**Background:** Legume seeds are a major source of dietary protein (20–45% protein content) and are increasingly important as plant-based protein alternatives to animal products. However, legume proteins are generally less digestible than animal proteins due to several factors: the presence of protease inhibitors (which can comprise up to 10% of plant protein content), high protein concentration with low water content creating physical barriers, and inherent structural features of storage proteins such as glycosylation and oligomerization. These features likely evolved as defenses against herbivores and pathogens. Germination—triggered by water availability, oxygen, and appropriate temperature—initiates a program of metabolic activation, including the mobilization and breakdown of stored reserves. This review synthesizes the qualitative and quantitative changes in legume seed proteins during germination and their impact on protein digestibility.
**Methods:** This is a narrative review of the literature on legume protein changes during germination. The authors synthesized findings from studies employing various analytical approaches, including Kjeldahl nitrogen determination for crude protein content, electrophoretic analysis of polypeptide molecular weight distributions, in vitro protein digestibility (IVPD) assays, amino acid analysis, protease activity assays, and measurements of protease inhibitor activity. Studies across multiple legume species were compared, including chickpea (Cicer arietinum L.), lentil (Lens culinaris Medik.), pea (Pisum sativum L.), common bean (Phaseolus vulgaris L.), cowpea (Vigna unguiculata L.), fava bean (Vicia faba L.), mung bean, kidney bean, lupin (Lupinus albus L.), and soybean.
**Key Results:** Germination typically results in an increase in the dry weight percentage of protein across many legumes, likely reflecting both the breakdown of fats and carbohydrates and de novo synthesis of protein and free amino acids. Electrophoretic analysis showed that germination increased concentrations of low molecular weight peptides and decreased higher molecular weight proteins/peptides for all legumes studied except kidney bean. Storage proteins, particularly globulins (2S, 7S, and 11S fractions), undergo proteolytic breakdown during germination. Free amino acids increase markedly during germination—in L. culinaris, free amino acids rose from 2.2 mg/g to 48.6 mg/g dry weight, with asparagine accounting for a substantial portion of this increase. Total amino acids increased from 160 to 258 mg/g dry weight in lentil, with lysine and aspartate/asparagine increasing 4-fold. However, sulfur-containing amino acids (cysteine and methionine), already low in chickpea, declined further during germination. In vitro protein digestibility (IVPD) showed moderate increases after germination for all species examined except lentil. For chickpea, IVPD increased progressively: 68% in seed, 70% after soaking, and 72%, 76%, and 79% after 3, 4, and 5 days of germination, respectively. A recent study noted a doubling of free alpha-amino nitrogen after chickpea sprouting, reflecting increased amino acids and oligopeptides. Protease activity generally increases with germination time—mung bean endopeptidase activity showed a 10-to-15-fold increase by day 6, while carboxypeptidase activity increased by 50% over 6 days. Trypsin inhibitory activity typically reduces during germination but is not eliminated: 64% reduction in fava bean after 48 hours, up to 45% reduction in lentil after 10 days, and 19% reduction in cowpea after 8 days. Germination also decreases allergenicity—studies showed 40% reduction in pea immunoreactivity and 70% (78% in darkness) reduction in soybean immunoreactivity. Non-protein antinutritional factors (phytates, tannins, lectins, non-starch polysaccharides) can also be reduced by germination and simple processing techniques.
**Clinical Implications:** Germination represents an inexpensive, natural technique to improve the nutritional quality of legume proteins by increasing digestibility, reducing antinutritional factors, and decreasing allergenicity. The improved digestibility could help address protein-related malnutrition in growing human populations, particularly as plant-based diets become more prevalent. The review highlights that combining germination with other food processing methods (heating, fermentation, autoclaving) may yield further improvements in digestibility—for example, germination of Vigna unguiculata L. for 96 hours followed by autoclaving improved protein digestibility while completely destroying some antinutritional factors. Fermentation of soybean with Bacillus subtilis increased in vitro gastrointestinal digestibility by 1.52-fold and reduced trypsin inhibitor activity by more than 10-fold (from 27.33 TIU/g to 2.14 TIU/g). The authors call for further research using mass spectrometry-based proteomics and peptidomics to characterize the shifts in protein makeup, activity, and degradation during germination, which could guide the selection of more digestible legume varieties and the development of optimized food preparation approaches.