## 7 Extraction and Purification Methods for SvGIs Production  
Dehydration and extraction are the two basic steps that determine the extraction rate of SvGIs from the leaves of _Stevia rebaudiana_. The type of solvent and the manufacturing process have a large impact on the yield of its metabolites. Some other parameters which play a crucial role in the extraction process are the operating temperature (60-100 \({}^{\circ}\)C), exposure time (30 min to 24 h), devices, and materials (e.g., resins). The common dehydration or drying processes used are freeze-drying, vacuum drying, microwave drying, infra-red drying, sun-drying, and shade drying. The traditional methods of sun drying and shade drying are more prone to microbial contamination and have an adverse impact on their quality. In the case of stevia, convective and infra-red drying is suitable to increase its shelf life by restricting the growth of _Listeria imocua_. Thus, an effectively optimized drying condition is necessary to eliminate the adverse effect affecting the antimicrobial and antioxidant activity of the stevia extracts. Steviol glycosides are highly stable at normal temperature and pH. Both major and minor SvGIs are thermally and hydroditically stable at normal temperature and storage conditions. This characteristic of SvGIs makes them highly desirable ingredients for the food industry. The extraction efficiency of the SvGIs and other metabolites is dependent on the type of solvent and technique used for the extraction process. Water is observed to be the best solvent for stevia phytoconstituent extraction. The extracts obtained from the mixture of propylene glycol and water in the ratio (4:1) produced a maximum concentration of phenols, and flavonoids [97]. The use of organic solvent preserves the antioxidant property of the extracts but may cause fibroblast irritation when used as a food additive or in cosmetics, probably due to cytotoxicity. Thus, recently, the concept of greener options for SvGIs extraction has become more desirable. [PERSON] et al. [98] used a new solid-liquid extraction method employing a Naviglio extractor and water as solvent at room temperature. This method was effective in saving energy consumption as it yielded 1197.8 and 413.6 mg/L of Reb A and stevioside, respectively, in just 20 min. Extraction techniques along with temperature, extraction time, pressure, and particle size have a large influence on the extraction rate. Classical methods such as maceration and heat extraction are very lengthy, and thus, these methods are being eliminated. Various new approaches to improve extraction yields and purity of molecules have been proposed such as pressurized fluid extraction (PFE), microwave-assisted extraction (MAE), high voltage electrical discharge, pulsed electric \begin{table}
\begin{tabular}{l l l l l} \hline
**Target Enzyme** & **Strain Used** & **Methodology** & **Production Rate** & **Reference** \\ \hline \multirow{4}{*}{Reb D} & \multirow{4}{*}{_Pichia pastoris_} & One-pot synthesis utilized for & \multirow{4}{*}{Conversion rate} & \multirow{4}{*}{[PERSON] et al., 2020 [94]} \\  & & heterologous expression of & & \\  & & EUGT11 from _Oryza sativa_ forming & & \\  & & XE-3 transformant & & \\ \multirow{4}{*}{Syevioside} & \multirow{4}{*}{_Pichia pastoris_} & _TtbGal1_ and _MBlgl3a_ expressed in & & \\  & & Pichia pastoris & & \\ \multirow{4}{*}{Reb A} & \multirow{4}{*}{_E. coli BL21_} & Co-expression of endogenous & \multirow{4}{*}{
\begin{tabular}{} \end{tabular} } \\  & & prpD and malK in _E. coli_ improved & & 4.8 g/L (RebA) \\ \multirow{4}{*}{Reb M} & \multirow{4}{*}{_E. coli BL21_} & & 1.8 g/L (RebM) & \\  & & the expression of _Smt3-UGT76G1_. & & \\ \hline \end{tabular}
\end{table} 
Table 4: _Cont._field, and ultrasound-assisted extraction (UAE), and chromatographic techniques.