The effects of various thermal processes on protein quality, vitamins and selenium content in whole-grain wheat and white flour

Journal of Cereal Science6 (1987) 269-282

The Effects of Various Thermal Processeson Protein Quality, Vitamins and Selenium Content in Whole-grain Wheat and White Flour* B. HA.KANSSONH, M. JAGERSTADt, R. OSTEt, B. A.KESSON§ and L. JONSSONII

t University ofLund, ChemicalCentre, Departmentof AppliedNutrition, P.O. Box 124, S-22100 Lund, Sweden,§ University Hospital, Departmentof Clinical Chemistry, Lund, Swedenand II The SwedishFood Institute (SIK), Gothenburg,Sweden Received2 January 1987

Whole-grainwheat and white flour from wheat were subjectedto various thermal processes- steam flaking, autoclaving, popping, extrusion'cooking, and drumdrying - underboth mild and severeconditions.The effectsof processingon protein quality were evaluated(by comparing biological and chemical methods), and on retentionof vitamins (vitamin E, thiamin and folacin) and selenium.Proteinquality wasaffectedmostby poppingandautoclaving.Changesin biologicalvaluecorrelated significantly (P < 0,001) with available lysine measured according to both the fluorodinitrobenzene(FDNB) and the dye-binding procedure.Popping and autoclavingundermild andsevereconditionsalsoresultedin thegreatestlossesof thiamin and folacin. Vitamin E activity, however, was affected most in white flour after extrusioncooking and drum-drying, which destroyedthe major part of the vitamin E activity. Selenium retention was practically unaffected by the various thermal processes,except by drum-drying, which causeda 24% loss. No thermal process equally preservedprotein quality and process-sensitive vitamins. However, the lowtemperatureprocesses,like steamflaking and drum-dryingor thc high temperatureshort time processextrusion cooking, generally, with the exceptionof Vitamin E, retainedthe nutrient quality betterthan popping and autoclaving.

Introduction Cerealsareimportantcomponentsof man'sdiet providingmanynutrients:viz. protein, complex carbohydrates,certain vitamins and inorganic nutrients.However, it is well known that the nutrientcontentof cerealproductsis dependento n the milling method, extentof fortification and type of processing. The protein quality of cerealsis considerablylower than that of milk or egg protein,

* This paper is part III of a seriesof paper reporting physical, chemical and nutritional propertiesof processedwheat products,part II, J. Cereal Sci., 5 (1987) 67-72. ~ To whom correspondence should be addressed. Abbreviations used: FDNB = fluorodinitrobenzene;DBL = dye-binding lysine; DNP-Iysine= dinitrophenyl-lysine;DE = digestibleenergy;TD = true digestibility; BV = biological value; NPU = net protein utilisation; GE = gross energy; T = tocopherol; T-3 = tocotrienol; S-aa= sulphur-containing amino acids; Av.Lys = availablelysine; HPLC = high-performanceliquid chromatography. 14 $03.00/0 0733-5210/87/060269+

© 1987 AcademicPressLimited

270

B. HAKANSSON ET AL.

showing a biological value (BV) of only about60 % of that of animal protein1,2.The lower biological value of cerealproteinis dueto the low contentof the essentialamino acid, lysine. Lysine is not presentuniformly in the cerealgrain. Its concentrationis higherin theouterlayer(thealeurone)of thegrain,andthereforethe BV of cerealprotein decreaseswith decreasingmilling extractionrate1 ,2. In addition, various types of processingmay further decreasethe biological value of cerealproteins,mainly due to the Maillard reaction. Maillard-reactedlysine is no longer biologically available3• Cerealsare important-sourcesof vitamin E, thiamin and folacin along with several other B vitamins, exceptvitamin Bl2' The vitamins occur mainly in the bran and germ fractions. In the production of white flour the vitamins in the whole-grain are fractionatedinto the bran andgermfractionsso thatwhite flour haslessthanhalf the levels of vitaminspresentin thewhole-grain4- e, In Sweden,white flour is fortified with thiamin, riboflavin, niacin and pyridoxine. Other important vitamins including vitamin E and folacin are,however,not compensatedfor, althoughthesetwo vitamins andthiaminare the most process-sensitive vitamins. In the Scandinaviancountries,cerealsaregrown on selenium-deficientsoils, resulting in cereal products with extremely low selenium contenf,B. In addition, there are reports9,lO that selenium mightalso be lost during processing,therebyfurther lowering the seleniumconcentrationin the foods. Our investigationis part of a co-operativestudyll,12, of widely usedindustrial processesfor the manufactureof cerealproducts,suchas gruels, porridge and breakfast cereals.Whole-grain wheat and white flour have been subjectedto various thermal processesthatincludesteamflaking, autoclaving,popping,extrusioncookinganddrumdrying. Each processhas been carried out under mild and severeconditions, correspondingto the limits for commerciallyacceptableproducts.The main purposeof our investigationwas to compareseveralcurrently-usedprocessesto determinetheir effects on nutrient quality. Experimental

Materials Wheat samplesof whole-grain and white flour (70 % extraction) were from the samematerial describedpreviouslyll. No supplementationso f any nutrientsor food additiveswereincluded. Eachof the two raw materialswassubjectedto two or threethermalprocesses:the whole-grain wheatwas subjectedto steamflaking, autoclavingand popping, and the white flour to extrusion cooking and drum-drying as describedin detail previouslyll and summarisedin Table I. The processedmaterialwasstoredat -40°C until analysed.Theanalyseswereperformedwithin a few weeksexceptfor vitamin E, which was carriedout after 15 months.

Analyses Digestible energy(DE). Energy balancestudieswere performedwith weanling Sprague-Dawley rats in groups of five for a period of 5 days in parallel with the nitrogen balanceexperiments describedbelow. Gross energy (GE) of the dried diet and lyophilised milled faecespowder of Sprague-Dawleyrats were assayedby completecombustionusing a bomb calorimeter(Gallenkamp,Ballistic Bomb Calorimeter).No correctionsweremadefor dietaryfibre presentin the diet or faeces,but DE of the diet wascorrectedfor thatpart of the diet not originatingfrom the cereal sample,e.g. vitamin and mineral mixtures and additional sucrose.

NUTRIENT QUALITY OF PROCESSEDWHEAT CEREALS

271

TABLE I. Processingconditionsfor the whole-grainwheatand white flour samples(for full details, seeref. 11) Processingconditions

A. Whole-grainwheat Steamflaking Moisture (%), before Moisture (%), after Final temperature(0C) Processtime (min) Thicknessof flake (mm)

Mild

Severe

18·4" 10·0 89 18 0·7

27·6" % 79-81 25 0·35

Autoclaving Moisture (%), before Moisture (%), after Final temperature(0C) Processtime (min, heating+hold) Pressure(MPa)

13-3 8·8 130 7+20 0'20

14-4 150 20+5 0'29

Popping Moisture (%), before Moisture (%), after Final temperatureCOC) Processtime (min) Pressure(MPa)

13-3 8·6 240-260 10 0-69

13·3 9'3 240 13-15 1-37

B. Whiteflour Extrusion cooking Added water (g/100g) Moisture (%), before Moisture (%), after Temperature(0C) Feedrate (g/min) Screwspeed(rimin) Pressure(MPa)

10 24·6b 9'7 148e 318 200 4'14e

0 14-6 9'2 197e 318 200 2'41e

Drum-drying Moisture (%), before Moisture (%), after Processtime (min), suspension Processtime, drying Steampressure(MPa) Drum speed(r/min)

83b 3'7 35 60 0·69 13

77b 2'0 35 55 0'98 5

22"

" Moisture in raw material was 13'3%. b Moisture in raw materialwas 14'6%. e Mass temperatureand pressurewere measuredin the materialflow closeto the die inside the barrel.

272

B. HAKANSSON ET AL.

Protein quality. Nitrogen balanceexperimentswere performedwith weanling Sprague-Dawley rats in groupsof five describedpreviouslyby Nyman et al. u , with the exceptionthat the sucrose in the white flour diets was excluded.Nitrogen in diet, urine and lyophilised milled faeceswas measuredwith the Kjeldahl method using TecatorKjeltec equipment.True digestibility (TD), biological value(BV) and net protein utilisation (NPU) werecalculatedas describedby Eggum13• Availablelysine.Two methodswereusedto estimatethe availablelysine: the fluorodinitrobenzene (FDNB) and dye-binding lysine (DBL) procedures.The former was performed according to Booth14, but modified as describedby Noguchi et al. 15 Correction factors for loss of the DNPlysine during the reflux digestionwere not applied.The methodof Hurrell et al. 16 was usedin the latter procedure.Before analysisthe sampleswere milled during chilling in a Retschcentrifugal mill with 0'5 mm screenas proposedby Tameet alY All the analyseswere run in duplicate. Amino acids. Amino acid analysis after ion-exchangechromatographyof acid-hydrolysed,unoxidisedandperformic acid-oxidisedprotein wasperformedat the BiomedicalCentre,Uppsala, Sweden.Tryptophanwas measuredby the procedureof Osteet al. 18 Vitamin E. Tocopherolsand tocotrienolswereextractedfrom the cerealsamplesusing a saponification method,describedby Piironen et al. 19 Analysis of the various forms of tocopherolsand tocotrienolswereperformedby HPLC (Varian,M-5020B) with fluorescencedetection(Schimadzu, SpectrofluorometerRF-540).The excitationwasset at 290 nm and the emissionat 330 nm (both slits at 10 nm). The HPLC-columnwasa Varian Micro PakSi-5 (300 x 4 mm) andwaselutedwith hexane:diisoprophylether(93:7) at aflow rateof 3·0 ml/min. Thequantityof thetocopherolswas calculated from standardsusing DL-ct-Tocopherol(Merck) and RAC-P-Tocopherol(Supelco). Pure ct- and P-tocotrienolswere not available and were therefore quantified using the corre20 • All the analyseswereperformed spondingtocopherolstandards,asthesegive thesameresponse in duplicate. Thiamin. Assay of thiamin was carried out spectrofluorimetricallyusing thiochrome. Both extractionandassayof thiamin wereperformedaccordingto the AOAC method21 andin triplicate. Folacin. To extract folacin, samples(1 g) were suspendedin 0'1 M Na-phosphatebuffer, pH 6'0, (25 ml) containingfreshly addedascorbicacid (225 mg/IOO ml buffer). A suspension(100 ~l) of Termanyl(Novo, 120 L, Bacillus lichen/armis) wasaddedand the mixture was boiled for 10 min. Theboiled mixture wascentrifugedandanaliquot of thesupernatantwasuseddirectly for analysis of free folacin (seebelow). To hydrolyse the polyglutamateforms of folacin, samples(I ml) of the supernatantwere immediately treated with a solution (0'1 ml) containing 5 mg of lyophilised chicken pancreas (Difco) in 0'05 M Na-phosphatebuffer, pH 7'0, and centrifuged(4000 revImin x 10 min). After incubationfor 2 h at 37°C, the hydrolysis was stoppedby boiling for 5 min. Freeandtotal folacin wereanalysedusing Lactobacilluscasei(ATCC 7469)asthe testorganism accordingto a previousdescription22• Correctionswere madefor folacin presentin the enzyme preparations,Termamyl and the chickenpancreassuspension.All the sampleswere analysedin duplicate using two different amounts(dose-response) from a standardcurve of pteroylmonoglutamate.The assaywas repeatedonceand meanvaluescalculated. Selenium.Seleniumwas determinedin duplicatesamples(0'5 g) of material using a modification of the fluorimetric method describedby Koh and Benson23 • The NBS Bovine Liver Standard ReferenceMaterial l577a, used to check the validity, contained 0·72 ~g Se/g (given value 0·71 I!g/g).

Statistics Student'st-testwasperformedto determineif thechangesin digestibleenergyandproteinquality, as measuredbiologically, were significantly different from unprocessedmaterial.

273

NUTRIENT QUALITY OF PROCESSEDWHEAT CEREALS

Pearsoncorrelationcoefficients24 (one-tailed,12 observations)were usedto evaluatethe relationship betweenbiological value of the protein quality and analysesof availableand total lysine. Resultsand Discussion

Digestible energy(DE)

Table II showsthat DE was higher for the white flour comparedwith the whole-grain wheat(95 and 87 % respectively).Thesefigures agreewith resultsreportedby Pedersen and Eggum2 • The lower digestibility of whole-grainwheatis due to a higher contentof dietaryfibre oflow digestibility comparedwith thewhite flour. The 8 % higherdigestible energyof white flour is due to its high starchand lower dietary fibre content2,1l. TABLE II. Digestibleenergy,true digestibility, biological value and net protein utilisation of raw and processedwheat Processmaterial"

DE±s.D.

TD±s.D.

BV±S.D_

NPU±S.D.

(%)

(%)

(%)

(%)

Whole-grain wheat

Raw Steamflaked, m Steamflaked, s Autoclaved,m Autoclaved,s Popped,m Popped,s

87-6±2·0 88-0±1'4 88'O±O'7 89'8±O'1>I< 89'8±O'9>1< 85·9±1-0 84'5±1-3**

86·1 ±4'9 94'2±1'7""" 93·1± 1-6>1< 90·8±O·6 89·1 ± 1·4 86'5±2'5 74·9±1'9>1<>1<

63-3± Jo6 60'5±2'5 60'7±1,1* 53-6±2,2*** 56'1±5-2>1< 46·9± 4'1>1<>1<>1< 41'1± 1·9>1<>1<>1<

54·6±4·2 57'O±2'6 56-5±O'8 48'7±1'7>1<

95-4±O'4 95-6±O'7 95-6±0'3

92·1±2-4 95'9±1'7* 95'2±1·3>1< 93·7±3·0 95·0±O·I>1<

46·7±2·& 46-4±2'& 47'O±2'3 56'8± 3'9>1<>1< 52'O±1,9**

43·0±3·2 44'5±2'& 44'8±2'7 53'2±4'1*>I< 49'4±1-8**

50'O±S'3

40'6±4'1*>I<>I< 30·8±2'0***

Whiteflour

Raw Extruded,m Extruded,s Drum-dried,m Drum-dried,s

* Significantly different from the correspondingraw material.

* p < 0'05, ** P < 0,01, *** P < 0·001.

" m andsrefer to mild andsevereprocessing. The thermalprocessesh ad very little influenceon DE. Autoclavingincreasedthe DE contentof the material,while poppingdecreasedDE significantly, but not more than a few per cent.The processingseemsto havemuchgreaterimpact on proteindigestibility (TD) comparedwith DE, although both showedchangesin the samedirection (see below). Protein quality True digestibility (TD). Resultsof the nitrogenbalanceexperimentson growing rats are shown in Table II. Mildly heat-treatedwhole-grainwheatwas significantly (P < 0'05) more digestiblethan the raw sample.This is consistentwith the suggestionsthat pro-

274

B. HAKANSSON ET AL.

teolytic enzymeinhibitors are presentin unheatedwhole-grainwheat26 • Strongerheat treatment,like popping, reducedthe digestibility (P < 0'01). The digestibility of white flour was, in general,somewhathigher than that of whole-grainwheat. A small, but statisticallysignificant,increasein digestibility uponheatingthe white flour proteinwas also observed,suggestingthe presenceof enzyme inhibitors also in the endosperm fraction of the grain. Thereducedproteindigestibility of theseverelypoppedsampleis a featurecommonly observedwith strongly heatedproteins26 •27 • Formationof cross-linkagesin the protein molecularstructureshavebeenreported27 and may lead to incompletedigestiondueto reducedavailability of adjacentamino acids. In addition, someMaillard reactionproducts may interfere with the digestiveprocess26 , most likely due to inhibition of proteolytic enzymaticactivity28 and reduceddigestionof both exogenousand endogenous proteinsin the intestine. Biological value(BV), netprotein utilisation (NPU), availablelysineandfreeaminoacids. Among the processesapplied on whole-grain wheat, popping under both mild and severeconditions resultedin the greatestdecreaseof BY and NPU (P < 0,001). Unprocessedwhole-grainwheatshoweda BY of 63·3 %, which aftermild andseverepopping had decreasedto 46·9 and 41·1 % respectively.Correspondingfigures for NPU was a changefrom 54·6 to 40·6 and 30'8% respectively(Table II). Lossesof lysine measured chemically,eitheras total lysine or availablelysine (FDNB or DBL), rangedfrom 30 to 60% (Table III). Thus, decreasein protein quality was considerablyhigher when measuredchemically comparedwith biological evaluation. When feeding very low quality proteinwith lysineasthelimiting aminoacid,asin this case,BV will bemeasured 29 at maintenancelevel and may thus be overestimated , becausethe reductionof other essentialamino acids was comparativelylower. Severepopping lowered the concentration of the essentialamino acids by the following figures: lysine (60 %), sulphurcontaining amino acids (30 %), tryptophan (27 %), threonine (15 %) and histidine (14 %), while isoleucine, leucine, valine and phenylalanine+ tyrosine levels were unchanged.O therprocessesdid not affectthecontentof the aminoacid analysedafteracid hydrolysis (Table III). Autoclavingundermild andsevereconditionsdecreasedthe BY from 63'3 to 53·6and 56·1 % respectively,whereasNPU decreasedfrom 54·6 to 50·0 and 48·7% respectively. Steamflaking reducedBV from 63·3 to 60'5 and 60·7%, but NPU was unaffected.The decreaseof BV during autoc1avingand steamflaking correspondedto a slightly more pronounceddecreasein total lysine, FDNB-Iysine and dye-bindinglysine (Table III). Autodaving, especially,decreasedthe lysine level (as measuredby dye-binding) to a greaterextentthanexpected,indicatinglossesbetween30 and40 %, whereastotal lysine andFDNB-lysineindicatedlossescomparableto thoseshownby the biological method. White flour was processedby drum-drying and extrusion cooking. Drum-drying raisedthe BY of white flour from 46·7to 56·8 and52·0% aftermild andsevereprocessing respectively.Also NPU increasedsignificantly (P < 0'01)ascomparedwith theuntreated sample.On the otherhand,total lysine andFDNB-Iysinewasunaffectedby both drumdrying and extrusioncooking,whereasthe dye-bindinglysine methodindicatedlosses. Thus, no apparentexplanationof the increasein BY and NPU after drum-drying is available.

TABLE III. Essentialamino acids and availablelysine of raw and processedwheat (gj16g N) Z

Available lysine Processmaterial& Whole-grainwheat Raw Steamflaked, m Steamflaked, s Autoclaved,m Autoclaved,s Popped,m Popped,s

His

Ile

Leu

S-aabb

Phe+ Tyr

--~---

Thr

Trp

Val

Lys

FDNB"

DBL d

c:::

-l

:;tJ

til

Z

-l ,Q

c:::

2-2 2-3 2-2 2-2 2-2 2-2 1-9

3-4 3-4 3-4 3-4 3-4 3·5 B

6·7 6·9 6-8 6·8 7·0 6·9 6-5

4·4 4·5 4-7 4-3 4-3 4-0 3·1

7-3 7·5 7'5 7-5 7-6 7-6 7-3

B B B B 3-4 3-3 2·8

1-1 1·2

1-1 1-3 1-3 1-2 0·8

4-3 4·7 4-6 4-6 4·5 4-6

4-4

2·8 3·0 2-9 2-5 2·7 2·0 1·1

2·8 2·6 2-4 2-6 2-4 1·9 I-I

3·8 4·0 3-8 2'2 2·7 2-3

1·2

>-

l'

~ 0

"I1

"t:l

:;tJ

0

()

m C/.l m

V-l

Whiteflour Raw Extruded,m Extruded,s Drum-dried,m Drum-dried,s

2-0 2-3 2-3 2-2 2-2

3'5 3-7 3'8 3-6 3-7

&m ands refer to mild andsevereprocessing. Sulphur-containingamino acids(cys+meth). C Flnorodinitrobenzene_ d Dye-bindinglysine.

I>

7·0 7-1 7-3 7-0 7-0

4'5 4-3 4'3 4-4 4-4

7-8 7-4 7·6 7-4 7-4

3-1 2·8 3·0 2·8 2·8

1-2 H H H 1·2

4-2 4·3 4'3 4·1 4·2

2-1 2·1 2'1 2·2 2·1

2-2 2-2

2-3 2·4 2·3

2-8 2·3 2'5 2·6 2-2

t:J

::;:: :r: m

>-l

()

m

:;tJ tTl

>-

l' C/.l

tv

--l VI

276

B. HAKANSSON ET AL.

Thelossesof availablelysinein thevariousthermalprocessesthatwe haveinvestigated here might well be attributedto the Maillard reaction3. Reactionrate is increasedby high temperatureand low water activity, conditionswhich areespeciallycharacteristic of the poppingprocedure. Oneof theaimsof thepresentstudywasto comparethechemicalmethodsof analysing total andavailablelysine with the biological methodto evaluateprocess-induced effects on proteinquality. Thecorrelationcoefficientsshowedthe following values:BV against total lysine (r = 0'89; n = 12; P < 0'001); BV againstFDNB-lysine (r = 0'80; n = 12; P < 0'001) and BV againstDBL (r = 0'84; n = 12; P < 0'001). Also the threechemical methodsof analysinglysine were significantly correlated(P < 0·001); only the correlation betweenFDNB-lysineand DBL was slightly weaker(P < 0,01). Thus, both total lysine and availablelysine measuredwith the FDNB or DBL procedurecould be used asvaluablescreeningmethodsto estimatedamageof proteinquality in processedcereals. The DBL procedureis, however,less time consumingand uses lessnoxiouschemicals. The DBL procedure,on the other hand, showedhigher lysine concentrations,in some samplesabout 30 % higher than correspondingvalues measuredas total lysine or availablelysine accordingto the FDNB procedure.

Vitamin E Among the eight naturally occurring forms of tocopherolsand tocotrienols,four are representedin wheatcereals,namely (j,- and ~-tocopherol (a-T, ~-T) andcorresponding tocotrienols(a-T-3, ~_T·3)3o,31. Thedistribution of theseforms in the samplesareshown in Table IV. Unprocessedwhite wheat flour (70 % extraction) contained only 35-70% of the vitamin E presentin untreatedwhole-grainwheat (Table IV). Among the vitamin E isomers,a-T-3 was affectedmost; which accordswith the observationthat 80 % of the total a-T-3 in whole-grainwheatis locatedin the bran32. The concentrationof vitamin E for the untreatedwhole-grainis in agreementwith otherreports30,33. In white flour it is more complicatedto comparethe values with others becausethe amountsof the variousE vitaminsare highly dependenton the proportionof the total amountof germ tocopherolpresentin the white flour. The germ is extremelyrich in (t-T and ~_T32. Processingwhole-grainwheatundermild conditionsdid not reducethe amountsof any of the tocopherolsandtocotrienols.However,steamflaking andpoppingperformed under severeconditions resultedin 40-45% lossesof a-T and ~- T. Although steam flaking is a relatively mild heatingprocess,the substantiallossesmay havebeendue to great mechanicaldisruption during processing,providing a greater opportunity for enzymicactivity, which in turn may acceleratethe oxidation process34. Processinghad lesseffect on the concentrationof tocotrienols,with exceptionof steamflaking, where the concentrationof (j,- T-3 increased35 %. At present,we haveno otherexplanationof this increasedvalue otherthan that theremight havebeena concentrationof the bran when milling the sample. Extrusioncookinganddrum-dryingof the white flour greatlyreducedthe contentof tocopheroland tocotrienol. Only 15 % or less of (j,-T and a-T-3 were retainedin these samples,comparedwith the unprocessedwhite flour. The lossesof ~-T and ~-T-3 were lower, amountingto between63 and 88 %.

TABLE IV. Contentsof vitamins and seleniumof raw and processedsamplesof whole-grainwheat and white flour (Ilg or mg/IOO g dry matter) Z C

Vitamin Eb

Processmaterial&

ex-T (mg)

ex-T-3 (mg)

~-T

(mg)

~-T-3

(mg)

Thiamin (Ilg)

Free (Ilg)

Total (Ilg)

o-l

Folacin Total ex-T-equiv_ (mg)

:;tl tt1

Selenium (lJ.g)

Z

o-l

I:)

C

;J> l'

Whole-grain wheat Raw Steamflaked, m Steamflaked, s Autoclaved,m Autoclaved,s Popped,m Popped,s

1·79 1-68 H4 1-73 1-57 1-74 1-17

1·23 H6 0·67 1-19 HO 1-20 0-74

0-47 0-45 0·64 0'44 0-39 0'42 0-38

0-64 0-60 0-36 0·61 0-55 0-62 0-39

3-24 3-01 2-12 3-19 2-69 3-35 3-21

380 310 310 20 140 20 0

38 32 24 32 28 29

White flour Raw Extruded,m Extruded,s Drum-dried,m Drum-dried, s

0-81 0-17 0-09 0-04 0-08

0-52 0-08 0-03 0-01 0-03

0-16 0-02 0·01 < 0·01 0-01

0-33 0-\1 0-07 0-04 0-07

2-28 0·84 0-52 0·24 0-43

240 140 100 250 230

21 18 17 17

11

46 37 29 31 28 34 13

2-38 2-42 2-36 2-46 2·34 2-46

27 22 21 21 20

2-24 2-04 2-08 1-70 1·70

-

c

';:j

-<

0

"I1

'"tl :;0

0

(')

tt1

en en tt1

t:l ~

16

• m and s refer to mild and severeprocessing. ss b a-T-equiv_ = mg of a-tocopherolequivalentscalculatedaccordingto McLaughlin and Weihrauch , T = tocopherol,T-3 = tocotrienoJ. C Not analysed.

::x::

m ;J> o-l

(')

tt1 :;tl

m ;I>-

r

en

tv ....., .....,

278

B. HAKANSSON ET AL.

As vitamin E seemedto be less processstablein white flour comparedwith wholegrainwheat,it might besuspectedthattheparticlesizeof theraw materialsis influential. For instance,there is a possibility that vitamin E in white flour was more exposedto catalyticoxidisers,suchasiron andcopper,originatingeitherfrom the processingwater or from the processingequipment. The vitamin E assayswere performedon materialstoredfor 15 monthsat -40 °e. However,recentstudies,to bereportedelsewhere,haveshownthatwheatflour doesnot lose significantamountsof vitamin E activity overlong periodsat - 40 °e and that the loss of vitamin E occursduring drum-drying Thiamin

The thiamin contentin the samplesis alsoshownin TableIV. Unprocessedwhole-grain wheatcontained0·38 mg thiamin/100g, which is closeto the value of 0·4 mg given by food tables.The white flour containedonly 0·24mg/IOO g dry matter,indicating that 37 % was presentin the aleuronelayer. No fortification of any B vitamin to this white flour was doneprior to the processes. In general,theprocessingconsiderablyreducedthe amountof thiamin in whole-grain wheat,aswell asin white flour. Thelossesweresubstantial(63-100%) after autoclaving and poppingunder both mild and severeconditions.Surprisingly,the loss of thiamin was higher during mild autoclavingcomparedto severeautoclaving.In the latter case, thewhole-grainwheatwassoakedin waterbeforeautoclavingandthis resultedin higher moisturecontentof this material.Lossof thiamin is sometimesassociatedwith Maillard reactions.Interestingly, the mild autoclavingof whole-grain wheat resultedin lower proteinquality thandid severeautoclaving,indicatingthatmild autoclavingaffectedthe Maillard reactionmore,perhapsdueto morefavourableconditionswith regardto water activity. Extrusion cooking resulted in losses of 42 and 58 % for mild and severe conditions respectively.Steamflaking and drum-drying had a smaller effect on the thiamin content- up to 18 % of thiamin was lost. The results accord with the well 35.36. Thus, the known process sensitivity of thiamin, especially for heat-treatments degreeoflossesobservedin ourexperimentsmayreflecttheintensityof heating,although someinfluenceof the water activity is quite possible,too. Folacin

The total folacin valuefor whole-grainwheat,asshownin TableIV, is lower compared with others (46 J..lgjlOO g vs. 50-67 J..lg/IOO g)37-39. The free and total folacin values obtainedfor unprocessedwhite flour with an extractionrate of 70 %, however, agree with valuesobtainedin other investigations(17-32J..lgjlOO g). All the thermal processesdecreasedthe total folacin contentby at least 20 %, even underthe mild processingconditions.High-temperatureprocesseslike autoclavingand poppingreducedtotal folacin by 39 and72 % respectivelyunderthe intenseconditions and 33 and 26% respectivelyunder the mild processingconditions. Steamflaking of whole-grainwheatinducedlossesbetween20 and 37 %. Extrusioncooking and drumdrying of white flour showeda differenceof only 3 to 4 % in lossesbetweenmild and

TABLE V. Reductionin nutrient quality expressedin percentageof raw materialsafter mild and severeprocessingrespectively

~

Proteinquality Processmaterial"

Temp eC)

Time (min)

Whole-grain wheat Steamflaking(m - s) Autoclaving (m - s) Popping(m - s)

89 - 80 130- 150 250-240

18 - 25 27 -25 10 - 14

Whiteflour Extrusion cooking (m - s) Drum-drying (m - s) Milling b

Total lysine

Total a-T-equiv.

Thiamin

Folacin

Selenium

4-4

0-0 11 -4 29 - 61

6- 36 3 -12 3 -35

18 - 18 95 - 63 95 - 100

20-37 33 -39 26-72

0-2 0-2 0- ?~

~

§ o 'Tl

"" o

;l:l

<"'l

gJ

en

148 - 197 ? -

<5 60 - 55 -

• m ands refer to mild and severeprocessing. b Fractionatingwhole-grainwheatto white flour (70%). Not analysed. C

~ o

Biological value

15 -II 26 - 35

zc::

1-0 0-0

0-0 0-0

79- 89 95 -90

42-58 0-4

19 - 22 22-26

9-7 24-24

26

25

55

37

41

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B. HAKANSSON ET AL.

intenseconditions,which did not exceed26% totally (Table IV). It is well known that folacin is mainly destroyedby oxidation, especiallyat temperaturesexceeding100 °e. Lossesof folacin duringheatingcanbepreventedin thepresenceof ascorbicacid.Wheat cerealsare, however, poor sourcesof ascorbicacid. Another sourceof folacin losses could be by leaching;but sincethe wateraddedto someof the processeswas eliminated by evaporation,suchlosseswere unlikely to occur. Selenium

Steamflaking, autoclavingor poppingof whole-grainwheatdid not changethe selenium contentsignificantly (TableIV). Extrusioncooking reducedthe seleniumconcentration by lessthan 10 %, but drum-dryingdecreasedit by approx25 %. Therewasno consistent differencebetweenthe mild and the intenseprocessingconditions. Seleniumin wheatis to a largeextent,boundin proteins,anduponhydrolysisof highseleniumwheat, selenomethionine,b ut not selenocysteinewas a major component40 • Severalunidentified seleno-compoundswere observed.In anotherstudy, wheat flour contained14 % lessseleniumthantheraw wheat,but no changein seleniumcontentwas observedowing to processingwhentwo shreddedwheatcerealswereprepared41 • Drying a wheat cereal overnight at 100 e causeda 23 % loss of selenium, but boiling for 20 min resultedin no losso.YHi rantafound that4 % of the seleniumin rye grasswas lost upon drying at 100 °e for 4 days, and the loss increasedup to 20% at 200 °elO • 0

Conclusions The thermal processing-inducedlosses in protein quality and in concentrationsof vitamin E, thiamin, folacin and seleniumin whole-grainwheat and white flour (70 % extraction)aresummarisedin TableV. Datais given as rangebetweenmild and severe processingconditions,togetherwith valuesfor processingtimes and temperatures. Our resultsconfirm the widely acceptedview that when cerealproductsare expected to maintaingoodproteinquality andacceptableretentionof water-solublevitamins, the processesselectedshould be those that take place at low temperaturesor at high temperaturesduring a short period of time (HTST e.g. extrusioncooking). The new resultsfrom our study may have significantnutritional implications and indicate, that major lossesof vitamin E occur during such relatively mild processesas drum-drying and extrusioncooking. Becauseof the very low seleniumconcentrationof thesecereal samplesand becausevitamin E might partially replaceseleniumas a nutrient, a higher retentionof vitamin E in processedcerealproductsis needed.This is important with respectto increasinginterestin recentyearsconcerningthe protectiverole that vitamin E and seleniumare believedto play in the ageingprocess,as well as in certaindiseases. The authorswish to thank Mrs A.-K. Westessonand Mrs M. Birger, Departmentof Applied Nutrition, and Mr A. Pettersson,T he SwedishFood Institute, for skilful technicalassistancein conductingthe analyses.We are grateful to Mrs Elvy Hokby, SemperAB, for providing data regarding the industrial processes.G rants were awarded by the SwedishBoard for Technical Developmentand Director Albert Pahlsson'sFoundation.

NUTRIENT QUALITY OF PROCESSEDWHEAT CEREALS

281

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