6844

Physical, Chemical, and Dry-Milling Properties of Corn

of Varying Density and Breakage Susceptibility1

A. J. PEPLINSKI,2 M. R. PAULSEN,3 and A. BOUZAHER4

ABSTRACT Cereal Chern. 69(4):397-400

Seven lots of yellow dent corn varying in kernel density and breakage

susceptibility were tested for chemical composition, physical properties,

and dry-milling response. Whole-kernel nitrogen content varied among

lots from 1.35 to 1.48%, fat content ranged from 4.3 to 5.6%, starch

from 70 to 78%, and ash from 1.3 to 1.6%. Physical analyses showed

test weights of 732-762 kg! mJ (56.9-59.21b!bu), IDO-kernel weights from

29.0 to 32.2 g, 11-64% floating kernels, and 14-33% broken kernels.

Kernel breakage susceptibility correlated statistically (P:S 0.05) with test

weight, percentage of floating kernels, and percentage of stress-cracked

kernels. Percentage of floating kernels was a more precise indicator of

The importance of corn as a major food, feed, and export

commodity is demonstrated by the average U.S. production of

178 million metric tons (seven billion bushels) per year from 1977

to 1986 (USDA 1987). Corn varieties grown in the United States

vary in kernel chemical composition and in physical characteristics, such as density and hardness (Peplinski et al 1989). Corn

kernel characteristics can also be affected by sheller damage if

grain is harvested at high moisture content, by postharvest

handling, and by drying air temperatures (Peplinski et al 1975,

1982, 1989). Differences in kernel characteristics caused by genetic

inheritance, environment, or handling can influence the processing

and utilization of corn.

Corn breakage susceptibility and hardness have been studied

for some years with varying success. Paulsen et al (1983) investigated breakage susceptibility of midwestern corn genotypes;

Pomeranz et al (1984) looked at corn hardness as measured by

density and near-infrared reflectance; Stroshine et al (1986)

measured density and breakage susceptibility of selected hybrids;

and Kirleis and Stroshine (1990) checked the effect of hardness

on corn breakage susceptibility and dry-milling response.

The broad objective of this work was to study how kernel

density and breakage susceptibility are related to kernel chemical

composition, physical characteristics, and dry-milling response.

MATERIALS AND METHODS

Corn

Market-grade yellow dent corn was obtained from commercial

elevators in Illinois and was segregated by incoming truckload

before it was unloaded. Corn on each truckload was tested at

the elevator for test weight and breakage susceptibility and was

separated into four groups: high-test-weight kernels with low

breakage susceptibility; low-test-weight kernels with medium

breakage susceptibility; high-test-weight kernels with high breakage susceptibility; and low-test-weight kernels with high breakage

susceptibility (Paulsen and Hill 1985). Each truckload was

sampled by probing. Samples were checked immediately for test

'The mention of firm names or trade products does not imply that they are endorsed

or recommended by the U.S. Department of Agriculture over other firms or similar

products not mentioned.

'National Center for Agricultural Utilization Research. Agricultural Research

Service. U.S. Departme;t of AgriCUlture. 1815 N. University St., Peoria, IL 61604.

"Department of Agricultural Engineering. University of Illinois. Urbana 61801.

'Agricultural and Rural Development. Iowa State University, Ames 50011.

This article is in the public domain and not copyrightable. It may be freely

reprinted with customary crediting of the source. American Association of

Cereal Chemists, Inc., 1992.

kernel density, breakage susceptibility, and hardness than was test weight.

Dry-milling response showed that first-break grit yield was higher for

corn with test weight above 747 kg! mJ (58.0 Ib! bu), percentage of floating

kernels below 36%, and percentage of broken kernels below 16% as

measured by the Stein breakage test. Hull (pericarp) yield was positively

correlated (P:S 0.05) with breakage susceptibility, stress cracks, floating

kernels, and degermer throughput. The results indicate that corn is less

susceptible to breakage when its density is high, as measured by low

flotation and high test weight, and when the percentage of stress-cracked

kernels is low.

weight and breakage with a Wisconsin-type tester until 2,286-t

(90,000-bu) lots were obtained for each commercial milling run.

After sampling, the corn was transferred to holding bins, conveyed to a commercial dry-milling plant (Lauhoff Grain Co.,

Danville, lL), cleaned on 6.35-mm sieves, and sent to degermers.

Each corn lot was sampled at lO-min intervals before and after

cleaning to provide one 2-kg sample every 30 min. A representative

45.4-kg (lOO-lb) portion of the pooled 2-kg samples from each

lot was retained for testing at the Northern Regional Research

Center, Peoria, IL The corn was further blended and cleaned

by aspiration and screening over a 4.76-mm (l2/64-in.) roundhole-perforated sieve to remove fine materiaL This cleaning

removed I% or less fine material from each lot.

Analytical Procedures and Calculations

Ash, fiber, nitrogen, and starch contents of whole corn were

determined by AACC-approved methods (AACC 1983). The fat

content of whole corn and germ fractions was determined from

pentane-hexane extraction by the Butt procedure (AACC 1983),

and the fat content of the other roller-milled fractions was assayed

by gas chromatography (Black et al 1967) as modified by Nielsen

et al (1979). Moisture contents were determined gravimetrically

after lO-g ground samples were dried at 130° C for 60 min in

a forced-air oven. Yield of recoverable oil was calculated based

on yield of recovered germ fraction and its fat and moisture

contents and assuming that residual germ cake contained 5% oil

(db) (Reitmeier and Prusa 1990). Dry-milling product yields were

calculated with samples air-dried at 25° C and 50% relative

humidity to 9 ± 1.0% moisture. At this humidity and temperature,

the final water content of the dry-milled fractions ranged from

8 to 10%.

All physical tests were done on whole, sound corn kernels

equilibrated to 9 ± 0.5% moisture. Test weight was determined

in accordance with USDA Official Grain Standards (USDA 1977).

Weight per 100 kernels was determined for sound kernels visibly

free from chips, cracks, and other defects. Stress-cracked kernels

(sum of single, multiple, and checked kernels) were counted for

50-g samples of whole corn by the method of Thompson and

Foster (1963) under X3 magnification. Kernel breakage susceptibility was tested with a Stein breakage tester (model CK2) on

100-g portions of whole corn impacted for 2 min; breakage was

determined as the percentage of sample passing through a 4.76-

mm (l2/64-in.) round-hole-perforated sieve. Kernel hardness

index was determined on 150-g samples by the "floaters test"

(Wichser 1961) with a sodium nitrate solution of specific gravity

1.275 (Peplinski et al 1989); the hardest kernels were assigned

a value of 9 and the softest kernels a value of I. Kernel size

was determined by hand-screening of 200-g samples for 2 min

on 0.33-m (l3-in.)-square round-hole-perforated screens. All tests

were replicated two or more times.

Vol. 69, No.4, 1992 397
Statistical Analysis

Samples from corn lots A-G were tested with two or more

replications for every chemical and physical characteristic and

every roller-milling product yield and fat value. The general linear

models (GLM) procedure was used to obtain least significant

difference values, and correlation analyses were run to obtain

Pearson correlation coefficients between all pairs of variables

listed in Tables I-IV. Statistically significant coefficients (P ::;

0.05) are listed in Table VI. Analysis of variance with the GLM

procedure was used to compare values for corn lots with low,

medium, and high breakage susceptibility (Table V).

Degermation

Each corn lot at 9-12% moisture was tempered by addition

of water to 16% moisture, held for 16 hr, further tempered to

TABLE I

Chemical Composition of Whole Corn (db)

21% moisture, and held for 1.75 hr. Finally, to loosen the hulls

(pericarp), kernel moisture was increased to 24% for 15 min. Corn

was then ground in a horizontal rotor degermer (HRD). The

tempering steps allow adequate moisture penetration with minimal

internal stress-crack formation in the endosperm. Corn was fed

to the HRD while it operated at 1,750 rpm, and the net motor

load was kept constant at 0.26 kW. Corn throughputs from the

HRD ranged from 1.54 to 1.93 kg/ min. Details of the tempering

process and of the design and use of the HRD and the drymilling flow were published previously (Brekke et aI1972, Peplinski

et al 1984). The degermer throughput (broken pieces of endosperm, germ, and hull) was dried with 38°C (100° F) air in a

forced-air, flow-through tray dryer to 17 ± I% moisture. The

throughput was further processed by roller milling to low-fat foodgrade products (prime products), feed products, and corn germ

(Peplinski et al 1982, 1984, 1989). All tempering and roller milling

were done at 25° C and 50% relative humidity.

RESULTS AND DISCUSSION

'HTW and LTW indicate high and low test weight, respectively. LB,

MB, and HB indicate low, medium, and high breakage susceptibility,

respectively.

bLeast significant difference at P:::; 0.05.

Corn

Lot

A

B

C

D

E

F

G

LSDb

Type'

HTW-LB

HTW-LB

HTW-LB

LTW-MB

HTW-HB

LTW-HB

LTW-HB

Nitrogen Fat Starch Ash

(%) (%) (%) (%)

1.39 4.9 76 1.3

1.48 4.3 78 1.4

1.40 5.6 75 1.6

1.45 5.0 75 1.4

1.40 4.4 70 1.3

1.40 4.6 76 1.4

1.35 5.0 74 1.3

0.06 0.1 2 0.1

Fiber

(%)

2

2

2

2

2

2

2

0.1

Chemical Composition of Whole Corn

Corn lots differed in composition (Table I). Nitrogen content

ranged from 1.35% for lot G to 1.48% for lot B, a difference

of 10% based on lot G. Fat content varied from 4.3% for lot

B to 5.6% for lot C, a 30% difference based on lot B. Starch

ranged from 70 to 78% and ash from 1.3 to 1.6%. Fiber was

unchanged at 2%. These chemical composition data are similar

to previously reported values (Peplinski et al 1982, 1984, 1989).

Physical Analyses of Whole Corn

Corn test weights ranged from 732 to 762 kg/ m3 (56.9-59.2

Ib/bu) (Table II). Breakage susceptibility of these corn lots

increased sharply as test weight fell to 747 kg/m3 (58 Ib/bu) or

TABLE II

Physical Analysis of Whole Corn'

Test Weight Weight per Stress Stein

Corn 100 Kernels Floaters Hardness Cracks Breakage

Lot Typeb (kg/m3

) (Ib/bu) (g) (%) Index' (%) (%)

A HTW-LB 762 59.2 32.2 16 8 8 14

B HTW-LB 754 58.6 30.9 36 6 24 15

C HTW-LB 754 58.6 31.7 16 8 33 16

D LTW-MB 740 57.5 31.7 II 9 19 22

E HTW-HB 747 58.0 29.9 52 4 62 33

F LTW-HB 733 57.0 29.0 64 3 59 31

G LTW-HB 732 56.9 29.9 62 3 61 33

LSDd 8 0.6 1.5 4 2 3 3

'9 ± 0.5% moisture.

bHTW and LTW indicate high and low test weight, respectively. LB, MB, and HB indicate low, medium, and high breakage susceptibility, respectively.

'9 = hardest. I = softest.

dLeast significant difference at P:::; 0.05.

TABLE III

Degermer Throughput' and Yield of Roller-Milled Productsb

Low-Fat High-Fat

Degermer First-Break Second- and Third- Meal and Prime Meal and Degermer Bran

Corn Throughput Grits Break Grits Flour Products Flour Fines Meal Hull Germ

Lot Type' (kg/min) (%) (%) (%) (%) (%) (%) (%) (%) (%)

A HTW-LB 1.54 10 31 14 55 13 4 5 8 15

B HTW-LB 1.67 12 33 14 59 12 4 6 7 12

C HTW-LB 1.77 9 31 16 56 13 4 5 8 14

D LTW-MB 1.60 7 32 16 55 13 4 5 8 15

E HTW-HB 1.93 6 31 14 51 15 4 6 9 14

F LTW-HB 1.78 6 30 14 50 15 4 6 9 15

G LTW-HB 1.74 7 30 15 52 15 5 6 9 13

LSDd 0.2 1.5 2 1.4 3.3 1.2 0.5 1.2 I.l 1.9

'22.5 ± 1.0% moisture.

b9 ± 1.0% moisture.

'HTW and LTW indicate high and low test weight, respectively. LB, MB, and HB indicate low, medium, and high breakage susceptibility, respectively.

dLeast significant difference at P:::; 0.05.

398 CEREAL CHEMISTRY
less. We therefore used 747 kg/m) (58 Ib/bu) as the test weight

dividing high- and low-test-weight corn in this study. According

to this measure, lots A, B, C, and E fall into the high-test-weight

category. Paulsen and Hill (1985) suggested a dividing test weight

of about 733 kg/m) (57 Ib/bu) in their work on kernel breakage

and density.

Weight per 100 kernels was above 30 g for lots A-D. The

percentage of floating kernels was lowest (::; 36%) for these four

lots. Corn with a low percentage of floaters is denser than corn

with a high percentage of floaters and tends to have more horny,

vitreous endosperm, which is harder and more resistant to breakage than less dense, softer, floury endosperm. The floaters test

rated lots A, C, and D as hard, lots Band E as intermediate,

and lots F and G as soft. Stress-crack values for lots A-D were

33% or less, whereas lots E-G each contained 59-62% stresscracked kernels. Stress-cracked kernels tend to break or chip more

readily during handling than kernels without stress cracks. Kernel

breakage susceptibility, assessed by the Stein breakage test, ranged

from 14-16% for lots A-C to 22% for lot D and 31-33% for

lots E-G.

Degermation, Roller Milling, and Grading of Degermed Corn

Degermer throughputs from the HRD and yields of rollermilled products from the grinding and grading process are shown

in Table III. Throughput, or the output of ground corn from

the HRD at a constant net motor load of 0.26 kW, ranged from

a low of 1.54 kg/ min for lot A (corn with the highest test weight

and lowest stress-crack and breakage percentages) to 1.93 kg/

min for lot E. Throughput is influenced by kernel density, hardness, stress cracks, hull (pericarp) thickness, shape, size, and percentage of horny endosperm.

First-break grit yield, an indicator of endosperm hardness, was

highest for lots A and B (10-12%), intermediate for lot C (9%),

and lowest for lots D-G (6-7%). Prime products were highest

from lots A-D (55-59%) and lower from lots E-G (50-52%).

The prime products are a mixture of low-fat grits, meal, and

flour used by cereal, malted beverage, and other related food

processors. Yields of first-break grits and prime products were

directly correlated with density as measured by the floaters test

and negatively correlated with breakage susceptibility as measured

by the Stein breakage test.

The germ fraction containing the oil varied little among corn

lots; yields ranged from 12 to 15%. The remaining fractions are

generally high in fat content and have less economic value than

the lower-fat fractions; they are generally used in animal feed

products. Their yields were as follows: high-fat meal and flour,

12-15%; degermer fines, 4-5%; bran meal, 5-6%; and hull, 7-9%.

The fat content of roller-milled products did not appear to

be related to corn hardness or breakage (Table IV). Fat content

of the first-break grits, prime products, and germ fractions was

highest in lots B, E, and G and much lower in lots A, D, and

F; lot C had low fat content in the first-break grits and prime

products but high fat content in the germ fraction. Recoverable

oil yield from the germ fraction varied directly with the percentage

of floating kernels and inversely with test weight and ranged from

1.8 to 2.3 kg of oil per 100 kg of corn. These yields are comparable

to those previously reported (Peplinski et al 1989). The calculated

recoverable oil yields are an indicator of the oil that can theoretically be extracted from the germ by commercial oil processors.

Kernel Breakage Susceptibility

The seven corn lots were divided into three groups with low,

medium, and high mean breakage susceptibility for statistical

evaluation. Corns with low and high breakage susceptibility were

significantly different (P ::; 0.05) in all variables listed in Table

V. Test weight, 100-kernel weight, hardness index, and yields

of first-break grits and prime products increased as breakage susceptibility decreased. Corns with low breakage susceptibility had

the highest mean values for test weight (757 kg/m), 58.8 Ib/bu),

IDO-kernel weight (3 1.6 g), first-break grit yield (10.6%), and prime

product yield (57.0%). Corns with high breakage susceptibility

had the highest mean values for percentage of floating kernels

Yield'

TABLE IV

Fat Content of RoHer-Milled Products"

Second- and Low-Fat High-Fat

First-Break Third- Meal and Prime Meal and Degermer Bran Calculated

Corn Grits Break Grits Flour Products Flour Fines Meal Hull Germ Recoverable Oil

Lot Typeb (%) (%) (%) (%) (%) (%) (%) (%) (%) (kg/l00 kg of corn)

A HTW-LB 0.45 0.56 0.54 0.53 1.17 0.71 3.64 1.58 20.9 1.8

B HTW-LB 0.80 0.89 1.07 0.91 2.08 1.30 4.50 2.78 24.6 2.1

C HTW-LB 0.47 0.49 0.40 0.46 0.69 0.69 3.08 2.08 23.1 2.0

D LTW-MB 0.39 0.46 0.36 0.42 0.73 0.37 1.89 1.36 21.0 2.0

E HTW-HB 0.84 0.88 1.39 1.02 0.84 0.77 2.64 3.38 24.8 2.2

F LTW-HB 0.55 0.63 0.62 0.62 0.97 0.88 2.28 2.07 21.4 2.2

G LTW-HB 0.76 0.81 0.94 0.84 1.46 0.94 2.05 3.42 24.5 2.3

LSD' 0.2 0.2 0.3 0.14 0.5 0.15 1.6 1.5 2.5

"Moisture-free basis.

bHTW and LTW indicate high and low test weight, respectively. LB, MB, and HB indicate low, medium, and high breakage susceptibility, respectively.

'Least significant difference at P -:50 0.05.

TABLE V

Characteristics of Corn Lots with Low, Medium, and High Breakage Susceptibility'

Weight

Breakage per 100 Stress Degermer First-Break Prime

Susceptibilityb Test Weight Kernels Floaters Hardness Cracks Throughput Grits Products Hull

(%) (kg/ml

) (Ib/bu) (g) (%) Index' (%) (kg/min)d (%) (%) (%)

14.9 757 a 58.8 31.6 a 22.9 a 7.2 a 21.8 a 1.66 a 10.6 a 57.0 a 7.5 a

22.4 739b 57.4 31.6 a 10.8a 8.5a 18.8a 1.60 a 7.0b 54.3 a 7.8a

32.3 738 b 57.3 29.6 b 59.4 b 3.5 b 60.8 b 1.81 b 6.4 b 51.0 b 9.4 b

a Means in a column followed by different letters are significantly different (P -:50 0.05) as determined by protected (0.05) least significant difference.

Corn lots were tested at 9 ± 0.5% moisture unless otherwise indicated.

bCorn lots A-C (average breakage susceptibility 14.9%) were classified as low in breakage susceptibility; lot D (22.4%) was classified as medium;

and lots E-G (32.3%) were classified as high.

'9 = hardest, I =softest.

d22.5 ± 1.0% moisture.

'9 ± 1.0% moisture.

Vol. 69, No.4, 1992 399
TABLE VI

Correlation Coefficients' of Selected Variables

Weight Prime

Test per 100 Hardness Stress Stein Degermer First-Break Products

Variable Weight Kernels Floaters Index Cracks Breakage Throughput Grit Yield Yield

Weight per

100 kernels 0.73

Floaters -0.65 -0.96**

Hardness index 0.65 0.95** -0.99**

Stress cracks -0.69 -0.90** 0.89** -0.86**

Stein breakage -0.82* -0.85* 0.81* -0.77 0.90**

Degermer

throughput -0.31 -0.69 0.63 -0.59 0.85* 0.67

First-break

grit yield 0.73 0.59 -0.47 0.41 -0.70 -0.88** -0.55

Prime product

yield 0.69 0.69 -0.64 0.58 -0.78 -0.92** -0.56 0.95**

Hull -0.71 -0.88** 0.86** -0.82* 0.95** 0.97** 0.75 -0.83* -0.92**

'* and ** denote statistical significance at P"'5 0.05 and P"'5 0.0 I, respectively.

(59.4%), percentage of stress-cracked kernels (60.8%), degermer

throughput (1.81 kg/min), and hull yield (9.4%).

Correlations Among Variables

Correlation coefficients were calculated for all variables

reported in Tables I-IV, and some statistically significant

coefficients are listed in Table VI. Hull yield and Stein breakage

were each significantly correlated with seven of the other parameters. Hull yield was positively correlated with percentage of

floaters, stress cracks, and breakage and negatively correlated

with 100-kernel weight, hardness, first-break grit yield, and prime

product yield. Stein breakage was positively correlated with percentage of floaters, stress cracks, and hull yield and negatively

correlated with test weight, 100-kernel weight, first-break grit

yield, and prime product yield. Paulsen et al (1983) showed a

correlation between kernel thickness and density, and Kirleis and

Stroshine (1990) noted that kernel hardness was correlated with

breakage susceptibility.

Based on the strength of our data, we believe that the hull

(pericarp), because ofits amount, structure, or adherence to endosperm or germ, has a large but unexplained influence on kernel

hardness, breakage susceptibility, and milling response. The other

variables with statistically significant correlation coefficients (P

::; 0.05) were stress cracks, related to six variables; floaters and

100-kernel weight, five variables each; and hardness, four variables. Kernel chemical composition, fat contents of dry-milled

fractions, and the remaining fraction yields showed little or no

significant statistical correlation with other tested parameters and

are not shown in Table VI.

CONCLUSIONS

Seven corn lots varying in density and kernel breakage susceptibility differed extensively in chemical composition, drymilling response, and physical characteristics. Whole-kernel fat

content varied by up to 30% and nitrogen content by up to 10%.

Roller-milling response studies indicated that grit yield was

significantly higher for corn lots with test weight above 740 kg/ m3

(57.5Ib/ bu), fewer than 36% floating kernels, less than 22% kernel

breakage in the Stein test, and fewer than 34% stress-cracked

kernels. Recoverable germ oil varied directly with percentage of

floating kernels and inversely with test weight.

Corn hardness is related to breakage, density, and dry-milling

response and is very difficult to determine with present test

methods. The percentage of floating kernels correlates with hardness and breakage susceptibility more precisely than does test

weight. Corn kernels resist breaking and cracking when their

density is high, as measured by high test weight and low flotation,

and when the percentage of stress-cracked kernels is low. Hull

amount, structure, or adherence to endosperm or germ appears

to have a large but unexplained influence on kernel hardness,

breakage susceptibility, and milling response.

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[Received March 19, 1990. Revision received October 10, 1991. Accepted October 26, 1991.]

400 CEREAL CHEMISTRY