Interactions of Food Proteins - American Chemical Society

Chapter 17

Factors Influencing Heat-Induced Gelation of Muscle Proteins

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Denise M. Smith Department of Food Science and Human Nutrition, Michigan State University, East Lansing, MI 48824-1224

The gelation of muscle proteins is responsible for the textural attributes of many processed meat products. Successful modification of product texture requires an under-standing of the physicochemical or intrinsic properties of proteins and how proteins are influenced by environmental and processing conditions. Percentage of alt-soluble protein and source of the protein (skeletal, cardiac or smooth muscle) in a formulation determine the rheological and microstructural charac teristics of heat-induced gels. By binding water or interacting with salt-soluble meat proteins the addition of eater-soluble and insoluble proteins may modify gel characteristics. Rheological properties and microstructure of gels are determined by the chem ical properties of proteins in solution and thus can be modified by changing the pH and salt concentration. The temperatures at which salt-soluble muscle proteins unfold and reaggregate into a cross-linked network may alter gel properties due to interactions with other macromolecules. Non-destructive dynamic rheological properties of salt-soluble proteins can be monitored during a controlled heating process to help elucidate the relationship between the physical and molecular changes during gelation. The heat-induced gel forming properties of muscle proteins are one of the most important functional properties observed i n processed meat products 2, 3) and are responsible for the texture, waterholding, binding, and appearance of the meat products (JO. As the variety o f new meat products increase, the need to understand, modify and control protein gelation becomes more important (2). Currently, a t r i a l and error approach i s used by processors when making ingredient substitutions and process modifications. This approach i s time consuming and expensive. When the biochemical basis of protein gelation and factors which influence the properties 0097-6156/91/0454-0243$06.00Α) © 1991 American Chemical Society In Interactions of Food Proteins; Parris, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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INTERACTIONS OF FOOD PROTEINS

of f i n i s h e d gels are understood, processors w i l l be able to s c i e n t i f i c a l l y and economically manipulate protein properties to develop desired textures and y i e l d s i n processed meat products. The biochemical basis f o r the gelation o f s k e l e t a l muscle proteins has been reviewed by several authors (J[ 5), however, the gelation properties o f cardiac and smooth muscles have not been characterized. The e m u l s i f i c a t i o n properties of cardiac and smooth muscle proteins have been studied i n emulsion systems (6_), but not in gel systems. Recent evidence indicates that e m u l s i f i c a t i o n properties o f muscle proteins do not correlate well with function i n a meat product (7_) · A wide range of i n t r i n s i c and e x t r i n s i c factors influence the properties of muscle protein gels (jB). Researchers are j u s t beginning to assess the importance of ingredient i n t e r a c t i o n s on heat-induced gel q u a l i t y . The contribution o f the s a l t soluble s k e l e t a l proteins t o processed meat texture has been studied extens i v e l y , however the modifying influence of the sarcoplasmic and stroma proteins i s l e s s well understood. Non-muscle proteins used in combination with the s a l t soluble proteins can also influence g e l properties. The contribution of non-muscle proteins to the finished q u a l i t y o f muscle protein gels was reviewed by Foegeding and Lanier (9,). A wide range o f gel textures and microstructures can be produced from multicomponent gels OC^, JJ_, Î2). The contribution of each protein to the texture and microstructure of the g e l w i l l depend on concentration, pH, i o n i c strength and heating temperature (13)» thus the properties which can be engineered into m u l t i component gels are almost unlimited. The objective o f t h i s paper i s to demonstrate how the properties of muscle protein gels can be manipulated by changes i n processing conditions and i n t e r a c t i o n s between muscle and non-muscle proteins. Results i l l u s t r a t e how simple modifications i n pH and temperature modify the texture and water-holding properties o f muscle protein gels. This paper w i l l show how ingredient i n t e r a c t i o n s can have negative or p o s i t i v e e f f e c t s on muscle protein gel properties depending on the processing conditions and formulation selected.


f

Experimental Procedures Materials. S a l t soluble proteins (SSP) were extracted from chicken breast as described by Wang (14). The SSP was suspended i n the 0.6M NaCl, 50 mM Na phosphate sample buffer and adjusted to the desired pH. Beef semitendinosus muscle, heart, lung and spleen proteins were extracted into high i o n i c strength (0.6M NaCl, 0.05M Na phosphate b u f f e r , pH 7.0) soluble or SSP, low i o n i c strength (0.05M Na phosphate b u f f e r , pH 7.0) soluble (LIS, sarcoplasmic) and insoluble fractions as described by Nuckles et a l . ( 15). Whey protein concent r a t e s (WPC) produced commercially by u l t r a f i l t r a t i o n were heated to produce f i v e treatments which contained from 27% to 98% soluble prot e i n (16) as determined by the assay o f Morr et a l . (17). The WPC were freeze dried f o r storage. A l l dried ingredients were hydrated in the sample buffer 12 hr p r i o r to use i n experiments. Gel Preparation and Evaluation. Proteins were combined i n the desired concentrations and mixed i n a Polytron Homogenizer for 30 s at a speed of 6 i n an i c e bath. Heat induced gels were prepared at

In Interactions of Food Proteins; Parris, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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Heat-Induced Gelation of Muscle Proteins

65, 80 or 90°C (_14_, Jj> J 6 ) . Gel texture by f a i l u r e testing was measured on 10 mm diameter by 10 mm height cores using the Instron (Model 4202, Canton, MA) at a crosshead speed of 10 mm/min with a 50 Ν compression c e l l . The gels were compressed between two l u b r i c a t e d f l a t p a r a l l e l p l a t e s . Apparent stress (gel strength) and s t r a i n (deformability) at f a i l u r e were calculated from the force time curve as described by Hamann (18). Non-failure testing was performed using the Rheometrics f l u i d s spectrometer and p a r a l l e l p l a t e geo metry as described by Wang (14). Expressible moisture was determined as described by Jaurequi et a l . (19) except the gels were c e n t r i fuged at 755 x g for 10 min. Gels were prepared for scanning e l e c tron microscopy (SEM) as described by Smith (20) and observed using a JEOL 35 Scanning Electron Microscope (Japan Electronics) at 15 kV. A l l means are the r e s u l t of at l e a s t t r i p l i c a t e analyses.


f

Results and Discussion S o l u b i l i t y Differences of S k e l e t a l , Cardiac and Smooth Muscle Proteins. Tissues from s k e l e t a l , cardiac and smooth muscle e x h i b i t large differences i n protein content and d i s t r i b u t i o n and conse quently vary widely i n protein f u n c t i o n a l i t y and bind values. The causes for t h i s v a r i a t i o n have not been determined, however, the differences i n protein f u n c t i o n a l i t y may be due to differences i n protein content or composition. The protein f r a c t i o n composition of several beef muscles i s i l l u s t r a t e d i n Table I (15). Beef semitendinosus ( s k e l e t a l ) muscle contained at l e a s t twice as much SSP protein as the cardiac (heart) and smooth (lung, spleen) muscles. Beef heart and lung contained the highest quantity of insoluble pro t e i n s , while spleen contained the highest quantity of LIS proteins. Studies i n a frankfurter model system suggested that the larger the SSP f r a c t i o n as a percentage of the t o t a l p r o t e i n , the greater the firmness, cohesiveness and water-holding capacity of the meat product (15). Table I. Protein f r a c t i o n s of beef tissues expressed percentage of t o t a l p r o t e i n . (Based on data from Ref. 15 and 21) Tissue Semitendinosus

Low Ionic Strength Soluble 21.8

(%)

Salt Soluble (%) 44.9

as a

Insoluble (%) 28.2

Heart

30.1

21.2

48.7

Lung

36.3

9.6

54.1

Spleen

53.6

21.7

24.7

Functionality of SSP Fractions. The lowest protein concentration of the SSP f r a c t i o n required to form a gel (defined as apparent stress at f a i l u r e l e s s than or equal to 4.0 kPa) varied with the t i s s u e s . The SSP f r a c t i o n of beef semitendinosus muscle, heart, lung and spleen formed gels at protein concentrations of 6.0%, 5.0%, 4.0% and 3.0%, r e s p e c t i v e l y , i n 0.6M NaCl, 50 mM Na phosphate, pH 7.0 when


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heated to 70°C (21). The expressible moisture (33.5%) and deforma b i l i t y (0.50) o f the gels were not s i g n i f i c a n t l y d i f f e r e n t at t h e i r least concentration endpoints. In 6.0% SSP gels, strength and deformability s i g n i f i c a n t l y decreased and expressible moisture s i g n i f i c a n t l y increased in the following order: semitendinosus, heart, lung, and spleen (Table I I ) . Large differences i n gel microstructure were observed between the t i s s u e s . Results indicated that the gel forming properties o f the SSP f r a c t i o n s were not equivalent and thus f u n c t i o n a l i t y o f muscle tissues cannot be compared s o l e l y on the basis of the quantity of SSP i n the t i s s u e . Table I I . Texture and expressible moisture of beef s a l t soluble protein g e l s . (Based on data from Réf. 2λ) Apparent stress at f a i l u r e (kPa)

Tissue Semitendinosus Heart Lung Spleen

13.01® 9.23 6.89° 6.22° D

Apparent s t r a i n at f a i l u r e 81

°· κ 0.77° 0.70° 0.66 d

Expressible moisture (%) 19.3* 23.3 26.7* 33.7 b

d

Any two means within the same column followed by d i f f e r e n t l e t t e r s were s i g n i f i c a n t l y d i f f e r e n t at p< 0.05. Composition of SSP Fractions. Sodium dodecyl sulfate e l e c t r o phoresis o f the SSP f r a c t i o n s were performed on 12% acrylamide gels (22) to determine differences i n the protein composition of the SSP f r a c t i o n . Beef semitendinosus and heart SSP f r a c t i o n s contained s i g n i f i c a n t l y larger percentages o f myosin and a c t i n compared to the other two tissues (Table I I I ) (21). The actin:myosin mole r a t i o s for beef semitendinosus and heart were 6.0 and 5.0, r e s p e c t i v e l y . Beef lung exhibited the lowest r a t i o of 3.2. The quantity of myosin in the SSP f r a c t i o n was strongly correlated with gel strength (r=0.86), deformability (r=.80) and expressible moisture (r=-.82) (21). These r e s u l t s suggest a new method for measuring the bind constants or r e l a t i v e f u n c t i o n a l i t i e s of meat ingredients used i n least cost formulation c a l c u l a t i o n s by meat processors. It may be possible to predict the gel strength and other f u n c t i o n a l i t i e s of a l l muscle tissues based on the s i z e o f the SSP f r a c t i o n and amount of myosin i n that f r a c t i o n . Currently, very poor c o r r e l a t i o n s e x i s t between bind constants measured by e m u l s i f i c a t i o n capacity and the actual function of meat ingredients i n a formulation. Influence of Stromal and Sarcoplasmic Proteins. Other muscle pro teins can also modify the properties of SSP gels. The influence o f the sarcoplasmic or low i o n i c strength (LIS) proteins and the i n soluble or stroma proteins on the gel properties of SSP gels from beef s k e l e t a l , cardiac and smooth muscle tissues were determined (21). LIS and insoluble protein f r a c t i o n s were substituted i n t o SSP protein solutions at 8.33» 16.7» 25» 33.3 and 50% to prepare gels


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with a t o t a l protein content of 6.0% (w/w) i n 0.6 M NaCl, 50 mM Na phosphate, pH 7.0 and heated to 70°C.


Table III. Myosin and actin composition of beef tissue salt soluble protein fraction. (Adapted from Ref. 15)

Tissue

Myosin (*)

Actin (*)

Semitendinosus Heart Lung Spleen

50.7. 47.9 37.6° 22.6

21.6 20.6 10.5° 10.1°

b

d

K

Actin:Myosin Mole Ratio 6.0

a

b

5

·

0

Ηd

3.2 4.5°

d

Source: Lowey, S. and Risby, D. 1971. Nature 234:81. b,c,d Any two means within the same column followed by d i f f e r e n t l e t t e r s were s i g n i f i c a n t l y d i f f e r e n t at p< 0.05. The addition of the LIS f r a c t i o n to the SSP gels resulted i n a s i g n i f i c a n t decrease i n gel strength of beef semitendenosus muscle (Table IV). The largest decrease i n gel strength was observed at the 8.3% s u b s t i t u t i o n l e v e l . Substitution of 16.7% to 41.7% LIS proteins into SSP gels did not s i g n i f i c a n t l y decrease gel strength in comparison to 6% (w/o) protein gels made e n t i r e l y from SSP. Deformability decreased gradually as the LIS concentration i n the gels increased. Expressible moisture decreased when SSP gels were substituted with 8.3% LIS proteins, but increased at higher s u b s t i t u t i o n l e v e l s . LIS s u b s t i t u t i o n s up to 16.7% of the SSP proteins did not have a s i g n i f i c a n t deleterious e f f e c t on expressible moisture. Small q u a n t i t i e s of LIS or sarcoplasmic proteins had an adverse e f f e c t on the strength and deformability of the SSP g e l matrix. Sarcoplasmic proteins may i n t e r f e r e with SSP c r o s s - l i n k i n g during matrix formation as they do not form gels and have poor water-holding a b i l i t i e s (23). Venegas et a l . (24) reported that meat homogenates containing large q u a n t i t i e s of sarcoplasmic pro t e i n s have good water-holding capacity, but poor gel strength. Scanning electron micrographs of SSP and SSP/LIS protein gels revealed very d i f f e r e n t microstructures (21). The SSP gels ex h i b i t e d a fibrous network with d i s t i n c t globular chains, while the SSP f i b e r s i n the SSP/LIS gel network appeared to be coated with a layer of LIS proteins. I t i s possible that a small amount of the hydrophilic sarcoplasmic proteins may enhance the water-holding capacity o f SSP gels by increasing the surface area o f the protein f i b e r s i n the matrix. The addition o f 8.3% insoluble proteins to the SSP gels had no s i g n i f i c a n t e f f e c t on gel strength or deformability, however, higher substitutions decreased these properties (Table IV). Expressible moisture increased i n a l l t i s s u e s as the quantity of insoluble pro teins i n the 6% protein gels increased. The microstructure of 6% SSP gels were d i f f e r e n t from those of SSP gels substituted with 33% insoluble protein (21). The addition of insoluble proteins appeared

American Chemical Society Library 1155 of16th N.W.Parris, N., et al.; In Interactions FoodSt., Proteins; ACS Symposium Series; American Chemical Society: Washington, DC, 1991. Washington. D.C. 20036

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to interrupt the regular fibrous network causing large spaces i n the gel matrix. Table IV. Substitution o f 6.0% (w/w) semitendinosus s a l t - s o l u b l e protein gels with low i o n i c strength or insoluble protein f r a c t i o n s . (Based on data from Ref. 21) Apparent Stress at Failure (kPa)

Substitution (%) Downloaded by PENNSYLVANIA STATE UNIV on July 1, 2012 | http://pubs.acs.org Publication Date: February 19, 1991 | doi: 10.1021/bk-1991-0454.ch017

Low Ionic

Apparent Strain at Failure

Strength Protein Fraction r,

8.3 16.7 25.0 33.3 41.7 50.0

Expressible Moisture (%)

6.1 4.9 4.6 4.4 4.3 3.7

b e

e

e e

d

ι-» Λ

y

0.80® 0.68° 0.60 0.58 0.54 0.52 e

e

e

d

17.7° 19.5 20.8 25.5 30.8® 37.1 a e

d

f

Insoluble Protein Fraction 0.0 8.3 16.7 25.0 33.3 41.7 50.0

a, u,

13.0* 12.8 9.1° 6.7

0.81° 0.80 0.71* 0.68° 0.64 0.62 0.61

a

a

e

e

6

·

3

Η

5.1 4.1

e

d e

e

· κ 22.7° 24.7 28.5 35.5 37.5 40.7 1 9

3

e

d

e

1

g

u, c, i, g i t h i n the same column followed by d i f f e r e n t l e t t e r s were s i g n i f i c a n t l y d i f f e r e n t at p< 0.05. A

n

y t

w

Qm e a n s

w

Effect o f Temperature and pH on SSP Gel Properties. Temperature and pH have a large influence on the properties of SSP gels. Wang (14) examined the dynamic rheological properties o f 3.0% chicken breast SSP solutions at 4 pHs (4.5, 5.5, 6.5 and 7.5) during heating at a rate o f 1°C/min i n a Rheometrics f l u i d s spectrometer at 1% s t r a i n and frequency of 10 rad/sec. Gel r i g i d i t y as determined from the complex modulus at 80°C decreased i n the following order: pH 5.5 > 6.5 > 7.5 > 4.5. Expressible moisture did not follow the same pattern, as expressible moisture increased i n the following order: pH 6.5 < 7.5 < 5.5 < 4.5. Gel properties were probably influenced by the protein t r a n s i t i o n s observed during heating (Figure 1). The SSP solution at pH 4.4 did not undergo any detectable rheological t r a n s i t i o n s during heating, however, several t r a n s i t i o n s were observed for SSPs at the other pHs. As pH increased, the tempera ture o f the f i r s t t r a n s i t i o n increased from 35.5°C at pH 5.5 to 47.0°C at pH 7.5. The shape o f the t r a n s i t i o n s also d i f f e r e d with pH i n d i c a t i n g the influence of protein charge on gel properties. A range of microstructures were observed i n the gels. Repre-



SMITH


Figure 1. Effect of pH on the complex moduli of 3% solutions of chicken breast s a l t - s o l u b l e proteins i n 0.6 M NaCl heated at 1°C/min.


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250


sentative scanning electron micrographs of SSP gels heated to 55°C and 65°C are presented at 10,000 X magnification i n Figures 2 and 3» respectively. The gels prepared at pH 4.5 e x h i b i t highly aggregated, globular structures with no network formation and are s i m i l a r at both heating temperatures. These microstructures are consistent with the poor gel strength and high expressible moisture observed. Gels at pH 5.5 heated to 55°C were filamentous and exhibited an i r r e g u l a r network structure with large holes i n the matrix, while regular lacy networks were observed at pH 6.5 and 7.5. At 65°C, the protein network became thicker and more r e g u l a r l y spaced which i s i n d i c a t i v e of increased g e l strength and water-holding capacity. These r e s u l t s suggest i t i s possible to produce a range of properties i n muscle protein gels simply by c o n t r o l l i n g the pH and heating temperature. These factors are r e l a t i v e l y easy to control and could be used by meat processors to manipulate the textures and y i e l d s i n processed products. Influence o f Non-meat Proteins. Other proteins used i n a meat f o r mulation also influence the gelation properties o f the SSPs and the resultant texture and y i e l d i n a processed meat product. Three types of multicomponent gels have been defined by Tolstoguzov and Braudo (1983): f i l l e d , mixed and complex. Non-meat proteins may act as f i l l e r s within the i n t e r s t i t i a l spaces of the SSP gel network. F i l l e r s are defined as macromolecules which do not form a g e l matrix themselves, but f i l l the i n t e r s t i t i a l spaces o f a gel matrix (JO). Some authors have reported that f i l l e r materials increase g e l strength and have a minimal e f f e c t on gel deformability (13» 27» 26), however, mechanical properties o f f i l l e d gels depend on the properties of the matrix g e l , the shape, deformability and volume f r a c t i o n of the f i l l e r and the i n t e r a c t i o n s between the f i l l e r and matrix (_1£, J_1_, 28). F i l l e r s have been c l a s s i f i e d as active or i n a c t i v e . Inactive f i l l e r s do not strengthen the gel matrix (11) while active f i l l e r s increase matrix strength. A mixed gel i s one in which two or more d i f f e r e n t types o f macromolecules form gel matrices. Multicomponent gel theory predicts that mixed and f i l l e d gels w i l l have widely d i f f e r e n t properties and thus would expand the t e x t u r a l a t t r i b u t e s of meat products which could be obtained from a formulation (10, 11, 13, 26). Beuschel (16) investigated the gelation properties o f a WPC and chicken breast SSP when heated to 65°C or 90 C i n 0.6 M NaCl, 50 mM Na phosphate b u f f e r , pH 7.0. Chicken breast SSP g e l s , WPC gels and combination gels were prepared by heating mixtures i n 0.6 M NaCl, 50 mM Na phosphate, pH 7.0 to 65°C and 90°C. The two temperatures were selected to evaluate SSP/WPC gel properties heated below and above the denaturation temperature o f the major whey p r o t e i n , betal a c t o g l o b u l i n . Beta-lactoglobulin i s p r i m a r i l y responsible for the gelation of WPC. The two major whey proteins, alpha-lactalbumin and beta-lactoglobulin denature at approximately 68°C and 80°C, respect i v e l y , although the t r a n s i t i o n temperatures are influenced by pH, ionic strength and other factors (25). Myosin i n chicken breast SSP denatures at approximately 55°C. Wang (14) reported that heating SSP above 70°C did not improve gel strength. The temperature had a large influence on the properties of the



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Figure 2. Microstructure of chicken breast s a l t - s o l u b l e protein gels i n 0.6 M NaCl heated to 55°C at a) pH 4.5 b) pH 5.5 C) pH 6.5 and d) pH 7.5.

Figure 3. Microstructure of chicken breast s a l t - s o l u b l e protein gels i n 0.6 M NaCl heated to 65°C at a) pH 4.5 b) pH 5.5 C) pH 6.5 and d) pH 7.5.


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protein g e l s . Whey protein concentrate d i d not g e l at 65°C (Table V). Gels prepared with 4% SSP and 8% or 12% WPC protein were only s l i g h t l y stronger than gels prepared with 4% SSP alone. SSP gels prepared with 16% WPC were weaker than those prepared with 4% SSP probably due to the interference of WPC i n the SSP protein network. SSP gels were most deformable. At 65°C, deformability decreased as WPC concentration increased i n SSP gels (Table V I ) . With each 4% increase i n WPC the deformability decreased by 0.2 u n i t s . Expressible moisture was strongly influenced by WPC concent r a t i o n i n the gels (Table V I I ) . The addition of 8% and 12% WPC to 4% SSP gels caused approximately a 50% decrease i n expressible moisture at 65°C. Table V. Strength o f chicken s a l t - s o l u b l e protein (SSP) and whey protein concentrate (WPC, 98% soluble) gels (Based on data from Ref. 16)

Treatment SSP WPC

SSP:WPC

Apparent Stress at F a i l u r e (kPa) Temperature 65°C 90°C

Percent Protein in SSPrWPC Gels 4:0 8:0 0:8 0:12 0:16 0:20 4:8 4:12 4:16

24.03° 76.74 no gel no g e l no gel no g e l 27.78° 27.09 15.21° a

D

no gel 53.67° no g e l 15.32 28.07® 94.85° 46.67° 97.66° 134.29 1

3

, b, c, d, e, f i t h i n the same column followed by d i f f e r e n t l e t t e r s were s i g n i f i c a n t l y d i f f e r e n t at p< 0.05. A n y t W Q

m e a n s

W

The WPC probably functions as a p a r t i c u l a t e f i l l e r i n SSP gels at 65°C as t h i s temperature i s below the denaturation temperature for the whey proteins. Our r e s u l t s are consistent with f i l l e d g e l theory, where WPCs act as an i n a c t i v e f i l l e r within the SSP matrix. The addition of i n a c t i v e f i l l e r s , such as wheat gluten and soy prot e i n i s o l a t e s , do not usually increase g e l strength, but do increase water-holding capacity i n processed meat products (9_, 29» 30» 31 ) . Wheat gluten proteins and soy proteins do not denature at temperatures t y p i c a l l y used i n meat processing and probably act as f i l l e r s . The SSP gels heated to 90°C were weaker than gels prepared at 65°C The WPC formed gels at 90°C which increased i n strength with protein concentration. Gels prepared with SSP and WPC were stronger than WPC gels o f the same protein concentration (Table V). Gels prepared with 4% SSP and 12% WPC (16% protein gels) were 3.5 times stronger than 16% WPC g e l s . The e f f e c t decreased as protein concent r a t i o n increased. Gels prepared with 4% SSP and 16% WPC were only 1.5 times stronger than 20% WPC g e l s . At 90°C, deformability was


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constant in SSP/WPC gels and was dictated by the WPC (Table V I ) . The SSP/WPC gels bound more water than WPC gels at the same protein concentration (Table V I I ) . Gels prepared with 4% SSP and 12% WPC bound 4.3 times more water than 16% WPC gels. There appears to be an optimum combination of SSP and WPC proteins. The 4% SSP/12% WHC


Table VI. Deformability of chicken salt-soluble protein (SSP) and whey protein concentrate (WPC, 98% soluble) gels (Based on data from Ref. 16)

Treatment

SSP WPC

SSP:WPC

Percent Protein in SSP:WPC Gels

4:0 8:0 0:8 0:12 0:16 0:20 4:8 4:12 4:16

Apparent Strain at Failure Temperature 65°C 90°C

a

1.61 1.61 no gel no gel no gel no gel 1.20° 1.04° 0.81° a

no gel 1.61 no gel 0.89° 0.69 b,c a

8 8

b

AA »

c

b,c 0.83. b,c 0.72 1

a , D , c

A n y two means within the same column followed by different l e t t e r s were s i g n i f i c a n t l y different at p< 0.05.

gels exhibited the largest increases in gel strength and expressible moisture over the same concentration of WPC protein alone. A synerg i s t i c effect occurred in the gels when both SSP and WPC proteins were heated to temperatures above their denaturation temperatures. At 90°C, the SSP and WPC probably form mixed gels as both protein fractions are denatured and reaggregate to form gels below t h i s temperature. The proteins formed a gel matrix which was stronger and held more water that either protein alone. This type of r e s u l t has been reported in other mixed gel systems (32, 33). The degree of denaturation of non-meat protein additives i s another factor determining gel properties. Whey protein concentrates were heated to i n s o l u b i l i z e the proteins to produce five concentrates with s o l u b i l i t i e s ranging from 27% to 98% (16). Gels were prepared from 4% SSP/12% WPC. Interactions between the denatured WPC proteins and SSP occurred during heating which altered the properties of the gel matrix. The use of denatured WPC improved SSP gel strength and deformability at low temperatures compared to undenatured (98% soluble) WPC. Denatured whey proteins interacted with SSP to enhance the gel matrix at 65°C. At 65°C, gel strength and deformability increased as the extent of WPC i n s o l u b i l i z a t i o n decreased to 41%. Gels prepared with 41% i n s o l u b i l i z e d protein were over 3 times stronger than those produced with 98% soluble WPC. S i m i l a r l y , expressible moisture of SSP/WPC gels decreased as the extent of the WPC denaturation increased. It i s not clear whether the denatured WPC were acting as active f i l l e r s within the i n t e r -


254



s t i t i a l spaces of a SSP gel matrix or whether there was d i r e c t i n t e r a c t i o n between SSP/WPC protein f i b e r s to form a mixed or complex gel (28). The denatured WPC were probably functioning as active f i l l e r s i n the SSP gels as the denatured WPCs alone did not produce gels when heated to 65°C. Gel properties were very d i f ferent at 90°C. Gel strength decreased and deformability increased as the extent of WPC i n s o l u b i l a t i o n increased i n both WPC and WPC/SSP gels. Gel strength of 98% soluble WPC was approximately twice that of 41% soluble WPC at 90°C. I t i s evident that the denatured WPCs were not able to form a mixed gel matrix with the SSPs, as gel strength decreased to that of the SSPs alone. Table VII. Expressible moisture of chicken s a l t - s o l u b l e protein (SSP) and whey protein concentrate (WPC, 98% soluble) gels (Based on data from Ref. 16) Percent Protein in SSP:WPC Gels

Treatment SSP

4:0 8:0 0:8 0:12 0:16 0:20 4:8 4:12 4:16

WPC

SSP:WPC

Expressible Moisture Temperature 90°C 65°C a

38.0 13.1° no gel no gel no gel no gel 16.0*·'

17.7^' 21.2 b

32.3^ 16.8 63.5^ 41.8 31.8< 20.3° 17.8 7.3 5.7 d

e

d

€

€

a,b,c,d i t h i n the same column followed by d i f f e r e n t l e t t e r s were s i g n i f i c a n t l y d i f f e r e n t at p< 0.05. Any

t w Q

m e a n s

W

Conclusions The properties of muscle protein gels can be manipulated by a v a r i e t y of factors and these factors could be exploited by the meat processing industry when engineering new products. Proteins from s k e l e t a l , smooth and cardiac muscles produce a range of r h e o l o g i c a l properties i n heated-induced g e l s . The f u n c t i o n a l i t y of these proteins i n gels can be predicted based on the myosin content of the t i s s u e s . Gel properties are modified by the amount of stroma, sarcoplasmic and non-muscle proteins i n the gel system. A wide range of properties can be engineered into gels by proper s e l e c t i o n of pH, processing temperature and protein ingredients. The quantity and the extent of protein denaturation of each protein ingredient i n a SSP gel a l t e r s the r h e o l o g i c a l and water-holding properties of the g e l . Gel properties can be s u c c e s s f u l l y modified i f the biochemical basis for protein gelation i s understood and s c i e n t i f i c a l l y exploited.


17.

SMITH


255

Acknowledgments The technical assistance o f Bryan Beuschel, Rod Nuckles and Shue Fung Wang i s g r a t e f u l l y acknowledged.

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1. 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. 29.

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30. Randall, C.J.; Raymond, D.P.; Voisey, P.W. J. Inst. Can. Sci. Technol., 1976, 9, 216. 31. Sofos, J.N.; Noda, I.; Allen,C.E. J. Food Sci., 1977, 42, 879. 32. Morris, V . J . ; Chilvers, G.R. J. Sci. Food Agric., 1984, 35, 1370. 33. Thom, D.; Dea, I.C.M.; Morris, E.R.; Powell, D.A. Prog. Food Nutr. Sci., 1982, 6, 97.


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Interactions of Food Proteins - American Chemical Society

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