Tribo-charging and separation behaviors of protein-starch mixtures derived from legumes and cereals

Tribo-electrostatic separation (TES) is a novel green technique for dry fractionation of plant constituents. It relies on particles' distinct chargeability upon friction with one another or dissimilar materials. In this study, online charge analysis was employed with different insulating and metal contact tubes (Teflon, nylon, PVC, and copper) to quantify the tribo-charging behaviors of protein and starch single components of yellow pea and wheat, as well as their binary mixtures. The impact of particle concentration rate (g/h) at different tube lengths was also explored. The tribo-electrostatic separation of protein-starch mixtures with various weight ratios was then correlated to the chargeability results. According to particles’ tribo-charging properties, the following triboelectric series was deduced for the first time: (+) nylon >protein >PVC >starch >Teflon >copper (-). Nylon and copper had the highest affinity to acquire positive and negative charges, respectively, making the other contact particle/material oppositely charged. At low particle concentration rates (< 600 g/h), the particles gained more specific charge (nC/g) with longer tubes. The length of tubes, however, was inefficient at high concentration rates. The type of tribo-material, surprisingly, did not affect the protein separation, suggesting that charge transfer in laminar flow was more influenced by particle-particle interactions. The particle size was inversely related to its specific charge, and over-charged fine starch granules at low protein-starch weight ratios formed agglomerates, thereby increasing loss and reducing protein separation efficiency through TES. Due to insufficient particle-particle interactions at high weight ratios, large protein particles were weakly charged, leading to limited protein enrichment. Thus, the optimal protein-to-starch weight ratio for the TES process was determined to be 3:7. These findings offer new insights into the charging behavior of plant particles and provide valuable information for developing an efficient TES design for legumes and cereals.

Acknowledgment:
The project was funded through USDA-NIFA-AFRI Grant #2020-67021-31141.