soy and meat proteins as food emulsion stabilizers 1. viscoelastic properties of corn oil-in-water...

SOY AND MEAT PROTEINS AS FOOD EMULSION STABILIZERS

WATER EMULSIONS INCORPORATING SOY OR MEAT PROTEINS

1. VISCOELASTIC PROPERTIES OF CORN OIL-IN-

H. J. RIVAS and P. SHERMAN' Department of Food Science

Queen Elizabeth College (University of London) Campden Hill Road, London W8 7AH, England

(Manuscript received August 16, 1982; in final form June 7, 1983)

ABSTRACT

The creep compliance-time response of 50% ( wt/wt) corn oil-in-water emulsions has been studied at a constant stress of 47.8 dyne em-' over the pH range 2.5-7.5. The emulsions were stabilised by acid precipitated (APSP), 7S( 7SPRF) or lIS(11SPRF) soy protein fractions, water soluble ( WSMP) or salt soluble (SSMP) meat protein fractions. AN the emulsions exhibited viscoelasticity. WSMP emulsions exhibited the highest initial viscoelasticity parameter values and this was attributed to the strong interlinking of WSMP loops adsorbed on the surfaces of adjacent oil drops. The parameter values of all the emulsions increased during the first 5-7 days storage and then they decreased. At any selected pH the 7SPRF emulsions exhibited the largest relative increase in parameter values. The changes in viscoelasticity during storage were explained on the basis of the relative effects of further protein loop interlinkage and drop coalescence.

INTRODUCTION

The most important nutritional deficiency in developing countries especially in young children is protein, both in terms of quality and quan- tity. As a result there has been an intensive search for protein containing foods that will bridge the gap between protein requirement and protein

'Author to whom correspondence should be addressed.

25 1 Journal of Texture Studies 14 (1983) 251-265. All rights reserved. 0 Copyright I983 by Food & Nutrition Press, h e . . Westport Connecticut.

252 H. J. RIVAS AND P. SHERMAN

supply. Since the majority of the population in these countries are poor it is essential that the protein cost be low.

Animal protein, such as meat, is expensive and in short supply on a world-wide basis so that there is active interest in alternative sources of protein. At present, soy protein has attracted most interest as a substi- tute. In addition soy milk, coffee whiteners, and similar soy protein products, have assumed some importance as substitutes for traditional dairy products.

Published literature provides little information about the fundamental rheological properties of O/W emulsions stabilised by. proteins. This applies particularly to emulsions stabilised by soy protein, interest having concentrated on qualitative evaluation of their capacity as emulsifying agents and emulsion stabilizers (McWatter and Holmes 1979; Aoki et al. 1980; 1981). It has been claimed (Pearson et al. 1965) that soy protein isolate is not as good an emulsifier as milk protein but this view was refuted by Inklaar and Fortsin (1969).

A comparative study has now been made of the rheological properties of O/W emulsions stabilised with protein fractions extracted from soybean and meat. It represents part of a more extensive investigation into the relative performance of these protein fractions in O/W emulsions, their influence on rheological properties and on stability, and a comparison of the steric mechanisms whereby they stabilise emulsions. Although soybeans are used as meat extenders, or substitutes, attention was focussed on classical type emulsions and did not extend to meat-type “emulsions”.

EXPERIMENTAL

Materials: General

The water used for preparing emulsions was double distilled from an all Pyrex apparatus. Commercial grade corn oil was used as the oil phase. It had a density of 913.0 Kgm-3at 20°C (Leon Frenkel Ltd., Kent, England). The chemicals used were all of analytical reagent grade.

Preparation of Protein Fractions

The soybeans originated from the USA and were bought in a local supermarket. Fresh pork fillets (tenderloin) from a local butcher’s shop were used as the source of meat proteins.

SOY AND MEAT PROTEINS AS FOOD EMULSION STABILIZERS 253

Soybeans were cracked by passing them through a Christy grinder (Christy and Norris Ltd., England) and dehulled by screen separation. The coarsely ground material was further ground in a Gruphon grinder (Brook Motors Ltd., England) fitted with a 60 mesh screen. The resulting flour was defatted three times with hexane, air dried and stored in glass jars at 5°C until required. The approximate composition, derived by the McWatters (1978) procedure, was 9% moisture, 45% protein (N x 5.75), 10% oil, 6% ash and 39% carbohydrate (by difference). Fractionation of the flour into acid precipitated, 7s and 1 IS protein fractions followed the procedure of Saio and Watanabe (1973).

Extraction of water soluble and salt soluble fractions from pork fillets followed the procedure recommended by Saffle and Galbreath (1964).

Emulsion Preparation

50.0% (v/v) corn oil-in-water emulsions were prepared in which the aqueous phase contained 2.0% (w/v) protein. A standardised procedure was used for the preparation of all the emulsions. 200 ml of 2.0 (w/v) protein solution at 20"C, at the required pH, NaCl or methanol concentration, were introduced into an 800 ml beaker (approx. 10 cm diameter) and corn oil was then added dropwise and dispersed for 20 min with the aid of a mechanical stirrer. The coarse emulsion was passed once through a Rannie pressure homogeniser set at a pressure of 500 psi. The homogeniser had a cooling system which prevented a significant increase in temperature during homogenisation.

Some emulsions were prepared with the aqueous phase at pH 6.5. Each of these emulsions was divided into batches and the pH of each batch was adjusted to the desired level with 5M NaOH or HCl. In this way it was possible to examine the rheological properties of fresh emulsions at different pH's but having identical initial mean drop size and drop size distribution.

All emulsions were stored in a refrigerator at 5 O f 1 "C, after the pre- liminary examination, until required for further tests.

The following emulsions were prepared at 20.0' *OO.2"C. (1) Emul- sions stabilised by acid precipitated (APSP), 7S(7SPRF) and 11s (1 ISPRF) soy protein fractions or water soluble meat proteins (WSMP) at various pHs (2.5, 3.5, 5.5, 6.5 and 7.5). (2) Emulsions stabilised by APSP, 7SPRF, 1 lSPRF, or WSMP or salt soluble meat proteins (SSMP) in 0.5M NaCl at various pH's (2.5, 3.5, 5.5, 6.5 and 7.5). (3) Emulsions stabilised by a 50/50 mixture of WSMP and SSMP in O.5M NaCl at various pH's (2.5, 5.5 and 7.5).


Rheological Examination of the Emulsions

The creep compliance-time behaviour of each emulsion was investigated using a Deer rheometer (Deer Rheometer Ltd., Essex, England) at a constant shear stress of 47.8 dyne cm-’. This stress was within the linear region of the stress-strain relationship for each of the emulsions examined so that the parameters derived were independent of the applied stress.

Drop Size Analysis

The size distribution of the oil drops in each emulsion was determined with a disc centrifuge photosedimentometer MKIII (Joyce Loebl, New- castle, England) as described previously (Sherman and Benton 1980), and the mean volume diameter calculated.

Electrophoretic Mobility of OiI Drops in the O/W Emulsions. The electrophoretic mobility of the oil drops in the corn oil-in-water emulsions was determined as described elsewhere (Vernon Carter and Sherman 1980).

All tests were made at least in triplicate.

RESULTS

All the emulsions, irrespective of whether they were stabilised by protein fractions derived from soybeans or pork, and irrespective of storage time, exhibited viscoelastic behaviour at the constant shear stress of 47.8 dyne cm-’. This behaviour could be characterised (Inokuchi 1955) by six parameters (Tables 1-5) viz. an instantaneous elastic modulus E,, two retarded elastic moduli E, and E,, and their associated viscosities q , and q2 and a Newtonian viscosity qN.

Soy Protein Fractions

The data (Tables 1-3) show quite clearly that the parameter values are influenced by pH. In general, the optimum values, at all storage times and irrespective of the protein fraction involved, were found at pH 3.5-5.5. The isoelectric point of each soybean fraction is between 4.0 and


Table 1. Influence of pH on rheological parameters and mean drop size of O/W emulsions stabilized by acid precipitated soy protein. Temperature = 20.0*0.2'C

Eo El E2 Mean Aging (dyne. (dyne. (dyne. 7, 82 8N Volume Time cm-2x cm-2x cm-2x (Poise (Poise (Poise Diameter

pH (days) x ~ O - ~ ) xlO-') x I O - ' ) (pm)

2.5 1/24 I 3 7 9 13

3.5 1/24 1 3 7 9 13

5.5 1/24 1 3 7 9 13

6.5 1/24 1 3 7 9 13

7.5 1/24 I 3 7 9 13

0.70 0.90 1.45 1 S O 1.30 1 .00

1.20 1.50 2.00 2.70 2.60 2.50

1.10 1.30 2.00 2.80 2.80 2.70

0.65 I .00 1.20 1.30 1 .OO 0.90

0.35 0.40 0.50 0.50 0.40 0.30

1 .00 1.05 1.40 1.90 2.00 1.60

3.00 3.90 4.70 7.00 7.00 5.00

3.10 4.20 5.60 6.40 7.20 5.50

1 .00 1 .00 1.30 1.90 1.70 1.40

0.60 0.95 1.10 1 .00 0.90 0.70

1.15 1.10 1.20 1.70 1.60 1.30

5.20 5.40 5.90 7.20 5.80 5.10

5.80 6.00 6.50 7.00 6.00 4.50

1.20 1.20 1.50 1.80 1 .60 1.30

0.90 1 .00 1.10 1 .00 0.90 0.80

0.80 0.80 0.98 1.65 1.20 1.08

3.00 4.30 5.70 7.10 6.30 4.30

3.10 4.60 6.10 6.50 6.50 4.00

0.70 0.80 1.10 1.40 1 .00 0.70

0.40 0.70 0.80 0.70 0.50 0.40

0.23 0.22 0.18 0.17 0.16 0.12

1 .00 1 .00 1 so 1.50 1.10 0.80

0.80 1.20 1.60 1.40 1 .oo 0.50

0.20 0.23 0.23 0.26 0.17 0.10

0.10 0.12 0.20 0.15 0.15 0.10

2.32 3.54 3.95 4.00 4.15 3.40

2.40 3.00 5.80 8.80 7.50 6.50

2.50 5.00 8.00 9.00 11.15 7.00

1.70 2.10 3.10 5.40 4.50 3.20

1.30 1.40 1.80 2.00 1.80 1.30

0.290 0.320 0.325 0.350 0.370 0.395

0.390 0.360 0.370 0.385 0.385 0.401

0.350 0.370 0.375 0.380 0.380 0.404

0.310 0.340 0.352 0.375 0.380 0.417

0.305 0.305 0.346 0.375 0.390 0.422

5.0. At higher or lower pH the parameter values decreased progressively with increasing or decreasing pH.

During storage all the emulsions showed an initial increase in the parameter values for some days ( - 7 days) until optimum values were


Table 2. Influence of pH on rheological parameters and mean drop size of O/W emulsions stabilized by 7s soy protein fraction. Temperature = 20.0 +0.2"C

Eo El E2 Mean Aging (dy";. (dyne. (dfl;. '11 1 2 'IN Volume Time cm- x cm-*x cm- x (Poise (Poise (Poise Diameter

pH (days) X10-4) X10-4) x ~ O - ~ ) (pm)

2.5 1/24 1 3

'7 9

13

3.5 1/24 1 3 7 9

13

5.5 1/24 1 3 7 9

13

6.5 1/24 1 3 7 9

13

7.5 1/24 1 3 7 9

13

1.10 1.40 1.60 1.70 1.50 1.40

1 .so 2.10 3.00 3.10 3.10 2.70

1.60 2.30 3.20 3.20 3.10 2.90

1.20 1 .so 1.80 1.80 1.70 1.65

0.60 0.70 0.95 1 .00 0.80 0.60

1.80 2.00 2.10 2.00 1.80 1.70

5.00 5.80 7.10 7.70 7.10 6.20

4.50 6.00 7.50 7.40 7.40 6.70

1.70 2.10 2.40 2.00 1.90 1.60

1.40 1.60 1.70 1.70 1 .so 1.30

1.90 2.20 2.20 2.00 1.80 1 .so

5.50 6.00 7.00 7.20 7.00 6.20

5.20 6.50 7.30 7.50 7 .OO 5.50

1.90 2.20 2.40 2.40 2.10 1.90

1 .so 1.75 1.70 1.80 1.30 1.15

1.40 1.80 2.00 2.10 1.80 1 .so

5.00 5.80 7.30 8.00 6.60 5.50

4.50 6.50 7.10 7.20 6.70 6.30

1.20 1.70 2.00 2.10 2.00 1.90

0.80 1.10 0.80 0.80 0.60 0.45

0.40 0.45 0.40 0.50 0.30 0.20

1.10 1.20 1.70 1.80 1.10 0.90

0.90 1.30 1.80 2.00 1.70 1.40

0.40 0.45 0.50 0.50 0.45 0.30

0.20 0.30 0.48 0.30 0.20 0.15

2.60 3.20 4.20 5.00 5.00 4.50

4.60 5.10 7.00 9.50 9.00 8.10

4.50 5.40 7.10 8.20 9.00 8.70

2.50 3.30 5 .00 5.60 5.50 5.00

2.10 2.80 3.00 2.80 2.90 2.50

0.275 0.297 0.300 0.325 0.340 0.360

0.315 0.332 0.342 0.350 0.352 0.365

0.317 0.330 0.335 0.345 0.350 0.360

0.280 0.295 0.305 0.315 0.315 0.337

0.258 0.277 0.285 0.313 0.320 0.346

attained. This was then followed by a decrease in all values. The rate at which the parameter values increased was much influenced by pH and the nature of the protein fraction. The closer the pH to 3.5-5.5 the faster the initial increase in E,, and the slower the subsequent rate of decrease. This effect was even more pronounced for El and E2.


Table 3. Influence of pH on rheological parameters and mean drop size of O/W emulsions stabilized by 1 IS soy protein fraction. Temperature = 20.0 *0.2'C

Eo EI E2 Mean Aging (dyne. (dyne. (dyne. q , ?2 q N Volume Time c m - 2 x cm-2x cm-2x (Poise (Poise (Poise Diameter

pH (days) x ~ O - ~ ) x ~ O - ~ ) x ~ O - ~ ) (pm)

2.5 1/24 1 3 7 9

13

3.5 1/24 1 3 7 9

13

5.5 1/24 1 3 7 9

13

6.5 1/24 1 3 7 9

13

7.5 1/24 1 3 7 9

13

- 0.25 0.30 0.50 0.30 0.30

0.80 1 .OO 1.30 2.00 1.75 1.20

1 .00 1.70 2.30 2.30 2.00 1.80

0.50 0.70 0.90 1.30 1.15 0.80

- 0.13 0.20 0.40 0.30 0.25

- 0.20 0.30 0.60 0.50 0.35

2.00 2.50 3.20 4.00 3.20 2.50

2.20 3.50 4.70 4.50 3.90 3.20

0.90 1.50 1.10 2.50 2.10 1.70

- 0.10 0.20 0.50 0.50 0.30

- -

0.30 0.70 - -

1.90 2.70 3.30 3.60 3.60 2.75

3.60 4.80 5.20 5.00 4.25 4.50

1.10 1.90 2.00 2.00 1.80 1.80

- -

0.40 0.70 0.60 -

- 0.10 0.20 0.40 0.30 0.17

2.10 2.80 3.50 3.40 3.00 2.20

2.20 3.70 5.30 5.30 4.00 3.20

0.90 2.00 2.30 2.50 I .90 1.50

- 0.10 0.14 0.30 0.35 0.12

- -

0.10 0.10 - -

0.28 0.70 0.72 1 .00 0.70 0.70

0.60 0.90 1 .so 1 .so 1.30 1 .00

0.16 0.20 0.50 0.56 0.40 0.20

- -

1.10 0.15 0.20 0.12

- I .70 2.00 2.50 2.10 2.00

2.50 3.20 3.50 3.70 3.20 3.25

3.10 3.50 4.20 5.00 4.00 3.50

2.34 2.80 4.00 4.10 3.60 2.70

- 1 .00 1.50 2.20 2.00 -

0.340 0,360 0.380 0.415 0.450 0.495

0.355 0.370 0.383 0.405 0.420 0.450

0.360 0.370 0.390 0.410 0.430 0.465

0.330 0.350 0.370 0.410 0.450 0.480

0.305 0.330 0.350 0.400 0.430 0.475

'A dash indicates that the parameter value was too low to be measured.

Emulsions stabilised by the different soy protein fractions showed cer- tain differences in viscoelasticity at any pH during storage. For example, at pH 5.5 the E, values for lh old 7SPRF emulsions were approximately 45% and 60% higher than the values for comparable APSP and 1 lSPRF


emulsions. At some other pH’s similar differences were found but at pH 7.5 the E, values for 7SPRF emulsions were 80% higher than for APSP emulsions and the viscoelastic parameters of 1 lSPRF emulsions were too low to be measured. With the other viscoelastic parameters the trends were even more pronounced than for E,.

When the emulsions were stored all parameter values increased and then decreased with time, at all pH’s. The general order of parameter values was 7SPRF > APSP > 1 ISPRF.

Meat Protein Fractions

pH influenced the viscoelastic properties of freshly prepared WSMP emulsions in much the same way as that observed for soy protein emulsions, E,, and also the other parameters, exhibited their maximum values at pH 5.5 , the pH nearest to the isoelectric point (Table 4). SSMP emulsions, on the other hand, exhibited only slightly higher parameter values at pH 5.5 than at pH 2.5 or 7.5 (Table 5).

When stored, both WSMP and SSMP emulsions exhibited an increase in parameter values for about 5 days and this was followed by a decrease. For both series of emulsions the final E,, after 13 days storage, was highest at pH 5.5. In general, the parameter values for SSMP emulsions were lower than for WSMP emulsions of comparable pH and storage time.

Emulsions stabilised by 50/50 mixtures of WSMP and SSMP exhibited parameter values closer to those of SSMP emulsions than to WSMP emulsions (Table 6) at comparable pH and storage time.

The oil drops in all the emulsions exhibited minimum electrophoretic mobility at a pH around 4.5, the mobility increasing as the pH increased or decreased from this value (Table 7). At any pH the 1 lSPRF emulsion drops had the highest mobility and the WSMP emulsion drops had the lowest mobility.

DISCUSSION

The rheological data indicate that the oil drops in each emulsion, irrespective of the protein fraction with which it was stabilised, had a network structure at low stress. The precise nature and properties of the network depended on the concentration and configuration of the protein molecules adsorbed on the surface of the oil drops, mean drop size, drop size distribution and the electrical charge on the drop surfaces. Soy and


Table 4. Influence of pH on rheological parameters and mean drop size of O/W emulsions stabilized by water soluble meat proteins. Temperature = 20.0*0.2'C

_ _ _ _ ~ ~

Eo El E2 Mean Aging (dyne. (dyne. (dyne. q l 72 qN Volume Time cm-2x c m - 2 x cm-2x (Poise (Poise (Poise Diameter

pH (days) x I O - ~ ) x I O - ~ ) x I O - ~ ) brn)

2.5 1/24 1 5 9

13

3.5 1/24 1 5 9

13

5.5 1/24 1 5 9

13

6.5 1/24 I 5 9

13

7.5 1/24 1 5 9

13

1.40 1.40 2.00 1.90 1.80

2.00 2.10 2.50 2.40 2.10

2.60 2.70 3.00 2.75 2.45

2.00 2.10 2.50 1.85 1.60

1.15 1.23 1.30 1.05 0.95

1.56 1.73 1.54 1.30 I .oo

5.60 6.07 6.45 5.10 3.93

6.75 7.00 7.10 5.12 4.40

5.40 5.65 6.40 4.95 3.05

1.50 1.78 2.00 1 .so 0.98

1 .OO 1.85 1.90 1.55 1.35

4.25 4.70 5.00 4.25 3.75

5.50 5.65 5.75 4.90 3.90

5 .00 5.40 5.95 4.57 2.97

0.90 1.08 1.20 0.90 1 .00

1.56 1.90 1.59 1.17 0.80

6.10 6.00 5.80 4.08 3.14

6.76 7.70 6.40 4.09 3.90

5.42 6.20 6.70 4.45 2.80

1.31 1.60 1.40 1.06 0.63

0.15 0.37 0.28 0.23 0.14

0.85 1.20 I .23 0.85 0.65

1.10 1.40 1.14 0.85 0.68

1 .00 1.20 1.19 0.83 0.52

0.13 0.27 0.17 0.10 0.10

10.00 11.00 11.40 9.40 5 .00

20.90 21.90 22.90 17.50 9.40

22.90 22.90 25.50 20.00 13.00

20.00 20.00 23.00 16.00 3 .00

11.00 11.40 13.30 11.40 5.00

0.290 0.300 0.3 15 0.325 0.337

0.325 0.335 0.350 0.358 0.366

0.340 0.350 0.355 0.358 0.362

0.330 0.340 0.350 0.360 0.369

0.325 0.337 0.360 0.370 0.387

meat protein configuration in aqueous solution is influenced by pH. They are adsorbed on to oil drops in the same configuration as they exist in aqueous solution.

Adsorbed protein molecules project numerous molecular segments (loops) outwards into the thin film of aqueous phase separating adjacent oil drops. It is the longest of these segments which oppose close approach of oil drops, interlink via hydrophobic bonds into a network with a weak gel-like structure (Van Vliet et al. 1978; Sonntag et al. 1982) and greatly

260 H, J. RIVAS AND P. SHERMAN

Table 5. Influence of pH on rheological parameters and mean drop size of emulsions stabilized by salt (0.5 M NaC1) soluble meat proteins. Temperature = 20.0 *0.2’C

Eo El E2 Mean Aging (dyne. (dyne. (dy:;. ‘11 ‘12 ‘IN Volume Time c m - 2 ~ cm- X (Poise (Poise (Poise Diameter

pH (days) x ~ O - ~ ) x ~ O - ~ ) x I O - ~ ) (pm)

2.5 1/24 1 5 9

13

5.5 1/24 1 5 9

13

7.5 1/24 1 5 9

13

1.10 1.85 1.24 1.95 1.27 2.10 0.95 1.55 0.70 1.25

1.32 2.36 1.41 2.56 1.45 2.61 1.25 1.86 1 .05 1.75

0.95 1.85 1.00 1.91 1.00 1.87 0.95 1.54 0.77 0.95

1.60 1.66 1.76 1.95 1.77 2.14 1.30 1.39 1.21 1.12

2.70 2.59 2.95 3.09 3.10 3.30 2.15 1.95 2.37 1.75

1 s o 1.85 1.67 2.00 1.80 1.77 1.25 1.38 1.00 0.76

0.40 5.00 0.44 5.00 0.43 5.80 0.26 4.40 0.24 3.75

0.54 7.50 0.88 8.40 0.93 8.50 0.91 6.25 0.47 5.00

0.30 5.00 0.41 5.70 0.46 5.25 0.50 4.00 0.15 3.50

0.310 0.325 0.350 0.400 0.435

0.340 0.350 0.375 0.420 0.440

0.305 0.320 0.365 0.410 0.470

increase the viscoelasticity of the emulsions as compared with those in which stabilisation derives wholly from electrical repulsion and attrac- tion forces.

At their respective isoelectric points the soy and meat proteins are in their most compact configuration in aqueous solution and, conse- quently, they diffuse more quickly to the drop surfaces and form an adsorbed layer which has a higher protein concentration than at any other pH (Rivas 1982). Under these conditions, there is increased interlinking of loops, so leading to more pronounced viscoelasticity . The degree of interlinking in freshly prepared WSMP emulsions is obviously higher than in the other emulsions not only at the isoelectric point but also at all other pH’s investigated.

The contribution (I&,) of adsorbed polymer interaction to E, is given (Van Vliet et a/. 1978) by

where b is the effective number of chains provided by each adsorbed protein molecule, Tis the concentration of protein adsorbed per unit area of


Table 6. lnfluence of pH on rheological parameters and mean drop size of O/W emulsions stabilized by mixtures of water and salt soluble meat proteins (50% of each fraction). Temperature = 20.0 *0.2’C

Ell El EZ Mean Aging (dyne. (dyne. (dyne. q I ‘I2 ‘IN Volume Time cm-2x cm-2x cm-2x (Poise (Poise (Poise Diameter

pH (days) XlOW4) x ~ O - ~ ) x ~ O - ~ ) b m ) ~~

2.5 1/24 1.20 2.00 1.80 1.60 0.36 4.00 0.300 1 1.30 2.10 2.00 1.90 0.40 5.50 0,320 5 1.50 2.50 2.30 2.50 0.70 6.00 0.345 9 1.20 2.00 2.30 2.00 0.70 4.50 0.395

13 0.90 1.70 2.00 1.60 0.40 4.00 0.434

5.5 1/24 1.60 3.00 3.50 2.40 0.70 6.00 0.330 1 1.80 4.50 4.00 4.00 0.80 6.50 0.340 5 2.00 5.00 5.50 4.50 1.60 8.00 0.366 9 1.70 4.00 4.00 3.60 1.20 7.00 0.412

13 1.30 3.00 2.80 2.70 0.56 4.50 0.430

7.5 1/24 1.10 1.90 1.50 1.50 0.30 3.50 0.295 1 1.30 2.10 2.00 1.90 0.60 4.00 0.315 5 1.40 2.30 2.10 2.30 0.63 5.50 0.358 9 1.10 1.90 1.90 1.70 0.38 4.00 0.404

13 0.90 1.50 1.40 1.20 0.28 3.50 0.463

Table 7. Electrophoretic mobility data for acid precipitated soy protein, 7s soy protein fraction, 11s soy protein fraction and water soluble meat protein

Electrophoretic Mobility (mpxs - l at 1 volt cm-’)

PH APSP 7SPRF 1 1 SPRF WSMP

2.5 2.42 2.43 2.70 2.20 3.5 2.00 1.90 2.20 1.70 4.5 0 0 0 0 5.5 1.70 1.70 2.00 6.5 2.40 2.30 2.70 1.80 7.5 2.80 2.80 3.10 2.40

drop surface, v is the number average of effective network chains= bTNov/HM, His the distance between adjacent drops, Mis the protein’s molecular weight and 6 is the volume fraction of oil. On this basis the variations in viscoelasticity parameter values between emulsions stabil-

H. J. RIVAS AND P. SHERMAN

ised by different protein fractions can be attributed to three main fac- tors: (1) The rate of protein diffusion to the oil drop surfaces, which is influenced by molecular weight and the degree of reconformation after adsorption. (2) The availability of loops for interlinking. This depends on the concentration of protein adsorbed, and molecular configuration in the adsorbed state. (3) The nature of the intra- and inter-molecular linkages between adsorbed molecules.

WSMP adsorbs faster, and forms more concentrated and viscoelastic films than soy protein fractions under any of the conditions studied (Rivas, 1982). The main components of 7SPRF and 1 lSPRF are 7s and 11s globulins respectively. 7s globulin is more hydrophobic and has a lower molecular weight (180,000) than 11s globulin (350,000). Conse- quently, 7s globulin molecules diffuse to the oil-water interface more quickly than 11s globulin molecules and they are adsorbed in a more favourable conformation. The higher electrical charge on 1 1 s globulin molecules reduces their rate of adsorption and hinders the formation of a compact layer of adsorbed molecules. With SSMP the situation is quite different. Adsorption of the very long fibrous helical polypeptides myosin and actomyosin, the principle proteins, appears to follow the adsorption behaviour of &casein. Segments of the protein molecules have long trains of amino acid residues lying in the interface, with loops and tails projecting into the aqueous phase. The long loops of myosin and actomyosin, as a consequence of their high molecular weights, prevent such close approach of drops as in WSMP emulsions, for example. Conse- quently, there is less interlinking and a lower degree of viscoelasticity. The changes in 1 lSPRF emulsions on storage can be explained in a similar way. O/W emulsions prepared with aqueous phases containing mixtures of WSMP and SSMP exhibited viscoelasticity parameters closer in magnitude to those of SSMP than to WSMP emulsions because the non- globular proteins adsorb more quickly than globular proteins (Boyd et al., 1973; Graham and Phillips, 1976).

When the emulsions were stored there was further interlinking of adsorbed protein loops as the drops moved closer together and the loops were compressed and overlapped (Napper, 1977). During the first 5-7 days storage this process, which increases the viscoelasticity parameter values, exerted a greater influence than drop coalescence, which reduces the parameter values. At longer storage times drop coalescence was the dominant process. WSMP exhibited a relatively smaller increase in E,, and in the other parameter values, during the first 5-7 days storage. This can be attributed to one of two possible mechanisms: (1) Interlinking of adsorbed protein loops proceeded more quickly immediately after emul-


sion preparation than in emulsions stabilised by soy protein fractions and was almost complete after a relatively short time. (2) Steric stabilisation by the longer adsorbed protein loops prevented drops from moving as close together as in emulsions stabilised by soy protein fractions.

The changes in parameter values of SSMP emulsions when stored can be attributed to mechanism (1). Following adsorption the long loops of actin and actomyosin prevent such close approach of drops as in WSMP emulsions, as indicated previously. This means that loops are less likely to interlink and so the emulsions exhibit a smaller increase in viscoelasticity.

CONCLUSIONS

O/W emulsions stabilised by soy or meat protein fractions exhibited viscoelastic behaviour when subjected to shear stress of 47.8 dyne cm-’. The rheological parameters, both of the fresh emulsions and of stored emulsions, were influenced by pH..

WSMP emulsions exhibited higher parameter values when freshly prepared than APSP, 7SPRF, 11 SPRF and SSMP emulsions at any given pH. This is attributed to the greater degree of interlinking of loops of WSMP adsorbed on the surfaces of adjacent oil drops. The lowest parameter values were those for SSMP emulsions.

When stored, the parameter values of all the emulsions increased over the first 5-7 days and then decreased. Initially, further interlinking of protein loops as drops moved closer together, and loops were compressed and overlapped, exerted a greater influence on parameter values than drop coalescence. When no further interlinking was possible, or it proceeded at a very slow rate, drop coalescence exerted the major influence and the parameter values decreased.

The increase in parameter values during the first 5-7 days storage was most pronounced in the 7SPRF emulsions at any pH.

In both fresh and stored emulsions the highest parameter values were observed around the isoelectric point of the protein fraction utilised, at any given pH.

ACKNOWLEDGMENT

One of the authors (HJR) gratefully acknowledges the financial assis- tance provided by the Consejo Nacional de Investigaciones Cientificas y Tecnologicas of Venezuela.


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soy and meat proteins as food emulsion stabilizers 1. viscoelastic properties of corn oil-in-water...

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