Physical quality of coffee bean (Coffea arabica L.) as affected by harvesting and drying methods

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Ethiopia is naturally endowed with a suitable climate with a distinctive coffee profile and has the potential to produce large amounts of differentiated high-quality green coffee. But currently, Ethiopia’s coffee qualities are quite average and need special attention to produce high-quality coffee to be competitive in today’s world market (Asfaw, 2018). Coffee is the number one foreign exchange earning export commodity of Ethiopia. Almost 2% of the world’s coffee comes from Ethiopia. Over 60% of the country’s foreign exchange is obtained through the export of coffee. A quarter of the population is directly or indirectly engaged in the production, processing, and marketing of coffee (Chauhan et al., 2015).

Coffee is grown by 6.3 million smallholder farmers in Ethiopia in an area of 758,523 ha with a production of 4. 8 million qt and an average productivity of 6.36 qt/ha (CSA, 2020). Coffee is the most important commodity and there is huge potential to increase coffee production as the country is endowed with suitable agro-ecology, climatic, soil fertility, indigenous quality planting materials, and sufficient rainfall in the coffee- growing belts of the country. And, there is high national and international demand for the Ethiopian coffee product, increasing interest of private sector with high investment potential (Berhanu, 2017). Ethiopia produces a large volume of coffee beans every year with 397,500 tons in 2014 alone, and ranking first in Africa and fifth in the world (ICO, 2015). However, coffee supplied and traded in the local market is usually has a lower quality. Coffee on the local market is mainly coffee destined for export through the Ethiopian Commodities Exchange (ECX) market but failed to meet ECX’s quality standards (Asfaw, 2018) for export and got rejected. Quality is an important attribute of coffee and it is currently becoming even more important than in the past as coffee industry is generally going through a worldwide surplus production crisis (Petit et al., 2007).

Wollega is also a potential coffee growing area of Western Ethiopia (Stieger et al., 2002). Though coffee quality is affected in several ways, the agronomic practices followed during harvesting, processing, and handling practices also influence its quality. According to Desse’s (2008) report, poor harvesting practices such as stripping, collecting dropped fruits from the ground, improper postharvest handling practices such as bad processing and drying on the bare ground resulted in the low-quality green coffee bean. Among them, type of harvesting and drying methods used are important. However, there is little information on the effect of different practices such as harvesting methods and drying surface on coffee quality. Therefore, this study was initiated to investigate the influence of harvesting methods and drying surfaces on the physical quality attributes of coffee in Begi district West, Wollega of Ethiopia.

Description of the study area: The study site was in Begi district, West Wollega zone, Oromia Regional State of Ethiopia which is one of the major coffee-producing districts. The selected district represents the agro-ecological zones where coffee is produced. The agroecology of the area is semi-humid and the annual rainfall ranged between 1300-1500 mm per year and the mean annual temperature is 20-280C. Geographically it is located between latitude of 9o 26’North and longitude 34o32’East at altitude range of 1768 meters above sea level.

Treatments and experimental design: The local land race of coffee (Coffea arabicaL.) was used in the present study. The study consists of two factors viz., the harvesting method and drying surfaces. Two harvesting method viz., selective and strip harvesting were tested. Under strip harvesting method, cherries were harvested when 75% of the cherries reached at full ripe stage whereas in selective picking the cherries were harvested as they attained full red ripe stage. Six drying surfaces viz., bamboo mats, bare ground, cemented floor, mesh wire, plastic sheet and tin sheet were tested. The cherries harvested using both methods were spread out to dry in the sun on the six drying surfaces. They were stirred regularly to promote even drying, prevent fermentation and the development of mold in each treatment. Then each sample cherries were dried till their outer shell skin became dark brown and brittle. When the approximate moisture content of 11.5% was attained, dried coffee cherries were collected and de-hulled with mortar carefully and cleaned (Boot, 2006). Each of the drying surfaces had an area of 1m x 1m = 1m2.

Laboratory analysis: Clean coffee bean sample of 500 g was taken from each treatment combination based on sampling procedure set by Ethiopian standard (ESBN 8.001), which is on the basis of drawing 3 kg per 10 tons. Representative samples were assigned an arbitrary code in order to secure an unbiased judgment and brought to coffee quality laboratory of the Jimma Agricultural Research Center where the green coffee beans were evaluated for different raw quality attributes. The moisture content of the sample was checked using Electronic Rapid Moisture Tester (HE 50, Germany) to make the uniform required moisture level of all samples.

Data collection: The data on length of drying period (days), weight of dried coffee berry (g), bean moisture content (%), dried bean weight (g), primary defect (count), secondary defect (weight), odor, coffee aroma and coffee flavor were collected according to their respective procedures.

Data Analysis: The various coffee quality data collected were subjected to analysis of variance using statistical procedures as described by Gomez and Gomez (1984) using SAS 9.3 version. The differences between and among treatment means were compared using the least s ignificance difference test at 5% of significance when the ANOVA shows the presence of s ignificant difference.

Bean size screen (% ): The main effect of harvesting methods and drying surfaces as well as their interaction were highly significantly (P<0.01) (Table 1) influencing the bean screen size. Moreover, the interaction effect of harvesting methods and drying surfaces on the total percentage of bean size retained above screen size 14 ranged from 90% to 73%. The highest beans retained above screen were recorded with wire mesh drying surfaces with selective harvesting methods (90%). However, it was at par with cemented floor. The result indicated that coffee beans harvested in selective picking and treated with different drying surfaces met the export standards except when selective beans dried on tin surfaces (82.3%) (Table 2). The present finding is in agreement with Mekonnen ( 2009) who reported the highest percentage of beans retained above screen were recorded when different varieties of coffee beans were harvested. All the interaction of strip harvesting methods with respective drying surfaces ranged from 75.67% to 73% (Table 2) which failed under the category of rejected commercial coffee based on ECX (2010) standard. According to ECX (2010), any Ethiopian coffee export shall have a minimum of 85% of bean weight remaining on the top of screen 14 (Table 2). Similarly, Mohammedsani et.al. (2017) reported bean size was significantly influenced by harvesting methods and the interaction of harvesting and postharvest processing methods. Selective harvesting of red fruits produced a uniform bean size that is above the minimum required bean screen size. To improve quality coffee, traders practice some value-adding activities like removing the defect and undersized beans thorough cleaning and sorting (Anteneh, 2011), and Belete (2014) indicated coffee with larger beans usually get a good grade and fetch a higher price than smaller ones. The current study confirmed the report of Getachew et.al (2015) who indicated drying coffee on wire mesh and bamboo mats with a thin layer of thickness earned above screen size of beans (>85%).

Dried bean weight : The result showed a significant difference in 100 bean weight due to the main effect of harvesting methods but a non- significant result was obtained due to the main effect of drying surfaces and their interactions (Table 1). And, from this study, the highest 100- bean weight was recorded when coffee was harvested by selective methods (16.51 g) and the lowest recorded in strip harvesting methods (15.39 g) (Table 3). Similarly, Vaast et.al. (2006) indicated harvesting methods significantly influenced the bean weight of coffee due to the lower biochemical composition of the bean, hence reducing the cup quality. This study confirms also the finding of Mohammedsani et.al., (2017), the highest bean weight was obtained from selective harvesting compared to strip harvesting. This study showed the selective harvesting method was 7% more than strip harvesting (Table 3). Another report by Boot (2006) showed that the weight of ripe cherry was more by 20% than that of immature cherry. This might be due to the fact that on bamboo, cement, and mesh wire there was a gradual moisture loss and less burning effect, whereas on a tin bed, there was a burning effect on coffee berry which may decrease the weight of coffee seed. The result regarding drying surfaces was supported by Mohammedsani et.al., (2017). And, report of Wintegens, (2004) and Yigzaw (2014) showed that Arabica coffee average bean weight with values ranging between 9.2 g and 18.2 g.

Primary defects: The analysis of variance revealed that the main effect of harvesting methods and drying surfaces were highly significant (P<0.01) on the primary defect. And, the interaction effect of harvesting methods and drying surfaces were also significant (P<0.05) for primary defect (Table 1). The highest percentage of many defected beans was recorded on selective harvesting methods and drying on a wire mesh (15) and the lowest number of a defected bean is recorded from strip harvesting with drying on plastic (5) (Table 2). This might be because unripe cherries lead to light-green beans, which when dried, become black and these beans are counted as defective in strip harvesting. This study is in agreement with the finding of Bee et al.,(2005).

Table 1
Mean squares values of raw quality attributes of coffee as affected by harvesting methods and drying surfaces in Begi district West Wollega Zone Ethiopia

* Significant at P<0.05** highly significant at P<0.01, ns= non-significant difference, Numbers in parenthesis indicates degree of freedom. CV (%) = coefficient of variation in percent, Sed (±) = Standard error of difference.

Table 2
Bean size screen using ECX 2010 standard

ECX (2010) stated that Moisture and screen analysis are the two requisites before grading any coffee. The moisture content should be less than 11.5 percent, while the size of the bean should be above screen size 14 for 85 percent of the bean sample.

Table 3
The main effect of harvesting method and drying surfaces on raw and physical quality attributes of coffee in Begi district Ethiopia

Means followed by the same letter(s) within rows and columns are not significantly different at P d” 0.05 level of significance, LSD= Least significant differences=Non-significant, CV (%) = coefficient of variation in percent

Similarly, with the report of Barel and Jacquet (1994), selective harvesting of coffee produced the best quality coffee by decreasing the percentage of defective coffee beans. Also, Berhanu et al., (2014) also indicated that inappropriate post-harvest management practices increased the number of defective coffee beans. Moreover, Tesfaye (2006) and Negussie et.al. (2009) stated that properly processed coffee is with very few defective beans.

Secondary defects

The result showed that there was a highly significant (Pd”0.01) variation of secondary defects due to the main effect of harvesting methods and drying surfaces. However, the interaction effect of harvesting methods and drying surfaces did not significantly affect secondary defects (Table 1). Selective harvesting had a high mean value of 13.17% indicating relatively pure coffee beans. However, the lower mean value (6.83%) was recorded from strip harvesting (Table 3), which indicated a high number of secondary defects due to improper harvesting. This showed that selective harvesting had more coffee beans free from secondary defects as compared to strip harvesting in dry- processed coffee. This is because selective harvesting involves only picking off the red, fully ripe, and normal cherries carefully from the tree while strip harvesting involves collecting of entire coffee bean just by one pass through cropping season. This result is in line with Hicks (2002) who described that although selective picking is more expensive, it can produce the best results of coffee by reducing the number of defects thereby increase the overall quality of coffee which is competent in the world market. And, Hicks (2002) reported that coffee that has been inappropriately dried would become brittle and produce too many broken beans that are considered as a secondary defect during hulling. Similarly, Olamcam (2008) result showed that the coffee well harvested and properly processed has no or very few broken beans and free of foreign matter.

Length of drying periods: The analysis of variance revealed that the length of drying periods was highly significantly (P<0.01) different due to the main effect of drying surfaces and harvesting methods and significant (P<0.05) difference due to the interaction effect of both factors (Table 1). Higher length of drying periods (41.67 days)was recorded from the interaction of wire mesh drying surfaces with selective harvesting method and the lowest (20.33 days) was recorded from the interaction of tin drying surfaces with strip harvesting method but statically at par with the interaction of plastic drying surface with strip harvesting method (20.67) (Table 4). Harvesting red cherry would prolong the drying periods than harvesting in a strip. Besides, at the full maturity stage, there might be an increment of moisture and the development of luxurious mucilage. This result agrees with the findings of Berhanu et.al. (2014) that the shortest time drying periods were recorded when coffee was dried in bricks off the floor then raised bed. FAO (2006) and Martin et al. (2009) also reported coffee dried on a flat surface more quickly than that dried on raised-bed surfaces like mesh wire and bamboo mats.

Table 4
Interaction effect of the harvesting method and drying surfaces on the primary defect length of drying in days and dried coffee berry weight at Begi West Wollega Zone Ethiopia

LSD = Least significant difference, CV= Coefficient of variation

The result revealed that the interaction of harvesting methods and drying surfaces were highly significant (P<0.001) difference for coffee bean size and dried coffee berry weight while significant (P<0.05) variation for primary defects, length of drying period. The main effect of harvesting methods and drying surfaces were highly significant on bean size, primary defect, secondary defect, length of the drying period, and dried coffee berry weight. Coffee beans harvested by selective harvesting and treated under different postharvest processing methods had 85%, except when coffee beans size dried on and above the minimum required bean size for export coffee as compared to strip harvesting beans in which all beans are recorded under rejected coffee due to many small beans (<76%).

The highest (16.51 gram) dried bean weight was verified in selective harvesting as well the lowest (13.59 gram) was in strip harvesting. Primary and secondary defects were highly significantly influenced by harvesting methods and drying surfaces. The highest length of drying period (41.67 days) was recorded from the interaction of wire mesh drying surfaces with selective harvesting method and the lowest (20.33 days) was recorded from the interaction of tin drying surfaces with strip harvesting method but statically at par with the interaction of plastic drying surfaces with strip harvesting method (20.67). The odor was significantly influenced due to the main effect of coffee harvesting methods and drying surfaces. The highest scale of the odor was recorded from selective harvesting and the lowest from strip harvesting. Acidity and flavor were affected by harvesting methods and selective harvesting produced a high raw quality of all attributes. The finding suggests that coffee physical quality could be better improved by the selective picking of red cherries. Moreover, drying coffee on bare ground highly reduced raw abnormal color and unpleasant odor.

CONFLICT OF INTERESTS: The authors declare that there is no conflict of interests.

The authors thank the Ethiopian Coffee and Tea Authority for their technical support in coffee quality analysis.