Selenium biofortification during soybean germination and its effect on soybean nutrients and functional properties

English. Selenium (Se) is an essential element playing an important role in the immune system in humans. However, an estimated one billion people have insufficient dietary Se intake, especially vegetarians or people who eat more vegetables than meat, because vegetables have less Se. Therefore, promoting the Se levels in vegetables through biofortification is an ideal way to improve the Se daily intake. Inorganic Se can be transformed into organic Se, which has been reported to have lower toxicity and high bioavailability. Soybean has been often studied as a dietary Se source, biofortification during soybean germination is a simple, rapid, and efficient method for improving Se contents in sprouts. The present study investigated the influence of Se biofortification on the growth and quality of soybean sprouts, and provided novel insights into Se metabolism during germination, further revealed Se distribution in different dominant protein fractions and its effects on their structure and functional properties. Our findings would provide a better understanding of Se-enriched soybean sprouts and support their application in functional food product development. Firstly, the influence of Se biofortification on the sprout growth was investigated. Total Se and the other nutrient compounds, such as proteins, amino acids, total phenolic compounds, and flavonoids, were analyzed to study how they are affected by Se biofortification during germination. The results showed that the germination rates of soybean decreased dramatically after 80 mg/L of selenite solution soaking, while soaking below 60 mg/L of selenite solution had no significant influence on sprout growth. Se biofortification increased the contents of total phenolic compounds, flavonoid compounds, and amino acids (both total and essential), especially in low-concentration Se treatment. Total Se content increased by 87.3 times under the concentration of 60 mg/L selenite solution treatment, indicating soybean germination after selenite solution soaking is an efficient method for Se biofortification. Se species transformation and distribution during soybean germination were further analyzed using HPLC-ICP-MS. Results showed that a large portion of selenite was transformed into organic Se during 24 h of germination, Methylselenocysteine (MeSeCys) and selenomethionine (SeMet) were the dominant Se species, MeSeCys decreased during the germination, but SeMet had an opposite trend. Meanwhile, 89.3% of the different species of Se were bound to soybean, indicating that selenite could be transformed into seleno-amino acids and incorporated into proteins. Moreover, the dominant protein fractions 7S and 11S have different Se enrichment abilities, it is much easier to combine Se with 11S than with 7S during soybean germination. Finally, the effects of Se on the structure and functional properties of soybean protein fractions (7S and 11S) were explored, and the air-water interfacial property was studied to unravel the mechanisms of foaming properties. The results showed that Se biofortification did not make protein fractions subunits degrade or aggregate, while the Se decreased α-helix in 7S, and Random coil in 11S, but increased β-Turn in 11S. The Se also had no significant effects on the water- and oil-holding capacities in protein fractions but increased the foaming capacity (FC) and emulsion activity index (EAI). The Se treatment also significantly increased the antioxidant capacity in 7S but not in 11S. Furthermore, germination and Se treatment can influence the lag time, initial adsorption rate, equilibrium surface tension, and dilatational modulus. FC showed positive correlations with surface tension decrease, dilatational elasticity and dilatational modulus, whereas FC was negatively correlated to surface tension; foaming stability (FS) was positively correlated to viscosity and lag time. This study indicated that Se biofortification could impact the protein fractions’ structures, and improve the foaming properties and EAI of protein fractions. The present research confirmed that Se-enriched soybean sprouts had the promising potential for organic Se supplementation. Se was mainly incorporated into dominant protein fractions and impacted the structure and properties. These investigations provided novel insights into Se species transformation at various time points during soybean germination, which will help us to obtain more target Se species through germination processing. Moreover, the study about Se distribution in different germinated soybean dominant protein fractions and its effects on protein fractions structure, functional properties, air-water interfacial properties, and antioxidant activity provided substantial evidence for developing efficient Se-enriched food supplements.

3. Good health and well-being