ABSTRACT
Stevioside, a white crystalline compound derived from a small perennial shrub, stevia, has been used for centuries as a safe bio-sweetener for patients suffering from diabetics, obesity and other medical complaints. However, its high cost of production renders it inaccessible to the poor and underprivileged. The potential of membrane-based pressure driven processes, such as microfiltration (MF), ultrafiltration (UF) and nanofiltration (NF), has been investigated for the separation and purification of the stevioside from crude stevia leaf extract to make it affordable to the common populace, which constitutes the main objective of this work. After dewaxing with hexane at 60°C, stevia was extracted using hot water at 120°C. The aqueous extract was pre-filtered by a filter press to obtain a turbid stevia solution (20 L), which was subjected to MF using a polypropylene micron cartridge (5 µm) and ceramic tubular module (0.2 µm) for removing suspended solids at a trans-membrane pressure of 0–2 kg/cm2 and temperature of 29°C. Permeate from MF was subjected to UF system having polyethersulfone membrane of 10 kDa MWCO to reject high molecular weight impurities and permeate stevia. UF reject containing remnants of stevia extract was subjected to a series of five diafiltration (DF) steps using deionized water to extract the maximum amount of stevia with each DF step, continuing until 90–95% of permeate containing stevia was recovered. The permeate coming from each DF step was combined and fed to a NF membrane of 150 Da MWCO at a feed pressure of 21 kg/cm2 until a permeate recovery of 93% was achieved. During the NF step, the high M.Wt. (829) stevia was concentrated while low molecular weight impurities, like salts, passed through into permeate. The resultant NF reject, enriched in stevia, was subjected to evaporation and crystallization to yield high purity stevia product.
