Natural deep eutectic solvents pretreatment as an aid for pectin extraction from apple pomace

Three natural deep eutectic solvents (NADES; choline chloride (CC): lactic acid (LA), CC: oxalic acid (OA), CC: urea (U)) in the ratio 1:2 were tested as a pretreatment of apple pomace prior to hot water extraction of pectin. The pretreatment duration and temperature (for 0.5–2h, at 40–80 °C) was adjusted to limit cell wall polysaccharides losses and degradation. Extraction yields and sugar composition were used to evaluate the impact of the pretreatment on pectin extraction and the molecular weight and degree of esterification of pectin were characterized. Results showed that CC:LA and CC:U pretreatments led to close overall recovery yields with the control using water instead of NADES. Instead, CC:OA led to polysaccharides degradation and loss. The cell wall monosaccharides composition was affected after CC:LA and CC:U pretreatments at 60 °C and above, while it was preserved by CC:LA pretreatments at 40 °C for 1h. Moreover, CC:LA pretreatment facilitated hot water extraction of a large amount (33.1%–56.1% of uronic acid recovery) of high Mw HM pectin comparable with pectin obtained through classical method. CC:U pretreatment led to saponification and affected pectin composition by introducing choline. Thus, CC:LA pretreatment of apple pomace followed by water extraction offers a “green” alternative for mineral acid pectin extraction while CC:U allows functionalizing pectin in apple pomace prior to hot water extraction.