Effect of Plant Ash on Nutrient Quality and Antioxidant Activity of African Yam Bean Seed Porridge in Relation to Cooking Time

The present study aimed to investigate the nutrient quality, antioxidant 
activity and sensory acceptability of hard-to-cook African yam bean (AYB) seeds 
cooked for 5 h 
(control) and for 2 h with plant ash. The plant ash were unripe plantain peel 
ash (UPA), corn cob ash (CCA) and UPA and CCA mixture. Standard methods of 
Association of Official Analytical Chemists (AOAC) were used for the proximate and mineral analysis, DPPH, 
ABTS and FRAP assays were used to investigate the antioxidant activity. A 
completely randomized design with one way analysis of variance (ANOVA) was used 
for data analysis. The results showed that 
cooking of AYB seeds for 2 h with 8 g/100mL plant ash solution improved the nutrient contents, antioxidant activity 
and sensory scores of the porridge. Higher values of proteins, ash and energy 
were obtained in 8 g/100mL 
UPA/CCA and in 8 g/100mL 
CCA additives. However, moisture, fat and fibre contents were not affected by 
plant ash additives and were higher in 5 h cooked porridge. Cooking with 8 g/100mL plant ash solution 
for 2 h significantly increased mineral elements in the porridge. Phenolic 
compounds (total polyphenols and flavonoids), and antioxidant activity (DPPH, 
ABTS and FRAP) showed higher values with plant ash additives. This study 
reveals that addition of plant ash did not only reduce the cooking time of AYB 
seed by 60%, but also improved the nutrient quality, antioxidant activity and 
sensory acceptability.


Introduction
African yam bean (AYB) (Sphenostylis stenocarpa) is an herbaceous leguminous plant with increasing nutritional interest in Nigeria and West African countries.
As an important diet in West African countries, it complements the lack of proteins from cereals, roots and tubers. The amino acid profile of AYB seed is comparable to other legumes like cowpea, soybean and pigeon pea [1]. It is also rich in minerals and fiber [2]. However, utilization of AYB seed is limited due to the hard-to-cook (HTC) phenomenon arising from high temperature and relative humidity during storage [3], even up to nine months. This results in long cooking time and high energy expenditure, sometimes, overnight by the local people.
Secondly, AYB seed contains some anti-nutritional factors and oligosaccharides which can provoke flatulence and chronic health effects.
Several authors have described the HTC phenomenon to changes occurring in the legume hull, cell wall and cotyledon in relation to legume hardening during storage [3] [4]. Others have linked phenolic compounds to hard-to-cook (HTC) phenomenon in legumes [5] [6].
To reduce the long cooking time and save energy and money, HTC legumes may be cooked using different salt solutions [7]. Trona, known by various names, for example, Akanwu in Igbo language of Nigeria is an alkaline salt that has been used by the locals in West Africa to tenderize or soften AYB seed hull and degrade phenolic compounds, thus reducing cooking time of AYB seed. Due to the purgative effect of trona to many users, plant ash sources have been discovered for use as alternatives tenderizer. Plant ash is obtained after complete incineration of some plant materials. Such materials include dried unripe plantain peel, corn cob, palm bunch and palm inflorescence [8] [9]. Legumes are potential sources of antioxidant compounds. The predominant antioxidant compounds in most legumes are flavonoids, tannins and phenolic acid, and according to Lin and Lai (2006), dark coat seeds possess high amount of phenolic compounds and antioxidant activity. The aim of this research was to determine the effect of solutions of unripe plantain ash (UPA) and corn cob ash (CCA) on the nutrient quality, antioxidant activity and sensory acceptability of African yam bean seed (AYB) porridge in relation to cooking time.

Sources of raw materials
African yam bean (AYB) seed (Sphenostylis stenocarpa) and corn cobs were purchased from Ubani market in Umuahia Metropolis of Abia state, Nigeria.

Sample Preparation
Production of alkaline plant ash The unripe plantain peel and corn cob ash were produced according the method of Abubakar et al. [10]. Matured unripe plantain peel was washed in water were added and it further boiled for additional 15 min. Four cooking stoves were used to achieve the cooking process.

Determination of cooking time
The method described by William et al. [12] was used to measure the cooking time. Cooking timing of 1.5 kg of AYB seeds commenced when the cooking water began to boil at 100˚C. After boiling for 15 min, two to three seeds were taken with spatula to determine the degree of softening by compression of the seed in between two fingers. This process was continued until the seeds were fully softened with no gritty or grainy feeling in between two fingers. Thereafter, cooking time was recorded.

Analysis and reagents
The proximate, mineral and sensory analysis was done in Food Chemistry laboratory, Department of Food Science and Technology, Michael Okpara University of Agriculture, Umudike while the antioxidant activity analysis was done in Biochemistry laboratory, National Root Crops Research Institute, Umudike.
All the reagents were of analytical grade.

Proximate and mineral analysis
The proximate composition of the AYB porridge was determined according to the AOAC method for nutrient analysis [13]. Iron was determined by the method of James [14]. Calcium was determined by EDTA Versanate complexometric method of James [14]. Potassium was determined by a serial dilution of K solution of 0, 2, 4, 6, 8 and 10 ppm by flame photometry [15].

Antioxidant activity
Total polyphenol was determined by the method described by Jagadish et al. [16] with slight modification. Half (0.5) mL of AYB porridge methanolic extract was diluted with 10 mL of distilled water and 2.5 mL of 0.2 N Folin-Ciocalteu phenol reagent added to it in a 25 mL volumetric flask. After 5 min, 2 mL of 2% Na 2 CO 3 solution was added and the mixture was made up to 25 mL volume with distilled water and allowed to stand for 90 min. The absorbance was measured at 780 nm versus the prepared blank. Gallic acid was used as standard for the calibration curve. Total polyphenol content was calculated as mg Gallic acid equivalent per gram fresh weight of sample. Flavonoids content was estimated by AlCl 3 method described by Jagadish et al. [16]. One and half (1.5) mL of the porridge extract was added to 5 mL of distilled water and 0.3 mL of 5% NaNO 2 in a 10 mL volumetric flask. It was properly mixed and after 5 min, 1.5 ml of 2% methanolic AlCl 3 solution was added and rested for 5 min, then 2 mL of 1 mol dm −3 NaOH was added. The volume was made up to 10 mL with distilled water, shaken vigorously for 5 min and incubated for 10 min. The absorbance was read at 367 nm against the reagent blank. Flavonoid content was calculated using a standard calibration curve prepared from quercetin. The flavonoid content was expressed as mg quercetin per gram of fresh extract. ABTS + was prepared by adding solid manganese dioxide (80 mg) to a 5 mM aqueous stock solution of ABTS (20 mL of 75 mM Na/K buffer at pH 7) according to the method of Seeram et al. [17]. AYB porridge extract was diluted appropriately in Na/K buffer pH 7 and mixed with 200 µL of ABTS• + radical cation solution. The absorbance was read at 750 nm after 5 min in spectrophotometer (Jenway digital, England).

ABTS values were calculated from Trolox standard curve and expressed as Trolox
Equivalents in µM. The DPPH radical scavenging activity of the porridge was determined by the method described by Manzocco et al. [18].

Results and Discussion
Effect of UPA and CCA additives in relation to cooking time on AYB seed porridge There was substantial decrease in cooking time of AYB seeds as a result of plant ash additives. Changes in hardness of AYB seed during cooking as measured by compression between two fingers were noticeable. Ojimelukwe [21] and Mubaiwa et al. [22] have shown that the lesser the compression, the softer the rence also made by [22], especially in brown or dark coated leguminous seeds [23]. Baiyeri et al. [24]  Das et al. [25] has opined that foods possessing ash content of 3% and above are ideal for mineral nutrition. Therefore, alkaline plant ash of vegetable origin may be used as a mineral supplement in cases of established mineral deficiency. As it is the case, AYB seeds as legumes have low carbohydrate and energy contents. In comparison to cereals, roots and tubers with high carbohydrate content and energy value, AYB seeds can be used as food for diabetics and convalescence.
These results clearly demonstrate that plant ash additives, being alkaline, can degrade and solubilize some hard-to-cook factors and phenolic compounds in AYB seeds [3]. Plant ash used in this study reduced the cooking time and fuel by 60% and positively improved the availability of some nutrient contents.
Effect of plant ash on the mineral content of AYB seed porridge in relation to cooking time  [10]. Mineral nutrition is important in maintaining the health status of the individual. These mineral salts, Ca and K are important regulars of the acid-base balance of the body, Fe works together with Zn mineral to improve the immune defense system, while Ca and P are important in bone mass development especially in children [26]. The mineral nutrients from AYB porridge can contribute substantially to mineral nutrition of the consumers. The hard-to-cook (HTC) phenomenon in legumes has been linked to high content of phenolic compounds in beans [6] [27]. To reduce the HTC phenomenon in relation to cooking time and fuel, plant ash was used to tenderize the AYB seed hulls resulting in higher phenolics from solubilized tannins into the porridge. Tannins can be degraded at 100˚C (cooking temperature) and increase phenolic compounds [28]. Boiled raw dry mung bean seeds (Phaseolus aureus), and soaked and cooked legumes with sodium bicarbonate caused the reduction of tannins from 3.3 mg/g to 45.5% [29], and 66% [30], and would increase the to softening of HTC legumes was reported by del-Valle et al. [31]. They further stated that chelating and ion-exchange agents can affect the softening of soaked beans. In addition, cooking at 100˚C may have effect on AYB seed matrix, and causing the release of bound phenolic compounds from the seed structure that influence total phenolic increase in leguminous seeds [32]. The concentrations  of phenolic compounds (Table 3) indicates the positive contribution of plant ash to antioxidant activity in AYB seed porridge.
Effect of plant ash on the sensory properties of AYB seed porridge in relation to cooking time Table 4 shows the effect of plant ash on sensory properties of AYB seed porridge in relation to cooking time. Several traditional methods are used for processing of AYB seed to acceptable meals. The use of kanwa, (alkaline rock salt), NaHCO 3 or some other salts to cook legume seeds in order to reduce the cooking time was reported by Uzogara et al. [7]. Akanwu (Igbo language) is also used to make sponge in abacha (shredded tapioca) snack, and today it is not acceptable by the generality of the people because of its organoleptic quality and health risk factors. This has resulted in an alternative alkaline salt like plant ash for this and other purposes. Variation in cooking time and plant ash additive significantly (p < 0.05) affected the mean sensory scores. The sensory mean scores ranged from 5 (neither like nor dislike) to 8 (like very much). Preference was more for UPA additive, followed by CCA before the control. According to Dehghan-Shoar et al. [33], sensory scores of foods are a combination of the organoleptic properties like colour, aroma, taste and texture. In this study, colour, mouth-feel, taste, texture and general acceptability were highest in AYB porridge cook for 2 h with UPA. So the use of these plant ashes greatly affected their higher acceptability scores for colour, mouth-feel, taste and texture. The results imply that 8 g/100mL plant ash inclusion in cooking of 1.5 kg AYB porridge for 2 h can be used to replace the traditional method of AYB seeds cooked for 5 h. Overall, alkaline salt used in this study decreased cooking time, increased nutritive value, improved sensory properties of HTC AYB seeds and reduced energy usage by 60% and household income.

Conclusion
The effect of plant ash on nutrient quality, antioxidant activity and sensory acceptability of AYB seed porridge in relation to cooking time was studied. The results showed that cooking of AYB seeds for 2 h with plant ash (8 g/100mL) significantly improved the nutrient and sensory qualities, especially the protein, h with plant ash showed higher concentrations of DPPH, ABTS, FRAP, total polyphenol and flavonoids. It is established that cooking with plant ash caused a breakdown and solubilization of phenolic compounds in AYB seed hull and cotyledon resulting into higher antioxidant compounds in the porridge. Sensory evaluation revealed that AYB seeds cooked for 2 h with plant ash, especially UPA was more preferred in overall acceptability. Cooking of AYB seeds with plant ash for 2 h was able to handle the HTC problem.