Extraction of Citrus maxima (Burm.) peel oil and pectin via acidic deep eutectic solvents: Storage effects and RSM optimization

Landmark fruit Citrus maxima (Burm.) is highly productive, but by-products can cause environmental pollution. This study simulates the storage conditions of Citrus maxima at various stages after harvest and models the nutrient content and changes in different by-products. An organic acid deep eutectic solvent (OADES) was configured using six organic acids as hydrogen bond donor (HBD) in combination with choline chloride (ChCl); and the yields of essential oils and pectin were optimized using response surface methodology. The study showed that the OADES configured with oxalic acid as the HBD had the highest extraction rate. The results of electronic nose (E-nose) combined with GC-MS showed that the volatile odors of the essential oils were mainly from β-Myrcene (38 %), D-Limonene (37 %), and β-Pinene (4.3 %). When the raw materials were stored for more than 30 days, the yields of essential oils were decreased by 27.68 %, and the composition was mainly a decrease of 4.3 % in the proportion of sesquiterpenes and an increase of 7 % in alcohols. After 60 days of storage, the yield decreased by 27.68 %, the composition was mainly mono sesquiterpenoids decreased by 21.8 % and alcohols increased by 20.8 %. In addition, throughout the storage period (0–60 d), the pectin yield increased and then decreased, with a denser structure and more thermally stable and methylated pectin compared to the pre-extraction grapefruit peel. The research results not only provide new methods for green and energy-saving extraction technologies, but also offer valuable insights for optimizing the storage and reproduction practices in the citrus industry.