Influence of plasticizers on the properties of soy-based bioplastic matrices for controlled release of iron

The global increase in horticultural production, driven by population growth and the resulting high food demand, has promoted the adoption of intensive cultivation systems that require large amounts of fertilizer. However, the low efficiency of conventional practices and their environmental impacts have driven the development of alternative strategies, such as controlled-release fertilizers. Soy protein isolate (SPI) bioplastics were developed, and the influence of plasticizers on their properties was analyzed. Bioplastic matrices were processed by compression molding at 300 bar using this by-product combined with different concentrations of water and glycerol (5, 10, and 15 wt%) as plasticizers and 5 wt% of iron sulphate heptahydrate as a micronutrient. Thus, physicochemical, mechanical, morphological and functional properties were assessed. Moreover, crop analyses were conducted to assess the potential biofortification effects of these systems. The results show that both plasticizers led to systems with similar elastic modulus (6.22 and 6.03 for systems 10/0 and 0/10, respectively). When glycerol is used, the systems exhibit greater instability than water-plasticized systems, as reflected by steeper slopes of the elastic moduli (70 and 120 for systems 10/0 and 0/10, respectively). Overall, these products demonstrated high potential for horticultural applications. They enabled optimal micronutrient assimilation by plants, which is reflected in increased biomass and crop weight, showing values from 33.3 to 41.5 g, matching or exceeding the positive control values (30.8 ± 5.3 g), while positively contributing to the soil, by-product valorization and nutrient delivery to soils within a circular-economy context. • SPI bioplastics acted as biodegradable iron controlled-release fertilizers. • Water and glycerol contents modulated functional, mechanical and structural traits. • Iron-loaded matrices matched or exceeded conventional fertilization with lower doses. • Bioplastic systems increased pepper yield and leaf greenness.