Notes 4 of fe 501 physical properties of food materials

FE-501
PHYSICAL PROPERTIES OF
FOOD MATERIALS
ASSOC PROF. DR. YUS ANIZA YUSOF
DEPARTMENT OF PROCESS & FOOD ENGINEERING
&
FACULTY OF ENGINEERING
UNIVERSITI PUTRA MALAYSIA

OPTICAL PROPERTIES OF FOODS
OPTICAL PROPERTIES OF FOODS

INTRODUCTION
• Optical properties of foods are those properties
which govern how food materials respond to
absorption of electromagnetic radiation in the range
of optical wavelengths and frequencies.
• These include visible light and color, but also
transmission, reflection and refraction of visible light.

REFRACTION
BASICS
• The speed of light is maximum in a vacuum.
• The speed of light is much lower when it must travel
through a material substance ( di ) and th speed
th
h
t i l bt
(medium), d the
d
will depend on the physical properties of the medium.
• 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.
d
• This results in the electromagnetic light beam changing
direction when it crosses the interface Between the two
different media, as shown in Figure 4.1

REFRACTION
BASICS

Figure 4.1. Refraction as a
consequence of different
speeds of wave propagation
through different materials

REFRACTION
BASICS
• A light beam that shown in Figure 4.2, where the light is
p
passing from medium 1 into medium 2, part of the light
g
p
g
“bounces back” (is reflected) at the interface, while the
other part is refracted as it enters material 2.
• According to Huygen’s principle, all points at the
interface are starting points of spherical waves
propagating through medium 2
2.
• If these waves have a lower propagation speed in
medium 2 than they did in medium 1, they will change
y
y
g
the direction of the light beam as a consequence.

REFRACTION
BASICS

Figure 4.2. Angles of reflection
(α) and refraction (β) when light
strikes an i
ik
interface of diff
f
f different
materials with refraction
indices n1 and n2

REFRACTION
BASICS
• The refraction angle β in Figure 4.2 can be calculated
with Snell’s law:
Snell s
(4.1)

(4.2)

REFRACTION
BASICS
• According to Snell’s Law, as the angle of incidence α
increases, so will the refraction angle β increase.
• When β = 90° , no light will enter the material at all,
and all incident light will be reflected.
g
• 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.

REFRACTION
MEASUREMENTS OF REFRACTION INDEX
• The refraction index can be determined
experimentally from Snell’s law by being able to
Snell s
measure the angle of incidence and the angle of
refraction in a light beam experiment.
• From the same experiment is also possible to
determine the critical angle of total reflection by
finding the angle of incidence at which the angle of
refraction goes to 90°

REFRACTION
• Figure 4.3 shows a schematic diagram of
refractometer based on measurement of the angle of
total reflection.
y j
g
g
g g ( g
• By adjusting the angle g of the incoming light (angle
of incidence α) until the detector (placed at position
8) gets a signal, the critical incident angle αG is
reached when the outgoing light has an angle of
refraction β = 90°

REFRACTION

Figure
4.3.
Measurement
of
refraction by total reflection: 1:
window, 2: sample, 3: cover, 4:
incoming beam, 5: reflected beam, 6:
refracted b
f t d beam, 7 t t l reflected
7: total
fl t d
beam, 8: detector

REFRACTION
• With n1 known, we can calculate the following using
equation (4.2)
equation (4.2)
(4.3)

• and so
( )
(4.4)

REFRACTION
• where

REFRACTION
• Because the refraction index n1 of the refractometer
material is known from equation (4.4),we can obtain
the refraction index n2 of the sample very easily.
p
• Table 4.1 shows some examples of refraction index
data.

REFRACTION

Table 4.1. Refraction indices of some materials

REFRACTION
• Laboratory refractometers (Fig. 4.4) are small hand‐
held instruments that are easy to use. They require
only a few drops of a liquid sample, and provide
results within just a few seconds.
• Th ability t measure th refraction i d easily and
The bilit to
the f ti index
il
d
quickly with handheld refractometers is very useful
in food technology applications.
gy pp
• For example, the sucrose concentration in fruit juices
and soft drinks can be related directly to the
refraction i d of the sample solution.
f
i index f h
l
l i

REFRACTION

Figure 4.4. Refractometers
Figure 4.4. Refractometers

COLORIMETRY
• Color and its measurement (colorimetry) involve
organoleptic perception of properties.
• The different colors we see with visible light are the
result of how our eyes perceive electromagnetic
radiation at different frequencies and wavelengths
wavelengths.
• The sensation of color occurs when light rays
(electromagnetic radiation) of a certain frequency and
wavelength strike the retina of the h
l
h ik h
i
f h human eye.
• The retina, in turn, transforms this sensation into a nerve
signal that is transmitted to our brain, and we perceive
color.

COLORIMETRY
LIGHT AND COLOR
• Visible light is electromagnetic radiation with
wavelength between 380 nm and 750 nm.
• Larger wavelengths belong to infra red radiation (IR)
and smaller wavelengths belong to ultraviolet
g
g
radiation (UV), and are invisible to the human eye.
• 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.

COLORIMETRY
LIGHT AND COLOR
(4.5)

• where

• The range of wavelengths for visible light can be further
subdivided into smaller ranges that are each responsible
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 4.2.
f d ll
d bl
li t d i T bl 4 2

COLORIMETRY
LIGHT AND COLOR
Table 4.2. Rough classification of visible light by color

• When light of different wavelengths (different colors)
is mixed together, we can produce any other color.
This is ll d ddi i
Thi i called additive mixing. O the other h d
i i
On h
h hand,
filtering can be used to eliminate certain wavelengths
in order to produce a different color with those
remaining. This is called subtractive mixing.

COLORIMETRY
LIGHT AND COLOR
• The perception of color is also dependent on particle
size. This is the result of what we call light scattering.
• As the particles become smaller and smaller, the
specific surface area of the particle aggregates
increases dramatically This causes more and more of
dramatically.
the incident light striking each particle to be fully
reflected.
• Our eye receives only the light that is reflected from an
object. When this light contains all the wavelengths in
the visible light spectrum, we are receiving the additive
mixing of all the colors, which is perceived as white.

COLORIMETRY
LIGHT AND COLOR
• Therefore, when we consider the physics of color, it
makes a difference whether the body is a radiating
body (emitting li ht) or i a non radiating b d
b d ( itti
light)
is
di ti
body
(absorbing light). The color of non radiating bodies
like food materials depends on several factors some
of which are listed in Table 4.3.
Table 4.3. Factors
influencing the color of
a material

COLORIMETRY
PHYSIOLOGY OF COLOR PERCEPTION
• There are two different types of light receptors on
the retina of the human eye that are called rods and
cones.
• The rods are sensitive to relative brightness and
darkness,
darkness while the cones are sensitive to colors
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 420 nm (blue), 535 nm (green) and 565
(
),
(g
)
nm (red).

COLORIMETRY
• Thus, we can simply say that we have cones sensitive
to blue, green and red light. By mixing and blending
the i l from all th
th signals f
ll three cone sensors, we can
perceive all colors made up of added mixtures of
light with these wavelengths
wavelengths.
• Figure 4.5 shows all colors which can be observed in
a so‐called color triangle. Here, each place in the
so called
diagram is a perception of color defined by x, y
coordinates.

COLORIMETRY
Figure 4.5. Chromaticity diagram. In the
triangle are all colors which can be
g
observed. Colors with maximum brilliance
are on the horse shoe curve. Point E in the
middle is zero brilliance (white)

COLORIMETRY
• As we move along the perimeter of the triangle in a
clockwise direction, we encounter a system of increasing
wavelength.
wavelength The points on the horseshoe like curve are
horseshoe‐like
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 f
d from the outer points of the d
h
f h 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. This is the point of
zero brilliance E.

COLORIMETRY
• The mixing of two colors on the color triangle can be
represent by drawing a straight line from one color to the
other in order to see what resulting color is possible
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.
ll d
l
t
l

COLORIMETRY
COLOR AS A VECTOR QUANTITY
• A convenient way to describe a color as a quantity is
to treat it like a vector with three components.
• Based on this vector system with three components,
we can indicate a color with numbers.
• For example, we can say color number 80‐70‐50 after
CIE or number 7:3:2 after DIN 6164.
• I thi way, th communication f d
In this
the
i ti for describing a color
ibi
l
is immune from problems with human perception
and subjective judgement This is important in
judgement.
technical applications.

COLORIMETRY
• For technical purposes we can describe a color by
three attributes;
–h
hue
– chroma
– brightness
Table 4.4. Terms used in colorimetry

COLORIMETRY
• The human eye can distinguish about 200 different
hues, 20–25 degrees of chroma and about 500
degrees of b i ht
d
f brightness. B combination of th
By
bi ti
f these we
can perceive some millions of different colors.

COLORIMETRY
LAB SYSTEM FOR COLOR QUANTIFICATION
• One of the first laboratory methods for quantifying colors is
the system devised by Munsell (Munsell, 1905). In this
system, a color is marked by a vector. The vector points to a
system a color is marked by a vector The vector points to a
place in the color space indicating the hue of the color.
• The length of the vector d indicates the distance from the
point of zero color, and quantifies the chroma of the color,
while the angle α gives the hue.
• Th vertical axis i scaled f th b i ht
The
ti l i is
l d for the brightness of th color
f the l
(see Figure 4.6). So, to describe the color of interest we
have to specify α and d of the vector and the value of
p y
brightness.

COLORIMETRY

Figure 4 6 Color as a point at
4.6.
the end of a vector. In the
Munsell system, an arrow
(vector) points to a place in
the color space. The arrow is
described by angle α and
length d. The vertical axis
represents the brightness
scale

COLORIMETRY
• Another presentation of the color vector is made in the
Judd–Hunter system (Judd & Wyszecki, 1967).
• The vector pointing to the place in the color space is
indicated with the coordinates a and b.
• The brightness L again is scaled on the vertical axis (see
Figure 4.7). This system often is called the L‐a‐b system.
The a‐axis and b‐axis are scaled from –100 to +100. So,
with the Judd–Hunter system we d
h h
dd
describe a color with
b
l
h
three numbers (L‐a‐b) which are all between 0 and 100.
Using the L‐a‐b system we see positive values of a
Lab
represent red,−a for green, +b for yellow, and −b for blue.

COLORIMETRY

Figure 4.7. Indicating a
color i the Judd–Hunter
l in h
dd
system(L‐a‐b system)

COLORIMETRY
• When we compare Figure 4.6 with Figure 4.7, we see
that they are based on the same idea of specifying a
point in the three‐dimensional color space, but there
point in the three dimensional color space but there
are differences in the terms used. So we can
ca cu ate α o a a d b by t e o o g
calculate α from a and b by the following:
(4.6)

• and the chroma d from
(4.7)
(4 7)

COLORIMETRY
• When we examine Figure 4.8,we will recognize the quantities
h
ll
h
a, b and α again. They describe the vector lying in the plane.
When we now use the vertical axis and let the vector also
point to the value for the brightness L, then we can get a
three‐dimensional vector pointing to our designated color.

Figure 4.8. Color as a point in a
three‐dimensional space of
polar coordinates

COLORIMETRY
• In Table 4.5 are some examples of L‐a‐b number sets with
bl
l
f
b
b
h
descriptions of the colors they represent. The Judd–Hunter
system is widely used, and is often called the L‐a‐b system.
y
y
y
Variations of the name include L*a*b* system and CIELAB
system.
Table 4.5. Characterization of colors by Judd–Hunter system

COLORIMETRY
COLOR MEASUREMENT
• Color measurements can be performed by visual
techniques or a class of spectrometric techniques.
Classic tri stimulus colorimeters belong to the class
tri‐stimulus
of spectrometric techniques, and try to adopt the
function of the human eye.
u ct o o t e u a eye
• Visual color measurement involves observing a
sample without instruments, but under controlled
p
,
conditions of illumination, along with reference to a
set of color standards with which to compare the
sample colors observed.

COLORIMETRY
COLOR MEASUREMENT
• Spectrometric measurements involve measurements
of the absorption of specified wavelengths by the
sample under controlled defined conditions of
sample under controlled defined conditions of
illumination.
• Tri‐stimulus techniques make use of three filters to
Tri‐stimulus techniques make use of three filters to
simulate the function of the three different types of
cones in the human eye retina.
y

COLORIMETRY
VISUAL COLOR MEASUREMENT
• When using visual color measurement techniques,
objects can be described as having the same color
when they show no observable difference in color
under identical conditions of illumination.
• For this reason visual techniques involve observing
reason,
the color of a sample and comparing it against
defined color standards under identical conditions of
illumination.
• This is called finding a color match, and falls into the
category of organoleptic (sensory) methods of food
quality analysis.

COLORIMETRY
VISUAL COLOR MEASUREMENT
• Color standards are commercially available in the
form of paper board tiles.
• F li id samples, colored standard solutions are
For liquid
l
l d
d d l i
used as matching fluids.
• Sol tions re ommended b the Ameri an and
Solutions recommended by
American
European Pharmacopoea for this purpose, include
CoCl2 (rose) FeCl3 (yellow) and CuSO4 (blue)
(rose),
(blue).

COLORIMETRY
TRI‐STIMULUS‐COLORIMETRY
• Based on the three types of cones in the retina of the
human eye, color measuring instruments have been
developed with three filters that function like each of
the three types of cones. With these types of
instruments, we can measure the intensity of the
st u e ts, e ca
easu e t e te s ty o t e
wavelengths transmitted through each of these
filters (Figure 4.9).
Figure 4.9. Tri‐stimulus colorimeter
(schematic).
Light
source
V
illuminates sample P Detector S
P.
reads the intensity of a frequency
given by filter F

ULTRAVIOLET (UV)
• Ultraviolet (UV) light is electromagnetic radiation
with a wavelength shorter than that of visible light,
but longer than soft X rays
X‐rays.
• It can be subdivided into near UV radiation with
wavelengths in the range (380–200 nm) far or
(380 200 nm),
vacuum UV (FUV or VUV) with wavelengths in the
range (200–10 nm), and extreme UV (EUV or XUV)
g (
),
(
)
with wavelengths in the range (1–31 nm).

ULTRAVIOLET (UV)
• When considering the effect of UV radiation on
human health and the environment, the range of
near UV wavelengths is even further subdivided into
UVA (UV α, 380–315 nm), also called long wave or
“black light; ”UVB (UV β, 315–280 nm), also called
b ac g t; U
(U
3 5 80
), a so ca ed
medium wave; and UVC (UV γ, < 280 nm), also called
short wave or “germicidal.”

ULTRAVIOLET (UV)
• Ultraviolet radiation is often used in connection with
visible spectroscopy or photometry (UV/VIS) to
determine the existence of fluorescence in a given
sample, and it is widely used as a technique in
c e st y, o a a ys s o c e ca st uctu e, ost
chemistry, for analysis of chemical structure, most
notably conjugated systems.
p
p
y,
• Perhaps more importantly, UV radiation has become
increasingly used as an effective disinfecting agent in
treatment of drinking water and in cold food
processing.

ULTRAVIOLET (UV)
ULTRAVIOLET (UV)
DISINFECTING DRINKING WATER
• One important application for UV radiation is in the
treatment of drinking water because it acts as a very
effective disinfecting agent
agent.
• Disinfection using UV radiation was historically more
commonly used in wastewater treatment applications,
but is now finding increased usage in drinking water
treatment. It used to be thought that UV disinfection was
more effective for bacteria and viruses which have more
viruses,
exposed genetic material, than for larger pathogens
which have outer coatings or that form spore states that
g
p
shield their DNA from UV light.

ULTRAVIOLET (UV)
ULTRAVIOLET (UV)
DISINFECTING DRINKING WATER
• However, it was recently discovered that ultraviolet
radiation can be somewhat effective for treating the
microorganism Cryptosporidium
Cryptosporidium.
• These findings resulted in the use of UV radiation as
a viable method to treat drinking water.
a viable method to treat drinking water

ULTRAVIOLET (UV)
ULTRAVIOLET (UV)
FOOD PROCESSING
• As consumer demand for fresh and “fresh like” food
products increases, the demand for non thermal
methods of food pasteurization is likewise on the
rise. In addition, public awareness regarding the
da ge s o ood bo e ess ( ood po so g) s a so
dangers of food‐borne illness (food poisoning) is also
raising demand for improved food processing
methods that assure safety to the consumer with
minimum loss in quality.

ULTRAVIOLET (UV)
ULTRAVIOLET (UV)
FOOD PROCESSING
• Ultraviolet radiation is used in several food processes
to inactivate (destroy) unwanted microorganisms
from liquid food products with suitable optical
properties (transparent).
• Among the most common applications today is the
use of UV light to pasteurize fruit juices by pumping
the juice over a high intensity ultraviolet light source.
j
g
y
g
• The effectiveness of such a process depends on the
UV absorbance of the juice.

REFERENCES
1. Judd DB, Wyszecki G (1967).Colour in Business,
Science, and Industry.Wiley, NewYork.
2. Munsell AH (1905) A C l
2 M
ll
Colour N
Notation. M
i
Munsell
ll
Colour Company, Boston MA.

THE END
THE END

RHEOLOGICAL PROPERTIES OF FOODS

Notes 4 of fe 501 physical properties of food materials

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