Plant-Based Protein Processing and Undesirable Consequences of Food Processing: A Focus on Protein Quality, Sulfur Amino Acid Bioavailability, and Quantification

Abstract

The nutritional quality of plant-based proteins (PBPs) is increasingly scrutinized as global dietary trends shift toward sustainable alternatives to animal-based proteins. This review critically evaluates the impact of food processing on essential amino acids (EAAs), with a particular focus on sulfur amino acids (SAAs), including cysteine and methionine. While processing techniques such as extrusion, autoclaving, fermentation, and high-pressure treatment can enhance the digestibility, functionality, and sensory properties of PBPs, they also induce undesirable chemical modifications. These include oxidation, racemization, and crosslinking, which reduce the biological availability of SAAs and compromise protein quality. Evidence from recent studies highlights significant losses of cysteine during processing, often attributed to the Maillard reaction and oxidative stress. Conventional quantification methods, such as performic acid oxidation, may overestimate SAA content due to the presence of pre-existing oxidized forms in processed foods. Alternative approaches, including Ellman's reagent-based assays, offer more accurate assessments of reactive cysteine and are better suited for evaluating true bioavailability. This review highlights the need for enhanced analytical methods to differentiate between total and biologically available SAAs, particularly in processed PBPs. Accurate quantification is crucial for refining protein quality metrics, informing food formulation, and developing correction factors to compensate for nutrient losses that occur during processing. Addressing these gaps will support the development of high-quality plant protein products and inform strategies to optimize processing conditions, ultimately enhancing the nutritional value of plant-based diets.