ENGINEERING PROPERTIES OF FOODS

FOOD ENGINEERING – Engineering Properties of Foods - Barbosa-Cánovas G.V., Juliano P. and Peleg M.
ENGINEERING PROPERTIES OF FOODS

Barbosa-Cánovas G.V. and Juliano P.
Washington State University, USA

Peleg M.
University of Massachusetts, USA
Keywords: Food engineering, engineering property, physical, thermal, heat, electrical,
foods, density, porosity, shrinkage, particulates, powders, compressibility, flowability,
conductivity, permittivity, dielectric, color, gloss, translucency, microstructure,
microscopy, diffusivity, texture
Contents
1. Introduction
2. Thermal Properties
3. Optical Properties
4. Electrical Properties
5. Mechanical Properties
6. Properties of Food Powders
7. Role of Food Microstructure in Engineering Properties
Glossary
Bibliography
Biographical Sketches
To cite this chapter

Summary

The engineering properties of foods are important, if not essential, in the process design
and manufacture of food products. They can be classified as thermal (specific heat,
therma UNESCO - EOLSS
l conductivity, and diffusivity), optical (color, gloss, and translucency), electrical
(conductivity and permittivity), mechanical (structural, geometrical, and strength), and
food powder (primary and secondary) properties. Most of these properties indicate
changes in the chemical composition and structural organization of foods ranging from
the molecular to the macroscopic level. Both modern and more conventional
measurement m
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ethods allow computation of these properties, which can provide
information about the macrostructural effects of processing conditions in fresh and
manufactured foods. Mathematical models have been fitted to data as a function of one
or several experimental parameters, such as temperature, water content, porosity, or
other food characteristics. Most engineering properties are significantly altered by the
structural differences between foods. Several microscopy, scanning, and spectrometric
technologies permit close visualization of changes in structure at different levels
without intrusion. Microstructure studies have increased understanding of several
changes detected in foods resulting from treatment in emerging and conventional unit
operations, by relating these changes to engineering property characterization data and
models. In the future, structure–property modeling could lead to the synthetic
production of natural materials with improved characteristics, provided advances in
genetic engineering and biotechnology are incorporated into the food engineering field.
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FOOD ENGINEERING – Engineering Properties of Foods - Barbosa-Cánovas G.V., Juliano P. and Peleg M.
1. Introduction
The word engine, derived from the words engineer and engineering, comes from the
Latin word for talent, ingenium. From the onset of the Industrial Revolution to the
beginning of the twentieth century, the term was used almost exclusively to describe
power machines. Those who designed, built, and operated these machines became
known as engineers, and their profession, or expertise, as engineering. In today’s
technological world, the meaning of the term has expanded to include not only such
disciplines or activities as chemical, medical, polymer, or food engineering, but also
genetic engineering and social engineering. Although these disciplines have little to do
with engines, they heavily rely on the ingenuity from which the term was originally
conceived.
It is difficult to define what exactly constitutes an engineering property of a certain food.
In general, however, any attribute affecting the processing or handling of a food can be
defined as an engineering property. Since many properties are related, there is usually
an arbitrary element in their classification. Traditionally, they are divided into the
following categories:
•
Thermal properties such as specific heat, conductivity, diffusivity, and boiling
point rise, freezing point depression.
•
Optical properties, primarily color, but also gloss and translucency.
•
Electrical properties, primarily conductivity and permittivity.
•
Structural and geometrical properties such as density, particle size, shape,
porosity, surface roughness, and cellularity.
•
Mechanical properties such as textural (including strength, compressibility,
and deformability) and rheological properties (such as viscosity).
•
Others, including mass transfer related properties (diffusivity, permeability),
surface tension, cloud stability, gelling ability, and radiation absorbance.
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Nearly all of the above properties are manifestations of a food’s chemical composition
and structural organization over several orders of length scales—from the molecular to
the macroscopic. A change in either composition or structure usually results in a
simultaneous change in several properties. Hence it is difficult, if not impossible, to
control a single property in isolation. Moreover, properties can be intrinsic, and
primarily co
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ntrolled by the material itself (for example, structural properties like density)
and response properties, varying according to the external conditions to which the food
is exposed (including colorimetric properties like hue).
Food materials or biological materials in general can display large compositional
variations, inhomogeneities, and anisotropic structures. Composition can change due to
seasonal variations and/or environmental conditions, or in the case of processed foods,
properties can be affected by process conditions and material history. For example,
North Atlantic fish show dramatic compositional changes in their protein and moisture
contents throughout the seasons. Cereals that are puffed up under different moisture and
temperature conditions can vary widely in density and cell-size distribution, and
exposure of such products to moist atmospheres, sometimes for short periods only, can
have dramatic effects on their crispness. Therefore, in many cases the data found in
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FOOD ENGINEERING – Engineering Properties of Foods - Barbosa-Cánovas G.V., Juliano P. and Peleg M.
published lists for engineering properties of foods can only be considered as
approximate values. Nevertheless, these tabular values are still very useful since a safety
factor is added to almost all calculations or designs of food processes and/or operations.
An understanding of what affects the engineering properties of foods is essential for
their proper interpretation and successful utilization. Therefore, one should always pay
attention to the conditions under which the reported properties were determined,
especially when response properties are involved.
Early physical property analyses of food products required constant uniform values and
were often oversimplified and inaccurate. Nowadays, computational engineering
techniques, such as the finite element method, are much more sophisticated and can be
used to evaluate non-uniform properties (for example, thermal properties) that change
with time, temperature, and location in food products that are heated or cooled.
Improvements measuring the compositions of foods are now allowing predictions of
engineering properties that are more accurate than previously, since they can be
predicted from existing numerical and empirical models of the food’s composition,
temperature, and porosity. There has always been a tendency to make general
correlations in predicting properties of food materials for use in process design
equations. A myriad of mathematical functions have already been fitted to experimental
data, and models are bringing order to experience with the goal of clarifying which
components or interactions are important in a food system.
The Engineering Properties of Foods topic covers different sets of engineering
properties that are described in greater detail in specific articles, each with wide
applications to food engineering and useful for product characterization and equipment
design in food manufacture. Basic definitions, common methods, parameter dependence,
modeling, and food engineering applications will dictate the basic pattern followed
within most sections. The final section will define how engineering properties and
microstructure are related, because foods are complex in both structure and composition,
this being the main reason for variability during property determination.
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2. Thermal Properties
Most processed and fresh foods receive some type of heating or cooling during handling
or manufacturing. Design and operation of processes that involve heat transfer require
special atten
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tion due to the heat-sensitivity of foods. Thermal properties of foods are
related to heat transfer control in specified foods and can be classified as
thermodynamic properties (enthalpy and entropy) and heat transport properties
(thermal conductivity and thermal diffusivity). Thermophysical properties not only
include thermodynamic and heat transport properties, but also other physical properties
involved in the transfer of heat, such as freeze and boiling point, mass, density, porosity,
and viscosity. These properties play an important role in the design and prediction of
heat transfer operations during the handling, processing, canning, storing, and
distribution of foods.
Heat can be transferred three different ways: by radiation, conduction, or convection.
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FOOD ENGINEERING – Engineering Properties of Foods - Barbosa-Cánovas G.V., Juliano P. and Peleg M.
•
Radiation is the transfer of heat by electromagnetic waves (as in a microwave
oven).
•
Conduction is the transfer of thermal energy due to molecular oscillations (for
example, heating of food by direct fire through metal containers).
•
Convection is the transfer of heat by bulk movement of molecules in heated
fluids such as liquids or gases (for example, air in heated oven or in tank during
juice evaporation).
Although all three types of heat transfer can take place simultaneously, generally only
one is predominant, depending on the state of the food and the heating system. In many
heat transfer processes associated with storage and processing, heat is conducted
through the product; heat is transferred by forced convection between the product and a
moving fluid (for example, hot air during tray drying), which surrounds or comes in
contact with the product.
Basic definitions of thermal properties of foods related to conduction within the product,
with reference to properties associated with forced convection through the surface (such
as surface heat transfer coefficient), will be mentioned in this section. Measuring
techniques will be briefly described, as well as parameters involved during processing
applications.
2.1. Definitions
The thermal properties of foods can characterize heat transfer mechanisms in different
unit operations involving heating or cooling. Specific heat, thermal conductivity,
thermal diffusivity, boiling point rise, and freezing point elevation are defined as
follows:
(a)
Specific heat, Cp, is the amount of heat needed to raise the temperature of unit
mass by unit degree at a given temperature. The SI units for C
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(kJ kg–1 K–1). Specific heat of solids and liquids depends upon temperature but
is generally not sensitive to pressure. It is common to use the constant pressure
specific heat, Cp, which thermodynamically represents the change in enthalpy H
(kJ Kg–1) for a given change in temperature T when it occurs at constant pressure
P:
C = (? H / ? T) (1)
p

Only with g SAMPLE CHAPTER
P
asses is it necessary to distinguish between Cp and Cv, the specific
heat at a constant volume. Assuming there is no phase change, the amount of
heat Q that must be added to a unit mass M (kg of mass or specific weight
kg/m3) to raise the temperature from T2 to T1 can be calculated using the
following equation:
Q = MC (T -T ) (2)
p
2
1
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FOOD ENGINEERING – Engineering Properties of Foods - Barbosa-Cánovas G.V., Juliano P. and Peleg M.
(b)
Thermal conductivity, ? , represents the quantity of heat Q that flows per unit
time through a food of unit thickness and unit area having unit temperature
difference between faces; SI units for ? are [W m–1 K–1]. The rate of heat flow
Q through a material by conduction can be predicted by Fourier’s law of heat
conduction. A simplified approximation follows:
Q = ? A(T ? T ) / x (3)
1
2

where A is the surface area of the food, x is its thickness, T1 is the temperature
at the outer surface where heat is absorbed, and T2 is the temperature at the
inner surface. In other words, ? represents the ability of the food to transmit
heat. Unlike specific heat, ? depends on mass density.
(c)
Thermal diffusivity,?, SI units [m2/s], defines the rate at which heat diffuses by
conduction through a food composite, and is related to ? and Cp through
density ? [kg/ m3] as follows:
? =? /?C (4)
P

Thermal diffusivity determines the speed of heat of three-dimensional
propagation or diffusion through the material. It is represented by the rate at
which temperature changes in a certain volume of food material, while
transient heat is conducted through it in a certain direction in or out of the
material (depending if the operation involves heating or cooling). Eq. (4)
shows that ? is directly proportional to the thermal conductivity at a given
density and specific heat. Physically, it relates the ability of the material to
conduct heat to its ability to store heat.
In liquid foods, boiling refers to water evaporation, in which water changes from the
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liquid phase to steam or vapor phase, and water vapor pressure equals the external
pressure. Liquid foods contain high molecular weight solids that cause the boiling point
to be elevated above that of pure water. The boiling point rise, ?Tr, is known as the
increase in boiling point over that of water in a given liquid food. As the vapor pressure
of most aqueous solutions is lower than that of water at the same temperature, the
boiling temperature (boiling point) of the solution is higher than that of pure water.
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During freezing, water in the food changes to ice while heat is removed by a
refrigeration system. During heat removal, the unfrozen water will still contain
dissolved food solids. The presence of dissolved solids will depress the initial freezing
point a certain amount ?Tf below the expected solidification temperature for pure water.
Freezing point depression is defined as the temperature reduction ?Tf. Both the boiling
point rise and the freezing point depression of a food are related to its solutes
concentration.
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FOOD ENGINEERING – Engineering Properties of Foods - Barbosa-Cánovas G.V., Juliano P. and Peleg M.
2.2. Thermal Variations in Properties and Methods of Determination
Precision and accuracy of measurement are important factors in determining thermal
properties variations. In commercial heating or cooling applications, computer
techniques nowadays provide accuracies of 2–5 percent for most heat-transfer
calculations, which provide much lower relative errors than practical boundary
condition determinations (for example, air temperature and velocities).
Several methods are known for measuring specific heat and Cp and thermal conductivity
? experimentally. Cp measurement of foods can be determined by methods of mixtures
and differential scanning calorimetry (DSC). For methods of mixtures, a calorimeter of
known specific heat is used and Cp is determined from a heat exchange balance. In the
DSC method, the sample is put in a special cell where the temperature is increased at a
constant heating rate. The specific heat of the food is obtained from a single heat
thermogram, which relates heat flow as a function of time or temperature. Two
experimental methods to determine ? are the Fitch method and the line source method.
In the Fitch method, a solid slab of a certain food receives heat from one layer and
conducts it to a copper plug. Conductivity k is obtained from the food’s temperature as a
function of heat conduction time. The line source method is based on the use of a
thermal conductivity probe to measure a temperature–time relation on a thin cylindrical
food piece to which constant heat is applied.
Thermal diffusivity ? is usually either determined by direct experimental methods or
estimated through Eq. (4). Several direct methods for ? determination can be based on a
one-dimensional heat conduction equation where geometrical boundary conditions are
defined. For instance, an apparatus can be used where the sample is located in a special
cylinder and immersed in a water bath at constant temperature. Thermocouples located
at the center of the sample (axis) and surface of cylinder measure temperature at
different heating times. Transient temperature variations are used for the analytical
solution. Indirect methods, although they might yield more accurate diffusivity values,
p).
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require more time and instrumentation for the three-parameter determination (?,? , and
C
Boiling point elevation ?Tr at a certain external pressure can be determined from a
thermodynamic equation using the latent heat of vaporization and molar fraction of the
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food. However, the use of these equations requires knowledge of the proportions of
specific components of the foods that cause changes in the boiling points. In many cases,
estimates for specific components present in higher concentrations can be used.
Sometimes reference liquids under the same vapor pressure conditions can be compared
with the food, and charts can be used to determining boiling points at different
saturation concentrations. On the other hand, freezing points Tf in foods can be directly
determined from the freezing curve (or cryoscope) method without using component
concentrations. ?Tf value can be derived from the temperature plateau after initial
temperature depression (or supercooling) on a time-temperature plot. Furthermore, DSC
can also be used to determine the onset, peak, and end of freezing.
Foods show extended variability in composition (mainly water, proteins, carbohydrates,
fat, ash, and fiber) and structure, and can be turned into even more complex composite
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FOOD ENGINEERING – Engineering Properties of Foods - Barbosa-Cánovas G.V., Juliano P. and Peleg M.
materials when heated together, as in the case of many canned and packed foods, pastry,
confectionery, and a wide variety of prepared foods. Thermophysical properties depend
on the chemical composition of the structure, determined by the physical arrangement
and phase distribution of a system. Thus, heat transfer by conduction may take place in
several forms depending on the tortuosity of the material, which may vary at different
locations. As porous materials contain a gaseous phase, the value of the thermal
conductivity ? , specific heat Cp, and thermal diffusivity ? will depend on the internal
and external pore space represented by its porosity (see Mechanical properties).
Thermophysical properties are significantly influenced by changes in water content and
temperature. During drying, the transfer of heat into food products is accompanied by
simultaneous diffusion of water through the product to the surrounding air, provoking
differences in thermophysical properties at different regions of the food. Pore size and
distribution not only affect heat transfer because of air retention, but also because of the
affinity pores have to retain water. The smaller the pore diameter, the greater the surface
tension forces, and the more affinity it has for water. Specific heat Cp of foods is
drastically influenced by water content. For example, specific heat has been found to
vary exponentially with water content in fruit pulps at above ambient temperatures.
Furthermore, nonaqueous components show lower Cp. The specific heats of oils and fats
are usually about one-half the specific heat of water, while the specific heat of dry
materials in grains and powders is approximately one-third to one-fourth that of water.
As a result of solute water interactions, the Cp of each individual component in a food
differs from the Cp of a pure component, and usually changes with the concentration of
soluble solids. The same occurs with thermal conductivity ? , where water shows
greater relative magnitudes in comparison to other food constituents. Thus, both ? and
Cp increase with increased moisture content. It is common to find a linear relation
between thermal conductivity and moisture content at ambient conditions.
The effect of temperature on thermophysical properties is not easy to establish because
solids (or semisolids), liquid foods, and food emulsions undergo structural changes.
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ophysical properties of foods change dramatically during the freezing process.
Specific heat changes are difficult to predict when free water becomes solid. Bound
water or unfrozen water has a different Cp than bulk-frozen water, and ice has a Cp of
about one-half that of liquid water. Thus, Cp below freezing is approximately half that
of Cp above freezing. Continuous changes in the fraction of frozen water as temperature
varies below the freezing point explain this similarity. In fact, specific heat can be
utilized to predict the state of water in frozen foods. Thermal conductivity, however, has
been found to be high when temperatures allow water to be in liquid or solid state at
very low or high temperatures. Yet when temperatures are within the range of –10 º to
0 ºC, ?
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shows its lowest values. Freezing point depression has been modeled with the
initial freezing point as a function of water content using linear and quadratic equations.
Some thermophysical property models for food systems have been developed as a
function of water content or temperature. Additionally, as composition greatly differs
between one food and another, other models are linear combinations of water, fat,
protein, carbohydrate and/or ash content, and temperature. Cp has been measured at
different temperatures in fresh and dried fruits, meats, cereal grains and cereal products,
oils and fats, powders, and other dry foods. Although linear correlations of Cp with
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FOOD ENGINEERING – Engineering Properties of Foods - Barbosa-Cánovas G.V., Juliano P. and Peleg M.
concentration are known in liquid foods, variations are often neglected for engineering
calculations at near room temperature.
General correlations also predict thermal conductivity ? , of food materials for use in
process design equations. Linear, quadratic, and multiple correlations of moisture,
temperature, and composition can be found for ? in food materials. Some models
consider that different components of foods (for example, fibers) are arranged in layers
either parallel or perpendicular to the heat flow. In products such as meats, heat is
usually transferred parallel to fibers and ? is dependent on the direction of the heat
flow. More general in nature are the randomly distributed models, which consider that
the food is composed of a continuous phase with a discontinuous phase dispersed within
(solid particles being in either regular or irregular array). In porous materials, porosity
must be included in the model because air has a ? much lower than that of other food
components. Models including density or porosity, and pressure, have been developed
in fruits and vegetables, meat and meat products, dairy products, cereals, and starch.
Several models for predicting ? in foods have also appeared in literature; however, most
are product specific and a function of water content or temperature. Although the
influence of carbohydrates, proteins, fat, and ash on thermal diffusivity has been also
investigated, it was found that temperature and water content are the major factors
affecting ?. Above freezing temperatures, diffusivity varies linearly with temperature or
water composition in some foods, while this is not valid at below-freezing temperatures.
2.3. Food Processing Applications
Food thermal properties play an important role in the quantitative analysis of food
processing operations. Numerous food processing unit operations are heat or energy
sensitive, and the most well known are shown in Table 1. Heat exchanged and resulting
temperature–pressure relations must consider the minimization of reactions such as
browning, vitamin loss, and oxidation reduction in order to preserve the acceptability
and nutritional value of the food. Thermal properties are useful when evaluating
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capacities of drying systems, or studying the effect of product shrinkage or internal
cracking with the aid of mathematical and numerical drying models. Enthalpy and
specific heat are required to calculate the heat load in food processing operations.
Specific heat measurement allows evaluation of the structure of foods (for example, fat
polymorphism in chocolate).
Operation
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Description Examples Heat transfer medium
Processing
conditions
Pasteurization Removal
of
Milk, fruit
Hot water, steam,
63–85 ºC
pathogenic
juices, beer,
electricity
microorganisms;
eggs
15 s–30 min
increasing shelf
life
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FOOD ENGINEERING – Engineering Properties of Foods - Barbosa-Cánovas G.V., Juliano P. and Peleg M.
Sterilization
Sterilization of
Meat, fish, soup, Hot water, steam
100–125 ºC
solids and liquid;
vegetables, fruit,
Canning
6-month shelf life
milk, cream,

15 min–2 h
custard,
desserts, soup




UHT
Sterilization of
Hot water, steam direct
135–150 ºC
processing
fluids and aseptic
or indirect
packaging
1–10 s
Evaporation Removal
of
Milk, fruit
Steam 40-100
ºC
water; production
juices, coffee,
of liquid
cheese whey
2 s–2 h
concentrate
Dehydration Removal
of
Milk, potato,
Hot air, steam, hot
150–250 ºC
water; production
vegetables, fruit, water, electricity
of dried material
meat, fish
with low water
activity
Cooking and
Cooking foods;
Catering
Steam, hot air,
1 min to several
baking
baking cereal
operations,
microwaves
hours
based foods
bread, meat,
pipes, cakes
Frying Immersion
in
hot
French fried
Hot oil
100–150 ºC
oil
potatoes,
doughnuts,
crisps
Chilling Reducing Dairy products,
Cold air, indirect
10–0 ºC
temperature to
meat, fish, fruit,
refrigeration (ammonia)
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just above
vegetables,

freezing point
frozen desserts
Freezing Reducing
Cryogenic fluids (liquid

temperature to
nitrogen)
well below
freezing point
Adapted from: Le
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wis M.J. (1987). Physical Properties of Foods and Food Processing
Systems. Chichester, UK: Ellis Horwood.
Table 1. Unit operations involving heat transfer in foods
During processing involving heat, temperature within a food changes continuously,
varying not only the food Cp but also the ? . When conduction of heat is involved,
thermal conductivity is important to predict or control the heat flux and processing
times. In a processing system, it is necessary to predict the time end-point of processing
to ensure the efficiency of the equipment. It is also desirable to heat and cool foods as
rapidly as possible to improve the economics of the process by increasing the capacity
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FOOD ENGINEERING – Engineering Properties of Foods - Barbosa-Cánovas G.V., Juliano P. and Peleg M.
and delivering a better quality product. All processing-time prediction models need the
thermal conductivity data of food where energy transfer is involved. The speed of heat
propagation or diffusion through the material is also related to processing times.
Therefore, thermal diffusivity can also participate in processing-time estimation of
processes like canning, cooling, freezing, and frying.
The equilibrium freezing point can be used for the prediction of thermophysical
properties because of the discontinuity exhibited at that point. Accurate freezing point
data can also be used to calculate other colligative properties such as effective
molecular weight, water activity, bound, free, and frozen water, and enthalpy below
freezing point. Knowledge of freezing point is important for analyzing freezing and
thawing times of frozen foods. Freezing point data can be used to ascertain chemical
purity with regard to whether a sample differs from a natural or desired condition. The
increase in boiling point or boiling point rise (?Tr) of liquid foods is a property of
interest in evaporators or other types of heat exchanger equipment design and operation.
It is worth mentioning the role of the surface heat transfer coefficient, as it is one of the
important parameters necessary to design and control food processing equipment where
fluids (air, nitrogen, steam, water, or oil) participate. Although it is not a property of a
food, it is used to quantify the transfer rate of heat by convection from a liquid or a gas
(especially boiling liquids and condensing vapors) to the surface of the foods. It plays
an important role when evaluating the effectiveness of heat transfer in processes where
hot water or steam is applied through the evaluation of the overall resistances during
heat transfer.
3. Optical Properties
Optical properties are those material properties resulting from physical phenomena
occurring when any form of light interacts with the material under consideration. In the
case of foods, the main optical property considered by consumers in evaluating quality
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is color, followed by gloss and translucency or turbidity among other properties.
“Color” is the general name applied to all sensations arising from the activity of the
retina, and is related to visual appearance of food (shape, size, surface and flesh
structure, and defects).
3.1 Definitions
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Optical properties are related to consumer judgment on food appearance and produce
some kind of visual effect. Among these, color, gloss and translucency can be defined
as follows.
•
Gloss is the name given to light specularly reflected from a plain smooth
surface. It can be defined by a goniophotometric curve, which represents the
intensity of light reflected at the surface at different angles of incidence and
viewing.
•
Color is essentially a beam of light composed of irregularly distributed energy
emitted at different wavelengths. Depending on the type of illumination, the
same material can show different light qualities and produce different
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