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Mechanical Testing of Agricultural Products

An introduction to some applications of stress-analysis techniques in agricultural research

by Peter W. Voisey

nBSTRAcT--Stress-analysis techniques are becoming in- creasingly important in research into methods of produc- ing, handling and processing agricultural products. Measurement of texture has been neglected in quality control of foods. Stress-analysis techniques are being applied to the problem of developing standards for tex- ture evaluation.

Introd uction This art icle is presented to show some of the un- usual appl ica t ions of s t ress-analysis techniques to problems in agr icul tural research. Mechanica l and rheological tes t ing have been appl ied to agr icul tural products in the pas t and ac t iv i ty in this field has increased in recent years. The areas where such tests are used cover a wide range.

Invest igat ions , such as de termining the elastic and viscoelastic proper t ies of agr icul tura l products so t ha t the behavior of the product can be ex- pressed mathemat ica l ly , leads to appl ica t ion in research. Mechanical models can be const ructed to s t imula te produc ts to ob ta in a be t t e r under- s tanding of thei r behavior. An ins t rument can then be devised to measure and compare a par t icu- lar p roper ty of qua l i ty of different f rui t or vege- tab le varieties. The effect of exper imenta l t r ea t - ments can then be measured in genetic breeding programs and processing methods, and de te r iora t ion during storage or harves t ing evaluated . A knowl- edge of the physical proper t ies of a p roduc t is re- quired for the design of harvest ing, processing and storage equipment . For example, fruit , such as berries, is now being harves ted pneumat ica l ly so tha t the force required to blow the frui t off the bush must be known. When fruit , such as apples, is harves ted one must know how high the frui t can be piled wi thout bruising those a t the bo t tom.

Peter W. Voisey is Instrumentation Engineer, Engineering Research Serv- ice, Research Branch, Canada Departmen to[ Agriculture, Ottawa, Can. Paper was presented at January, 1965, meeting of Ottawa Valley Section of SESA.

Let tuce is being harves ted mechanical ly , and a device is under deve lopment which selects only firm heads for harvest . As mechaniza t ion of food product ion increases products are subjec ted to a wider range of impac t loads. The mach inery must be designed to l imit impac t to a level which does not cause damage. Therefore, these levels must be determined.

Food Texture Most food is graded by size, flavor and ap-

pearance, bu t l i t t le is done with regard to texture. M a n y foods t o d a y are processed by a cont inuous system. The consumers ' t as tes are becoming more discerning, and the processor must ma in ta in qua l i ty to generate consumer loyal ty . The qua l i ty of agr icul tura l products is not a lways the same; the products are affected b y weather, t ime, place, soil conditions, farming pract ice and var ie ty . The pro- cessor must know the qua l i ty of the produc t en- ter ing his p lan t so t h a t he can modify his process accordingly.

Tex ture of the raw and finished p roduc t can be eva lua ted by sensory methods but , if done by one man, the resul ts may be biased. Tas t e panels are widely used and provide an es t imate of consumer reaction. However , t hey are expensive and re- quire skilled analysis. Mechanica l tes t ing can re- place some tas te -pane l eva lua t ions of t ex tu re bu t in i t ia l ly the two must be correlated. For bo th re- search and indust r ia l appl icat ions, a simple in- s t rument producing a single index re la ted to tex ture is preferred. However , consider French-f r ied po- ta toes which have crisp exteriors and soft interiors, it is obvious one index is not enough. M a n y of the object ive measurements of t ex ture used t o d a y are crude, and thei r results not fully unders tood. An improved ins t rument based on modern hardware may produce different results. The improved

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Fig. 1--A recording mixer used to examine the quality of bread dough by the energy absorbed

Fig. 2--The energy required to shear a single wire through cheese measures firmness (also butter and similar products)

instrument 's mechanical performance, calibration and accuracy may be known within 0.1 percent but proving tha t it is measuring the physical property more accurately is difficult. The over-all picture of quality is complicated; such factors as moisture content, matur i ty , defects and composition must be added to texture.

Bioengineering Problems The engineer dealing with biological materials

instead of steel has some additional problems. First of all, he must learn some biology. Why do fruits and vegetables grow in a particular way, how do you know they are ripe. Practical points of farming, economics of production and processing must also be taken into account.

Biological materials are unstable and are affected by temperature, humidi ty and other variables. The test specimen grows naturally, and the measure- ment must be designed around the specimen which may only be available a few days each year. The stress conditions in the material under natural or processing conditions are seldom known with any degree of accuracy which makes mechanical simula- tion a problem. The physical constants of the ma- terial are seldom known and, if they are, the de- gree of accuracy is limited by the wide natural vari- ations tha t occur. The physical property measured may show variations as high as 300 percent due to natural variations, individuals in the same batch ripening at different times and variations in sample size or internal structure.

A further problem exists in the field of definitions of texture. For example, texture, a mechanical property sensed by the mouth, is described by many words (at least 28) in the English language, ranging from chewy to viscous.

Different Approaches Are Used In general, mechanical tests used on agricultural

products fall into the following categories: com- pression of the whole or large part, shearing, pene- trat ion by a rigid punch, energy in grinding or mix- ing and viscosity measurements. An example is the Kramer shear press 1 which is widely used. I t subjects a bulk sample first to compression and then shear by a multiple punch which compresses the sample (for example, peas) and forces them through slots in the bot tom of a cell. Hand-held penetrom- eters 2 and "mechanical thumbs ''3 are inexpensive and suitable for many purposes such as determining firmness and ripeness of apples by recording re- sistance to penetration. Other devices, such as succulometers 4, squeeze the juice from the product, and the quant i ty of juice is a measure of tenderness. Mechanical false teeth ~ have also been used for

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textural measurements on a wide var ie ty of prod- ducts.

A general idea of the work done in agriculture and the wide field covered is shown by the follow- ing key words abstracted from literature on testing agricultural products: bending of lettuce leaves; but ter consistometer; ice cream--penet ra t ion; cookies-- impact loading; stickiness of c a n d y ; grain--hardness, shearing and rupture energy; energy required to cut grass; ult imate bending s t rength of pasture plants; mechanical bruising of apples; energy to shear pineapples; firmness of peaches and onions; potatoes--shearing, bruising, elastic properties; force to separate cucumber from stem; strength of tobacco leaves; tomatoes - - sk in strength, bruising, firmness; spaghet t i - -bending, tensile; marshmallows--viscosity, elasticity; meats - - shear ; tensile strength of fluid columns such as mayonnaise, tomato ketchup and so on.

Stress-analysis techniques are thus being applied to many different products for a multi tude of reasons. In some cases, a simple test such as the resistance of the product to puncture by a die may be all tha t is required but a greater proport ion of the problems are being approached from a stress analyst ' s viewpoint to determine the fundamental properties governing the behavior of a product. '; Engineers and biologists are joining together to solve today ' s problems in food production. Labo- ratories are being organized specifically for me- chanical testing of agricultural products. 7 A re- cent example is the mechanical harvesting of to- matoes in California. I t was necessary to breed new varieties of tomatoes with the required me- chanical properties before a machine could be de- signed to harvest them without causing excessive damage.

Some of the stress analyst 's tools can be used in the conventional manner. For example, photo- elastic techniques can be used with models of fruits and vegetables to s tudy stress distribution. One area of increasing importance is the s tudy of prod- uct behavior under impact where brittle coatings and models could play an impor tant part. How- ever, it is seldom tha t the most useful tool, the strain gage, can be applied directly to the product. Problems in bonding, changes in stress distribution caused by the gage and lack of knowledge of the thermal and elastic constants of the product limit strain gages to transducer applications.

Current Work in Ottawa Examples of current work in Ottawa are illus-

trated. The energy absorbed during mixing is impor tant in many phases of food production for both the design of processing machinery and the s tudy of the product 's reaction to the process. The

Fig. 3--Apparatus for recording the burst pressure of tomato-skin diaphragms

Fig. 4--Puncture testers used for measuring the strength of tomato skins and tenderness of sweet corn

Fig 5--Creep tester for tomato skin

recording mixer shown in Fig. i is used to record the development of bread dough during mixing, and provides the baking technologist with a picture of the strength of the dough. Firmness is a major component of cheese quality which is generally monitored in the manufactur ing process by squeez- ing a sample by hand. The energy absorbed in

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Fig. 6--Recording crack propagation in egg sheEIs using high.speed movie techniques

shearing a wire through the cheese is correlated to firmness and the simple appa ra tus (see Fig. 2) can put such measurements on an object ive basis.

Toma toes are susceptible to skin cracking during growth when it rains af ter a long dry spell. The sudden up take of water into the fruit increases the internal pressure causing the skin to break. I f cracking conditions do not occur during a growing season, new varieties cannot be ra ted visually for susceptibil i ty to cracking. This factor can be con- trolled genetically and is one of m a n y propert ies which govern suitabil i ty of varieties for production. Cracking is controlled by skin s t rength and this has been measured by recording the burst ing pressure of d iaphragms (see Fig. 3) and tensile tests of skin samples removed f rom the fruit. Prepara t ion of the specimen is t ime consuming, and the most pract ical method found so far is the resistance of the skin to punc ture (see Fig. 4). Correlat ion between skin s t rength and susceptibil i ty to cracking has been found, but it does not appea r to be the only factor involved. Evidence t h a t the abil i ty of the skin to creep under load is impor tan t . Creep tests

on t oma to skin (see Fig. 5) are difficult to accom- plish as the skin dehydra tes quite rapidly.

The s t rength of egg shells has been investigated for m a n y years but, to date, a sat isfactory method of measuring s t rength has not been found. The hen 's egg is an amazingly strong complex structure, bu t millions of dollars are lost each year because of breakage. The distr ibution of stresses through the shell under stat ic and impact conditions (see Fig. 6) are being studied to find a method of pre- dicting the s t rength of eggs wi thout breaking the shell.

References 1. Kramer, A. , and Aamlid, K . , "'The Shear'Press, A n Instrument for

Measuring the Quality of Foods. 3. Application to Peas," Proc. Amer. Soc. Hort. Sci., 61, 417-423 (1953).

2. Haller, M. H., "'Fruit Pressure Testers and Their Practical Applica- tions," U.S.D.A. Circ. 627 (1941).

3 Magness, J . R., "'An Improved Type of Pressure Tester for the Determination of Frui t Maturity," U.S.D.A., Circ. 350 (1925).

4. Kramer, A. , and Smith, R. H., "The Succulometer," Canning Trade, 68, 7-8 (1946).

5. Proctor, B. E., Davison, S., and Brody, A. L., "'A Recording Strain Gage Denture Tenderometer for Foods," Food Tech., 9, 471 (1955).

6. Mohsenin, N . N . , Cooper, H. E., and Tukey, L. D., "'Engineering Approach to Evaluating Textural Factors in Fruits and Vegetables," Trans. Am. Soc. Agr. Eng., 6, 85-92 (1963).

7. Mohsenin, N . N . , "How Much Can an Apple Take," Instrumenta- tion, 18, 10 12 (1965), Honeywell, Iac., Philadelphia, Pa.

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