## 3 Results and Discussion
### Quality of Products  
The kinetics of drying is an important parameter that allows the estimation of the operational regimes, times of processes, and costs. However, in the case of fruits and vegetables, the quality of the received products is a key parameter determining the real suitability of a particular drying operation. For this reason, each operation conducted in this research schedule of drying was judged in terms of the quality of the obtained products. A comprehensive analysis of the products' quality was presented in this section. The amount of total polyphenol compounds in dried apple cubes produced by pure convective drying (CV) was at the level of 1177 mg/kg DM (Figure 7). Application of both ultrasound and microwaves caused enhancement of phenolic compound content in the apple products. In the case of ultrasound-assisted convective drying (CVUS), the increase in phenolic compounds was about 17%, but it was not a statistically significant difference. Application of microwaves (CVMW1 and CVMW2) during the convective process meaningfully influenced the polyphenol content, causing the greatest increase in these compounds, about 26%. Three classes of polyphenols were detected in the investigated dried apple cubes (Table 5). It is noticeable that flavon-3-ols, represented by (+)-catechin, (\(-\))-epicatechin, and oligomeric procyanidins, were the dominant class of phenolic compounds in the investigated apple samples, accounting for 70% of the total polyphenols detected. They were in the range of 806 mg/kg DM (CV) to 1055 mg/kg DM (CVMW1). Our results correspond with data reported by other researchers, which also indicated that flavonids are the dominant phenolic group present in different cultivars of apples [38, 39, 40]. The second important group is phenolic acid, represented by chlorogenic acid (267 mg/kg DM and 325 mg/kg DM for drying schedule CV and CVMW1, respectively), which accounted for about 22% of the total detected polyphenols. The last dihydrochalcons group, represented by phloridine and phloretin xyloglucoside, constituted only 8% of the found phenolic \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline
**Symbol** & **a** & **SE** & **b** & **SE** & **k (I/s)** & **SE** & **n** & **SE** & **D\({}_{\text{eff}}\) (m\({}^{2}\)/s)** & **SE** \\ \hline CV & 1.02 & 3.72\(\cdot 10^{-3}\) & \(-\)3.79\(\cdot 10^{-6}\) & 2.39\(\cdot 10^{-7}\) & 2.14\(\cdot 10^{-5}\) & 2.98\(\cdot 10^{-6}\) & 1.22 & 1.59\(\cdot 10^{-2}\) & 7.39\(\cdot 10^{-9}\) a & 2.10\(\cdot 10^{-10}\) \\ CVUS & 1.05 & 4.83\(\cdot 10^{-3}\) & \(-\)3.26\(\cdot 10^{-6}\) & 1.88\(\cdot 10^{-7}\) & 6.86\(\cdot 10^{-5}\) & 9.16\(\cdot 10^{-6}\) & 1.12 & 1.54\(\cdot 10^{-2}\) & 9.06\(\cdot 10^{-9}\) a & 3.11\(\cdot 10^{-10}\) \\ CVMW1 & 1.04 & 4.75\(\cdot 10^{-3}\) & \(-\)5.05\(\cdot 10^{-6}\) & 6.47\(\cdot 10^{-7}\) & 2.53\(\cdot 10^{-5}\) & 4.53\(\cdot 10^{-6}\) & 1.29 & 2.20\(\cdot 10^{-2}\) & 1.33\(\cdot 10^{-8}\) b & 6.26\(\cdot 10^{-10}\) \\ CVMW2 & 1.07 & 2.40\(\cdot 10^{-3}\) & \(-\)9.04\(\cdot 10^{-6}\) & 5.28\(\cdot 10^{-7}\) & 1.16\(\cdot 10^{-4}\) & 8.41\(\cdot 10^{-6}\) & 1.19 & 9.58\(\cdot 10^{-3}\) & 2.26\(\cdot 10^{-8}\) c & 1.10\(\cdot 10^{-9}\) \\ CVMWUS & 1.06 & 3.35\(\cdot 10^{-3}\) & \(-\)7.80\(\cdot 10^{-6}\) & 5.11\(\cdot 10^{-7}\) & 6.96\(\cdot 10^{-5}\) & 7.45\(\cdot 10^{-6}\) & 1.19 & 1.35\(\cdot 10^{-2}\) & 1.57\(\cdot 10^{-8}\) b & 3.95\(\cdot 10^{-10}\) \\ \hline \end{tabular}  
Different letters in the columns indicate statistically significant difference at \(p\) < 0.05, according to Tukey post-hoc test. a, b, k, n—Midilli–Kucuk model variables, D\({}_{\text{eff}}\)—diffusion coefficient, SE—standard error.  
\end{table} 
Table 4: Midilli–Kucuk model parameters and effective diffusion coefficient values for individual processes. compounds. Moreover, these compounds proved to be the most resistant to the impact of ultrasound and microwaves during the drying process.