EVALUATION OF SOME TEXTURAL PROPERTIES OF FOODS WITH THE INSTRON UNIVERSAL TESTING MACHINE

Abstract. Analysis of published chewing force patterns indicates that the 'first bite' involves an approximately linear application of force. The Instron Universal Testing Machine also offers a linear pattern of force application making it possible to obtain information about textural properties identified during the 'first bite' from force-compression tests. However, the data obtained in such tests are influenced not only by the applied force but also by the rate at which it is applied. Both variables must be taken into account when analysing the data, since the applied force in chewing and also the rate of force application depend on the textural characteristics of the food being evaluated. Based on a correlation of sensory responses with instrumental force-compression-rate of force application data, a procedure is described for establishing the mechanical force conditions that should be used in Instron tests so that they simulate those associated with the sensory evaluation.     

DEVELOPMENT OF A HIGH-MOISTURE SHELF-STABLE GRATED CARROT PRODUCT USING HURDLE TECHNOLOGY

Carrot (Daucus carota) can be highly perishable and difficult to preserve fresh for long periods at ambient temperature and humidity. Studies on grated carrots were carried out for the development of a shelf‐stable moist‐preservation process based on infusion of additives using hurdle technology. Additives comprising 3.0% sodium chloride (to reduce water activity [A_w] to 0.94), 0.9% citric acid (to reduce pH to below 4.5), 1.0% sodium citrate and 0.2% sodium benzoate (as antimicrobial agents) were followed by partial dehydration. The product was stored in flexible polymeric pouches. The physical, chemical and pathological stability was monitored during storage at ambient temperatures (19–33C). The partially dehydrated grated carrots (moisture 66.2%) were acceptable for more than six months at ambient temperature with the retention of carotenoid up to 82.5%. The product was microbiologically safe throughout the study.     

IDENTIFICATION OF STIMULI CONTROLLING THE SENSORY EVALUATION OF VISCOSITY I. Non-Oral Methods

Flow properties of several liquid and semi-solid foods were characterized objectively with a coaxial cylinder viscometer. At the same time, a sensory evaluation panel compared the viscosities of these samples by tilting the containers and also by stirring the contents with a spoon. Correlation of the instrumental and sensory evaluation data established the shear stress-shear rate conditions prevailing during sensory evaluation of viscosity. The stimulus responsible for viscosity evaluation by tilting the container is the shear rate (0.1–40 sec^-1) developed at a shear stress (60–600 dyne cm^-2) related to the flow properties of the sample, whereas in stirring tests the stimulus is the shear stress (10²–10⁴ dyne cm^-2) developed at a particular rate of shear (90–100 sec^-1). In the latter type of test, the shear rate varies to some degree with the flow characteristics of the sample. Since different stimuli are involved in viscosity assessment by tilting the container or by stirring, it is possible for a series of samples to be given different ranking orders by the two methods of evaluation.     

VARIATION IN STIMULI ASSOCIATED WITH ORAL EVALUATION OF THE VISCOSITIES OF GLUCOSE SOLUTIONS

Abstract. Previous observations suggesting that different stimuli are associated with oral evaluation of the viscosity of fluid and of viscous foods have been confirmed. The change in stimuli begins at a viscosity of about 70 cp. In the initial study sensory evaluation of viscosity was by paired comparisons, while in this study distance measures and ratio scaling were used. The agreement between the data indicates that the observations are independent of the evaluation procedure employed.     

IDENTIFICATION OF STIMULI CONTROLLING THE SENSORY EVALUATION OF VISCOSITY II. Oral Methods

The stimulus associated with oral evaluation of the viscosity of liquid and viscous foods appears to embrace a range of shear rates extending from 10^-1 to 1000 s^-1. The value associated with a particular food depends on its flow characteristics. For liquid foods the stimulus appears to be the shear rate developed at an approximately constant shear stress (～ 100 dyn cm^-2), whereas for viscous foods it appears to be the shear stress developed at an approximately constant shear rate (～ 10 s^-1). Additional factors may be involved when evaluating highly viscous non-Newtonian foods or foods with an oil continuous medium. These are ‘plug flow’ or the spreading coefficient, respectively.     

STRESS RELAXATION DURING FORCE-COMPRESSION STUDIES ON FOODS WITH THE INSTRON UNIVERSAL TESTING MACHINE AND ITS IMPLICATIONS

Abstract. The time required to apply the force in force-compression studies with the Instron Universal Testing Machine is long enough for short time relaxation processes to operate, possibly via intermolecular attraction forces. A theoretical analysis is presented whereby stress relaxation can be determined for the stress-strain region in which the Boltzmann Superposition Principle is valid. Application of the analysis is demonstrated using force-compression data obtained on cheeses of different textural properties. After correction for stress relaxation, the calculated and experimental relaxation modulus-time curves agree satisfactorily over a much longer time interval for Edam cheese than for Cheddar cheese. The latter cheese becomes non-linear at much smaller compressions than does Edam cheese, possibly by the propagation of cracks through the sample during compression. Firmness is one of the textural properties evaluated during the first bite of mastication. Since the time associated with the first bite is longer than that required for application of force during the first compression of a sample with the Instron in this study, it is possible that the sensory evaluation of firmness is made under conditions involving stress relaxation.     

Densification and dielectric properties of SrO–Al2O3–B2O3 ceramic bodies

The influence of SrO (0·0–5·0 wt%) on partial substitution of alpha alumina (corundum) in ceramic composition (95 Al₂O₃–5B₂O₃) have been studied by co-precipitated process and their phase composition, microstructure, microchemistry and microwave dielectric properties were studied. Phase composition was revealed by XRD, while microstructure and microchemistry were investigated by electron-probe microanalysis (EPMA). The dielectric properties by means of dielectric constant (ε _r ), quality factor (Q × f) and temperature coefficient of resonant frequency (τ _f ) were measured in the microwave frequency region using a network analyser by the resonance method. The addition of B₂O₃ and SrO significantly reduced the sintering temperature of alumina ceramic bodies to 1600 °C with optimum density (∼ 4g/cm³) as compared with pure alumina powders recycled from Al dross (3·55g/cm³ sintered at 1700 °C).     

THE INFLUENCE OF WORK SOFTENING ON THE VISCOELASTIC PROPERTIES OF BUTTER AND MARGARINE

Abstract Creep compliance-time studies at low stresses give more information about the structural changes in work softened butter and margarine than the Cone Penetrometer. They show that margarine undergoes a greater structural change than butter. Margarine loses most of its instantaneous elasticity in an unrecoverable manner. However, most of the Newtonian viscosity loss is recovered. Butter loses much less of its instantaneous elasticity, and most of what is lost is recovered during aging, but less of the Newtonian viscosity which is lost is recovered. Retardation spectra derived from creep compliance-time curves for both butter and margarine before work softening show a peak at ∼10⁴ sec. In contrast to margarine, butter does not lose this peak when work-softened. For margarine, the peak slowly redevelops during aging and occurs at ∼10² sec. Past interpretation of Cone Penetrometer data has been based on the simplified concept of primary irreversible bonds and secondary reversible bonds, and the changes in their relative proportions during work softening. The present approach avoids the allocation of bond strengths into these arbitrary categories, and is based on the realization that butter and margarine contain a wide range of bond strengths.