## 2 Materials and Methods
### Moisture Content (MC), Water Activity (\(a_{w}\)), and Sorption Isotherms  
The MC and \(\mathrm{a_{w}}\) was determined using the gravimetric method described by [11]. A 2 g sample of green rooibos extract (\(\beta\)-GRE and GRE) was heated at 100 \({}^{\circ}\)C for 60 min using a Denver Instrument Infrared Moisture Analyser (Colorado, USA). The \(\mathrm{a_{w}}\) was measured at 25 \({}^{\circ}\)C using a Novasina LabMaster-\(\mathrm{a_{w}}\) meter (Lachen, Switzerland).  
The water sorption isotherms were determined based on a gravimetric method described by [16]. The method was based on the use of saturated salt solutions to maintain a fixed relative humidity (RH). Five salts were chosen to achieve RH values in the range of 10-85%. Freeze-dried green rooibos extracts (\(\beta\)-GRE and GRE) were weighed (0.2 g) into 5 mL glass vials with rubber stopper caps and placed into desiccators containing saturated solutions of the following salts: LiCl, KCOOCH\({}_{3}\bullet\)5H\({}_{2}\)O, MgCl\({}_{2}\bullet\)6H\({}_{2}\)O, K\({}_{2}\)CO\({}_{9}\bullet\)2H\({}_{2}\)O, and NaCl, achieving water activities (\(\mathrm{a_{w}}\)) values of 0.11, 0.22, 0.33, 0.43, and 0.75 at 25 \(\pm\) 1 \({}^{\circ}\)C, respectively. The saturated salt solution water activity values (\(\mathrm{a_{w}}\)) were confirmed using a Novasina LabMaster-\(\mathrm{a_{w}}\) meter (Lachen, Switzerland). The samples in the desiccators were kept in an oven at 25 \(\pm\) 1 \({}^{\circ}\)C for equilibration. After reaching the equilibrium condition (difference < 0.0005 g), the moisture content of the samples was determined by drying the samples in vials in a vacuum oven (735 mmHg) at 70 \({}^{\circ}\)C until constant weight. Analysis was conducted in triplicate, and the equilibrium moisture content of the samples was reported as g/100 g dry solids. The moisture content as a function of \(\mathrm{a_{w}}\) was plotted for each sample, and the curves obtained were fitted with the Guggenheim-Anderson-de Boer (GAB) model.  
Monolayer values were computed from the moisture sorption data by using mathematical modelling. The GAB model, as described by the following equation, is considered the best fitting model for a host of foodstuffs and reportedly provides accurate fittings over a wide range of water activity up to at least 0.9 and better evaluates the amount of water tightly bound to primary adsorption sites.  
The GAB model is described mathematically by (Equation (1)):  
\[m/m_{0}=a_{w}GK/[(1-a_{w}K)(1-a_{w}K+a_{w}GK)] \tag{1}\]  
where \(m\) and \(m_{0}\), respectively, are the moisture content and the content of water tightly bound to primary adsorption sites as a monomolecular layer, \(\mathrm{a_{w}}\) is the water activity, and \(\mathrm{G}\) and \(\mathrm{K}\) are constants related to the energies of interaction between the first and distant sorbed molecules at the individual sorption sites.  
To calculate the monolayer values from moisture adsorption data, the GAB equation (Equation (1)) was transformed into the quadratic form (Equation (2))  
\[a_{w}/m=Aa_{w}{}^{2}+Ba_{w}{}^{2}+C \tag{2}\]  
to which the least-squares multiple linear regression method was applied to obtain the following regression coefficients in Equation (3) (A), Equation (4) (B), and Equation (5) (C).  
\[A =(k/m_{0})[1/G)-1] \tag{3}\] \[B =(1/m_{0})[1-(2G)]\] (4) \[C =1/m_{0}GK \tag{5}\]as well as the square of the multiple correlation coefficient (r\({}^{2}\)). The monolayer values were obtained from these coefficients according to (Equation (6)).  
\[M_{0}=\frac{-b\pm\sqrt{b^{2}-4 ac}}{2a} \tag{6}\]