Notes 4 of fe 501 physical properties of food materials

FE-501PHYSICAL PROPERTIES OFPHYSICAL PROPERTIES OF

FOOD MATERIALSFOOD MATERIALSASSOC PROF. DR. YUS ANIZA YUSOF

&DEPARTMENT OF PROCESS & FOOD ENGINEERINGFACULTY OF ENGINEERING

UNIVERSITI PUTRA MALAYSIA

OPTICAL PROPERTIES OF FOODSOPTICAL PROPERTIES OF FOODS

INTRODUCTIONINTRODUCTION

• Optical properties of foods are those propertieswhich govern how food materials respond towhich govern how food materials respond toabsorption of electromagnetic radiation in the rangeof optical wavelengths and frequencies.

• These include visible light and color, but alsotransmission, reflection and refraction of visible light.

REFRACTIONREFRACTIONREFRACTIONBASICS

REFRACTIONBASICS

• The speed of light is maximum in a vacuum.• The speed of light is much lower when it must travel

th h t i l b t ( di ) d th dthrough a material substance (medium), and the speedwill depend on the physical properties of the medium.

• When a beam of light (electromagnetic waves) crossesthe interface between two different media, the differentphysical properties of these media will cause the lightwaves to travel at different propagation velocities in each

dmedium.• This results in the electromagnetic light beam changing

direction when it crosses the interface Between the twodifferent media, as shown in Figure 4.1


REFRACTIONBASICS

Figure 4.1. Refraction as aFigure 4.1. Refraction as aconsequence of differentspeeds of wave propagationthrough different materials


REFRACTIONBASICS

• A light beam that shown in Figure 4.2, where the light ispassing from medium 1 into medium 2, part of the lightp g p g“bounces back” (is reflected) at the interface, while theother part is refracted as it enters material 2.

• According to Huygen’s principle, all points at theinterface are starting points of spherical wavespropagating through medium 2propagating through medium 2.

• If these waves have a lower propagation speed inmedium 2 than they did in medium 1, they will changey y gthe direction of the light beam as a consequence.


REFRACTIONBASICS

Figure 4.2. Angles of reflection(α) and refraction (β) when light

ik i f f diffstrikes an interface of differentmaterials with refractionindices n1 and n2


REFRACTIONBASICS

• The refraction angle β in Figure 4.2 can be calculatedwith Snell’s law:with Snell s law:

(4.1)

(4.2)


REFRACTIONBASICS

• According to Snell’s Law, as the angle of incidence αincreases, so will the refraction angle β increase.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 iscalled the critical angle of total reflection.

• It can be used for measurement of refraction indices.

REFRACTIONREFRACTIONREFRACTIONMEASUREMENTS OF REFRACTION INDEX

REFRACTIONMEASUREMENTS OF REFRACTION INDEX

• The refraction index can be determinedexperimentally from Snell’s law by being able toexperimentally from Snell s law by being able tomeasure the angle of incidence and the angle ofrefraction in a light beam experiment.

• From the same experiment is also possible todetermine the critical angle of total reflection byfinding the angle of incidence at which the angle ofrefraction goes to 90°



• Figure 4.3 shows a schematic diagram ofrefractometer based on measurement of the angle ofrefractometer based on measurement of the angle oftotal reflection.

• By adjusting the angle g of the incoming light (angley j g g g g g ( gof incidence α) until the detector (placed at position8) gets a signal, the critical incident angle αG isreached when the outgoing light has an angle ofrefraction β = 90°



Figure 4.3. Measurement ofFigure 4.3. Measurement ofrefraction by total reflection: 1:window, 2: sample, 3: cover, 4:incoming beam, 5: reflected beam, 6:f t d b 7 t t l fl t drefracted beam, 7: total reflected

beam, 8: detector



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

(4.3)

• and so(4.4)( )



• where



• Because the refraction index n1 of the refractometermaterial is known from equation (4.4),we can obtainmaterial is known from equation (4.4),we can obtainthe refraction index n2 of the sample very easily.

• Table 4.1 shows some examples of refraction indexpdata.



Table 4.1. Refraction indices of some materials



• Laboratory refractometers (Fig. 4.4) are small hand‐held instruments that are easy to use. They requireonly a few drops of a liquid sample, and provideresults within just a few seconds.Th bilit t th f ti i d il d• The ability to measure the refraction index easily andquickly with handheld refractometers is very usefulin food technology applications.gy pp

• For example, the sucrose concentration in fruit juicesand soft drinks can be related directly to thef i i d f h l l irefraction index of the sample solution.



Figure 4.4. RefractometersFigure 4.4. Refractometers

COLORIMETRYCOLORIMETRY

• Color and its measurement (colorimetry) involveorganoleptic perception of properties.

• The different colors we see with visible light are theresult of how our eyes perceive electromagneticradiation at different frequencies and wavelengthsradiation at different frequencies and wavelengths.

• The sensation of color occurs when light rays(electromagnetic radiation) of a certain frequency and

l h ik h i f h hwavelength 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 perceivesignal that is transmitted to our brain, and we perceivecolor.

COLORIMETRYCOLORIMETRYCOLORIMETRYLIGHT AND COLORCOLORIMETRYLIGHT AND COLOR

• Visible light is electromagnetic radiation withwavelength between 380 nm and 750 nm.wavelength between 380 nm and 750 nm.

• Larger wavelengths belong to infra red radiation (IR)and smaller wavelengths belong to ultravioletg gradiation (UV), and are invisible to the human eye.

• The speed of light is the speed at which the lightwaves propagate, and can be calculatedmathematically as the product of wavelength andffrequency.


• where

(4.5)

• The range of wavelengths for visible light can be further subdivided into smaller ranges that are each responsiblesubdivided 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 f d ll d bl li t d i T bl 4 2of red, yellow and blue are listed in Table 4.2.


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.Thi i ll d ddi i i i O h h h dThis is called additive mixing. On the other hand,filtering can be used to eliminate certain wavelengthsin order to produce a different color with thosein order to produce a different color with thoseremaining. This is called subtractive mixing.


• The perception of color is also dependent on particlesize. This is the result of what we call light scattering.

• As the particles become smaller and smaller, thespecific surface area of the particle aggregatesincreases dramatically This causes more and more ofincreases dramatically. This causes more and more ofthe incident light striking each particle to be fullyreflected.

• Our eye receives only the light that is reflected from anobject. When this light contains all the wavelengths inthe visible light spectrum, we are receiving the additivemixing of all the colors, which is perceived as white.


• Therefore, when we consider the physics of color, itmakes a difference whether the body is a radiatingb d ( itti li ht) i di ti b dbody (emitting light) or is a non radiating body(absorbing light). The color of non radiating bodieslike food materials depends on several factors somelike food materials depends on several factors someof which are listed in Table 4.3.

Table 4.3. Factors influencing the color of a material

COLORIMETRYCOLORIMETRYCOLORIMETRYPHYSIOLOGY OF COLOR PERCEPTION

COLORIMETRYPHYSIOLOGY OF COLOR PERCEPTION

• There are two different types of light receptors onthe retina of the human eye that are called rods andcones.

• The rods are sensitive to relative brightness anddarkness while the cones are sensitive to colorsdarkness, while the cones are sensitive to colors.

• There are three different types of cones, which havepigments that are sensitive to different wavelengthspigments that are sensitive to different wavelengthsof light. These include wavelengths with absorptionmaxima of 420 nm (blue), 535 nm (green) and 565( ), (g )nm (red).



• Thus, we can simply say that we have cones sensitiveto blue, green and red light. By mixing and blendingth i l f ll ththe signals from all three cone sensors, we canperceive all colors made up of added mixtures oflight with these wavelengthslight with these wavelengths.

• Figure 4.5 shows all colors which can be observed ina so‐called color triangle. Here, each place in thea so called color triangle. Here, each place in thediagram is a perception of color defined by x, ycoordinates.



Figure 4.5. Chromaticity diagram. In thetriangle are all colors which can begobserved. Colors with maximum brillianceare on the horse shoe curve. Point E in themiddle is zero brilliance (white)



• As we move along the perimeter of the triangle in aclockwise direction, we encounter a system of increasingwavelength The points on the horseshoe like curve arewavelength. The points on the horseshoe‐like curve arethe points of maximum brilliance. These are the spectralcolors (colors of the rainbow). At the bottom line of thecolor triangle, we have purple colors which are notspectral colors (not components of the rainbow).

d f h f h d• Moving inward from the outer points of the diagram tothe center of the triangle, we come to colors with lessbrilliance, to pale colors and at last to the point where abrilliance, to pale colors and at last to the point where acolor is so pale that it appears white. This is the point ofzero brilliance E.



• The mixing of two colors on the color triangle can berepresent by drawing a straight line from one color to theother in order to see what resulting color is possibleother in order to see what resulting color is possible.

• For example, mixing red and green will produce a linegoing through the region of yellow colors.going through the region of yellow colors.

• This illustrates the principle of additive mixing of colors.All lines passing through the point E representpossibilities for reaching a perfect white.

• Colors which result in white when mixed together arell d l t lcalled complementary colors.

COLORIMETRYCOLORIMETRYCOLORIMETRYCOLOR AS A VECTOR QUANTITY

COLORIMETRYCOLOR AS A VECTOR QUANTITY

• A convenient way to describe a color as a quantity isto 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 afterCIE or number 7:3:2 after DIN 6164.

I thi th i ti f d ibi l• In this way, the communication for describing a coloris immune from problems with human perceptionand subjective judgement This is important inand subjective judgement. This is important intechnical applications.



• For technical purposes we can describe a color by three attributes;

h– hue

– chroma

– brightnessbrightness

Table 4.4. Terms used in colorimetry



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

COLORIMETRYCOLORIMETRYCOLORIMETRYLAB SYSTEM FOR COLOR QUANTIFICATION

COLORIMETRYLAB 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 asystem, 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 thepoint of zero color, and quantifies the chroma of the color,while the angle α gives the hue.

Th ti l i i l d f th b i ht f th l• The vertical axis is scaled for the brightness of the color(see Figure 4.6). So, to describe the color of interest wehave to specify α and d of the vector and the value ofp ybrightness.



Figure 4 6 Color as a point atFigure 4.6. Color as a point atthe end of a vector. In theMunsell system, an arrow(vector) points to a place inthe color space. The arrow isdescribed by angle α andlength d. The vertical axisrepresents the brightnessrepresents the brightnessscale



• Another presentation of the color vector is made in theJudd–Hunter system (Judd & Wyszecki, 1967).

• The vector pointing to the place in the color space is• The vector pointing to the place in the color space isindicated with the coordinates a and b.

• The brightness L again is scaled on the vertical axis (seeThe brightness L again is scaled on the vertical axis (seeFigure 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,

h h dd d b l hwith the Judd–Hunter system we describe a color withthree numbers (L‐a‐b) which are all between 0 and 100.Using the L‐a‐b system we see positive values of aUsing the L a b system we see positive values of arepresent red,−a for green, +b for yellow, and −b for blue.



Figure 4.7. Indicating al i h ddcolor in the Judd–Hunter

system(L‐a‐b system)



• 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 therepoint in the three‐dimensional color space, but there are differences in the terms used. So we can calculate α from a and b by the following:ca cu ate α o a a d b by t e o o g

(4.6)

• and the chroma d from

(4 7)(4.7)



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

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


COLORIMETRYLAB SYSTEM FOR COLOR QUANTIFICATIONbl l f b b h• In Table 4.5 are some examples of L‐a‐b number sets with

descriptions of the colors they represent. The Judd–Hunter system is widely used, and is often called the L‐a‐b system. y y yVariations of the name include L*a*b* system and CIELAB system.

Table 4.5. Characterization of colors by Judd–Hunter system

COLORIMETRYCOLORIMETRYCOLORIMETRYCOLOR MEASUREMENTCOLORIMETRY

COLOR MEASUREMENT

• Color measurements can be performed by visualtechniques or a class of spectrometric techniques.Classic tri stimulus colorimeters belong to the classClassic tri‐stimulus colorimeters belong to the classof spectrometric techniques, and try to adopt thefunction of the human eye.u ct o o t e u a eye

• Visual color measurement involves observing asample without instruments, but under controlledp ,conditions of illumination, along with reference to aset of color standards with which to compare thesample colors observed.

COLORIMETRYCOLORIMETRYCOLORIMETRYCOLOR MEASUREMENTCOLORIMETRY

COLOR MEASUREMENT

• Spectrometric measurements involve measurements of the absorption of specified wavelengths by the sample under controlled defined conditions ofsample 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

COLORIMETRYCOLORIMETRYCOLORIMETRYVISUAL COLOR MEASUREMENT

COLORIMETRYVISUAL COLOR MEASUREMENT

• When using visual color measurement techniques,objects can be described as having the same colorwhen they show no observable difference in colorwhen they show no observable difference in colorunder identical conditions of illumination.

• For this reason visual techniques involve observing• For this reason, visual techniques involve observingthe color of a sample and comparing it againstdefined color standards under identical conditions ofillumination.

• This is called finding a color match, and falls into thecategory of organoleptic (sensory) methods of foodquality analysis.

COLORIMETRYCOLORIMETRYCOLORIMETRYVISUAL COLOR MEASUREMENT

COLORIMETRYVISUAL COLOR MEASUREMENT

• Color standards are commercially available in theform of paper board tiles.

F li id l l d d d l i• For liquid samples, colored standard solutions areused as matching fluids.

• Sol tions re ommended b the Ameri an and• Solutions recommended by the American andEuropean Pharmacopoea for this purpose, includeCoCl2 (rose) FeCl3 (yellow) and CuSO4 (blue)CoCl2 (rose), FeCl3 (yellow) and CuSO4 (blue).

COLORIMETRYCOLORIMETRYCOLORIMETRYTRI‐STIMULUS‐COLORIMETRY

COLORIMETRYTRI‐STIMULUS‐COLORIMETRY

• Based on the three types of cones in the retina of thehuman eye, color measuring instruments have beendeveloped with three filters that function like each ofdeveloped with three filters that function like each ofthe three types of cones. With these types ofinstruments, we can measure the intensity of thest u e ts, e ca easu e t e te s ty o t ewavelengths transmitted through each of thesefilters (Figure 4.9).

Figure 4.9. Tri‐stimulus colorimeter(schematic). Light source Villuminates sample P Detector Silluminates sample P. Detector Sreads the intensity of a frequencygiven by filter F

ULTRAVIOLET (UV)ULTRAVIOLET (UV)• Ultraviolet (UV) light is electromagnetic radiationwith a wavelength shorter than that of visible light,but longer than soft X raysbut longer than soft X‐rays.

• It can be subdivided into near UV radiation withwavelengths in the range (380–200 nm) far orwavelengths in the range (380 200 nm), far orvacuum UV (FUV or VUV) with wavelengths in therange (200–10 nm), and extreme UV (EUV or XUV)g ( ), ( )with wavelengths in the range (1–31 nm).

ULTRAVIOLET (UV)ULTRAVIOLET (UV)• When considering the effect of UV radiation onhuman health and the environment, the range ofnear UV wavelengths is even further subdivided intonear UV wavelengths is even further subdivided intoUVA (UV α, 380–315 nm), also called long wave or“black light; ”UVB (UV β, 315–280 nm), also calledb ac g t; U (U β, 3 5 80 ), a so ca edmedium wave; and UVC (UV γ, < 280 nm), also calledshort wave or “germicidal.”

ULTRAVIOLET (UV)ULTRAVIOLET (UV)• Ultraviolet radiation is often used in connection withvisible spectroscopy or photometry (UV/VIS) todetermine the existence of fluorescence in a givendetermine the existence of fluorescence in a givensample, and it is widely used as a technique inchemistry, for analysis of chemical structure, mostc e st y, o a a ys s o c e ca st uctu e, ostnotably conjugated systems.

• Perhaps more importantly, UV radiation has becomep p y,increasingly used as an effective disinfecting agent intreatment of drinking water and in cold foodprocessing.

ULTRAVIOLET (UV)ULTRAVIOLET (UV)ULTRAVIOLET (UV)DISINFECTING DRINKING WATERULTRAVIOLET (UV)

DISINFECTING DRINKING WATER• One important application for UV radiation is in the

treatment of drinking water because it acts as a veryeffective disinfecting agenteffective disinfecting agent.

• Disinfection using UV radiation was historically morecommonly used in wastewater treatment applications,but is now finding increased usage in drinking watertreatment. It used to be thought that UV disinfection wasmore effective for bacteria and viruses which have moremore effective for bacteria and viruses, which have moreexposed genetic material, than for larger pathogenswhich have outer coatings or that form spore states thatg pshield their DNA from UV light.

ULTRAVIOLET (UV)ULTRAVIOLET (UV)ULTRAVIOLET (UV)DISINFECTING DRINKING WATERULTRAVIOLET (UV)

DISINFECTING DRINKING WATER

• However, it was recently discovered that ultravioletradiation can be somewhat effective for treating themicroorganism Cryptosporidiummicroorganism Cryptosporidium.

• These findings resulted in the use of UV radiation as a viable method to treat drinking watera viable method to treat drinking water.

ULTRAVIOLET (UV)ULTRAVIOLET (UV)ULTRAVIOLET (UV)FOOD PROCESSING

ULTRAVIOLET (UV)FOOD PROCESSING

• As consumer demand for fresh and “fresh like” foodproducts increases, the demand for non thermalmethods of food pasteurization is likewise on themethods of food pasteurization is likewise on therise. In addition, public awareness regarding thedangers of food‐borne illness (food poisoning) is alsoda ge s o ood bo e ess ( ood po so g) s a soraising demand for improved food processingmethods that assure safety to the consumer withminimum loss in quality.

ULTRAVIOLET (UV)ULTRAVIOLET (UV)ULTRAVIOLET (UV)FOOD PROCESSING

ULTRAVIOLET (UV)FOOD PROCESSING

• Ultraviolet radiation is used in several food processesto inactivate (destroy) unwanted microorganismsfrom liquid food products with suitable opticalfrom liquid food products with suitable opticalproperties (transparent).

• Among the most common applications today is the• Among the most common applications today is theuse of UV light to pasteurize fruit juices by pumpingthe juice over a high intensity ultraviolet light source.j g y g

• The effectiveness of such a process depends on theUV absorbance of the juice.

REFERENCESREFERENCES1. Judd DB, Wyszecki G (1967).Colour in Business,

Science, and Industry.Wiley, NewYork.

2 M ll AH (1905) A C l N i M ll2. Munsell AH (1905) A Colour Notation. MunsellColour Company, Boston MA.

THE ENDTHE END

RHEOLOGICAL PROPERTIES OF FOODS

Notes 4 of fe 501 physical properties of food materials

Technology

angle of refraction

refraction of visible

refraction basicsfigure

refraction angle increase

refraction index n1

refraction index n2

refraction indices n1

speed of light