Mixed Food Waste as a Low-Cost Carbon Source for Kombucha Bacterial Cellulose

Food waste is an abundant yet underutilized resource for sustainable biomaterial production. In this study, mixed fruit and vegetable waste (orange, pineapple, carrot and lemon) was evaluated as a low-cost carbon source for bacterial cellulose (BC) production via Kombucha fermentation. Fermentation from unhydrolyzed juice (UJ) yielded the highest amount of BC (6.17 g/L) surpassing the yield obtained from 2% acetic acid hydrolyzed juice, t = 30 min (HJ-30 min) at 5.44 g/L. The yields achieved using the mixed food waste substrates (UJ and HJ-30 min) were substantially higher than those from refined sugar (RS) and 2% acetic acid only (Ac2) controls. Acetic acid hydrolysis (2% v/v, 30 or 100 min) significantly reduced the initial sucrose concentration from 41.75 g/L to <7 g/L, converting it into glucose and fructose, and decreased the pulp particle size. Importantly, this mild pretreatment did not generate the inhibitory byproduct 5-hydroxymethylfurfural (5-HMF). However, the hydrolyzed medium was associated with Kahm yeast formation, which is attributed to the significant drop in sucrose concentration that caused an imbalance for beneficial yeast growth. Fourier transform infrared spectroscopy (FTIR) confirmed the presence of characteristic cellulose-specific functional groups in all samples. X-ray diffraction (XRD) analysis revealed that BC from UJ and HJ-30 min had higher crystallinity indices compared with the controls (RS and Ac2). All BC samples exhibited high molecular weights and degrees of polymerization (>2.7× greater than that of microcrystalline cellulose (MCC)), and scanning electron microscopy (SEM) micrographs revealed characteristic nanofibrillar morphologies. These results demonstrate that unhydrolyzed mixed food waste is not only sufficient but preferable for BC production, offering a low-cost, minimally processed pathway to obtain high-quality, high-purity biopolymers with numerous applications.