Promoting Food Security through Value-added Processing of Lesser-known Indigenous Fruits

O. C. Aworh

Introduction

In Nigeria and other African countries, there are hundreds of little-known indigenous crops that contribute to food security and play vital roles in the nutrition of the people, particularly the rural populace. Among these is a wide variety of wild indigenous fruits and vegetables that enrich the diet of the rural populace. These lesser-known crops of considerable horticultural and nutritional significance are available at certain critical periods of the year when the more common sources are very scarce or completely unavailable (Okigbo 1977). These crops could thrive with little care and without the use of costly agricultural inputs such as fertilisers, herbicides and pesticides (Okafor 1981).  Notable among these lesser-known crops are numerous fruits of Nigerian trees that grow wild in the rain forest of the humid tropics of southern Nigeria and the woodland savanna zone of the country or are cultivated on compound farms (Figure 1). These fruits that are consumed in the fresh state are rich sources of nutrients including ascorbic acid, provitamin A carotenoids, minerals and nutraceuticals with health-promoting benefits. Regrettably, many of these indigenous fruit trees that are also of considerable environmental significance are threatened by deforestation arising from uncontrolled wood felling activities. Moreover, because of poor postharvest handling practices, these highly perishable fruits suffer considerable postharvest losses, reducing their contribution to food security. There is therefore the need for value-added processing of these fruits into shelf-stable products to reduce postharvest losses and promote their utilisation and cultivation. 

Value-added Processing

Apart from water, carbohydrates (mainly sugars) are the predominant chemical components of most of the lesser-known Nigerian tree fruits, except for a few such as the African pear (Dacryodes edulis) that is rich in fat (Table 1). Consequently, these fruits are suitable for the production of fruit juices, jam and other preserves as well as fruit leathers / candies.

Figure 1 (l to r) African Pear, African Star Apple and African Mango

Table  1. Typical proximate composition (g per 100 g edible portion) of some lesser-known Nigerian Fruits

Production of Fruit Juices

Fruit juices are essentially the liquids expressed by pressure or other mechanical means from the edible portions of fruits. All natural fruit juices are low in solids, usually 5-20 % of fresh weight. Nigeria’s fruit juice industry relies heavily on the importation of concentrates from conventional fruits including citrus, apples and pineapples costing the nation millions of dollars in foreign exchange, even though Nigeria produces a wide variety of conventional and indigenous fruits suitable for juice production. Whilst theoretically fruit juices can be produced from all edible fruits, in practice not all edible fruits are suitable for commercial juice production. Juice expressed from African star apple is too thick and too harsh in flavour for direct consumption without dilution while that expressed from hog plum is too acidic. Consequently, African star apple and hog plum are best processed into juices known as nectars containing 25-50% fruit, with added water and sugar or they may be mixed with other fruits in various fruit juice blends. 

Laboratory studies have shown that fruit juices can be produced from African star apple (C. albidum) and hog plum (S. mombin) using simple procedures suitable for home preparation or small-scale commercial production. The procedure developed for African star apple juice involves first blanching washed fruits in hot water to facilitate manual peeling with knives. Juice extraction may be with a home blender or a pulper / finisher after removal of the seeds. The extracted juice which has an attractive deep red color is then pasteurised (Okoro 2001). Similarly, hog plum juice was highly rated for color but lowly rated for taste in sensory evaluation tests (Ogunmuyiwa 2001). Blending African star apple juice or hog plum juice with pineapple (Ananas comosus) juice considerably improved the taste (Okoro 2001).

Jam-making

Jams, jellies and marmalades, sometimes referred to as preserves or conserves, are concentrated products made from fruits with added sugar. Concentration and the addition of sugar increase the soluble solids of the products to a level that inhibits microbial growth and spoilage. The basic ingredients used in jam-making are fruit, sugar, pectin, acid and water. The sugar and acid cause the pectin to form a gel in which the fruit is suspended.

Laboratory trials have shown that jams can be produced from lesser-known Nigerian tree fruits including African star apple (C. albidum), African mango (I. gabonensis), hog plum (S. mombin) and black plum (V. doniana). Some of these fruits have sufficient pectin and acid that no other ingredients other than sugar and water need be used to produce good quality jam (Inoh et al. 1977, Aina 1991).  After harvesting and washing, the fruits are peeled and cored followed by slicing, mashing and boiling with sugar addition in a steam-jacketted kettle. Jam made from African star apple by the open-kettle process had an attractive deep red colour and that made from hog plum had a golden yellow colour. Consumer acceptance tests, with over 50 assessors, indicated no significant differences in preference between hog plum jam and commercial pineapple (Ananas comosus) and mango (Mangifera indica) jams.  Using African star apple in combination with pineapple to produce a mixed fruit jam containing 50% African star apple and 50% pineapple improved consumer acceptance (Adekoya 1999). Compared to African star apple and hog plum jams, African mango jam was less preferred, especially in terms of flavor and consistency. Jam made from African mango had a somewhat gummy consistency and it would appear that the fruit has less prospects for commercial jam-making. 

Production of Fruit Leathers / Candies

Osmotic dehydration allows for the development of a variety of new shelf-stable food products from perishable fruits thus reducing postharvest fruit losses and ensuring that seasonal fruit products are available throughout the year. Osmotic pre-treatment of fruits, by dipping them in sugar solutions prior to hot-air drying produces relatively inexpensive intermediate moisture fruit products with good color, flavor and texture characteristics, and shelf stability at tropical ambient temperatures. Fruits dried with infused sugar are sometimes referred to as candies due to the large amount of sugar added to the fruit. They are also referred to as leathers because the infused sugar imparts a leathery texture to the dried product as opposed to the woody texture of non-osmosed dried fruit products. Fruit leathers / candies are sufficiently moist to be eaten without rehydration, yet shelf-stable (Cheman and Taufik 1995).

Results of laboratory studies on the production of fruit leathers / candies from African star apple (C. albidum) revealed that the best products, from the standpoint of consumer acceptance, cost and energy savings, were obtained when ripe African star apple fruit pulps were immersed in sugar syrup [60°B, 1:4 ratio (w/w) of fruit pulp to sugar syrup] at tropical ambient temperature (27°C) for 8 h followed by hot-air drying at 60°C. Fruit pulps dipped in sugar syrups of lower concentrations (44°B and 52°B) received  lower scores for sensory quality attributes than those dipped in 60°B sugar syrup (Falade and Aworh 2004, 2005). Similar studies with African mango (I. gabonensis) indicated that coating African mango slices (10 mm thick) by immersion in 5-10 % (w/w) gum guar solution prior to osmotic dehydration promoted water extraction from the fruit pieces but reduced solute (sugar) infusion into the fruit pieces. Although osmo-dried African mango slices received consistently better scores for color, taste, chewiness and overall acceptability relative to controls that did not receive osmotic pre-treatment, osmo-dried products from African mango were generally less preferred than those from African star apple.

Conclusion

Shelf-stable value-added products can be produced from lesser-known indigenous tree fruits using simple technologies suitable for small-scale commercial production. Promoting the production of these products including fruit juices, jams and fruit leathers / candies would reduce postharvest losses, increase utilisation and cultivation of these valuable but under-utilised crops and contribute to reducing widespread food insecurity in Nigeria and many other African countries.

References

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Dr Ogugua Charles Aworh is Professor and former Head of the Department of Food Technology, University of Ibadan, a Fellow of the International Academy of Food Science and Technology and Chairman of the Body of Fellows of the Nigerian Institute of Food Science and Technology; Email: ocawo51@yahoo.co.uk