Alginate–Chitosan Encapsulation of Fenugreek Seed Oil: Effects of Chitosan Molecular Weight on Physicochemical Properties and In Vitro Bioactivity

Fenugreek seed oil contains bioactive constituents of interest for functional-food applications, but its susceptibility to environmental degradation may limit processing and storage. This study developed an alginate–chitosan encapsulation system and examined how chitosan molecular weight affected the physicochemical properties and in vitro bioactivity of the resulting formulations. Fenugreek seed oil was obtained by mechanical pressing and Soxhlet extraction, yielding 2.81% and 5.50% (v/w), respectively. The oil was incorporated into calcium-alginate beads (T1), which were left uncoated or coated with high-molecular-weight chitosan (HMC; T2) or low-molecular-weight chitosan (LMC; T3). The formulations were characterized by color measurement, microscopy, scanning electron microscopy, Fourier-transform infrared spectroscopy, thermogravimetric analysis, and particle-size analysis of oil droplets released from the polymer matrix. Free and encapsulated oil samples were also exposed to UV-C irradiation and evaluated using antioxidant assays, an albumin-denaturation inhibition assay, and an α-amylase inhibition assay. Dry-product recovery ranged from 76.72% for T2 to 94.79% for T1, with T3 showing a higher recovery than T2. SEM showed that T2 had the smoothest and most uniform dried morphology. FTIR spectra were consistent with interactions between alginate and chitosan while retaining characteristic bands associated with the oil. The chitosan-coated formulations also showed altered thermal-degradation profiles compared with alginate alone. After irradiation, T3-UV showed DPPH radical-scavenging activity of 63.03% and albumin-denaturation inhibition of 94.95%, whereas T2-UV showed the highest α-amylase inhibition. These findings indicate that chitosan molecular weight influences formulation recovery, morphology, thermal behavior, and measured in vitro activity. Further work using matched non-irradiated controls, standardized oil-equivalent concentrations, release testing, and gastrointestinal models is required before functional-food delivery claims can be established.