Determination of maize kernel hardness: comparison of different laboratory tests to predict dry-milling performance

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    Research ArticleReceived: 17 March 2010 Revised: 29 April 2010 Accepted: 30 April 2010 Published online in Wiley Interscience: 3 June 2010

    (www.interscience.wiley.com) DOI 10.1002/jsfa.4027

    Determination of maize kernel hardness:comparison of different laboratory teststo predict dry-milling performanceMassimo Blandino,a Mattia Ciro Mancini,a Alessandro Peila,a Luca Rolle,bFrancesca Vanaraa and Amedeo Reyneria

    Abstract

    BACKGROUND: Numerous foods are produced frommaize, and grain hardness has been described to have an impact on grainend-use value, and in particular for dry-milling performance.

    RESULTS: Thirty-three samples of commercial hybrids have been analysed for test weight (TW), thousand-kernel weight(TKW), hard : soft endosperm ratio (H/S), milling time (MT) and total milling energy (TME) through the Stenvert hardness test,coarse : fine material ratio (C/F), break force (HF) and break energy (HWF) through the puncture test, floating test (FLT), kerneldimensions and sphericity (S), protein (PC), starch (SC), lipid (LC), ash (AC) content and amylose : amylopectin ratio (AS/AP).

    Total grit yield (TGY) has been obtained through amicromilling procedure and used to compare the efficiency of the tests topredict the dry-milling performance. TW, H/S, MT, TME, C/F, FLT, S, PC, SC and AS/AP were significantly correlated with eachother. TW has been confirmed to be a simple estimator of grain hardness. Among the hardness tests, C/F was shown to be thebest descriptor ofmaizemilling ability, followed by FLT. A good correlationwith TGY has also been observedwith H/S, MT, TMEand PC, while SC, S and AS/AP seem to play a minor role. The puncture test (HF and HWF) did not offer good indications on theimpact of hardness on kernel grinding properties.

    CONCLUSION: This study can be considered as a contribution towards determining kernel properties which influence maizehardnessmeasurement in relation to the end-use processing performance.c 2010 Society of Chemical Industry

    Keywords:maize kernel; dry-milling; hardness; endosperm; texture analysis

    INTRODUCTIONGrain hardness is an important grain quality attribute that plays arole in the processing of cereal grains and in the end-use qualityof cereal grain products.1 Maize (Zea mays L.) is dry-milled toproduce a range of flours and grits which are further processedfor snacks, breakfast cereals and cooked or extruded products.2,3

    Maize hardness has been shown to have a remarkable influenceon the efficiency of the extraction yield and quality of the finalproduct.4 Maize for dry-milling and alkaline cooking processesshould be hard, with large kernels and with pericarps and germsthat are easy to remove during the process.5,6 On the other hand,wet millers prefer soft maize grain, which usually requires lesssteeping and leads to a better starchprotein separation.7

    The physical and biochemical aspects of maize hardnesshave been described in numerous publications. As far as thebiochemical contribution to hardness in maize is concerned, boththe protein and starch compositions have been associated withmaize hardness.8 Although the protein content comprises a lowerproportion of the total kernel composition compared to starch,it would appear that it plays a significant role in influencinghardness,3 and the variation in zein classes has, in particular,been linked to differences in hardness.9 On the other hand, somestudies10 have not shown any link between grain protein content

    (PC) and hardness. Fox and Manley11 suggest that the type ofhardness test adopted may be influenced by protein, and sometests could be more influenced by the endosperm structure,thereby giving a stronger correlation with the protein content. Atpresent very few studies have carried out multiple hardness testsand linked the result to PC.

    Among the physical tests, the ratio of the cross-sectional areafrom the hard to the soft (H/S ratio) endosperm, which can bemeasured with different techniques,10 is probably the most directway of measuring the fraction of kernel which influences dry-milling processing performance to the greatest extent, althoughthis method is not practical and is time consuming.12 The othermethods used to assess whole grain hardness are empirical andgive an indirect measurement of the hardness that is generally

    Correspondence to: Massimo Blandino, Department of Agriculture, Forestryand Land Management, University of Turin, via Leonardo da Vinci 44, 10095Grugliasco (TO), Italy. E-mail: massimo.blandino@unito.it

    a DepartmentofAgriculture, ForestryandLandManagement,UniversityofTurin,10095 Grugliasco (TO), Italy

    b Dipartimento di Valorizzazione e Protezione delle Risorse Agroforestali FoodTechnology sector, University of Turin, 10095 Grugliasco (TO), Italy

    J Sci Food Agric 2010; 90: 18701878 www.soci.org c 2010 Society of Chemical Industry

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    correlated with H/S. The maize physical characteristics, kernelsize and shape, weight and density, resistance to grinding orto abrasion and quantification of coarse and fine material aftergrinding and sieving have all been linked to hardness and itssubsequent effects on processing.11 Other indirect available teststoestimatemaizehardnessarebasedontheviscosityof thegroundmaterial (Rapid ViscoAnalyser)13 or on near-infrared reflectance(NIR) and transmittance (NIT), both of which use whole kernels orkernels after the grinding step.14 Most of these methods providevariable informationontherangeofhardness fromamaizesample.Moreover, in spite of the importanceof hardness indry-milling andthe number of studies that have been published on this subject,there is still no generally accepted standard for the evaluation ofmaize kernel hardness and there is a need to evaluate new simple,rapid and reliable tests that could relate maize quality to productyields.15

    At present there is little data concerning a single-kernel testingmethodology for maize, whereas for wheat and barley the single-kernel characterization system (SKCS) has been shown to besuitable to determine hardness and provide an indication ofquality.16 One of the few methods that has the potential formeasuring single maize kernels, similar to the SKCS for wheat,is the compression or puncture test. This test, already used forwheat17 and other cereals,18 relies on a resistance measurementof single kernels and involves a rod being pressed into the kernels.Shandera and Jackson19 have used a single-kernel puncturetexture analysis to discover the components andassociative forcesthat are responsible for the endosperm structure of maize kernels,andGaytanMartnez et al.20 have studied thehardnessof 21maizecultivars in relation to texture, floating test, size and arrangementof starch granules within the endosperm.

    The objectives of this study were: (1) to increase the under-standing ofmaize quality factor correlations and their relationshipwith the yield of dry-milling products; (2) to evaluate the rangeof variation of maize grain hardness that occurs in commercialmaize hybrids that are normally cultivated in northern Italy; (3) tocompare the parameters obtained from the puncture test withother standard tests estimating maize grain hardness.

    EXPERIMENTALMaize sample collectionThirteen commercial maize hybrids, all of which are normally cul-tivated in northern Italy and processed for dry-milling foodstuffs,were strip-test sown infive sites in 2007. Theplot sizewas100 mbyeight rows, and the row spacing was 0.75 m. The geographic andmain agronomic information concerning the experimental fieldsare reported in Table 1. The experimental fields were cultivatedadopting the normal agronomic technique of each site. All 13

    hybrids were compared at site D, while five hybrids were selectedat the other sites. At harvest, 100 ears were collected for eachhybrid by hand from each strip at the end of maturity (moisturecontent of the grains between 20% and 26%) and shelled usingan electric sheller. The kernels were mixed thoroughly to obtain arandom distribution of the kernels and a 5 kg sample was slowlydried to 14% moisture and stored in a cool room at 7 C and30% relative humidity until required. Storage of the kernels, equi-librated with the air in the cool room, resulted in a meanmoisturecontent of 10.2% (range 9.210.9%) when tested. Before testing,all the samples were equilibrated to room temperature (25 1 C)in paper bags for 48 h.

    The33maizesamples,whichwere testedfor severalphysicalandchemical properties, are listed in Tables 2 and 3. All the comparedtests were performed only on typical, flat-shaped, whole kernelsof the middle part of the ear, free from defects, which wereselected visually from each sample. The compared tests and theirabbreviations are summarized in Table 4.

    Analytical methodsMoisture content and test weight (TW)The moisture content and TW of the stored and dried maizesamples were determined by means of a grain analysis meter(Dickey-John GAC2000, Colombes, France) using the suppliedprogramme. Calibration for moisture was checked using oven-drying techniques. The test weight was recorded as kg hL1 andthe moisture content as g kg1 on the wet weight.

    Thousand-kernel weight (TKW)One hundred kernels were randomly collected from each sampleand weighed using an electronic balance to assess the thousand-kernel weight; this process was repeated three times. Thereafter,the mean value was used to calculated the TKW.

    Hard : soft endosperm ratio (H/S)The H/S endosperm ratio in the grain samples was estimated bysectioning thekernelsandmeasuring thehardandsoftendospermareas visible at the cut surface.12 Dried kernels were sectioned justabove the topof the embryo region using secateurs. TheH/S ratioswere calculated for 15 kernels from each sample by measuringthe area of the cut surface and the soft endosperm region, usingtheir scanned images and an image analysis system, with ImageJsoftware (version 1.38), which calculates the percentage of hardand soft endosperm in each kernel.

    Stenvert testThis test was based on the method described by Stenvert21 andPomeranz et al.22 A 20 g sample of kernel was ground using a

    Table 1. Geographic and main agronomic information about the experimental fields

    Site LocationGeographiccoordinates Soila Altitude (m)

    Sowingdate Harvest date

    A Carignano 44 55 N, 07 40 E Sandy loam, Typic Udifluvents 236 12 April 2007 12 October 2007B Chivasso 45 14 N, 7 51 E Sandy, Typic Hapludalfs 209 10 April2007 11 October 2007C Feletto 18 N, 7 45 E Sandymedium texture, Mollic Hapluquepts 275 9 April 2007 4 October 2007D Vigone 44 51 N, 07 30 E Sandy, Mollic Hapluquepts 256 29 March 2007 1 October 2007E Villafranca 44 47 N, 7 33 E Sandymedium texture, Typic Udifluvents 253 4 April 4 2007 10 October 2007

    a USDA soil classification

    J Sci Food Agric 2010; 90: 18701878 c 2010 Society of Chemical Industry www.interscience.wiley.com/jsfa

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    Table 2. Total grit yield and results of hardness tests for all maize samples, ranked according to total grit yield

    HybridSite of

    cultivation

    Total grityield

    (g kg1)

    Testweight

    (kg hL1)

    Thousandkernel

    weight (g)

    Hard/softendosperm

    ratio

    Millingtimea

    (s)

    Totalenergya

    (J)C/F

    ratioa

    Breakforceb

    (N)

    Breakenergyb

    (mJ)Floating

    test

    HCP CECINA D 600 81.1 350 3.5 9.8 1366 1.4 253 119 2123

    Pioneer 3245 D 591 82.4 404 4.2 9.5 1379 1.3 266 119 2190

    Dekalb DKC 6309 C 583 78.6 430 2.0 9.0 1344 1.2 193 75 2530

    Dekalb DKC 6309 D 580 79.2 406 1.9 8.3 1352 1.2 236 104 2395

    Pioneer 3235 B 578 80.6 380 4.0 8.5 1130 1.4 219 88 2058

    Pioneer 3235 E 560 79.0 360 3.5 8.2 1151 1.1 222 96 2475

    Pioneer 3235 A 559 79.6 370 4.8 8.8 1235 1.1 225 94 2323

    Dekalb DKC 6309 A 557 80.1 435 3.5 9.0 1343 1.2 220 93 2428

    Pioneer X1132R D 555 80.5 403 1.9 8.7 1353 1.3 242 101 2158

    Pioneer X1132R B 552 79.1 395 1.0 9.1 1117 1.3 214 87 2368

    Pioneer 3235 C 550 78.6 405 3.1 8.1 1293 1.3 203 87 2323

    Pioneer 3235 D 550 80.8 372 4.4 9.1 1388 1.3 278 135 2088

    KWS Kuadro D 548 77.8 325 0.9 9.1 1198 0.9 259 124 2415

    Dekalb DKC 6309 B 542 79.8 410 3.7 8.9 1074 1.2 212 87 2343

    KWS Kermess D 535 77.2 353 1.6 8.1 1149 1.1 215 91 2425

    Syngenta NX6413 D 534 80.2 391 2.3 8.5 1216 1.1 300 155 2528

    Pioneer X1132R A 532 78.6 390 3.0 8.5 1226 1.2 198 77 2175

    HCP DORIA D 532 80.0 313 3.2 10.2 1520 1.3 233 98 2110

    Pioneer X1132R C 526 75.3 395 3.0 8.8 1151 1.0 162 60 2555

    Pioneer X1132R E 526 76.7 385 1.3 7.8 1270 1.0 148 56 2605

    Dekalb DKC 6309 E 522 78.8 380 2.1 9.1 1211 1.1 198 80 2600

    Pioneer PR34G44 D 501 79.4 444 2.5 8.4 1259 1.0 211 86 2425

    Syngenta NX7034 D 493 76.1 421 1.4 8.9 1065 1.0 277 145 2653

    Dekalb Tevere E 484 73.3 370 1.2 6.7 1037 0.8 180 78 3225

    Dekalb Tevere B 478 75.2 385 0.8 7.9 1129 0.8 190 79 2890

    Syngenta NX7234 E 475 74.1 359 0.4 5.5 989 0.8 145 52 2775

    Dekalb Tevere D 448 73.1 388 0.4 7.8 1185 0.7 232 109 2835

    Dekalb Tevere A 444 75.6 385 0.3 7.2 1193 0.8 229 103 2935

    Syngenta NX7234 B 441 75.6 355 0.8 6.7 1068 0.8 184 75 2765

    Syngenta NX7234 A 416 75.1 380 0.6 6.9 1117 0.7 177 70 2775

    Syngenta NX7234 D 410 75.4 358 0.9 7.6 1029 0.7 208 91 3040

    Dekalb Tevere C 408 74.3 375 0.3 7.1 1084 0.6 199 81 3195

    Syngenta NX7234 C 404 73.5 365 0.2 6.2 939 0.5 178 71 3230

    Average 516 77.7 383 2.1 8.2 1199 1.0 215 93 2544

    Coefficient of variation (%) 11.1 3.3 7.5 66.5 12.8 11.1 23.6 16.9 25.6 13.2

    a Parameters that refer to the Stenvert hardness test.b Parameters that refer to the puncture test.

    Culatti micro hammermill (Labtech Essa, Belmont, Australia) fittedwith a 2 mmaperture particle screen at a speed of 2500 rpmwhenempty. The laboratory mill was equipped with a computerizeddata-logging system to log the instantaneous electric powerconsumption during the milling test, as reported by Mestreset al.23 and Li et al.12 Total milling energy (TME) and the millingtime (MT) taken to completely mill the 20 g kernel sample, weredetermined from these data. These parameters were determinedthree times for each maize sample.

    Particle size indexA 20 g kernel sample was ground using a Culatti micro-hammermill fittedwith a 2 mmaperture particle screen andwas sieved intotwo fractions using a Ro-Tap testing sieve shaker (WS Tyler Co.,Cleveland, OH, USA) with 8 in. diameter brass sieves. Sievemeshesof 500 and 700 m were chosen to represent the most commonproduct obtained in the milling industry: prime or large grits

    (7002000 m) and fine meal (

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    Table 3. Chemical and physical characteristics for all maize samples, ranked according to total grit yield

    HybridSite of

    cultivation

    Moisturecontent(g kg1)

    Proteincontent(g kg1)

    Starchcontent(g kg1)

    Lipidcontent(g kg1)

    Ashcontent(g kg1)

    Amylose/amylopectin

    rate

    Kernellength(mm)

    Kernelwidth(mm)

    Kerneldepth(mm) Sphericity

    HCP CECINA D 103 105 613 56 16 0.27 12.9 8.2 4.5 0.60

    Pioneer 3245 D 102 102 638 48 15 0.28 13.0 8.8 4.7 0.62

    Dekalb DKC 6309 C 104 104 629 58 17 0.34 13.4 9.6 4.5 0.62

    Dekalb DKC 6309 D 98 102 623 49 16 0.25 13.5 9.1 4.5 0.61

    Pioneer 3235 B 109 109 641 47 17 0.32 12.8 8.9 4.3 0.62

    Pioneer 3235 E 104 101 670 40 16 0.32 12.5 8.8 4.4 0.63

    Pioneer 3235 A 105 116 610 49 17 0.33 12.7 9.0 4.3 0.62

    Dekalb DKC 6309 A 101 109 602 55 17 0.34 12.9 9.1 4.4 0.62

    Pioneer X1132R D 92 107 615 52 17 0.29 13.5 8.7 4.6 0.60

    Pioneer X1132R B 109 110 635 52 18 0.32 13.4 8.8 4.5 0.60

    Pioneer 3235 C 105 114 612 57 19 0.31 12.9 8.9 4.5 0.62

    Pioneer 3235 D 99 113 618 47 16 0.35 12.8 9.0 4.6 0.63

    KWS Kuadro D 104 97 638 41 15 0.30 13.3 7.5 4.3 0.57

    Dekalb DKC 6309 B 104 99 635 49 16 0.30 12.9 9.1 4.6 0.63

    KWS Kermess D 92 93 656 31 14 0.28 13.6 8.1 4.3 0.57

    Syngenta NX6413 D 104 97 646 47 16 0.33 12.4 8.8 5.2 0.67

    Pioneer X1132R A 99 102 629 55 17 0.35 13.3 8.6 4.4 0.60

    HCP DORIA D 103 115 608 51 17 0.31 13.2 8.8 4.1 0.59

    Pioneer X1132R C 104 106 644 53 18 0.35 14.0 8.5 4.5 0.58

    Pioneer X1132R E 98 113 642 55 19 0.35 13.3 8.6 4.6 0.61

    Dekalb DKC 6309 E 103 110 639 46 17 0.34 13.5 9.0 4.4 0.60

    Pioneer PR34G44 D 95 97 640 45 16 0.26 13.8 9.1 4.7 0.61

    Syngenta NX7034 D 99 97 639 48 16 0.32 13.1 9.3 5.1 0.65

    Dekalb Tevere E 108 88 664 47 16 0.28 13.4 8.6 4.3 0.59

    Dekalb Tevere B 104 92 649 49 16 0.27 13.2 9.1 4.2 0.60

    Syngenta NX7234 E 102 83 672 43 15 0.28 13.6 8.6 4.1 0.57

    Dekalb Tevere D 96 91 639 55 16 0.24 13.8 9.0 4.4 0.59

    Dekalb Tevere A 99 94 641 50 16 0.26 13.1 9.0 4.2 0.60

    Syngenta NX7234 B 103 81 693 41 15 0.26 13.7 9.0 4.0 0.57

    Syngenta NX7234 A 108 94 648 46 15 0.29 14.1 8.8 4.1 0.57

    Syngenta NX7234 D 99 86 657 47 14 0.24 14.1 8.9 4.0 0.56

    Dekalb Tevere C 104 94 644 52 17 0.27 13.4 8.9 4.3 0.60

    Syngenta NX7234 C 106 91 672 52 16 0.25 14.1 8.9 4.1 0.57

    Average 102 100 639 49 16 0.30 13.3 8.8 4.4 0.6

    Coefficient of variation (%) 4.3 9.5 3.2 11.7 7.3 11.9 3.4 4.2 6.2 4.2

    small cylinder into the tissue and measuring the evolution ofstressstrain. It is assumed to measure a mix of compression(under the plunger) and shearing.25 Hardness was expressed asthe break force (HF) evaluated in newtons and the break energy(HWF) evaluated in megajoules. HF corresponds to the resistanceof kernels to the penetration of the probe, while HWF measuresthe area beneath the deformation curve between force values 0and HF.26 The data were all acquired at 400 Hz and using TextureExpertExceedsoftware (TextureTechnologies, Scarsdale,NY,USA).Figure 1 shows a typical forcetime deformation curve of textureanalysis, obtained from the kernel puncture test.

    Floating test (FLT)This test was used to assess the density of the maize grain; thenumber of floating kernels (floaters) in a variable density solutionwas recorded. The method that was adopted is a modification ofthat proposed by Wichser.27 100 mL tetrachloroethylene (density1.62 g mL1) and 40 mL petroleum ether (density 0.653 g mL1)were added to an Erlenmeyer flask, and the solution density

    obtained was 1.34 g mL1. A sample of 50 kernels was putinto Erlenmeyer flasks; 5 mL petroleum ether was graduallyadded to the solution and the density of the solution wasdecreased until there were no kernels left floating. The numberof kernels floating at each addition of petroleum ether to thesolution was recorded and a precipitation curve was obtained.FLT measures the area beneath the precipitation curve and thisparameter is adversely correlated to the density of the kernels.These parameters were determined three times for each maizesample.

    Kernel dimensions and sphericity (S)Thespatial dimensionsof 50kernelsof eachhybridwerecalculatedby measuring the average length (L), width (W) and depth (D) ofthe whole kernels using a 0.1 mm precise gauge. These data wereused to calculate S by means of the following formula:22

    S =(

    Volume of solid

    Volume of circumscribed sphere

    )1/3=(LWD

    L

    )1/3

    J Sci Food Agric 2010; 90: 18701878 c 2010 Society of Chemical Industry www.interscience.wiley.com/jsfa

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    Table 4. List of abbreviationsof parameters analysedonmaize kernel

    Abbreviation Parameter

    TGY Total grit yield

    TW Test weight

    TKW Thousand-kernel weight

    H/S Hard/soft endosperm ratio

    MT Milling time

    TME Total milling energy

    C/F Coarse/fine ratio

    HF Break force

    HWF Break energy

    FLT Floating test

    L Kernel length

    W Kernel width

    D Kernel depth

    S Sphericity

    PC Protein content

    SC Starch content

    OC Oil content

    AC Ash content

    AS/AP Amylose/amylopectin rate

    MT, TME: parameters that refer to Stenvert hardness test.HF, HWF: parameters that refer to puncture test.

    Figure 1. A typical forcetime deformation curve, obtained from thetexture analysis puncture test of maize kernels.

    The sphericity values range from 0 (no three-dimensional object)to 1 (perfect sphere). The closer the sphericity is to unity, themorespherical the kernel; conversely, the lower the sphericity, theflatterthe kernel.

    Kernel compositionA grab sample of approximately 300 g of maize was groundto a fine flour using a Foss Tecator Cyclotec 1093 samplemill fitted with a 1 mm screen. The protein (PC), starch (SC),oil (OC) and ash (AC) contents were estimated by near-infrared reflectance spectroscopy, using a NIRsystems 6500monochromator instrument (Foss-NIRsystems, Silver Spring, MD,

    USA) that was calibrated for wet chemical methods. The protein,starch, oil andashcontentswereadjusted to15%moisture contentusing the NIR-predicted moisture content of the ground grain.

    Theamylose : amylopectin ratio (AS/AP)of themaizekernelswasestimated using a Megazyme commercial assay kit (MegazymeInternational Ireland Ltd, Wicklow, Ireland), based on the con-canavalin A precipitation procedure.28

    Micromilling procedureAccording to Yuan and Flores,29 a micromilling procedure wasused to process the maize grain sample and provide an indexof the efficiency of the quality tests for dry-milling processing.Twenty intact, whole kernels were soaked in distilled water for1 h at room temperature (25 1 C) and the bran and germwere removedmanually with a scalpel. The procedure was alwaysperformedbythesametrainedresearcher toensureastandardizeddetermination and avoid subjective determination. The obtainedendosperms were conditioned in an oven at 40 C for 48 h, andwere then ground and sieved using the same procedure as theparticle size index test. The total grit yield (TGY) correspondedto the percentage of the fraction from 2.000 to 700 m, whichwas chosen to represent the main products obtained in theconventional dry-milling industry.6

    Thetotalgrityieldwasexpressedasapercentageof thetotaldry-milled fractions (g kg1). Considering that this procedure achieveda good separation of the bran, germ and endosperm,29 andthat the grounding operations were conducted under standardconditions for all the compared maize samples, micromillingcan be considered to provide a good index of dry-millingperformance.

    Statistical analysesWhen present, replicates were averaged. The coefficient of vari-ation (CV) was calculated for each parameter. Simple correlationcoefficients were then obtained for all the quality factors rela-tive to one another (SPSS, Version 16.0, SPSS Inc., Chicago, IL,USA).

    RESULTS ANDDISCUSSIONIn Tables 2 and 3 the data for each analysed parameter are shownfor each maize sample, ranked for grit yield. Table 5 reportsthe correlation coefficients and their significance between theparameters of maize kernels analysed.

    Although the compared maize hybrids are ordinary com-mercial hybrids that are normally cultivated in northern Italy,their composition varied in starch (602693 g kg1), protein(81116 g kg1), lipid (3158 g kg1) and ash (1419 g kg1)content and the relative amounts of amylose and amylopectin(0.240.35). As expected, the starch (SC) and protein (PC) contentswere negatively correlated with each other (r = 0.79), whilePC was positive correlated with lipid (0.43) and ash (0.71). Theamylose : amylopectin ratio (AS/AP) was positively correlated withPC (0.68) andAC (0.56), while a negative relationshipwas observedfor SC (0.49).

    TGY was between 404 and 600 g kg1. Although the evaluatedhybrids are normally used in food processes, they showed remark-able differences in their aptitude to dry-milling transformation.Among the compared hybrids, HCP Cecina, Pioneer 3245, DekalbDKC 6309, Pioneer X1132R and Pioneer 3235 showed the highestgrit yield.

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    Table

    5.

    Correlationmatrixbetwee

    nthephysical

    andch

    emical

    param

    eterstested

    onthemaize

    kernelsa

    Parameter

    TGY

    TWTK

    WH/S

    MT

    TME

    C/F

    HF

    HWF

    FLT

    LW

    DS

    PCSC

    OC

    AC

    TW0.84

    TKW

    0.19

    0.19

    H/S

    0.75

    0.80

    0.15

    MT

    0.74

    0.79

    0.11

    0.68

    TME

    0.66

    0.72

    0

    .35

    0.56

    0.72

    C/F

    0.92

    0.91

    0.20

    0.78

    0.79

    0.72

    HF

    0.40

    0.57

    0.03

    0.37

    0.59

    0.44

    0.43

    HWF

    0.30

    0.44

    0.01

    0.27

    0.50

    0.35

    0.32

    0.98

    FLT

    0.85

    0

    .89

    0

    .08

    0.78

    0

    .77

    0

    .70

    0

    .93

    0

    .45

    0

    .33

    L0

    .60

    0

    .60

    0

    .06

    0.61

    0

    .44

    0.36

    0.55

    0

    .55

    0

    .53

    0.48

    W0

    .13

    0.01

    0.61

    0.07

    0.05

    0.02

    0.30

    0.04

    0.07

    0.13

    0.05

    D0.46

    0.46

    0.57

    0.35

    0.47

    0.30

    0.43

    0.57

    0.60

    0

    .35

    0.48

    0.16

    S0.51

    0.56

    0.52

    0.54

    0.45

    0.34

    0.50

    0.57

    0.56

    0

    .39

    0.80

    0.45

    0.82

    PC0.70

    0.69

    0.17

    0.68

    0.73

    0.71

    0.76

    0.24

    0.14

    0.72

    0

    .46

    0.05

    0.35

    0.44

    SC0

    .58

    0

    .63

    0

    .27

    0.56

    0

    .73

    0

    .78

    0

    .67

    0

    .43

    0.34

    0.63

    0.39

    0

    .08

    0.34

    0.40

    0.79

    OC

    0.12

    0.09

    0.40

    0.12

    0.22

    0.34

    0.18

    0.08

    0.09

    0.07

    0.07

    0.35

    0.20

    0.25

    0.43

    0

    .55

    AC

    0.34

    0.22

    0.38

    0.29

    0.27

    0.31

    0.37

    0.23

    0.28

    0.27

    0.24

    0.27

    0.24

    0.34

    0.71

    0

    .49

    0.71

    AS/AP

    0.51

    0.42

    0.17

    0.52

    0.47

    0.31

    0.50

    0.05

    0.03

    0.48

    0

    .48

    0.06

    0.40

    0.49

    0.68

    0

    .37

    0.22

    0.56

    aAbbreviations:se

    eTa

    ble

    4.Th

    edatareported

    inthetable

    arePe

    arso

    nproductmomen

    tcorrelationco

    effic

    ients.

    Correlationsignifica

    ntat

    P

    0.05

    ; c

    orrelationsignifica

    nta

    tP

    0.01

    .

    J Sci Food Agric 2010; 90: 18701878 c 2010 Society of Chemical Industry www.interscience.wiley.com/jsfa

  • 1876

    www.soci.org M Blandino et al.

    Considering the compared tests and parameters recorded,TGY was shown to be significantly correlated with C/F (0.92),FLT (0.85), TW (0.84), H/S (0.74), MT (0.74), PC (0.70), TME (0.66),L (0.60), SC (0.58), S (0.51), AL/AP (0.51), D (0.46) and HF (0.40),as reported in Table 5.

    Particle size indexThe correlation matrix shows that the percentage repartition intocoarse (C) and fine (F) material after standard milling (C/F) is thebest descriptor of maize milling ability. The data obtained fromour study showed that the C/F ratiowas closely correlatedwith theH/S ratio (0.78). Although the H/S ratio ranged from 0.2 to 4.8 andexhibitedaCVof67,while theC/F ratio ranged from0.5 to1.4withaCVof26, the latter isaconsiderably less time-consuming laboratorytest compared to quantification of the hard and soft fractions inthe endosperm. Moreover, considering that the differences inthe germ and pericarp content among commercial maize hybridsare generally low, the C/F ratio is also an obvious descriptor ofmaize milling performance.30,31 This test is a good estimator ofgrain hardness, since it can measure objectively and accuratelyand give an indirect but clear evaluation of the hard (H) and soft(S) fractions. In fact, coarse material is mostly obtained by millingthe hard endosperm fraction.32

    Floating testThe FLTwas significantly related to the physical kernel characteris-tics obtained from the other tests (C/F, TW, H/S, MT, TME, HF), thusconfirming the data reported by Pomeranz et al.24 Furthermore,this testwas less affectedby the shapeof thekernels than theothertests, e.g. TW and H/S, which showed a higher correlation withkernel sphericity. FLT was highly and negatively correlated withPC and AS/AP, while a direct relationship was observed with SC.

    The observed CV of the floating test was 13. Considering thecapacity of the FLT to discriminate maize samples according tohardness, quantification of the precipitation curve in a variable-density solution, applied in our experiment, seems to be moreefficient than the simple percentage of floaters observed in astandard solution.27,33 This study confirms that kernel density,measured by means of the FLT, is a good descriptor of kernelhardness and of maize milling performance.10,33

    Test weightOf all the compared tests, TW showed the third strongest corre-lation with TGY (0.84). It also demonstrated a close relationshipwith all the other tests used to estimated grain hardness (C/F, FLT,H/S, MT, TME, HF, HWF). On the other hand, no correlation wasfound between these parameters and TKW. This result confirmsthat kernel hardness does not depend on kernel weight alone butalso on its shape. TW is correlated negatively with kernel length(0.60) and positively with kernel depth (0.46) and increases withthe sphericity of grain (0.56).

    The collecteddata confirm that TW is a simple estimator of grainhardness12 and, since it is widely used, it is the first parameter thatneeds to be considered to evaluate the dry-milling performanceof maize hybrids. On the other hand, TKWwas not able to provideeffective information on grain hardness, thus confirming datareported by Pomeranz et al.24 and Mestres et al.10

    H/S ratioCompared to C/F, FLT and TW, the ratio of the cross-sectionalarea of hard and soft endosperm (H/S) has demonstrated a lower

    correlation coefficient with TGY (0.74). This parameter insteadshowed the highest CV of all the methods (66.5), thus confirmingthat H/S ratio is an efficient kernel hardness test, which is also ableto discriminate the kernels from ordinary commercial hybrids.29

    The correlation matrix shows a close correlation between H/Sandother physical kernel characteristics, such as testweight (0.80),FLT (0.78) and MT (0.68). Considering the chemical compositionof the kernel, H/Swas shown to be significantly related to PC (0.68)andAS/AP (0.52). This parameterwas also directly related to kernelsphericity (0.54): the thickness of the hard endosperm region ishigher in rounder kernels than in flat ones. The H/S ratio increaseswith an increase in kernel depth and a reduction in kernel length,confirming data reported byMestres et al.10 and Pomeranz et al.22

    The digital image processing software used in this experimentfor the estimation of H/S ratio allows simpler and more accuratemeasurements of the hard and soft areas of the cross-section to beobtainedthanwith theothermethods, suchasquantificationusingcamera lucida drawings, or enlarged photographs,34,35 which aresubjective and require sophisticated equipment and particularexpertise. However, this method has been confirmed to be timeconsuming and not practical for very large numbers of samples,although it is the most direct measurement of the proportionof hard endosperm available in maize kernels, since the internalendosperm tissue is viewed directly.

    Stenvert testAs reported by Li et al.,12 the parameters obtained using theStenvert hardness test milling time (MT) and total milling energy(TME) were highly correlated with H/S. Of these two parameters,MT appears to be a better descriptor of maize grain hardnessthan TME, since the correlation with H/S is closer (0.68 and 0.56for MT and TME, respectively) and the CV is moderately higher(12.8 and 11.1 respectively). In the sameway, MT showed a highercorrelation with TGY than TME. However, TME could providemoreobjective information, since the data are not recorded by anoperator, but through a computerized data-logging systemwhichlogs the instantaneous electric power consumption during themilling test.

    Kernel compositionIn this experiment, PC was significantly correlated with severalparameters obtained from tests used to estimate grain hardness,thus confirming the data reported by Dorsey-Redding et al.36 andLee et al.,15 but was also correlated with the shape of the kernel.Round kernels were higher in protein content than flat kernels,as reported by Pomeranz et al.22 Moreover, kernels with a highprotein content have been confirmed to have a higher graindensity (FLT) and produce more total grits.29

    The dry-milling yield and maize kernel hardness-associatedproperties measured in this study were negatively related to SCbut positively associated with AS/AP. These data are consistentwith previous studies.3,37

    Dombrink-Kurtzam and Knutson38 suggest that increasedamounts of amylose may result in increased compressibility ofthe starch granules in the hard endosperm, which leads to acompactedstateandapolygonal shape,whileagreaterproportionof amylopectin, potentially more crystalline, could lead to a lesscompressible and softer endosperm.

    No significant correlations were observed between TGY andOC or AS. Significant correlations between the dry-millingperformance of maize hybrids and ash or lipid content havebeen reported by Mestres et al.10 and Dorsey-Redding et al.36

    www.interscience.wiley.com/jsfa c 2010 Society of Chemical Industry J Sci Food Agric 2010; 90: 18701878

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    Hardness methods for testing maize kernel www.soci.org

    Sphericity (S)Although all the tests in our study have only been performed onthe typical flat-shaped whole kernels of the middle part of themaize ear to reduce variability among maize samples, a positivecorrelation between S and TGY was observed. Mestres et al.10

    reported that the dry-milling yield of maize hybrids could also bepredicted on the basis of sphericity, although this parameter wasnot significantly correlated with kernel vitreousness. Moreover, inthis dataset a significant correlation coefficient was establishedbetween S and the various hardness parameters. These dataconfirm that kernels with higher sphericity are generally higherin flint-like characteristics than flatter kernels22, although studiesexist that report a lower hardness ratio for round kernels than forflat ones.39

    Texture analysisAs far as the puncture test is concerned, only break force (HF) wascorrelated, though weakly, to TGY (0.40), while the relationshipbetween TGY and break energy (HWF) was not significant. Boththese parameters HF and HWF were correlated with TW, MTand TME, and HF showed a higher correlation coefficient thanHWF for all the relationships with these parameters. Only HF wassignificantly correlated with H/S (0.37), C/F (0.43) and FLT (0.45).

    In our experiment, HF andHWFwerenot significantly correlatedwith PC or AS/AP, while a negative correlation was observedbetween HF and SC. Moreover, both HF and HWF were highlycorrelatedwith S (0.57 and0.56, respectively) and inparticularwithD (0.57 and 0.60 respectively), while they were not significantlyrelated to PC. These observations suggest that HF and HWFare probably affected more by the kernel shape than by theirendosperm composition: kernels with a higher sphericity, as aconsequence of a greater depth, present a higher physical andstructural resistance to penetration to the probe than do flatterkernels. Themoderate correlation observed between HF and HWFwith other kernel hardness parameters, such as TW, MT, TME, H/S,C/F and FLT, and betweenHF andGY, is probably an indirect effectof the relationship that exists between these parameters and thesphericity of kernels. In the study conducted by Gaytan Martnezet al.,20 the resistance obtained from a puncture test throughtexture analysis showed a better correlation with the floating test(0.74), compared to our experiment. In the same study, since apositive correlation (0.59) was also observed between the grainresistance to a puncture test and starch granule size, the authorssuggested an important effect of the structural arrangement ofthe starch granules within the endosperm structure: soft maizestarch granules are predominantly spherical and loosely packedwithin aproteinmatrix,whilehardmaizehasmostlypolygonal anddensely packed starch granules. A similar result has been obtainedon wheat by Greffeuille et al.,17 who suggested that greaterstarchprotein matrix adhesion in hard grains compared to softones couldexplain thegreater energynecessary tobreak thegrain.

    In their study, Shandera and Jackson19 compared differentsolvents and heat treatments on kernels accurately selected,according to dimension, from two maize hybrids. The authorsunderlined that kernel shape, surface area and thickness couldhave an important effect on textural testing.

    Since the results of the puncture test on single kernels haveshown a very weak correlation with TGY, this test appears tobe an inadequate predictor of kernel hardness, when this kernelcharacteristic is used to distinguish maize genotypes accordingto their dry-milling behaviour and performance. On the other

    hand, if hardness is only considered for the mechanical resistanceof the whole grain to applied deformation,40 this test couldprovide informationon thepower required tobreakkernelsduringprocessing although, regarding dry-milling, correlation with theenergy required to mill grain (TME) was moderate.

    This method could be used to clearly show the difference inmaize endosperm texture when applied to a product which isable to resist compression forces without completely breaking.For example, the application of this method tomaize grits or mealextrudates has shown a clear relationship between the obtainedforcedeformation curve and the porosity and composition of theproduct tested.41,42

    CONCLUSIONThis research confirms that the dry-milling behaviour of maizeis clearly influenced by the various physical and chemicalcharacteristics of the kernel, which combine to determine itshardness. First, the results have indicated the significance of testweight as an index of maize hardness. The test weight has beenconfirmed to be a simple estimator of grain hardness and is thefirst parameter to consider in evaluating the grain hardness ofa maize hybrid. Among the hardness tests, C/F has been shownto be the best descriptor of maize milling ability, followed byFLT. Since both these indicators also show a close relationshipwith the proportion of hard endosperm in the kernel and a goodcapacity to discriminate among commercial maize hybrids, thesetests could be the most interesting to characterize maize lots fortheir dry-milling performance. A good correlation with total grityield after dry milling has also been observed for H/S, MT, TMEand PC, while SC, S and AS/AP seem to play aminor role. Puncturetest did not provide good indications concerning the impact ofhardness on kernel grinding properties.

    Atpresentnoquality criteria areuniversally recognizedbymaizekernel end users. In fact the hybrids in this study, which werechosen from the varieties normally cultivated and processed indry-millingprocesses, exhibited remarkabledifferences in totalgrityield. A forthcoming objective will be to adapt commercial maizevarieties according to their end uses and to breed new hybrids,not only as far as their agronomic performance is concerned,but also for their technological properties. In order to obtaina widely accepted hardness evaluation and milling behaviourstandard for maize kernels, it will be necessary to establish alimited list of simple, rapid and reliable tests, which could improvethe explanation of maize hardness measurements in relation toend-use value, using a multivariate approach that simultaneouslytakes into account the hardness associated with both physicaland chemical properties. The data of this study offer preliminaryinformation that could help provide the main grain traits thatdetermine end-use processing performance and reduce the risk ofmisclassification of maize hardness.

    ACKNOWLEDGEMENTSThe funds for this research were provided thanks to grants fromthe Provincia di Torino, Servizio Agricoltura.

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