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Impact of cooking methods on the chemical and antioxidant composition of some indigenous vegetables used in different food dishes in Southeast Nigeria

Journal of Ethnic Foods volume 10, Article number: 6 (2023) Cite this article

Abstract

Background

Some wild and domesticated vegetables of Ibo ethnic tribe of Southeast, Nigeria, namely Piper guineense, Ocimum gratissimum, Solanum melongena L., Gongronema latifolium, Gnetum africanum and Vernonia amygdalina, have gained interest in food culinary uses due to its nutritional, antioxidant potentials and health benefits. These vegetables are rich in fiber, minerals and phyto-nutrients and have significant health benefits against degenerative disorders. Due to these facts, cooking methods aimed at better retention of nutrients and antioxidant compounds were exploited.

Methods

Carefully selected fresh and shredded indigenous vegetables that are commonly used in different food dishes in Southeast Nigeria were cooked (blanched at 98 °C, 2 min and sautéed at 150 °C, 5 min). They were analyzed on a dry weight basis for minerals, vitamins, phytochemicals and antioxidant activity assayed by DPPH, ABTS and FRAP.

Results

Results exhibited wild variations showing that Ocimum gratissimum and Solanum melongena L had higher concentrations of functional minerals Zn, Fe, K and Ca. Vitamins B1 and β-carotene had higher concentrations in Solanum melongena L, Gnetum africanum and Vernonia amygdalina. Ocimum gratissimum revealed higher concentrations of TPC and TFC and maintains strong scavenging activity in ABTS and FRAP, while %DPPH manifested stronger activity in Solanum melongena L. Vernonia amygdalina exhibited higher phytochemicals concentrations, especially the alkaloid content.

Conclusion

Sautéed cooking retained more nutrients and had stronger antioxidant activity than the blanched method. Overall, these vegetables possess high concentrations of functional constituents that can make them be used to boost human nutrition and benefit the health of consumers.

Introduction

The daily diet in Southeast Nigeria is dominated by starchy staple foods where indigenous vegetables are the cheapest and most readily available sources of fiber, micronutrients, protein and antioxidant compounds [1]. Indigenous vegetables feature prominently in food security, socio-economy and livelihood activities of our rural people. They serve as foods to households and some of their income-generating activities, mostly the women are based on these crops [2]. Common indigenous vegetables include but not limited to Vernonia amygdalina (onugbu), Gnetum africanum(okazi), Ocimum gratissimum (nchuanwu), Solanum melongena L. (anara), Gongronema latifolium (utazi) and Piper guineense (uziza) [3]. Indigenous vegetables refer to those vegetable species which are peculiar to a particular region or locality [4].

The Ibo people of Southeast Nigeria (Fig. 1) relishes foods prepared with these vegetables in soups for daily meals and in special occasions like marriages, burials, children dedication or chieftaincy title coronation ceremonies. In most cases, these vegetables are used in combination with other ingredients for soups and sauces like in Vernonia amygdalina (Onugbu) (bitter leaf) soup, melon seed (egusi) soup, Gnetum africanum (okazi) soup, Solanum melongena L. (anara leaf) soup or the spices (Ocimum gratissimum (nchuanwu leaf), Gongronema latifolium (utazi leaf) and Piper guineense (uziza leaf) used for fish/chicken/goat meat pepper soup, also called Ngwongwo and Nkwobi (Fig. 2). As an integral part of the people’s meals, the soups are served and eaten with gari/fufu (fermented and gelatinized cassava paste), boiled or roasted yam and plantain, rice or just as spices in others like Abacha (tapioca/African salad) and sauces, and have been part of the Southeast Nigeria’s cuisine. There are also prepared, sold and served in restaurants and eateries. These vegetables abound along the coastal lines of the West African States like Ghana, Benin, Nigeria and Cameroun. Already, the vegetables are part of local and international commerce involving an estimated millions of dollars, especially among the Southerneast people in other parts of Nigeria and those in diaspora where they are marketed in dehydrated forms and utilized in diasporan kitchens.

Fig. 1
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Map of South Eastern Nigeria

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Fig.2
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Photographic images of soups cooked with indigenous vegetables. Each soup is named after the vegetable used in cooking it: A (G. latifolium (Utazi) Nkwobi cow leg), B (P. guineense (Uziza) leaf soup), C (G. latifolium (Utazi)Abacha/African salad), D (G. latifolium (Utazi) leaf goat meat pepper soup), E (G. africanum (Ukazi) soup), F (G. africanum Ukazi + Egusi soup), G (G. latifolium (Utazi) chicken pepper soup), H (S. melongena (Anara) leaf soup), I (Gari), J (Vernonia amygdalina (Onugbu) soup). Sources: https://9jafoods.com; https://cookpad.com; https://myactivekitchen.com and https://allnigerianfoods.com

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Before now, these indigenous vegetables were harvested mostly from the wild but are nowadays domesticated and grown as garden crops for economic and food safety reasons. A number of these vegetables are consumed cooked or raw as a single or mixed salad, or in soups, sauce, stew and porridge. Others serve as a spice for their aromatic flavor, bitter, hot or pepperish taste. In the light of these, they are in high demand in recent years not only as food, but also as remedial home medicine and for health-promoting benefits. Besides, they are in addition to the diversification of diets rich in vitamins and mineral elements, and to the improvement in functional health. In spite of the value addition to foods among the populous Ibo tribe of Southeast Nigeria (over 70 million people), the nutritional contribution to local diets, and health maintenance and protective properties, these vegetables have not been fully exploited for their biodiverse nutritional and health resources in addressing the complex food, nutritional and health problems of the indigenous population [5].

The eggplant (Solanum melongena) is native to Asia (India) where it grows in the wild. As trade routes opened, the eggplant was introduced to Europe by the Arabs and transported to Africa by the Persians. It requires a warm climate [6]. It is a staple in the cuisines of the Mediterranean region. Piper guineense is an evergreen plant found in rainforest edges. It is native to tropical Central and Western Africa. It may be found in the wild or cultivated in countries such as Nigeria Senegal, Southern Sudan, Zambia, and Tanzania. Both the leaves and the fruits are used as spices [7]. Gongronema latifolium Benth. belongs to the family Apocynaceae. It is a climber with greenish yellow flowers and heart-shaped leaves that exude white/milky latex when plucked. The vernacular names are utazi (Ibo), Arokeke (Yoruba) and Utasi (Ibibio and Efik). It also has medicinal value and is used extensively in African cuisines [7]. Ocimum gratissimum (clove basil) is found in tropical Africa, India, and Southeast Asia. It is cultivated in China, South America, the Caribbean, Australia, New Zealand and many Islands in the Indian and the Pacific region [8]. Vernonia amygdalina occurs in the wild in most countries of tropical Africa and can also be cultivated. It is a member of the daisy family that typically grows to a height of 2–5 [9]. It is used for the management of various diseases in humans and animals and for special cuisines despite its bitter taste. Gnetum africanum is found in the humid tropical forest regions of the Central African Republic, Cameroon, Gabon, Republic of the Congo, Democratic Republic of the Congo and Angola. It is used traditionally in the treatment of many illnesses [10].

Cooking methods have a significant influence on the bioavailability of food nutrients and gastronomy. According to Miglio et al. [11], cooking induces significant changes in the chemical composition, concentration and bioavailability of nutrients and bioactive compounds in vegetables. Cooking methods have both positive and negative effects on the nutrients and antioxidant potential of vegetables. The phenolic content and antioxidant activity of vegetables increased when they were blanched, boiled, fried, steamed or microwaved [11,12,13]. However, blanching at 98  °C for 2 min reduced the phenolics and antioxidant capacity of some vegetables [14, 15]. The proportion at which nutrients in vegetables are retained after cooking relative to the amount of nutrient present in the raw vegetables before processing is referred to as nutrient retention. The retention of nutrients after processing is important to ensure the consumption of foods with high nutrient density for proper human growth, development and warding off sicknesses [16]. This is important to the indigenous people’s nutrition whose major source of micronutrients to enhance their physiological health depends on these vegetables as food. Evaluating the concentration of essential nutrients of these vegetables in cooked form vis-à-vis the uncooked ones for their nutrient retention and health-promoting benefits becomes imperative.

The main objectives of this study were to evaluate some cooked (blanched and sautéed) indigenous vegetable for chemical, phytochemical contents, and antioxidant activity and their contribution to nutrition and health-promoting benefits. Figure 1 shows the map of the Southeast region of Nigeria where the populace utilizes these vegetables extensively for various culinary and medicinal purposes.

Materials and methods

Sources of raw materials

Fresh vegetables: Vernonia amygdalina (onugbu) or bitter leaf, Gongronema latifolium (utazi), Solanum melongena L. (anara) or garden egg leaf, Ocimum gratissimum (nchuanwu) or scent leaf, Piper guineense (uziza) or West African pepper leaf and Gnetum africanum (okazi) (Fig. 3) were procured from Ubani main market, Umuahia, Abia State, Nigeria. Standard reagents were obtained from the Biochemistry Laboratory of National Root Crops Research Institute, Umudike.

Fig. 3
figure 3

Indigenous vegetables used in analysis (and in soup preparation)

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Processing of vegetable samples

Fresh vegetables were processed using the modified method of Bamidele et al. [17]. The vegetables were separately sorted and washed in potable water manually. They were then picked up from the nodes, shredded with a stainless steel knife and divided into three parts of 100 g each.

Processing treatments

Table 1 shows the cooking methods and conditions. Blanched vegetables were done in a stainless steel pot, after which the water was allowed to drain off. King’s brand oil was used for sautéed vegetable samples. The oil was drained off and the vegetables dabbed with clean Whiteman’s paper to absorb excess oil. After processing, the vegetables were allowed to cool at room temperature. Both the uncooked and cooked vegetables were further dried at 50 °C and pulverized into powder for subsequent analysis.

Table 1 Cooking of vegetables by (a) blanched method, (b) sautéed method
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Determination of minerals content of vegetables

Calcium, magnesium and potassium contents of vegetable samples were determined by the method described by Onwuka [18], while zinc, iron and phosphorus contents were determined by the method of AOAC [19].

Determination of vitamins content of the vegetables

The spectrophotometric method described by AOAC [19] was used to determine the β-carotene contents of vegetable samples. Five grams (5 g) of milled vegetable was added to 30 mL of absolute ethanol and 3 mL of 5% potassium hydroxide. The mixture was boiled under reflux for 30 min and cooled rapidly with running water and filtered. Thirty milliliters (30 mL) of distilled water was added to the mixture and was then transferred into a separating funnel. Three portions of 50 mL of ether were used to wash the mixture, the lower layer was discarded, and the upper layer was washed with 50 mL of distilled water. The extract was evaporated to dryness and dissolved in 10 mL of isopropyl alcohol, and its absorbance was measured at 325 nm. β-carotene content of the vegetables was then calculated as follows:

$$\upbeta -\mathrm{carotene }(\mathrm{mg}/100\mathrm{g}) =\frac{100}{w}\times \frac{au}{as} \times \mathrm{c}$$

where: au = absorbance of the test sample, as = absorbance of standard solution, c = concentration of the test sample and w = weight of the sample.

Vitamins B1, B2 and B3 were determined by the method of AOAC [19]. Vitamin C was determined by the method of Okwu and Josiah [20].

Preparation of vegetable extracts

A 50:50 (v/v) acetone/water mixture was the solvent used for extraction according to the modified method of Ukom et al. [21]. About 0.5 g of powdered vegetable samples (triplicate) was mixed with 5 mL of the extraction solvent in a 50-mL BD Falcon tubes using Ultra-Turrax (1Ka T18 basic Staufen, Germany) for 10 s and then capped and remixed in a Vortex mixer (Fisher Scientific, USA) for 1 min. The vegetable samples were placed in a multi-purpose rotator (Barnstead International, USA) for 30 min and then centrifuged at 4 °C for 5 min at 6000 rpm (Eppendorf Centrifuge 5804R Hamburg, Germany). The vegetable extracts (2 mL), each was collected and stored in the dark at 4 °C for determination of total polyphenol, flavonoids, DPPH, FRAP and ABTS.

Determination of total polyphenol content (TPC) of vegetable extract

TPC of the vegetable extract was determined using a Folin--Ciocalteu reagent [22]. A 50 µL of the vegetable extract was added to 0.5 mL Folin--Ciocalteu reagent and vortexed, then 400 µL NaC03 solution (75 g/L) was added after a 3-min reaction time. Thereafter, the mixture was vortexed thoroughly and the absorbance was measured at 765 nm against a blank after 30 min of incubation at room temperature. All samples were prepared and measured in duplicate. Gallic acid was used as the standard, and TPC was expressed as mg gallic acid (GA) equivalent per 100 g sample.

Determination of total flavonoid (TFC) content of vegetable extract

The total flavonoid content of the vegetable extract was determined using the colorimetric AlCl3 method [23]. Briefly, 250 μL of extract solution was diluted with 1.25 mL distilled water and mixed with 75 μL of 5%NaNO2. After 5 min, 150 μL of 10% AlCl3 was added and then incubated for 6 min and 500 μL of 1 mol/L Na0H was subsequently added. The absorbance was measured immediately at 510 nm. Tannic acid was used as the standard, and TFC was expressed as mg tannic acid (TA) equivalent per 100 g sample.

Antioxidant activity of vegetable extract determined by 2,2-Diphenyl-1-picrylhydrazyl (DPPH) assay

DPPH radical scavenging activity of the sample extract was determined according to the method reported by Benzie and Strain [24}. A series of extracts (10, 20, 30, and 40 mg/mL) were prepared using distilled water, and the total volume was maintained at 1 mL. About 250 μL of 0.5 mM methanolic solution of DPPH was added and vortexed. The test tubes were incubated for 30 min in the dark at room temperature. The absorbance was measured at 517 nm. Distilled water was used as control. The scavenging activity of the sample extract was expressed as 50% inhibition concentration (IC50) (mg/mL) and was obtained by interpolation from linear regression analysis.

Antioxidant activity of vegetable extract determined by ferric reducing antioxidant power (FRAP)

The FRAP assay is based on the reduction of the Fe (III)-TPTZ complex to the ferrous form at low pH. This reduction was monitored by measuring the absorption change at 593 nm [25]. One milliliter of the working reagent was mixed with 20 μL of the extract, and the absorbance at 593 nm was recorded after 4 min of incubation at room temperature. The absorption of 1000 μM ferrous sulfate standard was also measured. FRAP values are expressed as mmol of Fe (II) equivalent per 100 g sample.

Antioxidant activity of vegetable extract determined by ABTS+ radical cation

For the ABTS assay, the method modified by Ukom et al. [21] was adopted. ABTS+ was produced by reacting 38.4 mg ABTS and 6.6 mg potassium persulfate in 10 mL of deionized water. The mixture was allowed to stand in the dark at room temperature for 12 h before use. The ABTS+ stock solution was diluted with ethanol to obtain an absorbance of 1.1 + 0.02 at 734 nm. Then, 30 µL of vegetable extract or Trolox standard and 200 µL ABTS+ solution was added together and allowed to react for 6 min before taking absorbance at 734 nm (µQuant, Biotech Instruments, USA). Data were expressed as Trolox equivalent dry weight (TE/gdw). The Trolox standard curve was linear between 50 and 400 µM Trolox.

Determination of phytochemical contents of uncooked and cooked vegetables

The spectrophotometric method of AOAC [19] was used for the determination of phytate. The saponin and tannin contents were determined using the methods of AOAC [19]. The phenol content of vegetable samples was determined by using Folin--Ciocalteu reagent according to the method of Odabasoglu et al. [26]. The gravimetric method of Harbone [27] was used to determine alkaloids.

Statistical analysis

One-way analysis of variance (ANOVA) was carried out on the data generated in this study using the SPSS version 22.0 software. The analyzed data were expressed as mean ± SD (standard deviation). The Duncan multiple range test (DMRT) method was used to compare the means of experimental data at a 95% confidence interval.

Results and discussion

Minerals composition of uncooked and cooked indigenous vegetables

Table 2 presents the results of the minerals composition of uncooked and cooked vegetables. As observed in some cases, cooking of fresh vegetables resulted in significant (p < 0.05) loss of minerals in comparison with uncooked ones. In particular, blanching caused the degradation of soluble minerals in some vegetables probably because of leaching into water and to a high processing temperature. Also, blanched vegetables would have had a minor degree of softening and opening up of the cell matrix, a situation that probably prevented further minerals extraction from the blanched vegetables. Sautéed process lost minimal and/or gained minerals than the uncooked treatment possibly due to better sautéed texture quality. It was considered that sautéed treatment (150 °C for 5 min.) tenderized the vegetables but affected little or no solubilization of minerals. Anara (garden egg) leaf possessed the richest source of Ca, K, Fe and moderate concentration of Zn. Following, utazi leaf had high concentrations of K, Mg and Ca and a moderate amount of P and Fe, respectively. On the other hand, nchuanwu (scent) leaf was the richest source of Zn and Fe. From the study, onugbu (bitter) leaf showed the least minerals concentration, followed by uziza leaf which was only high in Fe, and okazi leaf which was moderate in K and P. The vegetables are of immense minerals nutrition to the indigenous population. They play key functional roles in the human body. Ca, Mg and K act as important regulators of the acid–base balance of the body, Ca and P are essential for skeletal development, while Fe and Zn associate with β-carotene for improved immune function for infants and children. Other researchers support these views on minerals nutrition of vegetables [27,28,29].

Table 2 Mineral composition of uncooked and cooked indigenous vegetables (mg/100 g)
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Vitamins composition of uncooked and cooked indigenous vegetables

The results of vitamins composition of uncooked and cooked vegetables are shown in Table 3. Cooking had a significant impact (p < 0.05) on vitamins concentration. Looking at the β-carotene, sautéed cooking exhibited the highest retention more than uncooked and blanched methods. This agrees with some findings in the literature. Bernhardt and Schlich [30] and Miglio et al. [11] revealed that cooked leafy vegetables increased the release of β -carotene from the matrix by the disruption of carotenoid--protein complexes, leading to better extractability and higher concentration of β -carotene. In this study, blanching determined the highest decrease (mean 19.1%) of β-carotene concentration. Blanching of vegetables at 98 °C in 2 min caused insufficient softening and cellular matrix opening, and thus, a reduction in carotenoids release and extractability. In addition, Pinheiro San’Ana et al. [31] explained that temperature, rather than the presence of water, was a major factor influencing the carotenoids’ stability and concentration during the cooking treatment of carrot. This may suggest the higher β-carotene concentration obtained in sautéed vegetables, and without the involvement of water. The highest β-carotene retention was seen in okazi leaf, followed by anara (garden egg) leaf and onugbu (bitter) leaf in comparison with other vegetables. Green leafy vegetables are rich sources of carotenoids. β-carotene is fat-soluble vitamin that is important for normal vision, and immune competence for the good health of the indigenous people.

Table 3 Vitamin composition of uncooked and cooked indigenous vegetable (mg/100 g)
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Uncooked anara (garden egg) leaf was significantly (p < 0.05) higher than other vegetables in B1 vitamin irrespective of the cooking method, while blanched uziza leaf had the least value. Blanched vegetables retained about 46.2 and 62.7% of the uncooked and sautéed values. Due to cooking, vitamin B2 showed significant variation with blanched uziza and nchuanwu (scent) leaves exhibiting the least values, whereas uncooked utazi leaf posited the highest value. The same trend was observed in vitamin B3 where blanched uziza leaf was seen to have the least value, whereas uncooked utazi leaf manifested the highest value. The clear observation was that uziza leaf (possibly due to its texture) and blanched treatment obtained the least concentration of the B-group of vitamins, and in all cases, blanching retained less than 50% of uncooked and sautéed concentrations of B-group of vitamins. The loss of vitamins during the blanching process is consequential in their solubility into the blanching water and high temperature.

From Table 3, vitamin C varied with blanched okazi leaf showing the least value, followed by blanched anara (garden egg) leaf, whereas uncooked onugbu (bitter) leaf exhibited the highest value, followed by sautéed onugbu (bitter) leaf and blanched onugbu (bitter) leaf, respectively. It showed that onugbu (bitter) leaf (52.63–62.39 mg/100 g) possessed the highest concentration of vitamin C, followed by utazi leaf (10.24–20.03 mg/100 g) and nchuanwu (scent) leaf (7.15–15.78 mg/g) than the other vegetables.

The presence of Fe, Zn, β-carotene and B-groups of vitamins in high concentrations make these vegetables good sources of essential minerals and vitamins for enhanced nutrition and health benefits. Therefore, anara (garden egg) and nchuanwu (scent) leaves with greater contents of Fe, Zn and β-carotene can be used for effective nutrition in combating the incidence of Fe, Zn and vitamin A deficiency that are frequently the cause of morbidity and mortality in children across the Sub Sahara Africa. Little wonder that these vegetables are intuitively used traditionally to boost health status due to the functional role that Fe, Zn and vitamin A can play in hematopoiesis.

Antioxidant properties of uncooked and cooked indigenous vegetables

The effect of cooking on the antioxidant potential of the vegetables is presented in Table 4. Blanching induced a loss of phenolics than the sautéed treatment. Blanching resulted in %DPPH loss of between 6.4 and 36.2%, ranking in ascending order: 6.4% (okazi leaf) < 13.6% anara (garden egg leaf) < 14% (uziza leaf) < 18.5% nchuanwu (scent leaf) < 21.3% onugbu (bitter leaf) < 36.2% (utazi leaf), whereas, sautéed treatment increased the %DPPH scavenging power by 1.7% (okazi leaf) < 2.6% anara (garden egg leaf) < 3% nchuanwu (scent leaf) < 9% (uziza leaf) < 16% (utazi leaf) when compared to uncooked vegetables.

Table 4 Antioxidant properties of uncooked and cooked indigenous vegetables
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ABTS antioxidant activity exhibited greater losses in blanched vegetables and ranked as follows: 9.3% (utazi leaf) < 19.6% (uziza leaf) < 26% nchuanwu (scent leaf) < 26.6% onugbu (bitter leaf) < 33.5 (okazi leaf) < 41.8% anara (garden egg) leaf, whereas, sautéed treatment posited gains in uziza leaf (1.7%) < nchuanwu (scent leaf) (31.5%), when compared to the uncooked vegetable samples. Sautéed vegetables had a mean ABTS (µmTE/gdw) concentration of 20.3% higher than the blanched vegetables.

In this study, blanched treatment (98 °C, 2 min) applied to the vegetables did change their antioxidant scavenging activity, affecting a significant decrease in ABTS more than the %DPPH. These changes indicate that there were thermal instability and lixiviation of hydrophilic-soluble antioxidant compounds from the vegetable matrix into the blanching water [13].

FRAP is an important assay commonly used to determine the antioxidant properties of different food products. The result showed that vegetables cooked and assayed through this condition presented antioxidant properties starting from 3.42 to 21.89 MmolFeII/100 g. Although blanched treatment indicated the lowest concentration, it revealed minimal losses, retaining a mean concentration of 85% of uncooked and 84.8% of sautéed values. Even at high temperature of processing, the vegetables maintained high levels of FRAP antioxidant activity. This implies that Fe3+ was fairly stable in all the treatments as FRAP assay measures the reduction of ferric ion (Fe3+) to the ferrous ion (Fe2+) by donor electron in the samples. The FRAP values of blanched okazi leaf were the least, whereas sautéed nchuanwu (scent) leaf was the highest. The variation in the FRAP values could be attributed to the texture and pro-oxidant extractability of the individual vegetables.

The TPC values of the vegetables showed that blanched utazi leaf exhibited the least value, whereas, sautéed scent leaf performed the highest value. The result of the TPC is in line with that of Zhan et al. [32]. Similarly, the TFC of the samples saw blanched garden egg leaf observing the least value, whereas, sautéed scent leaf had the highest value. The TFC values are quite lower than the result reported by Zhan et al. [32], probably due to the hydrophilicity of flavonoids, texture and method of processing. In this study, scent leaf maintained higher concentrations of TPC (89.92 to 116.74 mgGAE/100 g) and TFC (33.86 to 41.42 mgTAE/100 g). As antioxidant compounds, TPC and TFC would have contributed to the strong antioxidant scavenging activity of the vegetable extracts in vitro.

Most sautéed vegetables were found to have higher antioxidant potential compared to uncooked and blanched vegetables. Turkmen et al. [33] and Mwebi and Ogendi [12] showed that several cooking methods caused increases in the phenolic content of vegetables. They attributed the increase to the loosening of antioxidant moieties and the dehydration of food matrix and enhanced extractability of the antioxidant compounds which have minimal or no water added to them during stir-frying. This may explain why higher antioxidant concentration and strong radical scavenging activity were observed in sautéed cooking for most of the vegetables. However, blanching substantially decreased the antioxidant concentration in the vegetables as was observed by [12]. The losses may be attributed to the solubilization of phenolic compounds into the blanching water [34]. Irrespective of the cooking method, the high TPC, TFC and antioxidant activity contents of the vegetables in the current study have significant relevance for health-promoting benefits against degenerative conditions and associated complications.

Phytochemical composition of uncooked and cooked vegetables

The result presented in Table 5 shows the phytochemical properties of uncooked and cooked vegetables. Phytochemicals are substances that interfere with the nutritional value of foods by reducing mineral absorption and protein digestibility and causing toxicity and health disorders when present in high concentrations [35]. These phytochemicals are mainly found in foods that are consumed in raw forms [36]. Food processing substantially reduced the phytochemical antinutrients to save levels. Results maintained significant variation (p < 0.05) in the phytochemical contents of the cooked vegetables. The alkaloid content varied from 0.06 to 6.38%. Sautéed anara (garden egg) leaf had the least value (0.06%), whereas uncooked onugbu (bitter) leaf had the highest value (6.38%). Also, tannin content varied from 0.02 to 4.09%. Sautéed anara (garden egg) leaf had the least value (0.02%), whereas uncooked nchuanwu (scent) leaf had the highest value (4.09%). These results agree with the report of Okon and Akpanyung [37]. The phytates content varied from 0.05 to 2.72%, with sautéed nchuanwu (scent) leaf having the least value (0.05%), whereas uncooked onugbu (bitter) leaf had the highest value (2.72%). The phytates content of the samples is low when compared to the result of Ekpo [38]. Also, the saponin content varied from 0.01 to 4.73%, with sautéed anara (garden egg) leaf having the least value (0.01%), whereas uncooked onugbu (bitter) leaf had the highest value (4.73%).

Table 5 Phytochemical composition of uncooked and cooked indigenous vegetables (mg/100 g)
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The phenol content varied from 0.02 to 3.20%, with sautéed okazi leaf having the least value (0.02%), whereas uncooked onugbu (bitter) leaf had the highest value (3.20%). Phytochemicals in foods must be rated on their toxicity levels. High contents of phytate and tannin exert negative effects on the bioavailability of protein and some mineral nutrients. According to Codex Alimentarius [39], the lethal dose of phytate is 25 mg/100 g [40], and tannin is 90 mg/100 g [41] which are far above the results in this study. These vegetables will not pose nutritional problems when they are consumed, whether raw or cooked. Accordingly, processing especially sautéed cooking effectively decreased the anti-nutritional factors.

In our modern days of cultural demand for foods, these vegetables that were sometime treated as food for the poor are nowadays sought after by the low  and mighty of the society due to their perceived health benefits. Not only the intuitive use of these vegetables by the ethnic locals for nutrition and health benefits for decades, but they are also being recommended for adequate consumption by nutritionists, dieticians and medical professionals with the hope to ameliorate the preponderance of degenerative disorders. For this also, the food products are in high demand in homes, eateries and restaurants. On these grounds, the results of these analysis are a boost to increasing the awareness and show the  cultural relevance of their continuous use traditionally as vegetable foods. Furthermore, the results can help advance these ethnic food vegetables in our modern days and an effect to increase their production, marketing and utilization as part of the historical values and preservation of our ethnic food system.

Nutrition and health benefits

Health benefits attributed to these ethnic vegetables are not only for improved nutrition, but also for medicinal purposes mostly due to their antioxidants and free radical scavenging properties. Free radicals are known to be responsible for the pathophysiological of several degenerative diseases such as diabetes, cancer and cardiovascular diseases [42]. Antioxidant and other phytochemicals in vegetables have the potential to dissolve free radicals in the human cells [43]. As evidence based and as well in this study, vegetables are the main sources of some functional minerals, vitamins, antioxidant and phyto-nutrients [44]. We can adduce their association with overall health benefits, reduced risk of diabetes, cancer, cardiovascular diseases, anemia and some other degenerative disorders due to their high content of antioxidants and phytochemicals as evidence [45]. Confirming their quantitative phytochemistry analysis and pharmaceutical interest, these vegetables contain flavonoids, alkaloids, saponins, tannins, phenols and carotenoids, all of which function as therapeutic, pharmacological, antioxidant, antibacterial, anti-inflammatory, hepatoprotective, aphrodisiac, anti-hypertensive and anticancer agents [46,47,48,49,50,51]. For example, Oyugi et al. [52] showed that extracts of Vernonia amygdalina inhibited the growth of human breast cancer cells and that the ethanolic extracts lowered blood sugar in rats [53]. Ijeh and Obioda [54] and Egedigwe [55] further posited that Vernonia amygdalina incorporation in the diet had an effect on the hypo-protection and serum lipid modulation in rats. Likewise, Gongronema latifolium (utazi) extract was found to be effective in the treatment of abscesses, boils, bacteremia and entero-infections caused by Staphylococcus aureus [56]. Furthermore, the hypoglycemic, hypolipidemic, nephroprotective, anticancer and immunomodulatory activities of Gongronema latifolium leaf extract were reported by Balogun et al. [57]. Ocimum gratissimum (nchuanwu) leaf was reported to be used in traditional medicine for the treatment of cough, pneumonia, fever, inflammation anemia, diarrhea, pains and fungal/bacterial infections [58]. Moreso, Igwe et al. [59] and De Zoysa et al. [60] did suggest that Solanum melongena (garden egg leaf) benefited patients who suffered from raised intraocular pressure (glaucoma), convergence insufficiency and in the treatment of inflammatory disease, heart failure, neuralgia, ulcer in nose and cholera. The positive effect of Piper guineense on hypolipidemic activity reduced cholesterol intake and induced body weight gain and increased body’s antioxidant defense system [61]. These literature reports validate the efficacy of our indigenous vegetables with regards to the boosting of human nutrition and health benefits, being one of the major reasons for their intuitive use among our ethnic population.

Conclusion

Indigenous vegetables are important sources of income, food security and nutrients to the people as part of their daily meals. However, cooking/culinary methods may either diminish or increase the bioavailability of nutrients. Essentially, vegetables contain antioxidant constituents and fiber which have gained prominence in scavenging free radicals in vivo and thus, boosting the physiological health of consumers. The awareness for the consumption of vegetables as functional foods which contain ingredients that provide additional health benefits beyond the basic nutritional requirements is in increasing demand. Therefore, the utilization of vegetables as a source of dietary antioxidant compounds has positioned them as supplements for the amelioration of the incidence of degenerative disorders. Thus, knowledge derived from this study as it affects some cooking methods can be used for advocacy, increase awareness and improve the health status of the indigenous population, as well as retard oxidative stress in cellular tissues and, therefore, the potential health benefits in the context of indigenous food vegetables.

Availability of data and materials

All data generated or analyzed during this study are included in this manuscript.

References

  1. Ishiekwene IC, Dada TE, Odoko J, Nwose EU. Promoting African indigenous vegetables and its medical nutrition properties: a mini-narrative review based on Ukwani communities of Delta State Nigeria. Integr Food Nutr Metabolism. 2019;6:1–6.

    Google Scholar 

  2. Oladele OI. Contribution of indigenous vegetables and fruits to poverty alleviation in Oyo State, Nigeria. J Hum Ecol. 2011;34(1):1–6.

    Article  Google Scholar 

  3. Ndie EC, Okaka JC, Okoli EC. Evaluation of vegetable consumption in South Eastern Nigeria. Intern J Nutr Metabolism. 2013;5(4):55–60.

    Google Scholar 

  4. Ogunrotimi DG, Kayode J, Odesola AF. Ethnobotany and conservation of indigenous vegetables in Ekiti State, Nigeria. Singapore J Sci Res. 2018;3(3):1065.

    Google Scholar 

  5. Agbugba IK, Thompson D. Economic study of tropical leafy vegetables in South-East of Nigeria: the case of rural women farmers. Am J Agric Sci. 2015;2(2):34–41.

    Google Scholar 

  6. Ansari AM, Hassan W, Prakash M. Home shop imprints nova Solanum melongena: production, cultivation and nutrition. In: Solanum melongena: production, cultivation and nutrition recent advances in biotic and abiotic stresses of Solanum melongena L. 2021; https://doi.org/10.52305/RKTC8440.

  7. Schippers RR. African indigenous vegetables. An overview of the cultivated species. Natural Resources Institute/ACP-EU Technical Centre for Agricultural and Rural Cooperation, Chatham, United Kingdom. 2000.

  8. Ugbogu OC, Emmanuel O, Agi GO, Ibe C, Ekweogu CN, Ude VC, Uche ME, Nnanna RO, Ugbogu EA. A review on the traditional uses, phytochemistry, and pharmacological activities of clove basil (Ocimum gratissimum L.). Heliyon. 2021;7(11):e08404. https://doi.org/10.1016/j.heliyon.2021.e08404.

    Article  Google Scholar 

  9. Akinloye AO, Oyeyemi OT. Oyeyemi IT, Akinlabi AA, Adewumi A, Aleshi. Vernonia amygdalina: a folkloric herb with anthelminthic properties. Beni-Suef Univ J Basic Appl Sci. 2018;7(1), 43–49. https://doi.org/10.1016/j.bjbas.2017.07.007

  10. Hortense Biye E, Balkwill K, Cron GV. A clarification of Gnetum L. (Gnetaceae) in Africa and the description of two new species. Plant System Evol. 2014. https://doi.org/10.1007/s00606-013-0879-6.

    Article  Google Scholar 

  11. Miglio C, Chiavaro M, Visconti A, Fogliano V, Pellegrini N. Effects of different cooking methods on nutritional and physicochemical characteristics of selected vegetables. J Agric Food Chem. 2008;56:139–47.

    Article  Google Scholar 

  12. Mwebi NO, Ogendi BMO. Effect of boiling, steaming, stir-frying and microwave cooking on the antioxidant potential of peppers of varying pungency. Cogent Food Agric. 2020;6:1834661. https://doi.org/10.1080/23311932.2020.1834661.

    Article  Google Scholar 

  13. Huarte E, Júaniz I, Cid C, de Pe˜na M. Impact of blanching and frying heating rate/time on the antioxidant capacity and (poly)phenols of cardoon stalks (Cynara cardunculus L. var. altilis DC). Inter. J. Gastronomy Food Sci. 2021; 26:100415.

  14. Uyan SE, Baysal T, Yurdagel O, El SN. Effects of drying process on antioxidant activity of purple carrots. Nahrung-Food. 2004;48:57–60.

    Article  Google Scholar 

  15. Amin I, Lee WY. Effect of different blanching times on antioxidant properties in selected cruciferous vegetables. J Sci Food Agric. 2005;85:2314–20.

    Article  Google Scholar 

  16. Sharif ZIM, Mustapha FA, Jai J, Mohd-Yusof N, Zaki NAM. Review on methods for preservation and natural preservatives for extending the food longevity. Chem Engr Res Bull. 2017;19:145–53.

    Article  Google Scholar 

  17. Bamidele OP, Fasogbon MB, Adebowale OJ, Adeyanju AA. Effect of blanching time on total phenolic, antioxidant activities and mineral content of selected green leafy vegetables. Curr J Appl Sci Technol. 2017;24(4):1–8.

    Article  Google Scholar 

  18. Onwuka GI. Food analysis and instrumentation, Theory and Practices, Naphthalin Prints, Lagos Nigeria, 2018.

  19. AOAC. Association of Official and Analytical Chemists, Official Methods of Analysis. 18th Edition. Association of Official Analytical Chemists, Washington DC, USA, 2010.

  20. Okwu DE, Josiah C. Evaluation of the chemical composition of two nigerian medicinal plants. Afr J Biotechnol. 2006;5:357–336.

    Google Scholar 

  21. Ukom AN, Ojimelukwe PC, Ezeama CF, Ortiz DO, Aragon IJ. Phenolic content and antioxidant activity of some under-utilized Nigeria yam (Dioscorea spp.) and cocoyam (Xanthosoma mafaffa Schott) root tubers. IOSR J Environ Sci Technol Food Toxicol. 2014;8(7):104–11.

    Article  Google Scholar 

  22. Singleton VL, Rossi JA. Colorimetry of total phenolics with phosphomolybdic-phosphotungstic acid reagents. Am J Enol Viticult. 2015;16(3):144–58.

    Google Scholar 

  23. Bao J, Cai Y, Sun M, Wang G, Corke H. Anthocyanins, flavonols, and free radical scavenging activity of Chinese bayberry (Myrica rubra) extracts and their colour properties and stability. J Agric Food Chem. 2015;53(6):2327–32.

    Article  Google Scholar 

  24. Benzie F, Strain J. The ferric reducing ability of plasma (FRAP) as a measure of antioxidant power: the FRAP assay. Analyt Biochem. 1996;239(1):70–6.

    Article  Google Scholar 

  25. Odabasoglu FA, Ali CA, Suleyman HK, Yalçın HM, Bayir Y. Comparison of antioxidant activity and phenolic content of three lichen species. Phytother Res. 2004;18:938–41.

    Article  Google Scholar 

  26. Harbone ZB. Phytochemical methods: a guide to modern techniques of plant analysis. London: Chapman and Hall; 2007. p. 113–85.

    Google Scholar 

  27. Tamègnon VD, Honoré SB, Roch CJ, Jean RK, Godfried D, Fernand G, Fidèle A, Joachim G, Salifou S, Jean-Marc A, Bertrand HR, Frédéric L, Michel B, Aléodjrodo PE. Phytochemical screening, nutritional and toxicological analyses of leaves and fruits of Solanum macrocarpon Linn (Solanaceae) in Cotonou (Benin). Food Nutr Sci. 2012;3:1595–603.

    Google Scholar 

  28. Tsado AN, Lawal B, Santali ES, Shaba AM, Chirama DN, Balarabe MM, Jiya AG, Alkali HA. Effect of different processing methods on nutritional composition of bitter leaf (Vernonia amygdalina). IOSR J Pharm. 2018;5(6):8–14.

    Google Scholar 

  29. Paul SH, Usman AA, Gana IN, Manase A, Adeniyi OD, Olutoye MA. Comparative study of mineral and nutritional composition of a multifunctional flora composite formulated from seven medicinal plants and their applications to human health. Eng Technol Open Access J. 2018;1(5):137–50.

    Google Scholar 

  30. Bernhardt S, Schlich E. Impact of different cooking methods on food quality: retention of lipophilic vitamins in fresh and frozen vegetables. J Food Eng. 2006;77:327–33.

    Article  Google Scholar 

  31. Pinheiro San’Ana HM, Stringheta PC, Cardoso Brandão SC, Cordeiro de Azeredo RM. Carotenoid retention and vitamin A value in carrot Daucus carota L. prepared by food service. Food Chem. 1998;61:145–51.

    Article  Google Scholar 

  32. Zhan L, Ma Y, Zhang C, Li L, Pang L, Haung X. Antioxidants and antioxidant capacity of leafy, stem and fruit vegetables including 50 spices. J Food Engr Technol. 2018;7(1):8–15.

    Google Scholar 

  33. Turkmen N, Sari F, Velioglu YS. The effect of cooking methods on total phenolics and antioxidant activity of selected vegetables. Food Chem. 2005;93(4):713–8. https://doi.org/10.1016/j.foodchem.2004.12.038.

    Article  Google Scholar 

  34. Chuah AM, Lee YC, Yamaguchi T, Takamura H, Yin LJ, Matoba T. Effect of cooking on the antioxidant properties of colored peppers. Food Chem. 2008;111(1):20–8. https://doi.org/10.1016/j.foodchem.2008.03.022.

    Article  Google Scholar 

  35. Samtiya M, Aluko RE, Dhewa T. Plant food anti-nutritional factors and their reduction strategies: an overview. Food Prod Process Nutr. 2020;2(6):1–14.

    Google Scholar 

  36. Hamid R, Masood A, Wani IH, Rafiq S. Lectins: proteins with diverse applications. J Appl Pharma Sci. 2013;3(4):93–103.

    Google Scholar 

  37. Okon EU, Akpanyung EO. Nutrients and antinutrients in selected vegetables produced in Nigeria. Pak J Nutr. 2015;4(5):352–5.

    Google Scholar 

  38. Ekop AS. Determination of chemical composition of Gnetum Africanum (Afang) seeds. Pak J Nutr. 2017;6(1):40–3.

    Google Scholar 

  39. Codex alimentarius. General standard for contaminants and toxins in Food. FAO/WHO Food standard, 2009.

  40. Nagel R. Living with phytic acid. Wise traditions in Food, Farming and the Healing Arts. The quarterly Journal of the Weston, 2010.

  41. Ifie I, Emeruwa C. Nutritional and anti-nutritional characteristics of lava of Cryctes monocros. Agric Biol J North America. 1992;2010:42–6.

    Google Scholar 

  42. Ramkumar KM, Rajaguru P, Ananthan R. Antimicrobial properties and phytochemical constituents of an antidiabetic plant Gymnema montanum. Food Chem. 2007;2007(1):67–71.

    Google Scholar 

  43. Erasto P, Grierson DS, Afolayan AJ. Evaluation of antioxidant activity and the fatty acid profile of the leaves of Vernonia amygdalina growing in South Africa. Food Chem. 2007;104:636–42.

    Article  Google Scholar 

  44. Dias JS, Ryder E. World vegetable industry: production, breeding, trends. Hortic Rev. 2011;8:299–356.

    Google Scholar 

  45. Keatinge JDH, Waliyar F, Jammadass RH, Moustafa A, Andrade M, Drechsel P, Luther K. Re-learning old lessons for the future of food: by bread alone no longer-diversifying diets with fruit and vegetables. Crop Sci. 2010;50(1):51–62.

    Article  Google Scholar 

  46. Mamta SM, Saxena J, Nema R, Singh D, Gupta A. Phytochemistry of medicinal plants. J Pharmacognosy Phytochem. 2013;1(6):177.

    Google Scholar 

  47. Dibulo CC, Madu KC, Ogbu PN, Onyeachu BI, Njoku DI. Proximate and phytochemical analysis of ethanolic extracts of Leaves of Piper guineense from South-eastern Nigeria. IOSR J Appl Chem. 2017;10(8):46–50.

    Google Scholar 

  48. Egharevba C, Osayemwenre E, Imieje V, Ahomafor J, Akunyuli C, Udu-Cosi AA, Theophilus O, James O, Ali I, Falodun A. Significance of bitter leaf (Vernonia amgdalina) in tropical diseases and beyond: a review. Malaria Chemother Control Elimin. 2014;3(1):1–10.

    Article  Google Scholar 

  49. Chinedu I, Uhegbu FO. Phytochemical analysis of Gongronema latifolium Benth leaf using gas chromatographic flame ionization detector. Inter J Chem Biomol Sci. 2015;1(2):60–8.

    Google Scholar 

  50. Usunobun U, Ekpemupolo IS. Studies on in vitro antioxidant activities, mineral composition and phytochemical screening of Gnetum africanum leaves. Saudi J Biomed Res. 2016;1(2):48–51.

    Google Scholar 

  51. Alara OR, Abdurahman N, Mudalip SKA. Phytochemical and pharmacological properties of Vernonia amygdalina: A review. J Chem Engr Ind Biotechnol. 2017;2:80–9.

    Google Scholar 

  52. Oyugi DA, Luo X, Lee KS, Hill B, Izevbigie EB. Activity markers of the anti-breast carcinoma cell growth fractions of Vernonia amygdalina extracts. Proc Soc Exp Biol Med. 2009. https://doi.org/10.3181/0811-RM-325.

    Article  Google Scholar 

  53. Atangwho IJ, Ebong PE, Eteng MU, Eyong EU, Obi AU. Effect of Vernonia amygdalina Del. leaf on kidney function of diabetic rats. Int J Pharmacol. 2007;3:143–8.

    Article  Google Scholar 

  54. Ijeh II, Obidoa O. Effect of dietary incorporation of Vernonia amygdalina Del. on AFB1-induced hepatotoxicity in weanling albino rats. Jamaican J Sci Tech. 2004;15:32–6.

    Google Scholar 

  55. Egedigwe CA. Effect of dietary incorporation of Vernonia amygdalina and Vernonia colorata on blood lipid profile and relative organ weights in albino rats. MSc., Dissertation, Dept. Biochem. 2010; MOUAU, Nigeria.

  56. Edim EH, Egomi UG, Uwen EF, Arhbong OE. A review on Gongronema latifolium (Utasi): a novel antibiotic against S. aureus related infections. Int J Biochem Biotech. 2012;1(8):204–8.

    Google Scholar 

  57. Balogun ME, Besong EE, Obimma JN, Mbamalu OS, Djobissie SFA. Gongronema Latifolium: a phytochemical, nutritional and pharmacological review. J Phys Pharm Adv. 2016;6(1):811–24. https://doi.org/10.5455/jppa.1969123104000.

    Article  Google Scholar 

  58. Akara E, Emmanuel O, Ude V, Uche-Ikonne C, Eke G, Ugbogu E. Ocimum gratissimum leaf extract ameliorates phenylhydrazine-induced anaemia and toxicity in Wistar rat. Drug Metabol Personalized Therapy. 2021;36(4):311–20.

    Article  Google Scholar 

  59. Igwe SA, Akunyili DN, Ogbogu C. Effects of Solanum melongena (garden egg) on some visual functions of visually active Igbos of Nigeria. J Ethnopharmacol. 2003;86(2–3):135–8. https://doi.org/10.1016/S0378-7841(02)00364-1.

    Article  Google Scholar 

  60. De Zoysa HKS, Pavithra NH, Raymond C, Waisundra VY. Paspanguwa herbal formula, a traditional medicine of Sri Lanka: a critical review. J Complem Med Alter Healthcare. 2017;3(2):555609. https://doi.org/10.19080/JCMAH.2017.03.555609.

    Article  Google Scholar 

  61. Nwozo SO, Lewis YT, Oyinloye BE. The effects of Piper versus Sesamum Indicum aqueous extracts on lipid metabolism and antioxidants in hypercholesterolemic rats. Iran J Med Sci. 2017;42(5):449–56.

    Google Scholar 

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Acknowledgements

The authors acknowledge the Laboratory staff of the Department of Food Science and Technology, Michael Okpara University of Agriculture, Umudike, and Biochemistry Laboratory of National Root Crops Research Institute (NRCRI), Umudike, Nigeria for the analysis of antioxidant activity.

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  1. Department of Food Science and Technology, College of Applied Food Sciences and Tourism, Michael Okpara University of Agriculture, Umudike, Nigeria

    Anthony Ukom, Miracle Albert, Philippa Ojimelukwe, Blessing Offia-Olua & Lilian Nwanagba

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All authors contributed to the manuscript. AU contributed 50%; MA contributed 20%; PO contributed 20%; BO contributed 5%; and LN contributed 5%. All authors read and approved the final manuscript.

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Ukom, A., Albert, M., Ojimelukwe, P. et al. Impact of cooking methods on the chemical and antioxidant composition of some indigenous vegetables used in different food dishes in Southeast Nigeria. J. Ethn. Food 10, 6 (2023). https://doi.org/10.1186/s42779-023-00170-x

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Journal of Ethnic Foods

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