(342f) Tribo-Charging and Separation Behavior of Milled Yellow Pea Flour and Plant Protein-Starch Mixture Models

Background

The Food and Agriculture Organization (FAO) projects a severe worldwide food crisis if the current food supply system is not modified by 2050. Existing food production methods, particularly protein, rely heavily on unsustainable animal-based products. However, due to the surging demand for vegetarian diets, plant-based proteins are promising alternatives or supplements to animal-based products.

Methods

In this study, the tribo-electrostatic separation (TES) technique, a novel and green protein fractionation method capable of preserving plant-protein’s bio-functionality, was combined with the milling process to optimize protein fractionation from yellow pea flour. Yellow pea's protein was first detached from other cellular bodies using the pin and ferkar milling systems equipped with 0.14- or 0.5-mm screen sizes. Next, the milled samples were pneumatically transferred to a Teflon tribo-charger tube at flow rates of 7 to 14 LPM, where particle-particle and particle-wall collisions resulted in the acquisition of charge by protein and starch at different levels. The charged particles were then fractionated in a chamber by being exposed to two oppositely-charged plates with voltage values ranging from ±3 to ±12 kV. The positively-charged plate (PCP) and chamber bottom (CB) attracted starch the most, while protein was mainly adsorbed to the negatively-charged plate (NCP). Finally, we statistically optimized the protein enrichment. Correlating each fraction's yield, protein content, and separation efficiency to operating parameters (milling type, milling screen size, airflow rate, and electric field strength) allowed us to verify the analytical predictions.

Results

According to General Linear Models (GLM), 0.14-mm pin-milled flour treated under laminar flow (7 LPM) and medium plate voltage (±6.5 kV) was predicted as the optimal conditions for protein enrichment on the NCP fraction. In this condition, protein content rose from ~20 to 57%, corresponding to 62% protein recovery. Furthermore, regardless of screen size, pin-milled flours had a greater specific surface area than the ferkar-milled flours. Therefore, protein particles of the finest pin-milled flour (0.14-mm screen size) experienced the highest number of collisions and gained the highest charge, allowing more protein particles to be attracted to the NCP fraction. As a result, the NCP fraction’s protein content and separation efficiency were maximized by using 0.14-mm pin milled flour.

NCP fraction recorded the highest values of yield and protein separation efficiency at higher plate voltages. However, it was ineffective on protein content due to electrode fouling at strong electric fields, which acted as a shield to the additional attraction of protein particles. Moreover, the weakly-charged protein-fiber matrices of the cell wall, which were accumulated on CB fractions at weak electric fields, were transferred to the NCP fractions upon increasing the plate voltage, rendering the electric field strength ineffective for enhancing protein enrichment.

The separation behavior of milled yellow pea particles differed at turbulent airflow as many starch granules were surrounded by over-charged protein particles, creating large agglomerations with a net positive change. Therefore, as the airflow rate increased, the resulting agglomerated complexes ended up in the NCP fraction, thereby elevating its acquired average particle size and decreasing its protein enrichment.

The tribo-charging and separation behavior of the major constituent particles of optimally milled yellow pea flours (protein/gluten and starch) and their binary mixtures at different weight ratios were further investigated to correlate the net acquired charge of particles to the level of their protein enrichment via TES. The results showed that TES could effectively enrich plant protein particles when the starting material has low or moderate protein content using nylon and polyvinyl chloride (PVC) tribo-charger tubes. Yellow pea protein-starch weight ratios of 1:9 and 3:7 at the optimal TES condition (7 LPM & ±6.5 kV) resulted in the highest protein enrichment using PVC and nylon tubes, respectively.

Significance

As a legume representative, the TES optimization study of protein enrichment from yellow pea flour can provide helpful information on improving sustainable fractionation methods from plant resources by significantly eliminating waste stream and energy loss typical of conventional wet fractionation techniques. It also promotes the production of premium-quality bio-functional plant-based proteins.

Acknowledgment

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