Empirical Instrumental Tests

The earliest applications of rheology to cheese, even before rheology was conceived as a scientific discipline, consisted of making subjective estimations of some of the more obvious properties and trying to describe these in plain language. Often the cheese-maker or the grader, though skillful and competent in judgment, would find it difficult to define precisely the terms being used. One can see that confusion can arise when one tries to distinguish between terms such as consistency and body, firmness and hardness, chewiness and meatiness. The grader's mind may be clear about what is understood by each, but communicating the precise meaning to the outside world is less easy. The difficulty is further compounded by the fact that these words do not translate precisely into other languages, making international communication more difficult. To some extent the development of texture studies, particularly in the United States, has clarified the semantic problem (9), but subjective assessment can never match the precision which can be attained with sophisticated instrumentation. Much of the earlier instrumental rheology sought to replace, or at least to reinforce, subjective judgment with numerical quantities which could be measured reproducibly by any competent technical assistant (10).

The earliest instruments were empirical (3) and, in general, intended to give some indication of the firmness or similar qualities. To this end, they were designed to simulate what the scientist perceived to be the mechanical action of the hand or fingers of the expert during examination of the cheese. They were generally unsophisticated, not too expensive, and made no pretence of measuring any precisely defined physical property. Because of this, their measurements cannot be analyzed and expressed in terms of fundamental rheological parameters as defined above. Nevertheless, some of them may still perform a useful function if their limitations are understood, because they possess the undoubted merit that their measurements are easily reproducible between different laboratories and different times.

One such instrument became known as the Ball Compressor (11). In the course of examining a cheese, one of the actions of any grader is to press either a thumb or a finger into the surface and, from the reaction which is felt, gain an impression of the firmness or springiness of the cheese. The Ball Compressor might in fact be described as a mechanical thumb. It consists of a metal hemisphere which is initially placed on the surface of a whole cheese and allowed to sink into the cheese under the action of a load; the depth of indentation can be measured and recorded after a fixed time. The load can then be removed and the recovery of the cheese observed. The instrument is shown diagrammatically in Figure 4.

Making a number of simplifying assumptions, it is possible to convert a reading obtained with the Ball Compressor into a modulus G (12,13), which may be considered to be the equivalent of the modulus of rigidity, using the formula

where M is the applied load, R the radius, and D the depth of the indentation. This formula has been arrived at assuming that the sample is a homogeneous isotropic elastic solid, the load is static, and there is no friction between the sample and the surface of the hemisphere. It is clear that every one of these conditions is violated. Cheese is far from homogeneous, particularly if it has a rind or has been salted by immersion in brine, causing the properties to vary from the surface to the center. It is not isotropic: the method of manufacture of some varieties, such as Cheddar, on which many of the earlier experiments were carried out, or Mozzarella, is intended to produce an oriented structure. Some of the objections, however, would appear to be much less serious in the case of other cheeses where uniformity of texture is the aim of the manufacturer. Cheese is not an elastic solid; it is viscoelastic. The indentation is not instantaneous and may not be completed within the duration of the experiment. Finally, if the load is removed,

Figure 4. The Ball Compressor (diagrammatic).

some indentation remains, showing that some of the energy has been dissipated in causing flow and not stored up as in an elastic solid. In spite of all these limitations, if the test proceeds at least until the indentation becomes so slow that it has virtually ceased, the use of equation 9 gives an indication of the magnitude of the firmness in readily understandable physical units. This enables the reader to compare measurements given in the early literature with those made more recently by more sophisticated means.

The Ball Compressor has been discussed at some length, not only because of its historical importance, but because it has the merit of being a nondestructive means of testing and does not require an operator to undergo any specialized training. Also, it is used on the whole cheese, whereas almost every other rheological test requires that a sample be cut from the whole. Using the Ball Compressor, it has been shown that in a whole cheese such as a Cheddar or a Cheshire weighing some 25 kg there are considerable variations in the firmness over the surface of the cheese (14) and that the firmness differs on the upper and lower sides. This difference is influenced by the frequency with which the cheese is turned in the store during its maturing period, and the time which has elapsed since it was last turned. The implication of this is that it is not possible, whatever instrumental measurement is made, to assign a single number to any property of the cheese, nor is it possible to assess the properties of the whole cheese by means of a measurement made at one local point in that cheese.

Clearly there is a need for some form of nondestructive test and for many purposes there is no reason why it should not be empirical, provided that the implications are understood. The Ball Compressor has the merits of cheapness and simplicity, but the time taken to obtain a representative reading limits its use to the research laboratory. The problem of devising a suitable test is as yet unsolved. It is unfortunate that the application of ultrasonic techniques, which have proved so useful in the nondestructive testing of many engineering materials, have proved unrewarding with cheese (15,16). This is because the dimensions of the cracks and other inhomogeneities in cheese are commensurate with, or sometimes even larger than, the wavelength of the ultrasound and large-scale scattering takes place. It is difficult for a pulse to penetrate the body of the cheese and both the velocity of propagation and the attenuation are more influenced by the scattering than by the properties of the bulk.

One other empirical test deserves to be mentioned on account of its simplicity. This is the penetrometer (17-19). It is not quite nondestructive, but very nearly so, since it only requires that a needle be driven into the body of the cheese; no separate sampling is required. A penetrometer test may take one of several forms. For example, a needle may be allowed to penetrate under the action of a fixed load (17,18), or it may be forced into the cheese at a predetermined rate (19) and the required force measured.

Whichever method is used, let the specific actions be considered. As the needle penetrates the cheese, that part of the cheese immediately ahead of it is ruptured and forced apart. If the needle is thin, the deformation normal to its axis is small, so that the force required to accomplish this may be ignored. On the other hand, the progress of the needle is retarded by the adhesion of its surface to the cheese through which it passes. This may be expected to increase with the progress of the penetration until a point is reached when the restraining force matches the applied load and further penetration ceases. If a suitable diameter for the needle and a suitable weight have been chosen, this test may be completed in a few seconds. The test will be more useful for cheese whose body is reasonably homogeneous on the macroscopic scale, as, for example, the Dutch cheeses. With cheeses such as Cheddar or the blue cheeses the heterogeneities are generally on too large a scale and the penetration becomes irregular. The needle may pass through weaknesses in the structure or even cracks and so give rise to the impression that the cheese is less firm than it really is, or the point of the needle may attempt to follow a line of weakness, not necessarily vertical, and as a result there will be additional lateral forces acting on the needle and its penetration will be arrested prematurely.

Measurements made with a penetrometer cannot be converted theoretically to any well-defined physical unit. Both the cohesive forces within the cheese and the adhesive forces between the cheese and the surface of the needle are a consequence of the forces binding the structure together. One may infer that these are related to its visco-elastic properties but there is no simple theory which attempts to establish these relations. It has been shown experimentally (20) that there is a statistically significant correlation between firmness as measured by the Ball Compressor and by penetration, but this differs among different types of cheese (21). It has also been shown experimentally that a curvilinear relation exists between the resistance to penetration and an elastic modulus of some Swiss cheeses, calculated from the results of a compression experiment (Fig. 5). While this gives some idea of the magnitude of the forces involved, it has little practical application since it refers only to one series of experiments on only one type of cheese. In the absence of a similar experimentally determined relation for the cheese in which one is particularly interested, penetrometer measurements can only be regarded as empirical.

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  • leonardo
    What is accompaniment in food processing?
    8 years ago

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