FOOD ENGINEERING

Lesson 7. PROPERTIES OF SOLID FOODS

Module 1. Rheology of foods

Lesson 7

PROPERTIES OF SOLID FOODS

7.1 Introduction

Solid foods are generally characterized in terms of stress - strain relationship. The stress may be of tensile, compressive, tangential (shear) or torsional (acting on a transverse cross section). The classification of solid foods is even more hazy than that of fluid foods. There are two major groups : elastic and non elastic. visco - elastic foods, mostly of semi - solid and solid nature, form an important group of non - elastic foods.

7.2 Elastic Solids

7.2.1 Hookean or linear elasticity

Elasticity is defined as the tendency of the product to recover upon unloading the shape and dimensions it had before loading. If there is no permanent deformation after unloading, the elasticity is said to be complete elasticity. Ideal or Hookean elasticity is characterized by a linear relationship between force (or stress) and deformation (or strain) starting at the origin (Fig. 7.1a) The body instantaneously returns to its initial form with no residual strain upon unloading.

Further, the Linear relationship is retraced when the sample is unloaded. The ratio of tensile stress to strain for these so-called Hookean bodies is termed Young’s modulus (E) or elongation modulus. The ratio between shear stress to shear strain in an ideal linear elastic solid is called shear modulus (G) or rigidity.

7.2.2 Non - Hookean or non - linear elasticity

In reality, most elastic solids exhibit a non-linear or non-Hookean elasticity, in which case the stress is not proportional to strain, and the linear dependence of stress on strain exists only at the lowest strain ‘levels. In general, at higher strain levels the loading-and-unloading cycle yields two separate traces describing a hystersis loop (Fig. 7.1b). Since the stress-strain relationship is curvilinear, the modulus of elasticity is frequently given as the tangent modulus, which is the slope of the stress-strain curve at any specified stress or strain.

Fig. 7.1 Linear (a) and Non-linear (b) elasticity: Stress-strain relationship

7.3 Non-Elastic Solids

A material may show elasticity, linear or non-linear, if the applied stresses and corresponding strains are small. However, for large deformations most solids are non-elastic. Non-elastic products may exhibit failure when stress exceeds the strength of the body.

Failure

Failure may be seen as fracture or rupture.

(i) Fracture : Cracking of hard materials such as hard cheese at low temperature ultimately resulting in two or more separate pieces is termed fracture. Elastic fracture is fracture without or with a very limited amount of flow (only in the region just around the crack) of the material, as in unripe fruit flesh, tubers etc., whereas plastic fracture is fracture accompanied by flow of material as may be seen in certain soft or semi-hard cheeses.

(ii) Rupture : This term refers to tearing (in pieces) of soft materials. Rupture point is sometimes defined as a point on the stress-strain or force-deformation curve at which the axially loaded specimen ruptures. The failure in rupturing materials such as certain cheese gels, cooked egg white etc. is characterized by a multitude of failure planes.

7.4 Plastic Solids

Certain non-elastic products may show yield value and tend to flow when the stress exceeds this point. Plasticity is found more frequently in semi-solid and soft products such as butter, spreads etc. rather than hard solids.

7.5 Viscoelstic Foods

Failure resulting in rupture, fracture or plastic flow usually involves relatively large stresses and large deformation in solid foods. On the other hand, small deformation in most solids and semi-solid products may reveal what is known as viscoelasticity. Certain, shear-thinning fluids such as age thickened sweetened condensed milk also exhibits viscoelasticity.

The reaction of a viscoelastic body to stress (or strain) consists partly of a viscous component and partly an elastic one. Since stress and strain are time-dependent, the response of the material is rate dependent.

Attempts have been made to classify food products on the basis of their rheological behaviour. However, the rheological phenomena in various foods are so complex that it is not simple to categorize them into distinct groups or classes. Yet the classification of foods based on the stress-strain rate relationship for fluid and semi-solid products, and stress-strain relationship for solids would greatly facilitate comprehending the rheological behaviour of various dairy and food products and relating it to their processing, handling and texture attributes.

7.6 Rheological Properties of Solid Dairy Products

Solid dairy products such as butter and cheese are valued for their textural characteristics. Admittedly, the texture of other products, solid, semi-solid or fluid, is an equally important determinant of their overall sensory acceptance, but probably because of their solid nature and its variability. Several empirical methods have been developed in attempts to best describe the product's rheology in relation to their textural properties. Considerable efforts have also been devoted to obtain information on the fundamental rheological properties of cheese and butter. However, owing to the complexities of the product texture, progress made in this regard is rather limited. Nevertheless, recent developments in rheological instruments hold out a definite scope for generating valuable information on the basic rheological parameters of these products. In the context of Indian dairy industry, texture and rheology of certain solid and semi-solid products such as paneer, khoa, chhana and milk sweets have been recognized to play an important role-in their acceptance which, in turn has a great bearing on the success of their production in modern dairy plants.

7.6.1 Rheology of cheese

Ever since the early report pertaining to the Davis's plastometer devised to measure deformations in cheese, butter etc. under compression several different rheometers have been developed. These include Devis's apparatus for measuring the crushing strength of cheese, a spherical compression device of Scott Blair and Coppen (employing a 3.8 cm sphere for compression of cylindrical cheese samples), Caffyn's ball compressor and sectilometer ( a write cutting apparatus), and several penetrometers. Some of those principles were incorporated into certain commercial instruments for routine analysis. Hoeppler consistometer was one such instrument also used for deriving certain fundamental measurements e.g. viscosity and elastic modulus of hard and semi - hard cheese varieties. Studies to correlate measurements obtain with instruments and sensory texture properties of cheese yielded varying results, the correlations generally being high for hardness but low for springiness and other attributes.During the late fifties and sixties, considerable interest was witnessed in employing rheological measurements for process control (in terms of raw cheese ingredients etc.) in the manufacture of processed cheese. This interest has been subsequently maintained to a great extent primarily because of increasing process variables and sustained and growing demand for this product .

With the advent of the new generation rheological instruments such as the Instron machine, Ottawa texture measuring system, Bohlin's rheometers and several texture analyses (e.g. Steuren's, Micro stable etc.), rheological measurements of cheese has undergone a dramatic change. Instruments providing non - destructive dynamic measurements have been used for viscoelastic characterizations of cheese offering better understanding of cheese texture.

In spite of the impressive advances registered regarding the rheological measuring systems there is a long way to go before rheological measurements become a substitute for sensory texture. More specific in relation to cheese is its no homogeneity arising primarily from its constituents and the manufacturing process. A recent IDF publication provides an extensive review of various aspects of the rheology and fracture properties of different varieties of cheese, where the arisotropic nature of cheese resulting from pressing of curd, considered a major factor contributing non-homogeneity.

Hard and semi - hard cheese varieties have been subjected to rheological measurements most frequently by uniaxial compression and to a lesser extent by deformation in tension or shear between two parallel plates. Sometimes three point bending of a cylindrical cheese sample has been employed to imitate the deformation seen while grading a cheese plug. The quantitative characterization of cheese can be achieved' in terms various moduli especially the compression modulus as well as the relative deformation at fracture and fracture stress in a compression test. Toughness or the energy of fracture (the energy required from the onset of compression till the cheese sample fractures ) is also a useful parameter.

Texture profile analysis (TPA) using compression between parallel plates has been extensively used for cheese texture characterization but the TPA parameters have been stated to be of only a limited value in cheese texture characterization.

Biaxial extesnsional viscosity determined by using compression between parallel discs has been found to be particularly applicable to process cheese. The so-called lubricated squeezing flow technique for the determination of extensional or elongation viscosity has been used to measure the melting properties of processed cheese as also of mozzarella cheese. To a limited extent oscillation measurement have been made of Gouda and other cheese to obtain dynamic moduli reflecting the viscoelastic character of the product.

Factors affecting cheese rheology

Cheese is a composite material. Its major constituents para-casein, fat and the aqueous phase, contribute each in a specific way to the structure and hence to the rheology of cheese. The para casein matrix imparts the product solidity though formation of 3-dimension structure. Thus composition is among the most important factors influencing cheese rheology. The moisture, fat and protein contents are major compositional variables in cheese. Fat is a key contributor of the temperature-caused variation in the rheological properties of cheese. The pH of Gouda cheese has been observed to affect the stress-strain curve. These rheological parameters of ripened cheese such as Gouda also vary with the period of ripening.

Regarding the rheological methodology for cheese the following are among the salient recommendation made by IDF.

a. A method that yields real and unequivocal rheological or fracture parameters should be used so that the results do not vary with test-piece size or with small variation in test conditions.
b. The sample preparation should be such that it docs not subsequently alter the product properties.
c. The type, extent and time scale, of deformation should be in accordance with the conditions during the actual use of the cheese (e.g. eating, cutting or storage).
d. Different mechanical methods may be used for the purpose of comparison and visible changes during deformation should also be observed.

7.6.2 Rheology of butter

Rheology measurements of butter are important from two main points of view : first, spread ability of butter as a functional texture attribute, and second, its pump ability or handling convenience. The most relevant factor in this connection is the high temperature coefficient for the consistency of this 'plastic' product, which necessarily needs to be stored under refrigeration in plants or in homes.

Several empirical instruments viz. cone penetrometer, sectilometer, extrusion devices such as FIRA- NIRD extruder etc. have been used for 'obtaining the subjective measurements that would correlate with the sensory assessment of spread ability of butter. The most common parameter that is sought to be measured by those methods is hardness of the product. Other less important texture measurements made on butter include its stickiness, oiliness and brittleness but nor all instruments give this information. Most popular the sectilometer is now available as a highly sophisticated microprocessor controlled instrument (Buttomat). The cone penetromer has frequently been used for routine purposes in New Zealand, Australia and UK. extruder thrust (From the NIRD instrument) has been found to correlate very well with sensory and spreadability of butter. Recently the Instron back extension test has been employed to compare the consistency of commercial butter made by different processes.

Beside temperature, compositional and other related factors such as moisture SNF, salting, ripening etc. have been found to influence the rheological behaviour of butter. The solid fat content seems to be the major determinant of butter rheology as also the state of fat crystals. Work softening, referring to the softening effect of working is important to butter handling. Rheological measurements such as yield stress have been used to study this phenomenon.
The processing parameters in conjunction with the physical properties or milk fat in butter govern the structure of butter (believed to be a dispersion of fat glubules and aqueous phase in a continuous phase of liquid fat) which is responsible for the typical rheological behaviour of the product.

7.6.3 Rheology of traditional milk products

Industrial production of traditional milk products is yet to come of age in this country. Although studies have been conducted on several technological and quality aspects of these products during the past four decades, their rheological behaviour has aroused little research interest in the past. Occasionally, certain empirical instruments such as cone penetrometer or similar contraptions for compression studies have been used. Process development studies in respect of rasogolla have benefitted by the use of cone penetrometer for determining the product's firmness. The penetrometer has been used for certain khoa - and chhana based sweet such as burfi, sandesh etc.

In the first ever attempt to study the viscoelastic behavior of a traditional product like paneer at PAU, Ludhiana. The relaxation times have been worked out employing compression in an Instron machine. A mechanical model has also been proposed to describe the viscoelastic behaviour of paneer. Khoa may be a solid or semi - solid product depending on its type and / or moisture content. However, at high temperature such as those encountered during its manufacture this product is essentially a pseudoplastic fluid, the relevant power law parameters being a function of total solids.

The viscosity of khoa at 300C as measured in Hoeppler consistomer has been found to range from 2.Ox103 to 7.6 x 103 poise with the product TS ranging from 56 to 72 %. Texture profile parameters of khoa determined as a function of composition have been studied and Texture Profile Analysis (TPA) hardness has been found to exhibit a significant correlation with the corresponding sensory attribute of the product.

In recent times investigations have been carried out at different Institutition with a view to generating the information on rheology of various indigenous products other than khoa viz., chhana and chhana-based sweets such as rasogolla, and sandesh, paneer, and khoa - based sweets such as kalakand, gulabjamun etc. Temperature and test conditions during parallel plate compression for 'TPA of these products are major factore affecting the measurements. Further, in most cases, the TPA hardness has been observed to be the single most important parameter.