PH—Postharvest Technology: The Effects of Grain Temperature on Breakage Susceptibility in Maize

Biosystems Engineering (2002) 82 (4), 415–421doi:10.1006/bioe.2002.0094, available online at http://www.idealibrary.com onPH}Postharvest Technology

The Effects of Grain Temperature on Breakage Susceptibility in Maize

Tae Hoon Kim1,2; L. U. Opara2; J. G. Hampton3; A. K. Hardacre4; B. R. MacKay5

1Seed Technology Centre, Massey University, Private Bag 11222, Palmerston North, New Zealand; e-mail of corresponding author:[email protected]

2 Institute of Technology and Engineering, Massey University, Private Bag 11222, Palmerston North, New Zealand; e-mail:[email protected]

3New Zealand Seed Technology Institute, P.O. Box 84, Lincoln University, Canterbury, New Zealand; e-mail: [email protected] & Food Research, Palmerston North Research Centre, Private Bag 11030, Palmerston North, New Zealand; e-mail:

[email protected] Institute of Natural Resources, Massey University, Private Bag 11222, Palmerston North, New Zealand; e-mail: [email protected]

(Received 20 March 2001; accepted in revised form 17 May 2002)

To understand the viscoelastic characteristics (i.e. hardness) of the maize (Zea Mays L.) grain in relation tohigh-temperature drying, a breakage tester (HT-I drop tester) was developed and single-grain breakage atvarious grain temperatures and times after drying was determined. Both hard and soft maize hybrid varietieshad minimal breakage at high grain temperatures (78–1108C), while decreasing grain temperature increasedbreakage exponentially. After drying at both 60 and 1208C, the percentage breakage measured at ambienttemperature increased rapidly during cooling in air at an ambient temperature of 208C and a relative humidityin the range 65–70%. Breakage reached a maximum after about 10min from the start of cooling. AMitscherlich function described the chronological development of percent grain breakage. Analysis of thefunction parameters for the extent (maximum) and rate of breakage indicated that there was a significantinteraction between variety and drying temperature for the development of grain breakage after drying. Theseresults indicated that grain temperature should be considered as a co-factor when assessing grain breakagesusceptibility. # 2002 Silsoe Research Institute. Published by Elsevier Science Ltd. All rights reserved

1. Introduction

Breakage susceptibility is one of the most importantphysical properties determining the usage and value ofmaize. Increases in grain breakage due to conventionalhigh-temperature drying increase broken maize and finematerial during subsequent handling, and lower the enduse quality (Watson, 1987). The breakage susceptibilityof maize grain, however, is moisture dependent (Herum& Blaisdell, 1981; Paulsen, 1983). Thus, it has beenrecommended that the moisture content of the sample(around 12–13%) should be consistent and reportedtogether with the percentage breakage when grain istested (Miller et al., 1981; Dorsey-Redding et al., 1990).On the other hand, grain temperature has been noted asa minor factor that affects grain breakage at the time oftesting. Herum and Blaisdell (1981) found that breakagesusceptibility increased greatly as moisture contentdecreased from 14 to 12% and increased slightly asgrain temperature decreased from 38 to 48C.

1537-5110/02/$35.00 415

Several grain physical factors related to breakagesusceptibility including grain moisture content and graintemperature have been studied (Herum & Blaisdell,1981; Miller et al., 1981; Watson et al., 1993). However,the effects of high grain temperatures (higher than408C) on breakage susceptibility have not yet beenstudied and data for the chronological change ofphysical characteristics of maize grain after high-temperature drying are not available. More data arecrucial for understanding the viscoelastic characteristics(i.e. hardness) of the grain in relation to high-temperature drying and may also provide importantinformation for measurement of grain hardness andbreakage susceptibility.In this study, a new impact drop tester was

developed for determining the breakage susceptibilityof individual maize grains at various grain temper-atures and the chronological development ofbreakage in maize grain. The objectives of the studywere to

# 2002 Silsoe Research Institute Published by

Elsevier Science Ltd. All rights reserved

TAE HOON KIM ET AL.416

(1) determine grain breakage at various grain tempera-tures;

(2) measure grain breakage at various times after dryingand

(3) develop an empirical model for grain breakage atvarious grain temperature and times after drying.

2. Material and methods

2.1. Development of the HT-I drop tester

To measure the breakage of an individual grain athigh grain temperatures, a new breakage tester (ModelHT-I drop tester) was developed (Fig. 1). An aluminiumdrop bar (588mm length; 13mm diameter; 0�2 kg) wasinserted into a steel tube (550mm length; 17�1mm

AB

MB

MG

S

ST

P

(a)

Fig. 1. (a) Diagram of HT-I drop tester; AB, aluminium bar (201S, stand; and dimensions; in mm

internal diameter; 19�5mm external diameter). The steeltube had 4mm diameter holes drilled at 5 cm intervalsfrom 5 to 50 cm.The drop-height of the aluminium barwas manually controlled by a pin inserted in the hole inthe middle of a steel tube. The steel tube was clamped toa laboratory stand. The test grain was placed in themiddle of the cast-iron base germ side down and themetal base was then inserted into the end of the tubefrom the side of the tube. The aluminium bar dropped,hitting the grain when the pin was manually removed atthe given drop height.The impact force on grain depends on the drop height

of the standard 201�0 g aluminium bar. In this experi-ment, the drop height used for the HT-I drop tester wasselected after preliminary experimentation (data notshown). The fixed drop height of the aluminium bar was

(b)

(c)

50

2.5

C C

Section view C−C

2.5

D D

Section view D−D

19

Ø53

19.7 12 10

Ø16

Ø20

�0 g); ST, steel tube; P, pin; MB, metal base; MG, maize grain;of (b) pin and (c) metal base

EFFECTS OF GRAIN TEMPERATURE ON BREAKAGE SUSCEPTIBILITY 417

20 cm, the impact energy Ei was 0�392 J based on thefollowing relationship:

Ei ¼ mgh ð1Þ

where: m is the mass of impactor (0�2 kg), g is theacceleration due to gravity (9�8m s�2) and h is the dropheight (0�2m).In practice, some of the impact (total) energy will be

lost due to rebound, such that the absorbed impactenergy Ea becomes more relevant to the damageobserved than total impact energy:

Ea ¼ mgðh1 � h2Þ ð2Þ

where: h1 is the original drop height and h2 is therebound height. However, it is assumed that h2 is verysmall and negligible (h1dh2) and thus the reboundenergy was ignored.

2.2. Maize varieties and physical characteristics of grain

In 1998, forty cobs (about 5 kg) of the two maizevarieties, Clint and P3902 at about 20–22% grainmoisture content were hand-harvested and hand-shelled.Before drying, grains of each variety were classified bysize and shape using a mechanical seed cleaner (OfficeClipper Tester and Cleaner No. 400/C, Seedburo, IL,USA). Grains were sorted in lots of 200 g for 30–60 s.Broken and very small grains from the shelled maizewere removed through a 6�75mm round-hole sieve.Grains were then divided into six categories based ontheir ability to pass through sieves of different sizes, i.e.small round, medium round, large round, small flat,medium flat and large flat. First, grain size wasdetermined by using two round-hole sieve diameters8�73 and 9�53mm. Small grains passed through 8�73mm,medium size grains were retained by the 8�73mm sievebut passed through 9�53mm, and large grains wereretained by the 9�53mm sieve. Grain roundness was thendetermined by using a 5�95mm slot-hole sieve (slotswere 19�05mm). Flat grains passed through this slot-

Tabl

Comparison of grain moisture content, 100-grain weight, bulk densP3902 an

Variety Moisturecontenty, %

Hundred-grainweight, g

Bulkkg

P3902 20�5 35�4 7Clint 22�2 37�5 7Significance ** *LSD (5%, df=4) 0�3 2�1

yAfter sieving.Note: Hundred-grain weight and bulk density were adjusted

significant or significant F at 50�05, 0�01, respectively, LSD, lea

hole size while round grains were retained. All theclassified grains were then collected in each labelled-plastic bag and stored in a controlled temperature roomat 5�18C and 90–95% relative humidity (r.h.) beforebeing tested.To ensure as far as possible that the breakage test

results were attributable to variety and grain tempera-ture effects only and not biased by grain size/shapedifferences, only medium size flat grains were selectedfor testing. Table 1 presents the physical characteristicsof grains of the two varieties. The AACC air ovenmethod (1038C, 72 h) was used to determine grainmoisture content (AACC, 1983). In this report, allmoisture contents are expressed on a wet basis (w.b.).Hundred-grain weight was determined by measuring theweight of 100 grains for each replicate and then adjustedto 14% grain moisture content (w.b.). Bulk density ofgrains was determined by using the method suggested byHardacre et al. (1997). The ratio of hard to softendosperm (H/S ratio) of grains was determined bythe method of Kirleis et al. (1984). After drying, eachgrain was sectioned just above the top of the embryoregion (about 2/3 of the distance from the tip cap to thecrown). The H/S ratio was determined by using 20grains per replicate:

H=S ¼ðT � SÞ

Sð3Þ

where: the dissected area of the grain was assumed as arectangular for calculating approximate total cut-sur-face area T in mm2, soft endosperm area S in mm2 andhard endosperm area H in mm2, respectively. Length,width, and thickness of 20 grains were measured byhand with a digital calliper (�0�01mm) by the methodof Martin et al. (1987).The harvest grain moisture content (w.b.) for the two

varieties was 20�5% for P3902 and 22�2% for Clint,respectively. Clint had a significantly higher grainweight, bulk density and hardness ratio than P3902(probability, P50�05). The two varieties had different

e 1

ity, hardness ratio, and grain dimension of medium flat grains ofd Clint

density,m�3

Hardnessratio

Length,mm

Width,mm

Thickness,mm

2�7 1�9 12�07 9�10 4�984�8 3�3 12�53 9�00 4�97* ** * * NS

1�6 0�8 0�30 0�10 }

to a grain moisture content of 14% (w.b.); NS, *, or **, non-st significant difference; df, degrees of freedom.

Table 2

Grain temperatures at the time of breakage testing of two maize

varieties dried at various drying air temperatures

Drying airtemperature, 8C

Grain temperature at the time of breakagetesting, 8C

P3902 (soft) Clint (hard) Average

120 109�0 110�8 110100 90�3 97�0 9480 77�5 78�8 7860 58�9 58�9 5940 39�7 40�2 4020 20�0 20�0 20


grain length and width (P50�05), but there was nosignificant difference in grain thickness between the twovarieties (Table 1).

2.3. The breakage test at various grain temperatures

Breakage susceptibility is defined in the US as thepotential for grain fragmentation when subjected toimpact forces during handling or transport (AACC,1983). In this report, the relative percent breakage B isexpressed as

B ¼ðTw � RwÞ � 100

Tw

ð4Þ

where: Tw is the total grain weight after impact and Rw isthe retained grain weight over 4�76mm round hole sieveafter sieving.To determine grain breakage at various grain

temperatures, six drying temperatures (20, 40, 60, 80,100 and 120oC) were selected, for two maize varieties.The test was replicated three times with 20 medium-flatgrains in each replicate. Each replicate was dried at eachof five drying temperatures in a single layer in a verticalflow (0�1m s�1), forced air oven (Clayson, Upper Hutt,New Zealand). A reference moisture measurement grainsample of 20 grains in a single layer in a separate dryingtray was placed at each test. At 30min intervals, thereference sample grains were weighed (�0�001 g) andtheir moisture content was calculated. Drying continueduntil the moisture of the reference grains reachedapproximately to 13% (w.b.).At each test, five grains were positioned germ side

down on an individual metal base plate and placed in anoven. When the reference grains reached the targetmoisture content of 13% (w.b.), each individual grainand base plate was removed from the oven andtested immediately. For completing a replicate, thisprocedure was replicated four times at each dryingtemperature (5� 4=20 grains per replicate). As acontrol, another sample of 20 grains from each varietywere dried at 208C in a controlled temperature room at20�18C and 65% r.h. until the target moisture contentwas reached.The impacted 20 grains of each replicate were stored

in an aluminium bin and weighed before sieving using4�76mm round-hole sieve. The grains that retained over4�76mm round-hole sieve were reweighed and thebreakage was calculated as described in Eqn (4).Using extra grains, grain temperature was measured

using a temperature probe and Squirrel data logger(Grant, 1250 series, Cambridgeshire, UK) at each test. Asmall hole was made from the crown or tip to the centreof the grain with a hand drill and the temperature probe

was inserted. Another two probes measured the ovenand ambient air temperatures. The measured graintemperature at the time of testing for the two varietieswas closer to drying air temperature as the temperaturedecreased (Table 2).

2.4. Breakage at various times after drying

Another set of experiment was carried out todetermine grain breakage times after drying. Threereplicates were tested at 0, 2, 4, 6, 8, 10, 20 and 30minafter drying at 60 and 1208C. Five grains (i.e. medium-flat) were tested at each time and the whole procedurewas repeated four times for each replicate. Within avariety, 480 grains were sampled to complete one set ofbreakage test (5� 8� 4� 3=480) at each drying tem-perature. Another 20 grains were selected for reference,arranged in a single layer in a separate drying tray andplaced in each test. Similarly, at 30min intervals, thereference grains were weighed (�0�001 g) and theirmoisture content (w.b.) was calculated. At each time, aprepared sample of 40 grains per test were arranged on asmall tray in the oven at a given drying temperature ofeither 60 or 1208C until the reference grain moisturereached about the target moisture content (13%, w.b.).At this stage, all the 40 grains were removed from theoven and placed at ambient temperature (20�18C and65% r.h.). At 0, 2, 4, 6, 8, 10, 20 and 30min after drying,a five-grain subsample was selected and tested. This wasrepeated four times for each replicate. The impacted 20grains of each replicate were collected in an aluminiumbin and weighed before sieving using 4�76mm round-hole sieve. The grains that retained over 4�76mm round-hole sieve were reweighed and the breakage wascalculated as described in Eqn (4).During drying and cooling, the temperature of the

maize grain was monitored using an extra grain.

0

10

20

30

40

50

0 20 40 60 80 100 120

Grain temperature,°C

Bre

akag

e, %

Fig. 2. Predicted percentage grain breakage for P3902 ( )and Clint ( ) in Eqn (5) at various grain temperatures: ,measured breakage value of P3902; , measured breakage value

of Clint

Table 3

Comparison of the parameters for breakage susceptibility ofmaize grain of two maize varieties as a function of grain

temperature at the time of testing; Bmax, maximum grain

breakage; b, coefficient

Variety Parameters for exponential model forgrain breakagey

Bmax b

P3902 67�8 0�0398Clint 38�8 0�0328Significance * NSLSD (5%, df=4) 26�2 }


2.5. Data analysis and modelling

A factorial arrangement of maize hybrid variety,drying temperature and post-drying timing of breakagemeasurement, organized in a completely randomizeddesign, was used to evaluate the effect of various graintemperatures and post-drying timing of measurement ongrain breakage. Data were analysed by analysis ofvariance using the general linear models procedure(PROC GLM) of SAS (SAS, 1985). Significant interac-tions between independent variables among maizehybrid variety, drying temperature and post-dryingtiming of breakage measurement were presented byplotting the data means on graphs and calculating leastsignificant difference (LSD) values appropriate fortesting at a significance level of probability P50�05.Data for the percentage breakage B as a function of

grain temperature at the time of testing were resulted inan exponential model [Eqn (5)] and the coefficients ofBmax and b from the fitted models were subsequentlyanalysed by ANOVA:

B ¼ Bmaxe�bT ð5Þ

where B is the breakage in %, Bmax is the maximumgrain breakage in %, b is a coefficient and T is the graintemperature in 8C at the time of testing.Another set of data, the percentage breakage of maize

grain at various times after drying were fitted to aMitscherlich function [Eqn (6)] (Seber & Wild, 1989)and the coefficients of Amax; k and c from the fittedmodels were analysed by ANOVA:

B ¼ Amaxð1� e�kðtþcÞÞ ð6Þ

where Amax is the maximum grain breakage in %, k is acoefficient, t is the time after drying at ambienttemperature in minute and c is the calculated ordinateintercept of the curve.

3. Results and discussion

3.1. Breakage of maize grain at various grain

temperatures

The breakage of maize grain was very sensitive to thegrain temperature at the time of testing (Fig. 2). Bothvarieties were plastic and had minimal breakage at highgrain temperatures of 78–1108C. However, decreasinggrain temperature increased breakage exponentially.The hybrid variety P3902 (soft) had a significantlyhigher percentage of breakage (P50�05) than Clint(hard) when the grain temperature was lower than 408C.The exponential model [Eqn (5)] for grain breakage

according to grain temperature at the time of testingsuccessfully predicted the real value of grain breakage

and comparison of the model parameters clearlyexplained the extent and the rate increases in grainbreakage at various grain temperatures. For example,comparison of the exponential function parameters(Table 3) indicated that the two varieties had similarrate increases in grain breakage as grain temperaturedecreased from about 110 to 208C, even though the twovarieties had different percentage grain breakage asgrain temperature was lower than 408C.The grain temperature has been noted as a minor

factor that affects grain breakage at the time of testing(Herum & Blaisdell, 1981). However, the results of theirexperiment and other published data (Miller et al., 1981)suggest that grain breakage changes with graintemperature, particularly at a range between 20 and408C. Thus, grain temperature also should be considered

40

30

20

10

0

120

90

60

30

0

Time after drying, min Time after drying, min(a)

Bre

akag

e, %

0 10 20 30

Gra

in te

mpe

ratu

re, °

C

Gra

in te

mpe

ratu

re, °

C

(b)

40

30

20

10

00 10 20 30

120

90

60

30

0

Bre

akag

e, %

Fig. 3. The predicted maize grain breakage at drying temperature of (a) 608C and (b) 1208C for: P3902 ( ) and Clint ( )in Eqn (6) during cooling at ambient temperature (20�18C and 65–70% relative humidity). , measured breakage value of P3902;

, measured breakage value of Clint; , grain temperature. The bar in the plot presents the least significant difference of theasymptote Amax 5�6 (probability P50�05, the degree of freedom was eight)


as a correction factor for an accurate measurement ofgrain breakage along with grain moisture content.

3.2. Breakage of maize grain at various times after drying

Percentage breakage of the two maize varietiesincreased rapidly for the first 10min after drying atboth 60 and 1208C, and had reached an asymptotic levelafter around 10min cooling at ambient temperature(20�18C and 65% r.h.) in a single layer (Fig. 3). TheMitscherlich function [Eqn (6)] (Seber & Wild, 1989)was fitted to the chronological development of percen-tage grain breakage after drying at 60 and 1208C and theparameters were compared to evaluate the rate andextent of grain breakage for the two varieties (Table 4).

TablComparison of the Mitscherlich function [Eqn (6)] parameters for

temperature (20�18C and 65–70% r.h.) after drying at 60 and 120

the intercept

Factor Mits

Variety Amax

P3902 30�5Clint 32�1Significance NSLSD (5%, df=8) }

Drying temperature, 8C60 28�4120 34�2Significance **LSD (5%, df=8) 4�0

InteractionsVariety�Drying temperature *

yNS, *, or **, Non-significant or significant F at 50�05, 0�01Note: LSD, least significant difference; df, degrees of freedom

The Mitscherlich model [Eqn (6)] successfully pre-dicted the real value of percentage grain breakage timeafter drying and its parameters provided a more preciseinterpretation for the effect of varieties and dryingtemperatures on the extent and the rate increases ingrain breakage time after drying. Comparison of theMitscherlich model parameters indicated there was asignificant interaction between drying temperature andvariety for the maximum value of Amax. As dryingtemperature increased from 60 to 1208C, the predictedmaximum value Amax of percentage breakage in Clintsignificantly increased (P50�05), indicating that Clinthad more heat sensitive endosperm characteristics thanP3902 (Table 4).Kirleis and Stroshine (1990) suggested that grains

might be plastic shortly after finishing high-temperature

e 4percentage breakage of maize grain during cooling at ambient

8C; Amax, maximum grain breakage in %; k, a coefficient; and c,of the curve

cherlich function parameters for grain breakagey

k c

0�204 0�5300�200 0�608NS NS} }

0�240 1�2690�164 �0�131NS **} 0�728

NS NS

, respectively..


drying and thus less stress cracking in grains after dryingat high temperature (>908C). The result of thisexperiment supports their idea of stress crackingmechanism. Stress cracking started to develop in grainsduring cooling some times after drying instead of duringdrying due to viscoelastic characteristics of grain. Grainsare soft when their temperature exceeds 608C shortlyafter finishing drying, but they become rigid within10min after finishing drying during cooling.The models developed in this study in relation with

the percentage grain breakage and grain temperature atthe time of testing might contribute to better under-standing of breakage in single grains. The HT-I droptester (Fig. 1) developed in this study is useful fordetermining grain susceptibility to stress cracking andbreakage. This grain breakage device has several benefitscompared to conventional breakage testers such as SteinBreakage tester (SBT) and Wisconsin breakage tester(WBT): (1) it can be used to define a more accurate grainbreakage for single grains at a given grain temperatureand moisture content; (2) it is simple and (3) it is aninexpensive tool that can be used by grain growers anddryer operators.

4. Conclusions

The exponential model for predicting grain breakagefor the hard (Clint) and soft (P3902) maize hybridvarieties at six grain temperatures at the time of testingindicated that the two varieties had minimal breakage athigh grain temperatures (from 78 to 1108C), whiledecreasing grain temperature increased breakage expo-nentially. The predicted values also indicated that P3902(soft) had a significantly higher breakage than Clint(hard) as the grain temperature fell below 408C.Grain breakage was also affected by the timing of the

test. Grain breakage increased rapidly after finishingdrying during cooling at ambient temperature. TheMitscherlich model successfully predicted the percentagegrain breakage time after drying and its parametersprovided a more precise interpretation for the effect ofvarieties and drying temperatures on the extent and therate increases in grain breakage time after drying. Asdrying temperature increased from 60 to 1208C, thepredicted maximum value of Mitscherlich model in Clint

significantly increased, indicating that Clint had moreheat sensitive endosperm characteristics than P3902. Asimple inexpensive single-grain breakage tester wasdeveloped, and is applicable to growers, dryer operatorsand researchers.

References

AACC (1983). Method 44-15A, moisture-air-oven methods,October 1981; Method 55-20, corn breakage susceptibility,November, 1981. In: Approved Methods of the AmericanAssociation of Cereal Chemists (AACC), St. Paul, MN

Dorsey-Redding C; Hurburgh C R; Johnson L A; Fox S R(1990). Adjustment of maize quality data for moisturecontent. Cereal Chemistry, 67, 292–295

Hardacre A K; Brenton-Rule R; Clark S M (1997). The analysisof maize grain quality in New Zealand and calibration oftesting methods. Report No. 367, Maize Grain QualityCommittee, New Zealand Manufacturers Association, NZInstitute of Crop & Food Research Ltd., Christchurch, NewZealand

Herum F L; Blaisdell J L (1981). Effects of moisture content,temperature and test variables on results with grain break-age testers. ASAE Paper No. 81-3030, St. Joseph, MI

Kirleis A W; Stroshine R L (1990). Effects of hardness anddrying air temperature on breakage susceptibility and dry-milling characteristics of yellow dent corn. Cereal Chem-istry, 67, 523–528

Kirleis A W; Crosby K D; Housley T L (1984). A method forquantitatively measuring vitreous endosperm area in sec-tioned sorghum grain. Cereal Chemistry, 61, 556–558

Martin C R; Converse H H; Czuchajowska Z; Lai F S;Pomeranz Y (1987). Breakage susceptibility and hardness ofcorn kernels of various sizes and shapes. Applied Engineer-ing in Agriculture, 3, 104–113

Miller B S; Hughes J W; Rousser R; Pomeranz Y (1981).Measuring the breakage susceptibility of shelled corn.Cereal Foods World, 26, 75–80

Paulsen M R (1983). Corn breakage susceptibility as a functionof moisture content. ASAE Paper No. 83–3078, St. Joseph,MI

SAS (1985). SAS User’s Guide: Statistics, Version (5th Edn.).SAS Institute Inc., Cary, NC

Seber G A F; Wild C J (1989). Nonlinear Regression, pp 327–342. Wiley & Sons, New York

Watson S A (1987). Measurement and maintenance of quality.In: Corn; Chemistry and Technology (Watson S A;Ramstad P E, eds), pp 125–183. AACC, St. Paul, MN

Watson S A; Kreider W D; Sciarini M J; Keener H M (1993).Development and evaluation of an automated grain break-age tester for determining corn breakage susceptibility.Cereal Foods World, 38, 570–575

PH—Postharvest Technology: The Effects of Grain Temperature on Breakage Susceptibility in Maize

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