Application of some physical separation techniques on the purification of steviol glycosides in the stevia (Stevia rebaudiana B.) plant extract

Nowadays it seems that the use of sweeteners in food formulations is increasing. With the addition of these sweeteners, the energy content of foods is reduced. In particular, energy-reduced foods suitable for combating obesity and diabetes can be obtained. However, some commonly used sweeteners are synthetic, causing distrust among consumers. This situation drives the consumer to search for natural resources and increases the tendency towards natural resources. Although sugar alcohols can be obtained from natural sources, their use in various food formulations remains limited due to their laxative effects. For these reasons, natural sweet compounds obtained from the stevia plant as an alternative to synthetic sweeteners and sugar alcohols attract the attention of developed societies. It is stated that these sweet compounds, called steviol glycosides, have approximately zero (0) calorie energy. It is also stated that its relative sweetness is 200-300 times that of sucrose. Many toxicological studies on stevia extracts report that these compounds do not cause any negative effects on health and prevent the development of some diseases. Some extraction and purification methods are used in the widespread production of stevia products in the world. Steviol glycosides can be purified to a high extent, especially by chromatographic methods. However, chromatographic methods are difficult and costly, causing the final product to be sold at very high prices. In addition, ethyl alcohol and similar chemicals are used in the currently applied methods for purifying steviol glycosides. Therefore, it is important to investigate the effectiveness of other methods used to enrich and purify steviol glycosides. It has been deemed noteworthy to investigate and demonstrate the potential of cost-effective physical separation methods that can be an alternative to common patented purification techniques. Within the scope of this thesis study, it is aimed to obtain steviol glycosides with acceptable purity (90 %) from aqueous stevia extracts by using physical separation methods. Electrocoagulation (EC), adsorption, and membrane filtration techniques were used for purification. In the electrocoagulation system, aluminum (Al) and 304L stainless steel (S) plates were used as anode (+) and cathode (-) as electrodes; Thus, four different electrode combinations were obtained. Direct current (DC) was used as the electrical source. Crude stevia extract was processed using electrocoagulation [Al(+)/S (-)], adsorption (polyvinyl polypyrrolidone-PVPP), electrocoagulation [Al(+)/Al(-)], ultrafiltration (30 kDa Permeate-P), nanofiltration (5 kDa Retentate-R) and mixed bed resin systems, respectively. In practice, the extract obtained from one process was fed to the other process respectively. Within the scope of the research, firstly, the optimization of the electrocoagulation system was carried out depending on the electrode type, electrode spacing, current density, voltage and time parameters. In addition, the effectiveness of the system was expressed by mathematical modeling using the data obtained. As a result of preliminary trials, it was determined that the most suitable electrode combination was the Al(+)/S(-) electrode combination. In the next stage, optimum values of EC parameters were determined at the point where the total steviol glycoside (TSG) concentration was maximum, using the response surface method (RSM); 1 cm electrode gap, 100 mA/cm2 current density, 30 V voltage, 60 minutes application time were determined as optimum conditions. The most suitable PVPP concentration was determined as 0.02 %. When the results were evaluated in the final purified sample, the total dry matter (TDM) was determined as 1.86 g/100 g (2.10 g/100 g in crude extract), the total mineral matter was determined as 0.22 g/100 g TDM (12.02 g/100 g TDM in crude extract), and the total steviol glycoside concentration was determined as 81.21 g/100 g TDM (42.70 g/100 g TDM in crude extract). In addition, color measurement was made and the L (lightness-darkness) value was determined as 99.21 (0.08 in crude extract), a (redness and greenness) value was determined as -1.02 (-0.01 in crude extract), and b (yellowness and blueness) value was determined as 4.09 (0.03 in crude extract). In addition, it was determined that there was a significant decrease in the total phenolic substance, total acidity contents, and turbidity value in the stevia extract samples taken from the intermediate stages of the process. At the same time, there was a significant increase in the transmittance value. In addition, the effectiveness of each application in purifying steviol glycosides compared to the previous process was determined. Purification activities determined in this context are respectively; was calculated as EC [Al(+)/S(-)] 34.52 %, adsorption (0.02 % PVPP) 4.83 %, second EC [Al(+)/Al(-)] 2.18 %, membrane filtration (5 kDa R) 20.51 % and mixed bed resin 21.05 %. The TSG concentration of crude stevia extract was found to be 42.70 g/100 g TDM. Based on this amount, when TSG concentrations are considered according to the purification efficiencies of the techniques used was reached values 57.44 g TSG/100 g TDM at the end of EC [Al(+)/S(-)] application, 60.21 g TSG/100 g TDM at the end of 0.02 % PVPP application, 61.52 g TSG/100 g TDM at the end of EC [Al(+)/Al(-)] application, 74.14 g TSG/100 g TDM at the end of membrane filtration (5 kDa R) application and 89.75≈90.0 g TSG/100 g TDM at the end of mixed bed resin application.