ABSTRACT

African leafy vegetables (ALVs) have a great potential in reducing the gap in nutritional status between low- and high-income households because of their nutrient densities and affordability. Cowpea is one of the major ALVs produced and consumed widely at the Coast and Western regions of Nigeria as a dual-purpose crop (grain and leaf). Although local cowpea accessions are preferred by farmers and consumers, there are still many challenges encountered. There is lack of sufficient information on nutrients and micro-nutrient densities and high postharvest losses. High post-harvest losses have also been reported as a result of perishability. The objective of this study was to evaluate and compare the nutritional and sensory attributes of five popular dual purpose local cowpea accessions and an improved variety developed by KALRO. In addition, the effect of blanching, solar drying and modified atmosphere packaging (MAP) on the shelf life and quality attributes of one superior cowpea accession was evaluated.

The cowpeas were planted in The University of Uyo field station during the short rains from October to December 2014 and long rains from March to May 2015. The cowpea leaves were randomly sampled in the experimental plots. One superior accession was chosen for post-harvest treatments. The treatments were, solar drying without blanching, blanching in pure water and solar drying, blanching in salty water and solar drying and fresh non-blanched leaves as control. The samples were then analysed for proximates, vitamins, minerals, anti-nutrients, sensory characteristics, colour change during processing and packaging, cumulative water loss and wilting in The University of Uyo and Jomo University of Agriculture and Technology laboratories.

In the first objective on evaluating and comparing nutritional attributes of local cowpea accessions with an improved variety, beta carotene content of M66 which is an improved variety was the lowest at 29.71mg/100g whereas Sura Mbaya had the highest beta carotene content at 36.4mg/100g. On the other hand, M66 had the lowest ascorbic acid content of 192.8 mg/100g whereas Usimpe Mtu Mdogo had the highest ascorbic acid content at 213.1mg/100g in season 1. The iron content of Usimpe Mtu Mdogo was the lowest at 395.9PPM compared to Mnyenze at 1034.3 PPM in season 1.

In the second objective of evaluating the efficacy of post-harvest treatments on the quality of fresh and processed cowpeas, it was found that blanching and dehydration had little effect on most proximate and mineral elements. However, vitamin and total phenolic contents were the most affected. Solar drying without blanching recorded the highest vitamin retention levels at 68.02% for beta carotene and 68.39% for ascorbic acid unlike blanching in pure water and solar drying at 55.58% for beta carotene and 21.08% for ascorbic acid and blanching in salty water and solar drying at 52.78% beta carotene and 20.24% ascorbic acid. In addition, solar drying without blanching recorded the highest retention total phenolic content at 149.91%. Blanching in pure water and solar drying and blanching in salty water and solar drying recorded retention levels of 62.58% and 65.79% of total phenolic content respectively. On the other hand, solar drying without blanching, blanching in pure water and solar drying and blanching in salty water and solar drying recorded a loss of 5.87%, 10.77% and 11.17% of oxalates and 37.22%, 69.98% and 58.7% of nitrates respectively.

In the samples subjected to MAP, the end stage of control, ordinary polythene bag and Extend bag under room conditions was 1 day, 4 days and 6 days respectively. By the end stage control, ordinary polythene bag and Extend^® bag had lost 28.84%, 0.93% and 3.27% cumulative weight for season 1 and 23.84%, 0.89% and 2.31% for season 2 respectively.

The results of the present study indicated that evaluated local cowpea accessions were comparable with each other but slightly superior to the improved variety developed by KALRO. Solar drying without blanching was found to be effective in maintaining the quality attributes of cowpeas. In addition, MAP was found to be effective in preserving quality of fresh cowpea leaves and improving shelf life. Therefore MAP and solar drying without blanching are simple and convenient technologies for preserving cowpea nutrients and improving shelf life.

                                                TABLE OF CONTENTS

CHAPTER ONE

INTRODUCTION

1.1.               Problem statement

1.2.               Justification of the study

1.3.               General objective

1.3.1.         Specific objectives

1.3.2.         Null hypotheses

CHAPTER TWO

LITERATURE REVIEW

2.0                 Nutrition status in Nigeria

2.1                 African leafy vegetables (ALVs) in Nigeria

2.2                 Botany and centre of origin of cowpeas

2.3                 Cowpea morphology

2.4                 Uses of cowpea

2.5                 Ecological growth requirements of cowpeas

2.5.1           Temperatures

2.5.2           Rainfall

2.5.3           Edaphic factors and fertilizer application of ALVs

2.5.4           Light intensity

2.6                 Nutritional composition of ALVs

2.6.1           Anti-oxidative activities of ALVs

2.6.2           Minerals constituents of ALVs

2.6.3           Other macro-nutrients

2.6.4           Anti-nutrient content in ALVs

2.7                 Post-harvest losses of cowpeas and other ALVs

2.8                 Post-harvest management

2.9                 Sensory properties of AIVs

CHAPTER THREE

EVALUATION OF NUTRITIONAL QUALITY ATTRIBUTES OF SELECTED SUPERIOR LOCAL LEAFY COWPEA ACCESSIONS AND AN IMPROVED VARIETY

3.1.               ABSTRACT

3.2.               INTRODUCTION

3.3.               MATERIALS AND METHODS

3.3.1.         Cowpea accessions under study

3.3.2.         Site description

3.3.3.         Experimental set up and preparation of cowpea leaves

3.3.4.         Moisture content

3.3.5.         Ash

3.3.6.         Minerals

3.3.7.         Crude protein content

3.3.8.         Crude fibre

3.3.9.         carotenes

3.3.10.     Ascorbic acid

3.3.11.     Nitrates

3.3.12.     Oxalates

3.3.13.     Total phenolic content

3.3.14.     Sensory evaluation

3.3.15.     Statistical analysis

3.4.               RESULTS

3.4.1.         Proximate analysis

3.4.2.         Beta carotene, ascorbic acid and total phenols

3.4.3.         Mineral elements

3.4.4.         Nitrates and oxalates

3.4.5.         Sensory evaluation

3.5.               DISCUSSION

CHAPTER FOUR

EFFICACY OF POST-HARVEST TREATMENTS IN MAINTAINING THE QUALITY ATTRIBUTES OF COWPEA LEAVES

4.1.               INTRODUCTION

4.2.               MATERIALS AND METHODS

4.3.               PARAMETERS ANALYSED

4.4.1.         Sensory evaluation

4.4.2.         Colour change evaluation

4.4.3.         Cumulative weight loss (percentage)

4.4.4.         Wilting

4.4.5.         Statistical analysis

4.4.               RESULTS

4.5.1.         Proximate analysis

4.5.2.         Beta carotene, ascorbic acid and total phenols

4.5.3.         Mineral elements

4.5.4.         Nitrates and oxalates

4.5.5.         Colour change during processing

4.5.6.         Sensory quality analysis

4.5.7.         Cumulative weight loss

4.5.8.         Wilting

4.5.9.         Colour change of fresh cowpea leaves in MAP

4.5.               DISCUSSION

4.6.               CONCLUSION

CHAPTER FIVE

CONCLUSION AND RECOMMENDATION

REFERENCES

APPENDICES

                                                          CHAPTER ONE

                                                            INTRODUCTION

1.0       Background to the Study

Agriculture has been the mainstay of Nigeria’s economy for decades. The sector contributes to nutrition, food security, employment and foreign exchange earnings. According to The Nigeria Economic Report (2013), agricultural sector directly accounts for about 26% of Nigeria’s Gross Domestic Product (GDP) and 27% indirectly through linkages with manufacturing, distribution and other service related sectors. The sector declined from a growth rate of 4.2% in the year 2012 to 2.9% in 2013 partly due to inadequate rainfall received in some growing regions in the country but increased to 3.5% in 2014 (Economic Survey highlights, 2014 and 2015). Agriculture industry accounts for 65% of Nigeria’s total exports, 18% and 60% of the formal and total employment, respectively. In addition it directly and indirectly supports the livelihood of 80% of the population living in the rural areas (Nigeria Economic Report, 2013). According to economic survey (2015), the value of marketed agricultural production declined marginally from Ksh 334.8 billion in 2013 to 333.2 billion in 2014.

The horticulture sector has been a major contributor to the good performance of the agricultural sector. In the year 2012 and 2013, the subsector contributed 26% and 25% respectively to the agricultural growth by value (Economic survey, 2014). By quantity, fresh horticultural produce contributed 205,700T in 2012, 213, 800Tin 2013 and 220,200T in 2014 becoming second after tea (Economic Survey highlights, 2014 and 2015). These statistics should be taken as an estimate to the actual horticultural growth since most of the horticultural commodities such as African leafy vegetables do not reach formal markets or export. In addition, many surveys do not inform of the farm gate prices of vegetables, farm level production quantity and value at the local and informal markets. Considering this situation, the horticultural subsector may be having far much more positive impact to the population than it is estimated.

The horticulture sector is made up of subsectors which include flower and ornamental, fruit and vegetable production. According to HCD report (2014), vegetables occupied the largest production portion at 32% by value, followed by flowers at 30%, fruits at 30%, and nuts at 5%. The major export destination is the European Union where the horticulture industry command about 30% market share. In general, vegetable production in Nigeria has been increasing steadily over the past years. In 2012, the quantity of vegetables exported was 66,352 tonnes valued at Ksh.20226 million. In 2013 exports increased to 77172 tonnes valued at Ksh.22923 million and in 2014, exports slightly reduced to 70335T valued at Ksh.18781 million (HCD Annual Report, 2014). Considering the report touched on exports, it means that the production is larger since the consumption in the country is larger compared to the exports. The major producing counties as reflected by exports include Meru, Bungoma, Murang’a, Kiambu and Kirinyaga in that order (HCD Annual Report, 2014).

Vegetable production is widely practised in the country. The development of the sector is due to the readily available markets with a higher marginal return per unit areas compared to cereal crops. Vegetable production has short growth cycles enabling farmers to have two to three seasons in a year. This combination, in addition to emerging health issues has placed vegetable production at a strategic position to expand even more.

Vegetables can be classified as exotic or indigenous depending on their origin, utilization and commercialization. Some exotic vegetables commercialized in Nigeria include kales, spinach, snow peas, French beans among others whereas indigenous vegetables include African nightshade, spider plant, cowpeas, amaranth among others. Major export vegetables comprise of exotic and Asian vegetables (EPC, 2014). Current trends have seen the incorporation of value addition strategies to ensure continuous availability and reduction of postharvest losses. Such strategies include canning, freezing, solar drying and/or roasting in addition to pre-packs for fresh produce meant for supermarkets.

Unlike other type of vegetables, trade and consumption of African leafy vegetables had been side-lined to serve the local population especially in the rural areas or among the poor and denied entry to formal markets. However, the HCD report,(2014) indicated that there has been a tremendous increase in production in ALVs in the country. This can be attributed to the awareness created to the population on the health benefits and nutritional superiority of these vegetables (Abukutsa, 2007) and value chain support by non-governmental organisations. As a result, in 2014 the acreage under ALVs increased by 10% and the yields and value rose by 5.6% and 6.2%, respectively (HCD Annual Report, 2014). According to AVRDC (2010), it is estimated that approximately 9000 tonnes of ALVs have been sold to formal and informal markets in the period between 2008 and 2010 in central Nigeria only.

Table 1: Performance of African Leafy vegetables 2012-2014

Source: HCD report (2014).

Nigeria, like many other tropical countries, is endowed with a great diversity of ALVs. These vegetables have a great social and economic importance for the local communities therefore making them part and parcel of their culture. The priority species grown and marketed in Nigeria include African nightshades (Solanum spp), amaranth (Amaranthus spp.), spider plant (Cleome gynandra), cowpeas (Vigna unguiculata), Ethiopian kale (Brassica carinata), ‘mitoo’ (Crotalaria ochroleuca and C. brevidens)), ‘kahuhura’ (Cucurbita ficifolia), jute plant (Corchorus olitorius) and pumpkin leaves (Cucurbita maxima and C. moschata). (Irungu et al., 2007). Among the ALVs, African nightshade accounted for 27% of market value followed by Spider plant and Cowpeas at 23% and 22% respectively in 2014 (HCD report, 2014). However, in terms of quantity produced, cowpea outperformed all other ALVs from 2012 to 2014 as shown in Table 1.

Cowpea (Vigna unguiculata) is an important grain legume in tropical and subtropical regions where a shortage of animal protein sources is often experienced (Tshovhote et al., 2003). Although a lot of emphasis has been put on the grain crop, the high potential in the vegetable has not been fully exploited (Abukutsa, 2003).

1.1       Problem Statement

Cowpea leaves has been viewed as a woman’s crop and therefore it has received little attention from stakeholders (Abukutsa, 2003).Nutritional information and quality among the cultivated cowpea varieties is scanty (Muchoki et al., 2007). However, the situation is worse among local cowpea accessions although they are preferred by farmers because of superior taste and palatability compared to improved varieties such as KVU, K-80 and M66 (KARI, 2010). The information available on the nutritional quality of cowpea leaves has been restricted to improved varieties and some few local accessions (Mamiro et al., 2011). However, the nutritional information available for the few lines studied has recorded a very large variability. For instance Mamiro et al., (2011) indicated that cowpeas crude protein ranges from 18 to 25%, Okonya and Maass (2014) found the protein content to be between 29.4 to 34.3% whereas, Ono et al., (1996) recorded as high as 43% crude protein content. On the other hand, ascorbic acid levels reported range of between 33.5mg/100g to 308 mg/100g (Muchoki et al., 2007; Ahenkora et al., 1998; Njoroge et al., 2015).

The potential of cowpea leaves has not been maximized due to post-harvest handling limitations (Affognona et al., 2014). It is estimated that post-harvest losses contribute to about 50% of total losses in the cowpea value chain (Masarirambi et al., 2010). The high losses can be attributed to lack of proper post-harvest knowledge, high perishability, poor processing practises and inefficient or high cost of post-harvest technologies. The situation is worsened during the periods of glut where production of these vegetables exceeds market demand.

1.2       Justification of the study

Local cowpea accessions have important significance to farmers although their nutritional quality has not extensively assessed (KARI, 2010). Nutritional profiling of superior local cowpea accessions study will compliment or add new information that will help in sensitizing the entire population to enhance utilization of cowpea vegetable. Ilelaboye et al., (2013) indicated that adequate nutritional information on ALVs will be useful for nutritional education to the public especially the vulnerable groups as a means to improving their nutritional status. The information will also enable further improvement of the local accessions which are thought to be adapted to wide range of climatic conditions (D’Andrea et al., 2007).

The high post-harvest losses on cowpea vegetable have led to led to decreased availability of the vegetable in households and markets (Shiundu and Oniang’o, 2007). The existing technologies to reduce post-harvest have been inefficient or expensive to the resource constrained farmer. Such technologies include sun drying, fermentation, charcoal cooling and refrigeration (Muchoki et al., 2007). However, very few technologies that suit small scale farmers been evaluated. Low cost methods of improving and lengthening shelf life such as modified atmosphere packaging and solar drying have been proposed (Chavasit et al., 2002) but have not been tested for ALVs. The reduction in post-harvest losses will be improved nutritional and food security and income.

1.3        General objective

To evaluate the nutritional quality attributes of local cowpea accessions and reduce loss of the quality attributes to improve the shelf life and enhance availability of quality vegetables and improve food security.

1.3.1    Specific objectives

1.            To compare the nutritional quality attributes of selected superior local leafy cowpeas accessions with an improved variety.

2.            To evaluate the effect of blanching, solar drying and modified atmosphere packaging on the quality attributes and shelf life of cowpeas leaves.

1.3.2        Null hypotheses

1.      Selected superior local cowpea accessions have similar nutritional content compared to the improved variety.

2.      Blanching, solar drying and modified atmosphere packaging does not preserve quality and does not improve the shelf life of cowpea leaves.