ORIGINAL PAPER

Effect of γ-Irradiation on Colour, Functional and PhysicochemicalProperties of Pearl Millet [Pennisetum glaucum (L) R. Br.] Cultivars

Kolawole O. Falade & Titilayo A. Kolawole

Received: 6 February 2012 /Accepted: 3 October 2012 /Published online: 10 November 2012# Springer Science+Business Media New York 2012

Abstract Effect of γ-irradiation dose (0–8 kGy) on seedcolour, functional and pasting properties of two selectedpearl millet cultivars (SOSAT and ZATIV) was investigated.Colour (L*a*b*) of the non- and γ-irradiated pearl milletcultivars was measured, and the deltachroma (ΔC), colourintensity (ΔE) and hue angle were calculated. Also, looseand tapped bulk densities, swelling capacity, water (WAC)and oil (OAC) absorption capacities of the flours weredetermined. Pasting characteristics were determined us-ing Rapid Visco Analyser, respectively. The effect ofγ-irradiation on L*, a* and b* values within ZATIVcultivar was almost never significant. ΔC and ΔE increased upto 4 kGy but decreased with increased γ-irradiation dose up to8 kGy. Loose and packed bulk densities, and WAC were notsignificantly affected by γ-irradiation. The OAC of theSOSAT (1.16–1.36 g/g) was not significantly affected butthe ZATIV (0.94–1.34 g/g) was significantly affected byγ-irradiation. The WACs of non-irradiated SOSAT andZATIV pearl millet flours were 1.42 and 1.33 g/g while theirradiated counterparts varied from 1.15 to 1.42 and 1.24 to1.39 g/g, respectively. Peak, trough, final, and setback viscos-ities decreased significantly (p<0.05) with increasedγ-irradiation dose. As irradiation dose increased, the peaktime of SOSAT and ZATIV pearl millet cultivars significantly(p<0.05) decreased from 5.84 to 5.07 and 5.58 to 4.94 min,respectively. However, pasting temperature of non-irradiated(61.80 °C) pearl millet was not significantly higher than theγ-irradiated (61.58–62.08 °C) samples.

Keywords Pearl millet cultivars . γ-Irradiation . Functionalproperties . Pasting characteristics . Colour parameters

Introduction

Pearl millet [Pennisetum glaucum (L) R. Br.] is a drought-tolerant cereal crop grown primarily as a food grain in Indiaand Africa (Freeman and Bocan 1973). It is known as darkmillet in Europe, Bajza in India, bubrush in Africa andMands forage in the USA (Freeman and Bocan 1973). Asa cereal for human food, pearl millet sustains the lives of thepoorest people in Africa and Asia (Ali et al. 2003), and isoften considered highly palatable and a good source ofprotein, minerals and energy (Durojaiye et al. 2010). Pearlmillet has a well-balanced protein, except for its lysinedeficiency, with high concentration of threonine and lower(but adequate) leucine than sorghum protein. Tryptophanlevels are generally higher in pearl millet than in othercereals (Chung and Pomeranz 1985). The nutritional prop-erties of pearl millet have received more attention than thoseof the other common millets, because it is the largest-seeded, most widely grown type (Hoseney et al. 1987).Pearl millet is nutritionally better than most other cereals;it has high levels of calcium, iron, zinc, lipids and high-quality proteins (Klopfenstein and Hoseney 1995).

Postharvest losses of millet could be high due to insectand pest infestation, and mouldiness. According to Olakojoand Akinlosotu (2004), insect or pest infestation is one ofthe major factors responsible for decline in quantity, qualityand germination potential of grains in storage. In fact, pestinfestation is not restricted only to the field, but continueseven into storage leading to wastage of a large fraction ofwhat should have contributed to national food supply. Post-harvest losses of grains, particularly pearl millet, vary sig-nificantly from region to region, with major losses being indeveloping countries of the world due to lack of properstorage and preservation facilities.

Food irradiation is a physical means of food processingthat involves exposing the pre-packaged or bulk foodstuffs

K. O. Falade (*) : T. A. KolawoleDepartment of Food Technology, University of Ibadan,Ibadan, Nigeriae-mail: kolawolefalade@yahoo.com

Food Bioprocess Technol (2013) 6:2429–2438DOI 10.1007/s11947-012-0981-8

to gamma rays, X-rays or electron beams (Arvanitoyannis etal. 2009). Irradiation protects foods by reducing parasites,food-borne pathogens and spoilage microorganisms, andeliminating pests and insects (Chen et al. 2007; Özden andErkan 2010). Proper irradiation in terms of intensity andtreatment time or irradiation rate can maintain, extend shelflife or improve the technological properties of food(Loaharanu 1989), while providing an alternative to chemicalconventional treatments (de Toledo et al. 2007). The Food andAgriculture Organization/International Atomic EnergyAgency/World Health Organization (FAO/IAEA/WHO1991a, b, 1992, 1994) announced that dosages of irradiationnot more than 10 kGY would not result in toxic danger inthe case of meat products. However, optimum dosagedepends on molecular chain or ligands characteristics ofthe biosystem under treatment.

Gamma irradiation generates free radicals and is capable ofhydrolyzing chemical bonds, thereby cleaving large mole-cules into smaller fragments of dextrin, a soluble gummysubstance from starch or fibril, such as protein (Pinto et al.2004; Lee et al. 2005) and fat (Kwon et al. 2000; Seisa et al.2004). It has been shown that γ-irradiation affected the phys-ical, functional and pasting properties of maize (Rombo et al.2001; Falade and Kolawole 2012), cowpea flour and pastes(Abu et al. 2005; Falade and Kolawole 2011), sweet potato(Falade et al. 2011), barley, wheat and rice (Zuleta et al. 2006;Wu et al. 2002). Since γ-irradiation of pearl millet prior tostorage presents an appealing option for reducing microbialcontamination and eliminating pests and insects, it is apt todetermine its effect on the grains. Moreso, in the semi-arid andthe arid areas of Africa, north of the equator, the staple food isfura and it is made from millet flour. Fura is produced bymixing flour with spices to form a paste, which is steamed forabout 30 min, kneaded into smooth dough and moulded intoballs (Jideani and Wedzicha 1995). The stiff dough producedis reconstituted into a porridge-like consistency with sour milkand consumed generally as an afternoon meal. While exten-sive literature abounds on the effect of processing on millet,there is paucity of information on the effect ofγ-irradiation on the physical, functional and physicochemicalproperties of the two selected pearl millet cultivars, which arethe most abundant cultivars in the region. Therefore theobjective of this research was to evaluate the effect ofγ-irradiation on the physical, functional and physicochem-ical properties of two selected pearl millet cultivars.

Materials and Methods

Source of Material

Millet cultivars, namely SOSAT and ZATIV, used for thestudy were obtained from the Kano State Agricultural and

Rural Development Authority, Nigeria. The grains werecleaned manually to remove all foreign matter and brokengrains, and then packaged in Ziploc® double zipper (26.8×27.3 cm) packages (Ziploc Brand Products, WI, USA).

γ-Irradiation Treatment of Millet Grains

The irradiation treatment was conducted in an irradiationfacility at National (Nigeria) Atomic Agency, Sheda Scienceand Technology Complex, Abuja, Nigeria, using a Cobalt-60 gamma source. Millet grains were portioned (1 kg) sep-arately in 100-μm polyethylene bags, sealed and irradiatedat of 2.0, 4.0, 6.0 and 8.0 kGy (±0.01 kGy) at a dose rate of1 kGy/h and at 22±2 °C. Non-irradiated millet (0 kGy)grains served as a control. Irradiated and non-irradiatedmillet grains were milled into flours using a hammer mill(Labmill No 275, Gibbsons Electric, Essex, UK) and sievedusing 425-μm Tyler screen. The latter was sealed in Ziploc®double-zipper high-density (26.8×27.3 cm, 100 μm)packages (Ziploc Brand Products, WI, USA) and keptfor analysis at 4–7 °C.

Hundred Kernel Weight

For each millet cultivar, hundred kernels were randomlyselected hundred (100) seeds and weighted on a digital toploading electronic balance (Model GF-2000, A&D companyLtd., Japan) of an accuracy of ±0.01.

Particle Size Distribution of Ground Millet

The particle size distribution of the milled millet was deter-mined using sieves of the Tyler series with aperture sizes212, 200, 180, 160, 150, 140, and 125 μm and pan. Onehundred grammes of the non- and γ-irradiated milled milletwas weighed and poured on the upper range of the Tylerseries. The set of Tyler sieves was given a thorough shakingfor 5 min. The oversize on each of the sieves was collectedand weighed. The percentage fraction on each of the over-size on each sieve was calculated.

Determination of Moisture Content

Moisture content of the millet cultivars was determined bydrying samples in a cross flow Gallenkamp Size two BSmodel OV-160 hot air oven (Gallenkamp, England, UK) at102 °C until constant weight was obtained (AOAC 1990).

Commission Internationale de l’Eclairage (CIE) L* a* b*Colour Evaluation of Non-Irradiated and Irradiated Millet

Colour (L*a*b*) parameters were determined using colourmetre (Color Tec PCMTM Color Tec Associates, Inc.,

2430 Food Bioprocess Technol (2013) 6:2429–2438

Clinton, NJ, USA). The instrument was first standardised(L*093.24, a*000.96, b*0−02.75) with a Business Xerox80-g/m2 white paper with 136 CIE whiteness D65. Multiplemeasurements (10 points) of L*, a* and b* parameters weredetermined by placing the sensor of the Colourimeter on thesample. From the data obtained, deltaChroma (ΔC), colourintensity (ΔE) and hue angle were calculated according toEq. 1, 2 and 3, respectively (Hunt 1991).

C ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

a*ð Þ2 þ b*ð Þ2q

ð1Þ

ΔE ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

ΔL*ð Þ2 þ Δa*ð Þ2q

þ Δb*� �2 ð2Þ

Hue angle ¼ Tan�1 b

að3Þ

Determination of Functional Properties of non-Irradiatedand Irradiated Millet Flours

The functional properties including loose and tapped bulkdensities, swelling capacity, water and oil absorption capac-ities of the flours obtained, after milling, for both γ-irradiated and non- irradiated grains were carried out asshown below.

Determination of Loose and Packed Bulk Densitiesof Grains and Flours

The method of Mpotokwane et al. (2008) was adopted withslight modification for the determination of bulk density. Ameasuring cylinder (100 mL) was filled with seeds or flourto (the 100-mL mark) and then the content weighed. Also,tapped bulk density was also obtained by following thesame procedure but tapping against the edge of the workbench for about 50 times prior to weighing. Bulk densitywas calculated as the ratio of the bulk weight and thevolume of the container (gram per cubic centimetre)(Asoegwu et al. 2006).

Swelling Capacity (Volume) of Non-Irradiatedand γ-Irradiated Millet Flours

The method described by Fleming et al. (1974) and Houssouand Ayernor (2002) was adopted for the determination of theswelling capacity (volume). Ten grammes of sample wasweighed into a 100-mL graduated measuring cylinder, andthe volume was noted. The measuring cylinder was filled tothe mark with distilled water and quickly stirred. The mix-ture was made to stand and the rise in the volume was noted

after time intervals of 1, 5, 10, 15, 20, 25, 30, 45 and 60 min.After the 60-min rest, the mixture was observed for swellinguntil the maximum volume was noted.

Water and Oil Absorption Capacity of Non-Irradiatedand γ-Irradiated Millet Flours

Water and oil absorption capacities of the flour were deter-mined following methods of Sosulski et al. (1976). Onegramme of flour sample was mixed with 10 mL distilled waterand 10 mL of refined soybean oil (specific gravity 0.9092) forthe determination of water and oil absorption capacities, re-spectively. The mixture was allowed to stand at 30±2 °C for30 min and then centrifuged (Sorvall® RC-6, Kendro labora-tory products, NC, USA) at 2,000×g for 30 min. Water or oilabsorption capacity was expressed as gramme of water or oilbound per gramme flour.

Pasting Properties of Non-Irradiated and γ-Irradiated MilletFlours

Pasting characteristics of irradiated and non-irradiated flourswere determined using a Rapid Visco Analyser (Model RVA3D, Newport Scientific, Narrabeen, Australia). Threegrammes of the flour samples was weighted into a driedempty canister; 25 mL of distilled water was dispensed intothe canister containing the sample. The mixture was thor-oughly stirred and the canister was fitted into the RVA asrecommended. The slurry was heated from 50 to 95 °C witha holding time of 2 min followed by cooling to 50 °C with2-min holding time. The rate of heating and cooling were ata constant rate of 11.25 °Cmin−1. Peak viscosity, trough,breakdown, final viscosity, set back, peak time and pastingtemperature were read from the pasting profile with the aidof Thermocline for Windows Software connected to a com-puter (Newport Scientific 1995).

Statistical Analysis

Experiments were carried out in triplicates and data weresubjected to analysis of variance. For difference betweenmeans, Duncan’s multiple range was used. Pearson correla-tions were used to relate functional and physicochemicalproperties of the irradiated maize. Statistical significancewas defined as p<0.05.

Results and Discussion

Effect of γ-Irradiation on Colour of Millet Cultivars

Visually, there were no observable changes in the colour ofthe non-irradiated or irradiated millet samples. The L*, a*

Food Bioprocess Technol (2013) 6:2429–2438 2431

and b* parameters varied with γ-irradiation dose and culti-var. The L* value of the SOSAT cultivar increased signifi-cantly with irradiation dose between 0 and 6 kGy. However,the L* value of ZATIV cultivar decreased but showed nosignificant difference with the range of 0–6 kGy (Table 1).Treatment of millet with higher γ-irradiation dose of 8 kGyresulted in significantly (p<0.05) lower L* value. The L*value of the SOSAT (47.11±0.74 to 49.41±0.82) cultivarwas higher than the ZATIV (45.53–46.54) cultivar, indi-cating lighter appearance. The effect of irradiation on L*,a* and b* values within ZATIV cultivar was almostnever significant. Falade and Kolawole (2011; 2012)reported similar result for some cultivars of maize andcowpea. The SOSAT cultivars showed a steady signifi-cant decline in a* value up to 6 kGy; however, theZATIV cultivar showed no significant decline. The a*value of the SOSAT cultivar (+3.60±0.32 to +5.18±0.16)was lower than ZATIV (+5.55±0.29 to +6.18±0.33) cul-tivar. Within each cultivar, γ-irradiation seemed not tohave any significant effect on b*. In fact, both SOSATand ZATIV cultivars showed similar b* values. The a*value showed significant (p<0.05) correlation with L*(−0.664) value and packed bulk density (0.982; Table 4).

Colour intensity and deltachroma values of both SOSATand ZATIV cultivars increased with increased γ-irradiationdose until 4 kGy, but decreased with increased γ-irradiationdose up to 8 kGy (p>0.05). Hue angle of ZATIV increased upto 2 kGy but decreased steadily thereafter up till 8 kGy. Thedecreased L*, deltachroma, colour intensity and hue anglecould indicate association between Maillard browning andapplied irradiation dose. The decrease in L* value was simi-larly observed by Abu et al. (2005) and Abu and Amanda(2009) for cowpea seeds and flour: irradiation produced aslight darkening of the cowpea seeds since L* values representthe degree of lightness. Irradiation effects are known to besimilar to those caused by heat treatment (Urbain 1986). Pimpaet al. (2007) showed colour change of native and irradiatedsago starches; a (redness) and b (yellowness) values were

increased with increased irradiation dosage. However, Kanget al. (1999) reported no noticeable changes in a (red–green)value of corn starch sample while the b (yellow–blue) valueincreased with γ-irradiation dose. Colour change could be dueto the caramelization reaction of monosaccharides obtainedfrom starch polysaccharide degradation as a result of thehigh-energy ray which is capable of hydrolyzing chemicalsbonds, thereby cleaving large molecules of starch into smallerfragments of dextrin, sugar and sugar acids that may be eitherelectrically charged or uncharged as free radicals (Greenwoodand Mackenzie 1963). Unfavourable conditions of water ac-tivity, reducing sugars and to some extent non-reducing sugars,as well as carbohydrate polymers, may take part in non-enzymatic Maillard browning reactions, leading to the forma-tion of browning compounds (Smith and Friedman 1984).

Functional Properties of Non-Irradiated and Irradiated MilletGrains and Flours

The particle size distribution of the milled millet determinedusing sieves of the Tyler series with aperture sizes 212, 200,180, 160, 150, 140, and 125 μm and pan varied from 35.96to 37.10, 22.58 to 33.32, 29.37 to 41.17, 0.05 to 0.08, 0.05to 0.10, 0.01 to 0.07, 0.00 to 0.01 and 0.00 to 0.03, respec-tively. Kernel weight, loose and packed bulk densities ofSOSAT and ZATIV cultivars were 1.04±0.02 g, 0.65±0.01and 0.62±0.02 g/cm3, and 1.03±0.02 g, 0.64±0.01 and0.61±0.02 g/cm3 respectively. Table 2 shows the functionalproperties of non-irradiated and γ-irradiated SOSAT andZATIV cultivars of millet. Both loose and packed bulkdensities of millet flour varied with cultivar and increasedirradiation dose. Between 0 and 2 kGy, loose bulk density ofboth cultivars decreased with increased γ-irradiation dose,thereafter there were mixed effects. Loose bulk density ofnon- and γ-irradiated SOSAT and ZATIV cultivars of milletvaried from 0.53 to 0.56 and 0.53 to 0.57 g/mL, respective-ly. Similarly, packed bulk density of the non- and γ-irradiated SOSAT and ZATIV cultivars varied from 0.66 to

Table 1 Effect of γ-irradiationon colour parameters of SOSATand ZATIV millet cultivars

Means±standard deviations in acolumn with the same letters arenot significantly different (p≤0.05)

Sample Dosage (kGy) L* a* b* ΔE ΔC Tan−1 b/a

SOSAT 0 47.21bc±0.83 4.81c±0.18 22.18±1.09 – – 77.76

SOSAT 2 48.15ab±0.52 4.81c±0.31 20.20±0.23 2.19 1.98 76.61

SOSAT 4 49.41a±0.82 3.94d±0.23 19.70±0.76 3.43 2.63 78.69

SOSAT 6 49.24a±0.48 3.60d±0.32 21.15±1.47 2.58 1.59 80.34

SOSAT 8 47.11bc±0.74 5.18bc±0.16 19.72±0.33 2.49 2.49 75.28

ZATIV 0 46.51bc±0.49 6.18a±0.33 19.43±0.39 – – 72.36

ZATIV 2 46.16bc±0.52 5.55abc±0.29 20.85±0.34 1.59 1.55 75.09

ZATIV 4 46.54bc±0.56 5.82ab±0.32 21.50±0.71 2.10 2.10 74.85

ZATIV 6 46.24bc±0.63 6.08a±0.28 20.81±0.63 1.41 1.38 73.71

ZATIV 8 45.53c±0.42 5.92ab±0.23 20.57±0.59 1.53 1.17 73.94

2432 Food Bioprocess Technol (2013) 6:2429–2438

0.67 and 0.66 to 0.60 g/mL, respectively. Packed bulk densityof both SOSAT and ZATIV cultivars were not significantlyaffected by γ-irradiation dose. Although, bulk densities gener-ally increased after γ-irradiation, γ-irradiation had no signifi-cant effect on both the loose and packed bulk density of themillet flours. Loose bulk density of maize flour ranged between0.51 and 0.55 g/cm3 for the non-irradiated and γ-irradiatedsamples (8 kGy), respectively, while their correspondingpacked bulk density were 0.59 and 0.69 g/cm3, respectively(Falade and Kolawole 2012).

Water absorption capacity (WAC) of millet varied withcultivar and irradiation dose, however, WAC was not sig-nificantly affected by γ-irradiation dose. Similarly, Faladeand Kolawole (2012) reported that γ-irradiation treatmenthad no significant effect (p>0.05) on the WAC of maizeflour. WAC of non-irradiated and γ-irradiated SOSAT (1.15±0.290–1.48±0.150 g/g) and ZATIV (1.24±0.020–1.39±0.085 g/g) cultivars varied. The WACs of non-irradiated

SOSAT and ZATIV millet flours were 1.42±0.060 and 1.33±0.020 g/g while the irradiated counterparts varied from 1.15±0.290 to 1.48±0.150 and 1.24±0.020 to 1.39±0.085 g/g,respectively. WACs of millet compared reasonably well withvalues reported for maize by Falade and Kolawole (2012) butwas lower than cowpea (Falade and Kolawole 2011). Thismay be due to the higher protein content in cowpea than inmillet and maize. Also, the differences in WAC of the grainsmay be attributed to the variation in their granule structure(Sandhu et al. 2004). The differences in degrees of availabilityof water binding sites among the starches of the grains mayhave also contributed to the variation in water absorp-tion capacity (Wotton and Bamunuarachchi 1978).Afoakwa (1996) reported that the major constituentsresponsible for the bulk of the water uptake are theproteins while contributing to a lesser extent the starchand cellulose at room temperature. Irradiation-inducedunfolding of non-polar protein sites and the ensuing

Table 2 Physical and functional properties of non- and γ-irradiated SOSAT and ZATIV cultivars of millet

Sample Dose (kGy) Loose bulk density Packed bulk density Water absorption capacity Oil absorption capacity

SOSAT 0 0.55ab±0.009 0.66±0.000 1.42±0.060 1.25a±0.020

SOSAT 2 0.53b±0.000 0.67±0.026 1.42±0.050 1.34a±0.045

SOSAT 4 0.54b±0.008 0.66±0.008 1.15±0.290 1.25a±0.045

SOSAT 6 0.56a±0.003 0.67±0.004 1.48±0.150 1.16 a±0.000

SOSAT 8 0.55ab±0.013 0.66±0.001 1.32±0.025 1.36a±0.015

ZATIV 0 0.55ab±0.003 0.66±0.009 1.33±0.020 0.94b±0.040

ZATIV 2 0.54b±0.001 0.69±0.003 1.39±0.085 1.34a±0.120

ZATIV 4 0.57a±0.004 0.68±0.005 1.30±0.020 1.31a±0.045

ZATIV 6 0.53b±0.005 0.66±0.000 1.24±0.020 0.94b±0.135

ZATIV 8 0.55ab±0.003 0.67±0.005 1.31±0.000 1.16a±0.015

Means±standard deviations in a column with the same letters are not significantly different (p≤0.05)

Table 3 Physicochemical properties non- and γ-irradiated SOSAT and ZATIV cultivars of millet

Dosage(kGy)

Peak viscosity(RVU)

Trough viscosity(RVU)

Breakdownviscosity (RVU)

Final viscosity(RVU)

Setback viscosity(RVU)

Peak time(min)

Pasting temp(°C)

SOSAT 0 98.71b±0.21 83.38a±1.71 15.33c±1.50 162.42b±13.75 79.04bc±15.46 5.84a±0.17 61.80a±0.15

SOSAT 2 74.25d±2.58 37.67c±2.09 36.58ab±0.50 120.09cd±3.41 82.42b±1.34 5.32bc±0.07 62.08a±0.03

SOSAT 4 71.33d±2.75 36.71c±8.79 34.63ab±6.05 104.165d±3.17 67.46bcd±5.63 5.30bc±0.18 61.93a±0.23

SOSAT 6 49.92f±0.835 13.92e±1.00 36.00 ab±0.17 57.67f±1.00 43.75e±0.00 4.99cd±0.00 61.58a±0.18

SOSAT 8 47.67f±0.665 16.46e±1.54 31.21b±0.88 56.92f±0.92 40.46e±0.63 5.07cd ±0.12 61.85a±0.35

ZATIV 0 113.21a±2.04 81.46a±2.29 31.75b±0.25 220.75a±5.5 139.29a±3.21 5.58ab±0.04 61.80a±0.35

ZATIV 2 82.38c±0.295 50.79b±0.21 31.59b±0.09 131.96 c±2.88 81.17b±2.67 5.18cd±0.03 62.08a±0.03

ZATIV 4 61.46e±0.29 22.25de±1.17 39.21a±1.46 84.25e±0.33 62.00cd±0.83 4.99cd±0.10 61.68a±0.03

ZATIV 6 63.29e±2.04 29.33cd±3.00 33.96ab±0.96 85.17e±2.92 55.84de±0.09 5.00cd±0.08 61.68a±0.23

ZATIV 8 51.75f±0.83 20.59de±0.67 31.17a±1.50 64.25f±0.67 43.67e±1.34 4.94d±0.05 61.73a±0.38

Means±standard deviations in a column with the same letters are not significantly different (p≤0.05)

Food Bioprocess Technol (2013) 6:2429–2438 2433

exposure of these sites may have led to a reduction inthe availability of polar amino groups for water binding(Zayas 1997), resulting in decreased WAC.

Oil absorption capacity (OAC) of millet varied withcultivar and irradiation dose; however, OAC was not signif-icantly affected by γ-irradiation dose. OAC of millet floursdecreased only at higher γ-irradiation dose. For SOSATcultivar, the effect of γ-irradiation on OAC was not signif-icant within the range of the dose (0–8 kGy), but ZATIVcultivar was significantly affected. Pearson’s showed thatthe parameters investigated showed no significant correla-tion with both WAC and OAC. This is contrary to thefinding of Abu et al. (2006a) who observed a significanteffect on the OAC of cowpea starch after irradiation.According to Abu et al. (2006b), oil absorption in starchrelies predominantly on the physical entrapment of oil with-in the starch structure, since starch does not possess non-polar sites akin to those found in proteins. The increase inOAC may be related to an increased ability of degraded and/or cross-linked starch to entrap more oil, physically, withincreasing irradiation dose. Abu et al. (2005) also reportedthat the increase in oil absorption capacity may be due toexposure of the non-polar protein sites. Oil absorption ca-pacity of flours is important for the development of newfood products as well as their storage stability, particularlyfor flavour binding and in the development of oxidativerancidity. The mechanism of oil absorption may beexplained as a physical entrapment of oil related to thenon-polar side chains of proteins. Both the protein contentand the type contribute to the oil-retaining properties of foodmaterials (Ravi and Sushelamma 2005). Furthermore, OACshowed significant (p<0.05) correlation with L* (0.991)value.

Physicochemical Properties of Non-Irradiatedand γ-Irradiated Millet Cultivars

Pasting properties of non-irradiated and irradiated milletgrains milled into flour varied with cultivar and dosage. Pastingproperties of non-irradiated and γ-irradiated SOSAT andZATIV cultivars of millet is shown in Table 3. Generally,pasting properties including peak, through, breakdown, finaland setback viscosities, and peak time and pasting temperaturevaried with cultivar and irradiation dose. Aside from pastingtemperature, pasting properties of both SOSAT and ZATIVcultivars of millet decreased significantly (p≤0.05) and consis-tently with increased γ-irradiation dose between 2 and 8 kGy.Furthermore, the peak, trough, final and setback viscosities ofboth cultivars decreased with increased irradiation dose.Generally, higher irradiation treatment resulted in lower peakviscosity. Peak viscosity, which shows the maximum swellingof the starch granule prior to its disintegration, has also beendescribed as the equilibrium point between swelling and T

able

4Pearson

’scorrelationanalysisbetweenfunctio

nalandpastingcharacteristicsof

γ-irradiatedmillet

cultivars

Loose

bulk

density

Packedbulk

density

L*

a*b*

Water

absorptio

ncapacity

Oilabsorptio

ncapacity

Peak

viscosity

Trough

viscosity

Breakdown

viscosity

Final

viscosity

Setback

viscosity

Peak

time

Packedbulk

density

0.923

L*

0.842

0.603

a*0.099

0.982

−0.664

b*−0.544

0.944

0.448

−0.590

Water

absorptio

ncapacity

0.776

−0.241

−0.260

0.494

0.483

Oilabsorptio

ncapacity

0.901

0.698

0.991

−0.410

−0.143

0.512

Peakviscosity

−0.884

−0.940

−0.499

0.942

0.793

−0.802

−0.607

Troughviscosity

−0.857

−0.930

−0.450

0.952

0.815

−0.798

−0.561

0.998

Breakdownviscosity

0.655

0.821

0.149

−0.953

−0.891

0.731

0.275

−0.931

−0.950

Final

viscosity

−0.909

−0.937

−0.554

0.934

0.750

−0.784

−0.657

−0.997

−0.992

−0.905

Setback

viscosity

−0.957

−0.924

−0.678

0.888

0.645

−0.745

−0.769

0.972

0.958

−0.823

0.986

Peaktim

e0.183

−0.115

0.679

0.494

0.601

−0.153

0.577

0.295

0.348

−0.622

0.234

0.078

Pastin

gtemperature

0.054

−0.021

0.417

0.637

0.260

0.146

0.312

0.344

0.381

−0.564

0.321

0.233

0.784

Allvalues

inbo

ldaresign

ificantat

p<0.05

2434 Food Bioprocess Technol (2013) 6:2429–2438

breakdown of the granules (Liu et al. 2006). It also provides anindication of the viscous load likely to be encountered duringmixing (Maziya-Dixon et al. 2004, 2005). Hoover (2001) statedthat granules with high peak viscosity have weaker cohesiveforces within the granules than those with lower values andwould disintegrate more easily. Peak viscosity is also a measureof the water-holding capacity of the starch in terms of theresistance of swollen granules to shear and the swelling perfor-mance of granules (Newport Scientific 1995). Peak viscosity isoften correlated with the final product quality.

Generally, ZATIV cultivar showed significantly higherpeak, trough, final and setback viscosities that the SOSATcultivar. Decrease in pasting characteristics of maize andcowpea cultivars have been reported (Falade and Kolawole2011, 2012). Similar observations were reported for irradi-ated cowpea (Abu et al. 2005), rice (Wu et al. 2002), maizeand kidney bean flours (Rombo et al. 2001). Similarirradiation-induced decreases in pasting properties have

been reported for rice starch (Wu et al. 2002), maize cultivars(Falade and Kolawole 2012), maize and bean flours (Romboet al. 2001). Decreased pasting characteristics could beattributed to changes in starch molecules such as starch deg-radation and debranching to simpler units (Rombo et al. 2001,2004), and Yu and Wang (2007) also attributed a decrease inpeak, trough and breakdown viscosity to the decrease in sizeof the starch granules in rice caused by irradiation. Decreasesin pasting properties, such as breakdown and setback values,result primarily from irradiation-induced starch degradationand may present opportunities such as ease of cooking andreduced starch retrogradation, respectively (Sabularse et al.1992; Abu et al. 2006a).

In both cultivars, setback viscosity decreased significant-ly (p<0.05) as γ-irradiation treatment dose increased. Agreater tendency to retrogradation was expected of thehigher setback viscosity values and therefore in non-irradiated and low (2.0–4.0 kGy) γ-irradiated millet flours.

Newport Scientific Pty Ltd

180

Vis

cosi

ty R

VU

Peak = 98.50

Hold = 81.67

0 kGy

Peak = 76.83

Hold = 39.75

2 kGy

Peak = 74.08

Hold = 45.50

4 kGy

Peak = 50.75

Hold = 14.92

6 kGy

0

60

120

240

0 3 6 9 2 15

Time mins

Peak = 47.00

Hold = 14.92

8 kGy

a

Newport Scientific Pty Ltd

Peak = 115.25

Hold = 83.75

0 kGy

Peak = 82.67

Hold = 51.00

2 kGy

Peak = 82.08

Hold = 50.58

4 kGy

Peak = 65.33

Hold = 32.33

6 kGy

0

60

120

180

240

0 3 6 9 12 15

Time mins

Vis

cosi

ty R

VU

Peak = 50.92

Hold = 21.25

8 kGy

b Fig. 1 Rapid Visco analyserpatterns of non-irradiated(0 kGy) and γ-irradiated aSOSAT b ZATIV cultivars ofmillet

10

20

30

0 10 20 30 40 50 60

Time (min)

Swel

ling

Vol

ume

(mL

)

Control

2 kGy

4 kGy

6 kGy

8 kGy

Fig. 2 Effect of irradiation onthe swelling volume of SOSATmillet cultivar

Food Bioprocess Technol (2013) 6:2429–2438 2435

Setback viscosity is calculated as the difference between thebreakdown viscosity and the viscosity at 50 °C and deter-mines the tendency of starch to retrogradation. Consequent-ly, millet grains subjected to higher doses (6.0–8.0 kGy) andpulverised into flours would show lower tendency to retro-grade. Starches with higher setback viscosity would tend tohave stiffer pastes than low setback viscosity (Seog et al.1987). Similar irradiation-induced decreases in pastingproperties have been reported for rice starch (Wu et al.2002), maize and bean flours (Rombo et al. 2001; Faladeand Kolawole 2012), cowpea starch (Abu et al. 2005; Faladeand Kolawole 2011) and yam and sweet potato flours(Falade et al. 2011). Irradiation-induced decreases in pastingproperties could be principally due to the starch polysaccha-ride degradation as a result of the high energy ray which iscapable of hydrolyzing chemicals bonds, thereby cleavinglarge molecules of starch into smaller fragments of dextrinand sugars. Setback viscosity showed significant (p<0.05)correlation with loose bulk density (−0.957), peak (0.972),trough (0.958) and final (0.986) viscosities.

Breakdown viscosity increased generally with increasedγ-irradiation dose. Breakdown viscosity is a parameter thatmeasures of the resistance to heat and shear of thedough. Pasting temperatures of non-irradiated (61.8 °C) andγ-irradiated (61.73-62.08 °C) millets were not significantlydifferent (Table 3). Breakdown viscosity showed significant(p<0.05) correlation with a* (−0.953) and trough viscosity(−0.950), while final viscosity showed significant correlationwith peak (−0.997) and trough (−0.992) viscosities (Table 4).Also, trough viscosity showed significant correlation with a*(0.952) and peak viscosity (−0.998). Moreover, peak timedecreased with increased irradiation dose for both millet culti-vars. For example, within a range of 0–8 kGy γ-irradiationtreatment, peak time of SOSAT and ZATIV cultivars signifi-cantly decreased from 5.84 to 5.07 and 5.58 to 4.94 min,

respectively. The peak time of non-irradiated millet flours(5.56–5.64 min) was higher than the irradiated samples(4.94–5.32 min). Pasting temperature of non-irradiated(61.80 °C) millet was not significantly higher than theγ-irradiated (61.58–62.08 °C) samples. Pasting temperatureof the millet cultivars was not significantly affected bythe γ-irradiation treatment. Pasting temperature gives the tem-perature at which a perceptible increase in viscosity occursand is always higher than gelatinisation temperature (Moorthy2002). Also, pasting temperature provides an indication of theminimum temperature required to cook a given sample, whichcan have implications on stability of other components in theflour, and also indicate energy costs (Newport Scientific1995).

Aside from slight differences, millet flours showed similarRVA patterns and trends with increased γ-irradiation dose.Similar patterns were observed in both SOSAT and ZATIVcultivars. Patterns characteristics of slow rise in viscosity up tothe peak and viscosity at holding, and thereafter a rise up tofinal viscosity, were observed for the non-irradiatedand γ-irradiated millet flours (Fig. 1a, b). Increasedγ-irradiation dose resulted in decreased sharpness ofthe rise to peak viscosity, holding and final viscosities.Flours of γ-irradiated millet showed steeper curvescompared to their non-irradiated counterparts.

Figures 2 and 3 show the effect of γ-irradiation dose onthe swelling volume (millilitre) of SOSAT and ZATIV culti-vars of millet, respectively. A mixed effect γ-irradiation onswelling volume was observed in the SOSAT cultivar.The SOSAT millet samples pre-irradiated at lower doses(2–4 kGy) and subsequently milled showed lower, whilethe samples pre-irradiated at higher doses (6–8 kGy)showed higher swelling volume than non-irradiated(control). Generally, pre-irradiated ZATIV flours showedhigher swelling volume than the non-irradiated sample.

10

20

30

0 10 20 30 40 50 60

Time (min)

Sw

ellin

g V

olu

me(

mL

)

Control

2 kGy

4 kGy

6 kGy

8 kGy

Fig. 3 Effect of irradiation onthe swelling volume of ZATIVcultivar

2436 Food Bioprocess Technol (2013) 6:2429–2438

Conclusion

Increased irradiation dose (0–8 kGy) resulted in mild to nosignificant effect on colour (L*, a*, b*) and functional(loose and packed bulk densities, WAC and OAC) ofSOSAT and ZATIV millet cultivars. Non- and γ-irradiatedpearl millet showed similar pasting pattern, however, higherdoses resulted in steeper curves. Thus, γ-irradiation treat-ment constitutes a means of modifying the pasting charac-teristics of pearl millet. Based on the results of theexperiments, γ-irradiation treatment of between 2 and4 kGy should be adequate as it would bring about little orno significant change in colour and function properties, anda minimal effect on the pasting characteristics of the grains.

Acknowledgment The authors are grateful to the University ofIbadan for the financial support for the project through the awardof MacArthur Multi-disciplinary Research Grant. Dr. S. A. Adesanmi,Mr, E. C. Akueche and staff of the National (Nigeria) Atomic EnergyCommission, Sheda, Abuja, Nigeria, are appreciated for guidance andhelp during irradiation of samples.

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