Texture-Structure Relationships in Texturized Soy Protein. III. Textural Evaluation of Extruded Products

Texture-Structure Relationships in Texturized Soy Protein. III. Textural Evaluation of Extruded Products T. J. Maurice', L. D. Burgess2 and D. W. Stanley Department of food Science, University of Guelph, Guelph, Ontario. tpresent address: General Foods Ltd., Cobourg, Ontario 'Present address: Omstead Foods Ltd., Wheatly, Ontario

Abstract Three mechanical and two sensory panel methods were evaluated for their ability to measure the texture of an extruded soybean meal product produced at varying extrusion temperatures. The instruments employed included the Instron Universal Testing Machine equipped to assess compression and stress relaxation, the Kramer Sheer Press and a modified Warner-Bratzler Shear device. All the mechanical tests correlated very significantly with sensory panel chew count and tactile evaluation and also with the temperature of extrusion. The Warner·Bratzler method was recommended on the basis of its close agreement with the sensory methods and its relative ease and speed of operation. Scanning electron microscopy of the extruded material showed that extrusion temperature had a strong influence on product microstructure. Increasing temperatures produced aligned fibers and a porous or spongy structure. These changes appear to be reflected in the response of the texture measuring devices.

Resume On a fait l'evaluation de trois methodes mecaniques et de deux methodes sensorielles pour l'appreciation de la texture d'un produit de tourteau de soja extrude a diverses temperatures d'extrusion. Les instruments employes comprennent l'appareil a mesurer Instron Universal equipe pour la compression et la relaxation de tension, la presse a cisaillement Kramer et un dispositif a cisaillement Warner-Bratzler modifie. Pour toutes ces mesures mecaniques, on a obtenu des correlations tres significatives avec la mastication quantifiee et l'evaluation tactile d'un jury sensoriel et aussi avec la temperature d'extrusion. La methode WarnerBratzler a ete recommandee a cause de sa forte correlation avec les methodes sensorielles et de son operation relativement facile et rapide. La microscopie electronique par balayage a montre que la temperature d'extrusion a eu une forte influence sur la microstructure du produit. Les temperatures plus elevees a donne lieu a des fibres orientees et a une structure poreuse et spongieuse. Ces changements semblent se refleter dans les donnees obtenues avec les appareils de mesure de la texture.

Introduction Texturized vegetable protein is being used increasingly in North America as an extender of red meat products. The major functional property of this food material is texture, however little attention has been paid to developing tests for the measurement of this parameter that can be related to any of its fundamental properties (Breene and Barker, 1976). In order to evaluate the texture of extruded products it is necessary to first define this quality parameter. This may be done empirically by both mechanical and sensory methods but because panel methods are time-consuming and thus costly there is a need for studies correlating panel and instrumental methods. This approach involves defining the parameters to be measured and eliminating extraneous test influences. This process has been termed 'a search for psychophysical analogs' (Kapsalis et al., 1973). A study of this type becomes much more valuable if the empirical data can be related to some intrinsic property of the test material. This may allow extrapolation or inferences to be made of a more basic nature. An example of this type of research may be seen in the work of SzczesCan. Inst. Food Sci. Technol. J. Vol. 9, No.4. 1976

niak and Smith (1969) who examined the texture of strawberries, not only for their mechanical and sensory characteristics but also for the relationship of textural properties to microstructure. This is the type of knowledge that is presently needed for texturized vegetable protein and it was the object of this research to investigate mechanical and sensory methods of measuring texture in an extruded soy product and to relate these measurements to observations of microstructure.

Materials and Methods Texturized vegetable protein-In a study of this type there is a great ad-vantage in producing the experimental material rather than purchasing commercially available products. This assures that the samples have been produced under standardized processing conditions and from the same starting ingredients. The extruded soy material used in this study was prepared from defatted soybean meal obtained locally. This material was ground, mixed and stored in sealed plastic barrels. Size distribution analysis showed that over 80% of the ground meal was retained between U.S. standard mesh numbers 10 and 40 sieves. Protein analysis (Kjeldahl N X 5.7) and moisture analysis (oven drying) gave 49.8% (d.b.) and 8.54%, respectively. Prior to extrusion the soybean meal was rehydrated to a final moisture level of 24% using distilled water. Mixing was performed with a variable speed mixer (Model S-30l, Hobart Manufacturing Co. Ltd., Toronto, Ont.) at speed 1 for 8 min. The mixed, rehydrated meal was placed in plastic bags and stored at 05°C overnight to ensure moisture equilibration. Refrigerated meal was allowed to come to room temperature (21°C) prior to extrusion. Experimental material was produced with a laboratory extruder (Model 2003, C. W. Brabender Instruments, Inc., Hackensack, N.J.) equipped with a 15.0 in screw (20: 1 LID ratio, 4: 1 compression ratio). A 0.125 in die orfice was used and screw speed was held constant at 60 rpm. This produced material about 1-3 cm in length. A temperature control unit was built to simultaneously sense, control and record temperatures at each of the three extruder heating sections. Time proportioning temperature control units were used to regulate temperatures ±2°C. A five step temperature range of 135-l80°C was used to produce a wide range of textures. Material was collected in plastic bags which were sealed and stored at 0-5°C until analyses were performed. Moisture content of the product ranged from 18 to 22% when sampled about 5 min after extrusion. Previous work (Cumming et al., 1972) has shown that rehydration ability is strongly related to extrusion temperature; to eliminate this variable the material was not rehydrated prior to texture measurement.

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Texture measurement-Texture of the experimental material was evaluated by three instrumental methods and two types of sensory panel assessment. A Kramer Shear Press (Model TPl, Food Technology Corporation, Reston, Va.) equipped with a 300 Iblroving ring with a full scale deflectIOn of 300 Ib was use . A 3.0 g sample was placed randomly in the CS-l cell and sheared at a ram speed of 10 cm/min and a ram force of 150 psi. Fifteen replicates were performed per sample. Shearing force was measured by peak height and work of shearing as peak area. A modified Warner-Bratzler Shear (Voisey and Hansen, 1967) was also used to shear the product. A separate strand of extruded material was chosen at random and sheared by a single blade. A shearing rate of 1.0 cm/ sec was used with a recorder speed of 1.0 cm/ min. Shearing force was measured as peak height. Twenty five replicates were performed per sample. An Instron Universal Testing Machine (Model TM, Instron Engineering Corp., Canton, Mass.) equipped with an Ottawa Texture Measuring System multiblade shearing cell (Voisey et al., 1972) was used to evaluate compression and stress relaxation. This cell has an upper plate containing ten parallel blades 1.25 cm apart and a lower plate of ten corresponding slots. Ten grooves tor test material are arranged in parallel but perpendicular to the blade slots thus allowing for up to one hundred simultaneous shearings. In these experiments only the upper plate was used. The blades of the crosshead cell were brought to a distance of 0.127 cm from a flat plate mounted on a 50 kg compression load cell, 50 kg full scale deflection, at a crosshead speed of 20 cm/ min. A separate strand of extruded material, no longer than 3 cm in order that only 2 blades contacted the product, was chosen randomly and tested individually. "Compression" was taken as the initial peak height while stress relaxation was determined by holding the crosshead a distance of 0.127 cm from the plate for 15 sec. Relaxation was calculated as the loss of stress after 15 sec divided by the original stress and was expressed as a percentage. Fifteen replicates were done for each sample. Samples for sensory evaluation were given to a 16 member texture panel in a flavorium equipped with separate cubicles. The panel was given several training sessions with similar extruded material prior to the experimental period. The entire experiment was performed twice. Two tests were performed, chew count and tactile evaluation. For chew counts the panelists were given two 2 cm pieces of material and asked to masticate them to a point where they were suitable for swallowing. The number of chews required to reach this point was recorded by the panelists. To allow for differences in chewing habits a standard sample of experimentally produced texturized soy was provided. This sample represented material considered to have a moderate degree of textural integrity and was used at each session. Relative chew counts were calculated by subtracting the number of chews for the standard from the chew count for the experimental material. In the tactile evaluation panelists were asked to appraise samples by pulling and twisting them. Again results were compared to the standard which was arbitrarily assigned a value of 5 on an unstructured 0-10 scale. Judges recorded their ratings by placing a vertical mark on a 10 174

cm horizontal line at a point chosen to reflect their opinion of the samples tactile strength as compared to the standard. The far left side of the line was labelled "lacking cohesiveness" and the far right "extremely cohesive." Data were recorded as the distance from the left hand, 0 end, of the scale to the panelist's mark. Microstructural anarysis-Samples of the extruded material produced at each temperature were examined by scanning electron microscopy to determine the effects of processin~ conditions on microstructure. The material was coated With a thin layer of gold and examined with an ETEC Autoscan microscope (ETEC Corporation, Hayward, Calif.) at 10 kv.

Results and Discussion The work of Cumming et al. (1972) indicated that temperature is a processing parameter of importance in the texturization process so this parameter was chosen to be varied in order to alter product texture. A range of 135180 a C was selected since it was found that extrusion temperatures outside this range produced either friable or burned products. Statistical examination of the results indicated that all measurements of texture, whether by mechanical or sensory means, correlated highly with extrusion temperature. Table 1 gives correlation coefficients between texture Table I. Correlation coefficients" between textural parameters and extrusion temperature and coefficients of variations. Method Instron peak height Instron stress relaxation Kramer peak height Kramer peak area Warner-Bratzler peak height Relative chew count Tactile evaluation

Correlation coefficient (r) .918* -.94S* .973** .970** .980** .993** .976**

Avg. coefficient of variation (%) IS.I 3.7 9.8 IS.7 13.S

*p:::; .OS; **p:::; .01. *Each data point for the S temperatures represents IS (Instron, Kramer), 16 (sensory panels) or 2S (Warner~Bratzler) replicates times 2 (number of experiments).

measurements and temperature. As the temperature of extrusion increases the product becomes more resistant to shear and compression indicating a greater cohesiveness and degree of structural integrity. The instrumental methods showed different degrees of variability over the temperature range used. The coefficient of variation «Standard deviation X 100) + mean value) for each method and temperature level was calculated and averaged over the range of temperatures to obtain an estimate of reliability. These data are given in Table 1. Instron compression (peak height) showed high variability (15.1%) that is probably due to difficulties encountered in keeping the instrument calibrated for the small (0.27 cm) distance from the test cell blades to the flat plate. Slight changes in this distance could produce significant errors. Instron stress relaxation, on the other hand, gave the lowest coefficient of variation. Because this parameter is calculated as a ratio it would be independent of variations in blade to plate distances. The variability in the Kramer values probably represents the complexity of these measurements. The latter portion of the Kramer peak J. Inst. Can. Sci. Technol. Aliment. Vol. 9. No.4. 1976

mainly represents extrusion of the product through the cell slots. Because the sample is placed randomly in the cell this factor may not be the same each time especially for products of differing diameters or shapes. Also, differing surface areas of the sample at equal weights may be a factor. Warner-Bratzler Shear values gave an average coefficient of variation of 13.5%. Part of this variation is likely due to differences in product diameter (Pool and Klose, 1969). Attempts were made to measure densities of the extruded material by physical measurements of diameter, length and weight but variability in diameter made it impossible to obtain a statistically valid measurement. One advanta~e of the Warner-Bratzler Shear is the rapidity with whIch tests can be performed. Both sensory evaluations also correlated well with extrusion temperature (Table 1). Chew count gave a non-significant correlation when not adjusted for the standard. Thus, subtracting each panel members' standard value from the chew count obtained for test samples appears to make allowances for individual chewing habits. Correlations of the instrumental with sensory panel texture evaluations revealed significant relationships (Table 2). All instrumental methods correlated sigTable 2. Correlation" of instrumental and sensory panel methods for textural evaluation of extruded soy protein. Instrumental method

Chew count

Instron peak height Instron stress relaxation Kramer peak height Kramer peak area Warner-Bratzler peak height

.994** -.974* .986** .985** .997**

Tactile evaluation .973** -.968* .941* .991** .990**

*p S .05; **p S .01. *Each data point for a sensory panel response represents IS (Instron, Kramer) or 25 (Warner-Bratzler) replicates times 5 (temperatures) times 2 (number of experiments).

nificantly with the sensory methods, however this does not mean that they are measuring the same property but only that they vary in the same way with changes in extrusion temperature. Thus, although methods were found that gave good predictions of texture panel responses, it is not obvious what property (or properties) of the material is

being measured. It would seem likely that a complex situation exists in which several attributes of the material are interacting since all the instrumental techniques correlated well although the mechanism of each test was different. Texture can be viewed as a direct consequence of microstructure which in turn originates from chemical composition and physical forces (Stanley and Tung, 1976). With this in mind it was of interest to investigate the microstructure of the extruded material to see whether the changes found in texture with increasing process temperature could be matched to a corresponding alteration in organization. Figure 1 shows cross sections of texturized soy protein extruded at 135, 165 and 180°C. As the temperature increases the product becomes less compacted and more spongy in appearance. At the highest temperature the walls of the cells vary in thickness from thin and sheetlike to dense and a generally porous structure is seen. The origin of these pores is thought to be the escaping steam which flashes off as the material leaves the extruder die. Longitudinal sections taken of the same material may be seen in Figure 2. At the lowest temperature the sample is dense and compacted in nature and the amorphous character of the soy grits are retained. At 165°C some partial alignment may be observed and a few fibers are seen but the density of the material is still high. By 180°C definite fibration and orientation of these fibers has occurred which seems responsible for the texture noted at this point. These observations of microstructure show a definite pattern that is dependent upon extrusion temperature, increasing in organization as temperature increases. Results from other experiments (Burgess and Stanley, 1976) indicate that fiber formation may be a result of a realignment of protein subunits that are disassembled due to the pressure and heat of the extruder environment. These subunits could then reform through an increased number of intermolecular bonds, leading to linear fibers. Thus, microstructure is another method of measuring texture of these products and texture may be seen to be dependent upon structure.

Conclusions 1) All mechanical methods were highly correlated with ex-

Fig. I. Microstructure of extruded soybean protein - cross sections. Amoc, B-16SoC, C-180°C (X60). Can. Ins!. Food Sci. Technol. J. Vol. 9, No.4, 1976

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Fig. 2. Microstructure of extruded soybean protein-longitudinal sections. A-135°C., B-165°C, C-180°C (XI80).

trusion temperature and sensory panel responses. On the basis of instrument cost, ease of operation and operation time required the modified Wamer-Bratzler Shear seems the instrument of choice. 2) Two structural changes occur as extrusion temperature increases, viz. an increase in aligned fibers reflected in enhanced cohesiveness and the development of a spongy texture as evidenced by an increase in porosity. 3) It appears from the SEM data that all the texture measuring devices used in these experiments were responding to the same microstructural element - the formation of fibers.

Acknowledgements Financial support for this project was partially supplied by the National Research Council and the Ontario Ministry of Agriculture and Food. Patricia Pierson supplied technical assistance with the scanning electron microscope.

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References Deeene. W. M. and Barker, T. G. 1976. Development and application of a texture measuring procedure for texturized vegetable protein. J. Texture Studies 6: 459. Burgess. L. D. and Stanley. D. W. 1976. A mechanism for thermal texlurization of soybean protein. Can. Inst. Food Sci. Techno!. J. 9: 228. Cumming, D. 8., Stanley. D. W. and deMan, J. M. 1972. Texture-structure relationships in text· urized soy protein. II. Textural properties and ultrastructure of an extruded soybean product. Can. Inst. Food Sci. Techno!. J. 5: 124. Kapsalis. J. G.• Kramer. A. and Szczesniak. A. S. 1973. Quantification of objective and sensory texture relations. In Texture Measurements in Food. Eds. A. Kramer and A. S. Szczesniak. D. Reidel Publishing Co., Dordrecht, Holland. Pool, M. F. and Klose, A. A. 1969. The relation of force to sample dimensions in objective measurement of tenderness of poultry meat. J. Food Sci. 34: 524. Stanley, D. W. and Tung, M. A. 1976. Microstructure of food and its relation to texlure. In Rheology and Texture in Food Quality. Eds. J. M. deMan, P. W. Voisey, V. Rasper and D. W. Stanley. AVI Publishing Co., Westport. Conn. , Szczesniak, A. S. and Smith, B. 1. 1969. Observations on strawberry texture-a three pronged ap. . . proach. J. Texture Studies I: 65. Voisey, P. W. and Hansen, H. 1967. A shear apparatus for meat tenderness evaluation. Food Techno!. 21: 355. Voisey, P. W., MacDonald, M. K. and Foster, W. 1972. The Ottawa texture measuring system -an operational manual. Engineering Research Service, Agriculture Canada, Engineering Specificalion 7024. Received March 23, 1976.

J. Inst. Can. Sci. Technol. Aliment. Vol. 9, No.4, 1976