in nonheated doughs. Cereal Chem. 69:1-6.HIBBERD, G. E. 1970a. Dynamic viscoelastic behavior of wheat flour

doughs. Part II. Effects of water content in the linear region. Rheol.Acta 9:497-500.

HIBBERD, G. E. 1970b. Dynamic viscoelastic behavior of wheat flourdoughs. Part III: The influence of the starch granules. Rheol. Acta9:501-505.

HIBBERD, G. E. and PARKER, N. S. 1975. Measurement of the funda-mental rheological properties of wheat-flour doughs. Cereal Chem.52: 1 r-23r.

HIBBERD, G. E., and WALLACE, W. J. 1966. Dynamic viscoelasticbehavior of wheat flour doughs. Part I: Linear aspects. Rheol. Acta5:193-198.

HOSENEY, R. C., FINNEY, K. F., POMERANZ, Y., and SHOGREN,M. D. 1971. Functional (breadmaking) and biochemical properties ofwheat flour components. VIII. Starch. Cereal Chem. 48:191-201.

LINDAHL, L., and ELIASSON, A-C. 1986. Effects of wheat proteinson the viscoelastic properties of starch gels. J. Sci. Food Agric. 37:1125-1132.

MATSUMOTO, S. 1979. Rheological properties of synthetic flourdoughs. Pages 291-301 in: Food Texture and Rheology. P. Sherman,ed. Academic Press: New York.

MEDCALF, D. G. 1968. Wheat starch properties and their effect onbread baking quality. Baker's Dig. 42(4):48-50, 52, 65.

MENJIVAR, J. A. 1990. Fundamental aspects of dough rheology. Chap.1 in: Dough Rheology and Baked Product Texture. H. Faridi, andJ. M. Faubion eds. AVI Publishing: New York.

MITA, T., and MATSUMOTO, S. 1984. Dynamic viscoelastic propertiesof concentrated dispersions of gluten and gluten methyl ester:Contributions of glutamine side chain. Cereal Chem. 61:169-173.

NAVICKIS, L. L. 1989. Rheological changes of fortified wheat and cornflour doughs with mixing time. Cereal Chem. 66:321-324.

NAVICKIS, L. L., ANDERSON, R. A., BAGLEY, E. B., and JASBERG,B. K. 1982. Viscoelastic properties of wheat flour doughs: Variationof dynamic moduli with water and protein content. J. Texture Stud.13:249-264.

SMITH, J. R., SMITH, T. L., and TSCHOEGL, N. W. 1970. Rheologicalproperties of wheat flour doughs III. Dynamic shear modulus and itsdependence on amplitude, frequency, and dough composition. Rheol.Acta 9:239-252.

SZCZESNIAK, A. S., LOH, J., and MANNELL, W. R. 1983. Effectof moisture transfer on dynamic viscoelastic parameters of wheat flour/water systems. J. Rheol. 27:537-556.

WATSON, S. A. 1984. Corn and sorghum starches: Production. Pages417-468 in: Starch: Chemistry and Technology, 2nd ed. R. L. Whistler,J. N. BeMiller, E. F. Paschall, eds. Academic Press: New York.

WEIPERT, D. 1990. The benefits of basic rheometry in studying doughrheology. Cereal Chem. 67:311-317.

[Received June 29, 1994. Accepted September 22, 1994.]

BREADMAKING

Effects of Certain Breadmaking Oxidants and Reducing Agentson Dough Rheological Properties'

WEI DONG3 and R. C. HOSENEY2

ABSTRACT Cereal Chem. 72(l):58-64

A dynamic rheometer was used to characterize the effect of glutathione, interchange. Addition of potassium bromate to dough resulted an increasepotassium bromate, and two ascorbic acid isomers on the rheological in G'and a decrease in loss tangent. L-threoascorbic acid was rheologicallyproperties of wheat flour doughs. During resting after mixing (dough more effective than D-erythroascorbic acid. This was explained by therelaxation), G' decreased and the loss tangent increased. The major factor presence of an active glutathione dehydrogenase in wheat flour that iscausing those changes was suggested to be a sulfhydryl-disulfide inter- specific for both glutathione and L-threoascorbic acid.change. Free radicals appeared to be involved in the sulfhydryl-disulfide

Sperling (1986) cataloged the causes of stress relaxation intofive categories: 1) a decrease in molecular weight caused by chainscission as a result of oxidative degradation or hydrolysis; 2)bond exchanges ongoing constantly in polymers, with or withoutstress (in the presence of a stress, however, the statistical rearrange-ments tend to reform the chains, so the stresses are reduced);3) viscous flow caused by linear chains slipping past one another;4) thirion relaxation as a reversible relaxation of the physicalcross-links or trapped entanglements in elastomeric networks; 5)molecular relaxation, especially near the glass transition tempera-ture (Tg), that tends to relieve any stress of chains during theexperiment.

The role of the sulfhydryl groups in dough chemistry has

'Contribution 94-432-J. Kansas Agricultural Experiment Station, Manhattan.2

Graduate research assistant and professor, respectively, Department of GrainScience and Industry, Kansas State University, Manhattan.

3Presently at Nabisco Brands, East Hanover, NJ.

a 1995 American Association of Cereal Chemists, Inc.

58 CEREAL CHEMISTRY

attracted the attention of many cereal chemists. The main premisehas been that these sulfhydryl groups are potentially capable ofundergoing a disulfide-sulfhydryl interchange that involves thecleavage or reformation of disulfide bonds mediated by sulfhydrylgroups in flour or by relatively small amounts of added sulfhydrylcompounds.

Reduced glutathione (GSH) and oxidized glutathione (GSSG)are both naturally occurring in wheat flour (Kuninori andMatsumoto 1964, Hird et al 1968, Tkachuk 1969). Gravelandet al (1978) reported that flour contained 5-7 mmol of sulfhydrylgroups and 11-18 mmol of disulfide per kilogram. Kuninori andSullivan (1968) studied disulfide-sulfhydryl interchange in wheatflour by adding radioactive glutathione. They reported that signifi-cant interchange took place in a flour-water dough, but not ina flour suspension. They postulated that mixing promoted thereaction of disulfide groups and GSH. Another possibility is thata free radical (GS-) is formed during mixing and may be involvedin the disulfide-sulfhydryl interchange. Reaction with (GS-) cancause session of protein disulfide forming a protein thiyl radical.

When interchain disulfide bonds are cleaved, the resulting

depolymerization of the gluten proteins decreases the molecularweight and thereby reduces the elasticity and increases theextensibility of dough. Evidence for the occurrence of the inter-chain disulfide bonds in gluten, particularly in the glutenin frac-tion, have been presented by Beckwith and Wall (1966) andYoshida et al (1980). Some of these interchain disulfide bondsmay be important in maintaining the gluten structure: a reductionof 3-4% of the disulfide bond with mercaptoethanol caused adepolymerization of 80% of the high molecular weight glutenproteins (Jones et al 1974).

It is generally accepted that the rheological properties of doughand its three-dimensional network are dependent on the arrange-ment and number of disulfide bonds and sulfhydryl groups ofthe protein. The vital contribution of disulfide bonds to doughstability has been shown in rheological studies by the additionof either sulfhydryl compounds or sulfhydryl-blocking reagents.A small amount of cysteine or reduced glutathione dramaticallyincreases the extensibility of dough. Bloksma (1972) showed thatboth the viscous and elastic component of dough deformationwere increased by reduced glutathione.

Bloksma (1972) studied the relationship between the sulfhydryland disulfide contents of dough and its rheological properties.He reported that only small fractions of the total sulfhydryl anddisulfide groups were rheologically effective, and these fractionswere much smaller than the chemically reactive ones. Jones etal (1974) estimated, on the basis of farinograph measurements,that -25-35% of sulfhydryl groups and 4-13% of the disulfidebonds were rheologically effective. However, a small decreasein crosslinking was sufficient to cause a considerable rheologicaleffect because of disappearance of rheologically effective groups(Bloksma 1972). In most breadmaking processes, after mechanicaldevelopment of the gluten network, the structure must be stabi-lized by oxidants. Minute amounts of oxidizing reagents suchas potassium bromate or dehydroascorbic acid (DHAA) improvethe handling and baking characteristics of wheat flour. Loafvolume increased, and bread had a better crumb grain (Jorgensen1939). Most current theories on the improver action of oxidantsagree that sulphydryl groups are involved in the reaction mech-anism. The bromate is assumed to oxidize low molecular SH-peptides (glutathione) and consequently hamper sulfhydryl-disulfide interchange of gluten molecules (Bloksma 1972).

The sulfhydryl-disulfide interchange reaction also is used toexplain the action of ascorbic acid, but the number of stepsinvolved in its improver action are higher than with bromate.

The four stereo isomers of ascorbic acid and their dehydro-formsdisplay quite different activities as improvers. Walther and Grosch(1987) reported that the specificity of the enzyme glutathionedehydrogenase (dehydroascorbate reductase, EC 1.8.5.1) wasresponsible for the difference in the improver action. L-threode-hydroascorbic acid was the best and D-threodehydroascorbic acidwas the worst substrate of the four stereoisomers. This enzyme,which was discovered in wheat by Kuninori and Matsumoto (1963,1964), oxidizes glutathione to its corresponding disulfide withDHAA as the oxidant. The enzyme appears to be specific forboth glutathione and L-threodehydroascorbic acid. Thus, the im-prover action of L-threodehydroascorbic acid is explained by theoxidation of glutathione to the oxidized form. The decrease inglutathione decreases the rate of sulfhydryl-disulfide interchangein the dough. The order of substrate specificity of glutathionedehydrogenase for the four stereoisomers corresponds well withtheir improver action in dough (Mair and Grosch 1979).

The purpose of this study was to characterize the effect ofpotassium bromate, glutathione, and ascorbic acid on the rheologyof flour-water doughs using a dynamic rheometer.

MATERIALS AND METHODS

Commercial bread flour, 12.1% protein (N X 5.7), 0.45 ash,(14% mc) from Ross Industries (Cargill, Wichita, KS) was used.The flour was fractionated into water-insoluble and solublefractions according to the procedure shown in Figure 1. Onepart of flour was suspended in three parts of distilled water andstirred continuously for 15 min. Then the suspension was centri-fuged for 20 min at 1,000 X g. The insoluble residue, glutenplus starch, was frozen and lyophilized. The water-soluble fractionwas boiled for 15 min and then frozen and lyophilized. Boththe water-insoluble and water-soluble fractions were ground in

12

10 F

8

Flour Water1 part 3 parts

Stir15 min

Centrifuge

1000 x G - 20 min

E

z

6

0.7 H

0.6 I

0.5Water soluble Water insoluble

Boil1 15 min

LyophilizeFig. 1. Flour fractionation procedure.

0.4Lyophilize 0 60 120

TIME (MIN)

Fig. 2. Dough rheological changes during resting time. Flour-water dough(0) tested immediately after mixing or after resting in a bowl coveredwith a plastic plate at ambient temperature (230 C) for various times.

Vol. 72, No. 1, 1995 59

0 t

0

I I I I

x . |~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

a Ross Mill until they passed a I0XX sieve. The water solublesand insolubles were blended and rehydrated by holding at 85%rh until the flour reached about 14% moisture. Mixing time andwater absorption were determined with a mixograph.

All chemicals used in the study were reagent grade. Potassiumbromate was from Baker & Adamson; L-threoascorbic acid fromFisher Scientific; and glutathione and D-erythroascorbic acid fromEastman Kodak. Also obtained from Eastman Chemical Productswas Tenox-4, a food-grade antioxidant containing 20% butylatedhydroxyanisole (BHA), 20% butylated hydroxytoluene (BHT),and 60% corn oil. Solutions of each chemical were prepared freshdaily. For the interaction studies, the two solutions to be studiedwere added to the water just before mixing.

The rheometer used was as described previously (Faubion etal 1985, Dreese 1988a). Dynamic measurements were taken atan oscillation frequency of 5 Hz and 1% strain. Dough was pre-pared with optimum mixing time and absorption except wherenoted. The doughs were tested immediately after mixing or afterresting in a bowl covered with a plastic plate at ambient tempera-ture (230C) for 60 min, 120 min, and 180 min. The results arepresented as plots of storage moduli G' and loss tangent versusstorage time. Data were evaluated using the Statistical AnalysisSystem (SAS Institute, Cary, NC).

RESULTS AND DISCUSSION

Effect of Resting TimeWhen a flour-water dough was rested at room temperature

for 180 min, it became softer. G' decreased, while the loss tangentincreased (Fig. 2). During resting, a number of factors may affectdough rheology: relaxation of the stresses introduced duringmixing, continued hydration of flour components, and redistribu-tion of water. It is also possible that an alteration of gluten andstarch may occur because of enzymatic action. Another possibility

12 F

10 F

8

6

0.7 F

z

z

H-

0.6 F

0.5 1

0.40 60 120 180

is that the sulfhydryl-disulfide interchange continues during doughresting and, as a result, the average molecular weight of the glutenprotein decreases.

Dough tested immediately after mixing had higher G' andsmaller loss tangent than did doughs that were allowed to restin a bowl for 15 min before testing (Fig. 2). Loading the doughin the rheometer immediately after mixing (-1 or 2 min required),did not allow sufficient time for the stresses to relax. Therefore,placing the dough between the parallel plates of the rheometerand allowing it to rest resulted in no change in the rheologicalproperties (Dreese et al 1988b).

However, dough allowed to rest in a bowl undergoes rearrange-ments that release the stress. As a result, the dough becomessofter, G' becomes smaller, and the loss tangent becomes larger.When doughs were rested in a bowl at room temperature for30 and 45 min before testing, the rate of decrease of G' wasmuch slower than that for dough during the first 15 min of resting.

Glutathione, both reduced and oxidized, are naturally occurringin wheat flour (Kuninori and Matsumoto 1964, Hird et al 1968,Tkachuk 1969). During mixing, disulfide bonds are ruptured andcreate thiyl radicals (MacRitchie 1975). This free radical can thenparticipate in and increase the rate of sulfhydryl-disulfideinterchange reactions. Because the viscoelastic properties of doughare primarily related to its continuous protein phase, a decreasein the average molecular weight of gluten protein would be

12

601

cL

b

10

8

6

0.70

0.65

z

z

I-

0.60

0.55

0.50

0 60 120 180

TIME (MIN)Fig. 3. Effect of antioxidant on dough rheology. Flour-water dough control(0) and flour-water dough with 1% Tenox-4 (0) based on flour weight.

60 CEREAL CHEMISTRY

TIME (MIN)Fig. 4. Effect of glutathione on dough rheology. Flour-water dough control(0). Flour-water dough treated with 5 ppm (0), 25 ppm (A), or 100ppm (A) glutathione based on flour weight.

o Control* Tenox-4 1%

I I I

I -- - I- - L-

expected to cause a reduction in G' and an increase in the losstangent.

To determine whether free radical-mediated sulfhydryl-disulfideinterchange reactions were major factors causing the dough tobecome softer during rest, a free radical scavenger was addedto the dough. By terminating the radicals, the scavenger wouldbe expected to materially decrease the rate of interchange in thedough. The mixing time increased significantly with the additionof 1% of Tenox-4, as shown previously by Schroeder and Hoseney(1978). Dough containing Tenox-4 did not decrease in G' afterthe first 60 min (Fig. 3). The rapid decrease in G'-during thefirst 60 min was considered to be a stress relaxation. The effectof Tenox-4 can be explained by its action as a radical scavenger,thus stopping the sulfhydryl-disulfide interchange during resting.Increasing the Tenox-4 content to 2 and 3% only slightly increasedthe G'. This experiment provides evidence to support the assump-tion that free radicals are involved in the sulfhydryl-disulfideinterchange during dough resting. These data indicate that strainrelaxation occurs rapidly (<60 min) and is followed by a slowdecay of G' brought about by free radical-mediated sulfhydryl-disulfide interchange reactions.

Obviously, these functional groups may interact with chargedresidues of proteins. However, the most reactive group inglutathione is the sulfhydryl of the cysteine side chain. This groupcan serve as a nucleophile, a reductant, and a scavenger of freeradicals. Reactions involving glutathione as a reductant usuallylead to formation of glutathione disulfide.

Addition of glutathione to a flour-water dough reduced G'and increased the loss tangent (Fig. 4). The doughs become rela-tively less elastic as the glutathione content was increased from5 ppm to 100 ppm, as expected. According to the continuousnetwork model of protein structure (Bloksma and Bushuk 1988),the elasticity of dough is at least partly due to disulfide crosslinksin the network of protein molecules. The storage modulus (G')would be expected to increase with increased disulfide bonds ingluten and the loss modulus (G") to increase with increased lowmolecular sulfhydryl content. The presence of glutathione in-creases the rate of thiol-disulfide interchange reactions, whichdecreases the size of large proteins and results in lower molecularweight. The actual occurrence of such a reaction has beendemonstrated by chemical experiments (Kuninori and Matsumoto1963, 1964).

Effect of GlutathioneGlutathione is a tripeptide, with two negatively charged car-

boxylate groups and one positively charged amino group.

12

01-

y%_.

10

8

6

0.7

zwC 0.6z

0.5

0 60 120 180

TIME (MIN)Fig. 5. Effect of potassium bromate on dough rheology. Flour-water doughcontrol (0). Flour-water dough containing with 10 ppm (0) or 50 ppm(A) potassium bromate.

12

0

C0y

%_.

10

8

6

0.8

zwC,z

0.6

0.4

0 60 120 180

TIME (min)Fig. 6. Effect of potassium bromate and glutathione on dough rheology.Flour-water dough control (0) (58% abs, pH = 4.9) containing 50 ppmKBrO3 . Flour-water dough (58% abs, pH = 4.9) containing 50 ppm KBrO3(0). Flour-water dough (58% abs, pH = 4.9) containing 100 ppmglutathione (A).

Vol. 72, No. 1, 1995 61

O KBrO3 50 ppm* KBrO3 50 ppm GSH 100 ppm

- v GSH 100 ppm

_~~~~~ -I

Y- v-I

' -

IoI f.,,

0, ,0

l

-

During the first 60 min of resting, the decrease in G' of doughswith different glutathione contents parallelled the decrease in G'of the control dough (Fig. 4). However, the G' of doughs contain-ing glutathione remained constant after 2 hr of resting, whereasthe G' of the control flour-water dough continued to decrease.As shown above, sulfhydryl-disulfide interchange continues whenthe dough rests for 180 min. Low molecular weight sulfhydrylgroups are more mobile than disulfide groups in protein. There-fore, added glutathione has more of a chance to react with lowmolecular weight sulfhydryl groups to form a stable disulfidebond (GS-SG). This can be viewed as free radical scavenging.As a result, this reduces the chance of intermolecular cross-linksbreaking and gives a constant G'.

Effect of Potassium BromatePotassium bromate has well-known effects on the rheological

properties of dough. A widely accepted explanation for its actionis that potassium bromate removes sulfhydryl groups by oxidizingthem to disulfide bonds (Bloksma and Bushuk 1988). Addingpotassium bromate to flour-water dough increased G' slightlyrelative to the control during the first 60 min (Fig. 5). As statedpreviously, sulfhydryl groups naturally occur in flour. The increaseof G' might be caused by a decrease of sulfhydryl groups inthe dough. Increasing the concentration of potassium bromatefrom 10 ppm to 50 ppm did not change G' significantly. One

12

ay

_E

10

8

6

0.7

z0 0.6zI--

0.5

0 60 120 180

TIME (min)Fig. 7. Effects of ascorbic acid on dough rheology. Flour-water doughcontrol (0). Flour-water dough treated with 50 ppm D-erythroascorbicacid (A) or 50 ppm L-threoascorbic acid (0).

62 CEREAL CHEMISTRY

reason could be that 10 ppm potassium bromate was sufficientto give the maximum effect (Fig. 5).

When the dough was mixed with 50 ppm potassium bromateand 100 ppm of glutathione, and the pH adjusted to 4.9, theG' and loss tangent were about equal to those for dough mixedwith 50 ppm potassium bromate alone at pH 4.9 (Fig. 6). Thisindicates that potassium bromate completely overcomes the effectof glutathione, presumably by oxidizing it to its disulfide.

Effect of Ascorbic Acid and its IsomersImmediately after mixing, the G' of doughs containing L-

threoascorbic acid or D-erythroascorbic acid was not changedfrom that of the control. With increased rest time, G' of thedoughs containing the two isomers increased slowly. L-threo-ascorbic acid was more effective than D-erythroascorbic acid (Fig. 7).The oxidizing effect of ascorbic acid can be explained by twosets of reactions. First, the ascorbic acid was oxidized to its corre-sponding dehydroascorbic acid. This reaction was very fast, beingcomplete during mixing.

In the second reaction, L-threodehydroascorbic acid oxidizedthe sulfhydryl groups to the corresponding disulfide. This reactionwas catalyzed by the enzyme glutathione dehydrogenase (EC1.8.5.1).

Because sulfhydryl groups were oxidized to a relatively stabledisulfide, they were less active in thiol-disulfide interchange.Consequently, G' increased slowly relative to the control. Thisis in agreement with the improver action of ascorbic acid as shownby Mair and Grosch (1979).

Dough mixed with 30 ppm glutathione and 50 ppm L-threo-ascorbic acid had a slightly lower G' than the dough containing

12

ca

y%_

10

81

6

0 Controlf 0* L-AA 50ppm

\ v L-AA 50ppmv GSH 30ppm

., % \

_,I

GSH 30ppm

0.65 _

I--zwC,z

I-5

0.60 _

0.55

0.50

0

'/

'

60 120 180

TIME (MIN)Fig. 8. Effects of L-threoascorbic acid and glutathione on dough rheology.Flour-water dough control (0). Flour-water dough treated with 50 ppmL-threoascorbic acid and 30 ppm glutathione based on the flour weight(0). Flour-water dough treated with 50 ppm L-threoascorbic acid (A)or 30 ppm glutathione (A).

ControlL-AA 50 ppmD-AA 50 ppm

*% %f_

I I I I

-

a II

0

I I I I

I

I I I I

12

b

10

8

6 , _ I

0.7

I-~~~~~~lv

0.5

0 60 120 180

TIME (MIN)Fig. 9. Effects of D-erythreoascorbic acid and glutathione on doughrheology. Flour-water dough control (0). Flour-water dough treated with50 ppm D-erythreoascorbic acid and 30 ppm glutathione (0). Flour-waterdough treated with 50 ppm D-erythreoascorbic acid (A) or 30 ppmglutathione (A).

only 50 ppm L-threoascorbic acid (Fig. 8). This indicated thatthe L-threoascorbic acid eliminated the effect of glutathione.

Dough mixed with 30 ppm glutathione and 50 ppm D-erythro-ascorbic acid, had essentially the same G' as dough containingonly glutathione (Fig. 9). This indicated that D-erythroascorbicacid was not used by glutathione dehydrogenase. In a similarexperiment, D-threodehydroascorbic acid did not oxidize gluta-thione (data not shown).

These Theological differences showed that L-threoascorbic acidis a much stronger improver in a dough than D-threoascorbicacid. Of course this was already known from baking experiments(Walther and Grosch 1987). This specificity suggests that theenzyme glutathione dehydrogenase catalyzes the oxidation ofglutathione to the corresponding disulfide with L-threodehydro-ascorbic acid as the oxidant, but not with D-threodehydroascorbicacid as an oxidant.

If the above is true, removal of the enzyme glutathione dehydro-genase should result in L-threodehydroascorbic acid being lessactive as an improver. Because the enzyme is assumed to be presentin the water-soluble part of flour, it would be inactivated afterthat fraction was boiled for 15 min. Therefore, the reconstitutedflour should be enzyme-free.

When reconstituted flour dough was mixed with 50 ppmglutathione and 100 ppm L-threoascorbic acid or mixed with 50ppm glutathione and 100 ppm D-erythroascorbic acid, no signifi-cant differences occurred in G' or loss tangent between the twotreatments. The G' of these two treatments were essentially thesame as that for dough containing only glutathione (Fig. 10).This shows that the reaction between sulfhydryl groups and

7

0.cL

by6

5

0.6

zw

z0.5

0.4

0 60 120 180

TIME (min)Fig. 10. Effects of D-erythreoascorbic acid, L-threoascorbic acid, andglutathione on reconstituted flour-water dough (0). Reconstituted flour-water dough treated with 30 ppm glutathione (0). Reconstituted flour-water dough treated with 50 ppm L-threoascorbic acid and 30 ppmglutathione (A). Reconstituted flour-water dough treated with 50 ppmD-erythroascorbic acid and 30 ppm glutathione (A).

dehydro-L-threoascorbic acid was catalyzed by an enzyme, pre-sumably glutathione dehydrogenase.

LITERATURE CITED

BECKWITH, A. C., and WALL, J. S. 1966. Reduction and reoxidationof wheat glutenin. Biochim. Biophys. Acta 130:155-162.

BLOKSMA, A. H. 1972. The relation between the thiol disulfide contentsof dough and its rheological properties. Cereal Chem. 49:104-117.

BLOKSMA, A. H., and BUSHUK, W. 1988. Rheology and chemistryof dough. In: Wheat Chemistry and Technology. Vol. 2, ed. Y.Pomeranz, ed. Am. Assoc. Cereal Chem.: St. Paul, MN.

DREESE, P. C., FAUBION, J. M., and HOSENEY, R. C. 1988a.Dynamic Theological properties of flour, gluten, and gluten-starchdoughs. I. Temperature-dependent changes during heating. CerealChem. 65:348-353.

DREESE, P. C., FAUBION, J. M., and HOSENEY, R. C. 1988b.Dynamic Theological properties of flour, gluten, and gluten-starchdoughs. II. Effect of various processing and ingredient changes. CerealChem. 65:354-359.

FAUBION, J. M., DREESE, P. C., and DIEHL, K. C. 1985. DynamicTheological testing of wheat flour doughs. In: Rheology of WheatProducts. H. Faridi, ed Am. Assoc. Cereal Chem.: St Paul, MN.

GRAVELAND, A., BOSVELD, P., and MARSEILLE, J. P. 1978. Deter-mination of thiol groups and disulfide bonds in wheat flour and dough.J. Sci. Food Agric. 29:53-61.

HIRD, F. J. R., CROKER, I. W. D., and JONES, W. L. 1968. Lowmolecular weight thiols and disulphides in flour. J. Sci. Food Agric.

Vol. 72, No. 1, 1995 63

19:602-604.JONES, F. K., PHILLIPS, J. W., and HIRD, F. J. R. 1974. The estimation

of rheologically important thiol and disulfide groups in dough. J. Sci.Food Agric. 25:1-10.

JORGENSEN, H. 1939. Further investigation into the nature of the actionof bromate and ascorbic acid on the baking strength of wheat flour.Cereal Chem. 16:51-60.

KUNINORI, T., and MATSUMOTO, H. 1963. L-Ascorbic acid oxidizingsystem in dough and dough improvement. Cereal Chem. 40:647-657.

KUNINORI, T., and MATSUMOTO, H. 1964. Glutathione in wheatand wheat flour. Cereal Chem. 41:252-259.

KUNINORI, T., and SULLIVAN, B. 1968. Disulfide-sulfhydrylinterchange studies of wheat flour. II. Reaction of glutathione. CerealChem. 45:486-495.

MacRITCHIE, F. 1975. Mechanical degradation of gluten proteins duringhigh-speed mixing of dough. J. Polymer Sci. Symp. 49:85-90.

MAIR, G., and GROSCH, W. 1979. Changes in glutathione content(reduced and oxidized forms) and the effect of ascorbic acid and potas-

sium bromate on glutathione oxidation during dough mixing. J. Sci.Food Agric. 30:914-920.

SCHROEDER, L. F., and HOSENEY, R. C. 1978. Mixograph studies.II. Effect of activated double-bond compounds on dough-mixingproperties. Cereal Chem. 55:348-360.

SPERLING, L. H. 1986. Introduction to Physical Polymer Science. Wiley-Interscience: New York.

TKACHUK, R. 1969. Involvement of sulfhydryl peptidase in the improverreaction. Cereal Chem. 46:203-205.

WALTHER, C., and GROSCH, W. 1987. Substrate specificity of theglutathione dehydrogenase (dehydroascorbate reductase) from flour.J. Cereal Sci. 5:299-305.

YOSHIDA, M., HAMAUZU, Z., and YONEZAMA, D. 1980. On re-activity of disulfide groups of gliadin and glutenin. Agric. Biol. Chem.44:657-661.

WEAK, E. D., HOSENEY, R. C., SEIB, P. A., and BIAG, M. 1977.Mixograph studies. I. Effect of certain compounds on mixing properties.Cereal Chem. 54:794-802.

[Received April 26, 1994. Accepted October 14, 1994.]

MISCELLANEOUS

Dielectric Properties and Water Mobility for Heated Mixturesof Starch, Milk Protein, and Water'

M. N. TSOUBELI, 2 E. A. DAVIS,2 and J. GORDON2

ABSTRACT Cereal Chem. 72(l):64-69

The dielectric properties of hydrated whey protein isolate (WPI), Ca- absorptivity of starch during heating but such an effect was not evidentcaseinate, and wheat starch, alone or in combination, were measured with Ca-caseinate. The dielectric properties of these systems were com-at ambient temperature and during heating to 900C. At lower moisture pared with their hydration properties by electron spin resonance usingcontents and ambient temperature, WPI exhibited higher dielectric proper- TEMPO, a water and oil-soluble noninteractive probe to correlate waterties than starch, whereas Ca-caseinate had a lower dielectric constant mobility results to dielectric relaxation phenomena. All systems werethan either WPI or starch. At higher moisture contents, the dielectric equally effective in slowing the motion of TEMPO. However, their di-properties were similar. At moisture contents of 30-80%, WPI showed electric properties differed, indicating that the dielectric properties areincreasing microwave absorption properties with increasing temperature; not only influenced by the water but by the macromolecules present asat higher moisture contents, microwave absorption by Ca-caseinate well.decreased with temperature. Adding WPI affected the dielectric loss and

Milk protein ingredients are used in many applications by thefood industry because they provide a wide range of functionalproperties and have high nutritional value. The influence of milkproteins in various food systems has been extensively studied.In one study, Pearce et al (1984) found that the adding nonfatdry milk solids to cake batter influenced properties such as aircell size, foam stability, and lipid emulsification. Other studieshave also addressed the functionality of milk proteins and theirincorporation into various products (Kinsella 1982). What is notclear, however, is the relationship between the functional proper-ties and the physicochemical interactions of the proteins withother ingredients that cause functionality modification. In cereal-based products, such interactions include interactions betweenmilk proteins, starch, lipids, and water. With the advance of micro-wave-heated products, a better understanding of the dielectricbehavior of milk proteins, alone and in combination with starch,

'Paper 20,770 of the Minnesota Agricultural Experiment Station projects 18-027and 18-063. Additional support from the Minnesota-South Dakota Dairy FoodsResearch Center.

2Department of Food Science and Nutrition, University of Minnesota, St. Paul.

© 1995 American Association of Cereal Chemists, Inc.

is important to controlling cereal product quality.Dielectric properties, the interaction of a material with electro-

magnetic radiation, can be expressed as: dielectric constant (k'),a measure of the ability to store the electromagnetic radiation,which is related to polarity; dielectric loss (k"), a measure ofthe ability of a material to transform or dissipate the electricenergy into heat; and absorptivity (/ Ri), a measure of how thewaves will be affected when they enter the material, defined asthe inverse of the penetration depth. The penetration depth pro-vides information on how far a wave will penetrate into a materialbefore it is reduced to 1 / e of its original intensity (Mudgett 1986).

In heating by electromagnetic energy, water is the primary com-ponent of a food system that interacts with electromagnetic radia-tion at 2,450 MHz. Water mobility has been assessed by manymethods for various systems. Electron spin resonance (ESR) usingthe noninteractive probe TEMPO has been used to assess watermobility in gluten and water systems (Pearce et al 1988) andwhey protein concentrate and water systems (Schanen et al 1990).In both studies, a splitting in the high field line of a typical three-line TEMPO spectrum indicated a partitioning of the probe intotwo water environments with differing mobility. Water self-diffu-sion coefficients (D) and dielectric properties have been also ob-tained (Umbach et al 1992) for conventional and microwave-

64 CEREAL CHEMISTRY