Introduction

Increasing the shelf life of fruit juices while reducing storage and transportation costs is a major concern in the food industry. Conventional concentration methods such as multi‑stage vacuum evaporation and freezing are widely used; however, they are energy‑intensive and can negatively affect juice quality. In contrast, membrane‑based processes offer energy‑efficient alternatives. Among them, forward osmosis (FO) has emerged as a promising technology due to its low operating pressure, ambient temperature operation, reduced membrane fouling, and potential to achieve higher final concentrations.

Forward Osmosis and the Role of the Draw Solution

Forward osmosis relies on the natural movement of water across a semi‑permeable membrane from a feed solution with lower osmotic pressure to a draw solution with higher osmotic pressure. A key challenge in FO‑based juice concentration is the selection of a suitable draw solution that combines high osmotic pressure with stability, low cost, non‑toxicity, biodegradability, and minimal reverse solute flux into the final product.

Deep Eutectic Solvents as Green Draw Solutions

In recent years, ionic liquids have been investigated as draw solutes because of their high osmotic pressure; however, concerns remain regarding cost, synthesis complexity, and environmental impact. Deep eutectic solvents (DESs) represent a newer class of ionic‑liquid‑like materials that share similar physicochemical properties while offering significant advantages, including simple synthesis, high purity, non‑reactivity with water, biodegradability, and low toxicity.

In this study, a citric acid–choline chloride (CA–CC) deep eutectic solvent was developed and evaluated as a novel draw solution for fruit juice concentration by forward osmosis. Choline chloride, a low‑cost and non‑toxic hydrogen bond acceptor, was combined with citric acid as a hydrogen bond donor to form a DES with very low water activity and high osmotic pressure.

Preparation and Optimization of CA–CC DES

The CA–CC DES was synthesized by mixing citric acid and choline chloride at different molar ratios (1:1, 1:1.5, and 1.5:1) and heating the mixture at 85 °C until a transparent liquid was obtained. The resulting DES was diluted with distilled water to prepare draw solutions with a concentration of 0.32 g/mL.

The osmotic pressure of each DES composition was determined using relative air humidity measurements at 25 °C. Among the tested formulations, CA1–CC1.5 exhibited the highest osmotic pressure (approximately 174 atm), comparable to that of a 1 M NaCl solution, and was therefore selected as the optimal draw solution.

Forward Osmosis Performance

FO experiments were carried out using a commercial cellulose triacetate (CTA) membrane. When deionized water was used as the feed solution, CA1–CC1.5 DES demonstrated the highest water flux among all tested DES compositions, confirming the strong correlation between osmotic pressure and FO performance.

Application to Fruit Juice Concentration

The applicability of CA1–CC1.5 DES for juice concentration was evaluated using apple and orange juices as feed solutions. The average water fluxes obtained were:

• 3.6 LMH for orange juice

• 2.9 LMH for apple juice

These results indicate that the osmotic pressure difference between the DES draw solution and the fruit juices is sufficient to effectively extract water and increase juice concentration without applying external pressure or high temperature.

Advantages Over Conventional Methods

The proposed FO process using CA–CC DES offers several advantages over traditional concentration techniques:

• Lower energy consumption

• Preservation of nutritional and sensory quality

• Reduced membrane fouling

• Use of a green, non‑toxic, and biodegradable draw solute

Conclusion

Citric acid–choline chloride deep eutectic solvent was successfully introduced as a sustainable and efficient draw solution for forward osmosis‑based fruit juice concentration. The CA1–CC1.5 DES showed superior osmotic pressure and water flux performance, enabling effective concentration of apple and orange juices. This approach presents a promising green alternative to energy‑intensive conventional technologies and highlights the potential of deep eutectic solvents in food processing applications.

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Based on a study presented at the 4th International and 29th National Iranian Food Science and Technology Congress, May 2023.