Drying gayo coffee under natural processing by using modified Hohenheim solar tunnel dryer in Gayo highland

Gayo highland has relatively low daily temperature which is not effective for drying coffee. The objective of this study was to examine the operation of modified type Hohenheim solar tunnel dryer with 33% absorber area for drying coffee under natural processing in Gayo highland. About 9kg coffee cherries were dried by using the dryer and about 1 kg was dried under sun drying method. Results showed that the drying temperature of modified type Hohenheim solar tunnel dryer was higher about 9 to 14°C than the temperature of sun drying method. However, the relative humidity was about 40% both inside the drying chamber and outside the dryer. In order to reach the moisture below 14%, the drying time of coffee cherries by using the Hohenheim solar tunnel dryer may take for 8—9 days (64—72h) whereas the drying time by using sun drying method needs about 40 days (320h). It was found that the drying rate of coffee cherries dried by using Hohenheim solar tunnel dryer was higher than that of dried by sun drying method. In conclusion, the modified type Hohenheim solar tunnel dryer is promising to be used by farmers in Gayo highland.


Introduction
Coffee is the most popular drink worldwide and it was spread around the world with high variation taste depending on its environment.Arabica Gayo Coffee planted in Gayo highland, Aceh, Indonesia since 1926 [1], is well-known for its specialty characterized by its strong flavor and fragrance among other coffee types in the world [2] and believed to be the top 10 best arabica coffee in the world [3].Gayo coffee becomes the local superior commodities which is very important for the economy of Aceh [4].It can be shown by the exported coffee beans or coffee by products yearly to many countries.
Coffee commonly is processed through 2 methods namely dry method and humid method [5].Dry method is also known as natural method.This method is preferable by small scale farmers due to its low cost.The coffee cherries are dried without being separated from the pulp.Interestingly, this procedure had resulted a new product called wine coffee since the fermentation step was happened when the beans were in the pulp [6].
As in other place worldwide [7], sun drying method is still used in the drying coffee Gayo highland, Aceh, Indonesia.The time needed may take up to 4 weeks depending on the local thus the method is inefficient and time consuming and also brings biological and chemical risky directly [5].In fact, drying 1297 (2024) 012018 IOP Publishing doi:10.1088/1755-1315/1297/1/012018 2 wine coffee needs longer time about 3 to 4 weeks when it is compared to natural method that needs only 2 weeks [8].However, this method has many disadvantages and needs to be improved, especially to warrant the quality and halal standard of the products.Olmos et al. [5] stated that common defects in coffee almonds related to inappropriate drying process for instance black coloration, crystallization and bleached beans.Application of drying technology is recommended as one of alternatives for quality risk management of Gayo coffee [9].
The average daily temperature in Gayo highland was 23-29°C [10].This situation is unfavorable for the drying process so that farmers often have to transport the coffee to lowland area for conducting the efficient drying process.To overcome these facts, the drying technology could be used as an approach to help the farmers.The mechanical drying could be used by flowing the hot air under a maximum temperature of 52°C, so the dryer unit should has ventilator, heating unit, and chamber [5].Since the small-scale farmers cannot afford such technology, as an alternative to the mechanical drying, traditional sun drying or the use of unrenewable fuels [5], the development of solar dryer takes important role.Moreover, this development will contribute positively to climate change, product quality and loss [5].
The cheaper and simpler technology such as modified type Hohenheim solar tunnel dryer could be used since this dryer was constructed for cheap cost dryer in Germany [11][12][13].We had developed this dryer under some modifications and evaluated on several agricultural products since 2015 till now [14][15][16][17][18][19] in Aceh lowland area.Therefore, the objective of this study is to examine the operation of modified type Hohenheim solar tunnel dryer in drying Gayo coffee under natural processing (dry method) in Gayo highland.

Materials and methods
The research was conducted in Mah Bengi Village, Gayo highland, Aceh, Indonesia.The coffee cherries were bought from the farmers about 15kg.The coffee cherries were washed and soaked for a night to sort the empty cherries signed by the floated ones.The coffee cherries were dried in two methods i.e., by using modified Hohenheim solar tunnel dryer and by using sun drying method.The drying was done following the natural processing procedures applied by Gayo farmers i.e., drying the coffee cherry whole without the intervention of water or machine.The drying process was started at 9.00am until 17.00pm since the dryer works based the availability of solar irradiation.

Figure 1. The drying process of coffee cherries by using modified Hohenheim solar tunnel dryer in Gayo highland
The dryer evaluated in this study was the modified type Hohenheim solar tunnel dryer [10].The first designed was built in 2015 sizing 6m length and 2m width [14].The second designed was built in 2018 sizing 2m length and 1 m width [18].The dryer used in this study has 6m length, 0.8m width, 1.2m 2 absorber area (33%), and 3.6m 2 drying area (66%).The drying areas consisted of 9 trays sizing 0.4m 2 .The trays were numbered 1 to 9 based on its position to the absorber area; the higher the number the faster its distance from absorber.During the drying process, the coffee cherries was completely protected from contamination.
The parameters observed were temperature, relative humidity and weight changes of coffee cherries.About 1kg of coffee cherry was spread on each box and the temperature and relative humidity was read hourly at the central point of each box.The weight of coffee cherries was also measured every hour.After the drying process completed on the tenth day, the coffee cherries were hulled in the local milling unit to produce green beans.The moisture of green beans were evaluated by using air oven method [20].The moisture wet basis (m) was calculated following the Equation 1 [21,22] based on the weight changes, the moisture dry basis (M) was calculated following the Equation 2, and the drying rate of coffee bean (D R ) was analysis by using Equation 3 [22][23][24].
Where m1 is the initial weight, m2 is the constant weight and Δt is the interval time of observations.

The temperature of modified type Hohenheim solar tunnel dryer
The drying process was conducted for 10 days as can be seen in Fig. 2. The average temperature drying chamber temperature of Hohenheim solar tunnel dryer was 35.6°C, while the average ambient temperature was about 25.8°C.It shows that the dryer could raise the drying temperature of coffee cherries for about 9.7°C.However, the temperature deviation inside the dryer (6.7°C) was still higher than the temperature deviation in ambient (2.5°C) since the solar irradiation had fluctuated along the day.
The temperature in the drying chamber was stable by deviation of 2-2.9°C (Table 1).Therefore, the drying process could run properly along the tunnel.Results also showed that the distribution of temperature among the trays decreased as the distance of the trays increased from the absorber area.
Tray number 1 has highest temperature because it is located closed to the absorber area, while tray number 9 has lowest temperature as it is located closed to the air outlet.However, previous design of solar tunnel dryer type Hohenheim has about 50% absorber area [13,17].Increasing the drying area is proposed as the expectation of increasing drying capacity by small scale entrepreneurs [25].This study showed that reducing absorber area to 30% could be effective in Indonesia as tropical country with higher daily solar insulation.
Generally speaking, the temperature in the drying chamber was relatively higher than the ambient temperature.In corresponding to this result, previously study had also confirmed that the drying temperature can be increased significantly by using Hohenheim solar tunnel dryer [10].From several studies conducted in lowland area, the temperature of drying chamber of Hohenheim solar tunnel dryer ranged from10 to 20°C [14][15][16].Therefore, the use of Hohenheim solar tunnel dryer is expected to faster the drying process of coffee in Gayo highland.

The relative humidity of modified type Hohenheim solar tunnel dryer
The second important parameter in drying is relative humidity.The low humidity will support the drying process as the ability of the air to take the water would be high.Gayo highland has cold temperature around 22 to 25°C during the day, the relative humidity in this area tends to be low about 40-45%.Fortunately, according to Figure 3, it can be seen that the average relative humidity inside the drying chamber was lower than the humidity of environment.Although, the difference between both conditions was not significantly signed by the value of standard deviations that is overlapped for all day observation.This fact explains that the use of Hohenheim solar tunnel dryer will improve the drying process both by higher temperature and lower humidity.The average relative humidity in drying chamber of Hohenheim solar tunnel dryer was approximately 42.3%, whereas the average relative humidity at environment was 45.5%.The same results was confirmed in the previous study under the same location [10].The relative humidity of Hohenheim solar tunnel dryer in lowland area was 51.5% where the ambient humidity was above 60% [15].Since Hohenheim solar tunnel dryer is a type of air dryer, the drying chamber condition is directly influenced by the environment situation.Overall, from all studies had been done, the relative humidity in the drying chamber of Hohenheim solar tunnel dryer was lower than the relative humidity at environment.This performance is very important to faster the drying process of products.

The moisture changes of coffee cherry
The drying process had been conducted for 10 days and the weight loss of coffee cherry were recorded hourly.According to our chemical analysis, the final moisture of coffee cherries dried by using Hohenheim solar tunnel dryer were below 14%, while the final moisture of coffee cherries dried by sun drying were 24%.According to our calculation based of weight loss recorded, the moisture changes of coffee cherries is shown in Figure 4. Since we had observed the data from 9 trays in Hohenheim solar tunnel dryer, we had found that there were 2 trends of data represented by tray 1 and tray 6.It can be seen that the drying process on tray 2-5 was similar to tray 1and the drying process on tray 7-9 was similar to tray 6.The drying process can be shown as the linear model with high R-square above 90% so that the equation can be used to estimate the drying time needed.The highest slope of the model was obtained on tray 1 with the moisture will decrease about 0.8418 %wb for an increase of drying time of 1h, while on tray 6, the moisture will decrease about 0.7231 %wb for an increase of drying time of 1h.In contrast, during the sun drying of coffee cherries, the moisture will decrease about 0.1872 %wb for an increase of drying time of 1h.The drying time on tray 1 would be faster that the drying time on tray 6 as the temperature on tray 1 was higher than that of on tray 6.
By the initial moisture of coffee cherries about 70%, the total drying time to reduce moisture at 14% is about 8 days (64h) on tray 1, 9 days (72h) on tray 6, and 40 days (320h) by using sun drying method.Therefore, the use of Hohenheim solar tunnel dryer could the drying time of coffee cherries.It was supported by higher temperature as well as lower relative humidity inside the drying chamber.However, previous study showed that the duration of drying coffee under sun drying method was approximately 3 weeks (21 days) or more depending on the local weather [5].Corresponding to Gayo highland situation, the study showed that the total drying time could reach 40 days due to low daily temperature in Gayo highland.

The drying rate of coffee cherry during the drying process
As can be seen in Fig. 4., the moisture of coffee cherries dried by using Hohenheim solar tunnel dryer was decreased significantly than the moisture of coffee cherries dried by sun-drying.This phenomenon can be explained by Fig. 5. Since the drying rate of coffee cherries dried by using Hohenheim solar tunnel dryer was relatively higher than that of sun-drying, the use of dryer for coffee can faster the drying time.The maximum drying rate of Hohenheim solar tunnel dryer was 13.8%dw/h, while the maximum drying rate of sun-drying was 6.9%dw/h.The average drying rate of Hohenheim solar tunnel dryer at tray 1 was 3.8%wb/h, and the average drying rate of Hohenheim solar tunnel dryer at tray 6 was 3.4%wb/h.In comparison, the drying rate of sun-drying was 1.8%wb/h.

Conclusion
The study showed that the use of modified Hohenheim solar tunnel dryer with 33% absorber area for drying coffee cherries under natural processing in Gayo highland is promising even though the local situation has low daily temperature.This conclusion is proved by higher temperature and lower relative humidity inside the drying chamber if it is compared to the open sun-drying.In addition, the performance of Hohenheim solar tunnel dryer was also better since the moisture of coffee cherries can be reduced significantly faster than that of open sun-drying and the drying process can be protected from contamination.This procedure would be effective for wine coffee producers in Gayo highland.

Figure 2 .
Figure 2. The drying temperature in drying chamber of Hohenheim solar tunnel dryer and at ambient for 10 days observation

Figure 3 .
Figure 3.The relative humidity in drying chamber of Hohenheim solar tunnel dryer and at ambient for 10 days observation

Figure 4 .
Figure 4.The moisture changes of coffee cherries during the drying process

Figure 5 .
Figure 5.The drying rate of coffee cherries in both drying methods

Table 1 .
The statistic descriptive of temperature in the Hohenheim solar tunnel dryer in 9 trays