## 3 Results  
### Microwave Heating Characteristics  
The typical evolution of temperature as a function of time at different mean fluid flow rates during continuous-flow microwave heating is shown in Figure 2, and it indicates typical lag periods prior to achieving a steady state. As explained in [PERSON] et al. [4], the nonlinearity in the time-temperature profile during the early phase of heating is contributed by the coil and the environment within the oven cavity. In the microwave heating set-up used, it took about a minute after turning the microwave on to reach the target temperature in the range 65 to 95 \({}^{\circ}\)C, while equilibrated steady-state exit temperatures were achieved after about 2 min. The mean exit temperature at the end of the first and the second oven as a function of flow rate and residence time are shown in Figure 3a,b. As expected, the exit temperatures after the second oven were higher than in the first due to the continuous flow of liquid from the first to the second oven. Further, higher flow rates resulted in lower exit temperatures. Figure 3b demonstrates the continuous evolution of temperatures through the first and the second ovens as a function of the overall residence time, ranging from 15-65 s, and reaching 50 \({}^{\circ}\)C to 95 \({}^{\circ}\)C after heating in the first and the second microwave ovens. The associated data on Reynolds and Dean numbers, mean heating rate, power absorbed, and power absorption efficiency are summarized in Table 1.  
Figure 2: Typical microwave heating profiles in the MW system as a function of flow rate. The heating rates varied from 1.09 to 1.27 \({}^{\circ}\)C/s depending on the flow rate and the exit temperatures. The microwave power absorption efficiency varied from 61% to 72%, with lower levels associated with higher exit temperatures and lower flow rates. This can be easily explained by the higher loss of heat to the microwave oven environment at the higher temperatures. The associated Reynolds numbers were considerably lower than the 2100, indicating the flow to be essentially laminar. However, the use of a helical coil creates secondary flow, which results in thorough mixing of the fluid as it passes through the system. It has been argued that, at high Dean numbers, the secondary mixing of fluids within a helical coil provides a \"perfectly mixed or plug flow profile\" for the flowing liquid [24]. The Dean numbers associated with the flow varied from 178-357 in the present studies. The exit temperatures achieved as a function of the Dean number and the flow rate numbers are shown in Figure 4a. The exit temperature as a function of flow rate at two pH levels are shown in Figure 4b. Figure 3: Sample temperatures at the microwave exit as a function of (**a**) residence time and (**b**) flow rate. The microwave heating patterns also depended on the pH of the test solutions. The mean exit temperatures (a) and the power absorbed (b) as a function of the Reynold number and the flow rate at different pHs are shown in Figures 5 and 6, respectively. The power absorbed and, hence, the exit temperatures were higher at a lower pH. Clearly, pH was an influencing factor. The differences in the heating patterns at different pHs were ascribed to the differences in the concentrations of various chemicals used for the preparation of the buffers and their influence on MW heating. Figure 7 shows the residual \(\alpha\)-amylase activity at the different sample exit temperatures as they exit from the MW oven. What is not apparent from the conventional residual activity plotted against temperature (Figure 7) is the fact that the flow rates employed for achieving the different temperatures (at each pH) are different. For the same exit temperature, the residence times under different heating conditions were also different.