Separation of tea saponin by two-stage foam fractionation

Abstract

A technology of two-stage foam fractionation for tea saponin recovery was studied. Effects of temperature, initial pH, loading liquid volume and superficial airflow rate in the two-stage foam fractionation process were investigated. The results showed that when the conditions of the first stage were pH 2.06, loading liquid volume 250 mL, superficial airflow rate 150 mL/min and temperature 65.0 °C, enrichment ratio and recovery percentage of tea saponin were 3.47 and 45.1%, respectively, and the residual solution was added to the second stage as initial solution. When the conditions of the second stage were temperature 30.0 °C and superficial airflow rate 200 mL/min, recovery percentage of tea saponin reached to 65.2%, and the foamate was added to the first stage as initial solution. Total recovery percentage of tea saponin reached to 80.1% by the two-stage foam fractionation.

Introduction

Tea seed is used to produce tea-seed oil that has significant value in food industry and health promotion. During the process of the oil production, plenty of tea-seed cake is produced and always treated as waste residues. However the residues contain a large amount of tea saponin, a kind of pentacyclic triterpenoid [1], the structure of which is shown in Fig. 1. Tea saponin is a new commercial source of saponin and it has been widely used in food, chemical, pesticide, building materials, hair care products and so on [2], [3], [4] because it is a natural surfactant with the performances of emulsifying, dispersion and wetting, and it also has many bioactivities such as antimicrobial, anti-inflammatory, antioxidant, anti-allergic, etc. [5], [6], [7], [8]. So it is very important to separate tea saponin from tea-seed cake.

At present, fractionation technology of tea saponin from tea-seed cake is mainly composed of its leaching and purification [9], [10]. Methods of its leaching include water leaching, ethanol leaching and water–ethanol leaching. Methods of its purification include precipitation, resin adsorption and membrane separation. However, these methods have disadvantages of low yield, high cost and large investment. So it is very important to explore a new technology with high yield, low cost, small investment and pollution-free in particular.

Foam fractionation technique has many advantages such as simple equipment, small investment, low energy consumption and environmental compatibility and therefore it has attracted many researchers’ attention. Early in the 20th century, it had been used in metallurgical industry. In the field of biochemical engineering, researchers applied the technology to separate ions [11], molecules [12] etc. Then the applied range expanded to extract proteins [13], [14], [15], enzymes [16], [17], fermentation products [18] and the active principles of traditional Chinese drugs. The molecular structure of tea saponin determines its surface activity, and makes it have good foamability, which lays the foundation for the application of foam fractionation.

It is very important to consider effect of temperature on performances of foam fractionation because the foam created from the tea saponin aqueous solution has good stability. At room temperature, the tea saponin aqueous solution has the high bulk viscosity, which increases flowing resistance of the interstitial liquid between bubbles in the foam layer. So foam drainage velocity is slow and it is hard to contribute to internal reflux in the foam layer towards the bulk liquid player [19]. Thus the foamate volume increases and enrichment ratio of tea saponin decreases. If the operating temperature of foam fractionation for separating tea saponin increases, the bulk solution viscosity will decrease and foam drainage will be enhanced. Then the coalescence of bubbles in the foam layer is enhanced. Furthermore, foam drainage is improved in turn due to the coalescence of bubbles in the foam layer. So the foamate volume will decrease. Therefore the effect of temperature on performances of foam fractionation will be investigated firstly for improving enrichment ratio of tea saponin. Then effects of initial pH, loading liquid volume and superficial airflow rate on foam fractionation performances of tea saponin will be investigated. Furthermore, a technology of two-stage foam fractionation will be developed. The first stage is aimed at increasing enrichment ratio of tea saponin as high as possible and the second stage is aimed at increasing recovery percentage of tea saponin. So the two-stage foam fractionation technology can increase both enrichment ratio and recovery percentage of tea saponin.