Chestnut burrs as a sustainable source of cellulose for Pickering emulsion stabilisers

As the global population grows, increasing food consumption drives a significant rise in agri-food waste. In the northern Trás-os-Montes region of Portugal, one of the world's largest chestnut producers, this waste includes valuable by-products such as chestnut burrs, which are composed of 63% cellulose. This study aims to valorise chestnut burrs by extracting compounds for incorporation into high-added-value products. Cellulose was extracted through physical and chemical pre-treatments, alkali hydrolysis and bleaching. The process parameters were adjusted to enhance the extracted cellulose content and purity. Extracts were characterised by thermogravimetry and Fourier Transform Infrared Spectroscopy, and parameters such as particle size, zeta potential and wettability were also assessed and compared with commercial microcrystalline cellulose. Temperature was identified as the key parameter in alkali hydrolysis, with optimal conditions obtained for 10% NaOH at 100 °C for 1 h and a biomass-to-solvent ratio of 1:20 g/mL. Bleaching under optimal conditions (2% NaClO at 35 °C) removed an additional 7% lignin and enhanced the whiteness of the final product. The cellulose obtained has a purity of 72%, with a particle size of 55 μm, a negative surface charge, hydrophilic behaviour, an estimated molecular weight of 13.2 kDa, and a crystallinity of 56.5%. When applied at 0.8 wt% of the aqueous phase in 20:80 oil-in-water emulsions, these particles stabilise Pickering emulsions with a 12 mm average droplet size, remaining stable for at least 30 days. These findings demonstrate that chestnut burrs are a viable and sustainable source of cellulose with high potential as a natural Pickering emulsion stabiliser for use in food, cosmetics, and pharmaceuticals.