Adhesion and Cohesion - ACS Symposium Series (ACS Publications)

6

Adhesion and Cohesion J. S. WALL and F. R. HUEBNER

Downloaded by FUDAN UNIV on April 16, 2017 | http://pubs.acs.org Publication Date: March 6, 1981 | doi: 10.1021/bk-1981-0147.ch006

Northern Regional Research Center, Science and Education Administration, Agricultural Research, U.S. Department of Agriculture, Peoria, IL 61604

Cohesion and adhesion are essential functional properties of certain of the constituents of food mixtures i f we wish to convert them to sight-appealing shaped products with acceptable textures. Parker and Taylor (1) define adhesion as the use of one material to bond two other materials together and cohesion as the joining together of the same material. The development of textured foods based on vegetable proteins, multi-component breakfast foods, and some prepared meat specialty items has required at least one ingredient, usually protein, to serve as binder to hold components together. The binding agents may function before or after cooking the ingredient mix; cooking establishes additional adhesive and cohesive interactions among protein, lipid, and carbohydrate components of foods. In this paper, we will explore the measurement of and the basis for the cohesive and elastic properties of a commonly used component of foods that excels in these characteristics, wheat gluten. Gluten constitutes from 10 to 16% of wheat flour, from which it may be separated by Martin, batter, or Raisio processes (2, 3). The separated wheat gluten is 70 to 80% protein, of which 85% is insoluble in saline solution. We shall also seek to correlate some of the basic concepts developed in studies of gluten to other protein systems, such as those of soybean protein isolates and concentrates. A good example of the contribution of protein to adhesion and cohesion of a multi-component system is wheat flour dough. Khoo et al. (4) have observed with the scanning electron microscope that dough consists of starch granules held together by a matrix of hydrated gluten protein, which is stretched into coherent films (Figure 1). These films are not artifacts of microscopy since isolated gluten is an excellent film former. Whole gluten purified by solubilization in dilute acids can be dispersed in lactic acid, which acts as a humectant and plasticizer, and cast as a film as shown by Wall and Beckwith (5). Such films would serve as edible coatings. Film-making properties are characteristic of many polymers, This chapter not subject to U.S. copyright. Published 1981 American Chemical Society

Cherry; Protein Functionality in Foods ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

112

PROTEIN F U N C T I O N A L I T Y IN FOODS

i n c l u d i n g p r o t e i n s , and are a good measure of cohesive s t r e n g t h . Gluten p r o t e i n s have a l s o been tested as i n d u s t r i a l adhesives f o r bonding paper or wood (5). J u s t as g l u t e n p r o t e i n s a s s o c i a t e w i t h s t a r c h , so does i t bond to other p o l a r m a t e r i a l s , such as c e l l u l o s e . Many of the concepts developed i n i n d u s t r i a l adhesives apply to food a p p l i c a t i o n s .


Experimental

Procedures

Many instruments are a v a i l a b l e to provide fundamental i n f o r m a t i o n on the adhesive and cohesive p r o p e r t i e s of food products of d i f f e r e n t forms and at d i f f e r e n t stages of preparat i o n . Voisey and de Man (6) c l a s s i f y such instruments i n t o two main c a t e g o r i e s : (a) l i n e a r motion instruments which g e n e r a l l y measure e x t e n s i b i l i t y and t e n s i l e s t r e n g t h or compress i o n and f l e x i n g s t r e n g t h , and (b) r o t a r y motion devices which measure r e s i s t a n c e to flow or mixing of v i s c o u s s o l u t i o n s or p l a s t i c masses. The l i n e a r motion instruments u s u a l l y rupture the s t r u c t u r e s and so are best used to measure a s p e c i f i c stage of product development. In c o n t r a s t , the r o t a r y instruments o f t e n may be used to examine t r a n s i t o r y e f f e c t s on p h y s i c a l p r o p e r t i e s induced by temperature v a r i a t i o n , chemical a d d i t i v e s , or s t r e s s e s caused by mixing. The most widely used l i n e a r motion analyzers are the Instron U n i v e r s a l T e s t i n g machines which range from l a r g e f l o o r models to t a b l e instruments s u i t a b l e f o r most food t e s t s (7, 8). Force i s a p p l i e d by d r i v e screws to a crosshead which i s moved downward at a s p e c i f i e d r a t e . An upper f o r c e measurement c e l l measures extending f o r c e s on m a t e r i a l s clamped between i t and the moving crosshead (Figure 2). Wall and Beckwith (5) used t h i s system to measure adhesion by wheat g l u t e n i n model systems. Data on f o r c e and d i s t a n c e t r a v e r s e d by the crossarm are recorded. F r a z i e r et a l . (9) used the I n s t r o n to compress dough or g l u t e n b a l l s at a given load and measure time r e q u i r e d f o r the b a l l s to r e l a x to a lower load at a constant deformation. Various attachments to the I n s t r o n permit measurement of l a t e r a l s t r e s s e s such as f l e x i n g . Rasper (10) has modified the I n s t r o n to permit dough extension measurements i n a temperature-controlled chamber c o n t a i n i n g f l u i d with d e n s i t y equal to that of dough. Instruments s p e c i f i c a l l y designed f o r dough e x t e n s i b i l i t y and t e n s i l e s t r e n g t h measurements are the Brabender Extensograph or Simon Extensometer (10). In the extensograph, a rod-shaped mass of dough i s clamped h o r i z o n t a l l y and a hook extends i t v e r t i c a l l y at a constant r a t e . Resistance and extension are recorded u n t i l the dough c y l i n d e r breaks. The area under the curve and the r a t i o of r e s i s t a n c e to e x t e n s i b i l i t y provide u s e f u l i n f o r m a t i o n as to the e l a s t i c i t y of the dough. Hydrated food products e x h i b i t i n g adhesive or cohesive p r o p e r t i e s are g e n e r a l l y h i g h l y v i s c o u s or p l a s t i c and, t h e r e f o r e ,


WALL AND HUEBNER


6.

Adhesion

and

Figure

Figure

2.

Tensile

strength

113

Cohesion

measurement with machine

1.

the

SEM of wheat flour after mixing (4)

Instron

Universal


dough

Testing


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PROTEIN FUNCTIONALITY I N FOODS

r e q u i r e s p e c i a l r o t a r y instruments f o r measurement of flow p r o p e r t i e s . A h i g h l y s o p h i s t i c a t e d instrument designed f o r elastomer and p l a s t i c research but h i g h l y u s e f u l f o r research on foods i s the Mechanical Spectrometer (Rheometrics Inc., Union, NJ). A considerable v a r i a t i o n i n shear r a t e s can be a p p l i e d i n a number of d i f f e r e n t geometries i n c l u d i n g r o t a t i n g cone and p l a t e between which a t h i n l a y e r of sample i s i n s e r t e d . The mechanical assembly and microprocessing u n i t permit a n a l y s i s of a broad spectrum of molecular responses y i e l d i n g data on v i s c o u s and e l a s t i c p r o p e r t i e s of the t e s t m a t e r i a l . A more g e n e r a l l y a v a i l a b l e l a b o r a t o r y instrument i s the Haake Rotov i s c o viscometer which a l s o features a cone-plate attachment. This modified instrument records torque r e q u i r e d to maintain constant r a t e of r o t a t i o n and provides data only on the v i s c o u s component of flow (8). I n the f l o u r m i l l i n g and baking i n d u s t r i e s , r e c o r d i n g mixers, designated farinographs or mixographs, are widely used f o r e m p i r i c a l e v a l u a t i o n of f l o u r performance during dough processing. In the Brabender Farinograph, f l o u r and water are mixed by r e v o l v i n g blades. The motor s h a f t i s connected to a dynamometer which measures r e s i s t a n c e to the mixing (11, 12). The chart records the dynamic process of dough making and breakdown with time. Water content (absorption) i s adjusted to give s i m i l a r r e s i s t a n c e s at maximum dough s t r e n g t h . The mixograph i s s i m i l a r i n concept to the farinograph but uses p i n s i n s t e a d o f blades and imposes a greater s t r e s s on the dough (11). While these methods can provide u s e f u l information f o r determining the f u n c t i o n a l performance of i n g r e d i e n t s used i n food, the c r i t e r i a f o r q u a l i t y must be e s t a b l i s h e d on the f i n a l product; thus, dough must be baked i n t o bread and meat analogues cooked and these foods subjected to o r g a n o l e p t i c e v a l u a t i o n f o r texture. Molecular

Interactions

Extensive s t u d i e s i n p r o t e i n chemistry (13) and s y n t h e t i c polymers (14) have e s t a b l i s h e d compositional f a c t o r s r e s p o n s i b l e f o r molecular a s s o c i a t i o n s i n v o l v e d i n adhesion and cohesion phenomena of p r o t e i n s and other polymers. Figure 3 summarizes types of f u n c t i o n a l groups i n p r o t e i n s p a r t i c i p a t i n g i n a s s o c i a t i v e i n t e r a c t i o n s and agents that d i s r u p t the bonds they form. E l e c t r o s t a t i c charges due to i o n i z e d a c i d i c or b a s i c amino acids influence protein s o l u b i l i t y . At extremes of pH, many p o o r l y s o l u b l e p r o t e i n s are d i s s o l v e d and t h e i r molecular s t r u c t u r e s unfolded due to surplus of s i m i l a r r e p e l l i n g charges. Gluten p r o t e i n s have few charged groups and so a r e poorly s o l u b l e i n n e u t r a l s o l u t i o n (15). Dispersions of other p r o t e i n s must be adjusted to t h e i r i s o e l e c t r i c p o i n t or have s a l t added to optimize cohesion and adhesion.



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WALL AND HUEBNER

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115

Cohesion

P o l a r groups c o n t r i b u t e g r e a t l y to adhesion of p r o t e i n s to carbohydrates and to t h e i r cohesion. In denatured or unfolded p r o t e i n s , such as animal glues, the peptide amide groups play an important r o l e i n adhesion; but i n the undenatured c o l l a g e n , most peptide groups are a s s o c i a t e d i n h e l i c a l conformations. In undenatured p r o t e i n s , s i d e chain amide groups from the amino a c i d s glutamine and asparagine and hydroxyl groups of s e r i n e and threonine i n t e r a c t through hydrogen bonds. Gluten p r o t e i n s c o n t a i n over 33% glutamine and asparagine i n t h e i r amino a c i d composition (15). I n v e s t i g a t o r s of the chemistry of adhesion r e f e r to nonpolar i n t e r a c t i o n s i n v o l v i n g long c h a i n a l i p h a t i c or aromatic groups i n terms of Van der Waal or London f o r c e s (1). P r o t e i n chemists g e n e r a l l y use the term "hydrophobic bonding" to d e s c r i b e these i n t e r a c t i o n s , because i n aqueous systems nonpolar r e s i d u e s i n p r o t e i n s tend to r e t r e a t and a s s o c i a t e i n the m o l e c u l e s i n t e r i o r or w i t h other l i k e groups on adjacent molecules. Membrane p r o t e i n s e s p e c i a l l y have exposed hydrophobic groups which c o n t r i b u t e to a s s o c i a t i o n with l i p i d s and i n t e g r i t y of the membrane s t r u c t u r e . D i s u l f i d e bonds i n the amino a c i d c y s t i n e are important to the p r o p e r t i e s of many p r o t e i n s by m a i n t a i n i n g covalent i n t r a molecular bonds and c r o s s l i n k s between p r o t e i n chains (16). The opposing e f f e c t s of hydrogen bonding and n e g a t i v e l y charged amino a c i d s i d e chains on molecular aggregation of p r o t e i n s have been demonstrated by experiments with model systems (17). S y n t h e t i c p o l y p e p t i d e s were prepared c o n t a i n i n g both p o l a r hydrogen bond-forming glutamine r e s i d u e s and glutamic a c i d groups. As shown i n Figure 4, 2M and 8M urea helped s o l u b i l i z e the polymers i n aqueous s o l u t i o n s at low pH by d i s s o c i a t i n g hydrogen bonds between amide groups. But as the f r a c t i o n of glutamine residues was i n c r e a s e d i n the p o l y p e p t i d e s , i t was necessary to r a i s e the pH to induce more negative e l e c t r o s t a t i c charges on the glutamic a c i d r e s i d u e s i n order to d i s s o c i a t e the p o l y p e p t i d e s h e l d together by hydrogen bonds. The p a r t i c i p a t i o n of hydrophobic groups of n a t i v e wheat g l u t e n p r o t e i n s i n i n t e r m o l e c u l a r a s s o c i a t i o n s was demonstrated by Chung and Pomeranz (18) through use of hydrophobic gels as i l l u s t r a t e d i n F i g u r e 5. These workers introduced a s o l u t i o n of g l u t e n p r o t e i n s i n 0.01M a c e t i c a c i d i n t o a column of Phenyl-Sepharose-4B. L i t t l e p r o t e i n was e l u t e d by washing the column w i t h d i l u t e a c e t i c a c i d , but about 40% of the p r o t e i n was e l u t e d by a s o l u t i o n c o n t a i n i n g 1% of the detergent sodium sodium dodecyl s u l f a t e . Only a small amount of a d d i t i o n a l p r o t e i n was e l u t e d by other s o l v e n t s i n c l u d i n g 0.005M g l y c i n e NaOH i n 50% propylene g l y c o l . Thus, although hydrophobic bonds are i n d i v i d u a l l y weak, t h e i r combined s t r e n g t h i n p r o t e i n s , which can provide many such i n t e r a c t i o n s , can be c o n s i d e r a b l e . The r o l e of noncovalent bonds i n determining p r o t e i n s t r u c t u r e and aggregation has been confirmed by x-ray a n a l y s i s f


116

PROTEIN

FtMCtionai Groups Involved

Bond Type Physical Electrostatic -COO" N H +

3

Hydrogen Bond — C=0 HO— 1 m


°-vwv /WW_o

Salt Solutions Htfi or Low pH

Hydroxy!

Urea SoJrtow GuamdM Hydrochloride uineuiynwmamne

Lonf Aliphatic Detergents Chains Organic Solvents Aromatic

Covamnt Cystine

—s—s

Figure

3.

Disrupting Solvents

Carboxyl Amino Imidazote Gumbo

Phenol

Hydrophobe Bonds

F U N C T I O N A L I T Y I N FOODS

Reducing Agents Sulfite Mercaptoethanol

Types of bonds between protein

chains

60

ol 3.5

i

i

i

I

4.0

4.5

5.0

5.5

M i H i M H M pH f o r S o l u b i l i t y

Biochemistry Figure

4.

Influence of urea and pH on the solubilities of synthetic copolymers of glutamine and glutamic acid (\7)

polypeptide


6.

WALL AND HUEBNER

Adhesion

and

Cohesion

117

of p r o t e i n c r y s t a l s . At 2A r e s o l u t i o n , the s i t e s of the i n d i v i d u a l amino acids and the p r o x i m i t i e s of t h e i r f u n c t i o n a l groups can be deduced, and the nature of the bonding f o r c e s i n v o l v e d i n maintaining the f o l d i n g and a s s o c i a t i o n of polypeptide chains can be e s t a b l i s h e d . Studies on the s t r u c t u r e of the molecules of the enzyme t r y p s i n , the soybean t r y p s i n i n h i b i t o r , and the complex between the two p r o t e i n s i n d i c a t e that m u l t i p l e types of noncovalent linkages i n v o l v i n g hydrogen bonds, hydrophobic bonds, and e l e c t r o s t a t i c a t t r a c t i o n s p a r t i c i p a t e i n the molecu l a r a s s o c i a t i o n (19).


Molecular S i z e and

Shape

Not only do the kinds and amounts of f u n c t i o n a l groups on p r o t e i n s govern the extent of p r o t e i n i n t e r a c t i o n s , but t h e i r l o c a t i o n on the molecule and t h e i r a c c e s s a b i l i t y to groups i n other molecules are important a l s o . These f a c t o r s depend on the s i z e and shape of the p r o t e i n molecules. Polymer and adhesive chemists concur that h i g h l y cohesive f i l m s and other s t r u c t u r e s are a t t a i n e d by high-molecular-weight molecules that allow extensive molecular i n t e r a c t i o n s and by those with numerous i n t e r m o l e c u l a r covalent c r o s s l i n k s (jL, 14). In c o n t r a s t , adhesion depends more on a c c e s s i b i l i t y of f u n c t i o n a l groups of the adhesive to the adhering m a t e r i a l s . As shown i n Figure 6, p r o t e i n s of the gluten complex can be separated by s o l u b i l i t y d i f f e r e n c e s i n t o s a l i n e - s o l u b l e albumins and g l o b u l i n s , 70% e t h a n o l - s o l u b l e g l i a d i n s , a c e t i c a c i d - s o l u b l e g l u t e n i n , and an i n s o l u b l e p r o t e i n r e s i d u e . Albumins and g l o b u l i n s are h i g h l y f o l d e d compact molecules. Much of t h e i r backbone chain i s i n v o l v e d i n h e l i c a l hydrogenbonded a s s o c i a t i o n s . F o l d i n g i s maintained by both i n t e r n a l hydrogen and hydrophobic bonds as w e l l as by d i s u l f i d e bonds. The g l i a d i n s , with few charged groups, a s s o c i a t e to y i e l d a syrupy mass on h y d r a t i o n . But the l a r g e asymmetric g l u t e n i n molecules form a tough, rubbery, cohesive mass when hydrated. The l a r g e s i z e of s o l u b l e g l u t e n i n molecules i s due to l i m i t e d d i s u l f i d e bonds between polypeptide chains. The i n s o l u b i l i t y of residue p r o t e i n i s a t t r i b u t a b l e to extensive i n t e r m o l e c u l a r disulfide crosslinks. Evidence f o r v a r i a t i o n i n molecular s i z e of gluten p r o t e i n s was obtained when p r o t e i n e x t r a c t s of f l o u r were chromatographed on agarose g e l f i l t r a t i o n columns (Sephadex C1-4B) i n t r i s b u f f e r c o n t a i n i n g sodium dodecyl s u l f a t e as shown i n Figure 7 (20). Most albumins (Alb) and g l o b u l i n s (Glob) as w e l l as g l i a d i n s e l u t e l a t e , i n d i c a t i n g they have molecular weights below 40,000. In c o n t r a s t , g l u t e n i n shows a broad spectrum of s p e c i e s with some components having molecular weight over 1 million. Use of the hydrophobic bond-breaking s o l v e n t sodium dodecyl s u l f a t e d i s r u p t e d aggregation of the p r o t e i n s . The


118

PROTEIN FUNCTIONALITY I N FOODS

2.0 Ptayt-Septeose C N 8 Column 1.6 x 20 cm 1-5 h

4 mg Protein Applied Solvents -

1.0 h

1.

0.01 N acetic acid (50 ml)

2.

1% Na Dod S0 4 in 0.01 N acetic

3.

0.1 M Ktycme-NaON (50 ad)

4.

50% propylene glycol in 0.005 M gtycine-NaOH

acid (150 ml) , 0.8 h


1

2

60

80

100

120

140

160

Tube No.

Cereal Science Today Figure

5.

Elution of acid-soluble wheat proteins from hydrophobic gel PhenylSepharose-4B by different solvents. NaDodSO = SDS (IS). H

Class

Figure 6.

Features

Solubility

Albumins and Globulins

Salt Solutions

Gliadin

70% Alcohol Solution

Glutenin

1% Acetic Acid s - s

Residue

Reducing Agents or Alkali

a

s s

hs-s-

s

F

hs-s

Types of proteins in wheat flour as separated by solubility


WALL AND HUEBNER

Adhesion

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Cohesion

Col: Soph CI-4B Tris Borate SDS

Water Soluble Ext. Alb

IJM^~J„

Gliadins

r


Glutenin Ppt. 70% EtOH

Whole Gluten AUC Ext. 0

i ml

\^

/ J

/ \ /

i i i i i i 1200 12501 1300 350 100 150 7 7 5 2*10 10 10* 10 30.000 10.000 Approx. Mai. Wt.

Journal of Agricultural and Food Chemistry

Figure 7. Fractionation of wheat proteins on sepharose-CL4B columns by gel filtration chromatography in 0.125 trisborate buffer, pH 8.9 and 0.1% SDS (20). AUC ext. refers to protein extracted from wheat flour with a solution containing 0.1M acetic acid, 3M urea, and 0.01M cetyltrimethyl-ammonium-bromide.

Alkyleted-Rodicod Gletoaie (4% W/V) 32 24

/

o

1* m

jr

/

7 16 _

Gleteaia

/

W w/v)

^

1

125

•

1

250 375 Shear Stress, dyaes/cai.

500

2

e

•32 OK

16 -

Gliadia (10% w / v w

Alkyleted-ledeced Gliedie (10% W/V)^

24

yy

^

^

^

^

^

8 - I ^ I J — - — r i

125

i

i

i

250 375 500 625 750 875 Shear Stress, dynes/cu.

i

1000

2

Cereal Science Today Figure 8.

Shear-stress

curves for native and alkylated-reduced solutions at low shear stresses (5)

gliadin and glutenin



120

PROTEIN FUNCTIONALITY IN FOODS

low-molecular-weight g l u t e n i n p r o t e i n f r a c t i o n was not separated i n other solvents and appears to c o n s i s t of membrane p r o t e i n s that tend to aggregate. The e f f e c t of molecular s i z e and shape on p r o t e i n cohesive s t r e n g t h was i n d i c a t e d by measurements of t e n s i l e strength and e l o n g a t i o n of f i l m s cast from l a b o r a t o r y preparations of wheat gluten, g l i a d i n , and g l u t e n i n (5). The measurements were made with a S c o t t Tester Model IP-2 at 20°C and 42% r e l a t i v e humidity. Values f o r f o r c e a p p l i e d to the f i l m ( t e n s i l e strength) and e l o n g a t i o n at time of f i l m rupture are l i s t e d i n Table I. G l u t e n i n , which c o n s i s t s of the l a r g e r , more asymmetric molecules, forms f i l m s with greater t e n s i l e strength than g l i a d i n f i l m s . G l i a d i n f i l m s s t r e t c h f u r t h e r than those from g l u t e n i n due to t h e i r having weaker molecular a s s o c i a t i o n s . Gluten, which i s a mixture of g l i a d i n and g l u t e n i n , has intermediate f i l m p r o p e r t i e s . Marked changes i n the v i s c o s i t i e s of g l u t e n i n and g l i a d i n s o l u t i o n s , as measured by cone-plate viscometer, occur a f t e r cleavage of t h e i r d i s u l f i d e bonds by a reducing agent (Figure 8). Native g l u t e n i n has high v i s c o s i t y , which i n d i c a t e s not only high molecular weight but a l s o a h i g h l y asymmetric s t r u c t u r e 05). Furthermore the h y s t e r e s i s or d e v i a t i o n of the v i s c o s i t y curves f o r i n c r e a s i n g and decreasing shear s t r e s s versus r a t e of shear provide evidence f o r non-Newtonian behavior of these molecules due to molecular i n t e r a c t i o n s . Glutenin v i s c o s i t y

TABLE I T e n s i l e Strength

and

Percent

of Wheat P r o t e i n

Elongation

Films

T e n s i l e strength Film

lb/in

2

X 10

3

Elongation %

Glutenin

3.38

63

Gliadin

1.42

72

1.75

75

Whole g l u t e n

(laboratory

preparation)

From Wall and

Beckwith (5). Cereal Science Today


6.

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d e c l i n e s markedly when i t s i n t e r m o l e c u l a r d i s u l f i d e bonds are cleaved to l i b e r a t e i t s c o n s t i t u e n t p o l y p e p t i d e chains. The v i s c o s i t y of n a t i v e g l i a d i n i n s o l u t i o n i s much l e s s than that of g l u t e n i n . Reduction of the d i s u l f i d e s of g l i a d i n r e s u l t s i n a s i g n i f i c a n t i n c r e a s e i n i t s v i s c o s i t y due to u n f o l d i n g of the polypeptide molecule. Reduction of g l u t e n i n destroys i t s cohesive nature when hydrated, but the reduced p r o t e i n s are very s t i c k y and q u i t e adhesive.


Dough Rheology Because of the gluten p r o t e i n s , hydrated f l o u r can be worked i n t o an e l a s t i c - c o h e s i v e mass by mixing. The development of optimal dough p r o p e r t i e s with time during the mixing process can be followed on a mixograph. The mixograph r e c o r d i n g s i n F i g u r e 9 show that i n i t i a l l y the unoriented dough molecules offer l i t t l e resistance. As mixing proceeds, the asymmetric g l u t e n i n molecules are o r i e n t e d and a s s o c i a t e to i n c r e a s e dough s t r e n g t h . D i s u l f i d e - s u l f h y d r y l interchanges to f a c i l i t a t e rearrangement of the molecule and a c t u a l cleavage of the d i s u l f i d e l i n k s may a l s o take p l a c e during mixing (23). F i n a l l y , r e s i s t a n c e to mixing d e c l i n e s as polymer d i s r u p t i o n continues. The three curves show d i f f e r e n t mixing responses from f l o u r s of wheats of d i f f e r e n t breadmaking q u a l i t y as measured by Finney et a l . (21) and Finney and Shogren (22). The middle curve shows r e s i s t a n c e changes with time f o r dough from a f l o u r w i t h s u i t a b l e p r o p e r t i e s f o r breadmaking. I t e x h i b i t s a moderate dough development time and s t a b i l i t y time. The upper curve i s that f o r dough from a wheat whose g l u t e n i s o v e r l y s t r o n g , s i n c e i t r e q u i r e s longer mixing to achieve maximum dough s t r e n g t h , whereas the lower curve i s of a weak f l o u r dough w i t h s h o r t mixing time requirement but r a p i d breakdown of dough s t r e n g t h . The r e l a t i o n s h i p between d i f f e r e n t f l o u r s v a r y i n g i n dough s t r e n g t h and t h e i r composition of d i f f e r e n t p r o t e i n f r a c t i o n s was i n v e s t i g a t e d by Orth and Bushuk (24) and by Huebner and W a l l (25). The former workers found a c o r r e l a t i o n between the mixing time requirement of doughs and t h e i r t o l e r a n c e to mixing to t h e i r content of r e s i d u e p r o t e i n . As shown i n Figure 10, the l a t t e r workers analyzed f o r p r o t e i n content a s e r i e s of f l o u r s d e r i v e d from d i f f e r e n t Hard Red Winter wheat v a r i e t i e s that vary i n mixing time requirement (mixing s t r e n g t h ) . The stronger f l o u r s contained not only more r e s i d u e p r o t e i n but a l s o more of the higher molecular weight g l u t e n i n f r a c t i o n (Glutenin I ) . Changes o c c u r r i n g i n d i s u l f i d e bonds of wheat g l u t e n during dough mixing are supported by two o b s e r v a t i o n s . Mecham and Knapp (26) found that mixing i n the absence of a i r i n c r e a s e s the content of s u l f h y d r y l groups i n the dough. A l s o , there i s an i n c r e a s e i n e x t r a c t a b l e p r o t e i n d u r i n g mixing. This i n c r e a s e i n e x t r a c t a b l e p r o t e i n i s p r i m a r i l y high-molecular-weight


122



Cereal Chemistry Figure

10.

Yields of protein fractions isolated from hard red winter wheat differing in mixing strengths (25)


flours


6.

W A L L AND HUEBNER

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123

p r o t e i n according to g e l f i l t r a t i o n s t u d i e s conducted by Tsen (27), whose data i s given i n Figure 11. Apparently, mixing breaks down d i s u l f i d e bonds i n the h i g h l y c r o s s l i n k e d r e s i d u e p r o t e i n , thereby decreasing r e s i s t a n c e to mixing. A d d i t i o n of reducing agents such as c y s t e i n e which cleave d i s u l f i d e bonds weakens the dough (21), whereas a d d i t i o n of o x i d i z i n g agents such as bromate tends to strengthen i t by e l i m i n a t i n g s u l f h y d r y l groups (26)• F i g u r e 12 summarizes the c o n t r i b u t i o n of v a r i o u s g l u t e n p r o t e i n s to dough p r o p e r t i e s . The l a r g e asymmetric g l u t e n i n molecules have c o n s i d e r a b l e surfaces with numerous exposed f u n c t i o n a l groups to permit strong a s s o c i a t i o n by noncovalent forces. Fragments of h i g h l y c r o s s l i n k e d residue p r o t e i n s c o n t r i b u t e l a t e r a l cohesion and r e s i s t a n c e to laminar flow. During mixing the residue p r o t e i n s are probably degraded to y i e l d l i n e a r , more s o l u b l e molecules. The smaller g l i a d i n molecules are l e s s t i g h t l y bound and f a c i l i t a t e f l u i d i t y and expansion of the dough. These ideas are c o n s i s t e n t w i t h e x p e r i mental f i n d i n g s by Hoseney et a l . (28), who separated the g l u t e n p r o t e i n s from f l o u r s of d i f f e r e n t baking q u a l i t y and s u b s t i t u t e d g l i a d i n or g l u t e n i n p r o t e i n from good baking wheat f l o u r s f o r the same p r o t e i n f r a c t i o n i n poor f l o u r s i n recons t i t u t e d doughs. The g l i a d i n f r a c t i o n from good wheat f l o u r s appeared to improve l o a f volume, while the g l u t e n i n f r a c t i o n a f f e c t e d the mixing requirement and t o l e r a n c e . A l l components of dough must be present i n proper amounts f o r good breadmaking properties. I f the p r o t e i n i s too cohesive and tough, the dough w i l l not r i s e p r o p e r l y because expansion of trapped yeast-generated CO2 bubbles w i l l be minimal; but, i f the p r o t e i n matrix i s weak, the gas pockets w i l l break and the dough w i l l collapse. Uses of Wheat Gluten The unique p r o p e r t i e s of wheat g l u t e n p r o t e i n s have r e s u l t e d i n c o n s i d e r a b l e use of i s o l a t e d g l u t e n p r e p a r a t i o n s . Of the 20 m i l l i o n kg used i n the United S t a t e s , 69% i s used i n bakery products, 12% i n b r e a k f a s t foods, 9% i n pet foods, and 4% i n meat analogs (2). In baked goods, g l u t e n may be used to supplement weak f l o u r s to provide a d d i t i o n a l mixing s t r e n g t h and t o l e r a n c e . I t may be used i n s p e c i a l t y products, such as h i g h - f i b e r breads, where the added g l u t e n provides b e t t e r l o a f volume. A major use i s i n production of hamburger buns, where the supplemented g l u t e n i n c r e a s e s the s t r u c t u r a l s t r e n g t h of the hinge. When hydrated gluten i s heated above 85°C, the p r o t e i n i s denatured but r e t a i n s i t s shape and i t s r e s i l i e n c y (29). In bread and r o l l s , the gluten helps r e t a i n moisture i n the crumb and c o n t r i b u t e s to crumb s t r e n g t h . Gluten i s used i n many other foods where i t s adhesive and cohesive p r o p e r t i e s provide b e n e f i c i a l value (29). In b r e a k f a s t


124

PROTEIN FUNCTIONALITY IN FOODS 0.8

Mixing Time. min.

0.0

0.4

o 0.8

3.0

0.4 0


g- 0 8 S

0.4

. u

0

I

0.8

3

0.4

6.5

0 31.0 1.2 0.8 0.4

Figure 11. Gel filtration of acetic acid extracts of flour and of dough mixed different times (21)

100

200

150

Effluent, ml.

Cereal Chemistry

Dough Proteins

Figure 12.

Effect of wheat protein structures on molecular elastic properties

associations

and visco-



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c e r e a l s , i t i s used to bond the formulation p r i o r to cooking. The c o h e s i v e - e l a s t i c character of gluten i s the b a s i s f o r many vegetarian-simulated meat products where i t holds i n g r e d i e n t s and provides chewy texture. I t must be cautioned t h a t , because of i t s h i g h l y cohesive p r o p e r t i e s , n a t i v e gluten may not be compatible w i t h and serve as an adhesive f o r some i n g r e d i e n t s . In the case of meat chunks, however, gluten appears to be one of the best b i n d i n g agents among nonmeat p r o t e i n s . As shown i n Table I I , S i e g e l and h i s a s s o c i a t e s (30) have tested the adhesive s t r e n g t h of s e v e r a l p r o t e i n d i s p e r s i o n s i n s a l t and phosphate s o l u t i o n f o r b i n d i n g meat p a r t i c l e s . The bonded meat cubes were heated to 75°C before s l i c i n g , and t e n s i l e strength measurements were conducted on them. Only gluten and egg whites were b e t t e r than the s a l t s o l u t i o n c o n t r o l s i n cementing the meat pieces together. Thermal and A l k a l i n e Improvement of Adhesion i n Globular

Proteins

Most g l o b u l a r or albumin p l a n t p r o t e i n s e x h i b i t l i t t l e cohesive or adhesive p r o p e r t i e s i n t h e i r n a t i v e s t a t e . At higher pH, 11 or above, d i s u l f i d e bonds are cleaved, p r o t e i n u n f o l d i n g occurs, and f u n c t i o n a l groups p r e v i o u s l y a s s o c i a t e d w i t h i n the molecule become a v a i l a b l e f o r e x t e r n a l b i n d i n g . The paper s i z i n g and plywood i n d u s t r i e s use s t r o n g l y a l k a l i n e d i s p e r s i o n s of soybean p r o t e i n s as i n d u s t r i a l adhesives. But f o r food use, d r a s t i c a l k a l i n e treatment i s not d e s i r e d f o r i t r e s u l t s i n l o s s of c y s t i n e and formation of l y s i n o a l a n i n e (31). However, milder a l k a l i n e treatment i s used to denature soybean p r o t e i n s p r i o r to spinning f i b e r s (32). The p r o t e i n cohesion r e s u l t i n g when the f i b e r s are coagulated i n t o a c i d and s a l t i s necessary to maintain f i b e r i n t e g r i t y (Figure 13). In the i l l u s t r a t e d product "Bacos," egg albumin i s used as an adhesive to bond the f i b e r s i n t o a d e s i r e d f a b r i c a t e d meat-like product (33). Heat denaturation i s the most widely used and most important means of a l t e r i n g the adhesive and cohesive p r o p e r t i e s of globular proteins. E x t r u s i o n cooking i s growing i n use as a means of simultaneously shaping, t e x t u r i z i n g , and s t a b i l i z i n g p r o t e i n - r i c h products. Jeunink and C h e f t e l (34) have studied the nature of the changes i n the p r o t e i n that f o l l o w e x t r u s i o n cooking of f i e l d bean p r o t e i n concentrate. As shown i n Figure 14, the p r o t e i n i n the n e u t r a l concentrate i s 80% s o l u b l e i n n e u t r a l phosphate b u f f e r ; but a f t e r e x t r u s i o n , only 20% i s e x t r a c t e d by that s o l u t i o n . E x t r u s i o n denatured much of the p r o t e i n ; u n f o l d i n g of the chain permitted new f u n c t i o n a l group a s s o c i a t i o n s which rendered i t i n s o l u b l e . In the presence of the d i s s o c i a t i n g s o l v e n t , sodium dodecyl s u l f a t e (SDS), and the d i s u l f i d e breaking agent, d i t h i o t h r e i t o l (DTT), most of the p r o t e i n i s s o l u b i l i z e d . The aggregation of the extruded p r o t e i n e v i d e n t l y i s maintained by hydrophobic bonds and i n t e r m o l e c u l a r d i s u l f i d e l i n k s produced during heating.



126


Figure

13.

SEM of Bacos spun soybean protein fiber simulated Egg albumin used as adhesive for fibers f33J.

meat

product.

100

• 0.1 M phosphate butter. pH = 6.9 El 0.1 M phosphate butter. pH = 6.9 0.005% DTI

Figure 14. Protein solubility of initial and extruded field bean protein concentrates in different extraction solutions (26)

• •

0.1 M phosphate butter. pH = 6.9 5% (w/v) SDS 0.1 M phosphate butter. pH = 6.9 0.005% DTT 5% (w/v) SDS


Cereal Chemistry

WALL AND HUEBNER

Adhesion

and

Cohesion


TABLE II Meat Binding A b i l i t i e s of Various Nonmeat Proteins i n the Presence of 8% Salt and 2% Phosphate Binding a b i l i t y , Protein

grams

Wheat gluten

175.4

Egg white

120.3

Control

107.0

Calcium reduced dried skim milk

74.5

Bovine blood plasma

71.9

Isolated soy protein

66.7

Sodium caseinate

0

Journal of Food Science



128

protein functionality in foods

Cooking and texturizing vegetable proteins appears desirable if their properties for use in frankfurters or other proteinsupplemented meat products are to be optimized. Uncooked soybean or cottonseed proteins do not maintain the texture of prepared frankfurters when added at 10% or 30% levels to the formulations according to Terrell et al. (35). Stress deformation of frankfurters was measured on the Universal Instron Testing machine with a L.E.E.-Kramer press. All-meat frankfurters showed higher values of stress-deformation prior to rupture than products with added soy flour, soy concentrate, liquid-cyclone-processed cottonseed flour, or isolated cottonseed protein. The textured soy flour and textured cottonseed protein performed better than the uncooked proteins; at the 30% level, the addition of textured soy proteins resulted in frankfurters having a higher deformation strength than the a l l meat one. Conclusions Adhesion and cohesion are properties of many polymeric substances including proteins. The effectiveness of the proteins in bonding or shaping food ingredients is dependent on their composition and structure. Hydrophobic and hydrogen bonding functional groups on»amino acids associate with like groups within the protein to influence conformation or between molecules to result in aggregation. Disulfide bonds between proteins result in larger molecules or insoluble complexes. High molecular weight and random coil structure of protein result in more associations and thereby enhance adhesive and cohesive properties. Although these characteristics are inherent in native gluten proteins, functional properties of other proteins may be improved by chemical or thermal processing. There is no universally good adhesive for food constituents. Proteins that are highly cohesive may not blend well with certain other ingredients. It is necessary to examine the available proteins for optimum properties and to select the most satisfactory ingredient combinations. A number of instruments are available for measurements of textural properties of food ingredients or products, but the final criteria for acceptable performance must be taste-panel evaluations. The mention of firm names or trade products does not imply that they are endorsed by the U.S. Department of Agriculture over other firms or similar products not mentioned. Literature Cited 1.

Parker, R. S. R.; Taylor, P. "Adhesion and Adhesives;" Pergamon Press, London, 1966.


6. wall and huebner 2. 3. 4. 5. 6.

7.


8.

9. 10. 11. 12. 13.

14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28.

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Sarkki, M. L. J . Am. Oil Chem. Soc., 1979, 56, 443. Wookey, N. J . Am. Oil Chem. Soc., 1979, 56, 306. Khoo, U.; Christianson, D. D.; Inglett, G. E. Bakers Dig., 1975, 49(4), 24. Wall, J. S.; Beckwith, A. C. Cereal Sci. Today, 1969, 14(1), 16. Voisey, P. W.; de Man, J . M. Applications of Instruments for Measuring Food Texture, in "Rheology and Texture in Food Quality," J . M. de Man, P. W. Voisey, V. F. Rasper, and D. W. Stanley, Eds., Avi Publishing Co., 1975, p. 142. Bourne, M. C.; Mayer, J . C.; Hand, D. B. Food Technol., 1966, 20, 170. Voisey, P. W. Instrumental Measurement of Food Texture, in "Rheology and Texture in Food Quality," J . M. de Man, P. W. Voisey, V. F. Rasper, and D. W. Stanley, Eds., Avi Publishing Co., 1976, p. 79. Frazier, P. J.; Daniels, N. W. R.; Eggitt, P. W. R. Cereal Chem., 1975, 52 (3, part II), 106r. Rasper, V. F. Cereal Chem., 1975, 52 (3, part II), 24r. Shuey, W. C. Cereal Chem., 1975, 52 (3, part II), 42r. Shuey, W. C. "Farinograph Handbook." American Association of Cereal Chemists, St. Paul, Minnesota, 1972. Anfinsen, C. B.; Scheraga, H. A. Experimental and Theoretical Aspects of Protein Folding; in "Adv. Protein Chemistry," 29, C. B. Anfinsen, J . T. Edsall, and F. M. Richards, Eds., Academic Press, New York, 1975, p. 205. Van Krevelan, D. W. "Properties of Polymers, Their Estimation, and Correlation with Chemical Structure;" Elsevier, Amsterdam, 1976. Krull, L. H.; Wall, J . S. Bakers Dig., 1969, 43(4), 30. Wall, J . S. J . Agric. Food Chem., 1971, 19, 619. Krull, L. H.; Wall, J . S. Biochemistry, 1966, 5, 1521. Chung, K. H.; Pomeranz, Y. Cereal Chem., 1979, 56, 196. Sweet, R. M.; Wright, H. T.; Janin, J.; Clothia, C. H.; Blow, D. M. Biochemistry, 1974, 13, 4212. Huebner, F. R.; Wall, J . S. J . Agric. Food Chem., 1980, 28, 433. Finney, K. F . ; Tsen, C. C.; Shogren, M. D. Cereal Chem., 1971, 48, 540. Finney, K. F . ; Shogren, M. D. Bakers Dig., 1972, 46(2), 32. Axford, P. W. E.; Campbell, J. D.; Elton, G. A. H. Chem. Ind. London, 1962, 407. Orth, R. A.; Bushuk, W. Cereal Chem., 1972, 49, 268. Huebner, F. R.; Wall, J. S. Cereal Chem., 1976, 53, 258. Mecham, D. K.; Knapp, C. Cereal Chem., 1966, 43, 226. Tsen, C. C. Cereal Chem., 1967, 44, 308. Hoseney, R. C.; Finney, K. F . ; Shogren, M. D.; Pomeranz, Y. Cereal Chem., 1969, 46, 126.

RECEIVED September 5, 1980.


29. 30. 31. 32.

33.


34. 35.

Kalin, F. J. Am. Oil Chem. Soc., 1979, 56, 477. Siegel, D. G.; Church, K. E . ; Schmidt, G. R. J . Food Sci., 1979, 44, 1276. DeGroot, A. P.; Slump, R. J . Nutr., 1969, 98, 45. Smith, A. K.; Circle, S. J . Protein Products as Food Ingredients in "Soybeans: Chemistry and Technology I;" A. K. Smith and S. J. Circle, eds. Avi Publishing Co., Westport, Connecticut, 1972, p. 339. Wolf, W. J . Scanning Electron Microscopy Studies of Soybean Proteins; "IIT Symposium on Scanning Electron Microscopy" 1980 (in press). Jeunink, J.; Cheftel, J . C. J . Food Sci., 1979, 44, 1322. Terrell, R. N.; Brown, J . A.; Carpenter, Z. L . ; Mattill, K. F . ; Monagle, C. W. J . Food Sci., 1979, 44, 865.


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