Physico-chemical, techno-functional and structural properties of native and glycated proteins isolated from black soldier fly (Hermetia illucens) larvae

Abstract

Extracting the constituents of edible insects such as lipids and proteins for food applications is a promising approach to upturn global consumption in the next 2 – 3 decades. Black soldier fly (Hermetia illucens), an edible insect, rich in protein and lipids is one of the most promising insect species for incorporation in food products due to the environmental benefits, coupled with available technology for their rearing. The need to increase the global consumption of edible insects is paramount due to the high demand for protein sources for human consumption. This study aimed to extract and characterise native BSFL protein and to compare the effect of glycation on the physico-chemical, techno-functional and structural properties of BSFL-Glu (black soldier fly larvae-glucose) protein conjugates as a function of reaction temperature and time with a view to obtain novel proteins with superior functionalities for food application. The proximate compositions of freeze-dried BSFL (BSFL-FD), defatted BSFL (BSFL-DF), alkali and isoelectric precipitation BSFL protein concentrate (BSFL-PC1), alkaline extraction protein concentrate (BSFL-PC2) was established. BSFL protein concentrates (BSFL-PC1 and BSFL-PC2) displayed significantly higher (p < 0.05) protein content and lower ash (p < 0.05) compared to BSFL flours (BSFL-FD and BSFL-DF). The essential amino acid content of BSFL-PC1 was superior compared to BSFL-PC2. The high solubility at low pH values for the protein concentrates (BSFL-PC1, 95% and BSFL-PC2, 85%) makes them ideal candidates for use in acidic beverages. The foaming capacity (FC) of BSFL-FD (40%) was not statistically different (p > 0.05) than that of BSFL-DF (55%). The highest emulsion capacity (EC) was determined for the protein concentrates BSFL-PC1 (100%) and BSFL-PC2 (100%). Reaction mixtures containing black soldier fly larvae protein concentrate and glucose (2:1 weight ratio) were wet-heated at 50, 70 and 90°C for 2, 4, 6, 8 and 10 h, respectively, with an initial pH of 9. The ABTS+ radical scavenging activity of conjugates produced at 50°C ranged from 10.5 – 16.5% and exhibited the lowest ABTS radical scavenging activity when compared to those heat-treated at 70 and 90°C. Generally, BSFL-Glu conjugates heated at 90°C exhibited higher metal chelation activity compared to those at 50°C and 70°C. For the metal chelating activity of BSFL-Glu conjugates at 90°C, a significant increase was observed until maximum (64.45%) at 6 h, this was then followed by a slight decrease until the end of the heating period. The emulsion capacity of BSFL-Glu conjugates at 70°C ranged from 54.44 – 59.45%. The emulsion stability increased significantly (p < 0.05) as a function of reaction time, with conjugates produced after 8 hrs exhibiting the highest emulsion stability (35.89%). The zeta-potential (ζ) of BSFL-Glu conjugates heat-treated at 70°C ranged from -10.25 to -25.25 mV while the native BSFL protein ranged from -12.84 to -16.70 mV. The ζ-potential analysis revealed that the glycation reaction modified the surface charge density of the BSFL protein as a function of reaction time and temperature. In addition, an increase in thermal stability of the BSFL-Glu conjugates was observed by utilizing Thermo-gravimetric analysis (TGA) and differential scanning calorimetry (DSC) was observed for the conjugates compared to the native BSFL protein. The thermal denaturation peak temperature (Tp) for the heat-treated native BSFL protein at 50°C ranged from 72.23 – 73.81°C, with no significant differences (p > 0.05) between heating times (2, 4, 6, 8 and 10 h). Fourier transform infrared spectroscopy (FT-IR) analysis indicated that the most apparent structural changes in the BSFL protein were in the amide I and amide II region. The BSFL-Glu conjugates heated at 90°C for 10 h clearly shows an increased intensity in the amide I region (1624 cm-1). Well-separated clusters permitting differentiation between native BSFL and BSFL-Glu conjugates were observed by using principal component analysis (PCA) on FT-IR spectra. At 50, 70 and 90°C the first two principal components (PC1 and PC2) showed an accumulated total variance of 91, 96 and 95%, respectively. Soft independent modelling of class analogy (SIMCA), a supervised chemometric classification tool, was used to establish the best classification model for discrimination between native BSFL protein and BSFL-Glu conjugates based on PCA. The applied model was able to distinguish between native BSFL-Glu and BSFL conjugates with an accuracy of 91%. Taken collectively, the findings of this study clearly illustrate that the degree of glycation results in structural changes to the native insect protein and thus conjugation hold a possibility for delivering novel food components with enhanced functionalities and expand the application of BSFL-Glu conjugates in food applications.