Protein Extraction and Characterisation of Defatted Sal (Shorea robusta) Seed Cake under Different Conditions

Sal (Shorea robusta) seed cake, an underutilized by-product of oil extraction, represents a promising source of plant-based protein. This study investigated the influence of particle size, extraction solvent, and precipitation pH on protein recovery from defatted Sal seed cake. Protein extraction was performed using distilled water with alkaline pH adjustment, 0.25 M sodium hydroxide (NaOH), and 0.2 M potassium hydroxide (KOH) at two particle size fractions (<0.5 mm and 0.5–1.0 mm) and precipitation pH ranging from 2 to 5. Results revealed that particle size significantly affected extraction efficiency, with the finer fraction (<0.5 mm) consistently yielding higher protein recovery across all solvents. Among extraction media, alkaline solvents outperformed distilled water, with the highest protein yield (38.63%) obtained using NaOH at pH 4 for the <0.5 mm fraction. KOH extraction resulted in a maximum yield of 22.38% at pH 4–5, while distilled water extraction showed comparatively lower yields (maximum 18.67%). The lowest protein recovery (4.58%) was observed with distilled water at pH 5 for the 0.5–1.0 mm fraction. Overall, the study demonstrates that optimized particle size reduction combined with alkaline extraction and controlled precipitation pH significantly enhances protein recovery from Sal seed cake, supporting its valorisation as a sustainable plant protein source for food and feed applications.