Green and Sustainable Forward Osmosis Process for the Concentration of Apple Juice Using Sodium Lactate as Draw Solution

Abstract

China is the world's largest producer and exporter of concentrated apple juice (CAJ). However, traditional concentration methods such as vacuum evaporation (VE) and freeze concentration cause the loss of essential nutrients and heat-sensitive components with high energy consumption. A green and effective technique is thus desired for juice concentration to improve product quality and sustainability. In this study, a hybrid forward osmosis-membrane distillation (FO-MD) process was explored for the concentration of apple juice using sodium lactate (L-NaLa) as a renewable draw solute. As a result, commercial apple juice could be concentrated up to 65 °Brix by the FO process with an average flux of 2.5 L·m^-2·h^-1. Most of the nutritional and volatile compounds were well retained in this process, while a significant deterioration in product quality was observed in products obtained by VE concentration. It was also found that membrane fouling in the FO concentration process was reversible, and a periodical UP water flush could remove most of the contaminants on the membrane surface to achieve a flux restoration of more than 95%. In addition, the L-NaLa draw solution could be regenerated by a vacuum membrane distillation (VMD) process with an average flux of around 7.87 L∙m^-2∙h^-1 for multiple reuse, which further enhanced the long-term sustainability of the hybrid process.

Keywords: apple juice; concentration; forward osmosis; sodium lactate; vacuum membrane distillation.

Figure 1

Schematic diagram of FO set-up.

Figure 2

Schematic diagram of VMD set-up.

Figure 3

Effect of membrane orientation on FO performance. (T_FS = 25 °C, V_FS = 250 mL∙min⁻¹ (131.06 cm∙s⁻¹), V_DS = 500 mL∙min⁻¹ (12.82 cm∙s⁻¹)).

Figure 4

Effect of FS temperature on FO performance. (AL-FS, V_FS = 250 mL∙min⁻¹ (131.06 cm∙s⁻¹), V_DS = 500 mL∙min⁻¹ (12.82 cm∙s⁻¹)).

Figure 5

Effect of flow rate on FO performance. ((A–C): AL-FS, T_FS = 25 °C, V_FS = 250 mL∙min⁻¹ (131.06 cm∙s⁻¹); (D–F): AL-FS, T_FS = 25 °C, V_DS = 500 mL∙min⁻¹ (12.82 cm∙s⁻¹)).

Figure 6

Effect of DS on FO performance (A) at concentration of 1 M and (B) at maximum concentration. (T_FS = 25 °C, V_FS = 250 mL∙min⁻¹ (131.06 cm∙s⁻¹), V_DS = 500 mL∙min⁻¹ (12.82 cm∙s⁻¹)).

Figure 7

Osmolality of different draw solutions.

Figure 8

Effect of L-NaLa concentration on FO performance. (T_FS = 25 °C, V_FS = 250 mL∙min⁻¹ (131.06 cm∙s⁻¹), V_DS = 500 mL∙min⁻¹ (12.82 cm∙s⁻¹)).

Figure 9

Compositional characteristics of apple juices: (A) pH, total acid and total sugar; (B) total phenolic, total flavone and DPPH radicals scavenging rate (RSR).

Figure 10

Radar chart of electronic nose response data for the apple juices.

Figure 11

Effect of operating modes on FO performance. (T_FS = 15 °C; V_FS = 2 L·min⁻¹ (10.38 cm∙s⁻¹); V_DS = 2 L·min⁻¹ (2.22 cm∙s⁻¹)).

Figure 12

SEM-EDS and AFM results of membrane inner surface. ((A): original, (B): used; (I): SEM, (II): EDS, (III): AFM; Ra: average roughness).

Figure 13

XPS full scan results of membrane inner surface.

Figure 14

High-resolution XPS spectra of C1s (A) and O1s (B).

Figure 15

Characterization of the membrane after cleaning by UP water. ((A): SEM, (B): EDS; (C): AFM, (D): XPS full scan, (E): High-resolution XPS spectra of C1s; (F): High-resolution XPS spectra of O1s).

Figure 16

Performance of VMD regeneration of DS. (T = 75 ± 2 °C, V = 20–16 cm∙s⁻¹, P = −0.095 MPa).

Figure 17

UV/Visible spectra of L-NaLa solution before and after VMD regeneration.

Figure 18

FO performance of the regenerated L-NaLa solution. (T_FS = 25 °C, V_FS = 250 mL·min⁻¹ (131.06 cm∙s⁻¹), V_DS = 500 mL·min⁻¹ (12.82 cm∙s⁻¹)).