Lotus leaf (moisture content of 12%), green tea leaf (moisture content of 11%) provided by Dai Nam company (Ho Chi Minh city, Vietnam).

Accurately weight about 60 g of lotus leaves (size 1.0–1.6 mm) and 60 g of green tea leaves (size 1.0–1.6 mm) into 2000 mL Erlenmeyer flask, extracts by heat reflux method at a temperature of 60 ± 2 °C in 60 minutes in two times and medicinal herbs/solvent ratio was 1:15. All extracts are combined, concentrated to the liquid extract (1:8) and dried by Labplant spray-drying equipment to obtain instant tea powder. The parameters of the independent and dependent variables are presented in Table 1.

The performance of spray dryer for each experiment is calculated as the ratio of the weight of the instant tea powder obtained and the initial total solids (raw material and carrier mass) in the suspension prepared (Eroğlu et al. 2018).

The moisture content of the sample is gravimetrically determined with a moisture analyzer MA35 (Sartorius AG, Germany) at 105 °C. The moisture content should not be more than 10% (Lee et al. 2017; Eroğlu et al. 2018).

Total phenol content is determined by the Folin-Ciocalteu (FC) method and gallic acid is used as standard material (Eroğlu et al. 2018; Gunjal and Shirolkar 2020). FC 10% reagent is diluted with distilled water. The sample powder is diluted in methanol solution with a concentration of 1,000 μg/mL. 1 mL of the sample, 6 mL of distilled water and 0.5 mL of FC reagent are added in a 10 mL volumetric flask and shake well. After 5 min, add 1.5 mL Na₂CO₃ 20%, shake well and add distilled water until the total volume is 10 mL. Keep the samples in the dark at ambient temperature for 30 min, the absorbance of samples is determined by a Shimadzu UV2100 spectrophotometer at 760 nm. The total polyphenol content in instant tea powder is expressed as mg GA per g of instant tea powder (mg GA/g) and calculated by the formula:

$Y_3=\frac{C\times V}{m}$,

in which:

C: x value from calibration curve with gallic acid (mg/mL);

V: volume of test solution (mL);

m: mass of instant tea present in volume V (g)

Total flavonoid content is determined by the aluminum chloride colorimetry method and quercetin is used as the standard material (Rahmati et al. 2020). Powder sample is diluted in methanol solution with a concentration of 1,000 μg/mL. 1 mL of the sample and 4 mL of distilled water are added in a 10 mL volumetric flask. Then, add 0.3 mL NaNO2 5%, 0.3 mL AlCl3 10% and 2 mL NaOH 1M, shake well and add distilled water to bring volume to 10 mL. The absorbance of samples is determined with the wavelength of 510 nm. The total flavonoid content in instant tea powder is expressed as mg QE per g of instant tea powder (mg QE/g) and calculated by the formula:

$Y_4=\frac{C\times V}{m}$,

in which:

C: x-value from the calibration curve with quercetin (mg/mL);

V: volume of test solution (mL);

m: mass of instant tea present in volume V (g)

Some criteria have been evaluated are appearance, moisture content, microbiological contamination and active ingredient content, in which, total phenol content and total flavonoid content are determined with the formula for Y₃ and Y₄ that has been illustrated above.

Twenty experimental (F₁-F₂₀) are designed according to the D-optimal model using Design Expert software (version 6.0.6, Stat-Ease Inc., Minneapolis, USA). The data are analyzed by BCPharSoft software to investigate the cause-effect relations and optimized preparation process. The optimized process is experimentally repeated in triplicate for further validation. The predicted data created by BCPharSoft software are compared with the observed response data from the optimized process using SPSS version 26.0 (SPSS, Inc., Chicago, IL, USA).

The preparation process of instant tea powder was designed by Design Expert software including 20 experiments. These results corresponding to the experiments were summarized in Table 2.

Analyzing the cause-effect between the conditions of the preparation process and the properties of instant tea powder. The data in Table 2 were used as inputs for BCPharSoft to investigate the cause-effect relations and optimize the process.

The results of the accuracy of model statistics from BCPharSoft outputs were presented in Table 3.

Table 3 demonstrates that all R₂ training and R₂ test values were more than 0.9, indicating that the models were extremely reliable. These models have the potential to be utilized for multivariate optimization.

For a better understanding of the cause-effect linkages between the independent and dependent variables, three-dimensional (3D) response surface plots of the fit models were displayed. Each 3D figure depicted the impacts of two independent factors on the dependent variables at the same time while keeping the third variable constant.

With the ideal circumstances as shown in Table 1, drying performance (percent) – Y₁ has to be as high as feasible. When all X factors are considered in the 3D diagram in Fig. 1, it can be seen that the alcohol content – X₁ is high (level 3- 60%), the carrier mass (g) – X₂ needs to be low (10 g), the inlet air temperature (°C) – X₃ has little effect on drying performance (percent), but X₃ should not be low, and the flow rate (rpm/min) – X₄ uses at a low level (4 rpm).

Figure 1. 

Response surface plots showing the effects of (a) alcohol content (X₁) and carrier mass (X₂); (b) alcohol content (X₁) and inlet air temperature (X₃); (c) alcohol content (X₁) and flow rate (X₄); (d) carrier mass (X₂) and inlet air temperature (X₃); (e) carrier mass (X₂) and flow rate (X₄); (f) inlet air temperature (X₃) and flow rate (X₄) on drying performance (Y₁).

Water or alcohol with concentrations ranging from 10% to 90% were commonly used in the extraction of therapeutic plants. However, alcohol is the most commonly used solvent because it can dissolve a wide range of bioactive chemicals and has a good storage stability (Tungmunnithum et al. 2018; Abubakar and Haque 2020; Wan-Li and Wen-Chuan 2020). The solvent used is determined by the kind of plant, the section of the plant to be extracted and the nature of the bioactive chemicals. Polar solvents like water and alcohol are used to extract polar chemicals, whereas nonpolar solvents like hexane and dichloromethane were used to extract nonpolar compounds (Li et al. 2018). The bioactive components in lotus and green tea leaves are polar compounds, and alcohol concentrations of 40%, 50%, and 60% are utilized in the extraction of lotus and green tea leaves. Table 2 shows that the drying performance of instant tea powder ranged from 17.49% to 30.75%. As the alcohol concentration rises, so does the drying performance (Y₁), as shown in Fig. 1.

Increasing the carrier mass reduces the ability of the drying chamber walls to stick, resulting in improved drying performance (Eroğlu et al. 2018; Sidlagatta et al. 2020). Recent research has revealed that if the carrier mass is too large, raising the viscosity reduces drying performance (Erolu et al. 2018). This is also true for the research findings in Fig. 1a, d, e.

The drying performance and the input air temperature are in the proportional relationship. If the input air temperature is too high, the completed product melts and sticks to the product receiver, limiting drying performance (Lee et al. 2017; Li et al. 2018; Özdikicierler et al. 2019; Gunjal and Shirolkar 2020; Thumthanaruk et al. 2021). However, if the inlet air temperature is too low, it will be difficult to dry the extract. Fig. 1b, d, and f indicate that the selected temperature levels of 160–170 °C has minimal influence on drying performance, but the input air temperature (X₃) is chosen to be 165 °C to have little effect on other dependent variables.

Fig. 1c, e, f illustrate that when the flow rate decreases, the efficiency of extract evaporation increases, lowering the likelihood of particles clinging to each other and to the chamber wall and therefore boosting drying performance (Saha et al. 2019; Latifi et al. 2020). Increasing the cycle speed, on the other hand, reduces drying performance. This is similar to the findings of Dhritiman Saha et al.

Moisture content (percent) – Y₂ should be as low as feasible under the required conditions shown in Table 1. If all X factors are taken into account in the 3D diagram in Fig. 2, the alcohol content value – X₁ is at a high level (level 2 – 50% or 3 – 60%), the carrier mass (g) – X₂ needs to be around 10 g, the inlet air temp (°C) – X₃ is around average 165 °C, and the flow rate (rpm/min) – X₄ is at a low level (4 rpm).

Figure 2. 

Response surface plots showing the effects of (a) alcohol content (X₁) and carrier mass (X₂); (b) alcohol content (X₁) and inlet air temperature (X₃); (c) alcohol content (X₁) and flow rate (X₄); (d) carrier mass (X₂) and inlet air temperature (X₃); (e) carrier mass (X₂) and flow rate (X₄); (f) inlet air temperature (X₃) and flow rate (X₄) on moisture content (Y₂).

Moisture content is a significant aspect in instant tea processing, and it is connected to drying performance (Rahmati et al. 2020). The moisture content is in inverse proportion with the drying performance. At the same time, moisture content has a significant impact on final tea quality measures such as flow, stickiness, stability, and oxidation reaction (Özdikicierler et al. 2019; Rahmati et al. 2020). According to Saha et al. (2019), at low moisture content, the flavor and aroma of the finished tea may be diminished or lost, but at excessive moisture content, the finished product may be sticky or inappropriate for long-term storage. Table 2 shows that the moisture level of instant tea powder made from lotus leaves and green tea leaves ranges between 4.33 and 5.99%. Fig. 2 shows that all independent factors have a substantial impact on moisture content. Fig. 2a–c indicate that a high alcohol level of 50–60% leads in a low moisture content. This can be explained by employing high alcohol concentration to enhance the active component content of the extract or increasing the number of solids in the extract, hence lowering moisture content (Lee et al. 2017; Sidlagatta et al. 2020).

Simultaneously, increasing the carrier mass in the extract increases total solids while decreasing the quantity of water available for evaporation and the moisture content (Fig. 2a, d, e). Because a large carrier mass clogs the spray gun during preparation (Lee et al. 2017), the carrier mass (X₂) should be as low as 10 g for a lower moisture (Y₂) value (Fig. 2d). According to Zokti et al. (2016), when the moisture level of instant tea increases, the stability of the completed product improves. The moisture content of soluble tea generated when dries at various input air temperatures is shown in Table 2. According to Lee et al. (2017), the temperature of the inflow air rises, resulting in higher heat transfer to the grain and increased evaporation rate. At high intake air temperatures, there is a temperature difference between the spray droplet and the drying air, which improves the dynamics of water evaporation (Gunjal and Shirolkar 2020). Normally, the air temperature ranges between 150 and 220 °C. Lee et al. (2017) and Saha et al. (2019) As seen in Fig. 2 f, the moisture content decreases as the incoming air temperature and flow rate rise. However, when the inlet air temperature is low, the drying performance is also low because the extract has not yet dried. On the contrary, the extract is dried before being placed in the collecting jar (Lee et al. 2017; Huaman-Castilla et al. 2019).

To achieve desired conditions in Table 1, the total phenol content (mg GA/g) needs to be as high as possible. X factors are considered in the 3D diagram in Fig. 3, which can be seen that the alcohol content (X₁) is on level 3 (60%), the carrier mass (X2) is around ≥ 10 g, inlet air temperature (X3) is set at about 165 °C, and the flow rate (X4) is on a low level (4 rpm/min) giving the increase of total phenol content (mg GA/g).

Figure 3. 

Response surface plots showing the effects of (a) alcohol content (X₁) and carrier mass (X₂); (b) alcohol content (X₁) and inlet air temperature (X₃); (c) alcohol content (X₁) and flow rate (X₄); (d) carrier mass (X₂) and inlet air temperature (X₃); (e) carrier mass (X₂) and flow rate (X₄); (f) inlet air temperature (X₃) and flow rate (X₄) on total phenol content (Y₃).

The total phenol content of instant tea powder ranged from 20.66 to 45.96 mg GA/g, as shown in Table 2. The relationship between alcohol content and the total phenol content is proportional. (Huaman-Castilla et al. 2019). The carrier mass also affects the properties of instant tea, when the carrier mass is low, a sticky instant tea powder can be obtained, if the carrier mass is high, the total phenol content in the finished product can be reduced (Eroğlu et al. 2018). Fig. 3a, d, e show a suitable carrier mass of 10 g bringing high economic efficiency. On the other hand, the total phenol content is less affected by inlet air temperature (Fig. 3b, d, f), however the inlet air temperature should not be too low because it is difficult to dry the extract, so the inlet air temperature (X₃) is selected around 165 °C. When the inlet air temperature is too high, it leads to a change in the structure of the finished tea powder and the decomposition of phenolic compounds and reduces the total phenol content (Lee et al. 2017; Drozłowska et al. 2021). When the flow rate is increased, the size of the particles is increased, but the efficiency of the evaporation process, the drying performance and the total phenol content are reduced (Fig. 3c, e, f) (Aute and Shirsand 2019).

According to conditions in Table 1, total flavonoid content (mg QE/g) also needs to be as high as possible. The value of alcohol content (X1) is used on level 2–3 (50- 60%), the carrier mass (X2) needs to be around ≥ 10 g, inlet air temperature (X3) is about average 165 °C, while the flow rate (X4) also on a low level (4 rpm/min), which increased total flavonoid content (Y4).

Total flavonoid content of instant tea powder ranged from 31.66 to 71.32 mg QE/mg as shown in Table 2. Similar to total phenol content, total flavonoid content increased with the increase of the alcohol content (Fig. 4a), this is similar to the results of study of Bansal et al. (2014), but it is inversely proportional to the carrier mass as the results shown in Fig. 4d, e. Increasing the inlet air temperature did not increase the total flavonoid content (Fig. 4f), but to avoid affecting other dependent variables in the preparation process, the inlet air temperature should be selected around 165 °C. At the same time, the flow rate (X₄) should be selected as low as 4 rpm to ensure other optimal conditions.

Figure 4. 

Response surface plots showing the effects of (a) alcohol content (X₁) and carrier mass (X₂); (b) alcohol content (X₁) and inlet air temperature (X=); (c) alcohol content (X₁) and flow rate (X₄); (d) carrier mass (X₂) and inlet air temperature (X₃); (e) carrier mass (X₂) and flow rate (X₄); (f) inlet air temperature (X₃) and flow rate (X₄) on total flavonoid content (Y₄).

BCPharSoft program optimizes the preparation process by setting variables X₁, X₂, X₃, and X₄ to 60%, 10g, 165 °C, and 4 rpm/min, respectively. Three replicated batches of the improved method are created to confirm the validity of the optimization approach. Table 4 displays the experimental outcomes.

To compare observed and predicted data, the statistical program SPSS 26 (SPSS, Inc., Chicago, IL, USA) is utilized. A one-sample T-test reveals no statistically significant difference between the anticipated and observed data (p > 0.05), indicating that the ideal findings are compatible with the results predicted by the BCPharSoft program.

Determining the extraction and spray drying parameters in the instant tea powder manufacture process is deemed crucial for the development of medicinal herb products. The modeling of experiments is critical in the preparatory procedure. Most formulas/procedures are formerly based on randomly altering variable values while holding other variables constant to analyze the effect of certain variables of the procedure, however this strategy is incorrect. Today, the introduction of intelligent software in process optimization assists in overcoming the aforementioned shortcomings.

BCPharSoft OPT program optimizes the following parameters: 60% alcohol percentage, 10g carrier mass, 165 °C inlet air temperature, and 4 rpm flow rate. The R₂ test and R₂ train values are used to examine the cause-effect relationship. In general, if the R₂ training value is more than 95% and the R₂ test value is greater than 70%, the model is acceptable. If the R₂ test value is greater than 100%, the model’s predictability is improved (Singh B et al. 2005). According to Table 3, the values Y₁, Y₂, Y₃, and Y₄ exhibited high compatibility based on the R₂ test (R₂ training = 99.8% > 95%). The values Y₁, Y₂, Y₃, and Y₄ have strong predictability (R₂ test = 99.6 percent > 85 percent) based on the value of R₂ test.

The bulking properties of a powder are dependent upon the preparation, treatment, and storage of the sample, i.e., how it was handled. The tapped density is an increased bulk density attained after mechanically tapping a container containing the powder sample. Bulk density and tapped density of instant tea powder in 3 analytical batches are in a good precision with the averages, which are 0.26 ± 0.001 g/mL and 48 ± 0.001 g/mL, respectively. The detailed results are shown in Table 5.

The surface features and morphology of optimum samples are determined by SEM, illustrated in Fig. 5.

Figure 5. 

SEM images of the optimization of instant tea powder. (a) x3000, (b) x10000.