Application of Nanofiltration Membrane in Steviol Glycoside Purification and Desalination

1. Introduction

Stevia rebaudiana leaves contain multiple glycoside components including stevioside and Rebaudioside A. After hot water leaching, the crude extract carries leaf fibre debris, protein, pectin, plant pigments and a great quantity of inorganic salts from raw materials and extraction water. If these salt ions and heterogenous impurities cannot be removed effectively, the finished stevia powder will have obvious bitter aftertaste, poor whiteness and unstable dissolution performance.

Conventional production mostly uses resin column adsorption-desorption. Although high-purity products can be obtained, the resin needs frequent regeneration with acid and alkali, producing large volumes of salty wastewater. Multiple evaporation steps also cause huge energy consumption and partial thermal degradation of heat-sensitive glycosides. In recent years, membrane coupling technology has become a preferred upgrading scheme for stevia factories. Ultrafiltration removes macromolecular colloids and suspended solids for pre-clarification; subsequently, nanofiltration realizes targeted desalination and concentration, greatly reducing the load of downstream resin and evaporation units.

2. Separation Principle of Nanofiltration Membrane for Steviol Glycoside

Industrial spiral wound nanofiltration elements are mostly made of thin-film composite polyamide material, with pore size between ultrafiltration and reverse osmosis, and typical molecular weight cut-off around 200–400 Da. The molecular weight of main steviol glycosides ranges from 600–950 Da, so they are intercepted on the retentate side. Small molecules such as sodium salt, potassium salt, chloride ion and monosaccharide can penetrate the membrane layer and enter the permeate stream.

Different from RO which rejects almost all salts, NF has selective separation performance: high rejection to organic molecules with larger molecular weight, partial rejection to multivalent ions and low rejection to monovalent ions. This characteristic perfectly matches the requirement of stevia processing: concentrating target glycosides and draining most monovalent salts synchronously in one stage of cross-flow circulation. The whole system runs at room temperature, avoiding thermal decomposition of active ingredients.

3. Integrated Membrane Process Flow

Crude stevia leaf extract → pre-filtration (bag filter 50–80 mesh) → UF feed tank → security filter (5 μm) → ultrafiltration module (removal of protein, pectin and suspended particles) → UF permeate tank → NF feed pump → cartridge filter (3 μm) → spiral nanofiltration module

Retentate of NF: steviol glycoside concentrated liquid, sent to decolorization, resin refining and spray drying Permeate of NF: salty wastewater with low glycoside content, can be recycled for leaf leaching or sent to wastewater treatment station CIP station: shared by UF and NF units with independent cleaning recipe

Ultrafiltration is an indispensable pre-treatment for nanofiltration system. Colloid and pigment pollutants intercepted by UF will not enter the NF element, effectively alleviating organic fouling on the polyamide selective layer and extending the service life of nanofiltration membranes.

4. Core Advantages of Nanofiltration Desalination

First, low energy consumption. The membrane system removes most water and salt before evaporation. The volume of feed liquid entering evaporator is greatly reduced, cutting steam consumption significantly compared with full evaporation of crude liquid. Second, reduce resin dosage and regeneration frequency. Most soluble salt impurities are separated by NF, so the resin only plays a fine polishing role, acid-alkali consumption and salty wastewater output drop sharply. Third, stable product taste and color. Low-temperature operation prevents browning and glycoside degradation; the desalination effect reduces bitter salt taste and improves the whiteness and solubility of final stevia powder. Fourth, continuous automatic operation. The system is equipped with TMP, conductivity and online Brix sensors. It can adjust circulating flow and concentrate discharge automatically, easy to match with continuous production lines of large stevia manufacturers.

5. Fouling Control and Cleaning Scheme

The main pollutants on NF membrane surface are residual polyphenol pigments, trace protein and calcium/magnesium scale. Long-time operation will cause flux decline and salt rejection rate drift. Standard CIP sequence:

1. Permeate water rinse to drain residual material liquid

2. Alkaline cleaning (food-grade NaOH + surfactant) to remove organic pigment and protein fouling

3. Rinse to neutral

4. Acid cleaning (citric acid) for inorganic scale removal

5. Final rinse with pure water, standby

Key operation control points: control feed hardness before entering NF; keep cross-flow velocity within reasonable range to reduce concentration polarization; avoid over-high recovery ratio which causes salt saturation and scaling.

6. Summary and Prospect

Nanofiltration coupled with ultrafiltration is a mature green separation technology for steviol glycoside production. It balances product yield, purity and production cost well and helps enterprises meet increasingly strict sewage discharge standards. At present, this set of membrane technology is also extended to other plant sweetener and polyphenol extraction industries, such as mogroside purification. With the development of high anti-fouling NF membrane materials, the operation cycle and service life of membrane elements will be further improved, making membrane separation the mainstream core unit for natural plant extraction.