Uapaca kirkiana, an indigenous fruit tree in sub-Saharan Africa: A comprehensive review

Abstract

Uapaca kirkiana is an indigenous fruit tree that grows in hot and dry areas in sub-Saharan Africa. The tree tolerates dry conditions, high temperatures and acts as a food source for people that live in drier conditions were exotic fruit trees can hardly survive. The tree produced fruits that are rich in essential minerals and has a potential to act as a source of vitamin C. Ripe fruits are eaten raw and mostly sold at local and roadside markets with no processing. Traditionally the fruit can be processed into various products which include alcoholic and non-alcoholic beverages and traditional cakes. The fruit has a potential to improve nutritional status, food security, and livelihoods of rural dwellers especially in arid and semi-arid areas. However, the contribution of the fruit to human nutrition is often not recognised. The fruit, like most indigenous fruits, has limited and out-dated data on nutrient composition. Indigenous knowledge on handling and uses of the fruit needs to be upgraded. This review attempts to contribute to this effort by evaluating the existing evidence on its nutritional potential, functional and bioactive properties, processing, postharvest handling with the aim of identifying possible areas of research and its utilisation.

Keywords:

• nutritional value
• functional properties
• fruit processing
• fruit marketing
• fruit postharvest handling

PUBLIC INTEREST STATEMENT

Sub-Saharan Africa region consists of many but different species of fruits that have adapted to different climatic conditions. Fruits are good sources of essential micronutrients in the diet. However, there is limited of scientific knowledge on nutritional composition and benefits of most indigenous fruits. Furthermore, sub-Saharan Africa has the most nutritionally insecure people, especially children and women who suffer from hidden hunger. Food insecurity at most rural households is approximately over 30%. At the same time, there is high rate of malnutrition and micronutrients deficiency in sub-regions, especially Southern Africa due to limited access to food and utilization of fruits. Many processing technologies have been developed to add value in quality, mineral bioaccessibility, shelf life and postharvest handling on fruits. The utilisation of indigenous fruits in the production of functional foods has great potential to improve nutrition, and help mitigate mineral deficiency problems in most rural communities in sub-Saharan Africa.

Competing interest

The authors declare no competing interest.

1. Introduction

Mineral undernutrition, particularly iron and zinc deficiencies have been reported to be the prevalent nutritional problems worldwide (Platel & Srinivasan, 2016). Sub-Saharan Africa is a home to some of the most nutritionally insecure people in the world; notably, women and children suffer from insufficient intake of protein and energy, and lack of micronutrients (Stevens et al., 2012; World Health Organization [WHO], 2017). A WHO report (WHO, 2015) states that iron deficiency is the most common micronutrient deficiency that affects over 30% of the global population with women and children representing a greater percentage of effected individuals in developing nations. In sub-Saharan Africa, especially in Zimbabwe, the prevalence of stunting was 26% in 2018 and it remains above the acceptable global threshold of 20% (National Nutritional Survey [NNS], 2018). Consumption of indigenous fruits has a great potential to improve nutrition and help to mitigate against mineral deficiencies problems in most rural communities in sub-Saharan Africa.

Indigenous fruits are mainly used to supplement the diet by many rural folks in sub-Saharan Africa (Akinnifesi et al., 2004; Bille et al., 2013; Legwaila et al., 2011; Mithöfer & Waibel, 2003; Mpofu et al., 2014; Nhukarume et al., 2010; Saka et al., 2004). Consumption of wild fruits such as Uapaca kirkiana is important amongst the poor and vulnerable groups in the community (Cunningham, 2002; Maghembe et al., 1998; Tiisekwa et al., 2004) because they cannot afford to buy food to feed themselves. More so, Mander and Le Breton (2006) and Garrity (2004) reported that 80% of the rural population in sub-Saharan Africa depend on traditional plants for medicinal use as a form of primary health care. The Plant for a future (pfaf) (2020) and Orwa et al. (2009) have reported a traditional medicinal claim on the potential use of U.kirkiana roots, bark, and leaves in the treatment of stomach-ache and dysentery.

U. kirkiana is closely related to other species such as U. bengelensis, U. nitida, U. pilosa, and U. robynsii, which are not commonly found in Zimbabwe (Ngulube, 1996) but found in other areas of Southern Africa (Figure 1).

Figure 1. Distribution of U.kirkiana species in Southern Africa

Figure 2. Distribution of U. kirkiana fruits in Zimbabwe

U. kirkiana tree is mainly distributed in semi-dry and dry areas although it can grow in some relatively wet areas of Zimbabwe (Figure 2). The species, U. kirkiana (Figure 3(a)) is one of the most dominant and abundant wild fruit tree found in Zimbabwe (Kadzere et al., 2001). It produces fruits that ripen during the period October to February (Figure 1(a)).

Figure 3. U. kirkiana tree (a), U. kirkiana fruits (b), and U. kirkiana pulp and seed (c)

The fruit is known by different names from different locations and countries; wild loquat (English), msuku/nkusu in Malawi, Tanzania, and Zambia, and mahobohobo or mazhanje in Zimbabwe (Akinnifesi et al., 2004). The fruit is oval shaped (Figure 3(c)), and contains seeds and a yellowish pulp (Moombe et al., 2014). Unripe fruits usually have a relatively sour taste (Kadzere et al., 2001). The ripe fruit has a sweet taste (Maroyi, 2014; Mithöfer & Waibel, 2003). Traditional foods (wild fruits) of sub-Saharan Africa have a history of being safe and nutritious and affordable (Mpofu et al., 2014). Wild loquat fruit can produce a pulp yield of 28.6% of total fruit weight (Ndabikunze et al., 2010). Domestication of the fruit tree can therefore be suggested (Katsvanga et al., 2007; Maghembe et al., 1993; Mwamba, 1988).

Data on the multipurpose Uapaca kirkiana fruit tree has been published by other authors (Akinnifesi et al., 2004; Ngulube, 1996). A review on U.kirkiana fruit tree by Ngulube (1996) focused on the taxonomy, tree propagation and resource potential in food, wood and medicinal uses. Furthermore, Plant for a future (pfaf) organisation database (Plant for a future [pfaf], 2020) has similar data on cultivation, edible uses (Facciola, 1998; Karalliedde & Gawarammana, 2008), medicinal (Ecocrop database, 2020), and agroforestry uses (World Agroforesty Centre, 2020) of U. kirkiana fruit tree. This review is aimed at providing more data and information on U. kirkiana fruit by evaluating the nutritional potential, functional and bioactive properties, postharvest handling and traditional uses, fruit marketing, and identifying possible research gaps on the fruit.

2. Nutritional composition of U. kirkiana fruit pulp

Macronutrient composition of U. kirkiana fruit compares well with other wild fruits from sub-Saharan Africa as shown in Table 1.

Table 1. Nutritional composition of some indigenous fruits of Southern Africa per 100 g EP

Fruit Name & Botanical family	Energy (kcal/KJ)	Water (g)	Proteins (g)	Fats (g)	CHOs (g)	Fibre (g)	Ash (g)	Reference
Adansonia digitata (Malvaceae)	1380^a	10^a	2.3^a	0.7^b	74.9^a	7.8^a	4.8^b	(^aOsman, 2004; ^bAbdel et al., 2011)
Sclerocarya berrea (Anacardiaceae)	2703^a	87^a	0.9^b	0.5^a	3.7^b	2.9^b	3.8^b	(^aJaenicke & Thiong’o, 2000; ^bThiongo’s, Kingori and Jaenicke, 2002)
Uapaca kirkiana (Phyllanthaceae)	523^a	72^b	0.3^a	0.4^a	28.7^a	2.0^b	0.8^b	(^aMalaisse & Parent, 1985; ^bSaka et al., 1992)
Ziziphus mauritiana (Rhamnaceae)	184^a	92^a	0.35^a	0.7^a	8.3^b	1.2^b	0.8^a	(^aSaka et al., 1992; ^bLockett et al., 2000)
Tamarindus indica (Fabaceae)	1160^a	28^a	3.8^b	1.1^b	60.4^b	6.0^b	2.4^a	(^aSoloviev et al., 2004; ^bFentahun & Hager, 2009)
Strychnos cocculoides (Loganiaceae)	1315.4^a	78.8^a	3.2^a	0.3^a	16.8^a	25.2^a	0.5^a	(^aNgadze et al., 2017)

Metabolisable energy calculated based on protein 4 (kcal/17 KJ), fat 9 (kcal/37 KJ), CHOs 4 (kcal/17 KJ), Fibre 2 (kcal/8 KJ); Protein values obtained using Kjeldahl method^; Fat values by Soxhlet method^; CHOs calculated by difference method; Fibre represents crude fibre.

^a,bSuperscripts indicate the source of the data.

2.1. Carbohydrates

The fruit has a total carbohydrate content that range from 28.7 to 92 g/100 g edible portion (EP) (Ngulube, 1996), which is greater than that found in other wild fruits such as Ziziphus mauritiana and Irvingia edulis. In a study by Saka and Msonthi (1994), the whole fruit was found to contain 86.5% total carbohydrate, 8.4% fibre, 1.1% fat, 1.8% crude protein, and 27.4% dry matter.

A review by Sufi and Kaputo (1977) on specific sugars noted that U. kirkiana fruit contains 41 g/100 g glucose, 27 g/100 g fructose, 15 g/100 g sucrose, 2 g/100 g xylose, and traces of galactose, raffinose, and ribose. These sugar levels are higher than those of tropical fruits such as apples, which were found to contain 5.7 g/100 g fructose, 0.6 g/100 g glucose, and 0.57 g/100 g sucrose (Ngadze et al., 2017). Studies by Ngulube (1996), and Malaisse and Parent (1985) reported that related species of U. kirkiana had a carbohydrate content of 89 g/100 g, 82.9 g/100 g, 88.3 g/100 g, and 91.2 g/100 g for U. bengelensis, U. nitida, U. pilosa, and U. robynsii species, respectively. These observations explain the fact that other related species of U. kirkiana have a relatively same carbohydrate content found on other wild fruits found in the sub-Saharan Africa such as Adansonia digitata and Tamarindus indica.

A review on carbohydrate data indicated that U. kirkiana fruit has a higher carbohydrate content (28.7 g/100 g EP) as compared to that of most tropical fruits as shown in Table 2. Furthermore, U. kirkiana fruit compares well in energy provision and has a high caloric value of 523 Kcal/KJ. Ngulube et al. (1996) reported energy values of 13,780 KJ/kg, 14,200 KJ/kg, 14,620 KJ/kg, 14,820 KJ/kg, and 14,620 KJ/kg in U. bengelensis, U. kirkiana, U. nitida, U. pilosa, and U. robynsii, respectively. In a comparative study by Stadlmayr et al. (2013) the U. kirkiana fruit had high energy values of 523 Kcal kJ-1, which was higher than that of Ziziphus mauritiana (184 Kcal kJ⁻¹), Vitex doniana, (474 Kcal kJ⁻¹), and Irvingiaga bonensis, (364 Kcal kJ⁻¹). This suggests U. kirkiana fruit is a relatively excellent source of energy as compared to other tropical fruits, such as apples, oranges, and mangoes.

Table 2. Comparison of the nutritional content per 100 g edible portion (EP) of selected tropical fruits and Uapaca kirkiana

Fruit Name & Botanical family	Water (g)	CHOs (g)	Protein (g)	Fibre (g)	Fat (g)	Calories (kcal/KJ)	Vitamin (mg)	References
Wild loquat (U.kirkiana) Euphorbiaceae	72^a	28.7^a	0.3^a	2.0^b	0.4^a	523^b	16.8^b	(^aMalaisse & Parent, 1985; ^bSaka et al., 1992)
Apples (Malus pumila) Rosaceae	85.3^a	15.25^a	0.19^a	2.7^a	0.36^a	207^a	5.7^a	(^aJensen et al., 2015)
Mangoes (Mangifera indica) Anacardiaceae	84^a	15^a	1.0^a	1.6^a	0.4^a	255^a	53^a	(^aKazii et al., 2015)
Oranges (Citrus X sinensis) Rutaceae	87^a	10.6^a	1.0^a	1.8^a	-	198^a	49^a	(^aKazii et al., 2015)
Avocados (Persea Americana) Lauraceae	81^a	7.0^a	2.0^a	0.2^a	15.4^a	523^a	8.80^a	(^aUSDA, 2011)
Peaches (Prunus persica) Rosaceae	89^a	7.9^a	1.0^a	1.4^a	-	151^a	8^a	(^aKazii et al., 2015)

^a,bSuperscripts indicate the source of the data.

2.2. Protein

Protein content data amongst the major indigenous fruits of sub-Saharan Africa indicates that U. kirkiana fruit has a protein content range of 0.3 g/100 g EP to 0.9 g/100 g EP (Jones et al., 2006; Ngulube, 1996). Furthermore, Malaisse and Parent (1985) reported a protein content of 0.8 g/100 g for U. nitida and 0.4 g/100 g for U. kirkiana. This low protein content data on U. kirkiana fruit explains that the fruit cannot be used as a protein source, rather protein supplement can be found on other wild fruits such as Adansonia digitata.

2.3. Fat

The fat content in U. kirkiana fruit was noted to be as high as 1.1 g/100 g EP (Saka et al., 1992). The fat content is higher as compared to data recorded by other authors and amongst other indigenous and tropical fruits as shown in Tables 1 and 2. The fat content in all the indigenous fruits shown in Table 1 are not significantly different (p < 0.05) using least significant difference (LSD) test on the means. However, the U. pilosa species has the highest fat content of 3.4 g/100 g (Malaisse & Parent, 1985; Ngulube, 1996). There is need to verify such data on the fat content of the fruits using modern analytical method.

2.4. Dietary fibre

The dietary fibre content of U. kirkiana fruit is indicated in Table 1. Dietary fibre intake reduces the risk of stroke (Steffen et al., 2003; Whelton et al., 2005), hypertension (Anderson, 2004; Keenan et al., 2002; Montonen et al., 2003), diabetes (Birketvedt et al., 2005; Lairon et al., 2005), and obesity (Brown et al., 1999; Watzl, Girrbach, & Roller, 2005), and potentially improves the immune system (Schley & Field, 2002). The relatively high fibre content in the edible portion of the fruit could be explained with the reported higher micronutrient contents (Table 2). Stadlmayr et al. (2013) noted an ash content of 3.2 g/100 g EP in U.kirkiana fruit. This ash content is relatively high in comparison to the data reported by other authors in Table 1. The fruit has an ash content that was comparable to that of Ziziphus mauritiana and Irvingia edulis.

2.5. Water content (Moisture)

Comparative studies on the water content showed that U. robynsis, a related species of U. kirkiana has the highest water content of 83.0 g/100 g (Ngulube et al., 1996). Findings from literature indicate that fresh U. kirkiana fruit has a water content of more than 50%, suggesting a high water activity in the fruit, which can affect its microbiological activity, shelf life, and storage quality.

2.6. Mineral content

The mineral content of U. kirkiana fruit and other indigenous fruits are shown in Table 3. From the reviewed mineral content data presented in Table 3, it is evident that U. kirkiana fruit can be used as an important source of iron and zinc because its iron content (11.8 mg/100 g EP) is higher than that in most indigenous fruits and its zinc content shows a no significant difference (p < 0.05) with respect to wild fruits when an LSD test was used.

Table 3. Comparative mineral content of selected indigenous fruits of Southern Africa

Mineral composition of selected indigenous fruits of Southern Africa per 100 g EP
Fruit Name & Botanical family	Ca (mg)	Fe (mg)	Mg (mg)	P (mg)	K (mg)	Na (mg)	Zn (mg)	Vit. C (mg)	References
Adansonia digitata (Malvaceae)	401^a	8.1^a	285^a	62^b	2120^b	20.4^a	1.87^a	273^b	(^aOsman, 2004; ^aAbdel et al., 2011)
Uapaca kirkiana (Phyllanthaceae)	17^a	11.8^a	39^a	15^a	375^b	10^b	1.3^a	16.8^a	(^aMalaisse & Parent, 1985; ^bSaka et al., 1992)
Sclerocarya berrea (Anacardiaceae)	67^a	8.0^a	33^a	-	601^a	2.7^b	5.19^a	167^b	(^aJaenicke & Thiong’o, 2000; ^bThiongo’s, Kingori and Jaenicke, 2002)
Ziziphus mauritiana (Rhamnaceae)	23^a	0.82^a	5^b	19^b	256^a	6^a	0.03^b	13.6^b	(^aSaka et al., 1992; ^bLockett et al., 2000)
Tamarindis indica (Fabaceae)	192^b	4.4^b	66^a	87^a	933^b	30.2^b	3.1^a	15.5^b	(^aSoloviev et al., 2004; ^bFentahun & Hager, 2009)
Strychnos cocculoides (Loganiaceae)	46.5^a	70.5^a	137.2^a	116.5^a	959.2^a	4.5^a	0.4^a	34.2^a	(^aNgadze et al., 2017)

Mineral values were obtained using atomic absorption spectroscopy and some calorimetric methods.

^a,bSuperscripts indicate the source of the data.

A comparative analysis of mineral content of U. kirkiana to tropical fruits revealed that U. kirkiana is an excellent source of iron, zinc, magnesium, and sodium, as shown in Table 4. This makes the fruit a potential source of essential minerals and can be used to improve iron and zinc nutrition upon consumption of the fruit. However, in terms of nutrition, it is not just enough to determine the total mineral content; it is therefore important and necessary to determine the bioaccessibility of these minerals. Minerals are important for the normal functioning of an organism. Barros et al. (2012) noted that minerals such as iron, calcium, zinc, copper, sodium, potassium, magnesium, boron, manganese, and sulphur were present in the fruit pulp and peel of citrus fruits and this compares well with most wild fruits found in sub-Saharan Africa as shown in Table 3.

Table 4. Comparison of mineral content of selected exotic fruits and Uapaca kirkiana per 100 g EP

Fruit name & Botanical family	Fe (mg)	Zn (mg)	Mg (mg)	Ca (mg)	K (mg)	P (mg)	Cu (mg)	Na (mg)	References
Wild loquot (U.kirkiana) Euphorbiaceae	11.8^a	1.3^a	39^a	17^b	375^a	15^a	0.1^b	10^a	(^aMalaisse & Parent, 1985; ^bSaka et al., 1992)
Apples (Malus pumila) Rosaceae	0.18^a	0.04^a	6.0^a	7.0^a	115^a	12.0^a	0.041^a	-	(^aJensen et al., 2015)
Mangoes (Mangifera indica) Anacardiaceae	0.50^a	0.30^a	7.0^a	7^a	250^a	13^a	0.11^a	2^a	(^aSoloviev et al., 2004)
Oranges (Citrus X sinensis) Rutaceae	0.40^a	0.20^a	11^a	29^a	145^a	24^a	0.05^a	-	(^aSoloviev et al., 2004)
Avocados (Persea Americana) Lauraceae	0.61^a	0.68^a	29.0^a	13.0^a	507^a	54^a	-	8.0^a	(^aFentahun & Hager, 2009)
Peaches (Prunus persica) Rosaceae	0.20^a	0.1^a	6.0^a	6^a	186^a	19^a	0.07^a	-	(^aSoloviev et al., 2004)

^a,bSuperscripts indicate the source of the data.

No research has been conducted on the mineral bioaccessibility of U. kirkiana fruit. Therefore, there is a need to carry out more mineral assays and to ascertain their bioaccessibility and/or bioavailability in the human body upon consumption. Estimates of the contribution of the fruit pulp on recommended daily allowances were calculated per consumption of 100 g of the served fruit. The calculation has indicated that U.kirkiana fruit pulp can be able to deliver more than 100% of the RDA for iron in age groups 1–9 years and 9–13 years (Table 5). This further supports the importance of the fruit as a good source of iron.

Table 5. U. kirkiana composition with recommended dietary allowance (RDA) and adequate intake (AI) for children (1–9 yrs), children (9–13 yrs), adolescents (14–18 yrs), pregnant females (all ages)

		*CHOs	Protein	Fibre	Zn	Fe	Mg	Ca	Vit C
		g/day	g/day	g/day	mg/day	mg/day	mg/day	mg/day	mg/day
U. kirkiana composition		28.7**	0.3**	2.0**	1.3***	11.8***	39***	17***	16.8***
RDA children (1–9 yrs)	Man Women	80^b 80^b	25^b 25^b	10^a 10^a	5^b 5^b	7^b 7^b	130^b 130^b	1000^b 1000^b	52^b 52^b
% contribution of the fruit in children		35	1.2	0.2	26	168	30	1.7	32
RDA children (9–13 yrs)	Man	130 ^b	34 ^b	12 ^a	8 ^b	8 ^b	240 ^b	1300 ^b	45 ^b
	Women	130 ^b	34 ^b	12 ^a	8 ^b	8 ^b	240 ^b	1300 ^b	45 ^b
% contribution of the fruit in children		22.0	0.8	16.6	16.2	147.5	16.2	1.3	37.3
RDA adolescents (14–18 yrs)	Man	130 ^b	55 ^b	16 ^a	11 ^b	11 ^b	410 ^b	1300 ^b	75 ^b
	Women	130 ^b	52 ^b	17 ^a	9 ^b	15 ^b	360 ^b	1300 ^b	65 ^b
% contribution of the fruit in adolescents		22.0	0.5	16.6	13	90.7	10.1	1.3	24
RDA pregnant	All ages	175 ^b	41 ^b	28 ^a	12 ^b	27 ^b	350 ^b	1000 ^b	85 ^b
% contribution of the fruit in pregnant females		16.4	0.7	7.1	10.8	43.7	11.1	1.7	16.7

^aAdequate intake (AI) values (Food and Nutrition Board, 1997, 2001). ^bRecommended dietary allowances (RDA) values (Food and Nutrition Board, 2005)

*CHOs- carbohydrates, **Unit of measurement = g/100 g, ***Unit of measurement = mg/100 g.

2.7. Ascorbic acid (Vitamin C)

Vitamin C content studies revealed that U. kirkiana fruit has an ascorbic acid content of 14.5 mg g⁻¹ (Stadlmayr et al., 2013), while Saka and Msonthi (1994) reported a vitamin C content of 16.8 mg/100 g of fresh fruit. In a study by Ndabikunze et al. (2010) on vitamin C and mineral contents of indigenous fruits of the Miombo woodlands of Tanzania, U. kirkiana fruit pulp had a vitamin C content of 20.8 mg/100 g, which was lesser than that of V. mombassae (41 mg/100 g) fruit. The WHO (1999) reported that the daily body requirement of vitamin C is between 45 mg to 80 mg. Consuming 247 mg of vitamin C daily improves iron absorption by 35% (Cook & Reddy, 2001). Therefore, the U. kirkiana fruit can aid in iron absorption because of its relatively high vitamin C content.

3. Functional properties of the fruit

U. kirkiana fruit has a relatively high pulp yield of 283.4 ± 3.91 g kg⁻¹, compared to S. berrea (161.9 ± 1.97 g kg⁻¹), A. digitata (202.4 ± 4.4 g kg⁻¹), and V. mombassea (186.0 ± 4.59 g kg⁻¹) (Ndabikunze et al., 2010). Ndabikunze et al. (2010) also noted that U. kirkiana fruit pulp has a high total soluble solids (TSS) content of 169 ± 0.14 g kg⁻¹ as compared to S. berrea (133.0 ± 0.19 g kg⁻¹), A. digitata (116.3 ± 0.16 g kg⁻¹), and V. mombassea (123.3 ± 0.16 g kg⁻¹). The results for TSS values obtained by Ndabikunze et al. (2010) of the U. kirkiana fruit were in agreement with values found for mangoes (140 g/100 g) by Soloviev et al. (2004). This suggests the potential use of the fruit as an ingredient in jam and juice production.

Ndabikunze et al. (2010) noted that U. kirkiana pulp possessed a pH value of 4.67 ± 0.04. The pH values of the fruit pulp observed by Ndabikunze et al. (2010) are higher as compared to pH range 3.0–3.5 which is recommended for jam and juice making. This explains that when using the fruit in jam or juice processing, the pH must be balanced with the addition of other ingredients. The mean total titratable acidity (TTA) of the fruit pulp was 0.5 ± 0.02 g kg⁻¹ (Ndabikunze et al., 2010). The acid content of ripe fruit pulp affects the biotransformation of nutrients during processing and product stability in juices. Therefore, there is need to improve the acid levels when processing the U. kirkiana in juice processing (FAO, 1999).

4. Bioactive compounds

U. kirkiana fruit contains tannins, which gives it an astringent taste (Muchuweti et al., 2006). Tannins reduce blood pressure, speed up blood clotting, reduce serum lipid levels, and adjust immune responses (Bele et al., 2010). For example, in wine, tannins exhibit potent antioxidant effects against low-density lipoprotein (LDL) (Bele et al., 2010). Muchuweti et al. (2006) reported that ripe and unripe sun-dried U. kirkiana pulp had tannin concentrations of 2 mg/100 g and 2.5 mg/100 g, respectively.

Golding et al. (1998) reported that the biochemical events that occur during the ripening of banana involve the conversion of starch into sugars, flesh softening, and aroma development (Gross et al., 1976), and in some cases, tannin biotransformation by UV light produces bioactive metabolites. The embryo part of the U.kirkiana fruit has the highest tannin concentration of 4.5 mg/100 g (Muchuweti et al., 2006). Flavonoid concentration was found to be 40 mg/100 g and 30 mg/100 g in sun and oven-dried pulp samples, respectively (Muchuweti et al., 2006). Flavonoids are non-nutrient, bioactive compounds (Harnly et al., 2006) that are absent in the fruit flesh (Aherne and O’Brien, 2002), and help to inhibit the oxidation of low-density lipoproteins (LDL) cholesterol (Chen et al., 2007; El-Sayed et al., 2006; Hollman et al., 2010; Huxley & Neil, 2003; Rapizzi et al., 2004).

5. Fruit processing

Over the years, the U. kirkiana fruit has been mostly eaten fresh, but is sometimes processed into juices, squashes, wines, sweet beer, porridge, jams (Figure 3), and cakes (Ngulube et al., 1995). The fruit is used for the production of Masuku, a local brew in Zambia (Muchuweti et al., 2006). Furthermore, there is no evidence of commercial processing of the fruit in Zimbabwe. Processing of fresh fruit is necessary because of the high rate of fruit perishability. Production of U. kirkiana juice involves cutting the fruit skin, pulping the crude mixture in a mortar and pestle, sieving it through an 800 μM sieve, diluting the pulp with cool boiled water, adding sucrose and preservatives, pasteurising at 90°C for 15 min, and cooling and storage (25–32°C) (Ndabikunze et al., 2010).

Pasteurisation reduced the vitamin C content in U. kirkiana fruit juice by 55% (from 4.5 ± 0.324 to 2.1 ± 0.330 mg/100 g) (Ndabikunze et al., 2010). The observed loss in vitamin C levels was attributed to the high pasteurization temperatures (Cradall et al., 1990). During juice storage, the vitamin C content decreased significantly (by 40%) with time. The same trend was observed in citrus fruits (Kadzere et al., 2006). However, despite the reported decrease in the vitamin C content over time, the vitamin C content of U. kirkiana juice was within the allowed amounts recommended for an adult (60 mg per day, Lutham, 1997). It is important to note that most households are unaware of the new processing technologies that may be suitable to meet their needs. Most people rely on their indigenous knowledge systems of processing the fruit at a household level. In addition, there is a need to upgrade and improve the processing techniques. Tiisekwa et al. (2004) recommended that if households are trained on good processing methods in their locality, the fruit could be better processed for home consumption, even at a commercial level.

6. Sensory qualities

Moombe et al. (2014) reported that U. kirkiana was the most preferred indigenous fruit and had an overall mean ranking score of 3 out of 5 in Zambia. In Malawi, the fruit was most preferred because of its sweetness and nutritional value (Campbell et al., 1997; Haq et al., 2008; Ngulube et al., 1995; Ntupanyama et al., 2008; Saka et al., 2008). Ntupanyama et al. (2008) reported the fruit was liked for the following parameters: preference level of sweetness (38%), vitamins (23%), snack value (18%), hunger satisfaction (16%), leisure/habit (3%), and thirst quenching (2%). Consumers regarded taste, cleanliness and flavour as major indicators on which to base their liking of the fruit (Ramadhani & Schmidt, 2008). Studies indicated that 55% of the consumers selected the fruit based on taste, while 32% selected the fruit based on fruit colour (Ntupanyama et al., 2008).

Jam made from the fruit had a higher mean acceptance score of 4.0 out of 5, as compared to that of Strychnos cocculoides fruit jam (Saka et al., 2007). Juices made from U. kirkiana had a higher colour score of 5.36 as compared to those of V. mombassea (4.56), S. berrea (4.63), and A. digitata (4.87) fruits (Ndabikunze et al., 2010).

7. Fruit marketing

U. kirkiana fruit collectors, retailers and vendors usually collect the fruits for selling from naturally grown forests near their homes, and very few people collect the fruits from trees at their homesteads. The marketing system of the fruits is not characterized by a clear separation of marketing activities. This is because anyone can become a fruit collector, a vendor and a consumer at the same time thereby making it difficult to identify the fruit collectors who sale to vendors, identify vendors who then sale to consumers. Fruit collectors tend to use scotch carts, bicycles, buses, and sometimes transport the fruit to markets by hired pick-up trucks.

In most situations, vendors and retailers buy the U. kirkiana fruits from fruit collectors and from the people with the fruit tree at their home and transport them to urban markets where they sell them to other vendors, and to customers (Figure 2). In this review, retailers are defined as formal traders with permanent selling places in urban markets, and who pay the tax. Literature shows that as compared to retailers, vendors sold most of the indigenous fruits. Vendors are the informal traders who sell fruits along the highways, on roadsides, in streets, and at the peripheries of the market places, where they do not have to pay taxes (Ramadhani & Schmidt, 2008).

Unlike most exotic fruits (Mangifera indica, Prunus persica, Psidium guajava), which have a marketing system in place, these indigenous fruits (U. kirkiana and S. cocculoides) lack product differentiation at the production level (Figure 4) as there is no grading, packing or washing of the collected fruits prior to sale at the market (Ramadhani & Schmidt, 2008). Furthermore, the marketing process involves the sale of fruits of mixed sizes (small, medium, large), different colours (brown, yellow), and different levels of freshness. In addition, the marketing of indigenous fruits in Zimbabwe is conducted without an established formal pricing system. This resulted in U. kirkiana and other fruits such as S. cocculoides having varied prices based on the regional locations of the markets. Fruit prices in urban markets were higher than those in semi-urban areas/growth points and rural markets (Ramadhani & Schmidt, 2008).

Figure 4. U. kirkiana fruit marketing

8. Post-harvest handling

Post-harvest darkening of the fruit is due to the impact damage during harvesting, and is an undesirable quality characteristic of the fruit and affects the degree of liking by consumers. Subtropical fruits such as Litchi chinensis darken due to heat stress and the same is also true for the U. kirkiana fruits that undergo post-harvest darkening in October due to higher temperatures, sunlight intensities, and low relative humidity (Kadzere et al., 2006). The skin of the fruit is soft when fully ripened and the heat caused the fruit to darken (Hughes & Haq, 2003). The fruits are collected and carried in museke baskets and tswanda to reduce post-harvest losses in Zimbabwe and Zambia (Figure 5). It is evident that the harvesting methods used reduce fruit quality. Saka et al. (2004) supported these findings and reported that nutrient and quality loss of the fruit occurs at all stages, right from harvesting to marketing. Hughes and Haq (2003) reported post-harvest losses of 40–60%. Furthermore, post-harvest losses could be attributed to a lack of knowledge regarding fruit handling. The handling and uses of U.kirkiana fruit are summarised in Figure 6.

Figure 5. Traditional baskets used to collect U.kirkiana in Zimbabwe (a and b) and Zambia (c)

Figure 6. U. kirkiana fruit handling and use

9. Conclusions

U. kirkiana fruit has the potential to improve the nutritional status, food security, and the livelihood of rural populations in sub-Saharan Africa, especially in the drier, rural regions of Zimbabwe. However, the contribution of indigenous fruits to nutritional requirements and poverty reduction efforts is often unrecognised. The U. kirkiana fruit is underutilised in other parts of Southern Africa and its traditional claims with respect to its nutritional and health benefits have been proven to be true by this review. It is possible to improve the nutritional and economic benefits of the fruit at a household level by processing the fruit into different products and commercialising them as health food products. Indigenous knowledge systems regarding the processing of U. kirkiana fruit at a rural household level need to be verified, upgraded, and optimised as they are diverse as well as unreliable. Also, medicinal claims of the U. kirkiana fruit tree need scientific verification. This review has identified that the fruit has compounds that provides nutritive and health benefits. Furthermore, the fruit can be processed into functional foods which can be commercialised. The impacts of processing the fruits into products on micronutrients need to be documented; hence, the recommendation to assay the micronutrient bioaccessibility once the fruit and its products are consumed and determine customer preferences of the fruit-based food. The distribution and adaptability of U.kirkiana fruits in drier areas will render the fruit an importance source of nutrition, help in alleviating micronutrient deficiency, and improve food security.

Cover image

Source: Author.

Acknowledgements

The authors acknowledge the Central University of Technology, Free State, Department of Agriculture and Chinhoyi University of Technology, Department of Food Science and Technology for technical support and making the review possible.

Additional information

Funding

The project was financially supported by the Central University of Technology, Free State, Research Grant [RGS/grants 2018] through the Research Development and Postgraduate Studies.

Notes on contributors

Armistice Chawafambira

Armistice Chawafambira is a PhD student at Central University of Technology, Free State. He is professionally a Lecturer and Food and Nutrition Scientist under the Department of Food Science and Technology at Chinhoyi University of Technology. His key research areas are Functional foods, Human Nutrition, Dairy Technology and Food Safety.

Mahmood Moosa Sedibe

Mahmood Moosa Sedibe (PhD) is a Professor at Central University of Technology, Free State, South Africa in the Department of Agriculture. His research focus areas are Water Quality, Fertigation, Hydroponics, Soil Science, Sustainability, and Agronomy.

Augustine Mpofu

Augustine Mpofu (PhD) is a Senior Researcher at Chinhoyi University of Technology in the Department of Food Science and Technology. His research interests are Applied Microbiology, Fermentation Biotechnology, and Food Security.

Matthew Achilonu

Matthew Achilonu (PhD) is a Researcher at Mangosuthul University of Technology, Durban, KwaZulu-Natal, South Africa. His current researches are on Synthetic Chemistry, Organic Synthesis, Extraction and Isolation of Natural Products, and Phytochemical Analysis.