Papaya (Revised)

Introduction

Papaya is a widely cultivated fruit tree in the tropics and subtropics. It is a popular fruit in Kenya. Ripe papaya is a favourite breakfast and dessert fruit that is available year-round. It can be used to make fruit salads, refreshing drinks, jam, jelly, marmalade, candies and crystallised fruits. Green fruits are pickled or cooked as a vegetable. Young leaves are sometimes eaten. In some countries, seeds are used as vermifuge and to induce abortion (abortifacient). Pawpaw (Carica papaya) is a member of the family Caricaceae and the genus Carica. The genus consists of about 35 recognized species but Carica papaya is the one of the most economically significant species. 

It is native to lowland tropical areas of Central and South America, but is now grown in tropical to subtropical regions (Asia, Africa, Oceania, and North America). 
Papaya is a fast-growing, tropical fruit tree with a single stem or trunk and large, deeply lobed leaves. It thrives in well-drained soils and requires a warm climate with abundant sunlight to grow optimally. The plant is highly sensitive to frost and cannot tolerate cold temperatures. It prefers a moderate amount of rainfall and can withstand drought conditions for short periods.
Papaya is widely used as a food source due to its delicious, sweet-tasting fruit. The ripe fruit is consumed fresh, either on its own or in various culinary preparations such as salads, smoothies, and desserts.

Unripe green papaya is also used in savory dishes and pickles. Young leaves are sometimes eaten. In some countries, seeds are used as vermifuge and to induce abortion (abortifacient). Carpaine, an alkaloid present in papaya, can be used as a heart depressant, amoebicide and diuretic. The fruit and juice are eaten for gastrointestinal ailments; a fresh leaf poultice is used to treat sores. In some countries papaya is grown in sizeable plantations for the extraction of papain, an enzyme presents in the latex, collected mainly from the green fruit. Papain has varied uses in the beverage, food and pharmaceutical industries: in chill-proofing beer, tenderizing meat, drug preparations for digestive ailments and treatment of gangrenous wounds. It is also used in treating hides, degumming silk and softening wool. Papaya leaves are also used in traditional medicine for various purposes, including treating skin wounds and reducing fever. 
Papaya is highly valued for its rich nutritional content. It is an excellent source of vitamin C, vitamin A (beta-carotene), and folate. It also provides a good amount of dietary fiber, potassium, and antioxidants, making it beneficial for overall health and immune function.
In addition to being consumed fresh, papaya can be processed into various value-added products such as papaya puree, papaya juice, dried papaya, and papaya-based jams or preserves. These products are often exported to international markets, especially in regions where fresh papaya is not readily available year-round.
(Orwa et al., 2009, GBIF secretariat, 2021, Heuzé V., Tran G., 2015, Koul, B., et la., 2022)

Species account

Pawpaw is an evergreen, tree-like herb growing to a height of 2-10 m tall with a trunk 20cm across narrowing to a crown of leaves . Trunk: unbranched, contains white latex in all parts, 10-30 cm in diameter, hollow and spongy-fibrous tissue. Bark: Pale grey, smooth with prominent leaf scars of fallen leaves . Leaves: up to 60cm across, deeply palmately lobed, broadly toothed and held in hollow stalks 60 cm long. Flowers: plants can be dioecious (having male and female flowers in separate tree) while others are hermaphroditic (having both male and female flowers in one tree). Flowers are tiny, funnel-shaped, fragrant, yellow to white, males in long raceme, females in small clusters or sometimes solitary. Fruits; large fleshy berry, hanging in clusters attached to the stem top just below the leaves, smooth green skin that turns yellow or orange when ripe. Flesh: thick and succulent, ranging in color from yellow to red or orange. Seeds: numerous, small, black, round, wrinkled covered with gelatinous aril. Small latex vessels extend throughout the tree and are particularly abundant in fruit that has reached full size but has not yet begun to ripen.
(Orwa et al., 2009, Mbuya et al., 1994, Plantvillage (n.d), Koul, B., et la., 2022)

Pawpaw varieties
Major varieties include:
•     'Honey Dew'. This is an Indian variety of medium height that produces oval juicy medium size fruit.
•    'Kiru'. Is a Tanzanian variety that produces large fruits. It is a high yielder of papain.
•    'Mountain'. Originally the name for a variety grown at high altitudes with very small fruits only suitable for jam and preserves. Now the name is also used for a medium size variety with good fresh consumption qualities such as firm sweet tasting yellow flesh.
•    'Solo'. It is a Hawaiian variety that produces small round very sweet fruits with uniform size and shape. It is hermaphroditic.'
•    'Sunrise Solo'. Hawaiin variety that produces smooth pear shaped fruit of high quality, weighing 400 to 650 g. The flesh is reddish orange. This variety is high yielding.
•    'Sunset': Hawaiian variety with red flesh and having same characteristics as 'Solo'
•    'Waimanalo'. Hawaiian variety that produces smooth, shiny round fruits with short neck and is of high quality. The flesh is orange yellow, thick, sweet and firm.

Most of commercial varieties grown in Kenya are derived from Hawaii. A few are from India and some known as 'Mountain varieties' whose origin / source is a rather not explained. None of these have been reported to haveresistance to Phytophthora palmivora. However, information available claims that Hawaiian lines such as 'Waimanalo 23', 'Waimanalo 24' and 'Line 40' exhibit resistance to P. palmivora. 
Another serious disease problem with papaya is papaya ring spot virus. However, according to a recent report by PIP COLEACP (www.coleacp.org/en), there are no commercial papaya varieties, except for transgenic, which are tolerant or resistant to papaya ring spot virus or bunchy top virus. 

Ecological conditions

Papaya thrives in warm areas with adequate rainfall and a temperature range of 21-33degC. Its altitude range is similar to that of the banana, from sea level to elevations at which frosts occur (often around 1600 m). However, they grow best in areas below 1000 m. The quality and yield are low at higher altitudes. Frost can kill the plant, and cool and overcast weather delays fruit ripening and depresses fruit quality. Fruit tastes much better when grown during a warm sunny season. Evenly distributed annual rainfall of 1200 mm is sufficient if water conservation practices are employed. Plantations should be in sheltered locations or surrounded by windbreaks; strong winds are detrimental, particularly on sandy soils, as they cannot make up for large transpiration losses. 
Papaya grows best in light, well-drained soils rich in organic matter with soil pH of 6.0-6.5. It can tolerate any kind of soil provided it is well-drained and not too dry. The roots are very sensitive to waterlogging and even short periods of flooding can kill the plants. 

Agronomic aspects

Papaya is propagated by seed. To reproduce the desired characteristics, it is best to get seeds through controlled pollination. The fleshy outer layer of the seed coat (sarcotesta) enveloping the seed is removed because it inhibits germination. This is achieved by rubbing the seed together against a fine-meshed screen under running water. Thoroughly dried seeds stored in air-tight containers remain viable for several years. Seeds are sown in small containers (tin cans, plastic bags or paper cups) at the rate of 3-4 seeds per container. Use of sterilized soil minimizes losses resulting from nematodes and damping-off fungi. Germination takes 2-3 weeks. Another practice is to sow the seeds in sterilised nursery beds and to prick out at the 2-3-leaf stage, transferring 3-4 seedlings to each container. Seedlings are transplanted about 2 months after sowing when they reach the 3-4-leaf stage or 20 cm height, preferably at the onset of the rainy season. During transplanting, take care not to disturb the roots. Older seedlings recover poorly after planting out. 
Papaya needs adequate drainage and is often planted on mounds or ridges. Transplants must be watered regularly until they are established. Field spacings are in the order of 3 x 2 m to 2.50 x 1.60 m, giving densities of 1667 and 2500 plants/ha respectively. The same densities are obtained by planting in double rows spaced (3.25+1.75) x 2.40 m or (2.50+1.50) x 2 m. Thinning to one female or one hermaphrodite plant per hill is done when the plants reach the flowering stage. In the absence of hermaphrodite plants, 1 male plant per 25 - 100 female plants is retained as pollinator. 
Papaya plants grown from seed produce fruits of different shapes, sizes, colour and even taste. Vegetative propagation of papaya provides a solution to most of these problems. The clone is selected for higher productivity and good quality fruits besides agronomic qualities such as dwarfness for easy harvesting and good resistance to diseases. Propagation of papaya using tissue culture is fast gaining popularity, mainly because tissue culture has numerous advantages over other conventional methods of propagation. Tissue culture facilitates rapid production of disease free plants. In Kenya such plants are available from Kenya Agricultural Research Institute (KARI), Thika as well as several private companies. 
Planting holes of 60 x 60 cm and at least 50 cm deep are prepared with 1 bucket of compost and a handful of rock phosphate is mixed in with the dug out soil and returned around the plant. Firm the soil and water liberally and add mulch around the young plant. 

Intercropping
Papaya grows best when planted in full sunlight. However, it can be planted as an intercrop under coconut, or as a cash crop between young fruit trees such as mango or citrus. Low growing annual crops such as capsicums, beans, onions and cabbages are suitable good intercrops. 

Husbandry
Clean cultivation is standard practice and weed control, particularly around the small plants, is very important. If weeds are only slashed - resulting in a grassy weed cover - papaya plants suffer severe competition. Experimental work shows a very good response to mulching. Irrigation is needed to minimize the abortion of flowers and maintain growth during the dry season. Watering once a week is recommended. Papaya is a fast-growing crop that requires a lot of nutrients. The use of manure and mulch steadies the release of nutrients. Calcium deficiency depresses growth and fruit set and enhances fruit drop; liming (to a pH of about 6) is the remedy. 

Harvest, post-harvest practices and markets 

Harvest
The stage of physiological development at the time of harvest determines the flavour and taste of the ripened fruit. The appearance of traces of yellow color on the fruit indicates that it is ready for harvesting, but leaving them on the tree longer increases the fruit's sugar content. Fruits harvested early have longer post-harvest life, but give abnormal taste and flavour. The fruits also tend to shrivel and suffer chilling injuries when refrigerated. The fruit is twisted until the stalk snaps off or cut with a sharp knife. Yields per tree vary from 30 to 150 fruits annually, giving 35 to 50 tons of fruit per ha per year. A papaya plantation can be productive for over 10 years but the economical period is only the first 3 to 4 years. It is therefore advisable to renew the plantation every 4 years.
For papain production, latex is collected by tapping the green unripe fruit. Four longitudinal incisions, skin-deep and 2 to 3 cm apart are made with a sharp, non-corrosive rod (glass, plastic or horn). Latex is collected in a clean glass or porcelain container and dried, or a canvas covered tray fixed onto the trunk of the tree. The latex is later scraped off the canvas with a wooden scraper and dried. Fruits may be tapped once a week, until they show signs of ripening. The operation is best done early in the morning (before 10:00) because the latex flows slowly in hot weather. Tapping results in ugly scars on the fruit, although quality is unaffected. Tapped fruit can be processed or used as animal feed. The papain producing trees are productive for 2 to 3 years, with the first 2 years being the most productive. If kept longer production is uneconomical

Markets 
Papaya is primarily cultivated for its delicious and nutrient-rich fruit. Papaya is produced at commercial scale in 61 countries of the world (FAOSTat, 2018). Key producers of papaya include countries such as India, Brazil, Nigeria, Indonesia, and Mexico. The primary importers of fresh papaya include the United States, Europe, and Asia. 
The fruit is consumed fresh, either on its own or as an ingredient in various dishes such as salads, smoothies, and desserts. It is also processed into juices, jams, and dried fruit snacks. The global papaya market is driven by increasing consumer demand for healthy and exotic fruits.
In African countries, including Nigeria, Kenya, and Uganda, papaya is consumed as a staple fruit due to its accessibility and affordability. Local markets play a crucial role in distributing fresh papaya to urban and rural populations

Nutritional value 

Pawpaw is renowned for its delightful taste and numerous nutritional advantages. It is a low-calorie making it a suitable choice for those looking to manage their calorie intake. Papaya is a good source of carbohydrates, primarily in the form of natural sugars and dietary fiber which supports digestion and helps maintain healthy bowel movements.
Papaya is well-known for its high vitamin C content, which is an antioxidant that supports the immune system and skin health. Additionally, the presence of beta-carotene, a precursor to vitamin A, contributes to maintaining good vision, skin integrity, and immune function. The fruit also delivers vitamin E, which acts as an antioxidant, and contains folate (vitamin B9), important for cell division and particularly vital during pregnancy.
C. papaya is a source potassium, an electrolyte that helps regulate fluid balance, nerve signals, and muscle functions, as well as magnesium, a mineral crucial for various biochemical processes in the body in the body, including muscle and nerve function. Papaya contains a papain enzyme papain, which has been associated with digestive benefits. It can help break down tough protein chains found in muscle meat, making papaya a popular choice for tenderizing meat for thousands of years. The fruit also boasts antioxidants like carotenoids and flavonoids, which protect cells from oxidative stress and offer potential health benefits.
The nutritional content of papaya can vary as the fruit ripens. Unripe green papayas contain lower levels of sugars and some nutrients but are a source of other enzymes like chymopapain, which also has digestive properties. 
(Healthline (n.d), Webmed (n.d)

Furth reading:
•https://www.healthline.com/nutrition/8-proven-papaya-benefits#TOC_TITLE_HDR_8
•https://www.webmd.com/diet/health-benefits-papaya

Table 1: Proximate nutritional value of 100g of papaya

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

Nutritive Value per 100 g of edible Portion

*Percent Daily Values (DV) are based on a 2000 calorie diet. Your daily values may be higher or lower, depending on your calorie needs.