Sorghum threshing in Kenya is characterized by high grain breakages and accounts for grain loss of 4% of total production. Existing threshing methods are time consuming and yield a low throughput. Grain losses mainly occur due to grain damage and incomplete removal of grains from panicles. In order to reduce the losses, optimal levels of machine and operational parameters influencing threshing need to be established. A prototype spike tooth sorghum thresher was therefore developed using engineering principles with the objective of optimizing threshing performance of its threshing unit. The performance tests of sorghum threshing were conducted at three levels for drum diameters (200, 300 and 400 mm), spike spacings (50, 75 and 100 mm) and drum peripheral speeds (8, 10 and 12 m s-1) using a factorial experimental design. The data were subjected to graphical and statistical analysis of variance (ANOVA); and optimization was done using the Taguchi signal/noise ratio method. It was observed that threshing sorghum at spike spacing of 50 mm, drum diameter of 400 mm and drum peripheral speed 12 m s-1 produced the highest threshing efficiency of 96%. Minimum mechanical grain damage of 3% was obtained using drum diameter 400 mm, spike spacing 100 mm and drum peripheral speed 8 m s-1. Threshing sorghum at drum diameter of 400 mm, spike spacing 50 mm and drum peripheral speed of 10 m s-1 produced maximum throughput per unit energy consumption of 153 kg h-1 (kWh)-1. Although throughput of the thresher increased as the drum peripheral speed was increased from 8 to 12 m s-1, throughput per unit energy consumption reached a maximum at 10 m s-1. From the study it was concluded that optimal threshing performance could be attained with drum diameter 400 mm, spike spacing 50 mm and peripheral speed 10 m s-1. The study recommends that the performance of sorghum threshers be based on throughput per unit energy consumption rather than throughput as throughput does not take into account the energy consumed during threshing. This recommendation could be extended to other grain threshers since their principles of design and operation are the same. Future studies could be done to determine the influence of drum length, concave clearance, feed rate, sorghum variety and moisture content on the performance of sorghum threshers.

Background to the study
Grain sorghum (Sorghum bicolor (L.) Moench) is the world’s fifth most important cereal crop after maize, rice, wheat and barley. In Africa it is second after maize in terms of importance and is dietary staple food for more than 500 million people in more than 30 countries (Muna et al., 2016). The world annual sorghum production is over 60 million tons, out of which Africa produces about 20 million tons (Sale, 2015). According to Indexmundi (2017), Kenya produced 150,000 tons of sorghum in 2017.

One of the biggest challenges facing the agricultural sector in Kenya is meeting the growing demand for food grains to feed its increasing population. With maize production under threat of armyworm attack, sorghum presents a viable alternative of maintaining the country’s grain supply. The crop is becoming a source of livelihood for farmers in arid and semi arid regions as it gives them an opportunity to harvest from low productive land with minimal input (Oyier et al., 2016).

Sorghum is important to food security in Kenya because it is drought resistant among cereals and can withstand periods of high temperature and water-logging (Takuji and Baltazar, 2009). It is characterized by an extensive root system, waxy bloom on leaves that reduces water loss and the ability to stop growth in periods of drought and resume it when the stress is relieved (Muui et al., 2013).

Sorghum is used for food, feed and beverage production (Muui et al., 2013). It is used to manufacture wax, starch, syrup, alcohol and edible oils (Agrama and Tuinstra, 2003). As food, the grain is used in making fermented and non fermented porridge and ugali (Ministry of Agriculture, 2012). The grain has high levels of iron and zinc used to reduce micronutrient malnutrition (Gerda and Christopher, 2007).

Despite the food and nutritional value of sorghum, its threshing is still a challenge for Kenyan farmers. In Kenya, sorghum grain post-harvest losses amount to 15% of the total production, out of which 24% occur during the threshing and cleaning stages (FAO, 2012).

According to Kamble et al. (2003), mechanical sorghum threshing suffers high grain damage and imperfect threshing process. They estimate these losses to amount to four percent of total throughput while Azouma et al. (2009) reported manual threshing losses as 8%. These losses are due to incomplete removal of grains from heads and grain damage. Damaged grains are prone to fungal attack, pose less resistance against pests leading and reduce the grade and marketability of the grain (Wang et al., 1994). Further, Spokas et al. (2008) reported that when grains are broken their germination rate is reduced by 10%.

The threshing unit plays a key role in determining the performance of a thresher (Sudajan et al., 2005). Its performance is influenced by machine characteristics as well as physical and mechanical properties of sorghum (Osueke, 2013). Important crop factors influencing sorghum threshing and drying are bulk and solid density, moisture content, angle of repose; major, intermediate and minor diameters and coefficient of friction of the grains (Sessiz et al., 2008). Machine parameters include drum speed; concave clearance; type, number and shape of threshing spikes; type of sieve and feed rate (Muna et al., 2016).

Presently, the performance evaluation of sorghum threshers is based on machine throughput, threshing and winnowing efficiencies, grain mechanical damage, cleaning efficiency and throughput capacity (Wangette, 2015). A mechanical thresher should have high threshing efficiency, low mechanical grain damage and a high throughput per unit energy consumption.

Statement of the problem
Current performance evaluation of sorghum threshers is based on machine throughput, threshing and winnowing efficiencies and grain mechanical damage. However, research data on throughput per energy consumption of a spike tooth sorghum threshers are limited. Different researchers have reported different optimal values of these parameters with inconsistent recommendations. Studies on the influence of drum speed on threshing performance have been on threshing drum angular speed rather than drum peripheral speed. Further, few researchers have given consideration to the influence of other inputs during threshing such as energy on threshing performance.

Broad objective
The broad objective of this study was to optimize the performance of a prototype spike tooth sorghum threshing unit in terms of drum diameter, spike spacing and drum peripheral speed.

Specific objectives
i. To determine the influence of drum diameter, spike spacing and drum peripheral speed on the threshing performance of a spike tooth sorghum threshing unit.

ii. To optimize the threshing performance of the spike tooth sorghum threshing unit.

Research questions
i. How do drum diameter, spike spacing and drum peripheral speed influence the threshing performance of a spike tooth sorghum threshing unit?

ii. What drum diameter, spike tooth spacing and drum peripheral speed of the sorghum thresher results in optimal threshing performance?

Optimization of the selected parameters of the threshing unit is a vital step towards reducing threshing losses, improvement of quality of grains and increased sorghum productivity. Lack of standards for the threshing unit has meant artisans use different levels of the design parameters contributing to inefficient threshing. This has hindered the uptake of mechanical threshing by farmers. Results from this study will enable standardization of the threshing unit configuration. This will spur the growth of sorghum mechanical threshing benefitting both artisans and farmers. Energy will be saved when threshers are designed based on the combination of parameters producing optimal throughput per unit energy consumption.

Scope and limitation
The study covered the development and subsequent performance evaluation of prototype spike tooth sorghum thresher to establish the levels of drum diameter, spike spacing and drum speed that would result in optimal threshing performance. Crop factors (variety and moisture content), machine factors (feeding chute angle, cylinder type, spike shape and size. concave size, shape and clearance) and operational factors (feed rate and machine adjustment) affecting sorghum threshing were not covered.

The performance indicators were limited to grain mechanical damage, threshing efficiency and throughput per unit energy consumption. Cleaning efficiency, germination percentage and grain scatter loss were not studied. The sorghum crop variety used was andiwo grown in Migori County.

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Item Type: Kenyan Topic  |  Size: 58 pages  |  Chapters: 1-5
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