Preparation and Characterization of Micro-Cellulose and Sub-Micro Nano-Cellulose Particles from Mango Peel Waste

Cellulosic biomaterials, such as mango peel, are waste by-products of the food processing industry. The extraction of cellulose from these waste materials could provide economic value while contributing to environmental sustainability through waste valorization. In this study, micro-cellulose was extracted from mango peel by hot water digestion followed by alkaline digestion. The resulting residue was mixed with ethanol, centrifuged and freeze-dried to obtain micro-cellulose. The extracted freeze-dried micro-cellulose contained 85 g/100 g holocellulose. Nano-cellulose was subsequently prepared by disc milling the micro-cellulose for 100 s, successively three times. The average particle sizes of micro-cellulose and nano-cellulose were 74 µm and 320 nm, respectively. Scanning Electron Microscopy (SEM) micrographs showed an irregular and shriveled morphology with a porous interconnected network. Transmission Electron Microscopy (TEM) micrographs showed that the nano-cellulose particles exhibited spherical, triangular and brick shapes. Water absorption decreased from 86 to 81 g/g dry solids, whereas solubility increased from 6 to 11 g/L when micro-cellulose was transformed into nano-cellulose (p < 0.05). The BET-monolayer moisture of micro-cellulose (0.041 g/g dry solids) and nano-cellulose (0.038 g/g dry solids) were significantly different (p < 0.02). Transformation of micro-cellulose to nano-cellulose resulted in an increase in the onset glass transition temperature from 140 °C to 146 °C, while relaxation at the glass transition decreased from 102 J/g to 63 J/g. Similar trends were also observed in the solids-melting peak temperature and enthalpy. Crystallinity of micro-cellulose was 80.7%, whereas that of nano-cellulose was 76.5%. The bands at 1262 cm−1 and 670 cm−1 shifted to high, and 1378 cm−1 and 1233 cm−1 shifted to low in the case of nano-cellulose. Fourier Transform Infrared (FTIR) band intensities at 3450 cm−1, 1562 cm−1, 963 cm−1, 897 cm−1, 838 cm−1, and 670 cm−1 increased, whereas the bands at 1646 cm−1, 1378 cm−1, 1233 cm−1, 1200 cm−1, and 998 cm−1 decreased when micro-cellulose was converted to nano-cellulose. These findings suggest that some molecular bonds in functional groups were disrupted. Physicochemical and thermal properties, together with the extent of functional group alterations, indicated that the extracted micro-cellulose and nano-cellulose from mango peel have potential for applications in food products, biomaterials, edible coatings and biodegradable packaging.