Postharvest Treatments of African Leafy Vegetables POSTHARVEST ...

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Postharvest Treatments of African Leafy Vegetables POSTHARVEST TREATMENTS OF AFRICAN LEAFY VEGETABLES FOR FOOD SECURITY IN KENYA: A REVIEW Gogo EO1,2*, Opiyo A2, Ulrichs C1 and Huyskens-Keil S1 1 Humboldt-Universität zu Berlin, Faculty of Life Sciences, Division Urban Plant Ecophysiology, Lentzeallee 55-57, 14195 Berlin, Germany 2 Egerton University, Department of Crops, Horticulture and Soils, P.O. Box 536, 20115 Egerton, Kenya *Correspondence: [email protected] Abstract African leafy vegetables (ALVs) play a significant role in food security of smallholder farmers in rural and urban/peri-urban areas. In the human diet, they serve as vital sources of protein, fiber, minerals and vitamins as well as health promoting secondary plant compounds. In addition, ALVs create employment opportunities for women and youth in the society, hence providing a viable option to alleviating food insecurity. Consumption of ALVs in African countries, including Kenya has increased recently. However, while farmers still rely on the traditional methods of handling of these vegetables along the value chain, the magnitude of postharvest losses of ALVs in Kenya can reach up to 50%, being attributed to inadequate conditions during transport, storage and marketing. Inadequate postharvest handling and facilities for storage and transport, inappropriate processing methods, insufficient hygiene conditions in the markets and poor infrastructure aggravate these problems, causing massive losses along “the field to consumer” chain. Despite all these impressive qualities and importance of ALVs to the society, studies are still limited especially on postharvest handling and preservation techniques. Thus, the paper reviews various studies on common postharvest handling and postharvest treatment of traditional ALVs grown and consumed in Kenya. This paper can therefore provide a basis for future studies on appropriate postharvest treatments of ALVs with emphasis on contributing to food security concerns. Keywords: Postharvest handling, processing, preservation, indigenous African leafy vegetables Introduction Average consumption of vegetables in Sub-Saharan African countries ranges from 27 kg to 114 kg per person per year, far below the WHO/FAO minimum recommended levels of 146 kg per capita per year (400 g per day, fruit and vegetables combined) (Ruel et al., 2005). It is therefore obvious that micronutrient deficiency is a serious problem in Sub-Saharan Africa (Nyaura et al., 2014). African leafy vegetables (ALVs) are cheap and readily affordable to many low-income communities in rural, peri-urban and urban areas. They are not only rich in macro- and micro-nutrients but

Afr. J. Hort. Sci. (June 2016) 9:32-40

also possess bioactive compounds with antioxidant potential (Kamga et al., 2013). Therefore, they can be crucial for the food and nutrition security of poor families, especially during the drought periods (Ngugi et al., 2007). The number of commonly produced and consumed ALVs in Kenya is enormous. At least 200 varieties have been documented but only a few have been recognized (Schippers, 2002). Many of these are either cultivated or gathered from the wild, and consumed by most rural people and presently urban and peri-urban dwellers (Onyango and Imungi, 2007). A wide variety of these

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vegetables are available in all parts of Kenya, with more consumption being concentrated in the Coast, Nyanza and Western Provinces (Kimiywe, 2007). Despite these beneficial attributes, ALVs have generally been neglected by researchers and national agricultural programs because of their association with poor rural lifestyles, which means they are often regarded as a low-status food. This contributed to severe postharvest losses which are also a result of lacking knowledge on appropriate postharvest treatment and preservation technologies of ALVs (Kinyuru et al., 2012). ALVs have been reported to suffer postharvest losses of up to 50 %. Major postharvest losses reported in Kenya are due to insufficient preharvest conditions, insect pest and diseases, poor storage conditions, and poor handling along the value chain. Postharvest handling and treatments (e.g cleaning, sorting, grading, cold storage, packaging, blanching, drying, and fermentation) in Kenya have been reported to improve shelf-life and quality as well as reducing microbial contamination of ALVs (Ayua and Omware, 2013). On the other hand, other studies indicate that drying and blanching may reduce quality of ALVs (Kasangi et al., 2010; Chege et al, 2014). However, farmers in Kenya still rely on traditional postharvest treatment methods. Some of these postharvest treatment methods include sprinkling cold water on leaves to maintain freshness, sun drying, poor packaging (gunny bags and non-perforate


polythene bags). These unfavorable conditions are even more serious during rainy and dry seasons where these vegetables exist in abundance and scarcity, respectively. During this period when ALVs are scarce, many rural, peri-urban and urban dwellers have a limited availability of leafy vegetables. Therefore, this contributes to lack of dietary diversity or malnutrition of the local populations. Currently, there are opportunities to use ALVs in the country to expand the local food base, improve health, enhance food security and generate income (Kamga et al., 2013). In recent years, nutritionists have strongly emphasized their use in the human diet due to their health-promoting qualities, including management of HIV and AIDS (Friis et al., 2002). In addition, ALVs have several advantages over their exotic counterparts such as; well adapted to local climatic conditions, more resistant to pests and diseases, can be intercropped easily since they are found growing in the wild as weed, some have medicinal properties, have high yield potential and have high nutritional value (Table 1). This paper aims at reviewing the various postharvest treatment and preservation methods (e.g packaging, blanching, drying, and fermentation) presently used in Kenya to preserve nutritional and health related quality compounds of ALVs, providing a basic knowledge for the improvement of simple and affordable postharvest technologies in order to contribute to food security concerns in Kenya in the future.

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Postharvest Treatments of African Leafy Vegetables Table 1: Leaf yield and nutritional value of selected Kenyan indigenous vegetables, per 100 g fresh edible portion Scientific name

Yield ton ha-1

ßCarotene (mg) 6-8

Vit. A (mg)

Vit. C (mg)

Ca (mg)

Fe (mg)

5.4-10

Crude protein (g) -

5.7

70-100

152-400

10-15

Dry matter (g) 15-20

Vigna Unguiculata (L.) Walp. Solanum scabrum L. Gynandropsis gynandra L. Amaranthus spp.

30-80

3-6

8-10

8.8

40-140

250-442

5-17

18-22

10-13

5-10

6-19

8.71

130-180

262-434

11-15

15-20

45

4-5

5-10

10.7

90-160

480-800

5-15

11-15

Source: Abukutsa-Onyango, 2003 Postharvest treatment of ALVs The most serious threat to the survival of humanity is the ever-increasing gap between population growth and food supply (Shiundu and Oniang’o, 2007). Changing lifestyles have led to the increasing use of convenient foods at home and in food outlets. Postharvest treatments of vegetables have led to high convenience and nutritional value which is advantageous to consumers and food services (Smith and Eyzaguirre, 2007). However, postharvest treatment of agricultural produce is one of the central problems facing developing countries, including Kenya. Owing to the lack of and/or inadequacy of postharvest treatment technologies, large quantities of urgently needed food, especially ALVs are lost. Onyango et al. (2007) reported 3.1 %, 3.5 %, 4.2 % and 5.5 % losses of Gynandropsis gynandra, Solanum scabrum, Vigna unguiculata and Amaranthus spp., respectively, due to wastage as a result of excessive wilting, in Nairobi groceries alone. These problems will be aggravated by the increasing dietary needs of growing populations in these countries as the majority of the population in Kenya currently depends on such ALVs. In Africa and Kenya in particular, the lack of and/or inadequacy of postharvest treatment technologies exists with many vegetable varieties (especially ALVs) resulting in wastage during the in-season (November to May) and limited supply during


the off-season (June to October) accompanied by high prices (Habwe, 2008), because most locally available vegetables are seasonal and not available year-long. ALVs cannot be marketed fast enough when they are in-season owing to their high perishability and thus, limited shelf-life and storability. This forces farmers to sell soon after harvest (Shiundu et al., 2007). Accordingly, market sellers and supermarkets strive to sell all the supplies on the day of delivery and whatever remains at the end of the day may be discarded as having lost saleable value. This was reported to be a major problem of ALVs sold in Nairobi contributing to heavy postharvest losses (Onyango et al., 2007). In addition, wilting was also indicated to be a challenge as ALVs deteriorate faster especially at ambient temperature where trading of these vegetables is mostly done. Appropriate preservation and adequate storage methods are necessary in order to extend the consumption period of these nutrient-rich foods all year round (Habwe et al., 2008). According to Smith and Eyzaguirre (2007), there is a need to develop and promote appropriate handling and processing techniques to minimize postharvest losses and ensure regular supplies of ALVs from the production areas to consumers in peri-urban and urban centers. Commonly applied postharvest treatments of fresh and preserved ALVs in Kenya are presented in Table 2.

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Table 2: Postharvest treatment of selected African leafy vegetables (ALVs) in Kenya ALV

Consumed fresh X

Dried

Blanched

Fermented

X

X

X

Shelf-life (days)** 3

Gynandropsis gynandra L. 4-8 Vigna unguiculata (L.) 4-8 Walp.

X X

X X

X X

X X

3 4

-

Solanum scabrumL.

X

X

X

X

3

-

Amaranthus spp.

Harvest stage* 4-8

4-8

Storage Reference temperature (°C) 5 Chege et al., 2014; Nyaura et al., 2014; Habwe et al., 2008

*Weeks from sowing to harvest **At ambient conditions, all listed ALVs can only keep for one day. Shelf-life indicated is for fresh ALVs.


AVRDC, 2014 Habwe et al., 2008; Muchoki, 2007; Kasangi et al., 2010 Habwe et al., 2008

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Postharvest Treatments of African Leafy Vegetables Preparation and storage of ALVs for fresh consumption (marketing) Just like other leafy vegetables, ALVs should be prepared for market or preservation as soon as possible after harvesting. Since these vegetables are highly perishable, the likelihood of spoilage increases rapidly as time passes. Therefore, these vegetables need to be transported to a nearby shade or cold store within the shortest time possible. Majority of farmers producing these vegetables are resource limited (Yadav and Sehgal, 2002); hence they cannot afford conventional cold stores. Most farmers normally prepare their vegetables under a tree shade or ordinary stores. Sometimes, some farmers sprinkle cold water on the vegetables in order to quickly remove field heat and maintain freshness for a longer time. At this point, these vegetables are sorted, cleaned and packed using farmer and consumer specific requirements. Cleaning is necessary to remove any dirt or residues. Unfortunately, many farmers forget to dry leaves after cleaning and therefore encounter higher prevalence of disease development, especially molds. However, at supermarkets these vegetables are stored in cold shelves at about 5-10 °C together with other vegetables and fruits. The knowledge of low temperature storage of ALVs is still limited along the value chain. Very low temperatures have been reported to cause chilling injury to ALVs (Nyaura et al., 2014). Mixing of these vegetables with ethylene producing fruits and vegetables (as in the case of most markets and supermarkets) will hasten their deterioration rate. In most cases, if these vegetables are not sold within 24 hours after harvest, the likelihood of deterioration is imminent. Some farmers have tried to sprinkle water and leave then in the open overnight. However, problems of disease development and thus microbiological contamination still hamper their effort. Moreover, nutritional quality is also highly affected by the different postharvest temperatures. For example, it has


been demonstrated that ascorbic acid declined by 88 % in vegetable amaranth when kept at room temperature after 4 days of storage while the lowest loss was observed at 5 °C (55 % loss) after 23 days of storage. Based on this study (Nyaura et al., 2014), it is suggested that shelf life extension and nutrient preservation of vegetable amaranth can be achieved through storage at temperatures of 5 °C. Packaging of fresh and processed ALVs The aim of packaging foods is to protect them against spoilage and microorganism decay, preserve their quality and provide convenience of handling. Packaging has become a specialized issue, resulting in the development of new products, such as laminated paper and plastic materials, appropriate to different food products. Combined with improved traditional methods of processing, packaging of foods can reduce postharvest losses and prolong storage life (Ayua and Omware, 2013). In order to protect food against undesired alteration, it is necessary to use appropriate food product physiologically adapted film packaging material. The properties of the packaging material are determined by its permeability for gases (O2, CO2, ethylene, water vapor) and the degradative agents permeating from outside (Nyaura et al., 2014). Packaging should also minimize oxidation, undesired aroma substances, decline in nutrient and health promoting compounds, or the transfer of energy (light, heat). Currently, Xtend®, formally Active® bags are now being used as emerging technologies in packaging of fresh fruits and vegetables in Kenya. These polyethylene films possess the ability to control exchange of gases such as oxygen, carbon dioxide, and ethylene through the walls of the film. Nyaura et al. (2014) reported higher vitamin C content of vegetable amaranth using film packaging bags such as Xtend® and stored at low temperatures (5 °C). In addition, these bags have the ability to

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extend shelf-life of vegetable for several weeks, depending on the vegetable type. Packaging plays an indispensable role in modern society. However, the knowledge of appropriate packaging for ALVs is limited. Farmers still store ALVs in normal nonperforated clear polythene bags. Due to the high physiological activity of ALVs, e.g. high transpiration and respiration rates, water condenses inside such polythene bags, thus creating avenues for microbial development. Other farmers/traders tie their ALVs in bunches for marketing. However, this compromise on hygienic conditions of such vegetables as they are exposed to dust and dirt due to poor sanitation in the marketing outlets and frequent handling by consumers who want to buy the produce. Sometimes, they are also packed in gunny bags for long distance transport. However, at fresh state, these vegetables are very fragile and brittle and therefore subject to damage during transportation. Preservation Technologies Since ALVs are produced mostly during rainy season, they become scarce during dry season. Therefore, it is important to look for appropriate preservation methods to ensure that these vegetables are available throughout the year and are of acceptable quality (Kendall et al., 2003). Blanching Blanching is defined as a short heat treatment prior to processing or preservation aiming at inactivating enzymes in vegetables. It can be done either by immersion in hot water or spraying steam. In some cases the blanching water is used repeatedly, with the purpose of building up the concentration of dissolved solids to the point where leaching losses are small. Dehydrated vegetables are blanched prior to drying in order to arrest undesirable enzyme action and that the dried products will refresh more readily. Enzymes


are sensitive to moist heat conditions, especially where temperatures range above the maximum for enzyme activity (Kendall et al., 2003). Moist heat instantaneously inactivates the enzymes. A new recipe was developed at Maseno University to preserve ALVs through blanching followed by freeze drying: Fresh prepared vegetables were placed into a wire basket and gently lowered into a pan of boiling water where they were left in for two minutes. Thereafter, basket with vegetables was removed from the boiling water and plunged into ice cold water to stop the cooking process. The blanched vegetables were then drained and packaged into polythene bags, labeled and then placed into the freezer to freeze dry (Habwe et al., 2008). Drying Drying or dehydration has been a mean of preserving foods from earliest times. Drying is a deliberate removal of water from food products. Water removal should be under controlled conditions causing minimum or no changes in the food properties. A major criterion of quality of dehydrated foods is that when they are reconstituted in water they are very close to, or virtually indistinguishable from the original food material used. The primary objectives in removing water from any food material are to reduce its weight and bulk, leading to economical transportation, handling and distribution; and to improve its keeping quality by reducing the water activity. Drying is used in production of convenient foods. It is inexpensive in energy forms and dried products are economical in their storage requirements. The principal disadvantages of dried products are that they require a longer cooking period than the fresh or canned products and do not retain their flavor. Sundrying, the most common preservation measure practiced by Kenyan ALVs farmers, is the oldest method of drying food and its cost is low (Oniang’o et al., 2008). The sun’s ultraviolet rays can also inhibit the growth of

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Postharvest Treatments of African Leafy Vegetables microorganisms. The radiant energy of the sun provides the heat to evaporate the water. Drying proceeds well in warm and dry weather, however, at night and during the rainy seasons sun-drying is not effective. The temperatures of vegetables during sun drying are usually 5–15 ˚C above ambient temperatures (Abukutsa-Onyango, 2003). The time of drying ALVs can be 3 to 4 days or longer depending on the product and prevailing weather conditions. In dehydration of ALVs, enzyme systems must be inactivated prior to drying. This is accomplished usually by blanching. Though drying leads to loss of a proportion of the water soluble vitamins, fat soluble vitamins like β-carotene are fairly well-retained (Chege et al., 2014). It is deemed as an economically feasible method for preservation at a local level. According to Kendall et al. (2003), solar drying is more efficient because drying time is shorter since the drying temperatures are higher. The bioavailability of key minerals such as iron and zinc is known to be significantly affected by phytic acid and tannin content of foods. The content of these anti-nutrients are altered by various processing methods including solar drying. Solar drying has been found to lower oxalic acid, phytates and polyphenols significantly. Solar drying was reported to concentrate per unit nutrients of vegetable amaranth of ß-carotene, iron and zinc (Chege et al., 2014). They also reported retention of 77.5 %, 94.3 %, and 95.4 % for β-carotene, iron and zinc, respectively. The quality of dried product is reflected not only in its texture, flavor and color, but also in its ability to rehydrate as closely as possible to the original raw material. The rehydration efficiency is determined by preparation and the method of drying. During rehydration, dehydrated vegetables are soaked in water for some time before cooking; otherwise, they are likely to remain tough and shrinkled. Enough water should be used to permit plumping up to approximately the


original volume of the fresh product, with enough water remaining to almost, but not quite, cover the vegetable. The same water should then be used for cooking of the vegetables to avoid nutrient loss. Factors that affect rehydration processes of the dehydrated products are time, temperature, air displacement, pH and ionic strength (Oniang’o et al., 2008). Fermentation Fermentation, one of the oldest known methods of preparing and preserving food, has been reported by several researchers to improve palatability, taste, aroma and texture; extend the keeping quality; increase nutritional value and improve safety of food products (Schippers, 2002; Habwe et al., 2008; Franz et al., 2014). Fermentation of ALVs is considered to be an effective, inexpensive and nutritionally beneficial household technology in the developing world. The fermentation process is known to be very effective in eliminating a number of anti-nutritional factors in foods such as phytic acid and glucosinolates (Ifesan et al., 2014). Moreover, fermented foods are considered to have health benefits, and in many regions they are believed to control some diseases (e.g. intestinal disorders) (Mathara et al., 2004). It improves digestibility and utilization of proteins and fatty acids; improves solubility of minerals. Fermentation also has anti-microbial activities and imparts flavors and functionality attributes into foods. Food fermentation can be used as a tool in alleviating nutritional defects at household level during food preparation or processing (Habwe, 2008). Brining or salting or pickling, which permit and favor fermentation, can preserve many vegetables, through action of lactic acid-forming microorganisms. Fermentation of cowpea leaves with the addition of the highest concentration of glucose (3 %) gave the highest concentration of lactic acid of 0.6 % and the lowest pH of

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4.7 (Kasangi et al., 2010). In addition, when compared with blanched solar dried and fresh solar dried, fermented samples had the least crude protein and moisture content of 13.2 % and 6.2 %, respectively while crude fibre, ash and soluble carbohydrates of fermented samples increased. A reduction of iron, calcium, magnesium and zinc was also observed on fermented cowpea leaves. Conclusion and Recommendation In spite of the abundance of ALVs, they remain under-exploited and under-utilized due to various constraints, including the application and knowledge about appropriate postharvest treatment and preservation techniques. The future of ALVs is dependent on a higher research impact on the optimization of handling procedures, postharvest treatments and processing technologies for ALVs in terms of food safety and nutritional and health related aspects. It is not enough to encourage local farmers to grow their traditional crops without adequate postharvest handling techniques. Successful postharvest handling and preservation of ALVs is important in the effort of creating sustainable livelihoods. ALVs could highly contribute to food security, to earn valuable foreign exchange by exporting, provide opportunities for import substitution, and generally benefit a large number of people. References Abukutsa-Onyango M.O. 2003. Unexploited potential of indigenous African vegetables in Western Kenya. Maseno Journal of Education Arts and Science 4:103-122. AVRDC Learning Center. Publication and fact sheets on indigenous vegetables: www.avrdc.org. Accessed on 22.12.2014. Ayua A., Omware J. 2013. Assessment of processing methods and preservation of African leafy vegetables in Siaya County, Kenya. Global Journal of Biology and Health Sciences 2:46-48.


Chege P., Kuria E., Kimiywe J., Nyambaka H. 2014. Retention of ß-Carotene, iron and zinc in solar dried amaranth leaves in Kajiado County, Kenya. International Journal of Sciences: Basic and Applied Research 13:329-338. Franz C., Huch M., Maina J., Abriouel H., Benomar N., Reid G., Galvez A., Holzapfel W.H. 2014. African fermented foods and probiotics. International Journal of Food Microbiology 190:84–96. Friis H., Gomo E., Michaelsen K.F. 2002. ‘Micronutrient interventions and the HIV pandemic’ in Friis H (ed) Micronutrients and HIV infection, CRC, Florida, pp. 220-245. Habwe F.O. 2008. Development of East African indigenous vegetable recipes and determination of their iron, copper and vitamin C contents. MSc thesis, Maseno University, Kenya. Habwe F.O., Walingo K.M., Onyango M.O.A. 2008. Food processing and preparation technologies for sustainable utilization of African indigenous vegetables for nutrition security and wealth creation in Kenya. International Union of Food Science and Technology 3:1-9. Ifesan B.O.T., Egbewole1 O.O., Ifesan B.T. 2014. Effect of fermentation on nutritional composition of selected commonly consumed green leafy vegetables in Nigeria. International Journal of Applied Sciences and Biotechnology 2:291-297. Kamga R.T., Kouame C., Atangana A.R., Chagomoka T., Ndango R. 2013. Nutritional evaluation of five African indigenous vegetables. Journal of Horticultural Research 21:99-106. Kasangi D.M., Shitandi A.A., Shalo P.L., Mbugua S.K. 2010. Effect of spontaneous fermentation of cowpea leaves (Vigna unguiculata) on proximate composition, mineral content, chlorophyll content and beta-carotene content. International Food Research Journal 17:721-732.

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Postharvest Treatments of African Leafy Vegetables Kendall P., DiPersio P., Sofos J. 2003. Drying vegetables. Food and nutrition series. Colorado State University Cooperative Extension. Fort Collins. USA. ISBN 9.308 Kimiywe J., Waudo J., Mbithe D., Maundu P. 2007. Utilization of medicinal value of indigenous leafy vegetables consumed in urban and peri-urban Nairobi. African Journal of Food Agriculture Nutrition and Development 7:15-19. Kinyuru J.N., Konyole S.O., Kenji G.M., Onyango C.A., Owino V.O., Owuor B.O., Estambale B.B., Friis H., Roos N. 2012. Identification of traditional foods with public health potential for complementary feeding in Western Kenya. Journal of Food Research 1:148-158. Mathara J.M., Schillinger U., Kutima P.M., Mbugua S.K., Holzapfel W.H., 2004. Isolation, identification and characterization of the dominant microorganisms of kulenaoto: the Maasai traditional fermented milk in Kenya. International Journal of Food Microbiology 94:269–278. Muchoki C.H.N. 2007. Nutritional, sensory and keeping properties of fermented solardried cowpea leaf vegetables. MSc. Thesis. University of Nairobi, Kenya. Ngugi I.K., Gitau R., Nyoro J.K. 2007. Access to high value markets by smallholder farmers of African indigenous vegetables in Kenya. Regoverning Markets Innovative Practice series, IIED, London. Nyaura J.A., Sila D.N., Owino W.O. 2014. Postharvest stability of vegetable amaranth (Amaranthus dubius) combined low temperature and modified atmospheric packaging. Food Science and Quality Management 30:66-72. Oniang’o R., Grum M., Obel-Lawson E. 2008. Developing African leafy vegetables for improved nutrition. Regional workshop, 6-9 December 2005. Rural Outreach Program, Nairobi, Kenya.


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