They found that conductivity along the stem was higher than that across the stem for bamboo shoots and sugarcane. However, the reverse was observed for lettuce and mustard stems.
NDL India: Engineering properties of foods, Fourth Edition
A microstructural examination revealed that two influences were important: the orientation of vascular bundles and the shape of parenchyma cells. When both types of tissue were present, the vascular bundles dominated the trend in electrical conductivity, since these are the primary modes of water and nutrient transport within plants.
However, in the absence of vascular tissue, the shape of the parenchyma cells was the dominant factor, explaining the different results between the different types of tissue. Effects of Temperature and Electric Field Strength 1. Gels and Noncellular Solids The electrical conductivity of noncellular solids tends to increase with temperature. The trend is generally a linear one, as observed by Yongsawatdigul et al. Various reasons may be advanced for such effects, including the breakdown of the gel, resulting in lower drag on ions, and enhanced conductivity at higher temperatures.
Electro-osmotic effects are unlikely in such cases, since, as noted by Yogsawatdigul et al. Solids with Undisrupted Cellular Structure For solids with a cellular structure, such as fruits, vegetables, and intact muscle foods, the electrical conductivity depends on temperature as well as electric field strength. As illustrated by Palaniappan et al. As the electric field strength is increased, the change in electrical conductivity becomes more gradual, until at sufficiently high field strengths, the familiar linear electrical conductivity— temperature relation is seen Figure 5.
This suggests that under the influence of electricity, the cell structure is broken down at lower temperatures than for conventional heating. This phenomenon has been termed electroporation or electroplasmolysis. Ohmic heating relies on the flow of alternating or other waveform current through a food material to heat it by internal generation. In recent years, both these technologies have been explored for a variety of other applications; hence a class of processes known as moderate electric field MEF processes is emerging. Equipment design and product safety assurance in both ohmic and PEF technologies depend on the electrical conductivity of the food in question.
Electromagnetic heating processes are governed by the material properties called dielectric properties. As microwave heating gains increasing use in food processing systems in industry and in the home, knowledge of dielectric properties becomes increasingly critical for consistent and predictable product, process, and equipment development. Figure shows the electromagnetic spectrum.
The frequencies allocated for microwave and RF heating are shown in Table When microwave energy is incident on a food material, part of the energy is absorbed by the food, leading to its temperature rise. Electromagnetic waves are composed of an electric field and a magnetic field. Dielectric properties can be categorized into two: dielectric constant and dielectric loss factor. The parameter that measures microwave absorptivity is the loss factor. The values of dielectric constant and loss factor will play important roles in determining the interaction of microwaves with food.
The dielectric constant and loss factor of free water are predicted by Debye models and shown in the first and second equations, respectively Mudgett, Debye models are expressed in terms of wavelength and temperature-dependent parameters. Water can exist in either the free or bound state in food systems. Free water is found in capillaries but bound water is physically adsorbed to the surface of dry material.
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Figure shows the variation of dielectric loss factor with moisture content. As can be seen in the figure, loss factor is constant in the bound region region I up to a critical moisture content Mc but then increases sharply for high moisture contents. Therefore, the effect of bound water on dielectric properties is negligible. The interaction of food components with water is a significant factor in affecting their dielectric properties.
The stronger the binding forces between protein or carbohydrates and water, the smaller the value of the dielectric constant and loss factor since free water in the system decreases. For this reason, adjusting the moisture content is the key factor in formulating microwaveable foods. The increase in water increases the polarization, which increases both dielectric constant and loss factor.
At low moisture contents, variation of dielectric properties with moisture content is small. For food materials having high moisture contents, bound water does not play a significant role and the dielectric properties are affected by dissolved constituents as well as water content.
Engineering Properties of Foods
In a recent study, it was shown that the dielectric loss factor of apples increased rapidly at water activity of around 0. Dielectric properties of foods decrease during drying, since free moisture content in the system decreases. Feng, Tang, and Cavalieri showed that both dielectric constant and loss factor of apples decreased during drying owing to the reduced moisture content in the food.
If the water is in bound form, the increase in temperature increases the dielectric properties. However, in the presence of free water, dielectric properties of free water decrease as temperature increases. Therefore, the rate of variation of dielectric properties depends on the ratio of bound to free moisture content. During thawing, both dielectric constant and loss factor show large increases with temperature. After the material thaws, dielectric properties decrease with increasing temperature for different food materials except for a salted food ham. The loss factor of ham shows a continuous increase during heating.
The increase in loss factor with temperature was also observed in turkey meat, which contains high amounts of ash Sipahioglu et al. The variation of dielectric loss factor of a salt solution or a salty material with respect to temperature is different because the loss factor of a salt solution is composed of two components: dipolar loss and ionic loss. Dipolar loss decreases with temperature at frequencies used in microwave processing. In contrast to dipolar loss, loss factor from ionic conduction increases with temperature owing to the decreased viscosity of the liquid and increased mobility of the ions.
At higher temperatures, ions become more mobile and not tightly bound to water, and thus the loss factor from ionic loss component increases with temperature. On the other hand, microwave heating of water molecules or food containing free moisture decreases with increasing temperature Prakash, The reasons for this are the rare hydrogen bonds and more intense movements which require less energy to overcome intermolecular bond at higher temperatures.
For materials containing both dipolar and ionic components, it is possible to observe first a decrease and then an increase in loss factor with temperature. There are limited dielectric properties data for foods below freezing temperatures. Sipahioglu et al. After melting took place, loss factor of ham increased with ash content Sipahioglu et al.
This can be explained by the fact that salts are capable of binding water which decreases the amount of water available for polarization. Dielectric properties data above the boiling point ofwater are also limited in the literature. Dielectric properties at high temperatures are important for microwave sterilization and pasteurization. The dielectric constant of all samples except noodles decreased as temperature increased at frequencies of and MHz. The increase in dielectric constant of cooked macaroni noodles with temperature is due to its low moisture content.
There was a mild increase in the loss factor of samples with temperature. Carbohydrate, fat, moisture, protein, and salt contents are the major food components. The presence of free and boundwater, surface charges, electrolytes, nonelectrolytes, and hydrogen bonding in the food product affect the dielectric properties. The physical changes that take place during processing such as moisture loss and protein denaturation also have an effect on dielectric properties.
Therefore, the investigation of dielectric behavior of major food components and the effects of processing on dielectric properties are important for food technologists and engineers to improve the quality of microwave foods, to design microwaveable foods, and to develop new microwave processes.
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Food components such as proteins, triglycerides, and starches have low dielectric activities at microwave frequencies. A Salt Solutions Salt is one of the major components in food systems, which is responsible for ionic conduction. Addition of salt to sturgeon caviar decreased dielectric constant but increased loss factor. The decrease in dielectric constant with the addition of salt is due to binding of water in the system which reduces the available water for polarization. On the other hand, addition of salt increases the loss factor since more charged particles are added to the system and charge migration is increased.
Nelson and Datta showed that the loss factor of salt solutions may increase or decrease with increasing temperature for different salt concentrations. B Carbohydrates Starches, sugars, and gums are the major carbohydrates in food systems. For carbohydrate solutions, the effect of free water on dielectric properties becomes significant since carbohydrates themselves have small dielectric activities at microwave frequencies.
Starch Variation of dielectric properties of starch with temperature depends on whether starch is in solid state or in suspension form. When the dielectric properties of different starches in powder form were measured at MHz, both the dielectric constant and the loss factor increased with temperature. The lower the bulk density, the lower the loss factor observed. Other researchers also found loss factors of other granular materials to be dependent on bulk density Calay et al.
For starch suspensions, the effect of free water on dielectric properties becomes significant. Dielectric constant and loss factor of different starch suspensions were shown to decrease as temperature and starch concentration increased Ndife et al. The dielectric properties of aqueous solutions are inversely related with temperature in the absence of ions. The increase in starch concentration decreases both the dielectric constant and loss factor since starch molecules bind water and reduce the amount of free water in the system.
Dielectric loss factor increased with increasing temperature and salt concentration. Dielectric constant of starch solutions containing no salt decreased with temperature. Salt ions affected the dielectric properties, especially the dielectric loss factor significantly. The dielectric constants of the salt solutions are known to decrease whereas the dielectric loss factor is known to increase with an increase in salt concentration. Gelatinization of starch is an important physical phenomenon that affects dielectric properties.
Sugar Sugar is an important microwave absorbing food ingredient as compared to other hydrocolloids. Sugars modify the dielectric behavior of water. The hydroxyl water interactions stabilize liquid water by hydrogen bonds and affect the dielectric properties of sugar solutions. Dielectric constant of glucose solution increased but the loss factor of glucose solution decreased with temperature.
Increasing glucose concentration decreased dielectric constant since less water was free to respond the electric field. There is a critical sugar concentration that affects the dielectric loss factor of sugar solution. However, at lower temperatures glucose solution became saturated at lower concentration and loss factor decreased with concentration. Gums Gums have the ability to bind high amounts of free water in the system. Therefore, depending on the amount of moisture bound to the gums, dielectric constant and loss factor of the system change. Charge of the gum is a significant factor in affecting its dielectric properties.
In the absence of water the effect of charge disappears. For microwaveable food formulations, it is important to know water binding capacity of the gums and viscosity of the solution to have an idea about the dielectric properties and microwave heatability of these formulations. When hydrocolloids are used in the range of 0. Since hydrocolloids can bind different amounts ofwater, food formulations containing one or more than one hydrocolloid are expected to have different amounts of free water in the system, which can affect polarization. Therefore, interaction of food with microwaves is expected to change in the presence of gums.
C Proteins Free amino acids are dielectrically reactive Pething, Free amino acids and polypeptides contribute to the increase in dielectric loss factor. The water adsorbed on the protein also affects their dielectric properties Dielectric properties of proteins change during denaturation. Protein denaturation is defined as the physical change of the protein molecule due to heat, UV, or agitation which results in a reduction in protein solubility and increase in solution viscosity McWilliams, During denaturation of proteins, since the structure of protein is disturbed, the asymmetry of the charge distribution will increase.
This will result in large dipole moment and polarization, which will affect the dielectric properties. Moreover, moisture is either bound by the protein molecule or released to the system during denaturation which shows a decrease or increase in dielectric properties, respectively. Therefore, dielectric properties of fats and oils are very low Figs. The effect of fat on dielectric properties of food systems is mainly the result of their dilution effect in the system.
The increase in fat content reduces the free water content in the system, which reduces the dielectric properties 6. Dielectric property measurement methods can be categorized as reflection or transmission types depending on resonant or nonresonant systems with closed or open structures Kraszewski, The most commonly used methods for dielectric measurement of foods are the transmission line method, coaxial probe method, cavity perturbation method, and free space transmission method. These include visible light and color, but also transmission, reflection and refraction of visible light. When a beam of light electromagnetic waves crosses the interface between two different media, the different physical properties of these media will cause the light waves to travel at different propagation velocities in each medium.
This results in the electromagnetic light beam changing direction when it crosses the interface between the two different media is called refraction and is shown in Figure below. If these waves have a lower propagation speed in medium 2 than they did in medium 1, they will change the direction of the light beam as a consequence. The angle of incidence causing this to happen is called the critical angle of total reflection. It can be used for measurement of refraction indices as detailed in the equation below.
Figure shows a schematic diagram of a refractometer based on measurement of the angle of total reflection. The different colors we see with visible light are the result of how our eyes perceive electromagnetic radiation at different frequencies and wavelengths. The sensation of color occurs when light rays electromagnetic radiation of a certain frequency and wavelength strike the retina of the human eye. The retina, in turn, transforms this sensation into a nerve signal that is transmitted to our brain, and we perceive color.
Before we can investigate the physical means by which this electromagnetic radiation reaches the eye, we must first take into account how color is perceived. The perception of color depends on many human conditions, such as age, health and visual abilities. The human eye has different receptors for light which have different spectral sensitivities. When we want to describe something we see in terms of alphanumeric figures,we have to look for quantities which allow us to describe in technical terms what the human eye would be sensing. So, let us first recall what is light and what is color.
Larger wavelengths belong to infra red radiation IR and smaller wavelengths belong to ultraviolet radiation UV , and are invisible to the human eye. Table The speed of light is the speed at which the light waves propagate, and can be calculated mathematically as the product of wavelength and frequency. The range of wavelengths for visible light can be further subdivided into smaller ranges that are each responsible for the different colors, such as the colors of the rainbow.
The wavelength ranges responsible for the primary colors of red, yellow and blue are listed in Table below. The rods are sensitive to relative brightness and darkness, while the cones are sensitive to colors. There are three different types of cones, which have pigments that are sensitive to different wavelengths of light. These include wavelengths with absorption maxima of nm blue , nm green and nm red.
Thus, we can simply say that we have cones sensitive to blue, green and red light. By mixing and blending the signals from all three cone sensors, we can perceive all colors made up of added mixtures of light with these wavelengths. Figure 7. Here, each place in the diagram is a perception of color defined by x, y coordinates. As we move along the perimeter of the triangle in a clockwise direction, we encounter a system of increasing wavelength. The points on the horseshoe-like curve are the points of maximum brilliance. These are the spectral colors colors of the rainbow. At the bottom line of the color triangle, we have purple colors which are not spectral colors not components of the rainbow Moving inward from the outer points of the diagram to the center of the triangle, we come to colors with less brilliance, to pale colors and at last to the point where a color is so pale that it appears white.
We can represent the mixing of two colors on the color triangle by drawing a straight line from one color to the other in order to see what resulting color is possible. For example, mixing red and green will produce a line going through the region of yellow colors. This illustrates the principle of additive mixing of colors. All lines passing through the point E represent possibilities for reaching a perfect white. Colors which result in white when mixed together are called complementary colors. Based on this vector system with three components, we can indicate a color with numbers.
In this way, the communication for describing a color is immune from problems with human perception and subjective judgment. By combination of these we can perceive some millions of different colors. He was an artist and published his Color Notation to have a rational way to describe color in In this system, a color is marked by a vector. See all. Item Information Condition:.
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May be very minimal identifying marks on the inside cover. Very minimal wear and tear. See all condition definitions - opens in a new window or tab Read more about the condition. About this product. EDT ; Rizvi, Syed. Furthermore, significant developments have taken place in thearea of high-pressure processing HPP , and the process has been approved by the Food and Drug Administration FDA for pasteurization of food. Kinetic data related to HPP play a crucial role for validating the pressure-assisted pasteurization.
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On the basis of these developments, three new chapters: "Microstructural Properties of Foods," "Glass Transition in Foods," and "Kinetics and Process Design for High-Pressure Processing" have been added in the fourth edition. Most of the existing chapters were revised to include recent developments in each subject. The chapter on colorimetric properties of food was removed from the earlier edition. Fruits and vegetables are usually graded depending on size, shape, and density. Impurities in food materials are separated by density differences between impurities and foods.
Knowledge of the bulk density of food materials is necessary to estimate floor space during storage and transportation Mohsenin, ; Rahman, When mixing, transportation, storing, and packaging particulate matter, it is important to know the properties of bulk material Lewis, The surface areas of fruits and vegetables are important in investigations related to spray coverage, removal of residues, respiration rate, light reflectance, and color evaluation, as well as in heat-transfer studies in heating and cooling processes Mohsenin, In many physical and chemical processes, the rate of reaction is proportional to the surface area; thus, it is often desirable to maximize the surface area.
Density and porosity have a direct effect on the other physical properties. Volume change and porosity are important parameters in estimating the diffusion coefficient of shrinking systems. Porosity and tortuosity are used to calculate effective diffusivity during mass transfer processes. Mechanical properties of agricultural materials also vary with porosity. This chapter provides terminology, measurement techniques, and prediction models of selected mass-volume-area-related properties. Mass-volume-area-related properties of foods.