Journal of the Sciellce of Food alld Agriculture J Sci Food Agn'c 81:1223-1227 (online: 2001)DOl: 10.1002/jsfa.934

Emulsifying activity and emulsion stability ofcorn gluten meal tY Victor WU*Fermentation Biochemistry Research Unit, National Center for Agricultural Utilization Research, Agricultural Research Service, U.S.

Department of Agriculture, Peoria, IL 61604, USA

Abstract: Corn gluten meal with good emulsifying activity and emulsion stability may have potential infood uses. Commercial corn gluten meal from various producers had a pH around 4 and showed noemulsifying activity. Reducing the particle size to below 15/-lm or increasing the pH to 6.6 (with particlesize above 53/-lm) did not improve the emulsifying activity. When the particle size was reduced to below44/-lm and the pH was adjusted to 6.6 or above, good emulsifying activity and good emulsion stabilitycould be obtained. The emulsifying activities of corn gluten meals «44/-lm) from four differentproducers at around pH 8 ranged from 49.3 to 51.5 and the corresponding emulsion stabilities rangedfrom 39.7 to 49.5. There was no significant variation in emulsifying activity between pH 6.9 and 7.8when NaOH, LiOH and KOH were used to adjust the pH, but LiOH- and KOH-adjusted meal ataround pH 7.8 showed better emulsion stability than NaOH-adjusted meal. Good emulsifying activityof corn gluten meal could be obtained in NaCI solutions at sufficiently high pH. Published in 2001 forSCI by John Wiley & Sons, Ltd.

Keywords: corn gluten meal; emulsifying activity; emulsion stability

INTRODUCTIONWet milling separates corn into starch, protein, germand bran fractions. Corn gluten meal, containing aminimum of 600 g kg-I protein, is the main proteinfraction. This protein consists mostly of alcohol­soluble zein (680 g kg-I) and alkali-soluble glutelin(270 g kg-I) but is low in salt-soluble globulin(l2gkg- I).1 Corn gluten meal has poor functionalitycompared to other commercial proteins such as thosefrom milk and soy. The poor functionality of corngluten meal is related to its low solubility in aqueoussystems at the pH and ionic strength of most foodproducts. Kinsella2 found that proteins generally haveto be in solution or in fine suspension to show goodfunctional properties.

Mannheim and Cheryan3 used a protease (Alcalase)to hydrolyse corn gluten meal. They found thatincreasing the degree of hydrolysis increased the initialfoam volume but decreased foam stability. Also, corngluten meal hydrolysates showed higher moisturesorption capacities than unhydrolysed corn glutenmeal.

Commercial corn gluten meals have a pH around 4and show no emulsifying activity. This paper reportsthe effects ofpH, particle size and ionic strength on the

emulsifying actIvity and emulsion stability of corngluten meal.

MATERIALS AND METHODSCorn gluten meals were supplied by four commercialproducers (A-D). Unless stated otherwise, the materi­al used was from commerical producer A.

Soy protein isolate, Supro 610, was supplied byRalston Purina Company (St Louis, MO, USA). Cornzein was from Sigma Chemical Co (St Louis, MO,USA).

The meal was ground in an Alpine Model 160Z pinmill (Natick, MA, USA) at 14000rpm and airclassified by particle size in a Pillsbury laboratory­scale air classifier (Minneapolis, MJ."J, USA). The firstset point for the air classifier was 15 ~lm to obtain fineand coarse fractions. The coarse fraction was then airclassified with an 181lm set point to get fine and coarsefractions. The coarse fraction from the 181lm set pointwas air classified with a 24 ~lm set point to yield fineand coarse fractions. The coarse fraction from the24 11m set point was air classified with a 30 Ilm set pointto obtain fine and coarse fractions. The >30 Ilmfraction was then separated into various particle size

* Correspondence to: Y Victor Wu, Fermentation Biochemistry Research Unit, National Center for Agricultural Utilization Research,Agricultural Research Service, US Department of Agriculture, Peoria, IL 61604, USAE-mail: wuyv@mail.ncaur.usda.govt This article is a US Government work and is in the public domain in the USA. Names are necessary to report factually on available data;however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by USDA implies no approval ofthe product to the exclusion of others that may also be suitable.(Received 26 January 2001; revised version received 5 March 2001; accepted 25 June 2001)Published online 15 August 2001

Published in 2001 for SCI by John Wiley & Sons, Ltd. J Sci Food Agric 0022-5142/20011$30.00 1223

YVlflu

fractions by 203 mm diameter stainless steel USAStandard Testing Sieves (WS Tyler, Mentor, OH,USA) with openings of37, 44, 53, 63, 74, 88,105,125and 149 J.lm as required.

The pH values of the meals were between 4.0 and4.3. Sodium hydroxide solution (400gkg- l

) wasnormally used to adjust the pH to higher values. AfterpH adjustment the wet material was dried in anIsotemp Model 655F forced-air oven (Fisher Scien­tific, Pittsburgh, PA, USA) at 90°C overnight, groundin a Varco Model 228 electric dry food grinder(Belleville, NJ, USA) and sieved to the desired particlesize. Potassium hydroxide (400 g kg-I) and lithiumhydroxide (120 g kg-I) solutions were also used.Hydrochloric acid solution (100gkg- l

) was used tolower the pH.

Ash, moisture and nitrogen contents were deter­mined by AACC approved methods. 4 Protein contentwas calculated as N x 6.25. Starch content wasdetermined by a polarimetric method in calciumchloride solution containing stannic chloride. 5 Fatcontent was determined by petroleum ether extrac­tion. 6 All composition experiments were conducted induplicate, and the values are reported on a dry matterbasis.

The solubility of com gluten meal at various pHvalues and ionic strengths was determined by stirring0.15 g of meal in 15 ml of water together withhydrochloric acid or sodium hydroxide solution. Themixture was stirred magnetically for 25 min andcentrifuged at 1300 x g for 20 min, and the supernatantwas analysed for nitrogen by the Kjeldahl method.4

Nitrogen solubility was calculated as the total nitrogenin the supernatant divided by the starting nitrogen inthe com gluten meal sample. Sodium chloride solu­tions were used instead of water for higher ionicstrengths.

Emulsifying actIVIty and emulsion stability weredetermined by the method of Yasurnatsu et al. 7 Theyused 7 g of soybean product, 100 ml of water and100 ml of soybean salad oil, homogenised at10000 rpm for 1min and centrifuged at 1300 x g for5 min. Emulsifying activity was calculated as100 x (height of emulsified layer) / (total height ofmixture in tube). For emulsion stability the emulsionprepared as above was heated for 30 min at 80°C,cooled with tap \vater for 5 min and centrifuged at1300 x g for 5 min. Emulsion stability was calculatedas 100 x (height of remaining emulsified layer) / (totalheight of mixture in tube). We used the sameproportion of com gluten meal/water/soybean oil, ie7:100:100 (w/v/v), but normally used only 1.05g ofmeal in order to conserve material. The estimatedvalues of emulsifying activity and emulsion stabilitywere averages of four measurements each. The effectof ionic strength on emulsifying activity and emulsionstability was determined by substituting NaCI solu­tions for water according to the method ofYasumatsuet al. 7

The data were subjected to analysis of variance.

1224

Means were compared by t-tests ofpairs ofleast squaremeans. s

RESULTS AND DISCUSSIONThere is no standard test of emulsifying power which isof general applicability. 9 Kinsella et allo stated thatsolubility, in addition to surface hydrophobicity, isimportant for the emulsifying properties of proteins.They used an emulsifying activity index, the area ofinterface stabilised per unit weight of proteins, whichdepends on the protein concentration in the aqueousphase, the volume fraction of the oil phase and theturbidity of the emulsion measured at 550 nm. II Sincecom gluten meal is practically insoluble in water (seebelow) and has a bright yellow colour, any methodbased on soluble protein or any optical method fordetermining emulsifying properties is not suitable.After careful consideration of the various methodsavailable, we chose that used by Yasurnatsu et al,7

because their method only involves measuring theheight of the emulsified layer and the total height of themixture in the tube.

Proximate compositionThe compositions of the four com gluten meals aregiven in Table 1. Protein content ranged from 670 to

730gkg- l, starch content from 100 to 180gkg- l

, fatcontent from 20 to 54 g kg-I and ash content from 13to 24g kg-I. This range of composition of com glutenmeal from different producers can arise from proce­dural differences as \vell as variations in the composi­tion of the com used.

Effect of particle size and pH on emulsifying activityand emulsion stabilityTable 2 gives the emulsifying activity and emulsionstability of com gluten meal at pH 6.6 and 4.0, the as­is pH of commercial sample A. After drying in theforced-air oven, the pH value was lower than theadjusted value if this was above pH 4 but higher thanthe adjusted value if this was below pH 4. The pH of6.6 in Table 2 was the value measured after pHadjustment and drying. As the particle size increasedfrom <15 ~lm at pH 6.6, the emulsifying activitydecreased from 56.2, gradually at first, then rapidlyto below 10 for a particle size greater than 53 f!m. Theemulsion stability at pH 6.6 decreased more rapidly

Table 1. Composition (g kg- 1 dry basis) of corn gluten meal

from various producers (A-D)

ProteinProducer (Nx 6.25) Starch Fat Ash

A 715a 126b 19.6d 24.2aB 733a 102c 49.3b 13.3cC 687b 176a 31.7c 13.5cD 673b 126b 54.3a 18.9b

Numbers followed by different letters in a column are

significantly different (P < 0.05).

J Sci Food Agric 81:1223-1227 (online: 2001)

Numbers followed by different letters in a

column are significantly different (P < 0.05).

Table 2. Effect of particle size on emulsifying

activity (EA) and emulsion stability (ES) of corn

gluten meal at pH 4.0 and 6.6

and was 10 or below for a particle size greater than18 ~lm. At pH 4.0, both emulsifying activity andemulsion stability were below 8 for all particle sizes.In comparison, soy protein isolate (Supro 610) withgood emulsifying properties had an emulsifyingactivity and emulsion stability of 59.4 and 57.2respectively. The results in Table 2 show that,although commercial corn gluten meal at pH 4.0 hadpoor emulsifying properties even for small particlesizes, good emulsifying activity and emulsion stabilitycould be achieved by both increasing the pH to 6.6 anddecreasing the particle size to <15 ~lm. In comparison,soy sodium proteinate at pH 5.3 had little or noemulsifying capacity, but reasonably good emulsifica­tion was obtained at a pH value of about 6.9. 12

Particle size(urn) pH

24-30 4.018-24 4.015-18 4.0<15 4.074-88 6.663-74 6.653-63 6.644-53 6.637-44 6.630-37 6.624-30 6.618-24 6.615-18 6.6<15 6.6

EA

1.411.716.787.ge3.016.ge6.6e

46.4cd45.5d47.4cd46.2cd48.3c51.9b56.2a

ES

1.411.716.8e7.5cde2.313.817.5cde

10.0c9.6cd9.6cd6.287.1e

41.1 b50.8a

Emulsion propel1ies of C0171 gluten meal

(around 4) and higher pH (around 8) for a particle sizeless than 44 11m. The emulsifying activities at aroundpH 8 were all good (near 50), although there weresome statistical differences among the four producers.Likewise, the emulsion stabilities at around pH 8 wereall good, with values close to 50 for producers C and Dand close to 40 for producers A and B. Theemulsifying activity and emulsion stability of corngluten meal from all four producers at around pH 4were poor, with values of 9 or below.

Effect of pH on emulsifying activity and emulsionstability of corn gluten meal «44f..lm)The effect of pH (adjusted by NaOH and HCl) on theemulsifying activity and emulsion stability of corngluten meal «4411m) is shown in Table 4. Bothemulsifying activity and emulsion stability were poor(below 6) from pH 3.2 to 6.2. Above pH 6.5 theemulsifying activity increased substantially but theemulsion stability did not. At pH 6.9 there was anotherlarge increase in emulsifying activity but only amoderate increase in emulsion stability. At pH 7.8there was a large increase in emulsion stability but nofurther increase in emulsifying activity. No significantincrease in emulsifying activity was observed at pH 9.0and 10.4; however, the emulsion stability was muchlower at pH 9.0 and 10.4 compared with the value atpH 7.8.

Effect of using different alkalis for pH adjustment onemulsifying activity and emulsion stability of corngluten meal «44f..lm)Table 4 also shows the effect of using LiOH, NaOH

Table 4. Effect of pH and different alkalis on emulsifying

activity (EA) and emulsion stability (ES) of corn gluten

meal «44Ilm)

Numbers followed by different letters in a column are

significantly different (P < 0.05).

Emulsifying activity and emulsion stability of corngluten meal from different producersTable 3 shows the emulsifying activity and emulsionstability of the four corn gluten meals at as-is pH

Table 3. Emulsifying activity (EA)

and emulsion stability (ES) of corn

gluten meal «44,lm) from various

producers (A-D)

Producer pH EA ES

A 4.0 5.5e 3.6d7.8 49.3c 39.8b

B 4.0 2.91 2.9d8.0 50.0bc 39.7b

C 4.3 7.9d 9.Oc8.3 51.5a 49.5a

D 4.3 5.ge 6.1cd7.7 50.5b 48.3a

Numbers followed by different letters

in a column are significantly different

(P<0.05).

pH Adjusted by

3.18 HCI3.95 None4.89 NaOH5.62 NaOH5.87 NaOH623 NaOH6.32 KOH6.33 NaOH6.47 NaOH6.53 NaOH6.81 (soy isolate) None6.91 NaOH7.13 LiOH7.26 KOH7.65 LiOH776 KOH7.80 NaOH9.02 NaOH9.80 (zein) NaOH10.40 NaOH

EA ES

2.8e 3.0gh5.58 3.6gh2.88 2.7gh1.48 1.7h2.4e 2.7gh4.ge 4.0g

14.4d 3.6gh5.1 e 4.51g6.38 4.0g

35.5c 2.7gh56.3a 55.6a49.7b 9.0848.8b 39.1c49.8b 39.4c49.3b 45.9b50.5ab 45.5b49.3b 39.8c49.0b 6.5150.3b 45.1b53.9ab 16.0d

J Sci Food Agric 81:1223-1227 (online: 2001) 1225

YVWu

Table 5. Effect of ionic strength on emulsifying

activity (EA) and emulsion stability (ES) of corn

gluten meal «44~lm)

Effect of ionic strength on emulsifying activity andemulsion stability of corn gluten meal «44,.,..m)Table 5 shows that there were no differences(P> 0.05) in the emulsifying activity of com glutenmeal «44Ilm) in 0 and 109 kg- 1 NaCI solutions atpH 7.8 and 9.0, but the emulsifying activity wasmarginally higher (P =0.046) in Ogkg-1 NaCI at pH10.4 compared with the value in 109 kg- 1 NaC!. Theemulsifying activity was lower (P < 0.05) in 20 g kg- 1

NaCI (35.2-36.9) than in 10 and Og kg- 1NaC! at pH7.8, 9.0 and 10.4. There was a large decrease inemulsifying activity in 50 g kg- 1 NaCI at pH 7.8, 9.0and 10.4.

The maximum emulsion stability (39.8 in Table 5)of com gluten meal (< 44 Ilm) was observed in 0 g kg- 1

NaCI at pH 7.8. Increasing the pH above 7.8

and KOH for pH adjustment on the emulsifyingactivity and emulsion stability of com gluten meal.There were no significant differences (P> 0.05) inemulsifying activity between pH 6.9 and 7.8 forLiOH-, NaOH- and KOH-adjusted samples. In com­parison, soy isolate at pH 6.8 had an emulsifyingactivity of 56.3, statistically higher (P < 0.05) than thevalues of 48.8-49.8 but not statistically higher(P > 0.05) than the value of 50.5 for com gluten meal(Table 4).

The emulsion stability of soy isolate was 55.6 (Table4), significantly higher (P < 0.05) than the values of45.9 and 45.5 for com gluten meal at around pH 7.7,adjusted by LiOH and KOH respectively. Theemulsion stabilities of com gluten meal at pH 7.1,adjusted by LiOH, at pH 7.3, adjusted by KOH, andat pH 7.8, adjusted by NaOH, ranged from 39.1 to39.8 and were significantly lower (P < 0.05) than thevalues of 45.9 and 45.5 for the LiOH- and KOH­adjusted samples at pH 7.6 and 7.8 respectively. At pH7.8 the emulsion stability of the KOH-adjusted samplewas higher (P <0.05) than that of the NaOH-adjustedsample (45.5 vs 39.8 in Table 4).

decreased the emulsion stability. The emulsion stabi­lity was higher in 10 than in Ogkg-1 NaC! at pH 9.0but higher in 0 than in 10gkg-1NaCI atpH 10.4. Theemulsion stability was 5 or below in 20 and 50 g kg- 1

NaCI at pH 7.8,9.0 and 10.4.

Effect of pH and ionic strength on solUbility of corngluten mealNo trend in the nitrogen solubility of com gluten mealwas observed at various pH values and ionic strengths,since less than 109 kg-1 nitrogen was soluble in waterbetween pH 2 and 10.4 and in 10, 20 and 50gkg- 1

NaCI at pH 7.8, 9.0 and 10.4 (results not shown).The layers formed in the emulsion studies were

(from top to bottom) oil, emulsion, water and solids.Good emulsifying activity or emulsion stability wasusually observed with no oil layer or minimal oil layer.If no separate oil layer is formed, only creaming anddrainage will occur. 13 It is possible that the com glutenmeal particles themselves stabilise the droplets. 14

Casella and Whitaker15 reported the isoelectricpoint of zein as pH 6-7. Wall et aZ 16 showed fromisoelectric focusing of zein that most bands werebetween pH 7 and 8, with some between pH 8 and 8.5and between pH 6.5 and 7. Isoelectric focusing of comglutelin showed most bands between pH 6.5 and 8.5and some between pH 4 and 5.5. Since com glutenmeal consists of 680gkg- 1 zein and 270gkg- 1

glutelin,1 the isoelectric point of com gluten meal isprobably around pH 7-8. The highest emulsifyingactivity and emulsion stability ofcom gluten meal wereobserved at pH 7-8 (Table 4), where the meal has zeroor minimum net charge. Halling13 suggested that thecohesiveness of protein films tends to be maximal nearthe isoelectric point, and cohesive films tend to bemore stable. A positive net charge on com gluten meal(pH below isoelectric point) decreased the emulsifyingactivity more than a negative net charge (pH aboveisoelectric point), with good emulsifying activity beingmaintained at pH lO.4 (Table 4) but not below pH6.5. Higher sodium and chloride ion concentrationsfrom sodium chloride (Table 5) also lowered theemulsifying activity even at the isoelectric pH, possiblyby interaction with charged groups such as carboxyl,phenylhydroxyl and guanidyl in com gluten meal withzero net charge.ESEApH

Numbers followed by different letters are significantlydifferent (P < 0.05).

ooo

101010202020505050

7.809.02

10407.809.02

10407.809.02

10407.809.02

1040

49.3b49.0b53.9a48.6b49.0b50.3b36.9c35.5c35.2c5.3d5.2d1.4e

39.8a6.5e

16.0c10.1d26.6b6.8e1.4e3.8fg5.0ef2.0gh34fgo

CONCLUSIONSAlthough commercial com gluten meal shows noemulsifying activity at pH 4, good emulsifying activityand emulsion stability can be obtained by bothreducing the particle size and increasing the pH. Highionic strength decreased the emulsifying activity andemulsion stability of com gluten meal suspensions.

ACKNOWLEDGEMENTSI thank ED Deadmond and AM Kelly-Webb fortechnical assistance.

1226 J Sci Food Agric 81:1223-1227 (online: 2001)

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National Center Tor AgriculturalUtilizatIon Research, Peona, Illinois

J Sci Food Agn'c 81:1223-1227 (online: 2001) 1227