close to the experimental yield of 86.0 ± 1.2%.