Green leaves as a promising source for sustainable food protein: Seeking the productivity-functionality balance

Background: The adverse environmental impact of animal protein production, along with its related health concerns, predominantly the rising threat of antibiotics-resistant bacteria, are propelling the major current trend in food science: sustainable alternative proteins (mainly plant-based, cell culture-based, fermentation-based, and insect-based). Among plant-based proteins, green leaves are being spotlighted here as a promising, ubiquitous, but underutilized source of alternative protein, especially leaves of crops used for human consumption, which become waste or animal feed.Scope and approach: This article aims to review leaf protein extraction methods, their benefits versus environmental impact, their yield, cost, and the nutritional and functional quality of the extracted protein, as a promising raw material for the booming sustainable (alternative) protein industry. Key findings and conclusions: Utilizing an agricultural side stream is usually advantageous environmentally and economically. When considering leaf utilization, one must consider leaf source, extraction process, and product (mainly protein) functionality. Leaf source choice must consider protein content, availability (geography, season), alternative uses, etc. Extracting leaf proteins is an important challenge, as yield, quality, and functionality are critical for food application. Moreover, the environmental effect of the process should be examined to minimize waste, water use, and pollution. Each step of the extraction process should be comparatively studied among several methods, to establish the best process for each plant and purpose. Here we provide a detailed review of major steps and key parameters of the process, to facilitate choosing the right conditions for making sustainable food protein out of agri-waste leaves.