Microfiltration of stevia extracts by usingceramic tubular membranes

ABSTRACT MICROFILTRATION OF STEVIA EXTRACTS BY USING CERAMIC TUBULAR MEMBRANES Özge AĞAR M. Sc., Food Engineering Department Supervisor: Assist. Prof. Dr. Ali Emrah Çetin September 2022, 100 pages Stevia rebaudiana Bertoni (stevia) is a plant which contains zero-calorie and highly sweet (relative to saccharose) natural compounds called steviol glycosides. Steviol glycosides in high purity have been used as natural sweeteners in foods. The production of highly pure steviol glycosides from stevia leaves includes extraction, separation and purification steps. Steviol glycosides are first extracted from the leaves and the extract then undergoes a series of separation and purification processes such as coagulation/flocculation, column chromatography by using activated carbon, adsorption and ion-exchange resins and crystallization. Membrane filtrations are separation processes frequently used in industry and employed for the separation of molecules and/or particles from liquid or gas streams by using a selective-permeable barrier called membrane. Although polymeric membranes are frequently employed in the membrane filtration processes, they have lower mechanical, chemical and thermal stabilities and shorter lifetimes compared to ceramic membranes. Ceramic membranes hold a great potential for use in clarification and decolorization of the stevia extract as well as separation and concentration of the steviol glycosides during the production of high-purity steviol glycosides. The aim of this thesis was to prepare single-channel Al2O3 microfiltration membranes and to evaluate their performances in the microfiltration of stevia extract by using a cross-flow filtration system. For this purpose, Al2O3 microfiltration layer coated tubular Al2O3 supports were prepared and their performances in the microfiltration of the stevia extract were systematically evaluated by the experiments carried out at four different cross-flow velocities (0.22, 0.30, 0.38 ve 0.44 m/s) and three different transmembrane pressures (2, 3 and 4 bars). Performances of Al2O3 microfiltration membranes were monitored via measuring time-dependent permeate fluxes and % transmittance, color and refractive indices of the permeate samples collected in pre-determined time periods. The overall performances of these membranes in the clarification and/or decolorization of the stevia extract were also determined by measuring % transmittance, color and refractive index values and total solid, rebaudioside A and stevioside contents of the total permeate samples collected at the end of each filtration run. In all microfiltration experiments, the permeate fluxes measured at the 5th minutes of filtrations were found to be similar to each other and these fluxes decreased rapidly within the first 30 minutes, slowly thereafter and reached constant values. Increase in both cross-flow velocity and transmembrane pressure did not exhibit any significant effect on the improvement of the permeate fluxes due to the higher amounts of membrane-fouling substances present in the stevia extract. While % transmittance of the stevia extract was found to increase by 60 – 241 %, color, water soluble dry matter and total solid contents of the extract were found to decrease by 27 – 68 %, 23 - 55 % and 14 – 32 %, respectively as a result of microfiltration experiments. Additionally, it was observed that ceramic membranes showed 9 - 37 % and 9 - 41 % retentions in rebaudioside A and stevioside, respectively in the microfiltration experiments. When the highest increase in % transmittance, the highest % decreases in color and total solid content yet the lowest % decrease in °brix were considered, the maximum performance was observed in the microfiltration experiment performed at 0.22 m/s and 4 bar. This microfiltration experiment however resulted in 36 and 32 % retentions in rebaudioside A and stevioside. In terms of high-purity rebaudioside A production, the maximum performance was found at 0.44 m/s and 4 bar by considering the lowest % rebaudioside A yet the highest % stevioside retentions along with the other performance criteria. Keywords: Stevia rebaudiana Bertoni, ceramic membrane, microfiltration, steviol glycoside, Al2O3, cross-flow, clarification