ARTICLE IN PRESS

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doi:10.1016/j.jc

Abbreviations

able energy; AA

Chemists; CF,

fiber; TMEn, tr�Correspond

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Journal of Cereal Science 44 (2006) 342–346

www.elsevier.com/locate/yjcrs

Compositional variation amongst sorghum hybrids: Effect of kafirinconcentration on metabolizable energy

Israel Salinasa,�, Arturo Proa, Yolanda Salinasb, Eliseo Sosac, Carlos Miguel Becerrila,Manuel Cucaa, Miguel Cervantesd, Jaime Gallegosa

aGanaderıa, Campus Montecillo, Colegio de Postgraduados, 56230 Montecillo, Estado de Mexico, MexicobLaboratorio de maız INIFAP, Estado de Mexico, Mexico

cUniversidad Autonoma Chapingo, 56230 Chapingo, Estado de Mexico, MexicodInstituto de Ciencias Agrıcolas, Universidad Autonoma de Baja California, Mexicali, Baja California, Mexico

Received 20 March 2006; received in revised form 24 August 2006; accepted 29 August 2006

Abstract

Kafirins are stored proteins that negatively affect the nutritional quality of sorghum grain. Kafirin concentration and other chemical

characteristics were determined in 12 sorghum hybrids and varied significantly, from 58% (HB1) to 42% (HB12) as percent total protein.

Kafirin concentration correlated negatively with crude protein (CP) (�0.49), with acid detergent fiber (�0.40), apparent metabolizable

energy (�0.61), and true metabolizable energy corrected for N (�0.63). HB12 was the hybrid with the lowest content of kafirins, amylose

and tannins, and the highest content of apparent metabolizable energy. No differences were observed in the concentration of starch, but

differences were found in apparent metabolizable energy (3325–2944 kcal kg�1) probably due to a greater availability of starch, related to

differences in kafirin concentration.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Sorghum hybrids; Kafirin; Metabolizable energy

1. Introduction

Sorghum is the world’s fifth most important cereal; it ishigh-yielding and resistant to drought stress. Its proteincontent is higher than that of corn although its nutritionalprotein quality is lower (Dowling et al., 2002; Gualtieri andRapaccini, 1990). Sorghum grain quality is affected byfactors such as genotype, climate, soil type and fertiliza-tion, among others, which affect the chemical compositionand nutrient value (Ebadi et al., 2005). The crude protein(CP) content of sorghum grain is highly variable(5.44–12.9%).

e front matter r 2006 Elsevier Ltd. All rights reserved.

s.2006.08.008

: ADF, acid detergent fiber; AME, apparent metaboliz-

OAC, American Association of Official Analytical

crude fiber; CP, crude protein; NDF, neutral detergent

ue metabolizable energy corrected for nitrogen

ing author. Tel.: +55595 95 2 02 0 0 ext. 1707;

4 59 79.

ess: isreal2611@yahoo.com (I. Salinas).

The storage proteins in sorghum grain are mostlykafirins, which are prolamins that are soluble in aqueousalcohol in the presence of a reducing agent. According toWatterson et al. (1993), kafirins are the most abundantstorage proteins in sorghum grain. They are of lownutritional quality, very heterogeneous (Sastry et al.,1986), deficient in lysine, threonine and tryptophan, andrich in leucine, proline and glutamic acid (Duodu et al.,2003; Shewry et al., 1995). The kafirin content of sorghumgrain depends on agronomic and genetic factors (Hicks etal., 2001) and accounts for up to 70% of the total proteincontent of the grain (Hamaker et al., 1995). Kafirins aredeposited primarily in the endosperm during grain devel-opment (Shewry et al., 1995) forming protein bodies thatsurround starch granules and prevent access of amylasesduring digestion (Chandrashekar and Kirleis, 1988). Forthis reason, the quantity and nutritional quality ofsorghum grain protein depends mainly on kafirin concen-tration. Recent studies have focussed on increasing thedigestibility of proteins such as kafirins (Dowling et al.,

ARTICLE IN PRESSI. Salinas et al. / Journal of Cereal Science 44 (2006) 342–346 343

2002; Elkin et al., 2002). In broiler chickens, the quantityand nutritional quality of the proteins is important,especially in cereal-based diets, since proteins in the cerealscontribute more than 30% of the CP in the diet (Dowlinget al., 2002). Genetic studies on sorghum improve-ment have considered mainly agronomic traits but theireffects on nutritional quality are unknown. The objectiveof this study was to determine the chemical composi-tion and variability of metabolizable energy of 12commercial sorghum hybrids and their relationship tokafirin concentration.

2. Materials and methods

2.1. Materials

The sorghum hybrids studied were: HB1 through HB12,and were cultivated under similar commercial conditionsduring the fall-winter crop cycle, 2003–2004, in the state ofMorelos, Mexico, located at latitude 181300N and long-itude 981450W and at an altitude of 1030m. Samples ofgrain (�30 kg) from each hybrid, were collected directlyfrom the field during harvest. From each sample, sub-samples were taken for chemical analyses.

2.2. Chemical analyses

The chemical characteristics of the sorghum grainsstudied were estimated: CP and crude fiber (CF) asdescribed by AOAC (1990). Acid detergent fiber (ADF)and neutral detergent fiber (NDF) were quantified by themethods proposed by Van Soest (1963) and Van Soest andWine (1967). Starch was determined according to Herreraand Huber (1989). Amylose concentration was estimated

Table 1

Concentration of kafirins, protein, apparent metabolizable energy (AME), true

in twelve sorghum hybrids (dry matter basis)

Hybrid Kafirin (%)*a Crude protein (%)* AME (kcal

HB1 57.59 8.72 3000

HB2 57.52 6.61 2944

HB3 53.59 9.64 3196

HB4 50.93 10.05 2996

HB5 48.86 8.59 3268

HB6 47.61 11.42 3089

HB7 44.56 10.27 3159

HB8 44.29 9.88 3091

HB9 44.25 9.71 2999

HB10 43.83 9.51 3070

HB11 43.12 9.89 3325

HB12 42.35 11.19 3195

Mean 48.29 9.62 3119

Standart error of mean7 1.09 0.37 40

*pp0.05.

Extreme values are given in bold type.aAs protein percentage.bAs starch percentage.

by the method described by Juliano (1971). Kafirins wereextracted with aqueous ethanol (70%) containing 0.6% 2-mercaptoethanol as described by Landry and Moureaux(1970). Tannin content was determined by the method ofMaxson and Rooney (1972).Apparent (AME) and true metabolizable energy cor-

rected for nitrogen (TMEn) were estimated as described bySibbald (1986), using adult cecectomized roosters.Analysis of tannins and amylose were conducted in

duplicate for each hybrid studied and, for the othercharacteristics, in triplicate. For the determination ofAME and TMEn, each sample of sorghum was fed to sixLeghorn roosters housed in individual cages. Excreta fromeach rooster fed each hybrid were collected for 48 h andfreeze dried. From the mixed of excreta from two roosters,a compound replication was formed and AME and TMEn

were determined.

2.3. Statistical analysis

A completely random experimental design was used;least square means were compared and correlations wereobtained using the SAS program. The characteristicsmeasured in percentage were transformed to the arc-sinefunction (Steel and Torrie, 1988). The statistical modelused was

Y ij ¼ mþHi þ Eij,

where Yij is the response variable in the jth observation ofthe ith hybrid, m is the constant that characterizes thepopulation, Hi is the fixed effect of the ith hybridi ¼ 1,2y12; Eij is the effect of the random error of thejth observation of the ith hybrid j ¼ 1,2,3; Eij�IDN (0, s2).

metabolizable energy corrected by N (TMEn), starch, amylose and tannins

kg�1)* TMEn (kcal kg�1) Starch (%) Amylose (%)*b Tannins (%)*

3301 71.24 25.28 0.98

3325 73.71 26.38 1.01

3605 68.62 26.27 0.55

3397 67.73 26.83 1.02

3591 72.81 27.92 0.65

3593 69.53 26.67 0.62

3574 65.17 26.46 0.65

3510 70.68 27.33 0.75

3562 71.38 28.26 0.68

3424 71.21 27.11 0.55

3459 68.23 27.01 1.05

3540 67.58 25.36 0.68

3497 69.82 26.74 0.77

71 2.44 0.32 0.08

ARTICLE IN PRESS

Table 2

Crude fiber and neutral and acid detergent fiber in twelve sorghum

hybrids, expressed on dry matter basis

Hybrid Crude fiber

(%)

Neutral detergent

fiber (%)

Acid detergent

fiber (%)

HB1 1.96 33.14 7.94

HB2 2.67 22.22 5.86

HB3 4.17 28.62 9.51

HB4 2.23 24.24 9.02

HB5 2.10 25.47 6.80

HB6 1.94 26.01 9.47

HB7 1.71 24.75 7.47

HB8 1.84 19.47 7.51

HB9 2.24 22.57 8.02

HB10 3.06 24.87 9.07

HB11 2.85 26.35 7.13

HB12 2.73 35.18 9.52

Mean 2.45 26.07 8.07

Standart error

of mean 70.07 3.18 0.47

Pp0.05.

Extreme values are given in bold type.

Table 3

Correlations of the analyzed characteristics

Kafirins AME CP

TMEn �0.63 0.91

** **

AME �0.61 1

**

ADF �0.4 0.01 0.6

* NS **

CP �0.49 0.19 1

** NS

Starch 0.23 �0.08 �0.5

NS NS *

Amylose 0.28 0.11 �0.13

NS NS NS

NS ¼ not significant; TMEn ¼ true metabolisable energy nitrogen cor-

rected by N; AME ¼ apparent metabolisable energy; ADF ¼ acid

detergent fiber; CP ¼ crude protein.

*pp0.05.

**pp0.01.

I. Salinas et al. / Journal of Cereal Science 44 (2006) 342–346344

3. Results

The results are presented on a dry matter basis and indescending order of kafirin concentration (Tables 1 and 2).

The characteristics analyzed were statistically different(pp0.05), except for starch and TMEn. Kafirin contentvaried by 15 percentage units between HB12 and HB1(Table 1). The highest concentration of protein wasobserved in HB6 and the lowest in HB2. The extremeconcentrations of amylose were observed between HB9 andHB1 with a maximum variation of three percentage units.Tannin content varied widely among HB11, HB10 andHB3.

CF, NDF and ADF values for the sorghum hybrids arepresented in Table 2. HB7 had the lowest content of CF(pp0.05), while HB3 had the highest concentration; thedifference between the two hybrids was 2.5%. NDF andADF were highest in HB12 contrasting with HB8 and HB2which had the lowest concentrations, differences inpercentage units were 16 and 4, respectively.

The correlations observed are summarized in Table 3.Kafirins correlated negatively with TMEn, AME, ADF,and CP, while CP correlated negatively with starch.Positive correlations were observed between CP andADF, and between AME and TMEn.

4. Discussion

The nutrient composition and variability among sor-ghum hybrids available on the market were analyzed withspecial emphasis on the kafirins present in the grain (Tables1 and 2). Kafirin concentrations in the sorghum sampleswere similar to those reported by Hamaker et al. (1995).Evers and Millar (2002) point out that the protein matrixand the protein bodies found in the starchy endosperm of

the mature grain have a high kafirin content and that theseprolamin polymers are intermolecularly associated throughdisulfide linkages with the protein matrix composed ofglutelins (Wall, 1971). Duodu et al. (2002) report that evenafter fine milling of sorghum grain, wedges of proteinbodies are held together by matrix proteins with attachedstarch and/or cell walls. In addition, the matrix is lessresistant to digestion than protein bodies, which surroundthe starch granules and form a covering that impedesstarch gelatinisation and access by enzymes (Rom et al.,1992).In treated, cooked sorghum flour a reducing agent such

as sodium bisulfite or 2-mercaptoethanol increases starchgelatinisation by reducing intermolecular disulfide linkagesbetween the matrix and protein bodies (Rom et al., 1992;Zhang and Hamaker, 1998). In this study, the hybrids thathad lower kafirin content probably have higher gelatinisa-tion capacity, i.e. the starch granules become less stableand more likely to lose their original structure. This implieshigher values for AME, as was found for the hybrids HB12and HB11, in contrast to HB1, HB2 and HB4 (Table 1),which had higher concentrations of kafirins. Kafirinconcentration correlated negatively with TMEn and AME(Table 3), indicating that with higher kafirin concentra-tions, the availability of energy decreases. Also, whenkafirin concentration increases, grain hardness increasesdue to the rigidity of protein bodies (Marzhar andChandrashekar, 1995), which are difficult to separate fromstarch even with milling (Rooney and Pflugfelder, 1986).Endosperm hardness is thus most likely related to thequantity of kafirins present. Watterson et al. (1993)reported that vitreous (hard) endosperms contain up totwice as much protein rich in b-g kafirins as opaque (soft)endosperms. This suggests that hybrids with highernutritional value are lower in kafirin content and softer(although the latter trait was not evaluated in our study).

ARTICLE IN PRESSI. Salinas et al. / Journal of Cereal Science 44 (2006) 342–346 345

The negative correlation observed between CP andkafirins is similar to that reported by Hicks et al. (2001),who analyzed different fractions of kafirins (a, b and g),while in our study the total concentration of kafirins wasconsidered. Although no studies were found in which CP,kafirins and metabolizable energy were analyzed simulta-neously, it is known that CP deposition depends on geneticvariation, availability of water, soil fertility and environ-mental temperature (da Silva and Taylor, 2004), andkafirins are particularly sensitive to availability of nitrogen(Banda-Nyirenda et al., 1987).

In the hybrids analyzed, no statistical differences wereobserved in the content of starch, but there were differencesin AME, and therefore the energy content of sorghumseems to depend more on the availability than on thecontent of starch. Starch availability in the sorghum grainis influenced by interactions between protein and cell walls(Chandrashekar and Kirleis, 1988), non-starch polysac-charides (Kavitha and Chandrashekar, 1997) and tannins(Duodu et al., 2003). When sorghum starch is gelatinised,more disulfide bridges are formed in the protein matrixwhich further delays the digestion of starch and proteinitself (Zhang and Hamaker, 1998). In this study kafirinconcentration was the only fraction of the sorghum grainthat correlated negatively with TMEn and AME (Table 3);these results suggest that the energy value of sorghum graindepends on the kafirin concentration, which reducesavailability of starch. Zhang and Hamaker (1998) pointout that the lower the digestibility of kafirins, the lower thedigestibility of gelatinized sorghum starch. In contrast,Elkin et al. (2002), comparing mutant sorghum P851171(with highly digestible protein) and normal sorghumsP712N and 611Y observed lower values of TMEn andpoor feed conversion in mutant sorghum, because theendosperm had less starch and more non-starch polysac-charides than normal sorghums. Thus, it may be importantthat future studies in sorghum breeding to focus onincreasing protein digestibility and perhaps on reducingkafirin concentrations without affecting the agronomiccharacteristics.

A negative correlation between kafirins and ADF (Table3) was observed. Although this had not previously beenreported, Bach-Knudsen et al. (1988) point out that thecontent of dietary fiber in sorghum flour reduces digestibleenergy. Evers and Millar (2002) and Duodu et al. (2002)state that cell walls surrounding protein bodies embeddedin a protein matrix and starch granules are found in cerealendosperm, and therefore it is possible that these structuresmake access difficult for digestive enzymes. In this study nocorrelation was observed between ADF and the types ofenergy evaluated; thus, it is likely that when kafirinsincrease, the negative effect on energy is greater than theeffect of the reduction of ADF associated with an increasein kafirins. The positive correlation of ADF with CP issimilar to that reported by Bach-Knudsen and Munck(1985), who mention that significant amounts of proteinare associated with ADF. However, in agreement with the

correlations observed in this study (Table 3), when CP andADF increase, kafirin concentration decreases.The tannin content (Table 1) of the hybrids studied is

lower than that previously reported for bird-resistantsorghum, but it is considered medium-high by Lucbertand Castaing (1986). These authors state that when tanninconcentration is above 0.23% (low content), metabolizableenergy decreases by 40 kcal for every additional 0.1%,although sorghums with less than 1% tannin can be usedfor broiler chicken with no negative effect on feedconversion. The HB11 hybrid had the highest content oftannins as well as AME, possibly because of the lowerkafirin content with which indigestible complexes could beformed. It is important to take into account the capacity oftannins to associate with protein (they can precipitate up to12 times their own weight of protein; Butler et al., 1984)and their great affinity for proteins such as kafirins whichare rich in proline. In Table 3, four negative correlationsstand out: starch with CP, kafirins with CP, kafirins withTMEn and kafirins with AME. When starch concentrationdecreased, CP increased. When this occurs, kafirinconcentration also decreases. It is possible that when thekafirin concentration decreases, starch availability is high-er, and this resulted in higher levels of TMEn and AME.High kafirin concentrations negatively affect metaboliz-

able energy values in sorghum. HB12 and HB11 were thehybrids with the lowest concentrations of kafirin, and theyhad high content of apparent metabolizable energy (AME).Starch content was similar in all samples analyzed.Differences, however, were observed in the levels ofAME of the hybrids when kafirin concentration changed,suggesting that other factors such as the association ofkafirins with ADF can affect the energy content ofsorghum. Because starch availability depends on manyfactors, more studies are needed to determine, fromagronomic and nutritional perspectives, the impact of areduced kafirin concentration.

Acknowledgement

We thank D.V.M. Juan Carlos Gordoa Garcıa forassistance in performing the surgery on the roosters.

References

Association of Official Analytical Chemists (AOAC), 1990. Official

Methods of Analysis. Washington, DC.

Bach-Knudsen, B.K.E., Munck, L., 1985. Dietary fibe contents and

compositions of sorghum and sorghum-based foods. Journal of Cereal

Science 3, 153–164.

Bach-Knudsen, B.K.E., Kirleis, A.W., Eggum, B.O., Munck, L., 1988.

Carbohydrate composition and nutritional quality for rats of sorghum

to prepared from decorticated white and whole grain red flour. Journal

of Nutrition 118, 588–597.

Banda-Nyirenda, B.G.D., Vohra, P., Ingebretson, K.H., 1987. Nutritional

evaluation of some varieties of sorghums (Sorghum bicolor (L.)

Moench). Cereal Chemistry 64, 413–417.

ARTICLE IN PRESSI. Salinas et al. / Journal of Cereal Science 44 (2006) 342–346346

Butler, L.G., Riedl, D.J., Lebryk, D.G., Blytt, H.J., 1984. Interaction of

proteins with sorghum tannin: mechanism, specificity and significance.

Journal of the American Oil Chemists Society 61, 916–920.

Chandrashekar, A., Kirleis, A.W., 1988. Influence of protein on starch

gelatinization in sorghum. Cereal Chemistry 65, 457–462.

da Silva, L.S., Taylor, J.R.N., 2004. Sorghum bran as a potential source of

kafirin. Cereal Chemistry 81, 322–327.

Dowling, L.F., Arndt, C., Hamaker, B.R., 2002. Economic viability of

high digestibility sorghum as feed for market broilers. Agronomy

Journal 94, 1050–1058.

Duodu, K.G., Nunes, A., Delgadillo, I., Parker, L.M., Mills, E.N.C.,

Belton, P.S., Taylor, R.N., 2002. Effect of grain structure and cooking

on sorghum and maize in vitro protein digestibility. Journal of Cereal

Science 35, 161–174.

Duodu, K.G., Taylor, J.R.N., Belton, P.S., Hamaker, B.R., 2003. Factors

affecting sorghum protein digestibility. Journal of Cereal Science 38,

117–131.

Ebadi, M.R., Pourreza, J., Jamalian, J., Edriss, M.A., Samie, A.H.,

Mirhadi, S.A., 2005. Amino acid content and availability in low

medium and high tannin sorghum grain for poultry. International

Journal of Poultry Science 1, 27–31.

Elkin, G.R., Elaine, A., Hamaker, B.R., Axtell, J.D., Douglas, W.M.,

Parsons, C.M., 2002. Nutritional value of a highly digestible sorghum

cultivar for meat-type chickens. Journal of Agricultural and Food

Chemistry 50, 4146–4150.

Evers, T., Millar, S., 2002. Cereal grain structure and development: some

implications for quality. Journal of Cereal Science 36, 261–284.

Gualtieri, M., Rapaccini, S., 1990. Sorghum grain in poultry feeding.

World’s Poultry Science Journal 46, 246–254.

Hamaker, B.R., Mohamed, A.A., Habben, J.E., Huang, C.P., Larkins,

B.A., 1995. Efficient procedure for extracting maize and sorghum

kernel proteins reveals higher prolamin contents than the conventional

method. Cereal Chemistry 72, 583–588.

Herrera, S.R., Huber, J.T., 1989. Influence of varying protein and starch

degradabilities on performance of lactating cows. Journal of Dairy

Science 72, 1477–1483.

Hicks, C., Bean, S.R., Lookhart, G.L., Pedersen, J.F., Kofoid, K.D.,

Tuinstra, M.R., 2001. Genetic analysis of kafirins and their phenotypic

correlations with feed quality traits, in vitro digestibility, and seed

weight in grain sorghum. Cereal Chemistry 78, 412–416.

Juliano, B.O., 1971. A simplified assay for milled rice amylose. Cereal

Foods World 16, 334–340.

Kavitha, R., Chandrashekar, A., 1997. Content and composition of

nonstarchy polysaccharides in endosperms of sorghum varying in

hardness during four developmental stages. Cereal Chemistry 74, 22–24.

Landry, J., Moureaux, T., 1970. Heterogeneite des glutelines du grain de

mais: extraction selective et composition en acides amines des trois

fractions isolees. Bulletin de la Societe de chimie Biologique 52,

1021–1037.

Lucbert, J., Castaing, J., 1986. Utilisation de sorghos diferentes teneurs en

tannins pour l’alimentation des poulets de chair. In: Proceedings of the

7th European Poultry Conference, vol. 1, pp. 472–476.

Marzhar, H., Chandrashekar, A., 1995. Quantification and distribution of

kafirins in the kernels of sorghum cultivars varying in endosperm

hardness. Journal of Cereal Science 21, 155–162.

Maxson, E.D., Rooney, L.W., 1972. Two methods of tannin analysis for

Sorghum bicolor (L.) Moench grain. Crop Science 12, 253–254.

Rom, D.L., Shull, J.M., Chandrashekar, A., Kirleis, A.W., 1992. Effects

of cooking and treatment with sodium bisulfite on in vitro protein

digestibility and microstructure of sorghum flour. Cereal Chemistry 69,

178–181.

Rooney, L.W., Pflugfelder, R.L., 1986. Factors affecting starch digest-

ibility with special emphasis on sorghum and corn. Journal of Animal

Science 63, 1607–1623.

Sastry, L.V.S., Paulis, J.W., Bietz, J.A., Wall, J.S., 1986. Genetic variation

of storage proteins in sorghum grain: Studies by isoelectric focusing

and high-performance liquid chromatography. Cereal Chemistry 63,

420–427.

Shewry, R.P., Naiper, A.J., Tatham, S.A., 1995. Seed storage proteins:

structures and biosynthesis. The Plant Cell 7, 945–956.

Sibbald, I.R., 1986. The T.M.E. system of feed evaluation: methodology,

feed composition data and bibliography. Technical Bulletin. 1986-4E.

Animal Research Centre, Ottawa, Ont., Canada.

Steel, R.G.D., Torrie, J.H., 1988. Bioestadıstica: Principios y Procedi-

mientos, 2a edicion. McGraw-Hill, Mexico.

Van Soest, P.J., 1963. Use of detergents in analysis of fibrous feeds. II. A

rapid method for the determination of fiber and lignin. Journal of the

Association of Official Analytical Chemists 46, 829–835.

Van Soest, P.J., Wine, R.H., 1967. Use of detergents in analysis of fibrous

feeds. IV. Determination of plant cell-wall constituents. Journal of the

Association of Official Analytical Chemists 50, 50–55.

Wall, J.S., 1971. Disulfide bonds: determination, location and influence on

molecular properties of proteins. Journal of Agricultural and Food

Chemistry 19, 619–625.

Watterson, J.J., Shull, J.M., Kirleis, A.W., 1993. Quantitation of a-, b-,and g-kafirins in vitreous and opaque endosperm of Sorghum bicolor.

Cereal Chemistry 70, 452–457.

Zhang, G., Hamaker, R.B., 1998. Low a-amylase starch digestibility of

cooked sorghum flours and the effect of protein. Cereal Chemistry 75,

710–713.