Eggplant (Revised)

Introduction 

Solanum melongena, commonly known as eggplant or aubergine, belongs to the family Solanaceae and the genus Solanum. The Solanum genus consists of more than 1000 species and is widely distributed, with approximately 100 native African species. The genus Solanum includes many economically important plants, including tomatoes, potatoes, and eggplants. 
The exact origin of S. melongena is not entirely clear, but it is believed to be native to South Central China, Laos, Myanmar, Thailand, and Vietnam. Today, they are widely grown in many countries, especially in tropical and subtropical regions, where the climate is suitable for their growth..
Eggplants are widely utilized as a food source in many cuisines worldwide. Their versatile nature allows them to be prepared in various ways, including grilling, roasting, frying, and baking. They are used in a wide array of dishes, including curries, stews, salads, dips, and spreads. Eggplants are prized for their ability to absorb flavors from other ingredients, making them a popular addition to many cuisines around the world. The young and almost mature fruits are used as a vegetable. The fruit is a good source of vitamin A and C, potassium, phosphorous and calcium. They may be roasted, fried, stuffed, cooked as curry or pickled. Cooking for prolonged periods will destroy most of the vitamins. In Kenya production is done throughout the year and the bulk of the crop is exported. The fruits of the white varieties have medicinal value for diabetics. Eggplant has a cropping period of 4-7 months. In Kenya they are normally planted from beginning of June to end of December to correspond with the export season from October to May.
Eggplants are low in calories and fat but rich in dietary fiber, vitamins, and minerals. They are particularly abundant in antioxidants like anthocyanins, which give eggplants their characteristic purple color and have been associated with numerous health benefits. Beyond culinary uses, eggplants have also been utilized in traditional medicine in some cultures. They have been believed to have anti-inflammatory properties and have been used to treat various conditions, including digestive issues and skin problems. 
(Daunay, M. & Chadha, M., 2004, Lester, R.N. & Seck, A., 2004, Bukenya-Ziraba, R., 2004, Kew botanical gardens, North Carolina Extension Gardener (n,d)).

Species account

Solanum melongena is an annual herb to perennial shrub growing to a height of 150-200 cm, often much branched with a long taproot which extends deep into the ground. Its stems and leaves are densely covered with star-shaped (stellate) hairs and sometimes prickles. Leaves: alternate, simple, with a petiole (appendage which connects the blade to the stem) 6-10 cm long. Flowers: are hermaphrodite, generally smooth and shiny with many seeds. Fruits: Globular to ovoid, ellipsoid in shape and available in purple, green, white, and variegated colors. The deep purple eggplant has very high gloss. 
The fruit is consumed while still immature, displaying a glossy and vibrant appearance. However, once it reaches maturity, the flesh becomes bitter and fibrous, and the seeds harden. Certain cultivars are enjoyed raw.
(Plant village. (n.d), Daunay, M. & Chadha, M, 2004, Kew botanical gardens).

Varieties of S. melongena
Seeds of the below listed varieties are commercially available in all seed companies in Kenya. Information on seed companies in Kenya can be obtained from Horticultural Crops Development Authority (md@hcda.or.ke www.hcda.or.ke +2542088469)
•    "Black Beauty" It takes about 100 days from transplanting to harvest. The fruit is oval to heart shaped glossy purple to almost black. Fruit size is about 10 x 8 cm. It has a good shelf life. It is unsuitable in cold wet areas.
•    "Florida High Bush" From transplanting to harvest is about 100 days. Fruit shape is elongate oval. Fruit size is about 15 x 8 cm. It is unsuitable for cold wet areas.
•    "Ravaya" It is an early maturing (about 80 days), high yielding variety. The fruits are slender, purple-coloured and borne in bunches of 3-4. This variety is popular for fresh export market.
•    "Long Purple". It is ready for harvest about 70-80 days after transplanting. Fruit shape is cylindrical. Fruit size is about 11 x 5 cm. It is unsuitable for cold wet areas
•    ."Early Long Purple". From transplanting to harvest is 90 days. Fruit shape is long cylindrical. Fruit size is about 12 x 6 cm. It is unsuitable to cold wet areas.

Related species
Solanum aethiopicum commonly known as African eggplant, is native to Africa, domesticated from the wild Solanum anguivi Lam., via the semi-domesticated Solanum distichum Schumach. & Thonn. Over time, it has spread beyond its native lands and can now be found in different countries across Asia, Europe, and the Americas.
Solanum aethiopicum is a perennial herbaceous plant typically growing to a height of 1 to 2 m. Leaves: alternate and can vary in shape, from ovate to lanceolate, sometimes featuring lobes. Flowers: star-shaped flowers displaying color variation, from elegant white to captivating purple. Fruits: edible fruits, show diversity in shapes, sizes, and colors, including green, yellow, red, and purple. They find their place as a delightful vegetable in numerous culinary creations, such as stews, soups, and stir-fries, offering a unique flavor and texture to dishes.
The leaves, too, have their use as leafy vegetables in cooking and contribute valuable nutrients to diets. In traditional medicine, different parts of the plant have been employed to address various health issues, such as skin infections, stomach ailments, and respiratory problems 
Within Solanum aethiopicum, four cultivar-groups are recognized, three of which hold particular significance in Africa. 

•    Gilo Group: The Gilo Group includes cultivars that are widely grown and valued for their large, round, and glossy fruits. Depending on the location, preferences vary for cultivars with pure white, creamy white, pale green, dark green, brown, or purple fruits, as well as striped fruits in two or more colors. These eggplants are highly versatile and used in various culinary dishes, providing a mild, pleasant taste. The Gilo Group is known for its adaptability to different environmental conditions, making it a popular choice among farmers and a preferred option for local markets and household consumption in Africa.

•    Shum Group are mainly known for its smooth mature leaves with tiny glandular hairs and non-prickly nature. The fruit is subglobose, about 1 to 3 cm in diameter. Primarily, it is used as a leafy vegetable, but ripe fruits are occasionally consumed too. This group is most prevalent in Central Africa, particularly popular in Cameroon and Nigeria. However, its popularity is even higher in Uganda, where it is locally called 'nakati.' It thrives in warm, high-rainfall areas or when grown under irrigation.
•    Kumba Group: mainly found in hot, semi-arid regions of the Sahel. Its mature leaves are smooth, except for tiny glandular hairs, and they are not prickly. The fruit is depressed globose, deeply furrowed, and often divided into many small vesicles, measuring 5-10(15) cm in width. Both the fruits and occasionally the leaves are consumed. Its importance for Africa is not as pronounced as the other three groups.
•    Aculeatum Group: have thorny stems and leaves with stellate hairs when mature. Its fruit is subglobose and furrowed, measuring 3–8 cm in diameter. While it is not consumed as food, it is grown for ornamental purposes or used as rootstock for tomatoes or eggplants. The cultivar is not cultivated in Africa.
(Lester, R.N. & Seck, A., 2004, Schippers, R. R, 2000)

Solanum anguivi is native to Africa, widely distributed on the African continent and its neighboring islands and Arabia. It has been recorded from West Africa, as well as Central Africa, East Africa, southern Africa and Madagascar, but it probably occurs in all non-arid regions throughout tropical Africa. It grows mostly in the wild, but sometimes, e.g. in Uganda and Côte d’Ivoire, it is a semi-cultivated vegetable.
S. anguivi is an upright woody herb or shrub, growing to a height of 4 m. Stems and leaves armed with straight or somewhat curved spines, yellowish to brownish, sometimes purple near the base, up to 13 mm long, branches often purple tinged. All parts covered in stellate hairs. Leaves: rhombic-ovate, elliptic or lanceolate, thinly stellate hairy above, densely so below. The central ray of the stellate hairs often much longer than the lateral rays. Leaf margin subentire to triangularly lobed. Prickles usually present on the midrib and main veins. Flowers; arranged in clusters of up to 20 blooms and have a pale mauve or purple color, with a star-like shape. Fruits: 6-12 mm in diameter, spherical, green, turning yellow and glossy orange-red when ripe. Edible when mature.
(Hyde, M.A., et al.,2023, Bukenya-Ziraba, R., 2004).

Ecological information

Optimum day temperatures for eggplant are in the range of 25-35degC and night temperatures from 20-27degC. Eggplant is more susceptible to low temperatures than tomato and capsicum and it does not tolerate frost. It is tolerant to drought and excessive rainfall, but struggles to grow when temperatures exceed 30degC, and where water logging occurs. When temperature and humidity are high, eggplant becomes more vegetative. Eggplant does best in well-drained, sandy loam soils. The best environmental conditions are normally found in lowland areas with relatively little temperature variation. When grown at altitudes above 800 m, growth is retarded and yields reduced. The pH requirements range from 6 to 7 with the optimum being pH 6.4. 

Agronomic Aspects 

a.    Planting materials
Eggplant is commonly propagated through seeds, but it can also be propagated by rooting healthy shoots. African eggplant seeds can be obtained from seed companies or directly from farmers. It is advisable for farmers to produce their own seeds to ensure self-sufficiency. When cultivating plants for seed production, it is best to isolate them from other solanaceous plants such as tomatoes and potatoes, preferably on soils that have not recently grown these crops. For optimal seed production, a wider seed spacing of 60 cm x 20 cm is recommended, resulting in a seed rate of 1.5 kg seed/ha
Soaking the seeds in water for 24 hours speeds up germination. No treatment is needed when sowing fresh, vigorous seed in sterilized soil. Otherwise, soak seeds in warm water (50 degC) for 30 minutes, rinse them in cold water, and dry them before sowing. For information on hot-water treatment for seeds click here (Kalb et al., 2002). 

b.    Planting 
Disease-free seeds may be sown directly or started in a nursery. The vegetable can be grown as a pure stand or intercropped with other food crops. They can be grown alone or as intercrops with other food crops. To ensure optimal growth, it is important to properly prepare the land. This involves digging to loosen the soil and remove any hardpan. In drylands, terracing is recommended to retain rainwater within the farm. After digging, well-decomposed manure is incorporated into the soil to create a well-drained structure with fine particles.
The prepared land is then divided into beds, preferably one meter wide. In wet and humid areas, raised beds are suitable, while sunken beds work well in dry areas or seasons. Additionally, farmyard manure or compost is applied and thoroughly mixed. To sterilize the soil, prevent soil-borne diseases, and control weeds, a layer of grass or similar material can be burned on top of the bed. This also adds a nutrient-rich ash, especially potash, to the soil. (Kalb et al., 2002).
Nursery Bed management 
African eggplants thrive when initially grown in a nursery bed and later transplanted to the main garden. To create the nursery bed, select a location away from drainage channels or slopes, yet accessible to a water source. Use friable soil (soil that breaks or crumbles easily when handled). In humid areas, raise the nursery bed at least 20cm above the ground to prevent waterlogging, while in arid and semi-arid regions, make it sunken to reduce water loss, shaded seedbed is recommended. Before sowing an application of 3 to 5 kg of good compost per m2 is incorporated into the nursery seedbed. 
Seeds can be sown by broadcasting or in rows (row planting). Mix the seeds with fine soil at a ratio of about 1:10 (seed to soil). If broadcasting, combine the seeds with dried poultry manure, ash, fine soil, or sand at a ratio of 1:10 to 1:20. Create 2cm deep furrows spaced 15cm apart. Sow the seeds and lightly cover them with soil. Apply mulch and water through it, preferably in the evening. The seedlings emerge after 8-10 days. Before planting, fertilize the field with compost or farmyard manure.. Remove the mulch from the seedlings and place it between the rows. Provide shade over the nursery using light materials that allow sunlight to reach the seedlings.
Transplant the seedlings after 1-1.5 months of germination. The ideal transplant is a seedling with three to four true leaves, stocky and disease-free, and without flower buds. Begin hardening plants 6 to 9 days before transplanting to reduce transplanting shock. Slightly withhold water. Thoroughly water seedlings 12 to 14 hours before transplanting to the field. Transplanting should be done in the late afternoon or on a cloudy day in order to minimise transplanting shock. Space the plants 1m apart between rows and individual plants. Prior to transplanting, add two handfuls of well-decomposed manure per planting hole (Kalb et al., 2002). Transplant seedlings by digging a hole deep enough to bury a plant so that its first true leaf is just above the soil surface. Press the soil firmly around the root. Irrigate furrows immediately after transplanting. 
Watering newly transplanted plants well with compost tea or EM (effective microorganisms) will give the seedlings a good start. To prepare your own compost tea, mix 1 part of compost with 6 parts of water. Leave the mixture for one week. Strain and spray on seedlings to control fungal pathogens and prevent infection. 
Transplanting and spacing 
The ideal transplant is a seedling with three to four true leaves, stocky and disease-free, and without flower buds. Begin hardening plants 6 to 9 days before transplanting to reduce transplanting shock. Slightly withhold water. Thoroughly water seedlings 12 to 14 hours before transplanting to the field. Transplanting should be done in the late afternoon or on a cloudy day in order to minimize transplanting shock. Before transplanting, place a basal dose of 15 to 30 tons/ha compost or well-rotted farmyard manure in the planting furrows or planting holes and mixed with the soil. 
Transplant seedlings by digging a hole deep enough to bury a plant so that its first true leaf is just above the soil surface. Press the soil firmly around the root. Irrigate furrows immediately after transplanting. 
Watering newly transplanted plants well with compost tea or EM (effective microorganisms) will give the seedlings a good start. To prepare your own compost tea, mix 1 part of compost with 6 parts of water. Leave the mixture for one week. Strain and spray on seedlings to control fungal pathogens and prevent infection (Kalb, T et al, 2002).

Direct sowing
Direct sowing either in irrigation systems or rain-fed results in taller plants and when there is adequate room, in more and larger leaves and branches and better dry-matter content when compared with transplanting. The seeds are planted by broadcast or in lines. If by broadcast, one should ensure even distribution. A seed rate of 1.5kg/ha is used (Kalb et al., 2002).

Thinning
After germination, the strongest plants are retained, and the others removed for transplanting or consumption. Thinning should be done with the type of nightshade and hence its growth habit and size in mind. Four weeks after planting, the crop is thinned (starting with taller ones) to leave a spacing of about 15 cm from one plant to the other during the dry season. Thinning can be continued on regular basis until the desired spacing is reached, which depends a lot on the species. During thinning, the whole plant is uprooted. Thinned plants should be brought to the marketplace with their roots attached and moistened to retain their fresh appearance.
Crop rotation
Eggplants should not be planted after tomato, pepper, potato, or other solanaceous crops to prevent a recurrence of the same pests and disease pathogens. Rotate eggplants with other crops like onions, cereals or legumes. Planting eggplant after rice reduces the incidence of bacterial wilt and nematodes. 
 
Husbandry
Side dressing with groundnut cake is recommended 40 days after transplanting. Also at this period, remove three nodes at the tips of the plants to improve branching and to increase the number of fruits. Weed control should be shallow, to avoid damage to the roots. Tall-growing cultivars will also require support. Supplementary irrigation is required during dry periods. Mulching with dried plant materials reduces moisture loss and weed problems. Apply well-decomposed farm yard manure and neem cake as basal fertilizers.

Harvest, post-harvest practices and markets

Harvest
The first harvestable fruits appear 60-90 days after planting, when the skin becomes glossy, smooth, and fully colored. Harvest fruits when they are about two-thirds of their maximum size. Harvesting is done once or twice a week by cutting the fruit from the stem and leaving a short piece of stem on the fruit. For seed production, harvest only fully mature fruits from healthy and productive plants. Yields of 20 to 25 tons/ha of immature fruits can be expected. 

Post-harvest practices.
After harvesting, sort the eggplants, remove any damaged, overripe, or diseased fruits. Gently clean the eggplants by wiping off dirt and debris using a soft cloth or brush. Eggplant fruits are prone to rapid dehydration after being harvested, causing them to lose their color, brightness, and smoothness. It is advisable to sell the fruits within a few days after harvesting. To preserve freshness, store the harvested eggplants in a cool and well-ventilated area. When packaging, make sure to use ventilated containers to prevent moisture buildup and decay. If stored under controlled conditions, the fruits can be kept for up to 10 days, but it's essential to maintain a storage temperature above 15°C to avoid cold injury.
(Daunay, M.-C. & Chadha, M.L., 2004, Kalb et al., 2002)

Value chain and Markets 

Eggplant has a strong presence in the global market, and its popularity has been steadily increasing due to its versatility in various cuisines and its nutritional benefits. As a result, the demand for eggplant and its processed derivatives has been growing steadily in recent years. Major producers include China, India, Egypt, Turkey, and Iran. China accounts for more than 60 % of world's eggplant. These countries benefit from favorable climatic conditions and have well-established agricultural practices to support large-scale eggplant cultivation and production.
Africa contributes less than 4% to the global production and cultivation area of eggplant, with the majority, over 90%, coming from northern African countries. However, comprehensive data on eggplant cultivation in tropical Africa remains incomplete, and it may encompass various types of African eggplants, such as S. aethiopicum and S. macrocarpon. Local markets and street vendors play a significant role in selling eggplants to the general population, making them easily accessible to consumers. Additionally, supermarkets and grocery stores in urban areas also stock eggplants, making them available to a broader consumer base. With the increasing urbanization and changing dietary preferences in Africa, the demand for eggplants is expected to rise, presenting opportunities for further value addition in the form of processed eggplant products (Daunay, M.-C. & Chadha, M.L., 2004, Tridge.com).

Fresh Quality Specifications for the Market in Kenya
The following specifications constitute raw material purchasing requirements.

© S. Kahumbu, Kenya

Nutritional value 

Eggplant is a versatile vegetable that can be prepared by baking, roasting, grilling, or sautéing, and relish. 
The fruit vegetable is low in calories, making it an excellent choice for individuals aiming to maintain or lose weight. Eggplant is a great source of dietary fiber, which aids in digestion and promotes a healthy gut. Fiber helps regulate bowel movements, prevents constipation, and may even reduce the risk of colon cancer.
Except for vitamin A, eggplants are low in most nutrients. Eggplants are however rich in antioxidants, specifically nasunin, chlorogenic acid, and anthocyanins. These powerful compounds help protect the body's cells from damage caused by oxidative stress, reducing the risk of chronic diseases like heart disease and certain types of cancer.
Moreover, eggplant is a low-glycemic food, meaning it has a minimal impact on blood sugar levels. This quality makes it an ideal choice for individuals with diabetes or those aiming to manage their blood sugar levels. Including eggplant in your diet may also provide anti-inflammatory benefits. Studies have shown that certain compounds found in eggplant can help reduce inflammation in the body, potentially alleviating symptoms associated with conditions like arthritis. To maximize the nutritional benefits, it's recommended to consume eggplant with its skin intact, as it contains a significant portion of the vegetable's antioxidants and fiber.
(Naeem & Ugur, 2019, KFCT, 2018, healthline.com, medicalnewstoday.com).

Table 1: Approximate nutritional composition of 100 g of eggplant

Source (Nutrient data): FAO/Government of Kenya. 2018. Kenya Food Composition Tables. Nairobi, 254 pp. http://www.fao.org/3/I9120EN/i9120en.pdf

RE=retinol equivalents.

RAE =Retinol activity equivalents. A RAE is defined as 1μg all-trans-retinol, 12μg beta-carotene, or 24μg α-carotene or β-cryptoxanthin.

^a Lewis, J. 2019. Codex nutrient reference values. Rome. FAO and WHO

^b NHS (refers to saturated fat)

^c https://www.hsph.harvard.edu/nutritionsource/water/

^d British Heart Foundation

^e FDA

^f NIH

^g Mayo Clinic