Recovering citric acid from orange juice production residues

A good part of the world production of oranges is destined to the production of juices, generating a huge amount of residues (e.g. peels), which are mostly used for the production of animal feed. However, there is still a great amount that is deposited in landfills generating an undesirable environmental impact [1,2,3]. There are several studies that prove the biological activities (e.g. antimicrobial, antioxidant and anticancer) of orange peels, being important to study the best way to extract the compounds that grant such activities, thus creating high added value to these residues [4]. This study has as its main objective the optimization of citric acid extraction (high antimicrobial power) from orange peels, in order to value citrus by-products through their recycling into natural ingredients. For the optimization of the ultrasound-assisted extraction, a central composite design coupled with RSM was implemented by combining five levels of the independent variables ethanol concentration (0-100%), time (2-45 min), and ultrasonic power (50-500 W). The levels of citric acid were determined by UFLC-PDA and used as response. Design-Expert software was used for regression and graphical analysis of the data. The developed theoretical models were successfully fitted to the experimental data, statistically validated based on high F-values and R2 and used to predict the optimal conditions that maximize the recovery of citric acid from the citrus peels. With the optimal processing conditions - short time, high ultrasonic power and extraction solvent containing a low percentage of ethanol - yielding about 10 g of this compound per 100 g of dry peels. Orange peels revealed an interesting content of citric acid, although a deeper analysis of these ingredients is necessary to allow the industries to implement sustainable extraction techniques and to use citric acid recovered from citrus residues as a food preservative.To FCT (Portugal) for financial support through national funds FCT/MCTES to CIMO (UIDB/00690/2020), for the PhD grant (SFRH/BD/145467/2019) of F. Fernandes, the contract of L. Barros through the institutional scientific employment program-contract and the contracts of M. Carocho (CEECIND/00831/2018), S.A. Heleno (CEECIND/03040/2017) and J. Pinela (CEECIND/01011/2018) through the individual scientific employment program-contract. To MICINN for the financial support Ramón & Cajal given to researcher M.A. Prieto; to the Government of Galicia for the financial support of Axudas Conecta Peme to the IN852A 2018/58 NeuroFood project. To Kontaktotal - Consultores Associados, Lda. - Dep. CuraNatura for collaborating on this project.