Evaluation of the chemical,antinutritional and antioxidant properties of composite flour comprising native and modified acha ( digitaria exilis stapf) flour supplemented with mango kernel seed and soy cake flours

Abstract

Gluten-free flours that are nutritionally balanced with appropriate functional characteristics were developed by supplementation of native and modified acha flours with protein, dietary fiber and antioxidants-rich mango kernel and soy cakes flours. Acha flour was subjected to chemical and enzymatic modifications. The proximate, mineral compositions, bioactive and antinutrients properties of the composite flours were evaluated. The water content of the composite flours with native and chemically modified acha flour was between 7.62 and 9.30%, while that of enzymatic acha flour was between 10.12 and 10.79%. However, samples made with 20 and 30% incorporated mango kernel flour had around 13 and 19% increase in the protein content respectively, others including sample with enzymatically modified acha flour had lower protein content. On the other hand, all samples with enzymatically modified acha flour had between 83 and 100% increase in fibre content. The Na/K ratio of all the samples were less than one, as nutritionally required. Samples with enzymatically modified acha flour had best total flavonoid (0.03–0.77 mgGAE/g), total phenol (2.35–11.99 mgTAE/g) and DPPH radical scavenging activities (58.29–94.02%) contents. In addition, samples with enzymatically modified acha flour had the least antinutritional values. Although all the samples had values that were significantly (p ≥ 0.05) different, the samples had significant protein, dietary fiber, minerals and antioxidants contents, while the antinutritional contents were well lower than the standard.

Keywords

Acha flour antioxidants antinutrients composite flour flour modifications mango kernel flour

Introduction

Modification is a change in the physicochemical properties of native starch which could be physical, chemical or enzymatic and it is employed to facilitate enhanced characteristics of the starch which includes the proximate composition, gelatinization, retrogradation pasting properties amongst others (Ashogbon and Akintayo, 2014) that are not achievable with native flours. Several researchers (Adebowale et al., 2006; Vieira and Sarmento, 2008) have worked on starch modifications and it could be inferred that physicochemical properties of flours could also be enhanced through modification since they contain starch (Ramesh, 2007).

Composite flour encourages reduction in wheat utilization for baked products and subsequent use of locally available crops. This leads to reduction in the consumption of gluten primarily, while secondarily, encourages economic well-being of developing countries (Awolu et al., 2017a; Hasmadi et al., 2014). Acha is a traditional cereal cultivated in most West African countries including Nigeria but is however being underutilized despite its importance culturally, nutritionally and economically (Ballogou et al., 2013). Mango kernel seed is a by-product of the consumption or industrial processing of mango (Mangifera indica) fruits, but with high bioactive contents (Awolu et al., 2018; Bandyopadhyay et al., 2014). Mango kernel seed flours has been substituted into wheat and other non-wheat flours for the development of functional baked products (Awolu et al., 2018). Soybean (Glycine max), on the other hand, is a legume of major dietary and economic importance in Nigeria according to Bolarinwa et al. (2015). It is an excellent protein rich legume that also contains calcium, iron, phosphorus vitamins, all essential amino acids and essential minerals (Ihekoronye and Ngoddy, 1985; Jiang et al., 2000). Composite flours consisting soy beans, cereals and some tubers have been developed for the production of nutritional rich baked products (Awolu et al., 2017b). This study evaluated the effect of substitution of mango kernel seed and soy cake flours on the nutritional, antioxidant and antinutritional properties of the modified acha flours.

Materials and methods

Sample collection

Acha (Digitaria exilis Stapf) was purchased from Kaduna in Kaduna State while mango kernel seed and soybeans (Glycine max) were obtained from Akure, Ondo State, all in Nigeria. All reagents used were of analytical grade.

Preparation of acha flour

Acha flour was prepared using the method described by Ikujenlola (2014). The grains (1 kg) were washed thoroughly with water to eliminate dirt, oven dried for 12 h at 60 °C after which it was milled using the coffee mill attachment of the Moulinex domestic food blender into flour and packaged in a container for subsequent use.

Preparation of defatted mango kernel seed flour

Mango kernel seed flour was prepared using the method described by Bandyopadhyay et al. (2014) while the convention Soxhlet extraction method described by Awolu et al. (2018) was used for oil extraction. The mango seeds (1 kg) were collected, washed thoroughly and air-dried. They were then cracked manually and the kernels removed, chopped and dried at 55 °C. The kernels were subsequently milled using the coffee mill attachment of the Moulinex domestic food blender into flour and the mango kernel seed flour was defatted using n-hexane as solvent. For the soxhlet extraction, about 3 g of milled sample was placed in a soxhlet apparatus containing 150 ml solvent. A rotary vacuum evaporator (Eyela, A-1000S, Japan) was used to remove the excess solvent while the cake was collected, packaged in sealed polyethylene bag and stored at room temperature for further use.

Preparation of defatted soy cake

The method of Nwakalor and Obi (2014) was employed to produce soy cake flour with slight modifications while the conventional Soxhlet extraction method of Awolu et al. (2018) was used for oil extraction. About 1000 g of soybeans were sorted to remove all extraneous materials including stones and pebbles, rinsed and steeped for 10 h. It was then drained and precooked at 100 °C for 15 min, while the hulls were manually removed. Dehulled and cleaned soybeans seeds were dried in cabinet drier at 100 °C for 5 h, milled into flour using the coffee mill attachment of the Moulinex domestic food blender into flour and passed through a 250 µm sieve after which the soy cake was defatted using n-hexane as solvent. About 3 g of milled sample was placed in a soxhlet apparatus containing 150 ml of solvent for the defatting. Excess solvent was removed in a rotary vacuum evaporator (Eyela, A-1000S, Japan) while the cake was stored in a packaged sealed polyethylene bag at room temperature.

Chemical modification (acetylation) of acha flour

The procedure of Awolu and Olofinlae (2016) was adopted. Sample flour was mixed with distilled water (1.5 w/v) and magnetically stirred for 20 min. Having adjusted the pH of the slurry to 8.0 using 1 M NaOH, acetic anhydride (10.2 g) was added and left for 1 h maintaining the pH range at 8.0–8.5. The pH was finally adjusted to 4.5 using 0.5 M HCl, filtered, washed four times using distilled water and air-dried at 28 °C for 48 h.

Enzymatic modification of flour and starch using α-amylase

Enzymatic modification of acha flour using α-amylase was done using the method of Man et al. (2013) with some modifications. About 500 g sample was made into 35% (w/v) (dry basis) suspension and heated to 85 °C with continuous stirring until gelatinization occurred. Dried α-amylase (300 µg) was added to the suspension and held at 85 °C for 48 h in a made in UK Gallenkamp Economy Incubator (size 2) with model no IEG097 XXI 5 after which it was cooled to 25 °C. The suspension was centrifuged in a LD 3/5 Electric adjustable speed centrifuge; model no: 50 ml/8 centrifuge at 3000 r/min for 10 min. at 4 °C to obtain the undissolved residue which was washed thrice with double distilled water to eliminate residual enzyme after which it was frozen at −40°C and then freeze dried at −50°C and 0.1 mPa for 20 h. Dried flour was manually powdered, passed through 100 mesh sieve and packaged for further use.

Experimental design for composite flour formulation

The composite flours were formulated such that acha flour was used as a base; having a fixed quantity in all the composite flours while mango kernel seed and soy cake flour were varied (Table 1). The composite flours were coded NMS (Native acha-Mango kernel seed-Soy cake composite flour), CMS (Chemically modified acha- Mango kernel seed -Soy cake composite flour) and EMS (Enzymatically modified acha- Mango kernel seed -Soy cake composite flour).

Table 1.

Experimental design for composite flour formulation.

Flour samples	Acha flour (%)	Mango kernel seed flour (%)	Soy cake flour (%)
NMS 1	70	30	0
NMS 2	70	20	10
NMS 3	70	10	20
NMS 4	70	0	30
CMS 1	70	30	0
CMS 2	70	20	10
CMS 3	70	10	20
CMS 4	70	0	30
EMS 1	70	30	0
EMS 2	70	20	10
EMS 3	70	10	20
EMS 4	70	0	30

NMS: native acha-mango kernel seed-soy cake composite flour; CMS: chemically modified acha-mango kernel seed-soy cake composite flour; EMS: enzymatically modified acha-mango kernel seed-soy cake composite flour.

1 = 70% acha flour + 30% mango kernel seed flour + 0% soy cake flour.

2 = 70% acha flour + 20% mango kernel seed flour + 10% soy cake flour.

3 = 70% acha flour + 10% mango kernel seed flour + 20% soy cake flour.

4 = 70% acha flour + 0% mango kernel seed flour + 30% soy cake flour.

Proximate analyses of composite flours

The method of AOAC (2005) was used to determine the proximate composition of the composite flours. The method of AOAC (2006) was used. Total carbohydrate was determined as difference between 100 and the total sum of fat, moisture, ash, crude fibre and protein content.

Mineral analysis of samples

Mineral elements including zinc, iron, calcium, copper, manganese, sodium and potassium were evaluated according to the method of AOAC (1990).

Antioxidant analyses of composite flours

The total phenolic content of the samples was determined by the method of Singleton et al., (1999) while total flavonoid content was determined using a colorimeter assay developed by Bao (2005) and DPPH (1, 1-diphenyl–2-picrylhydrazyl) scavenging ability was evaluated as described by Gyamfi et al. (1999).

Anti-nutritional analyses of composite flours

The method of Makkar and Goodchild (1996) was used for tannin determination; oxalate content was measured using the method of Day and Underwood, (1986); phytate determination (AOAC, 2005), and trypsin inhibitor was determined using the method of Smith et al. (1980).

Results and discussion

Proximate composition (%) of native and modified acha composite flours

The results of the proximate composition of composite flours consisting native and modified acha-mango kernel and soy cake flours are presented in Table 2. The moisture content of native acha- mango kernel seed -soy cake composite flours (NMS 1–4) ranged between 7.62 and 8.27%. The moisture content increased with increased mango kernel seed flour and decreased soy cake flour. The moisture content of the chemically modified acha- mango kernel seed -soy cake composite flour (CMS 1-4) ranged between 9.03 and 9.30% while enzymatically modified acha-mango kernel seed-soy cake composite flour (EMS 1–4) had moisture content ranging between 10.12 and 10.79%. The increase in the moisture content of modified flours was as a result of the drying temperatures of the samples. However, the moisture contents are within acceptable limits, maximum of 15.5% (CODEX STAND, 1985). High moisture content in flour however, enhances microbial proliferation (Quiñones et al., 2015).

Table 2.

Proximate composition of native and modified acha composite flours.

Samples	Moisture (%)	Protein (%)	Fat (%)	Ash (%)	Fibre (%)	Carbohydrate (%)
Native flour
NMS 1	8.27 ± 0.01a	8.12 ± 0.02d	8.62 ± 0.02d	1.96 ± 0.01a	0.65 ± 0.01d	72.31 ± 0.02a
NMS 2	7.87 ± 0.02b	13.37 ± 0.06c	9.17 ± 0.02c	1.97 ± 0.01a	0.72 ± 0.02c	66.86 ± 0.02b
NMS 3	7.70 ± 0.01c	20.19 ± 0.02b	9.69 ± 0.12b	1.97 ± 0.00a	0.87 ± 0.01b	59.52 ± 0.02c
NMS 4	7.62 ± 0.02d	24.33 ± 0.04a	9.75 ± 0.02a	1.97 ± 0.01a	0.91 ± 0.02a	55.42 ± 0.01d
Chemically modified flour
CMS 1	9.30 ± 0.01a	9.63 ± 0.01d	7.45 ± 0.01d	1.91 ± 0.01c	0.73 ± 0.01c	70.97 ± 0.01a
CMS 2	9.08 ± 0.01b	15.14 ± 0.01c	9.22 ± 0.01c	1.93 ± 0.01b	0.83 ± 0.02b	63.80 ± 0.01b
CMS 3	9.04 ± 0.02c	19.37 ± 0.02b	10.91 ± 0.01b	1.92 ± 0.02bc	0.88 ± 0.02a	57.89 ± 0.00c
CMS 4	9.03 ± 0.01c	24.17 ± 0.02a	11.74 ± 0.03a	1.98 ± 0.01a	0.87 ± 0.01a	52.22 ± 0.01d
Enzymatically modified flour
EMS 1	10.77 ± 0.02ab	8.58 ± 0.01d	7.56 ± 0.01d	1.90 ± 0.01c	1.29 ± 0.01d	69.90 ± 0.02a
EMS 2	10.79 ± 0.01a	12.50 ± 0.01c	9.91 ± 0.01c	1.94 ± 0.02b	1.49 ± 0.00c	63.38 ± 0.02b
EMS 3	10.12 ± 0.00d	16.54 ± 0.01b	10.36 ± 0.01b	1.95 ± 0.00ab	1.59 ± 0.00b	59.31 ± 0.01c
EMS 4	10.32 ± 0.20c	22.84 ± 0.01a	10.90 ± 0.01a	1.97 ± 0.02a	1.70 ± 0.01a	52.47 ± 0.01d

Means of triplicate determinations ± standard deviation. Means with different superscripts on the same column are significantly different at (p < 0.05).

1 = 70% acha flour + 30% mango kernel seed flour + 0% soy cake flour.

2 = 70% acha flour + 20% mango kernel seed flour + 10% soy cake flour.

3 = 70% acha flour + 10% mango kernel seed flour + 20% soy cake flour.

4 = 70% acha flour + 0% mango kernel seed flour + 30% soy cake flour.

The carbohydrate content of the modified flours was lower compared with the native acha based flours which suggests that the modification process led to a depletion in carbohydrate content. Substitution of acha flour with mango kernel seed flour and soy cake further resulted in reduction of the carbohydrate content of the composite flour. The flours have sufficient carbohydrate contents for sufficient energy needs (Butt and Batool, 2010).

Samples NMS 1, CMS 1 and EMS 1 had the least protein content due to non-inclusion of soy cake but the protein contents were higher than protein content of 100% acha flour. Temple and Bassa (1991) reported a protein content of 7% in 100% acha flour while 5.75% was reported by Anyika (2003) which signified that substitution with mango kernel seed flour resulted in increased protein content. Samples NMS 4, CMS 4 and EMS 4 supplemented with soy cake flour had the highest (24.33%) protein content due to the increased amount of soy cake flour inclusion. Soy cake flour incorporation resulted in higher protein content than mango kernel seed flour incorporation.

Mango kernel seed and soy cake flours inclusion increased the fat content of the composite flours. The modified flour samples also followed the same trend of increased fat content with increase in soy cake flour substitution. The fat content of 100% acha flours ranged between 2.20 and 2.80% (Temple and Bassa, 1991). Badejo et al. (2017) reported fat content range of 8.74 and 9.38% for plantain- moringa flour blends and opined that they were low values that will not encourage rancidity during flour storage.

The results of the ash content of the NMS flours indicated that it ranged from 1.96 and 1.97%. Modifications reduced the ash contents. The ash content of 100% native acha flour as reported by Temple and Bassa (1991) was 1.08%. The flour samples, however, had better ash content compared with 100% acha flour.

The fibre content of NMS 1-4, CMS 1-4 and EMS 1-4 flour samples were, 0.65 and 0.91%, 0.48 and 1.39% and 0.73 and 0.88% respectively; 100% native acha flour had no fibre content as reported by Anyika (2003). Fibre content of the flours increased with increased inclusion of mango kernel seed flour; mango kernel seed contained more fibre than soy cake. The values obtained was within the recommended limit stated by Nigerian Raw Materials Research and Development Council (Olaoye and Onilude, 2008). Olapade and Aworh (2012) reported a range of 0.05 and 0.81% crude fibre in blends of fonio and cowpea and established that the values were high enough to give bulkiness to food and prevent colon diseases. Foods rich in dietary fibre also helps in insulin sensitivity improvement and other disease conditions (Slavin, 2008) however, high content (above 20%) could result in food indigestible in the human body (Ijah et al., 2014).

Minerals composition (mg/100 g) of native and modified acha composite flours

The result of the minerals composition of native and modified acha-mango kernel seed-soy cake composite flour is presented in Table 3. However, the most abundant of the minerals are phosphorus and potassium. The iron content of NMS 1-4, CMS 1-4 and EMS 1-4 flours ranged between 7.09 and 11.29 mg/100 g, 7.29 and 12.38 mg/100 g and 11.68 and 12.94 mg/100 g respectively. An increase in iron content was observed in EMS flours compared to NMS flours while CMS flours had a decrease in iron content except CMS 1. Iron content in NMS flours increased with increased soy cake flour substitution and lesser mango kernel seed flour while the reverse was observed in the modified flours and this suggests that modification is capable of reducing the iron content of the substituted flours.

Table 3.

Mineral composition (mg/100 g) of native and modified acha composite flours.

Sample	Zn	Fe	Ca	P	Mn	Cu	Na	K	Cd	Pb
Native flour
NMS 1	2.04 ± 0.10d	7.09 ± 0.12d	20.36 ± 0.62d	75.40 ± 0.62d	0.39 ± 0.00d	1.16 ± 0.07a	0.19 ± 0.00b	23.75 ± 0.20d	BDL	BDL
NMS 2	2.50 ± 0.20c	9.60 ± 0.11c	49.20 ± 0.00c	86.04 ± 0.22c	0.58 ± 0.01c	1.00 ± 0.10c	0.20 ± 0.03a	26.50 ± 0.20c	BDL	BDL
NMS 3	2.89 ± 0.00b	9.92 ± 0.20b	69.71 ± 0.07b	91.28 ± 0.43b	0.84 ± 0.02b	1.06 ± 0.07b	0.19 ± 0.02b	27.79 ± 0.11b	BDL	BDL
NMS 4	3.07 ± 0.04a	11.29 ± 0.09a	88.71 ± 0.13a	98.66 ± 0.01a	1.08 ± 0.01a	0.99 ± 0.20c	0.20 ± 0.01a	29.40 ± 0.00a	BDL	BDL
Chemically modified flour
CMS 1	2.08 ± 0.12d	12.38 ± 0.05a	22.77 ± 0.05d	80.00 ± 0.00d	0.50 ± 0.02d	0.79 ± 0.07d	0.20 ± 0.00d	0.79 ± 0.07d	BDL	BDL
CMS 2	2.20 ± 0.15c	8.30 ± 0.02b	52.19 ± 0.00c	89.05 ± 0.22c	0.70 ± 0.00c	0.90 ± 0.06c	0.20 ± 0.11c	0.90 ± 0.06c	BDL	BDL
CMS 3	2.50 ± 0.00b	7.80 ± 0.14c	80.20 ± 0.10b	94.22 ± 0.16b	0.90 ± 0.12b	1.20 ± 0.20b	1.20 ± 0.20b	3.08 ± 0.04b	BDL	BDL
CMS 4	2.62 ± 0.02a	7.29 ± 0.08d	85.89 ± 0.07a	99.16 ± 0.51a	1.12 ± 0.09a	1.40 ± 0.00a	1.40 ± 0.00a	6.10 ± 0.05a	BDL	BDL
Enzymatically modified flour
EMS 1	1.57 ± 0.20d	12.94 ± 0.20a	19.90 ± 0.13d	70.02 ± 0.21d	0.29 ± 0.07d	1.55 ± 0.10d	0.10 ± 0.01c	19.22 ± 0.13d	BDL	BDL
EMS 2	1.60 ± 0.12c	12.91 ± 0.07b	47.30 ± 0.00c	79.31 ± 0.35c	0.53 ± 0.49c	1.60 ± 0.20c	0.10 ± 0.00c	23.04 ± 0.14c	BDL	BDL
EMS 3	2.62 ± 0.17b	12.06 ± 0.03c	71.83 ± 0.14b	86.76 ± 0.17b	0.96 ± 0.12b	1.85 ± 0.10b	0.18 ± 0.09b	26.73 ± 0.00b	BDL	BDL
EMS 4	2.94 ± 0.11a	11.68 ± 0.10d	86.96 ± 0.20a	97.64 ± 0.53a	1.09 ± 0.00a	2.16 ± 0.12a	1.08 ± 0.10a	27.35 ± 0.12a	BDL	BDL

Means of triplicate determinations ± standard deviation; means with different superscripts on the same column are significantly different at (p < 0.05).

1 = 70% acha flour + 30% mango kernel seed flour + 0% soy cake flour.

2 = 70% acha flour + 20% mango kernel seed flour + 10% soy cake flour.

3 = 70% acha flour + 10% mango kernel seed flour + 20% soy cake flour.

4 = 70% acha flour + 0% mango kernel seed flour + 30% soy cake flour.

Zn: zinc; Fe: iron; Ca: calcium P: phosphorus; Mn: manganese; Cu: copper; Na: sodium; K: potassium; Cd: cadmium; Pb: lead, BDL: below detectable level.

The calcium content of NMS, CMS and EMS flours ranged between 20.36 and 88.71 mg/100 g 22.77 and 85.89 mg/100 g and 19.90 and 86.96 mg/100 g, respectively. CMS flours increased in calcium content with increased soy cake flour incorporation, while the calcium content reduced in EMS flours. However, calcium content increased with increasing soy cake flour supplementation. The enzyme action during the enzymatic modification of acha flour could have led to a reduction in the calcium content of the composite flours. The results obtained in this study is in agreement with the report of Garg et al. (2016) that iron and calcium content of flours increases with increase in soy bean inclusion. However, modification reduced the contents.

There was a drastic reduction in the potassium content of CMS flours with respect to NMS flours which could be as a result of acetylation of the acha flour. On the other hand, there was minimal reduction in potassium content of EMS flours compared with NMS flours. The potassium content of all the composite flours however, increased with increased soy cake flour substitution.

Other minerals present in the composite flours included zinc, manganese, copper and sodium. The zinc component of NMS flours ranged between 2.04 and 3.07 mg/100 g; CMS flours between 2.08 and 2.62 mg/100 g; and EMS flours between 1.57 and 2.94 mg/100g which varies directly with soy cake flour substitution. There was a reduction in the zinc content of the modified composite flours compared with the native acha composite flour which suggests that modification had a reducing effect in the zinc content of the flours.

Acetylation had an increasing effect in the manganese content of the CMS flours, while enzymatic modification rather had a reducing effect on the EMS 1 and 2 and an increase in EMS 3 and 4. All the flours however, had similar trend of increasing manganese content with increased soy cake flour which could be as a result of high manganese content in soy cake flour. NMS flours had copper content ranging between 0.99 and 1.16 mg/100 g while the modified flours had values ranging between 0.79 and 1.40 mg/100 g in CMS flours and 1.55 and 2.16 mg/100 g in EMS flours. Acetylation reduced the copper content of CMS 1 and 2 and increased the copper content of CMS 3 and 4. On the other hand, enzymatic modification led to an increase in the copper content of EMS flour samples.

There was a sharp increase in sodium content of CMS 3 and 4 and EMS 4. The study revealed that all the composite flours (Table 4) had Na/K within the recommended value (<1.0) but Ca/P (>2.0) did not (FAO/WHO, 1991); which implies that while the use of the composite flours may not trigger high blood pressure, there will be need for a complementation of calcium from other sources for the formation and maintenance of healthy bones. Regardless of the depletion in some of the mineral components of the flours, the values obtained could be regarded as sufficient to meet the essential body needs while the sodium content of the flours is minimal and may not pose any health challenges.

Table 4.

Mineral ratios of native and modified acha-mango kernel seed-soy cake composite flours.

Sample	Ca/P	Na/K
Native flour
NMS 1	0.27 ± 0.22d	0.01 ± 0.00a
NMS 2	0.57 ± 0.00c	0.01 ± 0.03a
NMS 3	0.76 ± 0.16b	0.01 ± 0.02a
NMS 4	0.89 ± 0.13a	0.01 ± 0.00a
Chemically modified flour
CMS 1	0.28 ± 0.05d	0.25 ± 0.07b
CMS 2	0.59 ± 0.22c	0.22 ± 0.01d
CMS 3	0.85 ± 0.16b	0.39 ± 0.04a
CMS 4	0.87 ± 0.14a	0.23 ± 0.00c
Enzymatically modified flour
EMS 1	0.28 ± 0.12d	0.01 ± 0.00b
EMS 2	0.60 ± 0.35c	0.01 ± 0.00b
EMS 3	0.83 ± 0.35b	0.01 ± 0.00b
EMS 4	0.89 ± 0.38a	0.04 ± 0.20a

Means of triplicate determinations ± standard deviation; means with different superscripts on the same column are significantly different at (p < 0.05).

1 = 70% acha flour + 30% mango kernel seed flour + 0% soy cake flour.

2 = 70% acha flour + 20% mango kernel seed flour + 10% soy cake flour.

3 = 70% acha flour + 10% mango kernel seed flour + 20% soy cake flour.

4 = 70% acha flour + 0% mango kernel seed flour + 30% soy cake flour.

Antioxidant properties of native and modified acha composite flours

The bioactive properties of native and modified acha flour based samples are presented in Table 5. There was a reduction in the phenolic content of CMS composite flours compared with the NMS composite flours which suggests that acetylation of acha flour reduced the phenolic content of acha flour. The phenolic content of the flours increased with increased substitution of mango kernel seed flour. Phenolic contents scavenge free radicals in the human body thereby promoting their health (Fang et al., 2002).

Table 5.

Antioxidant properties of native and modified acha composite flours.

Sample	Total phenol (mg TAE/g)	Total flavonoids (mg GAE/g)	DPPH (%)
Native flour
NMS 1	11.31 ± 0.03a	0.57 ± 0.01a	86.44 ± 0.09a
NMS 2	9.34 ± 0.03b	0.26 ± 0.01b	77.67 ± 0.09b
NMS 3	5.30 ± 0.03c	0.22 ± 0.01c	28.85 ± 0.09c
NMS 4	3.38 ± 0.03d	0.08 ± 0.01d	24.98 ± 0.09d
Chemically modified flour
CMS 1	11.19 ± 0.03a	0.57 ± 0.01a	79.97 ± 0.09a
CMS 2	5.81 ± 0.03b	0.17 ± 0.01b	78.46 ± 0.09b
CMS 3	4.40 ± 0.03c	0.16 ± 0.01b	72.22 ± 0.08c
CMS 4	4.03 ± 0.03d	0.12 ± 0.01c	23.96 ± 0.09d
Enzymatically modified flour
EMS 1	11.99 ± 0.02a	0.77 ± 0.01a	94.02 ± 0.09a
EMS 2	9.23 ± 0.03b	0.69 ± 0.01b	87.56 ± 0.09b
EMS 3	6.66 ± 0.01c	0.53 ± 0.01c	67.56 ± 0.09c
EMS 4	2.35 ± 0.01d	0.03 ± 0.01d	58.29 ± 0.09d

Means of triplicate determinations ± standard deviation; means with different superscripts on the same column are significantly different at (p < 0.05).

TAE: tannic acid equivalent; GAE: gallic acid equivalent; DPPH: 1,1-diphenyl-2picrylhydrazyl; NMS: native acha-mango kernel seed-soy cake composite flour; CMS: chemically modified acha-mango kernel seed-soy cake composite flour; EMS: enzymatically modified acha-mango kernel seed-soy cake composite flour.

1 = 70% acha flour + 30% mango kernel seed flour + 0% soy cake flour.

2 = 70% acha flour + 20% mango kernel seed flour + 10% soy cake flour.

3 = 70% acha flour + 10% mango kernel seed flour + 20% soy cake flour.

4 = 70% acha flour + 0% mango kernel seed flour + 30% soy cake flour.

The result of the total flavonoid content showed that acetylation reduced the flavonoid content while the enzymatic modifications increased it. The flavonoid content of the composite flours increased with increased mango kernel seed flour which implies that the substitution with mango kernel seed gave rise to the flavonoid content. Flavonoids are active antioxidants with good free-radical scavenging activities which prevent coronary heart diseases as well as cancerous activities (Yao et al., 2004).

The 2,2-diphenyl picry hydrazyl (DPPH) radical scavenging ability of the samples indicated that EMS composite flours had the highest radical scavenging ability. DPPH free radical is a measure of free radical scavenging activity where antioxidants donate hydrogen in food materials (Qiao et al., 2009).

The variation in the bioactive components of all the flour samples followed a similar pattern of increase in bioactive components with increase in mango kernel seed flour. Oil seeds and legumes have been reported to contribute to bioactive properties of composite flours (Olorunfemi et al., 2018). Similarly, mango kernel seed flour have been found to be rich in antioxidants compounds (Ashoush and Gadallah, 2011). Antioxidants have anti-inflammatory, anti-allergic and anti-cancer properties (Crozier and Ashihara, 2006).

Antinutritional properties of native and modified acha composite flours

The results of the antinutritional properties of native and modified acha-mango kernel seed -soy cake composite flours are presented in Table 6.

Table 6.

Anti-nutritional properties of native and modified acha composite flours.

Sample	Tannin (mg/g)	Oxalate (mg/g)	Phytates (mg/g)	Trypsin Inhibitor (mg/g)
Native flour
NMS 1	4.81 ± 0.01a	1.76 ± 0.05a	21.01 ± 0.41a	11.54 ± 0.00d
NMS 2	4.40 ± 0.01b	0.99 ± 0.00b	16.89 ± 0.41b	12.83 ± 0.10c
NMS 3	4.23 ± 0.01c	0.68 ± 0.05c	14.01 ± 0.00c	16.84 ± 0.01b
NMS 4	2.60 ± 0.01d	0.54 ± 0.00d	11.13 ± 0.42d	21.61 ± 0.46a
Chemically modified flour
CMS 1	4.76 ± 0.01a	2.03 ± 0.05a	17.72 ± 0.42a	12.90 ± 0.03c
CMS 2	4.60 ± 0.01b	1.04 ± 0.05b	14.01 ± 0.00b	13.12 ± 0.05c
CMS 3	4.46 ± 0.01c	0.50 ± 0.05c	12.38 ± 0.02c	17.53 ± 0.02b
CMS 4	1.29 ± 0.01d	0.36 ± 0.00d	6.42 ± 0.18d	23.36 ± 0.62a
Enzymatically modified flour
EMS 1	4.73 ± 0.01a	1.31 ± 0.05a	14.42 ± 0.41a	14.57 ± 0.00d
EMS 2	4.55 ± 0.01b	1.13 ± 0.05b	12.77 ± 0.41b	16.08 ± 0.02c
EMS 3	4.43 ± 0.01c	0.90 ± 0.00c	10.71 ± 0.00c	19.75 ± 0.01b
EMS 4	2.62 ± 0.01d	0.41 ± 0.05d	9.48 ± 0.42d	25.42 ± 0.56a
Critical values	30 mg/g	2.5 mg/g	50–60 mg/g	2.5 mg/g

Means of triplicate determinations ± standard deviation; means with different superscripts on the same column are significantly different at (p < 0.05).

1 = 70% acha flour + 30% mango kernel seed flour + 0% soy cake flour.

2 = 70% acha flour + 20% mango kernel seed flour + 10% soy cake flour.

3 = 70% acha flour + 10% mango kernel seed flour + 20% soy cake flour.

4 = 70% acha flour + 0% mango kernel seed flour + 30% soy cake flour.

The tannin content of NMS 1-4, CMS 1-4 and EMS 1-4 composite flours ranged between 2.60 and 4.81 mg/g, 1.29 and 4.76 mg/g and 2.62 and 4.73 mg/g respectively. Echendu et al. (2009) reported 0.0013 mg/g tannin content for 100% acha flour while a range of 56.5–75 mg/g was reported by Ravindran and Sivakanesan (1996) and Ashoush and Gadallah (2011) for mango kernel seed flour which suggests that substitution with mango kernel seed led to increased tannin content in the composite flour. However, defatting could have led to the reduced tannin content of the composite flours. Also, it was observed that modification had little or no effect on tannins. Tannins interfere with proteins, causing a decrease in digestibility and also prevent dietary iron absorption in the body (Gemede and Ratta, 2014).

The oxalate content of the flours ranged between 0.54 and 1.76 mg/g for NMS composite flours, 0.36 and 2.03 mg/g for CMS composite flours and 0.41 and 1.31 mg/g for EMS composite flours. Echendu et al. (2009) reported 0.0114 mg/g oxalate content for 100% acha flour while a range of 0.0119–0.042 mg/g was reported by Ravindran and Sivakanesan (1996) for mango kernel seed flour. Nwosu (2011) reported a range of 2–5 mg/g oxalate as dangerous and unfit in flours for consumption. Oxalates chelate calcium hence making it unavailable in the body (Barasi, 2003).

The phytate content of the flours containing modified acha flour (CMS and EMS) decreased compared with the flours containing native acha flour (NMS). Echendu et al. (2009) reported 0.0122 mg/g phytate content for 100% acha flour while a range of 0.054 and 4.87 mg/g was reported by Fowomola (2010) for mango kernel seed flour. Anti-nutritional factors such as phytates form complexes with metal ions such as zinc, iron and calcium in the human system hence inhibiting their bioavailability and the proteins needed for growth and development (Soetan and Oyewole, 2009). Nwosu (2011) reported 50–60 mg/g phytate level in flours as unsafe for human consumption.

The tannin, oxalate and phytate contents of all the flour samples increased with increased substitution of mango kernel seed flour which suggests that mango kernel seed flour contained more of the antinutritional properties. However, defatting could have contributed to the reduced antinutrient contents in the composite flours. The antinutritional contents of all the composite flours could be said to be lower than the amount considered unsafe for human consumption (Okwuonu et al., 2013).

The trypsin inhibitor content of NMS 1-4, CMS 1-4 and EMS 1-4 composite flours ranged between 11.54 and 21.61%, 12.90 and 23.36% and 14.57 and 25.42% respectively. Soy cake flour contains 15% (0.0023 g/kg) trypsin inhibitor (Ari et al., 2012). Increased substitution of the composite flour with soy cake flour led to increased trypsin inhibitor content. Modification however, reduced the trypsin inhibitor content of the flours containing modified acha flour (CMS and EMS). The trypsin inhibitor content of the flour samples is lower compared with the 2.50 g/kg content reported and considered dangerous in foods for human consumption by Nwosu (2011).

Conclusions

Native and modified (chemical and enzymatic) acha flours were utilized in the production of functional gluten-free composite flour substituted with mango kernel seed and soy cake flours. The proximate composition of the composite flour containing chemically modified acha flour showed better nutritional properties compared with the flours containing native and enzymatically modified acha flour. The mineral content of the composite flours was enhanced with the inclusion of soy cake and mango kernel seed flours. While all the composite flours showed enhanced antioxidant properties, modification aided the reduction of anti-nutritional properties. Overall, the composite flour containing the enzymatically modified acha flour had the best nutritional properties.

Footnotes

DECLARATION OF CONFLICTING INTERESTS

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

FUNDING

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iDs

Margaret A Olorunfemi

Olugbenga O Awolu

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