The outcomes of this study were classified and discussed under different headings as drying experiments, energy utilization, and protein and oil analyses in hazelnuts.
3.1. Drying Experiments
Drying Experiments: In this experimental study, time-dependent moisture content was obtained for each periodic drying. It is not feasible to present all the results graphically. Thus, in this article, the effects of drying air velocity during periodic drying are presented via graphs for each operating period for comparison. The other figures are given as
Supplementary Materials. The time-dependent moisture contents for three different air velocities and each operating period are shown in
Figure 4,
Figure 5,
Figure 6 and
Figure 7.
As seen in
Figure 4, the drying time under case 1 conditions was 1290 min at air velocities of 1.5 m/s and 1 m/s, while this duration increased by 9.6% at an air velocity of 0.5 m/s. It is evident from
Figure 5,
Figure 6 and
Figure 7 that as the air velocity decreases in case 6, case 11, and case 16, the hazelnut drying time increases by approximately 10%. As the case number increases, the drying time also increased by an average of 5%.
As observed in
Figure 5,
Figure 6 and
Figure 7, as the air velocity increases, the total drying time shortened between 10% and 36%. As the case number increases, when transitioning from an air velocity of 1 m/s to 1.5 m/s, the drying time decreases by 9–14%, while when transitioning from an air velocity of 0.5 m/s to 1 m/s, the drying time decreases by 7–14%. This decrease occurred approximately proportionally with the increase in the case number. The reduction in hazelnuts’ drying time due to the increase in air velocity during periodic drying is similar to the situation of continuous drying described in the literature [
12].
Although the drying process was conducted at three air velocities in the study, comparison graphs for working periods are provided for the air velocity of 1.5 m/s, which has the shortest drying times, in
Figure 8,
Figure 9,
Figure 10 and
Figure 11.
As depicted in
Figure 8, it can be observed that, compared to continuous drying, as the idle time increases in the 0.5 h working periods, the total drying time also increases. The increases in drying time compared to continuous drying for cases 1, 2, 3, and 4 are 19%, 44%, 58%, and 83%, respectively.
As seen in
Figure 9, it can be observed that, similar to
Figure 8, as the waiting time increases in the 1 h working periods compared to continuous drying, and the total drying time also increases. The increases in drying time compared to continuous drying for cases 5, 6, 7, and 8 are 22%, 33%, 55.5%, and 74%, respectively.
As the waiting time increases in the 1.5 h working periods, the total drying time for the 1.5 m/s air velocity also increased compared to continuous drying, as shown in
Figure 10. This time increase was 8%, 19%, 25%, and 42% for cases 9, 10, 11, and 12, respectively.
Compared to continuous drying, the total drying time increased as the idle time outside the oven increased in the 2 h operation period. This increase was 6%, 8%, 22%, and 22% for case 13, 14, 15, and 16, respectively (
Figure 11).
During periodic drying, as the oven working time increases, the hazelnuts’ drying time decreases, and as the waiting time outside the oven increases, the drying time also increases. Compared to continuous drying (1080 min), for a 1.5 m/s air velocity, the shortest drying time (1140 min) occurs in case 13, while the longest drying time (1980 min) occurs in case 4. Since there are no data on periodic drying in the literature, a comparison with the results obtained in this study could not be made.
The time-dependent mass losses during sun-drying and the temperature changes that occurred during the drying process are given in
Figure 12.
As observed from
Figure 12, hazelnuts dried in the sun over 4 days. The daytime ambient temperature (35.2 °C) and hazelnut surface temperature reached a maximum (53.7 °C) between 13:00 and 14:30. Due to the high humidity (highest at 93% and lowest at 75%) in the region, hazelnuts absorb moisture during the night. Hazelnuts can only release this moisture after the sun rises, typically within 2–3 h. The time-dependent mass loss curves in solar-drying and the emphasis on the difference between the hazelnuts’ top temperature and ambient temperature are consistent with the studies of Kandemir [
12]. The drying time of hazelnuts under sunlight is similar to the studies of Islam and Turan [
37].
The working time of the oven and the idle waiting time outside the oven of the hazelnuts according to the air velocities are given in
Table 2. Here, only the last times are given for each period.
As seen in
Table 2, the shortest operating time and the longest total drying time occurred during case 4 at an air velocity of 1.5 m/s. In all experiments, the shortest drying times compared to continuous drying were observed in the 0.5 h waiting periods.
The shortest working time and the longest waiting time for all three air velocities are observed in the 0.5 h working and 2 h waiting periods.
3.2. Energy Utilization
The results of this study included the M. Periodic Drying Model, aiming to reduce the drying energy costs in conventional drying systems and, consequently, reduce the carbon footprint during drying. These were examined in terms of their energy consumption and energy utilization. The specific energy consumption of the channel-type dryers used for drying hazelnuts is given in
Figure 13,
Figure 14 and
Figure 15 for continuous drying and periodic drying.
When the graphs in
Figure 13,
Figure 14 and
Figure 15 are examined, it can be seen that the lowest specific energy consumption per kg water mass that evaporated from hazelnut was obtained during the 0.5 h work and 2 h wait periods. In hazelnut drying, the lowest specific energy consumption per kg water mass was obtained as 645.1 kWh/kg water in Case 4 for 0.5 m/s air velocity, while the highest energy consumption was obtained as 3096.8 kWh/kg water in Case 16 with 1.5 m/s air velocity. The specific energy consumptions of continuous drying, depending on the air velocities (0.5 m/s, 1 m/s, and 1.5 m/s), were calculated as 1600.3 kWh/kg water, 3354.6 kWh/kg water and 4644.8 kWh/kg water, respectively. As seen in the graphs, the specific energy consumption for drying increases proportionally with the increase in air velocity. The specific energy consumption values obtained during continuous drying are similar to those of Motevali et al. [
31] and Nwakuba [
32]. However, there are no data in the literature related to the specific energy consumption values of periodic drying. Therefore, the results of periodic drying could not be compared with the literature findings. The specific energy consumption of periodic drying is lower than that of continuous drying for each period.
The energy utilization calculated from Equation (7) for periodic drying compared to the continuous drying at three different air velocities is given in
Table 3,
Table 4 and
Table 5.
Table 3 indicates that, at an air velocity of 0.5 m/s, the highest energy of periodic drying compared to continuous drying is 59.7%, obtained in case 4, while the lowest energy utilization is 3.3%, obtained during periods 13 and 14. At an air velocity of 1 m/s, the energy utilization compared to continuous drying is 61.5% in case 4, while the lowest energy utilization was calculated as 17.9% during periods 13 and 14 (
Table 4). Similarly, at a low air velocity of 1.5 m/s, the energy utilization compared to continuous drying is 61.1%, as obtained in case 4, with the lowest energy utilization determined to be 11.1% in case 13 (
Table 5).
It is evident from the energy utilization tables that, as the air velocity increases, the energy utilization from periodic drying compared to continuous drying increases. However, as the oven operation time increases, the energy utilization decreases, and while the energy utilization increases, the idle waiting time outside the oven increases.
3.3. Protein and Oil Analyses in Hazelnut
The hazelnut oil and protein percentages under different drying conditions are presented in
Table 6. As seen in
Table 6, the lowest oil percentage occurred under the drying conditions of case 10, at 61.46%. The highest oil percentage, 67.71%, was observed under the drying conditions of Period 5, with a 10.33% increase in oil percentage. The oil percentages obtained during sun-drying and continuous drying are similar to the other periodic drying conditions.
When compared to continuous drying (control sample) for all periodic drying conditions, the lowest protein percentage was obtained under the drying conditions of case 16 (14.8%), while the highest protein percentage was measured under the drying conditions of case 7 (17.8%). However, the protein values for sun-drying and periodic drying are similar.
Protein and oil content may vary according to the drying method, the variety, and the size of the hazelnut [
12,
37,
38]. In this study, in order to minimize variability, hazelnuts were harvested from a single orchard, hazelnut size was kept constant, and a single variety was used as the material. The results of the present study related to the oil and protein content of the hazelnuts were in good agreement with those of the literature. The values measured in all methods were between the values specified by Balık et al. [
13] and Sali [
38].