The use of food waste as a rearing substrate for black soldier fly larvae (BSFL) offers a promising pathway for producing sustainable insect-based animal feed. However, food waste is increasingly contaminated with microplastics (MPs), raising concerns about contaminant transfer along the production chain and the safety of resulting value-added products. This study evaluated near-infrared (NIR) spectroscopy as a rapid, non-destructive screening tool for MP detection, first at the food waste stage and subsequently within BSFL biomass.
At the food waste level, polyethylene (PE), polypropylene (PP), and mixed polymers were added to high-moisture matrices representative of waste streams, namely spinach and banana. Samples were analysed using benchtop and portable NIR instruments, with spectra evaluated using principal component analysis and partial least squares (PLS) regression. Two spectral regions, 11,520–7500 cm⁻¹ and 7500–4200 cm⁻¹, were compared to assess wavelength sensitivity in high-moisture systems, with comparable performance. Using all samples, the benchtop instrument showed strong quantitative performance (R²CV = 0.88, SECV = 0.44, RPD = 3.6), while the portable instrument showed moderate performance (R²CV = 0.67, SECV = 0.55, RPD = 2.4).
Building on this feasibility assessment, polymer detection was extended to BSFL reared on contaminated substrates. Polyethylene, polypropylene, polyethylene terephthalate (PET), polystyrene and polyvinyl chloride were incorporated at 1% (w/w), with PET additionally assessed across 0.1–5% (w/w). Fourier-transform NIR spectroscopy combined with PLS regression enabled reliable polymer identification and quantification within the biological matrix. The PLS model achieved strong predictive performance across the full dataset (R²CV = 0.84, RMSECV = 0.67% w/w, RPD = 2.6), with consistent results across production sites, batches and sample handling conditions.
Overall, these results demonstrate the potential of NIR spectroscopy for monitoring MP contamination across the food waste to insect protein production chain, supporting quality assurance and the safe development of circular bioeconomy systems.