Effect of Elderberry Extract as a Functional Filler on the Physicochemical Properties and Storage Stability of HHP-Induced Pea Protein–Psyllium Hydrogels

The aim of this study was to investigate the effect of elderberry extract on the physicochemical properties and storage stability of HHP-induced pea protein–psyllium hydrogels. Hydrogels were prepared with the addition of 2 g of dry elderberry extract per 100 mL of water used for hydrogel preparation (E2) and without its addition (E0). Hydrogels were tested after 1 and 30 days of refrigerated storage (4 °C). It was found that the addition of the extract did not affect the ability of the pea protein–psyllium system to form a hydrogel network. However, its stability increased after storage (VGI: E0 88%; VGI E2 90%). Hydrogels with the extract added were also characterized by lower fluidity (solid–liquid balance (SLB): E0 0.44; E2 0.37) and higher elasticity (elasticity index (EI): E0 0.022 nm−2; E2 0.030 nm−2) and macroscopic viscosity (MVI: E0 0.029 nm−2 s; E2 0.044 nm−2 s), as well as higher adhesion force (E0 0.04 N; E2 0.09 N) and lower spreadability (E0 2.3 N s; E2 3.5 N). It was also observed that the rheological properties of hydrogels with the addition of the extract strengthened after 30 days of storage. Among the microrheological parameters, a decrease was observed in SLB and FI (fluidity index), along with an increase in EI and MVI. Among the texture parameters, an increase was seen in strength, viscosity index, and adhesiveness force. The addition of the extract also affected the color of the hydrogels, causing a decrease in brightness (L*: E0 71.64; E2 6.60), an increase in the share of red color (a*: E0 4.48; E2 12.67), and a decrease in the share of yellow color (b*: E0 16.63; E2 0.41). At the same time, elderberry extract dyes immobilized in the pea protein–psyllium hydrogels underwent relatively few changes during storage (ΔE = 0.53). An important nutritional effect of introducing elderberry extract into hydrogels was the improvement of their phenolic and antioxidant profiles. The addition of this functional filler to pea protein–psyllium hydrogels allowed the maintenance of total anthocyanins during cold storage. Such hydrogels can be used to create new types of food with increased health value and stability.