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LWT - Food Science and Technology 58 (2014) 239e246

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LWT - Food Science and Technology

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Effect of g-irradiation on granule structure and physicochemicalproperties of starch extracted from two types of potatoes grownin Jammu & Kashmir, India

Adil Gani a,*, Shama Nazia a, Sajad A. Rather a, S.M. Wani a, Asima Shah a, Mudasir Bashir a,F.A. Masoodi a, Asir Gani b

aDepartment of Food Science and Technology, University of Kashmir, Sgr 190006, IndiabDepartment of Food Engineering and Technology, SLIET, Punjab, India

a r t i c l e i n f o

Article history:Received 20 April 2013Received in revised form3 March 2014Accepted 6 March 2014

Keywords:Potato starchg-IrradiationPhysicochemical propertiesPasting propertiesMorphological properties

* Corresponding author. Tel.: þ91 8803023830 (moE-mail address: adil.gani@gmail.com (A. Gani).

http://dx.doi.org/10.1016/j.lwt.2014.03.0080023-6438/� 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

In this study red and white potato starches were treated with g-irradiation of 0, 5, 10 and 20 kGy.Physicochemical, pasting and morphological properties of the irradiated starches were investigated.Apparent amylose content, pH, moisture, swelling power and syneresis decreased; whereas carboxylcontent, water absorption capacity and solubility increased with increasing irradiation dose. Pastingproperties also decreased significantly (p � 0.05) upon increasing the irradiation dose. Observation underscanning electron microscope (SEM) showed surface cracking of the starch granules by g-irradiationwhich increased with increase in irradiation dose. X-ray diffraction pattern remained the same uponirradiation but a decrease in relative crystallinity was observed with increasing irradiation dose.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction which involves the alteration of the physical and chemical char-

Potatoes (Solanum tuberosum L.) are a staple food cultivatedthroughout the world and in most parts are harvested once a year.Potato starch is widely used in various food products as a filling,thickening or stabilizing agent in order to improve structure,texture, consistency and appeal. Starch affects texture, viscosity, gelformation, adhesion, binding, moisture retention, film formationand product homogeneity. It is used mainly in soups, sauces,gravies, bakery products, dairy confectionary, snacks, batters,coatings and meat products (Davies, 1995). There are some non-food applications of starch also in the field of pharmaceuticals,textiles, alcohol-based fuels and adhesives. New uses of starchinclude low-calorie substitutes, biodegradable packaging materials,thin films and thermoplastic materials with improved thermal andmechanical properties (Billiaderis, 1998).

Native starch is a good texture stabilizer and regulator in foodsystems (Cousidine, 1982, p. 142), but limitations such as low shearresistance, low thermal resistance, thermal decomposition, andhigh retrogradation tendency, are not optimal in some industrialfood applications (Thomas & Atwell, 1999). Starch modification,

bile).

acteristics of the native starch to improve its functional character-istics, can be used to tailor starch to specific food applications(Hermansson & Svegmark, 1996). Starches are often modified byphysical, chemical and enzymatic processes to promote specificfunctional properties. The irradiation of food products is a physicaltreatment involving direct exposure to electron or electromagneticrays for their long-time preservation as well as for the improve-ment of safety and quality (Urbain, 1986). Many studies have beenconducted on the effect of g irradiation on potato starches (Ciesla &Eliasson, 2007; Ezekiel, Rana, Singh, & Singh, 2007).

Irradiation treatments do not induce a significant increase intemperature, require minimal sample preparation, are fast andhave no dependence on any type of catalysts (Diehl, 2002). Theapplication of ionizing radiation (g and electron beam) is reportedto generate free radicals that are capable of inducing molecularchanges and fragmentation of starch (Ciesla, Zoltowski, &Mogilevsky, 1991; Sokhey & Hanna, 1993). This unique propertyhas been suggested to be one of the main mechanisms underlyingphysicochemical changes in starchy food, like reduction of viscosityand high water solubility (Lee et al., 2003). During irradiationtreatments (as with g rays), the glycoside bonds (at chain endings)are broken down in starch granules, which is later accompanied bythe decomposition of macromolecules and the creation of macro-molecules with smaller chains (Ghali, Ibrahim, & Aziz, 1979; Raffi,

A. Gani et al. / LWT - Food Science and Technology 58 (2014) 239e246240

Agnel, Thiery, Frejaville, & Saint-Lebe, 1981; Raffi, Frejaville, et al.,1981). Raffi, Agnel, et al. (1981) have reported that all the radiolyticend products formed with starch irradiation are similar, irre-spective of the source fromwhich they are obtained (maize, potato,wheat, or rice).

The present study was undertaken to elucidate the effects of g-irradiation (0, 5, 10 and 20 kGy) on morphological and functionalproperties of potato starches (red and white). The changes in starchproperties were observed that would be required for some appli-cations in the foodandnon-foodsuchaspaper and textile industries.

2. Material and methods

2.1. Starch isolation

Two types of potatoes, white and red were obtained fromSKUAST-K, Jammu and Kashmir, India. The potatoes were washed,peeled, cut into small pieces and ground in a blender to form apaste. The resultant slurry was sieved through muslin cloth into abeaker. Starch suspension was left overnight and extracted bywashing 4e5 times with distilled water. The resultant slurry wascentrifuged at 3000 � g for 10 min (Eppendorf Centrifuge 5810Rmade in Germany). The isolated starchwas dried at 40 �C for 24 h inhot air oven (NSW-143; India).

2.2. g-irradiation

The starch samples (10 g/100 g moisture content) were packedin a polyethylene bag and irradiated using 60Co g source at ambienttemperature (20e40 �C). The doses were controlled at 0 (control),5, 10, and 20 kGy with a dose rate of 2 kG/h. The irradiationtreatments were performed at Baba Atomic Research CentreZakura, Srinagar, Jammu and Kashmir, India.

2.3. Carboxyl content and pH

Carboxyl content was determined as per the procedure ofMattison and Legendre (1952). To 0.5e1.0 g of starch, 25 ml 0.1 molequi/l HCI was added and the mixture was allowed to stand for30 min with occasional stirring. The slurry was filtered through afritted glass crucible and washed with distilled water until it wasfree from chlorine. The starch was then transferred to a 500 mlbeaker to which 300 ml distilled water was added. It was thenboiled for 5e10 min for complete gelatinization, followed bytitrationwith 0.1mol equi/l NaOH solutionwith phenolphthalein asindicator. Carboxyl content was calculated as follows:

milli� Eq: of acidity=100 g starch

¼ ðA� BÞ � 0:1 mol equi 1 ðNaOHÞ � 100=W

whereA ¼ Titrate value for sample.B ¼ Titrate value for blank.W ¼ Weight of dry sample in grams.Apparent (g/100 g) carboxyl¼milli-equivalents of acidity/100 g

starch � 0.045.The pH of starch slurry (40 g/100 ml) was determined using a

digital pH meter calibrated at 25 �C.

2.4. Apparent amylose and amylose leaching

Apparent amylose contents of the starch samples were deter-minedby themethodofWilliams,Kuzina, andHlynka (1970). A starchsample of 20 mg was added to 10 ml of 0.5 mol/l KOH and the

suspension was mixed thoroughly. The dispersed sample was trans-ferred to a 100ml volumetricflask and the volumewasfilled up to themarkwithdistilledwater. Analiquotof the test starch solution (10ml)waspipetted into a50mlvolumetricflask and5mlof0.1mol/l aq.HClwas added followed by 0.5 ml of iodine reagent. The volume wasdiluted to 50 ml and the absorbance was measured at 625 nm (UV-Spectrophotometer, Model U-2900 Hitachi, Japan). The content ofamylose was determined from a calibration curve developed usingstandard amylose and amylopectin blends from potato starch.

The amylose leaching of starch at 60 �C was measured accordingto the procedure of Chung & Liu, 2009. Amylose leaching wasexpressed as percentage of amylose leached per 100 g of dry starch.

2.5. Swelling power and solubility

A Starch sample 0.6 g (M0) was mixed with 30 ml of distilledwater. For swelling power temperature was maintained at 60 �Cwhile as solubility was calculated at 50, 60, 70, 80, and 90 �C. After30min stirring, the mixturewas centrifuged at 1500� g for 30min.The supernatant was carefully removed, and the swollen starchsediment was weighed (M1). The supernatant was evaporated anddried at 130 �C in an oven until constant weight (M2). Swellingpower and solubility was calculated from the equations givenbelow.

Swelling power ðg=gÞ ¼ M1=M0

Solubility ðg=100gÞ ¼ M2=M0

2.6. Water absorption capacity

Water absorption capacity (WAC) of the starches was deter-mined as described by Mishra and Rai (2006) in triplicate using2.5 g/100 g starch suspensions at temperature of 25 �C. Water ab-sorption capacity (WAC) was calculated from the equations givenbelow.

WAC ¼ ðW3 �W2Þ=W1

W1 ¼ Weight of sample.W2 ¼ Weight of empty centrifuge tube.W3 ¼ Weight of tube after centrifugation and decanting.

2.7. Light transmittance

Transmittance of light from potato starches suspension wasmeasured as described by (Perera & Hoover, 1999). A starch sus-pension of potato starch (1 g/10 ml suspension, w/v) was heated ina boiling water bath for 30 minwith continuous gentle stirring andthen cooled for 1 h in a 25 � 0.5 �C water bath. The samples werestored for 72 h at 4 � 0.5 �C and the turbidity was determined bymeasuring absorbance at 650 nm against water blank (UV-Spec-trophotometer, Model U-2900 2JI-0003, Hitachi, Japan).

2.8. Syneresis

Syneresis was determined in triplicate using 5 g/100 g aqueousstarch solutionmade by adding 5ml of distilled water to 0.25 g (dryweight basis) starch in a screw capped centrifuge. The suspensionwas heated in a boiling water bath for 30minwith constant stirringand then cooled to room temperature in an ice bath. After cooling,starch pastes were reweighed to determine the amount of starchpaste and then placed in freezer at �20 �C for 48 h. After thefreezing period, the samples were placed in 40 �C water for 1.5 hto thaw and equilibrate. Syneresis was measured in triplicate as

Table 1Carboxyl content, pH and apparent amylose content of gamma-irradiated potatostarches.

Cultivar Dose(kGy)

Amylose(g/100 g)

Carboxylcontent(g/100 g)

pH

Whitepotato

0 32.60a � 0.3 0.00a � 0.00 4.00a � 0.015 31.10b � 0.1 0.11b � 0.01 3.70b � 0.01

10 27.20c � 0.5 0.17c � 0.01 3.30c � 0.0220 24.20d � 0.2 0.20d � 0.03 3.20d � 0.01

Redpotato

0 32.00a � 0.2 0.00a � 0.00 4.90a � 0.015 28.40b � 0.1 0.08b � 0.01 4.56b � 0.01

10 25.20c � 0.4 0.09c � 0.01 4.31c � 0.0120 22.60c � 0.6 0.10d � 0.01 4.20d � 0.02

Values are means � standard deviation of three determinations (n ¼ 3).Values followed by different superscript letter in a column are significantly different(p � 0.05).

A. Gani et al. / LWT - Food Science and Technology 58 (2014) 239e246 241

g/100 g water released after centrifuging at 1500 � g for 30 min(Singh, Sandhu, & Kaur, 2004).

Syneresis ðg=100gÞ ¼ ðWt: of water released=Wt: of gelÞ�100

2.9. Pasting properties

Pasting properties of kidney bean starchwere determined using arapid visco-analyzer (RVA Starch Master TM, Newport Scientific,Warriewood, Australia). The test profile STD1 (Newport ScientificMethod 1, Version 5, 1997) was used for determination of pastingcharacteristics. The sample (3.0 g of starch) was dispersed in water(25.0 ml) and stirred in an RVA container initially at 960 rpm for 10 sand finally at 160 rpm for the remaining test. The temperature profilewas started from50 �C for1min followedbyramping the temperaturelinearly to 95 �C in 3min and 42 s, holding for 2min and 30 s, coolingthesystemto50 �C in3minand48 sandending theprocess in13min.The pasting curves obtained were analysed using an RVA StarchMaster Software setup Tool (SMST) to obtain the characteristic pa-rameters like peak viscosity (PV), final viscosity (FV) at 50 �C; pastingtemperature; breakdown (BD¼ PV-HPV), set back (SB ¼ FV-HPV).

2.10. Scanning electron microscopy

In order to study morphological characters of the Potato starchgranules, a scanning electron microscope (SEM, S-3000H, Hitachi,Japan) was used at an accelerating potential of 15 kV. The starchsamples were attached to the SEM stub using double-sided cello-phane tape. Each sample was coated with gold. The morphologicalcharacteristics were then studied from SEM Micrographs.

2.11. X-ray diffractometry

Starcheswere equilibrated above a saturatedpotassium sulphatesolution (K2SO4) in a desiccator for two weeks. The hydrated starchpowders were packed tightly in a circular aluminium cell and pat-terns were measured using a Philips X’PERT PRO XRD by exposingthe samples to the X-ray beam from an X-ray generator running at45 kV and 40 mA. The scanning regions of the diffraction angle 2Øwere 2e49�. Other operation conditions included: Step interval0.05, divergence slit size 0.4354, receiving slitwidth0.1. Crystallinityof the starcheswasquantitativelyestimated following themethodofNara and Komiya (1983) by using a software package (Orion-version6.0 Microcal Inc., Northampton, MA, USA). A line connecting peakbaselines was computer-plotted on the diffractogram. The areaabove the smooth curve was considered as the crystalline portionand the lower area between the smooth curve and a linear baselinewas taken as the amorphous portion. The ratio of the upper area tothe total diffraction area was calculated as the crystallinity.

2.12. Statistical analysis

Mean values, standard deviation, analysis of variance (ANOVA)were computed using a commercial statistical package SPSS 10.1(USA). These data were then compared using Duncan’s multiplerange tests at 5% significance level.

3. Result and discussion

3.1. pH, carboxyl content and apparent amylose

The results for pH, carboxyl content and apparent amylose aregiven in Table 1. pH values of native starches were determined to be4e4.9 after their isolation. These values are similar to those of

Chung and Liu (2009) in case of native potato starch and fall in thenormal range for irradiated starches (Rowe, Sheskey, & Quinn,2006). The carboxyl content increased and pH decreased as irra-diation dose was increased. A substantial increase in carboxylcontent and decrease in pH was observed at 20 kGy and such anincrease in carboxyl content and decrease in pH could be due to theformation of number of carboxylic acids and aldehydes duringirradiation of starch (Sharpatyi, 2003). Dose dependent reductionin pH was also reported by Chung and Liu (2009) and Sofi et al.(2013) in case of corn and bean starches respectively.

The apparent amylose content of the non-irradiated potatostarches of two varieties were 32.60 and 32.0 g/100 g, respectively.Apparent amylose reduced from 32.60 to 24.20 g/100 g and from32.00 to 22.60 g/100 g in irradiated white and red potato starches,respectively. Similar results have been reported by Chung and Liu(2009, 2010) and Yu and Wang (2007) in maize, potato and ricestarches respectively. However, other studies (Lee et al., 2006;Rombo, Taylor, & Minnaar, 2004) showed that the amylose con-tent determined by gel permeation chromatography increasedwithincreasing radiation dose. Since the amylopectin molecules weredegraded during irradiation, leading to an increase in low molec-ular weight fraction (amylose-like). Also contradictory to ourfinding was reported by Ezekiel et al. (2007) they reported thatstarch separated from potatoes irradiated at levels of 0.1 and0.5 kGy and stored at 80 �C showed 1e2% higher amylose contentthan control. This discrepancy could be attributed to the higherdosage of irradiation level applied in this study (20 kGy) than thatof their work (up to 0.1 kGy) or could be probably due to severedegradation of amylose fraction that reduces the iodine bindingability of amylose resulting in smaller values of apparent amylosecontent (Sokhey & Chinnaswamy, 1993).

3.2. Functional properties

3.2.1. Swelling power and amylose leachingSwelling power and amylose leaching of irradiated potato

starches are shown in Table 2. The swelling power of irradiatedstarches was reported to be lower when compared to their non-irradiated counterparts. Similar results were found by Abu,Muller, Duodu, and Minnaar (2005) who studied the functionalproperties of cowpea (Vigna unguiculata L.Walp) flours g-irradiatedat 2, 10, and 50 kGy. Some of the starch-related functional prop-erties of cowpea flours and pastes, like the swelling index, and gelstrength and viscosity, were found to be significantly reduced at allthe doses of irradiation and these effects were dose dependent,which has been attributed to radiation-induced degradation of thestarch. Similar results have been reported by several researcherswith various starches (Abu, Duodu, & Minnaar, 2006; Ezekiel et al.,

Table 2Functional properties of irradiated potato starches.

Cultivar Dose(kGy)

Swellingpowere

(g/100 g)

Amyloseleachinge

(g/100 g)

Waterabsorptioncapacitye (g/100 g)

Whitepotato

0 28.10a � 0.42 15.4a � 0.8 2.10a � 1.75 22.40b � 0.50 18.3b � 0.7 2.64b � 0.1

10 19.30c � 0.47 20.4c � 0.6 2.88c � 1.220 16.10d � 0.54 27.3d � 1.0 2.96d � 0.7

Redpotato

0 26.90a � 0.35 16.0a � 0.9 2.06a � 0.35 20.90b � 0.32 25.5b � 0.4 2.72b � 0.8

10 17.40c � 0.28 31.4c � 0.5 2.82c � 1.020 15.80d � 0.36 35.7c � 0.8 2.87d � 1.2

Values are means � standard deviation of three determinations (n ¼ 3).Values followed by different superscript letter (aed) in a column are significantlydifferent (p � 0.05).

e Swelling power, amylose leaching experiments were carried out at 60 �C tempand water absorption capacity at 25 �C.

A. Gani et al. / LWT - Food Science and Technology 58 (2014) 239e246242

2007). Swelling of starch results from the ability of starch to trapand retainwater within its structure (Gani, Masoodi, Wani, Salim, &Rehana, 2013; Whistler & Daniel, 1985). Such capability may bediminished substantially by breakdown of starch molecules upong-irradiation (Ashwar et al., 2014; De Kerf, Mondelaers, Lahorte,Vervaet, & Remon, 2001).

The increase in amylose leaching by g irradiation could also berelated to the production of low molecular weight fractions anddegradation of starch structures (Bao, Ao, & Jane, 2005; Chung &Liu, 2009, 2010).

3.2.2. Water absorption capacityThe results of water binding capacity of native as well as irra-

diated (white and red) potato starches are provided in Table 2. Theresults are in accordance with those of MacArthur and D’Appolonia(1984), Abu et al. (2006) who also reported the increase in waterabsorption capacity by g-irradiated in hard red spring wheat

Table 3Solubility (g/100 g) of gamma-irradiated potato starches.

Temperature

Cultivar Dose (kGy) 50 �C 60 �C

White potato 0 8.04ae � 0.4 9.14be � 0.65 15.20ad � 0.8 17.30bd � 0.5

10 25.50ac � 0.6 28.50bc � 0.420 39.30ab � 0.5 44.40bb � 0.6

Red potato 0 9.46ae � 1.0 10.90be � 0.35 16.0ad � 0.7 18.40bd � 0.2

10 26.90ab � 0.8 32.70bb � 0.320 41.50ac � 0.9 45.80bc � 0.3

Values are means � standard deviation of three determinations (n ¼ 3).Values followed by different superscript letter in a column and a row are significantly d

Table 4Syneresis (g/100 g) of gamma-irradiated potato starches.

Cultivar Dose (kGy) Day 1 Day 2

White potato 0 79.40ad � 0.2 81.20bd �5 75.20ac � 0.5 77.30bc �

10 74.10ab � 0.3 74.90bb �20 72.50aa � 0.1 73.10ba �

Red potato 0 84.60ad � 0.4 86.50bd �5 81.30ac � 0.8 83.10bc �

10 80.20ab � 0.5 81.80bb �20 77.10aa � 0.3 76.70ba �

Values are means � standard deviation of three determinations (n ¼ 3).Values followed by different superscript letter in a column and a row are significantly d

cultivars and cowpea flours and pastes. The increase in WAC withirradiation may be due to irradiation-induced damage or degra-dation of potato starch granules to simpler molecules such asdextrins, maltose and other sugars that have higher affinity forwater than starch. Other workers (Wu, Shu, Wang, & Xia, 2002)have also reported depolymerization of various starches followingapplication of irradiation.

3.2.3. SolubilityNon-irradiated potato starches appeared slightly soluble at

room temperatures and showed a slight increase with increasingtemperature (Table 3). The same trend was seen by Liu, Ying Ma,Xue, and Shi (2012) in case of maize starch. Solubilities of irradi-ated potato starches showed increase by rising of temperature andelevation of irradiation dose. When heated in the presence ofexcess water, starch granules were considerably swollen and watermolecules become linked by hydrogen bonding to the exposedhydroxyl groups of amylose and amylopectin. The results are inagreement with Lee et al. (2003), who reported application of gradiations, generate free radicals which they suggested the mainmechanism underlying physiochemical changes in starchy foodswith high water solubility. El Saadany, El Saadany, and Foda (1974)evaluated the effects of g irradiation on rice starch and from theresults obtained higher starch solubility was observed with radia-tion treatments. Solubilities of the irradiated starch samples fromtwo varieties of potato increased significantly when compared totheir non-irradiated counterparts. It was concluded that effect ofirradiation on solubility was more than the effect of heating onsolubility. The increase in breakage of bonds under the g-irradia-tion and the decrease in interchain hydrogen bonds explain theparticularly great increase in solubility. Irradiation decreased inter-chain hydrogen bonds and increased the hydrogen bonds withwater, which improved the solubility of starch processed by g-irradiation. Similar results have been reported by Liu et al. (2012)while studying the effect of g-radiation on maize starch (De Kerf

70 �C 80 �C 90 �C

2 12.80ce � 0.18 15.70de � 0.45 16.51ee � 0.596 23.20cd � 0.24 28.90dd � 0.43 32.22ed � 0.538 35.90cc � 0.15 43.10dc � 0.48 46.44ec � 0.520 52.70cb � 0.21 59.50db � 0.52 62.74eb � 0.601 13.70ce � 0.74 15.90de � 0.26 17.21ee � 0.835 25.30cd � 0.66 30.20dd � 0.25 34.74ed � 0.557 36.40cb � 0.61 43.80db � 0.19 46.53eb � 0.646 54.90cc � 0.75 63.20dc � 0.18 64.80ec � 0.67

ifferent (p � 0.05).

Day 3 Day 4 Day 5

0.7 82.00cd � 0.1 82.10dd � 0.3 82.36ed � 0.40.8 78.00cc � 0.2 78.20dc � 0.1 78.44ec � 0.10.2 75.70cb � 0.9 76.30db � 0.6 76.92eb � 0.70.6 73.70ca � 0.4 74.00da � 0.2 74.23ea � 0.30.1 87.00cd � 0.7 87.50dd � 0.8 87.95ed � 0.20.6 83.80cc � 0.1 84.20dc � 0.7 84.50ec � 0.50.9 82.40cb � 0.2 82.70db � 0.4 83.02eb � 0.80.2 77.30ca � 0.5 77.80da � 0.1 78.05ea � 0.8

ifferent (p � 0.05).

Table 5Transmittance (%) of gamma-irradiated potato starches.

Cultivar Dose (kGy) Day 1 Day 2 Day 3 Day 4 Day 5

White potato 0 1.22ad � 0.13 0.81bd � 0.09 0.53cd � 0.02 0.48dd � 0.01 0.46ed � 0.055 1.55ac � 0.16 1.16bc � 0.05 0.92cc � 0.11 0.52dc � 0.03 0.51ec � 0.06

10 2.05ab � 0.12 1.22bb � 0.07 1.16cb � 0.06 0.56db � 0.08 0.54eb � 0.0920 2.14aa � 0.11 1.28ba � 0.08 1.24ca � 0.12 0.63da � 0.09 0.61ea � 0.03

Red potato 0 1.01ad � 0.30 0.61bd � 0.04 0.37cd � 0.05 0.25dd � 0.02 0.23ed � 0.015 1.51ac � 0.35 1.09bc � 0.10 0.71cc � 0.07 0.41dc � 0.06 0.41ec � 0.04

10 1.75ab � 0.26 1.14bb � 0.06 0.82cb � 0.09 0.52db � 0.04 0.51eb � 0.0220 1.83aa � 0.29 1.11ba � 0.03 1.01ca � 0.04 0.60da � 0.07 0.72ea � 0.08

Values are means � standard deviation of three determinations (n ¼ 3).Values followed by different superscript letter in a column and a row are significantly different (p � 0.05).

Table 6Effect of irradiation dose on pasting properties and relative crystallinity of potato starches.

Starchsource

Irradiationdose (kGy)

Peakviscosity

Holdingviscosity

Breakdown viscosity Setbackviscosity

Finalviscosity

Pastingtemperature

Relativecrystallinity(g/100 g)

Whitepotato

0 1693h � 13.0 1039p � 10.5 654y � 17.0 3770z � 15.0 4809i � 11.5 73.30q � 0.1 33.1a � 0.35 1376g � 12.5 831� � 7.0 545z � 14.0 861a � 9.5 1692j � 13.0 73.15r � 0.3 32.4b � 0.5

10 1192f � 11.0 705n � 6.0 487w � 11.0 370b � 14.0 1075k � 8.5 68.5s � 0.1 31.7c � 0.220 444e � 14.0 147m � 8.0 297u � 9.0 109c � 17.0 256l � 15.0 72.4t � 0.2 30.3d � 0.4

Redpotato

0 2486d � 9.0 1730l � 12.0 756t � 15.0 3642e � 12.5 5372m � 7.0 73.05u � 0.2 32.9a � 0.25 2245c � 12.0 1591k � 13.5 654s � 8.0 725f � 6.0 2316n � 12.0 72.6v � 0.1 32.1b � 0.6

10 1692b � 8.0 1174j � 7.5 518r � 12.5 391g � 10.0 1565� � 13.5 68.0w � 0.1 31.5c � 0.320 949a � 13.0 573i � 11.0 376q � 11.0 80h � 9.0 653p � 9.0 73.10y � 0.2 30.2d � 0.4

Values are means � standard deviation of three determinations (n ¼ 3).Values followed by different superscript letter in a column are significantly different (p � 0.05).

Fig. 1. Pasting curve of native red potato starch showing PV (Peak viscosity), HPV (Hotpast viscosity) and FV (Final viscosity).

A. Gani et al. / LWT - Food Science and Technology 58 (2014) 239e246 243

et al., 2001) and Liu et al. (2012) in corn and potato starches withX-ray and electron-beam irradiation treatment.

3.2.4. SyneresisSyneresis refers to expulsion of liquid from the gel. The syneresis

of the two varieties of potato starches were observed for 5consecutive days (Table 4). Syneresis deceased with increase in g-irradiation dose. Gani, Tahir Gazanfar, Romee Jan, Wani, andMasoodi (2013) also observed steady decrease in syneresis withincreasing irradiation dose in lotus stem starches. However itincreased for both the varieties with each passing day in native aswell as irradiated starches. Increase in syneresis was reported bySrichuwong, Isono, Jiang, Mishima, and Hisamatsu (2011) with anincrease in number of freeze thaw cycles and also varied with thebotanical sources. Factors responsible for turbidity development instarches during storage have been previously identified by manyresearchers Craig, Maningat, Seib, and Hoseney (1989) and includeaggregates made of leached amylose, amylose and amylopectinchain lengths, intra or intermolecular bonding, granule swellingand granule remnants affect the syneresis of starches. The decreasein syneresis by irradiation as compared to native is because ofdecrease of apparent amylose content during irradiation (Chung &Liu, 2009, 2010; Yu & Wang, 2007). It could partly be related toamylopectin chain ratio which has higher water holding capacity,highly branched structure & shorter chains would retrograde in aslower rate (Srichuwong & Jane, 2007).

3.2.5. Light transmittanceTransmittance is the fraction of incident light (or other elec-

tromagnetic radiation) at a specified wavelength that passesthrough a sample. The light transmittance of gelatinized starchpastes (1 g/100 g) decreased sharply up to 4th day of storage andthen remained almost constant (Table 5). However, the decrease inlight transmittance was most pronounced between 1st and 2ndday. Decrease in light transmittance of starch paste with increase in

storage time may be due to the aggregation and slow recrystalli-zation of amylopectin (Gani, Masoodi &Wani, 2013). The results arein accordance with those found in case of Indian bean starches(Wani, Sogi, Wani, Gill, & Shivhare, 2010) and water chest nutstarches (Gani, Haq, Masoodi, Broadway, & Gani, 2010). The in-crease in transmittance of starch paste was a result of decreasedretrogradation of starch with the increase in g-irradiation dose.Amylose reorganization forms aggregates that reduce light trans-mittance of starch pastes (Tetchi, Amani, & Kamenan, 2007). Highamylose starches reassociate more readily than amylopectinstarches thereby resulting in more opacity (Bultosa, Hall, & Taylor,2002). Transmittance was found to increase with the increase inirradiation dose for both the samples. This may be due to thedisintegration of starch molecules on exposure to g-irradiationresulting in a clear solution.

Fig. 2. Scanning electron micrographs (AeD) of g-irradiated white potato starches. (A e 0 kGy), (B e 5 kGy), (C e 10 kGy), (D e 20 kGy).

A. Gani et al. / LWT - Food Science and Technology 58 (2014) 239e246244

3.3. Pasting properties

Pasting properties (peak, breakdown, final, holding and setbackviscosities) decreased considerably with increasing irradiation dose(Table 6 & Fig.1). Peak viscosity (cP) was equal to 444 and 949 cP forWhite and Red potato respectively at 20 kGy. It is mainly related toswelling of starch granules (Vandeputte, Derycke, Geeroms, &Delcour, 2003). Substantial reduction in swelling index in irradi-ated starch due to its degradation upon its irradiation is responsiblefor lowering of peak viscosity with increasing dose (Chung & Liu,2010; Gani, Bashir, Wani, & Masoodi, 2012; Yu & Wang, 2007).The variation in swelling index may account for differences in peakviscosity among different potato starches.

Fig. 3. Scanning electron micrographs (AeD) of g-irradiated red potat

The setback and final viscosity are largely due to re-ordering orpolymerization of leached amylose and amylopectin depolymeriza-tion (degradation) of starchmolecules (Abu et al., 2006; Pimpa et al.,2007;Wu et al., 2002) led to significant decrease in setback and finalviscosities. Similar results have been reported by Abu et al. (2006),Chung and Liu (2010). The difference in these viscosities amongdifferentpotato starchesmaybeassigned to theirdifference inextentof polymerization of leached amylose and amylopectin molecules.Breakdown results from the rupture of swelling granules. Decreasedtrend in breakdown was observed as the irradiation dose increasedand the valuewas equal to 297 and to 376 at 20 kGy. The higher valuewas recorded for red potato starch and lower forwhite potato starch.The decrease in breakdownviscositymight be because of decrease in

o starches. (A e 0 kGy), (B e 5 kGy), (C e 10 kGy), (D e 20 kGy).

Fig. 4. X-ray diffraction patterns of g-irradiated red potato starch.

A. Gani et al. / LWT - Food Science and Technology 58 (2014) 239e246 245

peak viscosity after irradiation. The results were in agreement withthose reported by Abu et al. (2006), Yu andWang (2007).

3.4. Microscopic observation

The SEM photographs of starch granules revealed round, ellip-tical and oval shapes in native as well as irradiated starch samplesof the two potato varieties. However, the number of fracturedgranules increased with increasing irradiation-dose (Fig. 2 & Fig. 3).The cracks appeared on the surface of the granule after irradiationwhich could be ascribed to disintegration of starch granules causedby these highly energetic and penetrating radiations. The defor-mation in starch granule has also been reported byWu et al. (2002).The deformation of granular structure appeared to be dose-dependent. While all the potato starch samples were not affectedequally but differ more or less in the extent of cracking. The maineffects of irradiation are degradation and cross linking (Nagasawa,Yagi, Kume, & Yoshii, 2004) which therefore could account forcracking of granule. Similar results have been reported by Chungand Liu (2010) with irradiated potato starches.

3.5. X-ray diffractometry

All starch samples displayed the typical B-type diffractionpattern (Figs. 4 and 5). The irradiated starch showed no change in

Fig. 5. X-ray diffraction patterns of g-irradiated white potato starch.

diffraction pattern compared to non-irradiated starch. However,the relative crystallinity decreased with increasing radiation dose(Table 6). Similar findings have been reported by Ciesla et al. (1991)who observed a decrease in relative crystallinity of irradiated po-tato starch at 5e20 kGy. They claimed that the decrease in relativecrystallinity was due to destruction of long range ordering linked tothe ordered structure of crystalline and amorphous regions instarch granules. Chung and Liu (2010) also observed a decrease inthe relative crystallinity of potato and bean starches after g-irra-diation at 50 kGy.

4. Conclusion

g-Irradiation modified the physicochemical properties of thestarch, including an increase in carboxyl content, solubility, trans-mittance and water absorption capacity, and a decrease in swellingindex, apparent amylose content and syneresis. The decrease inswelling index could be beneficial to improve the texture uponcooking. The two very important functional properties syneresisand transmittance that got altered have positive effect on foodformulations where reduction of syneresis and increase in trans-mittance during storage are important for retention of qualityattributes. The scanning electron micrographs showed crackingof starch granules in irradiated starch. X-ray diffraction pattern(B-type) remainedsame upon irradiation but a dose dependentdecrease in crystallinity was observed. The pasting propertiesdecreased drastically with increasing irradiation dose. This can alsobe exploited in foods where less viscosity is desirable.

Acknowledgements

Authors are thankful to Department of Biotechnology, Govt. ofIndia; Department of Physics, IIT, Delhi and Baba Atomic ResearchCentre, Zakura, Srinagar, J & K, India.

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