J. Anim Sci. 2007. 85:1695-1701. doi:10.2527/jas.2006-127
© 2007 American Society of Animal Science
Nutritional value for swine of extruded corn and corn fractions obtained after dry milling1,2
N. S. Muley,
E. van Heugten,
A. J. Moeser,
K. D. Rausch3 and
T. A. T. G. van Kempen4
Department of Animal Science and Interdepartmental Nutritional Program, North Carolina State University, Raleigh 27695
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Abstract
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The experiment was designed to assess whether corn fractions or extrusion of corn can result in feed ingredients with a greater nutritional value than corn. Corn grain (8.0% CP, 0.21% P, 9.8% NDF) was processed by extrusion (82.8°C, 345 kPa steam pressure for 12 s) or by dry milling to derive fractions rich in germ (13.1% CP, 1.19% P, 17.2% NDF), hulls (8.1% CP, 0.27% P, 32.6% NDF), and endosperm, namely tails (6.6% CP, 0.07% P, 3.6% NDF) and throughs (7.4% CP, 0.15% P, 4.5% NDF). Relative recovery in each fraction was 16, 20, 44, and 20%, respectively. Ileal digestibility of DM, P, and amino acids was determined using diets containing 7.0% CP from soybean meal and 5.3% CP from one of the test products. To allow for determination of standardized ingredient, ileal digestibility, basal endogenous AA losses were determined using a protein-free diet (74.6% cornstarch and 18.7% sucrose). Soybean meal ileal digestibility was determined using a diet (12.3% CP) based on soybean meal (23.3%). Eight barrows (27 ± 2 kg) fitted with T-cannulas were fed 8 experimental diets (5-d adaptation and 2-d collection period) such that each diet was evaluated in at least 5 barrows. Relative to corn (77.9 ± 1.2%), ileal digestibility of DM was greater for extruded corn (82.5%; P = 0.02), tails (85.9%; P < 0.01), and throughs (85.0%; P < 0.01), but it was lower for hulls (62.2%; P < 0.01) and germ (51.1%; P < 0.01). For P, corn (41.6 ± 9.5%), throughs (47.2%), and hulls (57.3%) had similar ileal digestibility, but germ (7.9%) had lower ileal digestibility (P = 0.02) than corn; tails (27.6%) and extruded corn (23.5%) were not different from corn or germ but were lower than throughs and hulls. For total AA, corn (84.7 ± 2.4%), throughs (84.3%), and hulls (85.8%) had similar ileal digestibility, but germ (76.6%) had lower ileal digestibility (P < 0.01) than corn; tails (82.0%) and extruded corn (81.7%) were intermediate. In conclusion, germ and hulls have a low ileal DM digestibility; germ also has low AA and P digestibility. Extrusion improved the ileal DM digestibility of corn. To maximize the ileal digestibility, removal of germ and hull from corn or extrusion of corn may thus be of interest.
Key Words: amino acid • corn • dry matter • ileal digestibility • phosphorus • swine
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INTRODUCTION
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In the United States, most pigs are fed diets based on corn and soybean meal (Baxter et al., 2003
). Despite a moderate fiber content (e.g., 5 to 10% NDF), 70% of the total dietary fiber in such diets may still be attributed to corn. This fiber plays a determinant role in ileal digestibility (Moeser et al., 2002). Therefore, processing of corn to reduce its fiber content is one way to improve its nutritional value and to lower nutrient excretion.
Dry milling of corn is a mechanical process designed to fractionate corn into germ, hull, and endosperm fractions. Although wet milling is more common for preparation of starches, dry milling is a routine industrial process used for the production of corn grits. An interesting aspect of dry milling is that the chemical and physical structure of the corn fractions is not altered. Thus, the hulls, germ, and endosperm are harvested in their native form (Alexander, 1987; Duensing et al., 2003). The nutritional value of these components for swine diets has not been evaluated.
An alternative approach to process corn grain includes heat processing via extrusion cooking (Hongtrakul et al., 1998). In this process, the chemical and physical nature of corn is altered. Extrusion also may affect palatability and flavor (Mercier, 1980; Bjorck et al., 1985).
The objective of this experiment was to determine the ileal digestibility of corn components as obtained by dry milling or by processing through extrusion, with the ultimate goal to identify processing techniques or corn fractions for improving performance and minimizing nutrient excretion in swine.
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MATERIALS AND METHODS
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All animal protocols were approved by the North Carolina State University Animal Care and Use Committee. Corn used for dry milling and extrusion was from the same source and was obtained from growers in east central Illinois.
Dry Milling Process
Dry milling was used for physical separation of corn grain into germ, hulls, and 2 endosperm fractions, referred to as tails and throughs. For this, corn was tempered from 15 to 22% moisture by the addition of water, which helped to achieve differential swelling of germ, pericarp (hull), and endosperm. Tempered corn was then passed through a degermination mill (model 0, Beall Degerminator Company, Decatur, IL). This mill has an abrasive action that scrapes off the hulls and a grinding action that separates the germ from the endosperm. Material exiting this mill was roller milled, causing the germ pieces to flatten and allowing their removal by a simple sieving process. Hulls were removed by aspiration. Throughs were derived from the material exiting the bottom of the Beall mill, and tails were derived from the material exiting the back of the Beall mill. Tails contain larger and throughs contain smaller pieces of endosperm.
Extrusion Cooking
For extrusion cooking, corn was preconditioned at 81°C using 345 kPa steam pressure. The extrusion itself was carried out in a Model 2500 Insta-Pro extruder (Insta-Pro International, Des Moines, IA). The barrel configuration was a single flight screw with 11R-11R-11R-11R steam locks. The temperature of the last chamber was 140 to 143°C. An extruder exit with 5 die openings was used. Final moisture of the extruded corn was 12%.
Ileal Digestibility
Given the low CP content of some of the fractions, the experimental ingredients were tested together with a high-quality soybean meal (52.8% CP, included at 13.2%) such that the CP content of the diet being evaluated was within the range of commercial diets. Therefore, in each of the test diets, soybean meal provided 7.0% and the test ingredients provided 5.3% CP to the diets. The ileal digestibility of this soybean meal was evaluated in a diet containing only soybean meal as a source of CP, whereas a protein-free diet was included to determine the basal endogenous losses (Tables 1 and 2). All diets contained 5% oil to improve palatability, 0.35% vitamin-mineral premix, and 0.3% chromic oxide as an indigestible marker; the remainder was starch and sucrose.
Animals and Housing
Ten barrows, with an initial BW of 27 ± 2 kg, were fitted with an ileal cannula. Before surgery, barrows were fasted for 24 h. Barrows were surgically fitted, under anesthesia, with simple T-cannulas at the caudal ileum (Moeser et al., 2002). The postoperative period was 14 d, during which the pigs were fed a commercial diet ad libitum. Thereafter, 8 cannulated pigs were used for determination of ileal digestibility in an 8 x 8 Latin square design. Due to problems with the suture material (premature absorption resulting in herniation in some barrows), not all barrows completed the experiment. The actual number of observations per treatment ranged from 5 to 7.
Barrows were placed in concrete, smooth-walled pens (1.5 x 1.0 m). Pigs were fed twice daily, 12 h apart. Feed allowance for each meal was calculated at 45 g/kg of BW0.75. Water was provided ad libitum. Each period consisted of a 5-d adaptation period and a 2-d (12 h/d for 2 d) collection period. During the collection period, ileal digesta were collected from the cannula into an attached plastic container (Nalgene, Rochester, NY). The collected material was transferred hourly to a freezer and stored at –20°C. Digesta samples were pooled by barrow and period.
Chemical Measurements and Calculations
Diets were analyzed in triplicate, and ileal samples were analyzed with single assays. Dry matter content was determined by drying to a constant weight in an oven at 60°C. Gross energy was determined in an adiabatic bomb calorimeter (Model C5000, IKA, Wilmington, NC). Chromium, NDF, P, and AA analyses of feed and freeze-dried digesta samples were conducted at the Experimental Station Chemical Laboratories, University of Missouri, Columbia, MO, using AOAC procedures (AOAC, 1995). Digestibility coefficients were calculated using the following equation:
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where Nd = the nutrient content in digesta, Crd = the chromium content in digesta, Npf = the nutrient content in digesta from the protein-free diet (AA only), Crpf = the chromium content in digesta from the protein-free diet, Ndoi = the nutrient content in digesta derived from the other ingredients (e.g., soybean meal), Nfti = the nutrient content in feed derived from the test ingredient, and Crf = the chromium content in the feed.
Statistical Analysis
Data were subjected to ANOVA using SPSS 8.0 (SPSS Inc., Chicago, IL). Each barrow was treated as an experimental unit. The effect of period and barrow was tested and was found to be not significant and was therefore removed before the final analysis. Estimated marginal means were compared pairwise. Regression analysis was conducted, and correlation effects were determined.
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RESULTS AND DISCUSSION
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Chemical Composition and Recovery of Processed Corn Ingredients
Processing corn by dry milling results in an incomplete separation of hulls, germ, and endosperm. In this study, 14% of corn weight was recovered as germ, similar to the 11% found by Watson (1987). In contrast, 18% of the weight was recovered as hulls, which is much greater than the 5% reported by Watson (1987). During dry milling, endosperm (84% of the corn weight according to Watson, 1987) was split in 2 fractions based on the size of the particles and the physical attributes of the equipment used. These fractions combined accounted for 56% of the weight of corn used, suggesting that a portion of the endosperm remained associated with hulls because its recovery was much greater than theoretical. It should also be noted that only 88% of the corn was recovered in fractions. Based on the quantity and composition of the fractions recovered, it can be estimated that most of the lost material was endosperm, likely throughs of very small size causing them to be lost in the system (e.g., through aspiration).
Each of these fractions has a unique nutrient profile (Table 3). As compared with corn, germ was greater in total AA (12.8 vs. 8.3%), hulls and germ were richer in NDF (32.6 and 17.2% vs. 9.8%), whereas tails and throughs were lower in NDF (3.6 and 4.5%). Germ was also greater in P (1.19 vs. 0.21%), whereas tails and throughs were lower in P (0.07 and 0.15%).
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Table 3. Chemical composition of corn, extruded corn, and processed corn products used in the formulation of the experimental diets
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The amino acid profile for each of these fractions was different from corn as well (Table 3
). Germ protein has a high content of albumin and globulin (Landry and Moureaux, 1980; Gupta and Eggum, 1998) and was relatively rich in Arg (297%) and Lys (330%) compared with corn protein. Hull protein was relatively high in Lys (148%) compared with corn protein. Tails (but not throughs) were relatively low in Lys (65%) compared with corn protein. Extrusion of corn is a thermal and mechanical treatment designed to improve digestibility of energy, and as such there were no strong effects on amino acid composition.
Ileal Digestibility Data
Ileal digestibility data for test ingredients were calculated after correcting for ileal digestibility of the soybean meal diet (DM, P, AA) and the protein-free diet (AA only). This procedure has a shortcoming that all variation in ileal digestibility coefficients observed for the diets are amplified because this variation is all attributed to the corn fraction of the diet. The statistical analysis was done on these derived values, and so further results are explained in this context.
Ileal Digestibility of Dry Matter
Ileal DM digestibility was improved (P = 0.02) in extruded corn compared with corn by 5.9% (Table 4). Several reasons can be considered for this, including altered microstructure (Ghorpade et al., 1997), starch gelatinization (Lue et al., 1991), reduced resistant starch content (Murray et al., 2001), reduced starch-protein complexes (Herkelman et al., 1990; Hongtrakul et al., 1998), and rupture of oil cells that may improve the availability of oils for digestion (Hull et al., 1968).
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Table 4. Ileal digestibility (%) of DM, P (apparent), and AA (standardized) in growing pigs fed diets containing corn, extruded corn, or corn fractions
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Diets based on tails and throughs also had a significant increase (P < 0.01) in ileal DM digestibility as compared with corn (9 to 10% relative increase, Table 4). The low NDF (less than half that found in corn) and the low phytate P content (data not shown) are expected to contribute substantially to this improvement in digestibility (Ravindran et al., 1999; Moeser et al., 2002).
Hulls and especially germ had reduced ileal DM as compared with corn (P < 0.01). These fractions both are high in NDF, which has been shown to have a strong negative effect on digestibility (Noblet and Perez, 1993; Moeser et al., 2002). In this study, hulls had the greatest NDF content (32.6%) but germ with 17.2% NDF had the lowest digestibility. For hulls, the decrease of 0.86% in ileal DM for each additional percent NDF in the diet calculated by Moeser et al. (2002) matched observed data well. In contrast, germ with an NDF content of 17.2% had an ileal DM digestibility that was lower than expected (51.1%) based on NDF effects.
Ileal Digestibility of P
Corn germ contains 11% ash compared with 1% in endosperm (FAO, 1992), and germ contains 78% of whole-corn minerals. The most abundant mineral is P, which in the germ is predominantly in the phytate form (Bressani et al., 1989). Phytate P is indigestible in the digestive tract of a pig as the intrinsic phytase activity in corn is low (Weremko et al., 2001). Germ indeed had the lowest ileal P digestibility (7.9%), which was significantly lower (P = 0.02) than that of corn (41.6%). No significant differences were observed between corn and other fractions. Extrusion, which is expected to destroy phytase activity, only numerically decreased ileal P digestibility (23.5 vs. 41.6 ± 9.5%). This is in line with the observation that corn is low in intrinsic phytase (Weremko et al., 2001).
Standardized Ileal Digestibility of Amino Acids
Extrusion had no effect on Lys content or ileal digestibility, suggesting that no Maillard complexes with Lys were formed that affected Lys content or ileal digestibility. This agrees with the observation of Hongtrakul et al. (1998). Ileal digestibilities of other AA were similar between corn and extruded corn, except for a greater Met digestibility in extruded corn (P < 0.01, Table 4).
The structural composition of corn grain is well documented. For example, in corn, the endosperm consists of starch granules embedded in a protein matrix (Gallant et al., 1997). Storage protein bodies called zein proteins contribute 60% of grain protein (Lending et al., 1988). The ileal digestibility of this protein is affected by factors such as structural organization, polyphenols, and phytic acid, and differences between fractions were thus expected.
The ileal digestibility of AA was reduced on average by 10% in germ (P < 0.05) compared with corn, with the exception of Arg and Lys where there was only a numerical drop in ileal digestibility. This reduction matched well with the predicted decrease in digestibility based on phytic acid content in the germ (Ravindran et al., 1999). For other corn fractions, there were no general trends for improvements or reductions in amino acid digestibilities, although some individual amino acids yielded significantly different digestibilities. For example, in tails ileal digestibility of Lys was significantly reduced (P < 0.01). This may be attributed to the compositional characteristics of zein proteins present in endosperm that are low in Lys (Crow and Kermicle, 2002). Hulls and throughs had a greater ileal digestibility for Met (P < 0.01). It is unclear why amino acid digestion in hulls was not affected by the high fiber content. One possible explanation is that the protein in the hull fraction is physically located such that hull fiber has no strong effects on its digestion. Another possibility is that hull fiber is inert (insoluble) from a digestibility standpoint.
Additivity of the Ileal Digestibility Values
Although it is assumed that ileal digestibility data are additive, published data on additivity itself are scarce (Hong et al., 2002). Although this study was not designed to assess whether ileal digestibility data are additive, testing both intact corn and recombined corn creates a unique test case for evaluating additivity (Table 5). This comparison shows that for all AA ileal digestibility of corn and recombined corn was within 1 SEM except for Arg, Lys, and Trp, with Lys and Arg having a difference larger than 2 times the SEM. The lack of agreement for Lys and Trp is likely caused by very low contents of these in some of the fractions, resulting in larger measurement or analytical errors, or both. The current data set thus provides no clear evidence that ileal digestibility data are not additive.
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Table 5. Additivity of the ileal digestibility of DM, P, and AA (predicted vs. actual ileal digestibility, %) in growing pigs fed corn, extruded corn, or corn fractions1
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In conclusion, fractionation of corn through dry milling results in hulls, endosperm (throughs and tails), and germ. Each of these fractions has a unique nutritional makeup and, as a result, unique digestibility. In this study, especially germ had a very poor digestibility of P, DM, and AA, whereas endosperm had a high DM digestibility. Extrusion of corn improved DM digestibility. These data thus suggest that corn varieties with a low germ content or extruded corn would be preferred for swine nutrition when the objective is maximizing nutritional value and minimizing nutrient excretion.
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Footnotes
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1 The use of trade names does not imply endorsement by the North Carolina Agricultural Service of the products named or criticism of similar ones that are not mentioned. 
2 Appreciation is expressed to the USDA Initiative for Future Agriculture and Food Systems (IFAFS) Program for partial funding and to Insta-Pro International (Des Moines, IA) for extrusion of the corn.
3 Department of Agricultural and Biological Engineering Sciences, University of Illinois at Urbana-Champaign, Urbana 61801.
4 Corresponding author: theovankempen{at}yahoo.com
Received for publication March 5, 2006.
Accepted for publication March 27, 2007.
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Abstract
INTRODUCTION
