Development of a continuous-flow system for microwave-assisted extraction of pectin-derived oligosaccharides from food waste

This paper addresses the current lack of a scalable process for the extraction of “hairy” pectins to upcycle biomasses, by describing the design methodology, building and testing of a continuous microwave-assisted process for potato waste pectin extraction. Hairy pectins have been shown to present prebiotic activity. Conventional pectin extraction methods are not suitable to produce them, as they lead to degradation of hairy regions, in part due to long heating times. Microwave heating is considered an alternative due to its selective and rapid heating. The 2 kW single mode system developed in this study achieved good temperature control of ±2.5 °C, and a stable target temperature in ≈1 min processing time at a feed flow rate of 250 mL min−1. Pectin yields of 40–45% (indicated by the galacturonic acid content) were achieved, with a feed residence time of only 0.81 s followed by 20 min cooling-down under stirring. These yields were 59.75% higher on average than control experiments in batch conditions or under continuous heating. This indicates that a short heating time is sufficient to allow pectin hydrolysis, after which the rate limiting diffusion step can proceed during cool-down, minimising pectin degradation caused by prolonged exposure to high temperatures. The presence of hairy pectin regions in the extract was confirmed by neutral sugar analysis. The high starch content in potato necessitated a de-starching procedure. A sieving procedure was implemented, which removed more starch than enzymatic de-starching, resulting in a higher purity pectin extract and the ability to collect starch as a separate value stream. This work proves the suitability of microwaves to be used as a fast heating method to extract pectin from biomass, avoiding degradation, using a scalable continuous mode of operation.