The mechanical properties of food materials are important to the design of harvesting

and handling equipment.

How does a mechanical picker tell a potato from a rock?

Is there an alternative to a thick skin in making a tomato amenable to mechanical harvesting?

What is the relationship between the natural frequency of vibration of a spe­cies of fruit and that of a fruit transport truck that will best ensure against damage to the fruit in transit?

What is the coefficient of restitution of a blueberry or a pea, and why is it important to know?

The expenses of getting food from the fields to the consumer are a major part of the cost of eating. Inefficiencies in that process, as well as the increasing production costs attributable to the intensive labor traditionally needed in harvesting and processing crops, are re­flected in costs to consumers. These kinds of problems in agricultural produc­tivity are becoming the province of an array of research disciplines. Scientists and engineers are seeking the limits imposed on mechanization of food han­dling and processing by the physical characteristics of the biological materials being handled. And researchers, often with the support of the National Science Foundation, have begun in recent years to study in earnest the physical proper­ties of biological materials in the effort to understand these natural limitations and devise ways to come to terms with them.

This basic and applied research into the properties of plant and animal materials is essential to the design of efficient food-handling machines and processes. Besides a general increase in productivity at every stage of food production, an understanding of the basic

physical properties of foods will result in reduced costs for handling and pro­cessing, reduction of damage and waste, savings in weight and bulk, improved shelf-life and stability, the development of new food materials, more objective standards of evaluation, and maintenance of quality under adverse conditions of handling, storage, and distribution. Each of these may be as important in the world food equation as, for example, an

increase in the world's available arable land (see "All That Unplowed Land" in Mosaic, Vol. 6, No. 3). Such knowledge can help determine the optimum time to harvest wheat, methods of separating potatoes from stones, detaching apples from trees, minimizing energy require­ments, and ways of preventing damage to all kinds of foods during collection, handling, and storage. It can sharply reduce waste and increase the productive capacity of the world's food sources.

As seen by the food engineer, one of the byproducts of agricultural mechani­zation has been mechanical damage to

MOSAIC September/October 1976 37

the crop during harvesting and handling —when it is dropped (impact), when it is piled up (quasi-static loading), and when it is shaken or struck in transit (vibrational bruising). And for such crops as apples, apricots, blueberries, cherries, peaches, plums, olives, toma­toes, cucumbers, strawberries, and coffee, even removing the fruit from the parent plant without damage is a challenging engineering problem. During apple har­vesting, for instance, the possibilities for injury to the fruit are numerous. Bruis­ing can occur when the apple hits the stem and limbs during shaking, when it is dropped, caught, or conveyed, when it is stored in a bulk bin, and when it is sorted, graded, transported, and packed. Mechanical action can also cause skin­ning, external cracking, and puncturing.

Shaking the tree The delicate stems that connect fruits

and nuts to trees are one of nature's ingenious timing devices. When the time is propitious for propagation of the par­ent plant, the stem gives way and the

ripe—but overripe by human standards —fruit falls. Among the more success­ful mechanical harvesting approaches has been to imitate nature by forcing the market-ready fruit to fall earlier, by shaking the tree. But the engineer who designs the equipment for such harvest­ing operations should know the strength of the tree bark and wood in order to specify the types of stresses that can be applied without causing excessive damage to the tree. The research involved in understanding the tree's mechanical be­havior may result in recommending modifications of the genetic structure of the tree itself, or more commonly in modifications of shaking or padding prac­tices. Another result might be the de­velopment of chemical or biological methods of toughening the fruit's tissue to reduce damage.

Researchers have found, for example, that the skin is the most important com­ponent related to mechanical strength of the tomato. Its ability to stretch—not its thickness—is the important factor in cracking resistance. Although several re­searchers have tried to understand just what causes bruising (impact damage) in fruit tissues, so far there is no satis­factory answer to this question.

Researchers have made progress, how­ever, in determining the kind of shaking that is most effective for different crops. In a series of studies of vibratory har­vesting by J. Robert Cooke and his asso­ciates at Cornell University, they found that by accurately determining the natu­ral frequencies of various freely sus­pended fruits, such as apples, peaches, citrus fruits, grapes, and coffee, the best frequency at which to operate a vibratory harvester can be mathematically deter­mined. These harvesters, which attach directly to the parent plant, vibrate the fruit onto a collecting canopy. Cooke has found that the fruit is most likely to fall when the tree is shaken at a precise multiple of the fruit's natural fre­quency. In fact, by choosing the proper vibration, the fruit may be separated either with or without its stem attached. In order to harvest fruit without stems attached, a pulsating frequency is used instead of continuous shaking.

In another study, Michael O'Brien, and his colleagues at the University of California, Davis, investigated the effect of vibrations during transportation on bruising. By measuring the fruit's mod­ulus of elasticity and density, they were able to propose an equation that describes the relationship between the depth of

the fruit in its shipping container and the fruit's natural frequency. If the natural frequency for a given fruit at a given depth falls in a range coinciding with the natural frequency of the fruit transport truck, the truck's vibrations can damage the fruit. In order to mini­mize this type of vibration bruising, the truck's suspension system should be designed so that the fruit's natural fre­quency falls outside the range of the vehicle's natural frequency.

Sorting the crops Vibration can also be used to sort

fruits. A traditional test for firmness of blueberries is to squeeze them. This gives one a rough measure of the berry's quasi-static firmness. The problem for the food engineer is to develop a rapid mechanized means of measuring this property. One such method has been developed by D. D. Hamann of North Carolina State University. Using blue­berries, he devised a sorting method based on low frequency sonic vibrations. The fruit is placed in a trough, one side of which vibrates at a given frequency and amplitude. The vibration causes the fruit to bounce away from the vibrating surface. If the fruit rebounds sufficiently —indicating an acceptable coefficient of restitution, a measure of firmness—it falls out of the trough and into a col­lector. Since the distance traveled by the fruit is very small, this method of sort­ing causes almost no damage.

Another way in which understanding the physical properties of foods can help increase productivity is in the separation of foods from undesirable materials. Stones and clods of soil, for example, are frequently harvested along with pota­toes. Food engineers have found that potatoes can be separated from the stones and clods by using the properties of resilience, rolling resistance, and hardness.

In 1972, W. R. Ellis of the University of Maine used a measure of hardness in a separating machine. As food and debris moved from one conveyor to another, they were dropped onto a steel plate. The plate's response, which was greater for harder objects, was electroni­cally detected, allowing the hard objects to be rejected by deflection of the plate.

Other methods of separation use elec­tromagnetic radiation and depend on other physical properties. An experi­mental color sorting system sorts objects according to their ability to reflect certain wavelengths of light. An X-ray separa-

38 MOSAIC September/October 1976

tor differentiates and sorts materials on the basis of density—potatoes allow the X-rays to pass through, while the more dense stones and clods resist X-ray penetration. A similar device uses infra­red radiation.

The property of light reflectance has also been used by Gerald Hook, Robert B. Fridley, and Pictiaw Chen of the University of California, Davis. They worked on the problem of removing the caps from strawberries. Although the capping itself is a relatively simple pro­cess, they found that the strawberries must be right side up when they enter the capping machine. They attempted to position the strawberries by using an optical scanning device which was able to discriminate between the red ripe fruit and the green cap.

Another physical property of food materials, electrical resistance, has been used for precise measurement of cotton fiber length. Also, electrostatic separa­tion, which makes use of a food ma­terial's ability to conduct electrical charge, has been used for seed cleaning.

An analytical look at cooking In recent years there has been a

marked trend towards commercial prepa­ration of foods, not only the traditional canning and freezing of vegetables and fruits, but the mass preparation of meals and meal components. School lunches, for instance, are reported to be a $4-billion business now, and an increasing share of it is being conducted out of central facilities where food is prepared in bulk or even "preplated" and frozen for distribution as individual meals. For better or worse, "fast" foods in both homes and restaurants have become big business that could serve the consumers' nutritional and aesthetic needs better if more were known about the processes of heating, cooling, drying, and freezing. Efficient and economical cooking are not enough—cooked food must also be ac­ceptable to consumers, which means that such aesthetic qualities as flavor, color, and texture must be considered. Also vital are the food's nutritional quality and safety and convenience qualities.

In order to understand the thermo­dynamic and transport behavior of foods during cooking, Eugenia A. Davis and Joan Gordon of the University of Min­nesota are creating mathematical models of foods. Such models will account for changes that occur during cooking, in­cluding changes in cellular structure, chemical composition, molecular state,

mineral distribution, and water emission rates. The ultimate purpose of the model is to enable engineers to design nutritious, safe, and convenient food processes.

Some of the data for Davis and Gor­don's study of cooking processes will be gathered by means of the scanning elec­tron microscope. They will use the in­strument to examine porosity and sur­face changes during cooking, including a detailed study of changes in the fiber structure of meats during cooking. The microscope is also being used to analyze the subcellular structure of various bio­logical materials. This involves taking a sample of meat at a given point during the cooking process, freezing the sample so rapidly that virtually all action within the cell stops instantly—including the diffusion of elements from one part of the cell to another—and observing thin sections of the frozen meat. Using this technique, the researchers hope to trace the levels of calcium, potassium, magne­sium, sulphur, sodium, and chlorine within the cell during the cooking pro­cess.

Davis and Gordon are also investiga­ting basic questions about the effects on foods during cooking of a wide variety of variables: heating rates, cooling rates, atmospheric and humidity variations, irradiation, water release and movement, mineral redistribution, vitamin degrada­tion, and tissue and cellular transfor­mations. In one study which related cooking processes to pollution, Davis, Gordon, and Elizabeth V. Marston, determined that the phosphorus content of the water in which various vegetables were cooked did not significantly affect the retention of nutrients in the cooked vegetables. But they found that vege­tables cooked in water containing high concentrations of calcium and magne­sium retained significant amounts of these nutrients.

Food textures One of the most important mechanical

properties of foods is vaguely defined as texture. Nuri Mohsenin of the Penn­sylvania State University, one of the pioneers in food physics, estimates that the English language contains about 350 terms that describe food qualities. Of these, about one-quarter refer to tex­ture: hard, soft, brittle, firm, tough, tender, crusty, sticky, gummy, fibrous, mealy, smooth, chewy, juicy, crisp, flakey, flabby, lumpy, oily, gritty, springy, and so on. But food engineers

are not satisfied with any dictionary definition of texture. What they want are well-defined physical concepts ex­pressed in units—precise, quantifiable, measurable definitions.

Mechanical tests of food texture are especially important because of the severe limitations of humans as testers. For example, the light-touch organs in hu­mans cease to respond to a constant stimulus after about half a second. Con­tinued response is obtainable only by moving the stimulus over the sensory surface. In hard touch organs, such as teeth, the adaptation time is longer, but still not long enough adequately to per­ceive and record the material's texture.

Therefore, the food engineer defines the tenderness of cooked meats, the mealiness of fruit, or the crispness of cornflakes in engineering terminology. These properties include resistance to shearing, tensile or compressive mod­ulus of elasticity, toughness (defined as the area under the stress-strain curve up to the point where the specimen ruptures under compression), stress relaxation, creep (increase of deformation under a constant force), complex shear modulus, and bulk compression. Bulk compres­sion, one test of texture, can give infor­mation on the proportion of air space in a food sample. For example, it is useful to know that as much as 25 percent of the apple's volume is occupied by tiny air spaces, compared with about two percent for the potato.

An accurate evaluation of texture can be basic to productivity. For example, firm cherries remain whole and plump during processing and baking, while soft cherries lose a large amount of juice, becoming torn and flabby. Consumers will not buy wilted lettuce, mealy apples, or soft cucumbers or bananas. They demand crispness, rigidity, and turgidity in fresh fruits and vegetables, and in the United States they demand tender meat. Texture evaluation can also be used to help understand the mechanical behavior of foods as a function of age. Examples are the staling of cake and the hardening of margarine. In some foods, such as ice cream, texure can be indicative of overall quality.

The study of physical properties is also important in understanding the macroproperties of masses of materials. Many of the processes involved in har­vesting and handling grain, for example, deal with huge masses of grain—not individual stalks or grains. In the same way, the physical properties of masses

MOSAIC September/October 1976 39

of bulk fruit and vegetables may be very different from those of individual units.

When to harvest The timing of a harvest is a key factor

in determining the ultimate productivity of the year's agricultural process—a fact that farmers have always known all too well. But until recently, all harvest deci­sions were based on purely subjective criteria—the farmer's evaluation of the crop's appearance, texture, and taste. When he was just beginning hi« investi­gation of the physical properties of foods, Mohsenin says, he visited a large vegetable farm. "The foreman of the pea farm stopped at the field, grabbed a bunch of pods, and shelled them. Then he dropped several peas on the fender of his car. From the jump of the peas he could tell whether they were ready to be harvested. The principle involved was the coefficient of restitution, the re­bound characteristics of the pea. It's a mechanical property that has to do with the pea's elasticity, which changes with time. Here was a physical measurement, so we wouldn't have to be dependent on the subjective assessment of one man."

The same type of subjective judgments are often made by those who judge the quality of fruit to be harvested. Typi­cally, Mohsenin says, "The worker goes to the field and takes a look at the fruit; he takes a measure of its color and size. Then he tries to pull it off the tree; he makes a measurement of detachment. Next he takes a bite out of it; he makes a measurement of texture, firmness, and taste—the sugar content. All of that could be put into some type of physical measurement."

However, each crop has its own characteristics and idiosyncrasies. For example, grains such as wheat and rice must be harvested before lodging occurs. In lodging, the stalks bend down, either because of wind or stalk characteristics, making harvesting so difficult that most of the crop is lost. Also, grain must be harvested before it is overripe, to prevent shedding. Grapes should be harvested after they have developed a high sugar content—but if harvested too late, when the sugar concentration is too high, the skin will crack, causing loss of juice. In addition, crops vary widely in their vulnerability to damage and deteriora­tion of quality.

Certain crops which do not mature uniformly, such as lettuce, must be har­vested selectively. Food engineers are

trying to exploit the physical character­istics of the crops themselves—size, shape, color, surface conditions, and mechanical properties (such as strength of attachment to the parent plant)—in order to mechanize selective harvesting. Machines are now in use which select and detach lettuce heads by "feeling" for their resistance to pressure and firmness; others employ X-ray and gamma-ray detectors to assess harvestability. How­ever, many researchers would prefer to avoid the problems of selective harvest­ing altogether by using growth regula­tors or breeding practices to develop crops of uniform size, shape, color, firm­ness, and ease of detachment.

Several well-defined mechanical prop­erties of fruits and vegetables have been related to readiness for harvest, but the physical and chemical changes proceed so slowly that it is almost impossible to select any one change as the sole index of the rate of ripening. For example, there is no single index of readiness for harvest of apples. But by measuring and mathematically combining such proper­ties as size, weight, color, specific gravity, firmness, sugar content, and ease of de­tachment, the apples' readiness for har­vest can be determined.

Electronic furrows In order to better understand the

changes in food materials that occur over time, a group of researchers at the Pennsylvania State University have de­veloped a computer model. Based on the experience of various growers, they as­signed numerical weight factors to vari­ous physical properties of the fruit; the mathematical combination of these fac­tors in a digital computer resulted in a maturity index. A similar model, using an analog computer, was based on the chemical reactions that take place inside stored food materials such as apples and meat. It had a prediction accuracy of 80 percent. Being able to predict the condition of the material both in the field and in storage helps adjust the time of harvest and packaging to the market situation.

Another type of computer model is being used for quality detection by Chen and his associates at the University of California, Davis. At present, many foods are sorted mainly by the use of human vision. The work is slow, tedious, and tiresome—and often inconsistent. Rather than developing a sorting ma­chine, this research team studied the

process of quality detection itself. They found that in order to design quality detection devices, one must first identify the primary property to be detected— such as maturity, texture, skin damage, or color. Since such properties are often impossible to detect without destroying or damaging the fruit, a secondary prop­erty must be identified. This secondary property must correlate well with the primary property, and must be easily detectable by machine. Using a digital computer, the team developed a program to simplify analysis of the numerous primary and secondary properties of various fruits.

The computer is also being used to supply information on the physical prop­erties of agricultural products. The Penn­sylvania Physical Properties Information System, headed by Mohsenin and Charles T. Morrow, is a computerized informa­tion retrieval system which supplies complete reference citations to users who supply appropriate keywords. The sys­tem, which contains some 8,000 refer­ences, may be used by anyone interested in the physical properties of foods.

The removal of organic wastes from large-scale animal production systems or from processing plants is not only an aesthetic and health problem, but an economic one as well. One method, which is relatively clean and economical, is hydraulic transport—removing wastes by means of a flowing water slurry. However, the flow properties are poorly understood.

Mohsenin, working with Mahesh Kumar and Howard D. Bartlett of Penn State, is investigating the flow proper­ties of animal waste slurries in order to provide design criteria for hydraulic waste removal systems. The materials, described as non-Newtonian fluids, are complex suspensions of solid material in liquid and cannot be handled by ordinary pumping equipment.

Hydraulic transport is also being studied for use in moving fruits and vegetables by means of water. In some cases, water transport can help mini­mize mechanical damage. It can also help to cool and preserve fruit, and in the case of potatoes, the product may be washed while it is being transported.

Eating the container Each year, U.S. consumers pay over

$100 billion for farm foods. Of this amount, almost nine percent pays for packaging materials. Besides this tre-

40 MOSAIC September/October 1976

nendous economic cost, there is an eco-ogical cost: the enormous volume of -vaste materials generated—paper, metal, jlass, plastics, wood, and textiles. Food engineers are investigating the use of 'convertible" packages. Such materials :ould, after fulfilling their packaging role, be used in some other capacity, mch as pet food, pet litter, mulch, or insulation. Some researchers are even working on edible containers and utensils.

In one study, Mohsenin and Milford A. Hanna tested a composite biomaterial fabricated from corn, alfalfa, soybean meal, and ammonium liquin sulfonate. They found that the material's mechani­cal properties compared well with those of corrugated cardboard—and that it could be used as pet food as well. In another study, Mohsenin and his asso­ciates developed a new kitchen appliance to shred styrofoam packages. The shredded material, which is structurally similar to the horticultural soil condi­tioner, perlite, is collected in a bag. When enough has accumulated, it can be mixed with garden soil to improve the soil's physical characteristics.

Inventing food One of the primary applications of

research in to the physical properties of foods is the development of new ma­terials. For example, research has led to the development of tomatoes which can better withstand the shock and im­pact of mechanical harvesting, long-stemmed grapes which can easily be cut from the vine, and hard-shelled eggs which resis t cracking. Food engineers are also working with plant breeders on redesigning the corn kernel so that the germ, w h i c h contains the oil, and the endosperm, which contains the solid material, can be more easily separated in the d r y milling process. Biological materials such as soy products are being used as " m e a t expanders" which are al­ready b e i n g used commercially in forms which do n o t require sophisticated struc­tural mechanics , such as in soups, sau­sage, h o t dogs, and hamburger. The expertise of textile researchers is being sought to develop the fiber structure or larger cu t s of meat, though the fabrica­tion of foods from nonbiological ma­terials r e m a i n s for the future.

For t h o s e developing new food ma­terials, devis ing standards for testing is often a problem. For example, in order to compare the hardness of egg shells, the p rope r ty being tested must be defined

in terms of an objective, measurable physical parameter. Standardization is needed both in definitions of physical properties and in instrumentation. The adoption of standard tests would allow researchers in different parts of the world who performed the same test on the same material to obtain comparable results. It would make communication among researchers easier.

Until recently, food science and food technology were the creation solely of agricultural scientists, biologists, and chemists. But the research is now at­tracting scientists from a variety of dis­ciplines : chemists, biologists, engineers, physicists, mathematicians, and physi­ologists. The agricultural engineer fits in as a sort of jack of all trades, bringing together the biological, chemical, and physical aspects of foods.

Although many food engineers are engaged in efforts to understand the fundamental physical properties of bio­logical materials, they are also concerned

with the applications of their research. "We as food engineers are always looking ahead for some application," Mohsenin says. "Either we study the properties of the material and then find applications for it, or we work the other way around; we have a need for a cer­tain property, so we search and find that property and then apply it in a design. If basic research means that you work on a problem without anticipating any use for the result, then that is not what we are doing."

The researchers who are now involved in understanding the properties of foods are, however, providing basic scientific information. The understanding they gain will lead to a greater efficiency in production, less waste, and food of higher quality at lower cost. •

Research discussed in this article is supported largely by the Solid Mechan­ics Program of the National Science Foundation.