## 3 Results and Discussion
### Assessment of Product Quality  
The water activity (\(a_{w}\)) of a food product is a crucial factor in determining its shelf life. A low \(a_{w}\) inhibits the growth of pathogenic microorganisms, such as fungi, yeasts, and molds, and prevents unfavorable biochemical reactions, including enzymatic and non-enzymatic processes, and oxidation. It is generally accepted that microbial activity ceases below an \(a_{w}\) value of 0.6, while the most unfavorable biochemical reactions are maximally inhibited for an \(a_{w}\) below 0.4 [41]. Figure 4 depicts the mean water activity values for each dried sample and the raw material. \begin{table}
\begin{tabular}{c c c c c} \hline
**Process** & \(k\) **(1/s)** & **\(\alpha\) (m/s)** & \(D_{\textit{eff}}\) **(m\({}^{2}\)/s)** & **adj. R\({}^{2}\) (\(\leftarrow\))** \\ \hline CV & \(1.75\times 10^{-4}\pm 6.02\times 10^{-6}\) & \(4.39\times 10^{-7}\pm 1.50\times 10^{-8}\) & \(4.44\times 10^{-10}\pm 1.52\times 10^{-11}\) & 0.9612 \\ CVUS & \(2.17\times 10^{-4}\pm 5.50\times 10^{-6}\) & \(5.43\times 10^{-7}\pm 1.38\times 10^{-8}\) & \(5.50\times 10^{-10}\pm 1.39\times 10^{-11}\) & 0.9619 \\ CVMW & \(3.29\times 10^{-4}\pm 2.07\times 10^{-5}\) & \(8.23\times 10^{-7}\pm 5.17\times 10^{-8}\) & \(8.34\times 10^{-10}\pm 5.23\times 10^{-11}\) & 0.9243 \\ CVMWUS & \(4.59\times 10^{-4}\pm 2.79\times 10^{-5}\) & \(1.15\times 10^{-6}\pm 6.99\times 10^{-8}\) & \(1.16\times 10^{-9}\pm 7.08\times 10^{-11}\) & 0.9520 \\ \hline \end{tabular}
\end{table} 
Table 3: Values for drying constant, \(k\); mass transfer, \(\alpha\); and effective diffusion coefficient, \(D_{\textit{eff}}\). From an analysis of the results, we can conclude that the safety and microbiological stability of the material were ensured, as the results for the obtained (dried) products were characterized by a water activity (\(a_{w}\)) value below 0.4. Differences between the particular drying procedures were negligible and did not affect the shelf-life of the material. The color of the samples is also a significant quality parameter. It is primarily responsible for determining the sensory properties of the product and indicating the so-called palatability. Additionally, color can indirectly indicate the safety of a product or the changes that occur in it, such as the color change in fruit during the ripening process. It must be acknowledged, however, that color perception is highly subjective, and the evaluation of products will invariably depend on the observer's preferences. Consequently, instrumental color measurement was introduced as a means of objectively determining changes in color relative to the raw material. Figure 4 illustrates the color differences (\(dE\)) for dried samples in relation to the raw material. It is assumed that a value above 4 units represents a perceptible difference in color as discerned by an average observer. As can be observed in Figure 4, all the dried products exhibited a color that was discernibly distinct from that of the raw material, as indicated by color difference index values exceeding 4 units. The application of microwaves in microwave-assisted convective drying (CVMW and CVMWUS) resulted in the most detrimental impact on product quality. The color difference index (\(dE\)) was, for these processes, visibly greater in comparison to the solely convective (CV) or ultrasound-assisted convective processes (CVUS) (Figure 4). Thus it turned out that the very significant achievements made in the drying kinetics led to the opposite results regarding the quality of the products. This is often observed in drying processes, for example, as found by [PERSON] and [PERSON] [39]. The color loss that occurs during microwave drying is usually caused by the high temperatures to which the products are exposed. Figure 5 shows thermal images specifically of samples taken at the 140 th minute of the CVMW and CVMWUS drying processes. The maximum material temperature recorded in the image is approximately 98 \({}^{\circ}\)C, indicating a considerable degree of overheating in the material.