Optimizing the Performance of a Manually Operated Groundnut (Arachis hypogaea) Decorticator


Shelling of groundnut pods using manual decorticators in Kenya is characterized by high kernel breakages and low shelling efficiencies. As a result, farmers get low income due to low cost of broken kernels and a lot of time is lost in the tedious shelling operation. To overcome this problem, pertinent parameters that influence shelling efficiency of manually operated groundnut decorticators were identified. Two manually operated decorticators were tested and modifications done on one of the decorticators to optimize its technical performance. Results of machine performance tests showed that for WBS (Wooden beater sheller) at a feed rate of 30 kg/hr and 22.6 mm clearance, shelling efficiency increased with decrease in moisture content for all the groundnut varieties. The highest shelling efficiency was 55.3% for ICGV 99568, 39.2% for ICRISAT Groundnut Variety (ICGV) 90704 and 29% for ICGV 12991 at moisture content of 5.92% wb. For RBS (Rod beater sheller) at a feed rate of 30 kg/hr and 22.6 mm clearance, the highest shelling efficiency was 58.3% for ICGV 99568, 42.7% for ICGV 90704 and 35% for ICGV 12991 at moisture content of 7% wb. Identification of the pertinent parameters showed that pod moisture content, clearance and sieve size influence performance of manually operated groundnut. Theoretical predictive models developed were optimized which showed that a maximum shelling efficiency of 88.73% can be achieved with percent damage of 4% when the sieve size is 11 mm and clearance is 16 mm with a regression coefficient of over 85%. With the modifications done on the WBS decorticator, the highest shelling efficiency of 87% was obtained at a clearance of 10 mm for ICGV 99568 which is the largest in size from the three varieties. The shelling efficiency of the modified decorticator is far above those of the RBS and WBS because the sieve sizes and clearances of the later were not optimized. The results of the theoretical optimization of the manually operated groundnut decorticator implies that farmers who shell for seeds can now obtain more seeds shelled with low breakage and therefore will get more income.

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N. Gitau, A. , Mboya, P. , K. Njoroge, B. and Mburu, M. (2013) Optimizing the Performance of a Manually Operated Groundnut (Arachis hypogaea) Decorticator. Open Journal of Optimization, 2, 26-32. doi: 10.4236/ojop.2013.21004.

1. Introduction

Groundnut (Arachis hypogaea) is a species in the family Fabaceae native to South America [1]. Its seed contains about 63% carbohydrate, 19% protein and 6.5% oil [2]. As the groundnut seed is contained in pod, which is usually developed underground, the pod is harvested by pulling or lifting the plant manually or by using a hoe as the mechanization system [3]. The pods are stripped from the haulms, dried, stored and processed. Shelling is a fundamental step in groundnut processing as it allows the kernels and hull to be used as well as other post harvesting technologies to take place such as oil extraction or in hull briquetting [4]. Shelling can generally be done by hand or machines. Hand shelling is the process in which the pod is pressed between the thumb and first finger so that the kernel is released. It is the most predominantly used method in Kenya’s smallholder agriculture. While hand shelling keeps the rate of Kernel breakage low, it is labour intensive, energy requirement is high [5] and leads to “sore thumb syndrome” when large quantities are handled. A decorticator is a machine for stripping the husk off kernels in preparation for further processing, storage or use as food. The machine can dramatically reduce the labour costs associated with decortications, cleaning and preparing groundnuts for further processing. Decorticators are basically classified as manual or motorized. Manual decorticators are powered by human hand while motorized decorticators are powered by a motor or an engine. Optimizing the performance of a manually operated groundnut decorticator is important so that the shelling efficiency is set at maximum possible and kernel breakage set at minimum possible. Manual shelling of groundnut is a time-consuming and tedious operation [6]. The few existing manual decorticators in Kenyan farms are imported and out of reach of the rural peasant farmers who are characterized by small holdings and low income. The power requirement of such decorticators is high and hence, the prime mover is very expensive. [4] developed and evaluated a hand operated groundnut decorticator and found out that the amount of groundnuts shelled from one hectare per man day is 14 Kg.

2. Materials and Methods

2.1. Identification of the Pertinent Parameters

Table 1 provides the pertinent parameters affecting the performance of groundnut decorticator.

2.2. Determination of Physical Properties of the Nuts

A bulk quantity of the pods of each of the three groundnut varieties used in this study were obtained from ICRISAT farms in Alupe, western Kenya. The pods were cleaned using the cyclone separator to remove dust and other unwanted materials. The varieties were referred to as ICGV 12991, ICGV 90704, and ICGV 99568. 100 pods from each variety were randomly selected and put in the bowls. For each pod, the axial dimensions of length, major diameter and minor diameter were measured using the vernier callipers reading to 0.05 mm and readings recorded. 1000 pod weight was determined by physically counting 1000 pods and then weighing in the electronic balance. Weight measurement was replicated three times and average weight compared. For determination of angle of repose, a pod was placed on the 70 mm by 70 mm metal sheet and one side of the sheet metal lifted until the pod just rolled down. The angle of the tilt was then measured using angle protractor and recorded. This was repeated for 10 pods and the average determined for each of the varieties. The bulk density of the pods was determined using the AOAC (1980) recommended method. This involved filling 1000 cm3 plastic container with the pods and then weighing the pods. The

Table 1. Pertinent parameters affecting efficiency of groundnut decorticators.

bulk density was calculated by dividing the weight by the volume. The pods were conditioned using the method of [10]. This involved soaking of the pods in clean water for a period of 48 h. At the end of soaking, the pods were spread out in thin layer to dry in natural air for about 8 h. The pods were then sealed in marked polythene bags and stored in that condition for a further 24 h. This enabled a stable and uniform moisture content of the pods to be achieved. The moisture content of the kernels was determined using electrical moisture meter. In this method, 100 g of nuts were placed in the moisture meter and moisture content read. Variation of moisture content was achieved through drying of the pods in the tray drier. Measurements of the moisture content using the moisture meter were taken at intervals of 5 minutes during the first two readings and intervals of 10 minutes for the other three readings. All the readings were recorded and mean and standard deviations determined.

2.3. Description of the Decorticators

Figure 1 shows the shelling unit of the wooden beater decorticator. The overall dimensions of the rod beater decorticator are: length of 800 mm, width of 250 mm and height of 940 mm. The shelling unit consists principally of a rotating cylinder and a stationary concave sieve. The cylinder is 190 mm in diameter and 200 mm in length. Twelve shelling metal bars each of 10 mm diameter and 200 mm long are mounted uniformly on the cylinder surface and parallel to its axis.

Figure 2 shows the wooden beater decorticator. The decorticator is powered manually and operates in the same manner as rod beater decorticator except that the impact, shear and compressive forces and tangential force are provided by wooded bars. The overall dimensions of the decorticator are: length of 745 mm, width of 270 mm and height of 1250 mm. A description of two essential units of the decorticator: the shelling and clean ing units are given in the following sections. The shelling

Conflicts of Interest

The authors declare no conflicts of interest.


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