Chemical composition and functional properties of flour produced from two varieties of tigernut ( Cyperus esculentus )

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INTRODUCTION
The search for lesser known and underutilized crops, many of which are potentially valuable as human and animal foods has been intensified to maintain a balance between population growth and agricultural productivity, particularly in the tropical and sub-tropical areas of the world.Tigernut (Cyperus esculentus) is an underutilized sedge of the family Cyperaceae which produces rhizomes from the base and tubers that are somewhat spherecal.In Egypt, it is used as a source of food, medicine and perfumes (De Vries, 1991).
Tigernut is commonly known as "earth almond", "chufa", "chew-fa" and "Zulu nuts".It is known in Nigeria as "Ayaya" in Hausa, "Ofio" in Yoruba and "Akiausa" in Igbo where three varieties (black, brown and yellow) are cultivated.Among these, only two varieties, yellow and brown, are readily available in the market.The yellow variety is preferred to all other varieties because of its inherent properties like its bigger size, attractive colour and fleshier body.The yellow variety also yields more milk upon extraction, contains lower fat and more protein and poss-*Corresponding author.E-mail: dekanmiscientist@yahoo.com.esses less anti-nutritional factors especially polyphenols (Okafor et al., 2003).Tigernut can be eaten raw, roasted, dried, baked or be made into a refreshing beverage called "Horchata De Chufas" or tigernut milk.It also finds uses as a flavouring agent for ice cream and biscuits (Cantalejo, 1997), as well as in making oil, soap, starch and flour.
Although many researchers have worked on tiger nut (Eteshola and Oraedu, 1996;De Vries, 1997;Cortes et al., 2005), there is paucity of information on the properties of its flour.This project therefore aimed at determining the chemical composition and functional properties of tiger nut flour in order to be able to explore its potentials in food formulation.

Sample preparation
The two varieties of tiger nut used (yellow and brown) were obtained from a local market in Yola, Adamawa state, Nigeria.The nuts were cleaned, sorted, washed, drained, dried in an oven and ground into flour.The flour samples were passed through a 45 µm mesh size sieve.The brown and yellow tigernut flour was tagged BTF and YTF, respectively (Table 1).Proximate analysis Kirk and Sawyer (1991) methods were used to determine moisture, protein, fat, crude fibre and ash contents while carbohydrate was calculated by difference.Energy values (KJ) were calculated by multiplying the amounts of protein and fat by the factors of 17 and 38 KJ g -1 respectively as reported by Paul and Southgate (1978).

Mineral analysis
The method described by AOAC (1990) was used for mineral analysis.The ash was digested with 3 cm 3 of 3M HCl and made up to the mark in a 100 cm 3 standard flask with 0.36 M HCl before the mineral elements were determined by atomic absorption spectrophotometer (PYE Unicon, UK, model SP9).Colorimetric method using phosphovanado molybdate was used for phosphorus determination.

Water and oil absorption capacities
Water and oil absorption capacities of the flour samples were determined by Beuchat (1977) methods.One gram of the flour was mixed with 10 ml of water or oil in a centrifuge tube and allowed to stand at room temperature (30 ± 2 o C) for 1 h.It was then centrifuged at 200 x g for 30 min.The volume of water or oil on the sediment water measured.Water and oil absorption capacities were calculated as ml of water or oil absorbed per gram of flour.

Foam capacity and foam stability
The method described by Narayana and Narasinga Rao (1982) was used for the determination of foam capacity (FC) and foam stability (FS).Two grams of flour sample was added to 50 ml distilled water at 30 ± 2 o C in a 100 ml measuring cylinder.The suspension was mixed and properly shaken to foam and the volume of the foam after 30 s was recorded.The FC was expressed as a percentage increase in volume.The foam volume was recorded 1 h after whipping to determine the FS as a percentage of the initial foam volume.

Bulk density
A 50 g flour sample was put into a 100 ml measuring cylinder.The cylinder was tapped continuously until a constant volume was obtained.The bulk density (g cm -3 ) was calculated as weight of flour (g) divided by flour volume (cm 3 ) (Okaka and Potter, 1979).

Swelling power
This was determined with the method described by Leach et al. (1959) with modification for small samples.One gram of the flour sample was mixed with 10 ml distilled water in a centrifuge tube and heated at 80 o C for 30 min.This was continually shaken during the heating period.After heating, the suspension was centrifuged at 1000 ×g for 15 min.The supernatant was decanted and the weight of the paste taken.The swelling power was calculated as: swelling power = weight of the paste / weight of dry flour

Pasting properties
Three grams of the flour sample was mixed with 25 ml distilled water in the canister of a Rapid Visco-Analyzer (RVA, model 3D; Newport Scientific, Sydney, Australia) monitored with RVA control software and operated.The following parameters were obtained from the plotted graphs: peak viscosity, pasting temperature, setback viscosity, breakdown viscosity, final viscosity and time to reach the peak viscosity.
Table 2 shows the mineral composition of the flour.YTF is higher in calcium, sodium and copper while BTF is higher in potassium, magnesium, manganese and iron.Both YTF and BTF have similar phosphorus and zinc values.The values of calcium found in the flours, 140 and 155 mg/100 g, are adequate for bone and teeth development in infants.The presence of other minerals such as iron is highly important because of its requirement for blood formation.

Functional properties
The water absorption capacity was 1.37 and 1.26 ml/g for YTF and BTF, respectively.The values obtained are lower than 3.4 ml/g reported for raw conophor flour (Odoemelam, 2003) but comparable to 1.7 ml/g reported for African yam bean (Eke and Akobundu, 1993).Water absorption capacity describes flour -water association ability under limited water supply.This result suggests that tigernut flour may find application in baked products e.g.cookies.The YTF and BTF were found to have oil absorption capacity of 1.07 and 1.13 ml/g.The result shows that tigernut may be a lower flavour retainer than raw winged bean (1.4 ml/g) and (1.2 ml/g) flours (Narayana and Narasinga, 1982) and African yam bean (1.42 ml/g) flour (Eke and Akobundu, 1993).The lower oil absorption capacity of tigernut flour might be due to low hydrophobic proteins which show superior binding of lipids (Kinsella, 1976).The foam capacity (FC) of BTF Oladele and Aina 2475 (11.07%) was higher than that of YTF (10.28%).The values are in line with 11.30 and 9% reported for pearl millet flour and quinoa flour, respectively (Oshodi et al., 1999), but comparatively lower than 20 and 40% reported for African breadfruit kernel flour and wheat flour respectively, (Akubor and Badifu, 2004).The low foam capacity may be attributed to the low protein content of the flour since foamability is related to the amount of solubilized protein (Narayana and Narasinga Rao, 1982) and the amount of polar and non-polar lipids in a sample (Nwokolo, 1985).The foam stability (FS) for YTF and BTF, 50.6 and 58.99%, are higher than 14.6 and 20% reported for soyflour and pigeon pea flour, respectively (Oshodi and Ekperigin, 1989), but comparatively lower than 60 and 80% reported for wheat flour and African breadfruit kernel flour (Akubor and Badifu, 2004), respectively.
The sample YTF has the highest bulk density of 0.62 g/cm 3 as shown in Table 3.The 0.55 g/cm 3 obtained for BTF compares favourably with 0.54 g/cm 3 reported for African breadfruit kernel flour but lower than 0.71 g/cm 3 reported for wheat flour (Akubor and Badifu, 2004).Bulk density is a measure of heaviness of a flour sample.

Pasting properties
From Table 4, the pasting temperature, peak viscosity, breakdown viscosity, final viscosity and setback viscosity of BTF are higher than YTF.The peak viscosity of YTF (9.75 RVU) is lower than that of BTF (16.33 RVU) but the time to reach peak viscosity is in contrast, 6.53 min for YTF and 5.33 min for BTF.Both YTF and BTF have low setback and breakdown viscosities.This shows that they are both stable to heat and mechanical shear.This could be attributed to the high fat content of the samples.Both YTF and BTF are characterized by little decrease in viscosity on cooling as indicated by setback viscosity values, a pointer towards low retrogradation property of the flour.

Conclusion
Tigernut flour is a rich source of oil and contains moderate amount of protein.It is also a rich source of some useful mineral elements such as iron and calcium which are necessary for body growth and development.The low bulk density, setback and breakdown viscosities show that tigernut flour can be used in food formulation with less fear of retrogradation.
*Values are mean of duplicate samples.