Drying characteristics, nutritional and anti-nutritional properties of locust bean seed

Abstract

The need to ensure availability of locust beans when off-season forms the basis of this study. The study investigated the drying characteristics, nutritional composition and some anti-nutritional contents of dried locust beans. Fermented locust beans were dried at 50°C, 60°C, 70°C and 90°C. The moisture contents of the fermented locust seeds were determined at different time intervals between 0 and 480 min. Lower moisture content was achieved in a shorter drying time at higher temperature. The dried locust bean samples had lower moisture contents (12.36–12.76%) compared to control sample (38.76%). Higher drying temperature resulted in low ash content of the samples but increased the fibre, fat and protein content of the samples. The dried samples had low mineral contents compared to the fresh sample, which was used as control. The most abundant mineral in the samples were found to be calcium and zinc. No significant difference was observed in the titratable acidity and the pH of the locust bean samples at varying drying temperature. Increase in temperature resulted to a reduction in the antioxidant, phenolic composition and the anti-nutritional properties of the dried locust bean samples. Panelists preferred samples dried at lower temperature (50°C and 60°C compared with the samples dried at high temperature. The study concluded that locust bean should be dried using a carefully selected temperature to avoid negative impacts on the nutrients of the beans.

Keywords:

• locust beans
• anti-nutritional factor
• antioxidant properties
• sensory properties
• mineral composition
• drying properties

PUBLIC INTEREST STATEMENT

Inadequate post-postharvest storage facilities in many African countries have increased the need to apply drying techniques as a way of preserving the perishable crops. Locust beans is a condiment of choice by many Africans, and it is expedient to ensure its availability all the year round. In drying the condiment, it is necessary to ensure that the nutrients are not adversely affected. The aim of this work is therefore to investigate the best drying temperature for the locust beans while still minimizing the nutrient loss.

1. Introduction

The locust bean tree (Parkia biglobosa) is a well-known leguminous forest tree in West Africa. The importance of the tree lies in the usefulness of the seeds to West African populace following the process of fermentation (Sanya et al., 2009). The fermented seeds of Parkia biglobosa hold different names in different countries, such as dawadawa or iru in Nigeria, soumbala in Mali and Guinea, netetou in Senegal and afitin, iru or sonru in Benin (Guttierez et al., 2000). In Nigeria, iru serves as a cheap source of proteins, minerals, vitamins, antioxidants and most importantly, an inexpensive condiment in the preparation of stew and soup (Soetan et al., 2014). The use of locust beans in food is popular because it impacts good sensory properties on the food such as flavour and taste.

However, the use of legumes such as locust beans is sometimes being limited due to the presence of some anti-nutritional factors such as tannin, saponin, oxalates, phytates and trypsin inhibitors that adversely affect the overall nutritional value. These anti-nutrients interfere with processes such as digestion, absorption and utilization of nutrients or affect the body’s metabolic rate or exert direct toxic effects (Soetan & Oyewole, 2009). The bioavailability of the essential nutrients could be reduced in the body by the presence of the anti-nutritional factors (Esenwah & Ikenebomeh, 2008). Traditionally, the fermented beans are sold, wrapped in polythene sheet or leaves in the wet form, thus limiting their shelf life. Drying the fermented beans would prolong the shelf life, reduce bulk packaging, and increase its use over time.

Studies have shown that fermentation and drying processes reduce the presence of secondary metabolites such as anti-nutrients while enhancing the nutrient composition such as minerals, vitamins and antioxidant properties (Osman, 2007; Soetan & Oyewole, 2009). Also, Olalusi et al. (2019) mechanically dried fermented locust beans at temperatures ranging between 45°C and 75°C and determined the moisture content, moisture ratio and drying rate, with a conclusion that the samples dried at the highest temperature (75°C) had the lowest drying rate. However, there is a dearth of information on the nutritional composition and the quantities of metabolites at different drying temperatures of locust bean in the literature. Therefore, this study was designed to investigate the effect of different drying temperatures on the anti-nutrient composition, antioxidant and sensory properties of fermented locust bean seeds.

2. Materials and methods

2.1. Collection of materials

The fermented African Locust Beans (Parkia biglobosa) were purchased from Sabo market in Ile-Ife, Nigeria. Other equipment, including cabinet dryer, pH meter, weighing balance, mortar, pestle and microscope and analytical grade chemicals were from the Department of Food Science and Technology, Obafemi Awolowo University, Ile-Ife.

2.2. Drying Process

The fermented locust beans were divided into four portions and each of the portions was dried at 50°C, 60°C, 70°C and 90°C in an oven until a constant weight was obtained. During the drying process, the weights of the samples were measured at 30 min intervals. The weights were used to calculate the moisture content of the locust beans samples at each drying time and temperature.

(1) $MoistureLoss=\frac{(W_1-W_2)}{W_1}\times 100$(1)

M.L= Moisture Loss (%), $w_1=massofirubeforedrying\left(g\right),$

$W_2=massofiruafterdrying\left(g\right)$

2.3. Preparation of fresh and dried samples for analysis

The fresh and dried locust bean seeds were macerated (for fresh samples) and milled to powder (dried samples) using the laboratory blender before being for chemical analysis.

2.3.1. Minerals analysis

The mineral composition of the samples was evaluated using atomic absorption spectrophotometric method (Odunlade et al., 2017) for calcium, magnesium, and iron while sodium will be determined using flame atomic absorption spectrophotometer.

2.3.2. Proximate analysis

The proximate composition (crude fat, protein, fiber, ash, moisture, carbohydrate) on the fresh and processed Iru samples was evaluated using the AOAC (2012) methods.

2.3.3. Physiochemical properties

The titratable acidity (TTA) and the pH of the samples were evaluated using the AOAC (2012) methods.

2.3.4. Determination of anti-nutritional factors

The anti-nutritional contents of the samples were determined using the established methods. The tannin content was determined using modified vanillin—hydrochloric acid (MV-HCl) method of Price et al. (1974), saponins were determined using the spectrophotometric method described by Brunner (1984) and the oxalate content of the samples were determined using the modified titrimetric method of Falade et al. (2004).

2.4. Determination of antioxidant properties of the locust beans samples

2.4.1. Determination of ferric reducing antioxidant power (FRAP)

The ferric reducing power of the samples was evaluated using the modified method of Benzie and Strain (1999). The locust bean was made in powdery form using laboratory mortar and pestle, and the aliquots of the samples were hydrated to 10 mg/ml in distilled water and iron II sulphate–heptahydrate as standard in milli molar iron (II) ion.

2.4.2. Determination of DPPH radical scavenging ability

The radical scavenging activity of the samples against the DPPH radical was determined using the modified method described by et al(Girgih et al., 2011) by hydrating the samples to a final concentration of 10 mg/ml in phosphate buffer at pH 7.0 with 1% Triton X.

2.4.3. Determination of Metal chelation activity

Metal (iron) chelating activity (MCA) of the samples was determined using the modified method of Xie et al. (2008) and hydrating the samples in distilled water.

2.4.4. Determination of total phenolic content

The total phenolic content of the samples was determined using the Folin–Ciocaltue phenol reagent method described by Gulcin et al. (2006). Locus beans were made in powered form before use. A 10-fold dilution of Folin–Ciocalteu reagent was prepared just prior to use. The samples were prepared in the final dilution concentration of 10 mg/ml using 50% methanol, hydrated for 1 h and centrifuged. To 100 µL of the supernatant was added 900 µL of distilled water to give a 10-fold dilution. Two hundred microliters of freshly prepared diluted Folin–Ciocalteu’s phenol reagent were added and the mixture vortexed. After allowing the mixture to equilibrate for 5 min, the reaction was then neutralized with 1.0 mL of 7% (w/v) Na₂CO₃ solution. After 2 h of incubation at room temperature, the absorbance was measured at 750 nm. A standard curve was prepared with a linear range of 0.0–0.1 µg/mL using gallic acid. The results were expressed as microgram gallic acid equivalent (µg GAE/ml) of juice and this was obtained by extrapolation from the standard curve. Distilled water was used as blank.

2.5. Sensory evaluation

Four different samples dried at different temperatures with the fresh iru (as control) were placed before 10 panelists. The panelists were selected based on their familiarity with iru and their ability to differentiate effectively between one sample and the other, following the method of Ihekoronye and Ngoddy (1985) and described by Ibeabuchi et al. (2013). The samples were coded and served in clean white plates at room temperature (28 ± 2°C) to the panelists in a sensory evaluation laboratory. Panelists were asked to evaluate the product based on the sensory properties the products and score each sample using a 9-point Hedonic scale where 1 = extremely unacceptable and 9 = extremely acceptable. Attributes evaluated include; iru appearance, texture, taste, colour, smell and overall acceptability.

2.6. Data analysis

Data obtained in triplicate from the tests were subjected to analysis of variance (ANOVA) and the significant difference was accepted at p < 0.05.

3. Results and discussion

3.1. Effect of drying time and temperature

The results of the effect of time and temperature on the moisture content of dried locust beans are shown in Figure 1. The results showed that the moisture contents of the samples ranged between 15.37% and 33.37%, 14.46% and 24.88%, 14.09% and 22.49% and 13.50% and 22.02% for samples dried at 50°C, 60°C, 70°C and 90°C respectively. There was a decreasing order in moisture content of the samples as the drying time increased. The moisture contents of the dried locust beans samples decreased as the temperature of drying increased from 50°C to 90°C. The build-up of heat as the drying progressed due to the rise in temperature may have enhanced moisture removal from the samples (Sanya et al., 2009). The results showed that an increase in the drying temperature also increased the amount moisture removed at each drying time interval (Ogunjimi et al., 2002). The results obtained in this study are similar to the observation on the drying kinetics of locust beans made in Benin republic, where increase in the drying time and temperature increased the amount of moisture removed from the samples and thus resulting in low moisture content. Locust beans dried at 90°C was achieved at a faster rate than the other drying temperatures and had the lowest moisture content (13.59–22.02%). The results also showed that the higher the drying temperature, the shorter the time required to attain the critical moisture content. This observation agreed with the submission of other authors (Maarten et al., 2012; Sanya et al., 2009) for different food materials.

Figure 1. Influence of drying temperature and time on the moisture content of fermented locust Beans.

3.2. Effect of drying temperature on the proximate composition

At the end of the drying process, the moisture content of the dried samples ranged between 12.36% and 12.76%. The results showed that increase in the drying temperature reduced the moisture content of the dried samples. Samples dried at 50°C had the highest final moisture and the least was obtained at the drying temperature of 90°C. These values were significantly (p < 0.05) different from one another. The percentage change in the moisture content of the samples ranged between 5.84% and 6.87%. The moisture content of the dried samples was lower compared to the value (12.00%) reported by Oladunjoye et al. (2007) for dried locust beans at 75°C. Moisture is one of the indicators of spoilage, the higher the moisture of food samples, the higher the tendency of spoilage of the food materials. The level of the moisture content in the locust bean samples in this study could guarantee an extended shelf life of the locust beans.

The ash content of the samples is presented in Table 1. The values ranged between 4.02% and 4.65% for the dried locust beans, while the control sample had the ash content of 5.03%. The results showed that the increase in the drying temperature reduced the ash content of the dried samples, although the values were not significantly (p > 0.05) different from one another. Samples dried at 50°C had the highest ash content, while samples dried at 90°C had the lowest ash content. The percentage decrease in the ash content of the samples ranged between 7.55% and 20.08% and the reduction in the ash content of the samples increased as the drying temperature increased. Previous works by Audu et al. (2004) and Oladunjoye et al. (2007) reported the ash content of dried locust bean at 5.31% and 4.45% respectively at different conditions. Ash content in food is very important as it measures the levels of mineral composition in foods.

Table 1. Proximate composition of processed locust beans

Samples	Moisture (%)	Ash (%)	Fibre (%)	Crude fat (%)	Crude protein (%)	Carbohydrate (%)
Raw	38.76 ± 1.86^a	5.03 ± 0.69^a	7.88 ± 0.04^b	8.98 ± 0.51^a	31.88 ± 0.93^b	7.47 ± 0.93^b
50°C	12.76 ± 0.96^b	4.65 ± 0.18^b	8.65 ±0.11^a	9.01 ±0.36^a	32.51 ± 0.90^a	32.42 ± 0.90^a
60°C	12.43 ±1.21^b	4.21 ± 0.20^b	8.76 ± 0.02^a	9.08 ± 0.59^a	32.68 ± 1.16^a	32.84 ± 1.16^a
70°C	12.41 ± 1.01^b	4.13± 0.13^b	8.77 ± 0.06^a	9.34 ± 1.35^a	33.45 ± 1.30^a	31.90 ± 1.30^a
90°C	12.36 ± 0.91^b	4.02 ± 0.12^b	8.81 ± 0.02^a	9.37 ± 1.14^a	33.52 ± 1.16^a	31.92 ± 1.16^a
% Change	5.84–6.87	7.55–20.08	9.77–11.80	0.33–4.34	0.98–5.14	76.58–77.25

Note: Values are reported as means ± s.d. of triplicate determinations. Values with different superscripts down the column are significantly (p<0.05) different from one another.

The fibre content of the samples ranged between 8.65% and 8.81% and as shown in Table 1. The results showed that the samples dried at 90°C had the highest value (8.81%) while the samples dried at 50°C had the least. The results also showed that increase in the drying temperature from 50°C to 90°C increased the fibre content of the samples, although the results were not significantly (p > 0.05) different from one another. The control sample had the lowest fibre content when compared with the dried samples. The difference in the fibre content of the samples with respect to the control ranged between 9.77% and 11.80% and the changes increased with increasing drying temperature. The high fibre content obtained in the sample dried at higher temperatures may be attributed to removal of water from the samples at higher temperature, resulting in sample concentration (Oladunjoye et al., 2007). The value of the fibre content of the dried locust beans was higher than 6.81% reported for inoculated locust beans but similar to 8.30% for the naturally fermented locust beans (Oladunjoye et al., 2007). Fibre in food is very important as it helps the body activities in the colon.

The crude fat content of dried locust beans ranged between 9.01% and 9.37% as shown in Table 1. The fat content of the raw sample was 8.98%. The values showed an increase in the fat content of the dried locust beans as the drying temperature increased from 50°C to 90°C. There was no significant (p > 0.05) difference in the fat content of the drying dried locust beans and that of the control. Samples dried at 90°C had the highest fat content, while sample dried at 50°C had the lowest fat content, but the raw sample had lower fat content than the sample that was dried at 50°C. The percentage increase in values of fat content of the samples ranged between 0.33% and 4.34%. The difference between the fat content of the dried and the control samples could be attributed to the concentration of fats at the surface and the inner matrix of the samples due to heating. The values reported for fat content in this study was lower, when compared with the 10.84% and 14.84% reported for naturally fermented and inoculated locust bean dried at 80°C (Oladunjoye et al., 2007).

The crude protein content of the dried locust beans ranged between 32.51% and 33.52% and that of the control sample was 31.88% (Table 1). The results showed that samples that dried at 90°C had the highest protein content and the sample that was dried with 50°C contained the lowest protein content. The protein content of the dried samples was higher than the raw sample. There was no significant (p > 0.05) difference in the protein content of among the dried samples, but the results showed that dried samples were higher significantly (p < 0.05) than the raw sample. The percentage increase in the protein of the samples was between the ranges of 0.98% and 5.14%. The percentage change in the protein content increased with increasing drying temperature of the samples. The high protein content obtained in the samples dried at higher temperature could be attributed to the increasing rate of water removal from the samples. This is because when water is removed from food samples, every other nutrient is concentrated and increased in quantities. The results also showed that the temperature at which the locust beans are dried is not high enough to destroy the protein content of the samples. The values are also similar to the 34.94% reported for inoculated locust beans and 34.11% obtained for naturally fermented locust beans (Oladunjoye et al., 2007). Protein is very important in the body as it is responsible for healing of wounds and repair of body tissues (Ogunjimi et al., 2002).

The carbohydrate content of the dried locust beans ranged between 31.90% and 32.84% and the value for the raw sample was 7.47%. There was no significant (p > 0.05) difference in the carbohydrate content of the dried locust beans, but the content of the dried locust beans was significantly (p < 0.05) higher than the control sample. The highest carbohydrate content was obtained in the samples dried at 60°C, while the lowest was obtained in the sample dried at 90°C. The change in the carbohydrate content ranged between 76.58% and 77.25%. The carbohydrate content of food samples are functions of other proximate compositions. Carbohydrate as a component of food supplies energy to the body in addition to the fat content of food.

The results of the proximate composition showed that the protein, fibre and fat contents increased at increasing drying temperature, while the ash and moisture content decreased. The values of the moisture contents of the dried samples were within the range of moisture that would guarantee the shelf stability of the dried samples.

3.3. Effect of drying temperature on the mineral composition of dried locust beans

The mineral composition of dried locust beans is shown in Table 2. The calcium content of the dried locust beans ranged from 711.56 to 745.16 mg/100 g and the value of the control sample was 745.61 mg/100 g. The highest calcium content was obtained in the samples dried at 50°C and the lowest calcium content was observed at the samples dried at 90°C. The results showed a decrease in the calcium content of the samples as the drying temperature increased from 50°C to 90°C. The values obtained for calcium content of samples dried at 50°C and 60°C were not significantly (p > 0.05) different from each other. Also, there was no significant (p > 0.05) difference between the calcium content of samples dried at 70°C and 90°C. The calcium content of the dried samples was lower than that of the raw sample. The percentage change in the calcium content was between

Table 2. Mineral composition of dried locust beans

Samples	Calcium (mg/100 g)	Magnesium (mg/100 g)	Sodium (mg/100 g)	Copper (mg/100 g)	Zinc (mg/100 g)
Raw	745.61 ± 0.06^a	85.43 ± 0.09^a	13.44 ± 0.04^a	21.40 ± 0.11^a	294.39 ± 0.13^a
50°C	745.19 ± 0.06^a	83.13 ± 0.03^a	13.27 ± 0.11^a	21.09 ± 0.06^a	293.85 ± 0.70^a
60°C	741.35 ± 0.06^ab	72.20 ± 0.20^b	13.02 ± 0.02^a	18.65 ± 0.29^ab	241.29 ± 0.06^b
70°C	721.32 ± 0.11^b	71.54 ± 0.15^b	12.14 ± 0.06^a	18.07 ± 0.35^ab	222.56 ± 0.30^c
90°C	711.56 ± 0.41^b	64.63 ± 0.12^c	9.21 ± 0.02^b	15.34 ± 0.14^b	210.28 ± 0.16^d
% Change	0.42–34.05	2.30–20.80	0.17–4.23	0.31–6.06	0.54–84.11

Note: Values are reported as means ± s.d. of triplicate determinations. Values with different superscripts down the column are significantly (p<0.05) different from one another.

was between 0.42% and 4.05%. The percentage change increased with increasing drying temperature. Calcium is important in the body for body building. The calcium content of the dried samples was lower than the 1023 mg/100 g reported for inoculated locust beans and 1300 mg/100 g obtained for fermented locust beans by Oladunjoye et al. (2007). This suggests that the variety of a particular locus bean has implication in the content of calcium in the sample. The recommended daily allowance for calcium is 1000 mg/100 g daily. The level of calcium in the samples could contribute to some quantities of calcium per day (Food and Nutrition Bulletin, 2011).

The magnesium content of the dried locust beans ranged between 64.63 and 83.13 mg/100 g and the magnesium content of the raw sample was 85.43 mg/100 g. The magnesium content of the dried samples decreased as the drying temperature increased from 50°C to 90°C. The dried samples were significantly (p < 0.05) different from each other. The raw samples was also significantly (p < 0.05) higher than the dried sample. The percentage change reduction in the magnesium content of the dried locust beans ranged between 2.30% and 20.80%. The percentage reduction in the magnesium content of the samples increased as the drying temperature of the samples increased from 50°C to 90°C. The value (82.61 mg/100 g) obtained for the magnesium content of the samples inoculated locust beans (Oladunjoye et al., 2007) was lower than reported values for the dried locust beans in this work. The difference may be due to the source and the processing methods of the locust bean samples. Magnesium is involved in the respiratory process in the body (Cordenunsi et al., 2004). The recommended daily allowance for magnesium is 175 mg/100 g per day (Food and Nutrition Bulletin, 2011). The levels of magnesium in this work can provide some quantities of magnesium for daily utilization.

The sodium content of the dried locust beans ranged between 9.21 and 13.27 mg/100 g. The values for the raw samples were between 13.44 mg/100 g. The highest sodium content was obtained in the samples dried at 50°C while the lowest sodium content was obtained in sample that was dried at 90°C. The raw sample had the highest sodium content. There was no significant (p > 0.05) difference in the sodium content of the samples dried at 60°C, 70°C and 90°C and the raw sample. Also, there was a decrease in the sodium content of the samples as the drying temperature increased. The decrease in the sodium content as the drying temperature increased may be attributed to the depletion of sodium element by the effect of heat the samples were subjected to during drying (Oladunjoye et al., 2007). The percentage decrease in the sodium content of the samples ranged between 0.17% and 4.23%. Sodium is very important in the body due to its activity in maintaining the osmotic concentration of the body. The sodium content of the dried samples was lower than the value (5.84 mg/100 g) reported for fermented and dried locust beans (Motarjemi, 2002). The recommended daily allowance for sodium is 5 mg/100 g (Food and Nutrition Bulletin, 2011). The levels of sodium in the dried samples can provide the amount of sodium needed for human being.

The copper content of the dried locust beans ranged between 15.34 and 21.09 mg/100 g and the value for the raw sample was 21.40 mg/100 g. The highest copper content was obtained in the raw sample. Among the dried locust bean samples, the sample with the highest copper content was obtained in sample dried at 50°C and the lowest copper content was obtained in samples dried at 90°C. The result showed that drying locust beans at high temperature affected the copper content of the samples. The sodium content of the raw sample and locust beans dried at 50°C was not significantly (p > 0.05) different from each other. The copper content of locust bean dried 60–90°C were not different (p > 0.05) significantly from one another. The percent change in the copper content of the samples ranged between 0.31% and 6.06% and the percentage in copper decreased as the drying temperature increased from 50°C to 90°C. The copper content of the dried iru samples were higher than 13.87 mg/100 g obtained for locust bean seeds dried at 95°C and fermented for 2 days (Soetan et al., 2014). Copper is a micronutrient but required in the body for enzyme production and biological electron transport (Soetan et al., 2014). The samples that have high copper content would have high ability to produce enzyme and electron transfer ability in the body. The recommended daily allowance for copper is 2.8 mg/100 g (Food and Nutrition Bulletin, 2011). The copper content of the dried locust beans can supply the quantity of copper that is required in the body.

The zinc content of the dried locust beans ranged between 210.28 and 293.85 mg/100 g. The raw sample had 294.39 mg/100 g zinc content. There was no significant (p > 0.05) difference in the copper content of the raw sample and locust beans dried at 50°C. The locust beans dried at 60°C, 70°C and 90°C were significantly (p < 0.05) different from one another. Like the other mineral elements, locust beans dried at 50°C had the highest zinc content and locust beans dried at 90°C had the lowest zinc content. The percentage reduction in the zinc content of the samples ranged between 0.54% and 84.11%. The percentage decrease in the zinc content increased at the drying temperature increased from 50°C to 90°C. The values of the zinc content were lower than 300 mg/100 g obtained for locust bean seed (Elemo et al., 2010). Zinc is an essential micronutrient associated with a number of enzymes, especially those associated with synthesis of ribonucleic acid. Samples that contain high content of zinc will have a high ability to synthesize ribonucleic acid. The recommended daily allowance of zinc is 500 mg/100 g (Food and Nutrition Bulleting, 2011). The quantity of zinc in the dried samples can supply some amounts of zinc in the body.

The mineral results showed that drying reduced the mineral content of the locust beans. Some reductions were significant, while others were not. The results have shown the level of mineral elements in the dried samples reduced as the drying temperature increased and this pattern could be attributed to the heat deterioration of the mineral during high drying temperature. However, the residual minerals in the dried locust bean samples can still supply some quantities of the recommended daily allowance of each of the mineral discussed in this work. At low temp (50) % reduction was negligible but this increased with increase in drying temp. Ca, Cu and Na had <7% loss, but Mg had 20% and Zn had up-to 84% when dried at 90°C.

3.4. Effect of drying temperature on some physicochemical properties of dried locust

3.4.1. Beans Samples

The results of the physicochemical properties of locust beans samples are presented in Table 3. The values of the titratable acidity (TTA) of the samples ranged between 0.04 and 0.042 mg LAC/g. The results showed that the highest value was obtained in the raw sample and the highest titratable acidity in the dried samples (0.40–0.42 mg lactic acid/g) was obtained in sample that was dried at 50°C. The lowest TTA value (0.40 mg lactic acid/g) among the dried samples was obtained in the sample that was dried at 90°C. The results showed that there was no significant (P > 0.05) difference in the TTA values of the raw sample and the dried locust beans samples. The non-significant values obtained in the TTA of the samples showed that drying did not cause any significant change in the product that would affect the acidity levels of the

Table 3. Some physicochemical properties of locust beans samples

Samples	Titratable acidity (mg lactic acid/g)	pH
Raw	0.042 ± 0.003^a	4.50 ± 0.02^a
50°C	0.042 ± 0.003^a	4.50 ± 0.03^a
60°C	0.041 ± 0.002^a	4.70 ± 0.02^a
70°C	0.041 ± 0.002^a	4.70 ± 0.04^a
90°C	0.040 ± 0.001^a	4.75 ±0.03^a

Note: Values are reported as means ± s.d. of determinations triplicate. Values with different superscripts down the column are significantly (p < 0.05) different from one another.

samples. The percentage decrease in the TTA of the dried samples when it is compared with the raw samples ranged between 2.34% and 4.12%. The titratable acidity is a measure of the acidity levels of food samples. The acid that is mostly present in locust beans is lactic acid. The values of TTA in the locust beans samples showed that drying may not cause changes in the pH levels of the samples.

The pH values of the locust beans samples ranged between 4.50 and 4.75. The raw sample had the lowest pH value and the samples that were dried with 90°C had the highest pH value. The results showed that there was no significant (p > 0.05) difference in the pH values of the raw sample and the samples that were dried. This showed that the drying of the locust beans did not cause any significant changes in the pH level of the locust beans. An inverse relationship between the pH and the TTA values of the locust beam samples was established and agreed with the observation of Oladunjoye et al. (2007) that the higher the pH values, the lower the TTA values. This is true in this work because samples that had the highest TTA values had the lowest pH value. The values of the pH of the locust beans samples showed that the raw and the dried samples are within the acidic range. The values pH values in this study were similar to the values (4.32–4.57) reported for inoculated and naturally fermented locust beans (Oladunjoye et al., 2007). The results showed that drying did not cause any change in the acidity level of the locust beans samples.

3.5. Effect of drying temperature on the anti-nutritional properties of dried locust beans samples

The anti-nutritional properties of dried locust beans are presented in Table 4. The saponin content of the dried locust beans ranged between 0.20 and 0.35 mg/100 g while the values obtained for the control sample was 0.36 mg/100 g. There was no significant (p > 0.05) in the

Table 4. Anti-nutritional factors (mg/100 g) of dried locust beans

Sample	Saponin	Tannin	Oxalate
Control	0.36 ± 0.05^a	0.74 ± 0.02^a	1.12 ± 0.05^a
50°C	0.35 ± 0.02^a	0.71 ± 0.04^a	1.05 ± 0.04^a
60°C	0.30 ± 0.05^a	0.62 ± 0.03^a	0.96 ± 0.05^a
70°C	0.21 ± 0.03^ab	0.54 ± 0.06^a	0.92 ± 0.04^a
90°C	0.20 ± 0.02^ab	0.52 ± 0.02^b	0.64 ± 0.07^b
% Difference	2.78–44.44	4.05–29.73	6.25–42.86

Note: Values are reported as means ± s.d. of triplicate determinations. Values with different superscripts down the column are significantly (p < 0.05) different from one another

saponin content of the raw sample and the samples dried at 50°C and 60°C, but these were significantly (p < 0.05) higher than sample dried at 70°C and 90°C. The saponin content of the control sample was the highest, but among the dried samples; samples dried at the highest temperature (90°C) had the lowest saponin content (0.20 mg/100 g). The results showed that increase in the drying temperature decreased the saponin content of the dried sample. The percentage change in saponin content of the samples ranged between 2.78% and 44.44%. The percentage decrease in saponin content increased with increasing drying temperature between 50°C and 90°C. The increase in the change in saponin content of the samples may be attributed to the effect of high temperature during drying. The values of the dried locust beans were lower than 0.51–0.78 mg/100 g reported for locust beans (Hassan & Umar, 2004). Saponin is a class of phytochemical that has some physiological importance in health. It is also classified as a form of anti-nutrient which can affect the digestion of some minerals in the body, such as phosphorous (Hassan & Umar, 2004). Samples that have high saponin content may have a negative effect on phosphorus digestion.

The tannin content of the dried locust beans ranged between 0.52 and 0.71 mg/100 g. The value of the raw sample was 0.74 mg/100 g. There was no significant (p > 0.05) difference in the tannin content of raw sample and samples dried at 50°C and 60°C. Also, there was no significant (p > 0.05) difference in the tannin content of the samples dried at 70°C and 90°C. The tannin content of the samples decreased as the drying temperature increased from 50°C to 90°C. The highest tannin content was obtained in the control sample and the lowest tannin content was obtained in the sample that was dried with 90°C. The results showed that increase in the drying temperature reduced the tannin content of the dried samples. The percentage decrease in the tannin content of the samples ranged between 4.05% and 29.73%. There was an increase in the percentage decrease in the tannin content as the drying temperature increased from 50°C to 90°C. The tannin content of the dried locust beans were lower than the 0.98–1.12 mg/100 g obtained for inoculated and dried locust beans and dried at 75°C (Oladunjoye, 2007). Tannin is an anti-nutrient that affects the digestion of protein in the body (Soetan, 2012). Samples that have high content of tannin would have minimum protein digestion. The lower tannin content of the dried samples showed that samples dried at 90°C had low tannin content and would have minimum effect on protein digestion.

The oxalate content of the samples ranged between 0.64 and 1.05 mg/100 g. The oxalate content for the raw sample was 1.12 mg/100 g. The samples showed no significant (p > 0.05) difference in the oxalate content of the raw and samples that were dried at 50°C, 60°C and 70°C. The highest oxalate content was obtained in the raw sample and the lowest oxalate sample was obtained in the samples dried at 90°C. The percentage reductions in the oxalate content of the samples ranged from 6.25% to 42.86%. The percentage decrease was higher as the drying temperature increased from 50°C to 90°C. The values obtained for the oxalate content are similar to the values (0.94–1.15 mg/100 g) reported for two varieties of locust beans dried at 85°C. Oxalate is an anti-nutrient that affects the proper absorption of zinc and copper in the body. Samples that have high oxalate content would have a positive effect on the digestion of zinc and copper in the body.

Generally, the results indicated that the dried locust bean samples had lower oxalate content compared to the control sample, and this probably may be attributed to the effect of heat which may have broken down the components of the anti-nutrient to lower non-toxic components during drying (Hassan & Umar, 2004).

3.6. Effect of drying temperature on the antioxidant properties of dried locust beans samples

3.6.1. DPPH (1, 1-diphenylpicrylhydrazine) radical scavenging activities

Values for the antioxidant properties of the dried and control locust bean samples are presented in Figure 2 (A-D). The DPPH radical scavenging activities of the dried locust beans samples ranged from 40.47% to 43.81% and the raw sample had a DPPH radical scavenging value of 43.89%. The control sample had the highest DPPH scavenging activities and among the dried samples, samples dried at 50°C had the highest activity and samples dried at 90°C had the lowest DPPH radical scavenging activities. The results showed that the DPPH of the sample decreased as the drying temperature increased from 50°C to 90°C. There was no significant (p > 0.05) difference in the DPPH radical scavenging activities of the raw sample and the sample dried at 50°C, but the samples dried at 60°C, 70°C and 90°C were insignificantly (p < 0.05) different from each other. The percentage decrease in the DPPH radical scavenging ranged between 0.18% and 7.79%. The results showed that the percentage reduction increased as the drying temperature increased. The lowest percentage reduction was found in the sample dried at 50°C. The low value obtained at the highest drying temperature may be attributed to the loss in the component that is responsible for the antiradical activities of the samples. The values obtained for the dried locust beans were lower than the values (65–75%) obtained for dried and fermented locust beans produced in Kano in the production of daddawa and the variation may be due to variety of the locust beans and even the processing conditions. DPPH is a free radical that is very dangerous to the body. The resultant effect of free radical in the body is the development of many chronic diseases such as cancer and obesity. Samples that have high DPPH radical scavenging activities would have better ability to scavenge the activities of the free radicals in the body. In this work, the control sample had the best ability to do this and the ability to scavenge DPPH reduced as the drying temperature increased.

Figure 2. Antioxidant (A) DPPH radical scavenging activities. (B) Ferric reducing antioxidant property. (C) Metal chelating properties. (D) Total phenolic contents of locust bean samples.

Bars with different alphabets are significantly (p > 0.05) different from one another.

3.6.2. Metal chelating activities

The metal chelation activities of locust bean samples ranged from 50.31% to 52.22%, and the value for the control sample was 52.45%. The results showed that the raw sample had the highest metal chelation activity and the locust beans dried at 50°C had the highest metal chelation activity among the dried locust beans sample. There was a decrease in the metal chelation activity of the dried locust beans as the drying temperature increased from 50°C to 90°C. There was no significant (p > 0.05) difference in the metal chelation activities of the dried locust beans. This means that increase in the drying temperature did not cause any significant change in the metal chelation activity of the locust beans samples. The percentage change in the metal chelation activities of the samples ranged between 0.44% and 4.08%. The percentage reduction in the metal chelation activities reduced gradually as the drying temperature increased from 50°C to 90°C. The locust bean that was dried at 90°C had the lowest metal chelation among the dried samples. The values for the metal chelation activities for the samples were similar to the values (43.54–57.43%) obtained for the metal chelation activities of locust beans used to produce ogiri. Metal chelation activity is an antioxidant method which is used to test the ability of food samples to bind metals that can cause oxidation in foods. Samples that have high metal chelation activities would have good properties to chelate or bind the metals that cause oxidation. The results showed there was no significant change in the ability of the raw samples and the dried samples to chelate metals.

3.6.3. Ferric reducing antioxidant properties (FRAP)

The results for the ferric reducing activities of locust beans samples ranged between 0.32 and 0.27 mMol. The value for the raw sample was 0.33 mMol as shown in Figure 4. The results showed that no significant (p > 0.05) difference in the FRAP was observed between control sample and the dried samples. The highest ferric reducing ability was obtained in the raw sample. Among the dried sample, locust beans dried at 50°C had the highest metal chelation activities and the sample that was dried at 90°C had the lowest ferric reducing ability. Just like other antioxidant assays, increase in the drying temperature reduced the ferric reducing ability of the dried sample. Ferric reducing ability is a very important antioxidant method that evaluates the ability of food samples to reduce the iron III to iron II to prevent food oxidation. The values are similar to 0.35–0.73 mMol obtained for the ferric ability of locust beans in the production of ogiri. Samples that have high ferric reducing ability would be a good antioxidant substance in reducing the effect of oxidation in the body.

3.6.4. Total phenolic activities

The phenolic activities of the locust beans samples ranged between 0.381 and 0.561 mgGAE/g and the value for the control sample was 0.589 mg GAE/g. Control sample had the highest phenolic content, while the sample dried at 50°C had the lowest phenolic activity. The results showed that there was no significant (p > 0.05) difference in the total phenolic activities of the control sample and samples dried at 50°C and 60°C but there was a significant (p < 0.05) difference in the phenolic activities of the samples dried at 70°C and 90°C when compared to the control sample. The results showed reductions in the phenolic activities of the samples as the drying temperature increased from 50°C to 90°C. Among the dried samples, locust beans dried with 50°C had the highest phenolic content and samples that dried at 90°C had the lowest phenolic content. The results showed that drying temperature had effects on the phenolic contents of the samples. This pattern may be attributed to the decomposition of the benzene rings holding the hydroxyl component of the phenols together as a result of the high temperature, compared to samples dried at lower temperature. The values obtained for the total phenolic content of the samples were higher than (0.231–0.234 mgGAE/g) obtained for naturally fermented locust beans (Oladunjoye et al., 2007) and similar to 0.210–0.421 mgGAE/g reported for fermented locust beans to produce ogiri and dried at 85°C. Phenolic content of food is an index of antioxidant properties in the food. Samples that have high phenolic content may have high antioxidant properties.

The results showed that drying of locust beans have some effects on the antioxidant ability of the samples. Some of the antioxidant properties are significantly affected by drying temperature while some are not. The antioxidant properties of the samples were still higher than some values in the literature and this shows

3.7. Sensory properties of locust beans samples

The results of the sensory properties of locust beans samples are presented in Table 5. The mean scores obtained for the sensory parameters; appearance, colour, texture, taste and general acceptability ranged from 6.11 to 7.54, 5.05 to 7.85, 7.01 to 8.05, 6.20 to 8.50 and 6.11 to 8.06, respectively. Among the samples, the control sample was most preferred in all the sensory parameters evaluated. The preference of the samples decreased as the drying temperature increased from 50°C to 90°C. The preference of the colour of the dried samples reduced as the drying temperature increased because the samples were turning to black in colour during drying. This observation was similar to the result of Zakari et al. (2015) in the production of cake using mixtures of locust beans and other materials. Similarly, the dried samples became harder as against the soft texture of locust beans that the panelists are familiar with and this may have affected the texture scores of the dried samples. The decrease in the texture preference might be due to the effect of drying temperature which made the samples hard. The increase in the drying temperature of the samples may also have removed the flavour-rich compound that is associated with the fermented seeds. This raw locust beans are what people are familiar with but the drying process may have removed the liquid and this may have affected the flavour scores of the samples. It is also important to note that the flavour-rich component on the fermented seeds is a mixture of different components that brings about the taste of the samples and this is what the consumer of locust beans first perceive, before chewing the beans. A substantial portion of the population do not like the odour of the fermented beans. Since drying eliminates this unpleasant odour, the dried beans should meet the nutritional needs of the populace. The pattern of results in this study was similar to the reports of Amao (2013) where locust beans dried at

Table 5. Sensory properties of the locust beans samples

Samples	Appearance	Colour	Texture	Taste	Overall acceptability
Control	7.54 ± 1.03^a	7.85 ± 0.54^a	8.05 ± 1.14^a	8.65 ± 1.13^a	8.06 ± 0.89^a
50°C	7.06 ± 0.89^a	7.50 ± 0.90^a	7.88 ± 0.89^ab	8.40 ± 0.87^a	7.12 ± 1.03^b
60°C	6.78 ± 1.11^b	6.60 ± 0.54^b	7.50 ± 0.77^b	8.20 ± 0.78^a	7.05 ± 1.33^b
70°C	6.50 ± 0.85^b	6.20 ± 0.99^b	7.32 ± 1.11^b	6.53 ± 1.04^b	6.54 ± 0.78^c
90°C	6.11 ± 0.86^b	5.05 ± 1.02^c	7.01 ± 1.04^b	6.20 ± 1.14^b	6.11 ± 0.78^c

Note: Values are reported as means ± s.d. of 20pannelist. Values with different superscripts down the column are significantly (p < 0.05) different from one another.

40°C were more preferred compared to samples dried at dried at 80°C due to the loss of taste/flavor component in the dried locust beans at high temperature. The results showed that drying of locust beans at high temperature affected the overall acceptability levels of the locust beans samples.

4. Conclusion

The study established the drying behavior and quality characteristics of fermented locust bean seeds as affected by different drying temperatures. There was minimal loss in the ash content of the dried samples compared with the control sample (fermented only). There were also losses in the mineral content of the dried samples as the drying temperature increased. The dried samples had higher protein, fat and fibre contents compared to the fresh samples. In a similar manner, increase in the drying temperature reduced the anti-nutritional properties and also minimally reduced the antioxidant properties of the dried samples. The panelists also preferred the fresh sample compared to the dried locust bean samples. However, the reduction in the sensory properties of the dried samples may have resulted from non-familiarity of the panelists with dried locust beans and this pattern could be corrected by re-hydration in water before cooking.

Acknowledgments

The authors appreciate Miss Omotayo Christianah for assisting in collecting the data for the research.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Notes on contributors

Akinsola A. Famuwagun

Akinsola A. Famuwagun is a research scholar and a university lecturer at the Department of Food Science and Technology, Osun State University, Osogbo, Osun State, Nigeria. His specializes in Food Chemistry/Food analysis and Food Biochemistry. He has well above 20 research articles in national and international publishing outlets and has attended many academic conferences.

Kehinde A. Taiwo

kehinde A. Taiwo is a Professor of Food engineering in the Department of Food Science and Technology, Obafemi Awolowo University, Ile-Ife, Nigeria. She has published over 100 research articles with many book chapters in both national and international outlet. She has also attended many national and international academic conferences.