Carbohydrate Composition and Nutritional Quality for ...

Carbohydrate and Fiber

Carbohydrate Composition and Nutritional Quality for Rats of Sorghum To Prepared from Decorticated White and Whole Grain Red Flour1'2 [ KNCJD ERIK BACH KNÃœDSEN,ALLEN W. KIRLEIS,*3 BJ0RN O. EGGOM AND LARS MUNCK* National Institute of Animal Science, Department of Animal Physiology and Biochemistry, Foulum Research Center, DK-8833 0rum Sonderlind, Denmark and *Carlsberg Research Laboratory, Department of Biotechnology, DK-2500 Valby, Copenhagen, Denmark

for humans when consumed as a stiff porridge ugali. Protein digestibility was only 46% and the stool energy loss was 766 kj/d for the sorghum porridge. Compar ative data from experiments with wheat-, rice-, potatoand maize-based staple foods processed in a similar way show digestibility of protein in the range of 66-81% and loss of energy in stool of 243-477 kj/d. The fecal bulkiness of sorghum is surprisingly high because its starch content is generally higher and its dietary fiber (DF) lower than those of other cereal staples (4, 5). In a previous study (5) we considered four dietary factors that might influence the nutritive value of sorghum foods: DF, polyphenols, resistant starch (RS) and unavailable endosperm protein kafirins. The DF fraction consisted of various nonstarch polysaccharide (NSP) residues (cellulose, hemicellulose, ÃŸ-glucan,pec tin, etc.) and lignin located in primary and secondary cell walls. These cell walls survive breakdown in the small intestine to a large extent, whereas a significant amount of NSP are broken down by the microflora in

INDEXING KEY WORDS: â€¢dietary fiber â€¢resistant starch â€¢polyphenols â€¢kafÃ-rÃ-nsâ€¢digestibility â€¢biological value â€¢sorghum â€¢to

â€¢This work was supported by the United Agency for International Development, International Sorghum and Millet Collaborative Sup port Research Program, Project Grant No. AID/DSAN/XII-G-0149, by a grant for a senior research worker from the Royal Veterinary and Agricultural University, Copenhagen and by the Carlsberg Foun dation. 2A preliminary report of this work was presented at the Thirteenth International Congress of Nutrition, Brighton, UK, August 18-23, 1985 (abs., p. 67). 3Present address: Department of Food Science, Smith Hall, Purdue University, West Lafayette, IN 47907.

Sorghum is the fifth most important cereal in terms of world production (1). In the semi-arid tropics of Af rica and Asia sorghum is the most important source of energy and protein. To, or tuwo, prepared as a stiff porridge product is a common sorghum food in West 0022-3166/88 $3.00 Â©1988 American Institute of Nutrition.

Received 8 July 1987. Accepted 5 January 1988.

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Africa with only slight modifications in pH among countries (Mali, Burkina Faso and Niger). In these areas these thick porridges serve as the principal form of di etary carbohydrate for at least 20 million people (2). A study by Maclean et al. (3) of preschool children caused doubt about sorghum's nutritional properties

ABSTRACT The carbohydrate composition and nutri tional quality of acid, neutral and alkali to, prepared from a decorticated white (DW) flour containing no polyphenols or whole grain red (WGR) flour high in polyphenols, were studied. The diets were characterized with regard to nonstarch polysaccharides, Klason lignin, resistant starch (RS) and amino acid content. The nutritional properties were studied in balance trials with rats. Digestible energy of DW flour was higher than that of WGR flour because of a lower dietary fiber (DF) content and a higher digestibility of DF. Recovery of cellulose in feces of rats fed diets containing DW flour was 45-59% and recovery of noncellulosic polysaccharides (NCP) was 17-31%. In rats fed diets de rived from WGR flour, recoveries were 76-83 and 5467% for cellulose and NCP, respectively. Cooking resulted in formation of appreciable amounts of RS. Twenty-three to fifty-six percent of the RS in DW to and 59-74% of RS from WGR to were recovered in feces. Endosperm protein kafirins formed complexes during cooking. The result was a lower true protein digestibility and higher biological value in to than in flour. Amino acid data revealed that the un available kafirins serve as a nitrogen source for the hindgut microflora. A high affinity of dietary polyphenols for proline and glycineresiduescan be postulatedfrom digestibleamino acid data. The net effect was a change in the excretory route of nitrogen from urine to feces. J. Nutr. 118:588597, 1988.

NUTRITIONAL

QUALITY OF SORGHUM TO

MATERIALS AND METHODS Sorghum materials. Red and white whole grain sorghums were commercially obtained from a market place in Burkina Faso in 1983. Decortication and milling. The white sorghum grain was decorticated on a Schule laboratory vertical abra sive polishing machine (Vertical Abrasive Polisher, Schule, Hamburg, FRG) to yield 72% decorticated grain. A preliminary docortication run with the red seeded sorghum yielded less than 50% decorticated grain with only partial pericarp removal. Thus, the red seeded sorghum was used as whole grain. Both sorghum sam ples were ground on an Alpine laboratory hammer mill (Alpin, Augsburg, FRG) to pass through a 0.5-mm screen. The flours were labeled decorticated white (DW) and whole grain red (WGR). Preparation of neutral to. The following procedure of Scheuring, Sidibe and Kanta (2) was applied in the present study. Six hundred milliliters of water was mixed with 400 g of sorghum flour. The mixture was stirred until homogenous and then swirled into 3.5 L of water that has been brought to boil in an aluminum pot on a hot plate. The mixture (called "bouillie") was stirred for ~8 min with high heat. The heat was reduced to a medium setting on the hot plate and ~1000 mL of bouillie was removed from the pot and put aside. Six hundred grams sorghum flour was added, 75-125 g at a time, to the bouillie in the pot. After each addition of flour the paste was vigorously stirred with a wooden spoon. When the paste thickened too much for easy stirring, a small amount of the set-aside bouillie was used. At the end of this process the paste was homo geneous and very thick. This step took about 10 min.

The heat was reduced to a low setting on the hot plate and the pot containing the thick paste was covered and allowed to cook for 12 min. The to was removed from the hot plate, placed into trays, quickly frozen and freezedried. The freeze-dried to was pulverized into a powder with a cone mill and stored in a freezer at - 20Â°C.The pH of DW and WGR neutral to was 7.4 and 7.0, re spectively. Acid to was prepared by using a tamarind solution instead of water. The tamarind solution was composed of a 15% (vol/vol) mixture of tamarind concentrate in water. The tamarind concentrate was prepared by soak ing 200 g of dried tamarind in 2 L water overnight and then boiling the mixture for 10 min. The boiled extract was filtered to remove the solid material and cooled to room temperature. The pH of the acid to was 5.0 and 4.7 for D W and WGR sorghum, respectively. Alkali to was prepared by adding 7 g of wood ash concentrate to the initial flour and water mixture. By this procedure, the pH of the alkali to was raised to 9.4 and 7.9 for DW and WGR sorghum, respectively. Diets for nutritional experiments with rats. Diets were composed of DW and WGR flour, to and casein supplemented with 10 g DL-methionine/kg dry matter (DM) (Table 1). Casein fortified with methionine pro vided 10% of dietary protein in all diets to stimulate diet consumption. The diets were kept constant in di etary nitrogen by adding a nitrogen-free mixture to make 15 g N/kg DM. Vitamins and minerals were added as described by Eggum (11). Nutritional experiments with rats. The experimen tal procedure for nitrogen balance and digestibility trials has been described by Eggum (11). Groups of five male Wistar rats, each animal weighing ~ 70 g, were used in the experiments with preliminary feeding periods of 4 d and balance periods of 5 d. Each animal received 150 mg nitrogen and 10 g DM daily in the diet throughout the preliminary and the balance periods. True protein digestibility (TD), biological value (BV),net protein uti lization, digestible amino acids, digestible energy and digestible DF constituents were used as criteria in the biological study. Digestibility of energy and DF con stituents was calculated by the difference, after correc tion for digestibility of a diet (g/kg DM) consisting of a nitrogen-free mixture (844.5 g), casein (99.5 g) and minerals and vitamins (56.0 g), of 91.7 Â±1.3% (mean Â±SEM)and excretion of NSP sugar residues in feces. Recoveries of NSP sugar residues when the nitrogenfree mixture was fed were arabinose, 33.5 Â±0.3%; xy lose, 38.9 Â±4.0%; mannose, 29.4 Â±0.2%; galactose, 78.1 Â± 1.5%; and glucose, 73.0 Â± 1.4%. Individual fecal and urine samples were analyzed for nitrogen and energy and used to estimate TD, BV, net protein uti lization and digestible energy, whereas calculation of digestibility of amino acids and recovered DF constit uents was based on analysis of pooled fecal samples. Analytical methods. Proximate analyses were per formed in accordance with standard AOAC methods


the hindgut (6-9). The polyphenols are also found in secondary cell walls of pericarp and testa of some sorghum varieties often closely associated with the lignin fraction (10). When high polyphenol sorghum flour is cooked, the polyphenols are dispersed from the ma trix and form complexes with dietary protein (5). The net effect, when consuming these diets, is a change in the excretory route of nitrogen from urine to feces (5). In contrast to DF and polyphenols, which are normal cell wall constituents, RS and unavailable kafirins are formed during cooking. In vivo these latter components act as a physicochemical complex that passes through the small intestine undigested. In the hindgut the RSkafirin complex is exposed to microbial degradation. Hence the lower energy digestibility of cooked sorghum porridge ugali compared to uncooked flour is due to a higher excretion of carbohydrates and protein in feces (5). The purpose of the present study was to determine the effects of cooking, media pH and grain polyphenol content on carbohydrate composition and nutritional quality of a West African sorghum porridge to.

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590

TABLE l Composition of experimental diets Diet Ingredient g/kg DM DW flour DW acid to DW neutral to DW alkali to WGR flour WGR acid to WGR neutral to WGR alkali toNitrogen-free mixture1Casein2Mineral

785.5 781.5 772.0 762.4 849.6 849.6 849.6

mixture1Vitamin mixture4148.510.040.016.0152.510.040.016.0182.010.040.016.0171.610.040.016.084.410.040.016.084.410.040.016.084.410.040.016.0849.684.410.040.016.0

(12) and amino acids as described by Mason, Bech-An dersen and Rudemo (13). Polyphenol assays for proanthocyanidins and flavan-3-ol monomers and polymers (vanillin assay) were carried out on combined methanol and acidic methanol extracts according to the colori metrie procedures described by Watterson and Butler (14) and Butler, Price and Brotherton (15). Starch was gelatinized and hydrolyzed to glucose monomers with a thermostable a-amylase (Termamyl, Novo A/S, Denmark) and an amyloglucosidase (cata logue no. A9268, Sigma Chemical, St. Louis, MO). The glucose monomers were quantified with a glucose ox idase reagent (catalogue no. 124036, Boehringer Man nheim, Mannheim, FRG) and converted to polysaccharides by the factor 0.9 following the description of Bach Knudsen, Aman and Eggum (16). Total mixed linked (l-Â»3),(l-Â»4)-ÃŸ-D-glucans(ÃŸ-glucans) were measured fluorometrically using the method of Jorgensen and Aastrup (17). Duplicate samples (50 mg) of sorghum flour were weighed into 50-mL cen trifuge tubes with screw caps. Ten milliliters deionized water was added and the sample was boiled in a water bath for 1 h. After cooling to room temperature, 10 mL perchloric acid (0.1 M) was added and the sample was boiled for a further 10 min to solubilize the ÃŸ-glucans. Finally, the suspensions were centrifuged (3000 x g, 10 min). Quantification was done by complexing with Calcoflour (Polysciences, Northampton, UK) and meas uring the ÃŸ-glucan-Calcoflour complex fluorometri cally. Excitation was 350 nm and emission 425 nm. The total dietary fiber (TDF) content was assayed by a gravimetric method based on enzymatic digestion of starch and protein as described by Asp et al. (18). Ash

and protein contents in residues were determined ac cording to standard AOAC methods (12). Acid deter gent fiber (ADF) was determined gravimetrically after extraction of starch, protein and hemicellulose with an acid detergent solution according to the method of Van Soest (19). Protein and ash were measured in the fiber residues. RS, i.e., starch not hydrolyzed by incubation with aamylase in the fiber determination, was dispersed from the fiber residues with 2 M KOH, according to the de scription of Englyst, Anderson and Cummings (20) and Bach Knudsen, Munck and Eggum (5). NSP of DF were determined as alditol acetates by gas-liquid chromatography for neutral sugars and by a decarboxylation method for uronic acid as described by Theander, Aman and Westerlund (21, 22). After quantitative removal of starch the polysaccharides were swelled with H2SO4 (12 M, 30Â°C,60 min), hydrolyzed with 0.41 M H2SO4 Â¡125Â°C, 60 min), reduced with KBH4 to alcohols and acetylated by using 1-methylimidazole to catalyze the reaction. myo-Inositol was used as an internal standard. The NSP constituent sugar values were converted to polysaccharides by the factor 0.9. Content of cellulose in NSP was calculated as Cellulose and noncellulosic

= NSPglucose - (ÃŸ-glucan+ RS), polysaccharides

(NCP) as

NCP = arabinose + xylose + mannose + galactose + uronic acid + ÃŸ-glucan. Lignin was defined as the residue resistant against H2SO4 and measured gravimetrically as Klason lignin


'Provided |in g/kg) sucrose 90.0, cellulose powder 52.0, soybean oil 52.0, potato starch (autoclaved) 806.0. 2Fortified with 10 g DL-methionine/kg DM. Provided (in mg) per kg diet: CuSO4-5H2O 9, K2HPO4 8752, NaF 20, CaCO3 2740, Ca3C12H10Ol4-4H2O 12,333, CaHPO4-2H2O 4512, KI 26, MgSO4 1532, MgCO., 1408, MnSO4-H2O 80, NaCl 3080, KC1 4988, A1NH4(SO4)2-12HIO 4. "Provided (in mg or lu) per kg diet: retinyl acetate 3200 lu, cholecalciferol 240 lU, thiamin 0.64, riboflavin 2, nicotinamide 6, pantothanic acid 2, a-tocopherol 0.3, pyridoxine hydrochloride 0.2.

NUTRITIONAL


(22). Pepsin digestibility of proteins from DW and WGR flour and to preparations were determined using the in vitro method described by Mertz et al. (23). Statistical methods. The rat data within DW and WGR diets were examined by a one-way analysis of variance model, as outlined by Snedecor and Cochran (24):

where Xit is the dependent variable (i.e., TD, BV, di gestible energy, etc.), \Â¡. is overall mean, a, is effect of treatment (i.e., pH of to preparations) and e;/ is a normal distributed random variable. Standard errors of means (SEM)were calculated according to SEM = SE\/Ã±,and 95% confidence interval according tox Â±t097sdi â€¢ SEM. n represents the number of animals in each groups (i.e., n = 5).

RESULTS

The NSP content (DM basis) was 20.9 g/kg in DW sorghum flour compared with 59.2 g/kg in WGR flour. Cellulose, arabinose and xylose residues of NCP were greatly reduced by decorticating the whole grain (Table 2). By contrast, the glucose residue of NCP (ÃŸ-glucan) was approximately the same irrespective of decortica tion. In DW flour only trace amounts of lignin were found. Cooking did not alter either NSP content and composition or Klason lignin level. However, in the cooked to, RS was identified at levels of 37-41 g/kg in DW- and 22-29 g/kg in WGR-based materials. The presence of RS was also responsible for the higher as sayed values for TDF and ADF in to products. However, DF (NSP + lignin) and TDF were approximately the same when the latter was corrected for RS. Protein and starch contents were higher, whereas ash and fat contents were lower in DW than in WGR flour (Table 2). DW flour contains 104.5 g protein/kg, whereas WGR flour contains only 85.0 g/kg. The lysine levels were 1.5 and2.8g/16gN, respectively, in the two flours (Table 3). Because the diets also contained 10% casein, the lysine content of DW flour diets was 2.2 g/16 g N and that of WGR flour diets was 3.3 g/16 g N. The ratio between essential and nonessential amino acids was generally more favorable in WGR than in DW flour. As expected, cooking had no significant influence on either protein or amino acid composition. On the contrary, crude fat and starch were generally lower in to than in the respective flours. The higher levels of ash in alkali

TABLE 2 Chemical composition of decorticated white Â¡DW)and whole grain red Â¡WGR) sorghum flour and acid, neutral and alkali to preparations


Chemical composition of flour and to products. The polyphenol analyses show that the white sorghum grain used for decortication did not contain condensed polyphenols (proanthocyanidins). On the contrary, the red sorghum grain can be classified as a high polyphenol sorghum variety containing a moderate amount of con densed polyphenols (11.3 and 20.1 optical density /g, based on proanthocyanidins and vanillin assay, respec tively) compared to a high tannin genotype BR-64 (40.5 and 63.0 optical density/g, respectively).

591

from flourNeutral8.0103.55.8788.538.06.411.62.11.71.01.21.64.018.0Trace18.057.249.4Alkalig/kg12.7102.55.2779.5 DW from flourNeutral15.987.025.5688.026.027.232.2 WGR ComponentAshProtein

flour7.1104.59.3852.3Trace8.412.52.01.81.31.42.04.020.9Trace20.917.213.3Acid8.4103.55.4800.541.05.213.42.21.91.41.32.64.018.6Trace18.6 flourDM15.385.034.5745.1Trace27.232.010.68.52.03.12.35.559.236.095.292.93

6.25)Crude (N x fatStarchResistant starchNSP1CelluloseNCP2ArabinoseXyloseMannoseGalactoseGlucoseUronic

acidTotal NSPKlason ligninDietary fiberTDF3ADF4DW

'NSP, nonstarch polysaccharides. JNCP, noncellulosic polysaccharides. 3TDF, total dietary fiber determined gravimetrically 4ADF, acid detergent fiber.

(includes resistant starch].

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BACH KNUDSEN ET AL.

Digestible energy and digestibility ofRS and DF con stituents. Digestible energy was significantly lower (P Amino acids composition of decorticated white < 0.001 )for WGR flour (81.8%) than for DW flour (96.8% ) and whole grain red flour and diets (Fig. 2). The lower digestible energy values of WGR flour were primarily due to a higher excretion in feces red:lour2.8 grain of DF (NSP + lignin), which was 103 g/kg consumed wkFlour1.5iteDiet DM for WGR flour compared with 7 g/kg consumed Amino acidLysine 1g/16 DM for DW flour. Fecal excretion of protein was also gN higher when WGR flour was fed than when DW flour 2.2 was fed, although the differences were less significant. ThreonineCystine 3.01.8 3.21.71.82.314.94.55.55.5 3.31.9 3.41.71.82.411.94.25.54.9 As a consequence of cooking to, digestible energy of the to diets was reduced by between 3.4 and 4.3% (ab MethionineHistidineLeucineIsoleucineValinePhenylalanine 1.52.215.44.45.45.5 1.82.412.04.05.34.8 solute units) compared with the respective flour diets. The lower digestible energy was attributed to a higher excretion of undegraded RS (from 6 to 22 g/kg con sumed DM) and higher excretion of protein (from 7 to 18 g/kg consumed DM). Aspartic acid 6.4 6.424.09.79.53.0 6.8 6.8 acidProlineAlanineArginine Glutamic 23.69.210.22.9 19.37.68.54.5 20.37.88.04.3 The fecal lignin values revealed fecal recovery of from 145 to 163% of intake when the WGR diets were fed (Table 4). If recovery in feces of 100% of intake is as sumed, the actual fecal lignin values should be reduced Glycine 2.6 3.4 2.5 3.64.83.7Diet3.3 by from 18 to 23 g/kg DM. SerineTyrosineDecori 4.84.3icated 4.94.4Whole 4.93.9 Recovery in feces of the various NSP constituents and lignin is shown in Table 4. Cellulose was generally more resistant to microbial breakdown than was NCP. When WGR diets were fed recovery of cellulose was tÃ´than in flour were due to the addition of wood ash to increase pH of the alkali to. Approximately 40 and 90% of the dietary protein in 0 DW- and WGR-based materials, respectively, resisted O Â» in vitro pepsin and pancreatin digestion (Fig. 1). About .-50 100 15% of the protein in DW flour was associated with the ADF fraction and 18% of the protein in WGR flour. i : 100 200 As a consequence of cooking to, protein associated with ADF increased to 31-41% in D W tÃ´materials and to TJ B 150 300 24-35% in WGR to materials. In vitro pepsin diges o (D tibility was significantly lower in WGR flour (52%) IL 200 400 than in DW flour (68%). As a result of cooking to, in A vitro pepsin digestibility declined to 38-55% in DW B 100 100 to and to 15-20% in WGR to. TABLE 3

2 -100 >â€¢ 5

a 90 .o

80

S, 85

70

a

a

90

O)

80

*â€¢*Â»Â«****

â€¢Â§ 60 Â°Ã¬ *

_

C

Â«

40

8 Â£ 20

Flour AcidNeutral tÃ´ tÃ´

Decorticated

fl Alkali tÃ´

white

60

80

Flour Acid Neutral Alkali tÃ´ to to Decorticated white

Flour Acid Neutral Alkali tÃ´ tÃ´ tÃ´

Whole grain red

FIGURE l Protein associated with total dietary fiber (TDF) and acid detergent fiber (ADF) of flour and acid, neutral and alkali to of decorticated white and whole grain red sorghum. ADFprotein(a), TDFprotcin(a + â€¢).

Flour Acid Neutral Alkali to to to Whole grain red

FIGURE 2 Partition of fecal DM in nonstarch polysaccharides (NSP), resistant starch (RS), lignin (Klason), protein and residue (A) and digestible energy (B] of flour and acid, neutral and alkali to of decorticated white and whole grain red sorghum. Note: The scales for fecal dry matter and di gestible energy are different for the two types of sorghum. V7M NSP; ES3 RS; ES3 lignin; fTÃ•Ã•TÃ•Ã•I protein; | | resi due. Vertical bars represent 95% confidence interval (n = 5). ***P < 0.05 compared to respective flour.


x

? 95 o C

NUTRITIONAL


593

TABLE 4 Recovery in feces (% of intake! of resistant starch, nonstarch polysaccharide constituent sugars, Klason lignin and dietary fiber (DF) when feeding diets composed of decorticated white or whole grain red flour and acid, neutral and alkali to1

Diet and componentDecorticated whiteResistant starchNSP2CelluloseNCP3ArabinoseXyloseMannoseGalactoseGlucoseUronic

SI-41"44-34-82"222a41-70a1179ab58b61b561Â»50-61bTrace78C71ab155a1

acidTotal NSPWhole redResistant grain starchNSP2CelluloseNCP3ArabinoseXyloseMannoseGalactoseGlucoseUronic Downloaded from jn.nutrition.org by guest on July 10, 2011

OS'44"70aTrace104a74-163Â»108Â»84"Acid50-57-28-47-21C23b78bTrace24Â»36-59"76"61"63'"57"44b65abTrace87b68ab157" Ie46ab64abTrace79-=65b146a97b79"SEM10.33.0

acidTotal NSPKlason ligninDF4DF5Flourâ€”45b17C27Â°29b12-=26CTrace14"28bâ€”83-67JOS-

'Values are means for five rats. Groups in the same row with the same superscript letter do not differ significantly (P < 0.05). 2NSP, nonstarch polysaccharides. 3NCP, noncellulosic polysaccharides. 4DF, dietary fiber. 5DF, DF calculated assuming 100% recovery of Klason lignin.

76-83% compared with 54-67% for NCP. Of the NCP sugar residues, arabinose and xylose were most resist ant, whereas only trace amounts of glucose (ÃŸ-glucan) survive breakdown. The recovery of all NSP constitu ents was significantly lower when DW diets were fed than when WGR diets were fed. Recovery of cellulose was 45-59% and of NCP 17-31%. By using the difference, 100-recovery, the digesti bilities of NSP and DF were 26-35 and 16-21%, re spectively, in WGR diets compared with 59-72% for NSP in DW diets. The DF calculation provides 100% recovery in Klason lignin in feces. A significant amount of RS resisted bacterial break down in the hindgut. Hence, 23-56% was recovered in feces when DW to diets were fed and 59-74% when WGR to diets were fed. TD, BV, net protein utilization and digestibility of amino acids. TD were 101.2 and 63.3% (P < 0.001 ) and BV 53.5 and 86.3% (P < 0.001) in DW and WGR flour, respectively (Fig. 3). After to of DW flour was cooked, TD was reduced to 94.3-94.8% and BV increased to 55.1-61.3% (only significantly different from flour at

the highest level), whereas net protein utilization re mained constant at 52-58%. The effect of cooking was even more pronounced when to was prepared from WGR flour. TD decreased to 43.8-47.1% and BV of neutral and alkali to increased to 95.7-96.6%. For acid to BV was 81.4%, which was lower although not significantly different from that of WGR flour (86.3% ). The decrease in TD in WGR to diets was not counteracted by an increase in BV. Net protein utilization was, therefore, significantly lower in WGR to diets (37.4-45.5%) than in WGR flour diets (55.4%). Digestibility of the five selected amino acids in DW flour was between 98 and 105%, whereas in WGR flour it was markedly lower with values of 82, 60, 60, 41 and 58% for lysine, threonine, glutamic acid, proline and glycine, respectively (Fig. 4). As a consequence of mak ing to, digestibility of all amino acids was greatly re duced. In to diets of DW flour, digestibility of lysine was 11-18%, threonine 10-11%, glutamic acid 4-5%, proline 3% and glycine 9-11% absolute units lower than those of the corresponding flour. For to prepared from WGR flour, the negative effect was stronger: lys-

594

BACH KNUDSEN ET AL. %7060n50| digestib lity,

10510095 100959085105100959085TEUTâ€”*< 0.001. "Only values from studies with DW and WGR diets (present study].

DISCUSSION


The nutritional implications of RS in sorghum-based foods have been discussed in recent investigations (4, 5). RS levels in WGR to diets (22-29 g/kg) were in accord ance with the amount of RS found in the stiff porridge ugali (5), whereas RS of to products derived from DW flour was significantly higher (37-41 g/kg). For compar ison, Englyst, Wiggins and Cummings (25, 26) have found RS levels of 7-44 g/kg in heat-processed products such as bread, cornflakes, porridges and boiled potatoes. The formation of RS takes place within hours after cooking (27). It involves the amylose fraction [linear (1â€”>4)-a-D-glucan], that associates very easily during cooling of the cooked product and therefore retrogrades rapidly. This has been confirmed by Berry (27) who found a good correlation between formation of RS and content of amylose. However, it is a matter of specu lation whether endosperm proteinâ€”kafirinsâ€”is in some way involved in the formation of RS. Rooney and Miller (28) found that in the peripheral endosperm of sorghum, the starch granules are embedded in a dense mixture of protein bodies (kafirins) and matrix protein, making the starch difficult to hydrolyze by enzymes. When considering the results from the present study as well as results from previous work (5) it is striking how well the levels of RS correlate with the amount of kafirinlike protein associated with the ADF fraction. The mechanism involved in the formation of RS could be that the hydrophobic milieu promotes formation of ir reversible amylose aggregates of starch molecules. It could also result from cross-linking of protein through the formation of â€”Sâ€”Sâ€”linkages that coat starch granules and make them less susceptible to enzyme attack. There is no doubt that RS and endosperm protein kafirins act as a physicochemical complex in the gut. In the small intestine this complex passes without being hydrolyzed by endogenous enzymes. This interpreta tion is based first on the in vitro studies of Hamaker et al. (29) who showed that after cooking kafirins are

much less digestible by pepsin than before cooking. On the basis of their findings these workers suggested that the kafirin proteins form complexes during cooking and that the complexed kafirin proteins are less accessible to enzymic attack. These findings fit very well with our in vivo results with rats because TD was reduced, BV was increased and net protein utilization remained constant in cooked as compared with uncooked sorghum products. The converse effect of cooking on TD and BV indicates an improved ratio of essential to nonessential amino acids in protein absorbed from small intestine. Second, in vivo studies with ileostomy patients and antibiotic-treated rats indicate that RS to a large extent survives breakdown in the small intestine (7, 30). In the hindgut, however, the RS-kafirin complex is ex posed to microbial fermentation; RS serves as energy substrate and undigested kafirins as nitrogen source. The fact that kafirin is transformed into bacterial biomass can be seen by the more significant effect of cooking on digestibility of the essential amino acidsâ€” lysine and threonineâ€”than of the nonessential amino acidsâ€”glycine and proline. Moreover, recovery in feces of galactose was appreciably higher (78-100%) when DW to diets rather than DW flour diets were fed (25% ). High levels of galactose (31) and lysine and threonine (32) are found in bacterial biomass. It is surprising to find that RS is more resistant to microbial breakdown than NCP cell wall polysaccharides. This indicates that very strong intermolecular bindings are involved in recrystallized RS complexes. The NSP and lignin content and composition of WGR are essentially those reported by Bach Knudsen and Munck (4) and Nyman et al. (33). As a result of decortication, pericarp, testa and aleurone tissues are re moved, as shown by the fact that the DW flour con tained only traces of lignin and only 20.9 g/kg of NSP. In particular, cellulose, arabinose and xylose residues of NCP are much lower in DW than in WGR flour. This is in good agreement with the general composition of lignified cell wall materials of cereals. In lignified cell walls high levels of cellulose, arabinose and xylose res idues are found (34).

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very unspecific procedure that can give recoveries in feces above 100% even if foods without polyphenols are fed (9). The polyphenols complex with dietary protein and in this way change the excretory route for nitrogen from urine to feces. This is seen when comparing the nitro gen-balance data for WGR flour with those from DW flour. Net protein utilization of the two flours is the same. However, TD of WGR flour is significantly lower than that of DW flour, whereas the opposite is the case with BV. Moreover, the effect of polyphenols on nitro gen balance is even stronger when the polyphenols are dispersed from the cell wall matrix during cooking and come in contact with the protein. The result is a strong reduction in TD, the effect being progressively stronger for proline and glycine residues than for lysine and threonine. Because of this selective effect of the polyphen ols, which has also been identified in other studies (5, 38), BV increases. In the latter situation, however, the increase in BV could not counteract the decrease in TD. One can only speculate whether the nonessential amino acids limit the utilization of essential amino acids and

thereby the utilization of total nitrogen when digesti bility is as low as in the present study. Neither in this study nor in the study of Bach Knudsen, Munck and Eggum (5) were we able to identify any beneficial effect of varying pH on protein balance data. An in vitro pepsin digestibility method has been in troduced as a screening method for protein digestibility in vivo in sorghum (23, 39). Although the absolute in vitro digestibility values are quite different from those found in vivo, the pepsin method correlates reasonably well with in vivo values. The same was found when correlating ADFp with TD. The reason is that the res idues after pepsin digestion and ADFP measure kafirin protein complexes formed during cooking (5, 29). How ever, it is surprising that the amount of protein asso ciated with TDF is so poorly correlated with TD even though physiologically relevant enzymesâ€”pepsin and pancreatinâ€”were used to remove proteins. CONCLUSION In recent papers (4, 5, 40), we have discussed the nutritional implications of processing sorghum into foods. Four dietary factors, DF, RS, undigested protein (kafirins) and polyphenols, were considered. These fac tors explain, first, the differences in digestibility be tween various raw materials and, second, the slight inferiority in TD and digestible energy found in cooked sorghum foods. We strongly believe that RS and en dosperm protein (kafirins) are responsible for the low protein and energy digestibility of cooked sorghum por ridges reported in experiments with preschool children (3). ACKNOWLEDGMENTS The authors thank Birgit S. Jensen for excellent tech nical assistance and Ingeborg Jacobsen of the National Institute for Animal Science for cooperation in the course of the work. LITERATURE CITED 1. MACKEY,J. (1981| Cereal production. In: Cereals: A Renew able Resource, Theory and Practice (Pomeranz, Y. & Munck, L., eds.), pp. 5-23, American Association of Cereal Chemists, St. Paul, MN. 2. SCHEURING, J. F., SiDffiE, S. & KANTA,A. (1982) Sorghum alkali to: quality considerations. In: Sorghum Grain Quality (Rooney, L. W., Murty, D. S. & Merlin, I. V., eds.|, pp. 24-31, International Crop Research Institute for Semi-Arid Tropics, Hyderabad, India. 3. MACLEAN, W. C., JR.,DEROMANA, G. L., PLACKO, R. P. & GRAHAM, G. G. (1981| Protein quality and digestibility of sorghum in preschool children: balance studies and plasma free amino acids. /. Nutr. Ill: 1928-1936. 4. BACHKNUDSEN, K. E. & MUNCK,L. (1985| Dietary fibre con tent and composition of sorghum and sorghum-based foods. /. Cereal Sci. 3: 153-164. 5. BACHKNUDSEN, K. E., MUNCK,L. & EGGUM, B.O. (1987| Effect


The primary cell walls of endosperm are much more susceptible to microbial breakdown than the lignified cell walls of pericarp and testa. This conclusion can be drawn by comparing the NSP digestibility of DW flour (72% ) with that of WGR flour (26% ). The reason is that cellulose and NCP in pericarp and testa are cross-linked to lignin, which reduces digestibility of these cell wall constituents. This is in agreement with a study of Nyman et al. (35) that compared 65% with 100% extracted flour of six different cereals. In the low extraction flour NSP digestibility was higher than that of the high ex traction flour. The lower digestibility of cellulose than of NCP is explained by structural differences. Cellulose consists of a single type of polymer, a (lâ€”Â»4)-ÃŸ-D-glucan with a high degree of polymerization. In the cell wall, cellu lose is present in the form of fibrils that have a highly ordered structure with long regions possessing crystal line characteristics. The configuration of the cellulose chains also permits a very close packing of the chains within the fibril and there is a high degree of hydrogen bonding. On the contrary arabinoxylan, the major polysaccharide constituent of NCP in cereals (34), car ries side chains of several different kinds. This makes the arabinoxylan residue less rigid, and hence easier to degrade than celluloses. The same tendency was found in a compilation of ten digestibility trails with humans (36). Lignin resists bacterial degradation in the large in testine (35, 37). However, the fecal lignin values found in the present study when WGR diets were fed were much greater than those in the diet. This is undoubt edly due to the interaction of polyphenols with fecal nitrogen, which then is recovered gravimetrically in fÃ¨cesas lignin. In this discussion it should be stressed that the measuring of lignin as "Klason lignin" is a

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