- Background to the study
Groundnut or peanut (Arachis hypogaea L.) is originated in South America. The botanical name for groundnut, Arachis hypogaea Linn., is derived from two Greek words, Arachis meaning a legume and hypogaea meaning below ground, which refer to the formation of pods in the soil. The crop is known by two names i.e. groundnut or peanut. It is known as groundnut in countries like Asia, Africa, Europe and Australia, while in North and South America it is known as peanut. The term groundnut refers to the pods with seeds that mature underground, it is also called peanut because it belongs to the leguminous family which includes other crops such as beans and peas. Groundnut is the third most cultivated oilseed in the world and plays a huge role in the economy of West African countries such as Gambia, Nigeria, Ghana and Senegal (Nwanosika, 2011).
The major pests which are groundnut borer, red flour beetle, termite, aphid, and diseases which include groundnut rosette, groundnut streak, bacteria wilt, anthracnose, fusarium wilt, pod rot etc. are one of the restriction in groundnut production. Groundnut cannot compete with weeds effectively, majorly during the first 3-6 weeks after planting. According to Olayinka and Etejere, (2016), uncontrolled weeds for at least 6weeks of planting have been reported to cause yield loss in groundnut productivity which is high as 70%. This is because weeding is required at an early stage of growth due to inability of the crop to develop canopy that shades the ground effectively for weed prevention.
Pod maturity of groundnut is not uniform therefore harvesting should be done when most of the pods are mature because waiting for all the pods to mature will result in growth of the already matured pods. Early or premature harvesting, lowers the yield, oil quantity and seed quality. Plants from immaturely harvested seeds germinate slowly and have low vigour and their growth and survival can be difficult under stressful conditions both biotic and abiotic. In addition to growth of pods, delayed harvesting could also leads to yield loss of over-mature pods during harvest owning to weakening of pegs, Aspergillus flavus infection, and aflatoxin contamination in pods or seeds (Ding et al., 2014).
Loss of seed viability is intense in groundnut harvested in summer season and about 50% viability could be lost within 4-5 months of storage. Storage of groundnut seeds is very essential in order to prevent contamination. Seeds with high oil content appear to lose their ability to germinate in a short time regardless of the precaution taken during harvesting and drying. High temperature and high relative humidity could lead to loss of viability and vigour of groundnut seeds. Environmental conditions could also affect the seed quality and storability of groundnut seeds.
Groundnut is one of the most popular commercial crop grown in Nigeria. According to Taru et al. (2008) major groundnut zones in Nigeria are the Sudan and northern Guinea Savanna where the soil and agro-climatic conditions are favorable. Groundnut is an crucial component of Nigeria diet. The total world groundnut output in 2008 was estimated at 34.8 million metric tonnes and Nigeria accounted for about 3.8 million metric tonnes (FAO, 2008). Groundnut is grown on 26.4 million hectares worldwide with a total output of 37.1 metric tonnes and an average yield of 1.4 metric tonnes per hectare. Nigeria is considered the third largest producer of groundnut in the world after China and India with an output of 16,114,231, 6,933,000 and 2,962,760 million metric tonnes respectively in 2011.
1.1.1 Proximate composition
Proximate composition is the term usually used in the field of food and it means the composition of moisture, crude protein, crude fiber, crude ash and Crude fat which are expressed as the content (%) in the food, respectively. The measured values of these 5 components in food are important factors to understand the nature and the properties of the subject food. Moisture is composed of water and volatile substances. Crude protein is composed of protein compounds, non-protein compounds and amino acids. Crude fibre is composed of cellulose, hemicellulose and lignin. Crude fat is composed of fats, fatty acids, sterols, complex lipids and fat-soluble dyes. Crude ash is composed of pure ash and organic residue.
1.1.2 Phytochemical composition
Phytochemicals are naturally occurring chemical compounds found in plants and which protect plant cells from environmental threat such as pollution, stress, ultraviolet exposure and pathogenic attack. Phytochemicals are not regarded as essential nutrients and they are not required for sustainance of life, but they have important properties to prevent or to fight some common diseases (Holst and Williamson, 2008). These compounds are also known as secondary plant metabolites and provide health benefits to humans. Phytochemicals are classified majorly as carotenoids, Polyphenols (which include phenolic acids, flavonoids, stilbenes or lignans and tannins), Terpenoids, Sterols, Cardiac glycosides, Saponins and Alkaloids. They are known to act as synergistic agents, allowing nutrients to be used more efficiently by the body. Some of the beneficial roles of phytochemicals are low toxicity, low cost, easy availability and their biological properties such as antioxidant activities, antimicrobial effects, stimulation of the immune system, and regulation of hormone metabolism and anti -tumour properties (Andre et al., 2010).