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

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<ul><li><p>1870</p><p>Research ArticleReceived: 17 March 2010 Revised: 29 April 2010 Accepted: 30 April 2010 Published online in Wiley Interscience: 3 June 2010</p><p>(www.interscience.wiley.com) DOI 10.1002/jsfa.4027</p><p>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</p><p>Abstract</p><p>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.</p><p>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).</p><p>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.</p><p>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</p><p>Keywords:maize kernel; dry-milling; hardness; endosperm; texture analysis</p><p>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</p><p>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</p><p>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</p><p>(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.</p><p>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</p><p> 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</p><p>a DepartmentofAgriculture, ForestryandLandManagement,UniversityofTurin,10095 Grugliasco (TO), Italy</p><p>b Dipartimento di Valorizzazione e Protezione delle Risorse Agroforestali FoodTechnology sector, University of Turin, 10095 Grugliasco (TO), Italy</p><p>J Sci Food Agric 2010; 90: 18701878 www.soci.org c 2010 Society of Chemical Industry</p></li><li><p>1871</p><p>Hardness methods for testing maize kernel www.soci.org</p><p>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</p><p>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.</p><p>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.</p><p>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</p><p>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.</p><p>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.</p><p>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.</p><p>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.</p><p>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.</p><p>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</p><p>Table 1. Geographic and main agronomic information about the experimental fields</p><p>Site LocationGeographiccoordinates Soila Altitude (m)</p><p>Sowingdate Harvest date</p><p>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</p><p>a USDA soil classification</p><p>J Sci Food Agric 2010; 90: 18701878 c 2010 Society of Chemical Industry www.interscience.wiley.com/jsfa</p></li><li><p>1872</p><p>www.soci.org M Blandino et al.</p><p>Table 2. Total grit yield and results of hardness tests for all maize samples, ranked according to total grit yield</p><p>HybridSite of</p><p>cultivation</p><p>Total grityield</p><p>(g kg1)</p><p>Testweight</p><p>(kg hL1)</p><p>Thousandkernel</p><p>weight (g)</p><p>Hard/softendosperm</p><p>ratio</p><p>Millingtimea</p><p>(s)</p><p>Totalenergya</p><p>(J)C/F</p><p>ratioa</p><p>Breakforceb</p><p>(N)</p><p>Breakenergyb</p><p>(mJ)Floating</p><p>test</p><p>HCP CECINA D 600 81.1 350 3.5 9.8 1366 1.4 253 119 2123</p><p>Pioneer 3245 D 591 82.4 404 4.2 9.5 1379 1.3 266 119 2190</p><p>Dekalb DKC 6309 C 583 78.6 430 2.0 9.0 1344 1.2 193 75 2530</p><p>Dekalb DKC 6309 D 580 79.2 406 1.9 8.3 1352 1.2 236 104 2395</p><p>Pioneer 3235 B 578 80.6 380 4.0 8.5 1130 1.4 219 88 2058</p><p>Pioneer 3235 E 560 79.0 360 3.5 8.2 1151 1.1 222 96 2475</p><p>Pioneer 3235 A 559 79.6 370 4.8 8.8 1235 1.1 225 94 2323</p><p>Dekalb DKC 6309 A 557 80.1 435 3.5 9.0 1343 1.2 220 93 2428</p><p>Pioneer X1132R D 555 80.5 403 1.9 8.7 1353 1.3 242 101 2158</p><p>Pioneer X1132R B 552 79.1 395 1.0 9.1 1117 1.3 214 87 2368</p><p>Pioneer 3235 C 550 78.6 405 3.1 8.1 1293 1.3 203 87 2323</p><p>Pioneer 3235 D 550 80.8 372 4.4 9.1 1388 1.3 278 135 2088</p><p>KWS Kuadro D 548 77.8 325 0.9 9.1 1198 0.9 259 124 2415</p><p>Dekalb DKC 6309 B 542 79.8 410 3.7 8.9 1074 1.2 212 87 2343</p><p>KWS Kermess D 535 77.2 353 1.6 8.1 1149 1.1 215 91 2425</p><p>Syngenta NX6413 D 534 80.2 391 2.3 8.5 1216 1.1 300 155 2528</p><p>Pioneer X1132R A 532 78.6 390 3.0 8.5 1226 1.2 198 77 2175</p><p>HCP DORIA D 532 80.0 313 3.2 10.2 1520 1.3 233 98 2110</p><p>Pioneer X1132R C 526 75.3 395 3.0 8.8 1151 1.0 162 60 2555</p><p>Pioneer X1132R E 526 76.7 385 1.3 7.8 1270 1.0 148 56 2605</p><p>Dekalb DKC 6309 E 522 78.8 380 2.1 9.1 1211 1.1 198 80 2600</p><p>Pioneer PR34G44 D 501 79.4 444 2.5 8.4 1259 1.0 211 86 2425</p><p>Syngenta NX7034 D 493 76.1 421 1.4 8.9 1065 1.0 277 145 2653</p><p>Dekalb Tevere E 484 73.3 370 1.2 6.7 1037 0.8 180 78 3225</p><p>Dekalb Tevere B 478 75.2 385 0.8 7.9 1129 0.8 190 79 2890</p><p>Syngenta NX7234 E 475 74.1 359 0.4 5.5 989 0.8 145 52 2775</p><p>Dekalb Tevere D 448 73.1 388 0.4 7.8 1185 0.7 232 109 2835</p><p>Dekalb Tevere A 444 75.6 385 0.3 7.2 1193 0.8 229 103 2935</p><p>Syngenta NX7234 B 441 75.6 355 0.8 6.7 1068 0.8 184 75 2765</p><p>Syngenta NX7234 A 416 75.1 380 0.6 6.9 1117 0.7 177 70 2775</p><p>Syngenta NX7234 D 410 75.4 358 0.9 7.6 1029 0.7 208 91 3040</p><p>Dekalb Tevere C 408 74.3 375 0.3 7.1 1084 0.6 199 81 3195</p><p>Syngenta NX7234 C 404 73.5 365 0.2 6.2 939 0.5 178 71 3230</p><p>Average 516 77.7 383 2.1 8.2 1199 1.0 215 93 2544</p><p>Coefficient of variation (%) 11.1 3.3 7.5 66.5 12.8 11.1 23.6 16.9 25.6 13.2</p><p>a Parameters that refer to the Stenvert hardness test.b Parameters that refer to the puncture test.</p><p>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.</p><p>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</p><p>(7002000 m) and fine meal (</p></li><li><p>1873</p><p>Hardness methods for testing maize kernel www.soci.org</p><p>Table 3. Chemical and physical characteristics for all maize samples, ranked according to total grit yield</p><p>HybridSite of</p><p>cultivation</p><p>Moisturecontent(g kg1)</p><p>Proteincontent(g kg1)</p><p>Starchcontent(g kg1)</p><p>Lipidcontent(g kg1)</p><p>Ashcontent(g kg1)</p><p>Amylose/amylopectin</p><p>rate</p><p>Kernellength(mm)</p><p>Kernelwidth(mm)</p><p>Kerneldepth(mm) Sphericity</p><p>HCP CECINA D 103 105 613 56 16 0.27 12.9 8.2 4.5 0.60</p><p>Pioneer 3245 D 102 102 638 48 15 0.28 13.0 8.8 4.7 0.62</p><p>Dekalb DKC 6309 C 104 104 629 58 17 0.34 13.4 9.6 4.5 0.62</p><p>Dekalb DKC 6309 D 98 102 623 49 16 0.25 13.5 9.1 4.5 0.61</p><p>Pioneer 3235 B 109 109 641 47 17 0.32...</p></li></ul>