Dry Fractionation of Pulses Using Air Classification for Protein-Enriched Fractions and Development of Low Moisture Extruded Functional Foods

Abstract

The growing demand for sustainable and clean-label plant proteins has increased interest in dry fractionation as an alternative to conventional wet extraction. This study optimized an integrated dry fractionation process combining high-speed impact milling and air classification to produce clean-label, chemical-free protein-enriched fractions from black gram (Vigna mungo) and green gram (Vigna radiata) under water-free conditions at temperatures below 40 °C. The effects of milling and air-classification parameters on particle size distribution and protein enrichment were systematically evaluated, revealing that mill speed primarily influenced coarse particle size represented by D90, while airflow significantly affected D10, D50, and D90, with internal classifier wheel speed ranging from 2,000 to 8,000 RPM showing minimal independent impact. Optimized conditions enabled recovery of fine fractions with sub-20 μm D90 and effective protein–starch separation, increasing protein content from 24.81% to 45.79–46.00% in black gram, corresponding to an enrichment of 84.6% and from 24.60% to 49.16% in green gram, corresponding to an enrichment of 87%. The fine fractions exhibited small median particle sizes, with D50 values ranging from 9.59 to 10.35 micrometres, high foaming capacity approximately 90%, and enhanced water absorption capacity ranging from 3.37 to 3.71 grams per grams. Protein-enriched fractions were incorporated into low-moisture extruded products using a twin-screw extruder at 22% feed moisture, 280 RPM screw speed, and a temperature profile of zones varies from 50–125°C. Increasing levels of pulse protein fractions substitution resulted in a reduction in torque from 28.6 Nm to values between 20 and 22.9 Nm, as well as a decrease in melt pressure from 135 bar to between 77 and 90 bar, indicating changes in melt rheology. The expansion ratio decreased from 2 to 1.4, while bulk density increased, producing denser yet structurally stable extrudates. A marked reduction in hardness from 308 N to values between 60 and 75 N demonstrated improved textural acceptability. Overall, the integration of dry fractionation and extrusion successfully produced clean-label, protein-dense functional foods without chemical processing. This research establishes a scalable and environmentally sustainable approach for developing indigenous plant-protein ingredients and high-protein extruded snacks, supporting the growing demand for nutritionally enhanced, plant-based foods.