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The Whys and Whats of Objective Texture Measurements
The Whys and Whats of Objective Texture Measurements Alina Surmacka Szczesniak Technical Center General Foods Corporation White Plains, New York, U.S.A.
The questions "why" and "what", followed by the question "how", are crucial to the solution of any technical problem. When related to objective texture measurements, the most important aspects of the question "why?" are : why is texture important?, why should we ~e concerued with it?, and why should we measure It objectively? The "what" relates to the specific characteristic which should be measured and to the ultimate purpose for which the data are being gathered - what will be done with the numbers once they are obtained. Will thev be used for quality control, for predicting con-Ru~er acceptance, or for research - applied or fundamental? The answer to these questions will determine how tbe measurements should be performed. However, the "how" still has many "whats" in it, since the methodology of objective texture measurements still presents a large number of problems and is partly art and partly science. Texture is important for a number of reasons. It is an important attribute of foods in that it
influefnces food habits. Our jaws and teeth have only a certain strenght and for that reason we do not normally chew on bones and cracks nuts with our teeth. Products difficult to manipulate in the mouth, because of their rheological properties, usually do not form part of our diet. There are many well accepted liquid and semi-liquid foods which are shear thinning, but none that are shear thickening. Texture affects consumer preference of accepted foods and every food manufacturer strives to make his product of the highest practical level of texture and flavor acceptance. Advertisements stress creamier and smoother puddings, crisper crackers, crunchier celery, and juicer, more tender meats. Texture is ofton taken as a sign of spoilage or quality deterioration on age. Soft cucumbers, wilted lettuce, soggy crackers or firm stale bread are considered not fit for human consumption. Food processors are well aware of the fact that texture affects processing and handling such a pumping, retorting, filling of containers and disintegration on shipment. Interestingly enough, texture also affects oral health. Observations made by dentists and physicians in various countries indicate that there is a relationship not only between nutrition and dental Presented at the workshop on Texture Measurements held within the 12th Annual Convention of the Canadian Institute of Food Technology.
150
carles but also between the texture of foods consumed and dental health. Neumann and DiSalvo (19.58, 1965a) observed among pr~mitive .tribes th.at eatmg habits which involve occaSIOnal hIgh chewmg loads lead to a low caries incidence inspite of poor overall nutrition. 'That pressure influences structure of organic and inoro'anic molecules is well known and Neumann and DiSalvo (1965b) formulated a theory link ing sclerosin a effects from occasional transmission of higher che~ing loads to increased tooth r~sistance aaainst destructive agents in the mouth. Usmg labo. . ratory animals for research on dental carIes, varIOUS workers observed that the physical state of food affects tooth structure and proneness to decay. Moulton (195;'») reported that lack of chewing stimulat~on results not only in increased incidence of dental capes, but also leads to inflamation of the gums (perIOdonitis), malocclusions, improper jaw develop~Ient. ?,nd oftentimes psychosomatic symptoms such as hp bltmg, lip and thumb sucking, and nail biting. ~
Research indicates that the consumer is highly aware of food texture and that in certain foods ~exture may be even more important than flavor. It IS only recently that texture became the subj~ct of separate studies. For many years it was studIed as. part ?f research on commodities. In consumer studIes or m sensory evaluation work with la~oratory panel~, texture was considered as an ill-defmed segment m the sensory property called "taste". Today, the trend is to treat texture and flavor separately in order to obtain a better description of the product and better guidelines for product development (or improvement) work. 'rable 1 shows the relative awareness of texture and flavor as affected by sex, socio-economic class and geographic location. The data show the result.s from two word association tests : one conducted WIth 100 employees of General Foods Corporation in Tarrytown New York (see Szczesniak and Kleyn 1963 for details and description of the technique) and the other conducted with 150 people outside of the Corporation selected from three different geographic locations. The data are expressed in terms of a texture/ flavor index, i.e. a ratio of free responses referring to texture to those referring to flavor. An index value of 1.00 means that there was exactly the same number of free responses dealing with texture as those dealing with flav'or. The obtained data indicate that women appear to be more texture conscious than men and that texture awareness increases with higher income and higher education. No definite conclusions can be drawn regarding the effect of the geographic region on texture awareness. Can. Inst. Food Techno!. J. Vo!. 2. No.4, 1969
TABLE 1 TEXTURE/FLAVOR INDEX Outside Group Total Group Sex Men Women Socio-Economic Class Upper Lower Lower Middle Upper Middle Geographic Location Chicago Denver Charlotte
GF Group
0.89
1.20
0.76 1.02
1.10 1.30
impossible task of working against a variable target; Objective texture measurements can define this target once and for all, thus, saving much product development time. There are many (tpplications of objective texture
measurements define target for product development or product improvement work guide product development or improvement work by defining differences between experimental samples and the target, both with respect to direction and magnitude quantify effects of process and formula variables assist in the development of alternate raw materials and alternate processes which will yield the same end result - ultimate purpose may be cost reduction or alternate suppliers assure consistency of quality in manufactured products. predict consumer acceptance provide fundamental information on basic aspects of texture.
0.60 0.95 1.20 0.96 1.04 0.63
Objective texturo measurements are needed to describe textural properties in terms of numbers so that both fundamental and applied aspects of texture could be defined and studied in a quantitative manner. 'fhese arc superi
Defining the tal'get and guiding produot development work is the most important function of objeotive texture measurements in an industrial research laboratory. Figure 1 shows an example of such an instance. It illustrates a case of how viscosity measurements
performed with the Brookfield Viscometer at different rates of shear were used to define characteristics of a soda fountain milk shake and to guide product formulation work in simulating the same properties with a dry mix to be added to liquid milk (Farkas and Glicksman 1967). The target was f'ound to be a shear thinning product which dropped in viscosity from 1000 cps at 3 rpm to 120 cps at 60 rpm. The requirementsr for the convenience dry mix were: 1) that it should perform well without the need for highspeed mixing equipment used in soda fountains, and 3) that it should perform well in hot or cold milk. Examination of shear and temperature dependence of
One extremely important advantage of instrumental measurements is that they are universally accepted as being completely objecthe, whereas sensory measurements - as analytical as they often manage to be - always ha\'e a connotation of opinion. As we all well know, one can argue with opinions. Anchoring of sensorJ' scales is a serious problem and good comparisons can be obtained 'only when experimelltal products are tested side-by-side with a control. Keeping the control constant in textural characteristics throughout all the tests is often an extremely difficult if not altogether impossible task. Batch-to-batch variations, changes due to storage, differences in raw materials - all contribute to shifts in the control. Even if it is 100% the same, personnel to whom the experimental product is being demonstrated will often claim that theY remember the control as being different at the last demonstration. As a result, product development people are often faceo with the J. Inst. Can. Techno!. Aliment. Vo!. 2. No 4. 1969
RHEOLOGICAL
PROPERTIES
(BROOKFIELD
OF MILK
VISCOMETER
SHAKE
LVT)
from Forko. and Glicksman, 1967
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,000 TARGET
CONVENIENCE
PRODUCT
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600
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36
12
60
30 SHEAR
RPM
RATE
Fig. 1
151
viscosity of various hydrocolloid solutions identified carboxymethyl cellulose and methyl cellulose as the desired ingredients. Viscosity of CMC decreases and that of ruethylcellulose increases with rising temperature. A combination of these gums resulted in only small viscosity fluctuations with temperatures in the formulated product as depicted in the right hand side of the Figure. Harold Corey of Rutgers University (formerly of Thomas J. Lipton I~abs) coined the expression "synthetic viscometry" for this approach to guiding product development work. The Brookfield Viscometer works reasonably well with systems which exhibit fairly simple flow properties such as Newtonian or pseudoplastic. More compli· cated systems, which possess definite structures, such as Bingham bodies or products with Ostwald viscosity, can be better defined using more refined methods for characterizing and quantifying flow properties. In our laboratory, we are using for this purpose a couette type viscometer which automatically plots shear stress versus a linearly increasing shear rate. This instrument was originally developed at the Thomas J. Lipton Labs in Englewood, New Jersey, and described by Creswick and Corey in 1964. Why objective textuTe measur'ements aTe needed will detennine uhat needs to be measuTed, which in turn will determine what type of an objective method will be most s/litable for the pm-pose. It must be borne in mind that text1tTe is a complex of a numbeT of different chamcteTistics. These can be classified into mechanical and geometrical characteristics and those which relate to the amount and rate of release of moisture and fat. The mechanical characteristics are those which determine the resistance of the food to applied forces. Softness, hardness, toughness, tenderness, crumbliness, etc. are in this category. The geometrical characteristics are those which refer to the geometry of the product, e.g. fibrousness, grittiness, lumpiness, and surface properties of these structures (Szczesniak 1963). Except for the human mouth, no single test method will pick up all of these characteristics. In defining a target, it is often desirable to use a number of different methods so that a number of different characteristics can be quantified, thus giving a fairly complete description. A single method is usually adequate for quality control purposes and for studying effects of processing and formula variables. In selecting this method, one must make certain that it will reflect adequately those textural characteristics which one wants to control and quantify. It may be resistance to shear, resistance to puncture, resistance to flow or resistance to crushing. Objective methods of texture measurements have become almost synonymous with instrumental methods of texture measurements, since these are most widely used. However, there are objective methods other than instrumental which are valuable in texture studies especially those dealing with more fundamental aspects of the problem. Notable among these are chemical and histological techniques which have contributed much to our present day knowledge of the structural basis for texture. In many cases these have been 152
correlated with instrumental measurements of resistance to applied forces and with sensory evaluation resulting in valuable information on how chemical composition and structure relate to textural characteristics. These techniques, however, are usually too complicated, too indirect and too time consuming to lend themselves to routine characterization of texture. Instrumental methods of texture characterization can be div'ided into fundctJ)'wntal, empiTical and imitative (Scott Blair 1958). Fundamental tests measure fundamental rheological properties such as modulus of elasticity, coefficient 'of viscosity, creep and relaxation time. Because of the complexity of food products and the necessity to relate the obtained measurements to functional properties, the fundamental rheological tests have been limited in their application. Pioneering work in tile use of fundamental rheological principles to study food products was done by Dr. Scott Blair and his co-workers, beginning in about 1930, at the University of Reading in England. It laid the foundation for food rheology and contributed much to our understanding of stress-strain relationships in foods. In the last 10 years, we have witnessed a resurrection of this type of an approach primarily through the research conducted by agricultural engineers (e.g. Mohsenin, 1\1orrow, Finney), chemical engineers (e.g. Charm) and physical chemists (e.g. Sherman, Corey, Sone) interested in texture measurements. This approach offers the advantage of measuring under well defined and standardized conditions fundamental properties in fundamental units of force, time, distance and in a manner lending itself to mathematical treatments. The two disadvantages of this approach are : lack of general knowledge on how these fundamental rheological properties relate to sensory texture and the fact that, in most cases ,they are highly dependent on stress and strain conditions. The situation is complicated by lack of knowledge on physics of mastication. 'When vigorously pursued this approach will provide much of the needed fundamental knowledge and sounder basis for simpler techniques. The tiCO most common ways of measu1-i,ng rheological p1'opertics of sol'ids are by application of mechanical force 'Or by resonance techniques. Several instruments, including the Universal Testing Machine (Instron), are available for stress-strain measurements using the principle of mechanical force application or mechanical deformation. l\1ore recently sonic techniques have been adopted to objective texture measurements (Abhott et al 1D68, Finney and Norris 1967, Finney et al 1967) and at least one acoustic spectrometer is available commercially (Nametre). There are two ways of using the sonic resonance method to measure the inner texture of fruits and vegetables. Sonic energy can be applied to the entire specimen or cylindrical sections can be vibrated at their natural frequencies. The resonance technique measures the Young's modulus of elasticity and internal viscosity. Measurements are influenced by shape, dimensions and density of the specimen. The most popular instrumental methods for measuring textural characteristics fall into the class of empirical tests. These tests measure parameters, often Can. Inst. Food Techno!. J. Vo!. 2. No.4, 1969
poorly defined, that practical experience indicates to be related to textural quality. Instruments used to perform such tests include : penetrometers which register the force required to penetrate the material, or the depth of penetraHon following impact; compressors which determine hardness or firmness of foodstuffs by measuring resistance to a compressing force; consistometers which measure the consistency of liquids and semi-solids by testing their resistance to flow; and shearing devices, which record the force needed to shear the test material. Examples of instruments in each of these categories are shown in Table 2. Objective methods for measuring texture can also be classified after Bourne (1966a) on the basis of the variables that are being measured. The instruments can thus be grouped into force, distance, time, energy, ratio or multiple variable measuring devices. 'While empirical tests are very useful and are widely used, caution must be taken in the 'interpr'etation of rewlts. Two main problems present themselves : a) is the instrument performing satisfactorily? is the reading "real"? and b) what does the instrument measure? The first problem deals with standardization and with accuracy of the measurements. Mechanical means such as weights and proving rings are very useful, but they only standardize the sensing and the recording elements in the instrument. This is satisfactory when, for example, the probe in contact with the food is a flat plunger. However, when the probe is a series of shearing blades, as is the case with the Allo Kramer Shear Press, the Pea Tenderometer or 1he Pabst Texture 'l'ester, the entire instrument must be standardized since misalignment or warping of the blades may affect the reading. The Tenderometer is usually standardized by means of a standard lot of peas set aside for that purpose. This is not very satisfactory. Yarious materials have been suggested for the AHo Kramer Shear Press such as cigarettes (Kramer 1967), filter paper (Binder and Rockland 1964) and spun protein fibers (Sharrah et aI1965). We have made a study of these and other potential standardizing materials and have found two materials to be promising : single sheets of aluminium foil 0.038" thick and Carbowax 1540 molded to proper shape. Of these, Carbowax appears to be more suitable since with continuous use of aluminum foil there may exist the danger of damaging the shear cell. Additional work needs to be done to confirm the potential usefulness of these materials. It is also possible that different standardizing materials may have to be developed for use with different test substances. The accuracy and ]Jrecision of texture measuring devices is being improved by utilizi,ng modern advctnces in instrument designing. Older devices satisfied theirs users with gravity applied forces, beam or spring deflections and single point scale ratings. More recently designed texture testing instruments utilize 8train gage transducers, electronic circuits and recorders to accurately detect and permanently record the resistance of the test material to mechanically applied forces. Peter W. Voisey of the Canadian Department of Agriculture, the Chairman of this workshop, made very significant contributions to this field J. Inst. Can. Techno!. Aliment. Vo!. 2, No 4, 1969
TABLE 2 EMPffiICAL TESTS Bloom Gelometer Fruit Pressure Tester ASTM Grease Penetrometer "Plumit" Compressors. Delaware Jelly Tester Modified Brinell Hardness Tester Ball Compressor Baker Compressimeter Gel Characterization Apparatus Bloom Consistometer Consistometers. Bostwick Consistometer MacMichael Viscometer Brookfield Viscometer Shearing Devices. - W'arner-Bratzler Press Pea Tenderometer Shear Press Miscellaneous. - SuccuJometer Food Mincer Fiberometer
1. Penetrometers. -
2.
3.
4. 5.
by modernizing a large number of older instruments such as the Warner-Bratzler Shear, the farinograph, and the Cherry-Burrel curd firmness meter. The question «what do the empirical tests measure" has been recently receiving increasing attention. Significant contributions in this area have been made by M. C. Bourne at the Geneva Experiment Station, Cornell University, New York and his introduction of the Instron to the field of objective texture measurements. The Magness-Taylor pressure tester is a very popular simple device used to determine the degree of fruit ripeness by measuring the maximum force needed to press its tip 5/16" into the fruit. Although used for over 40 years, the device is subject to criticism as to what exactly it measures. By mounting it in the Instron and recording force-distance curves for each punch, Bourne (1965) was able to show that three different situations can arise with apples. As shown in Figure 2, the force-distance curve on apples has a bioyield point (marked YP) following which the resistance of the tissue to penetration can continue to increase (curve A), can remain fairly constant (curve B), or can drop off (curve C). In the last two cases, the Magness-Taylor reading (marked MT) will be approximately equal to the yield point and will be almost independent of the penetration depth. In the first case, however, pressure test results are highly dependent on the depth of penetration and measure a force which is greater than the yield point by a variable amount. Based on this study, Bourne suggested that the depth of penetration of the pressure tester's tip ue greatly reduced so as obtain a consistent measurement of yield point in a manner correlatable with the thumb test. Another pertinent study, also conducted by Bourne using the Instron, elucidated the effect of punch dimensions on the magnitude of force required to puncture a food. Two sets of flat punches were used : one with a constant area and a variable perimeter, and the other with a constant perimeter and a variable area. For foods used in Bourne's study (mostly fruits and vegetables), the puncture force was found to depend on both the area and the perimeter in a manner
153
CHARACTER 1ST IC
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(5/16" DIAMETER
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FOR
SENSORY METHODS FOR MEASURING FIRMNESS
APPLES
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DISTANCE (Cm)
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which relates to the compression and shear resistance of the food. The puncture force was described by Bourne with the equation : F = K.P KcA C where P is the perimeter of the punch (cm) A is the area of the punch (cm 2 ) K. is the shear coefficient of the food (kg/cm) Kc is the compression coefficient of the food (kg/cm2 ) and C is a constant (kg). For a given material, the penetration force will depend on the perimeter and the area of the punch. The above equation explains why a simple doubling of the area of a punch usually fails to double the puncture force. This can happen only when the shear coefficient is zero or when both the area and the perimeter are double, and the constant C is negligible (Bourne, 1965b) . Bourne has, thus, demonstrated that penetration tests of cellular plant material involve both shear and compression. In a similar study conducted {)n process cheese, butter, margarine and peanut butter, deMan (1969) found that products with strongly bound network structures (e.g. process cheese) exhibit shear and flow, while those with weakly bound network structul'es (e.g. plastic fats) exhibit only flow during the movement of a punch. Many instruments are purported to measure firmness. Deformatron, puncture, penetration and sonic techniques have been used to measure firmness of foodstuffs. In a recent Research Note in the Journal of Food Science (1969), Bourne showed that there may be two unrelated types of objective "firmness" in apples : deform ability and bioyield. While spending his sabbatical leave in our laboratory this past year, Bourne did a study on sensory judgment of firmness. The study was designed to determine how people test for firmness without placing the food in the mouth. The tested foods represented a wide range of firmness, from soft whipped toppings and puddings to hard fresh carrots, The results shown
+
154
bread
~
Fig. 2
whips
.
E
+
puddings
VISCOSITY / CONSISTENCY
-.:
......
...!
Fig. 3
in Figure 3 (Szczesniak and Bourne 1969) revealed that how people test for firmness depends on the degree of firmness represented in the specific food product. With soft foods, such as whipped toppings and puddings, people think of firmness in tprms of viscosity/consistency.The.y generally test for firmness by stirring the product to measure resistance to flow. With foods 'of intermediate firmness (marshmallows, tomatoes, bread, lettuce), people use the deformability test, i.e. the amount of compression that the food undergoes when squeezed relatively gently in the hand. It is characterized by the absence of permanent crushing. With firm foods. such as apples or pears, most people found that the deform ability test could not detect differences in firmness. They changed to a puncture test characterized by sensing the force required to cause permanen t crushing. "Vith a very firm food (carrots), people found that neither the deformation nor the puncture test was satisfactory and resorted to tl bending test. These results suggest that, for proper correlation with sens'ory evaluation, objective tests based on different principles may have to be employed to quantify different degrees of firmness. Ross and Porter at the Eastern Utilization Research and Development Laboratory in Philadelphia, Pennsylvania recently completed a study of what the Allo Kramer Shear Press measures when used on French fried potatoes. Using the old-type straightedged blades and increasing the chart speed with an Estel'1ine Angus Speed Servo, they found that the reCan. Inst. Food Techno!. J. Vo!. 2, No.4, 1969
SHEAR
CURVE
OF
A
FRENCH
from Ross
and
FRIED
POTATO
STRIP
Porter,I968
y. YIELD POINT UJ
u
a:
B' CRUST BURST
t..
S' CRUST SHEAR
o
DISTANCE
Fig. 4
corded force distance cune had several peaks. 'l'hey studied these peaks by observing what happens to the French fried potato strip when subjected to similar forces in a specially constructed, open, shear-presslike machine, by testing fragments of the potato strip (removing the skin, removing the inside, serrating the skin) and by testing model systems. As shown in Figure 4, they found that the peaks correspond to the yield point, the crust burst, and the crust shear. The Allo Kramer Shear Press is a very popular texture measuring instrument and has been applied to testing a variety of foodstuffs such as fruits, vegetables, meat, baked goods, etc. The standard multiple blade cell used with the press is called the compressionshear cell implying that the test material is subject to both compression and shear. It is interesting to note that no fundamental inf'Ormation is available in the literature on what exactly happens in this cell, what are the types of forces that act on the material being tested. The Shear Press was originally developed for vegetables, primarily peas, and it is recommended that enough peas be used for the test to fill the entire cell. In other applications, for example in meat work, researchers have used a standard weight of the test food wilich only partially filled the cell. In our work with the Shear Press, we are often faced with the necessity of using a certain sample weight which - in many cases - has to be different from one experimental run to another because of density variations or simply because of limited sample quantity. It stands to reason that the amount of force necessary to shear through the sample must be related to the amount of the sample placed in the cell. One cannot directly compare shear forces when sample weights are different. For that reason, many publications reporting uata obtained with the Shear Press express the results in force units per unit sample weight assuming direct proportionality. vVe checked the correctness of this assumption experimentally and were most intrigued to find that the relaUonship between force and sample J. lnst. Can. Techno!. Aliment. Vo!. 2, No 4, 1969
weight is non-linear for most foods and is different for each foodstuff. The typical relationships identified are illustrated in Figure 5. Fitting mathematical models to equations fOl' these curves indicated that only two out of the 26 tested foods (sponge cake and processed rice) probably underwent pure shear and the others were subject to various combinations of shear-compression-extrusion. Very few foods (e.g. peanuts and bread) underwent no extrusion (only compression and shear), and several (e.g. raisins, American cheese) appeared to undergo no shear at all. This work will be published in detail elsewhere. These data plus visual -observations and additional experiments with the extrusion cell indicate that extrusion is an important phenomenon occurring in the shear cell of the shear press, a fact which does not appear to be generally recognized. The third category of instrumental texture measurements are imitative tests. These are performed under conditions simulating those to which the material is subjected in practice. Devices used are those which measure the properties of the material during handling and those which measure the properties of the food during consumption. In the first group are instruments such as alveographs, amylographs and farinographs which measure the handling properties of farinaceous materials. In the second group are instruments which attempt to simulate the chewing action of teeth. The first device of this type was described by Volodkevich in 1938. It was subsequently TYPICAL
BEHAVIOR IN
THE
OF
SHEAR
SAMPLE
VARIOUS
FOODS
PRESS
WEIGHT
Fii·5
155
modified by a nurllber of l'esearcherl'l. 'fhe MIT Denture Tenderometer and the General Foods Texturometer are refined versions of this general principle. The number of texture measuring instrnme,nts is very lO1'ge. Many were designed for specific purposes and specific foods, some lend themselves to wider applications. Several instruments are available C'Olumerdally, while others have only been described in the literature. Commercial instruments vary widely in price from less than $100 for a simple fruit pressure t(~ster to more than $10,000 for a refined model Illstron. The question which poses itself is, what il/strument should be used for specific purposes? A.s stated in the beginlling of my presentation the answer to that question depends on wby is the measurement needed and what particular characteristic must be measured. (Szczesniak 1966.) Research people, in my opinion, are great individualists and love to develop their own methods and their own gadgets especially in an area where few principles are firmly established and even fewer are generally accepted. Quality control people in the food industry are extremely time and simplicity conscious and often sacrifice the quality of the test f'Ol' the sake of saying time. If any general rilles can be established about selecting an instrument, fof' routine testing and quality control purposes, they are these. If at all possible, IISO a sophisticated, highly accurate and precise instrument (e.g. Instron) to establish/chat type of test will fit your pnl'pose and under what conditions the test should be performed. Use these data t'O guide you in th e selection of a simpler instl'nmeo1l t before you em bark on a very time-consuming task of designing one. If the instrument does not give results which are satisfactory, ask the question why? before you decide that the instrument is "no good". Perhaps the instrument is lIot designed to measure the property that you are interested in, perhaps the conditions need to be modified, or perhaps the device is not heing used in the correct manner. Above all, use (t little science. Warren Wearer, a past president of the A.merican Association for the Advancement of Science, said recently (quoted by Rloberts, 1967) : "Science is not technology, it is not gadgetry, it is not some mysterious cult, it is not a great
156
llIechallictll monster. Science is an adventure of the buman spirit. It is essentially an artistic enterprise, stimulated largely by curiosity, served largely by disciplined imagination, and based largely on faith in the reasonableness, order, and beauty of the universeI'; of which man is part."
References Abbott, J.A,. G,S. Bachman, R.F. Childers, J.V. Fitzgerald and F.J. Matusik. 1968. Sonic techniques for measuring texture of fruits and vegetables. Food Techno!. 22. 101. Binder. L.J., and L.B. Rockland, 1964. Use of the automatic recording shear press in cooking stUdies of large dry lima beans. Food Techno!. 18. 1071. Bourne, M.e. 1965. Studies on punch testing of apples. Food Techno!. 19, 113. Bourne, M.C. 1966a. Classification of objective methods for measuring texture and consistency of foods. J. Food Sci. 31, 1011. Bourne, M.C. 1966b. Measure of shear and compression components of puncture tests. J. Food Sci. 31, 282. Bourne, M.C. 1969. Two kinds of firmness in apples. Food Techno!. 23, 333. Creswick, N.• and H. Corey, 1964. A versatile recording viscometer. Paper presented at the 24th annual meeting of the Institute of Food Technologists, Washington, D.C. deMan, J.M., 1969. Food texture measurements with the penetration method. Paper presented at the 12th annual meeting of the Canadian Institute of Food Technologists, Ottawa, Ontario. Farkas, E., and M. Glicksman. 1967. Hydrocolloid rheology in the formUlation of convenience foods. Food Techno!. 21, 5J5. Finney, E.E., 1. Ben-Gera and D. Massie 1967. An objective evaluation of changes in firmness of ripening bananas using a sonic technique. J. Food Sci. 32, 642. Finney, E.E. and K.H. Norris, 1967. Sonic resonant methods for measuring properties associated with texture of Irish and sweet potatoes. Proc. Amer. Soc. Hort. Sci. 90. 275. Kramer. A. 1967. Personal communication. Moulton, R. 1955. Oral and dental manifestations of anxiety. Psychiatry 18 (3). Neumann. H.H. and N.A. DiSalvo 1958. Caries of Indians of the Mexican Cordillena, the Peruvian Andes and at the Amazon headwaters. Brit. Dent. J. 104, 13. Neumann, H.H. and N.A. DiSalvo 1965a. Caries absence among "primitives" N.Y.J. of Dentistry 35 (10), 355. Neumann, H.H. and N.A. DiSalvo 1965b. Response of fibrous proteins to compression stress : the load theory of carries immunity. Ann. N.Y. Acad. ScI. 131, 893. Roberts, W.O. 1967. Science, a wellspring of our discontent. Am. ScI. 55 (1), 3. Ross, L.R. and W.L. Porter, 1968. Interpretation of multiple-peak shear force curves obtained with F'rench fried potatoes. Am. Potato J. 45, 461. Scott Blair, G.W.• 1958. Rheology in food research. Adv. in Food Res. 8, 1. Sharrah. N., M.S. Kunze and R.M. Pangborn, 1965. Beef tenderness : a comparison of sensory methods with the Warner-Bratzler and L.E.E. Kramer Shear Presses. Food Techno!. 19. 136. Szczesniak, A.S.• 1963. Classification of textural characteristics. J. Food ScI. 28, 385. Szczesniak. A.S. 1966. Texture measurements. Food Techno!. 20, 1292. Szczesnialt, A.S. end M.C. Bourne 1969. Sensory evaluation of food firmness. J. Texture Studies in press. Szczesniak, A.S. and D.H. Kleyn 1963. Consumer awareness of texture and other food attributes. Food Techno!. 17, 74.
Received July 7, 1969.
Can. Inst. Food Techno!. J. Vo!. 2, No.4, 1969
The questions "why" and "what", followed by the question "how", are crucial to the solution of any technical problem. When related to objective texture measurements, the most important aspects of the question "why?" are : why is texture important?, why should we ~e concerued with it?, and why should we measure It objectively? The "what" relates to the specific characteristic which should be measured and to the ultimate purpose for which the data are being gathered - what will be done with the numbers once they are obtained. Will thev be used for quality control, for predicting con-Ru~er acceptance, or for research - applied or fundamental? The answer to these questions will determine how tbe measurements should be performed. However, the "how" still has many "whats" in it, since the methodology of objective texture measurements still presents a large number of problems and is partly art and partly science. Texture is important for a number of reasons. It is an important attribute of foods in that it
influefnces food habits. Our jaws and teeth have only a certain strenght and for that reason we do not normally chew on bones and cracks nuts with our teeth. Products difficult to manipulate in the mouth, because of their rheological properties, usually do not form part of our diet. There are many well accepted liquid and semi-liquid foods which are shear thinning, but none that are shear thickening. Texture affects consumer preference of accepted foods and every food manufacturer strives to make his product of the highest practical level of texture and flavor acceptance. Advertisements stress creamier and smoother puddings, crisper crackers, crunchier celery, and juicer, more tender meats. Texture is ofton taken as a sign of spoilage or quality deterioration on age. Soft cucumbers, wilted lettuce, soggy crackers or firm stale bread are considered not fit for human consumption. Food processors are well aware of the fact that texture affects processing and handling such a pumping, retorting, filling of containers and disintegration on shipment. Interestingly enough, texture also affects oral health. Observations made by dentists and physicians in various countries indicate that there is a relationship not only between nutrition and dental Presented at the workshop on Texture Measurements held within the 12th Annual Convention of the Canadian Institute of Food Technology.
150
carles but also between the texture of foods consumed and dental health. Neumann and DiSalvo (19.58, 1965a) observed among pr~mitive .tribes th.at eatmg habits which involve occaSIOnal hIgh chewmg loads lead to a low caries incidence inspite of poor overall nutrition. 'That pressure influences structure of organic and inoro'anic molecules is well known and Neumann and DiSalvo (1965b) formulated a theory link ing sclerosin a effects from occasional transmission of higher che~ing loads to increased tooth r~sistance aaainst destructive agents in the mouth. Usmg labo. . ratory animals for research on dental carIes, varIOUS workers observed that the physical state of food affects tooth structure and proneness to decay. Moulton (195;'») reported that lack of chewing stimulat~on results not only in increased incidence of dental capes, but also leads to inflamation of the gums (perIOdonitis), malocclusions, improper jaw develop~Ient. ?,nd oftentimes psychosomatic symptoms such as hp bltmg, lip and thumb sucking, and nail biting. ~
Research indicates that the consumer is highly aware of food texture and that in certain foods ~exture may be even more important than flavor. It IS only recently that texture became the subj~ct of separate studies. For many years it was studIed as. part ?f research on commodities. In consumer studIes or m sensory evaluation work with la~oratory panel~, texture was considered as an ill-defmed segment m the sensory property called "taste". Today, the trend is to treat texture and flavor separately in order to obtain a better description of the product and better guidelines for product development (or improvement) work. 'rable 1 shows the relative awareness of texture and flavor as affected by sex, socio-economic class and geographic location. The data show the result.s from two word association tests : one conducted WIth 100 employees of General Foods Corporation in Tarrytown New York (see Szczesniak and Kleyn 1963 for details and description of the technique) and the other conducted with 150 people outside of the Corporation selected from three different geographic locations. The data are expressed in terms of a texture/ flavor index, i.e. a ratio of free responses referring to texture to those referring to flavor. An index value of 1.00 means that there was exactly the same number of free responses dealing with texture as those dealing with flav'or. The obtained data indicate that women appear to be more texture conscious than men and that texture awareness increases with higher income and higher education. No definite conclusions can be drawn regarding the effect of the geographic region on texture awareness. Can. Inst. Food Techno!. J. Vo!. 2. No.4, 1969
TABLE 1 TEXTURE/FLAVOR INDEX Outside Group Total Group Sex Men Women Socio-Economic Class Upper Lower Lower Middle Upper Middle Geographic Location Chicago Denver Charlotte
GF Group
0.89
1.20
0.76 1.02
1.10 1.30
impossible task of working against a variable target; Objective texture measurements can define this target once and for all, thus, saving much product development time. There are many (tpplications of objective texture
measurements define target for product development or product improvement work guide product development or improvement work by defining differences between experimental samples and the target, both with respect to direction and magnitude quantify effects of process and formula variables assist in the development of alternate raw materials and alternate processes which will yield the same end result - ultimate purpose may be cost reduction or alternate suppliers assure consistency of quality in manufactured products. predict consumer acceptance provide fundamental information on basic aspects of texture.
0.60 0.95 1.20 0.96 1.04 0.63
Objective texturo measurements are needed to describe textural properties in terms of numbers so that both fundamental and applied aspects of texture could be defined and studied in a quantitative manner. 'fhese arc superi
Defining the tal'get and guiding produot development work is the most important function of objeotive texture measurements in an industrial research laboratory. Figure 1 shows an example of such an instance. It illustrates a case of how viscosity measurements
performed with the Brookfield Viscometer at different rates of shear were used to define characteristics of a soda fountain milk shake and to guide product formulation work in simulating the same properties with a dry mix to be added to liquid milk (Farkas and Glicksman 1967). The target was f'ound to be a shear thinning product which dropped in viscosity from 1000 cps at 3 rpm to 120 cps at 60 rpm. The requirementsr for the convenience dry mix were: 1) that it should perform well without the need for highspeed mixing equipment used in soda fountains, and 3) that it should perform well in hot or cold milk. Examination of shear and temperature dependence of
One extremely important advantage of instrumental measurements is that they are universally accepted as being completely objecthe, whereas sensory measurements - as analytical as they often manage to be - always ha\'e a connotation of opinion. As we all well know, one can argue with opinions. Anchoring of sensorJ' scales is a serious problem and good comparisons can be obtained 'only when experimelltal products are tested side-by-side with a control. Keeping the control constant in textural characteristics throughout all the tests is often an extremely difficult if not altogether impossible task. Batch-to-batch variations, changes due to storage, differences in raw materials - all contribute to shifts in the control. Even if it is 100% the same, personnel to whom the experimental product is being demonstrated will often claim that theY remember the control as being different at the last demonstration. As a result, product development people are often faceo with the J. Inst. Can. Techno!. Aliment. Vo!. 2. No 4. 1969
RHEOLOGICAL
PROPERTIES
(BROOKFIELD
OF MILK
VISCOMETER
SHAKE
LVT)
from Forko. and Glicksman, 1967
1000
,000 TARGET
CONVENIENCE
PRODUCT
800
000
I
:: 600
600
1\
g; ~
~
;: 400
.
.',',
,....,'. .',
400 \ \ \
200
200
o
MIX
36
o
12 SHEAR RATE
')..... ..........
7" C
-- :::.-:~:..-..: :--.:.:.---.: ~g:~
36
12
60
30 SHEAR
RPM
RATE
Fig. 1
151
viscosity of various hydrocolloid solutions identified carboxymethyl cellulose and methyl cellulose as the desired ingredients. Viscosity of CMC decreases and that of ruethylcellulose increases with rising temperature. A combination of these gums resulted in only small viscosity fluctuations with temperatures in the formulated product as depicted in the right hand side of the Figure. Harold Corey of Rutgers University (formerly of Thomas J. Lipton I~abs) coined the expression "synthetic viscometry" for this approach to guiding product development work. The Brookfield Viscometer works reasonably well with systems which exhibit fairly simple flow properties such as Newtonian or pseudoplastic. More compli· cated systems, which possess definite structures, such as Bingham bodies or products with Ostwald viscosity, can be better defined using more refined methods for characterizing and quantifying flow properties. In our laboratory, we are using for this purpose a couette type viscometer which automatically plots shear stress versus a linearly increasing shear rate. This instrument was originally developed at the Thomas J. Lipton Labs in Englewood, New Jersey, and described by Creswick and Corey in 1964. Why objective textuTe measur'ements aTe needed will detennine uhat needs to be measuTed, which in turn will determine what type of an objective method will be most s/litable for the pm-pose. It must be borne in mind that text1tTe is a complex of a numbeT of different chamcteTistics. These can be classified into mechanical and geometrical characteristics and those which relate to the amount and rate of release of moisture and fat. The mechanical characteristics are those which determine the resistance of the food to applied forces. Softness, hardness, toughness, tenderness, crumbliness, etc. are in this category. The geometrical characteristics are those which refer to the geometry of the product, e.g. fibrousness, grittiness, lumpiness, and surface properties of these structures (Szczesniak 1963). Except for the human mouth, no single test method will pick up all of these characteristics. In defining a target, it is often desirable to use a number of different methods so that a number of different characteristics can be quantified, thus giving a fairly complete description. A single method is usually adequate for quality control purposes and for studying effects of processing and formula variables. In selecting this method, one must make certain that it will reflect adequately those textural characteristics which one wants to control and quantify. It may be resistance to shear, resistance to puncture, resistance to flow or resistance to crushing. Objective methods of texture measurements have become almost synonymous with instrumental methods of texture measurements, since these are most widely used. However, there are objective methods other than instrumental which are valuable in texture studies especially those dealing with more fundamental aspects of the problem. Notable among these are chemical and histological techniques which have contributed much to our present day knowledge of the structural basis for texture. In many cases these have been 152
correlated with instrumental measurements of resistance to applied forces and with sensory evaluation resulting in valuable information on how chemical composition and structure relate to textural characteristics. These techniques, however, are usually too complicated, too indirect and too time consuming to lend themselves to routine characterization of texture. Instrumental methods of texture characterization can be div'ided into fundctJ)'wntal, empiTical and imitative (Scott Blair 1958). Fundamental tests measure fundamental rheological properties such as modulus of elasticity, coefficient 'of viscosity, creep and relaxation time. Because of the complexity of food products and the necessity to relate the obtained measurements to functional properties, the fundamental rheological tests have been limited in their application. Pioneering work in tile use of fundamental rheological principles to study food products was done by Dr. Scott Blair and his co-workers, beginning in about 1930, at the University of Reading in England. It laid the foundation for food rheology and contributed much to our understanding of stress-strain relationships in foods. In the last 10 years, we have witnessed a resurrection of this type of an approach primarily through the research conducted by agricultural engineers (e.g. Mohsenin, 1\1orrow, Finney), chemical engineers (e.g. Charm) and physical chemists (e.g. Sherman, Corey, Sone) interested in texture measurements. This approach offers the advantage of measuring under well defined and standardized conditions fundamental properties in fundamental units of force, time, distance and in a manner lending itself to mathematical treatments. The two disadvantages of this approach are : lack of general knowledge on how these fundamental rheological properties relate to sensory texture and the fact that, in most cases ,they are highly dependent on stress and strain conditions. The situation is complicated by lack of knowledge on physics of mastication. 'When vigorously pursued this approach will provide much of the needed fundamental knowledge and sounder basis for simpler techniques. The tiCO most common ways of measu1-i,ng rheological p1'opertics of sol'ids are by application of mechanical force 'Or by resonance techniques. Several instruments, including the Universal Testing Machine (Instron), are available for stress-strain measurements using the principle of mechanical force application or mechanical deformation. l\1ore recently sonic techniques have been adopted to objective texture measurements (Abhott et al 1D68, Finney and Norris 1967, Finney et al 1967) and at least one acoustic spectrometer is available commercially (Nametre). There are two ways of using the sonic resonance method to measure the inner texture of fruits and vegetables. Sonic energy can be applied to the entire specimen or cylindrical sections can be vibrated at their natural frequencies. The resonance technique measures the Young's modulus of elasticity and internal viscosity. Measurements are influenced by shape, dimensions and density of the specimen. The most popular instrumental methods for measuring textural characteristics fall into the class of empirical tests. These tests measure parameters, often Can. Inst. Food Techno!. J. Vo!. 2. No.4, 1969
poorly defined, that practical experience indicates to be related to textural quality. Instruments used to perform such tests include : penetrometers which register the force required to penetrate the material, or the depth of penetraHon following impact; compressors which determine hardness or firmness of foodstuffs by measuring resistance to a compressing force; consistometers which measure the consistency of liquids and semi-solids by testing their resistance to flow; and shearing devices, which record the force needed to shear the test material. Examples of instruments in each of these categories are shown in Table 2. Objective methods for measuring texture can also be classified after Bourne (1966a) on the basis of the variables that are being measured. The instruments can thus be grouped into force, distance, time, energy, ratio or multiple variable measuring devices. 'While empirical tests are very useful and are widely used, caution must be taken in the 'interpr'etation of rewlts. Two main problems present themselves : a) is the instrument performing satisfactorily? is the reading "real"? and b) what does the instrument measure? The first problem deals with standardization and with accuracy of the measurements. Mechanical means such as weights and proving rings are very useful, but they only standardize the sensing and the recording elements in the instrument. This is satisfactory when, for example, the probe in contact with the food is a flat plunger. However, when the probe is a series of shearing blades, as is the case with the Allo Kramer Shear Press, the Pea Tenderometer or 1he Pabst Texture 'l'ester, the entire instrument must be standardized since misalignment or warping of the blades may affect the reading. The Tenderometer is usually standardized by means of a standard lot of peas set aside for that purpose. This is not very satisfactory. Yarious materials have been suggested for the AHo Kramer Shear Press such as cigarettes (Kramer 1967), filter paper (Binder and Rockland 1964) and spun protein fibers (Sharrah et aI1965). We have made a study of these and other potential standardizing materials and have found two materials to be promising : single sheets of aluminium foil 0.038" thick and Carbowax 1540 molded to proper shape. Of these, Carbowax appears to be more suitable since with continuous use of aluminum foil there may exist the danger of damaging the shear cell. Additional work needs to be done to confirm the potential usefulness of these materials. It is also possible that different standardizing materials may have to be developed for use with different test substances. The accuracy and ]Jrecision of texture measuring devices is being improved by utilizi,ng modern advctnces in instrument designing. Older devices satisfied theirs users with gravity applied forces, beam or spring deflections and single point scale ratings. More recently designed texture testing instruments utilize 8train gage transducers, electronic circuits and recorders to accurately detect and permanently record the resistance of the test material to mechanically applied forces. Peter W. Voisey of the Canadian Department of Agriculture, the Chairman of this workshop, made very significant contributions to this field J. Inst. Can. Techno!. Aliment. Vo!. 2, No 4, 1969
TABLE 2 EMPffiICAL TESTS Bloom Gelometer Fruit Pressure Tester ASTM Grease Penetrometer "Plumit" Compressors. Delaware Jelly Tester Modified Brinell Hardness Tester Ball Compressor Baker Compressimeter Gel Characterization Apparatus Bloom Consistometer Consistometers. Bostwick Consistometer MacMichael Viscometer Brookfield Viscometer Shearing Devices. - W'arner-Bratzler Press Pea Tenderometer Shear Press Miscellaneous. - SuccuJometer Food Mincer Fiberometer
1. Penetrometers. -
2.
3.
4. 5.
by modernizing a large number of older instruments such as the Warner-Bratzler Shear, the farinograph, and the Cherry-Burrel curd firmness meter. The question «what do the empirical tests measure" has been recently receiving increasing attention. Significant contributions in this area have been made by M. C. Bourne at the Geneva Experiment Station, Cornell University, New York and his introduction of the Instron to the field of objective texture measurements. The Magness-Taylor pressure tester is a very popular simple device used to determine the degree of fruit ripeness by measuring the maximum force needed to press its tip 5/16" into the fruit. Although used for over 40 years, the device is subject to criticism as to what exactly it measures. By mounting it in the Instron and recording force-distance curves for each punch, Bourne (1965) was able to show that three different situations can arise with apples. As shown in Figure 2, the force-distance curve on apples has a bioyield point (marked YP) following which the resistance of the tissue to penetration can continue to increase (curve A), can remain fairly constant (curve B), or can drop off (curve C). In the last two cases, the Magness-Taylor reading (marked MT) will be approximately equal to the yield point and will be almost independent of the penetration depth. In the first case, however, pressure test results are highly dependent on the depth of penetration and measure a force which is greater than the yield point by a variable amount. Based on this study, Bourne suggested that the depth of penetration of the pressure tester's tip ue greatly reduced so as obtain a consistent measurement of yield point in a manner correlatable with the thumb test. Another pertinent study, also conducted by Bourne using the Instron, elucidated the effect of punch dimensions on the magnitude of force required to puncture a food. Two sets of flat punches were used : one with a constant area and a variable perimeter, and the other with a constant perimeter and a variable area. For foods used in Bourne's study (mostly fruits and vegetables), the puncture force was found to depend on both the area and the perimeter in a manner
153
CHARACTER 1ST IC
FORCE - DISTANCE
(5/16" DIAMETER
CURVES
FOR
SENSORY METHODS FOR MEASURING FIRMNESS
APPLES
TIP)
from Bourne. 1965
.. ..
E
6
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MT
..... ...-;
E
I
...
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yp
......
I:
lU U
2
PUNCTURE
pears
I
I
0
t..
I
Ie
iB
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apples
I
MT
4
FLEXURE
til
0
I
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carrots
I
I
I
I
I
I
I
I
I
I
I
o'---.....O-J...-'---.l--J.....J.-----'---'--'---'
lettuce
t
DEFORM A TlON
tomatoes marshmallows
V) V)
l.LI
Z
DISTANCE (Cm)
ex L!:
which relates to the compression and shear resistance of the food. The puncture force was described by Bourne with the equation : F = K.P KcA C where P is the perimeter of the punch (cm) A is the area of the punch (cm 2 ) K. is the shear coefficient of the food (kg/cm) Kc is the compression coefficient of the food (kg/cm2 ) and C is a constant (kg). For a given material, the penetration force will depend on the perimeter and the area of the punch. The above equation explains why a simple doubling of the area of a punch usually fails to double the puncture force. This can happen only when the shear coefficient is zero or when both the area and the perimeter are double, and the constant C is negligible (Bourne, 1965b) . Bourne has, thus, demonstrated that penetration tests of cellular plant material involve both shear and compression. In a similar study conducted {)n process cheese, butter, margarine and peanut butter, deMan (1969) found that products with strongly bound network structures (e.g. process cheese) exhibit shear and flow, while those with weakly bound network structul'es (e.g. plastic fats) exhibit only flow during the movement of a punch. Many instruments are purported to measure firmness. Deformatron, puncture, penetration and sonic techniques have been used to measure firmness of foodstuffs. In a recent Research Note in the Journal of Food Science (1969), Bourne showed that there may be two unrelated types of objective "firmness" in apples : deform ability and bioyield. While spending his sabbatical leave in our laboratory this past year, Bourne did a study on sensory judgment of firmness. The study was designed to determine how people test for firmness without placing the food in the mouth. The tested foods represented a wide range of firmness, from soft whipped toppings and puddings to hard fresh carrots, The results shown
+
154
bread
~
Fig. 2
whips
.
E
+
puddings
VISCOSITY / CONSISTENCY
-.:
......
...!
Fig. 3
in Figure 3 (Szczesniak and Bourne 1969) revealed that how people test for firmness depends on the degree of firmness represented in the specific food product. With soft foods, such as whipped toppings and puddings, people think of firmness in tprms of viscosity/consistency.The.y generally test for firmness by stirring the product to measure resistance to flow. With foods 'of intermediate firmness (marshmallows, tomatoes, bread, lettuce), people use the deformability test, i.e. the amount of compression that the food undergoes when squeezed relatively gently in the hand. It is characterized by the absence of permanent crushing. With firm foods. such as apples or pears, most people found that the deform ability test could not detect differences in firmness. They changed to a puncture test characterized by sensing the force required to cause permanen t crushing. "Vith a very firm food (carrots), people found that neither the deformation nor the puncture test was satisfactory and resorted to tl bending test. These results suggest that, for proper correlation with sens'ory evaluation, objective tests based on different principles may have to be employed to quantify different degrees of firmness. Ross and Porter at the Eastern Utilization Research and Development Laboratory in Philadelphia, Pennsylvania recently completed a study of what the Allo Kramer Shear Press measures when used on French fried potatoes. Using the old-type straightedged blades and increasing the chart speed with an Estel'1ine Angus Speed Servo, they found that the reCan. Inst. Food Techno!. J. Vo!. 2, No.4, 1969
SHEAR
CURVE
OF
A
FRENCH
from Ross
and
FRIED
POTATO
STRIP
Porter,I968
y. YIELD POINT UJ
u
a:
B' CRUST BURST
t..
S' CRUST SHEAR
o
DISTANCE
Fig. 4
corded force distance cune had several peaks. 'l'hey studied these peaks by observing what happens to the French fried potato strip when subjected to similar forces in a specially constructed, open, shear-presslike machine, by testing fragments of the potato strip (removing the skin, removing the inside, serrating the skin) and by testing model systems. As shown in Figure 4, they found that the peaks correspond to the yield point, the crust burst, and the crust shear. The Allo Kramer Shear Press is a very popular texture measuring instrument and has been applied to testing a variety of foodstuffs such as fruits, vegetables, meat, baked goods, etc. The standard multiple blade cell used with the press is called the compressionshear cell implying that the test material is subject to both compression and shear. It is interesting to note that no fundamental inf'Ormation is available in the literature on what exactly happens in this cell, what are the types of forces that act on the material being tested. The Shear Press was originally developed for vegetables, primarily peas, and it is recommended that enough peas be used for the test to fill the entire cell. In other applications, for example in meat work, researchers have used a standard weight of the test food wilich only partially filled the cell. In our work with the Shear Press, we are often faced with the necessity of using a certain sample weight which - in many cases - has to be different from one experimental run to another because of density variations or simply because of limited sample quantity. It stands to reason that the amount of force necessary to shear through the sample must be related to the amount of the sample placed in the cell. One cannot directly compare shear forces when sample weights are different. For that reason, many publications reporting uata obtained with the Shear Press express the results in force units per unit sample weight assuming direct proportionality. vVe checked the correctness of this assumption experimentally and were most intrigued to find that the relaUonship between force and sample J. lnst. Can. Techno!. Aliment. Vo!. 2, No 4, 1969
weight is non-linear for most foods and is different for each foodstuff. The typical relationships identified are illustrated in Figure 5. Fitting mathematical models to equations fOl' these curves indicated that only two out of the 26 tested foods (sponge cake and processed rice) probably underwent pure shear and the others were subject to various combinations of shear-compression-extrusion. Very few foods (e.g. peanuts and bread) underwent no extrusion (only compression and shear), and several (e.g. raisins, American cheese) appeared to undergo no shear at all. This work will be published in detail elsewhere. These data plus visual -observations and additional experiments with the extrusion cell indicate that extrusion is an important phenomenon occurring in the shear cell of the shear press, a fact which does not appear to be generally recognized. The third category of instrumental texture measurements are imitative tests. These are performed under conditions simulating those to which the material is subjected in practice. Devices used are those which measure the properties of the material during handling and those which measure the properties of the food during consumption. In the first group are instruments such as alveographs, amylographs and farinographs which measure the handling properties of farinaceous materials. In the second group are instruments which attempt to simulate the chewing action of teeth. The first device of this type was described by Volodkevich in 1938. It was subsequently TYPICAL
BEHAVIOR IN
THE
OF
SHEAR
SAMPLE
VARIOUS
FOODS
PRESS
WEIGHT
Fii·5
155
modified by a nurllber of l'esearcherl'l. 'fhe MIT Denture Tenderometer and the General Foods Texturometer are refined versions of this general principle. The number of texture measuring instrnme,nts is very lO1'ge. Many were designed for specific purposes and specific foods, some lend themselves to wider applications. Several instruments are available C'Olumerdally, while others have only been described in the literature. Commercial instruments vary widely in price from less than $100 for a simple fruit pressure t(~ster to more than $10,000 for a refined model Illstron. The question which poses itself is, what il/strument should be used for specific purposes? A.s stated in the beginlling of my presentation the answer to that question depends on wby is the measurement needed and what particular characteristic must be measured. (Szczesniak 1966.) Research people, in my opinion, are great individualists and love to develop their own methods and their own gadgets especially in an area where few principles are firmly established and even fewer are generally accepted. Quality control people in the food industry are extremely time and simplicity conscious and often sacrifice the quality of the test f'Ol' the sake of saying time. If any general rilles can be established about selecting an instrument, fof' routine testing and quality control purposes, they are these. If at all possible, IISO a sophisticated, highly accurate and precise instrument (e.g. Instron) to establish/chat type of test will fit your pnl'pose and under what conditions the test should be performed. Use these data t'O guide you in th e selection of a simpler instl'nmeo1l t before you em bark on a very time-consuming task of designing one. If the instrument does not give results which are satisfactory, ask the question why? before you decide that the instrument is "no good". Perhaps the instrument is lIot designed to measure the property that you are interested in, perhaps the conditions need to be modified, or perhaps the device is not heing used in the correct manner. Above all, use (t little science. Warren Wearer, a past president of the A.merican Association for the Advancement of Science, said recently (quoted by Rloberts, 1967) : "Science is not technology, it is not gadgetry, it is not some mysterious cult, it is not a great
156
llIechallictll monster. Science is an adventure of the buman spirit. It is essentially an artistic enterprise, stimulated largely by curiosity, served largely by disciplined imagination, and based largely on faith in the reasonableness, order, and beauty of the universeI'; of which man is part."
References Abbott, J.A,. G,S. Bachman, R.F. Childers, J.V. Fitzgerald and F.J. Matusik. 1968. Sonic techniques for measuring texture of fruits and vegetables. Food Techno!. 22. 101. Binder. L.J., and L.B. Rockland, 1964. Use of the automatic recording shear press in cooking stUdies of large dry lima beans. Food Techno!. 18. 1071. Bourne, M.e. 1965. Studies on punch testing of apples. Food Techno!. 19, 113. Bourne, M.C. 1966a. Classification of objective methods for measuring texture and consistency of foods. J. Food Sci. 31, 1011. Bourne, M.C. 1966b. Measure of shear and compression components of puncture tests. J. Food Sci. 31, 282. Bourne, M.C. 1969. Two kinds of firmness in apples. Food Techno!. 23, 333. Creswick, N.• and H. Corey, 1964. A versatile recording viscometer. Paper presented at the 24th annual meeting of the Institute of Food Technologists, Washington, D.C. deMan, J.M., 1969. Food texture measurements with the penetration method. Paper presented at the 12th annual meeting of the Canadian Institute of Food Technologists, Ottawa, Ontario. Farkas, E., and M. Glicksman. 1967. Hydrocolloid rheology in the formUlation of convenience foods. Food Techno!. 21, 5J5. Finney, E.E., 1. Ben-Gera and D. Massie 1967. An objective evaluation of changes in firmness of ripening bananas using a sonic technique. J. Food Sci. 32, 642. Finney, E.E. and K.H. Norris, 1967. Sonic resonant methods for measuring properties associated with texture of Irish and sweet potatoes. Proc. Amer. Soc. Hort. Sci. 90. 275. Kramer. A. 1967. Personal communication. Moulton, R. 1955. Oral and dental manifestations of anxiety. Psychiatry 18 (3). Neumann. H.H. and N.A. DiSalvo 1958. Caries of Indians of the Mexican Cordillena, the Peruvian Andes and at the Amazon headwaters. Brit. Dent. J. 104, 13. Neumann, H.H. and N.A. DiSalvo 1965a. Caries absence among "primitives" N.Y.J. of Dentistry 35 (10), 355. Neumann, H.H. and N.A. DiSalvo 1965b. Response of fibrous proteins to compression stress : the load theory of carries immunity. Ann. N.Y. Acad. ScI. 131, 893. Roberts, W.O. 1967. Science, a wellspring of our discontent. Am. ScI. 55 (1), 3. Ross, L.R. and W.L. Porter, 1968. Interpretation of multiple-peak shear force curves obtained with F'rench fried potatoes. Am. Potato J. 45, 461. Scott Blair, G.W.• 1958. Rheology in food research. Adv. in Food Res. 8, 1. Sharrah. N., M.S. Kunze and R.M. Pangborn, 1965. Beef tenderness : a comparison of sensory methods with the Warner-Bratzler and L.E.E. Kramer Shear Presses. Food Techno!. 19. 136. Szczesniak, A.S.• 1963. Classification of textural characteristics. J. Food ScI. 28, 385. Szczesniak. A.S. 1966. Texture measurements. Food Techno!. 20, 1292. Szczesnialt, A.S. end M.C. Bourne 1969. Sensory evaluation of food firmness. J. Texture Studies in press. Szczesniak, A.S. and D.H. Kleyn 1963. Consumer awareness of texture and other food attributes. Food Techno!. 17, 74.
Received July 7, 1969.
Can. Inst. Food Techno!. J. Vo!. 2, No.4, 1969