1. Electromagnetic Properties

2. Electromagnetic Properties Colour measurement methods and colour order systems Dielectric properties of foods

3. I NTRODUCTION : E LECTROMAGNETIC

4. INTERACTION OF OBJECTS WITH LIGHT  When electromagnetic radiation strikes an object, the resulting interaction is affected by the properties of an object such as colour, physical damage, and presence of foreign material on the surface.  Different types of electromagnetic radiation can be used for quality control of foods. For example, near-infrared radiation can be used for measuring moisture content, and internal defects can be detected by X-rays.  Electromagnetic radiation is transmitted in the form of waves and it can be classified according to wavelength and frequency. The electromagnetic spectrum is shown as Figure below:

6.  Electromagnetic waves travel at the speed of light and are characterized by their frequency ( f ) and wavelength ( λ ). In a medium, these two properties are related by: c = λf where c is the speed of light in vacuum (3.0 × 10 8 m/s)  Radiation can exhibit properties of both waves and particles. Visible light acts as if it is carried in discrete units called photons. Each photon has an energy, E, that can be calculated by: E = h f where h is Planck’s constant (6.626 × 10 −34 J·s)

7.  When radiation of a specific wavelength strikes an object, it may be reflected, transmitted, or absorbed (see figure below).  The relative proportion of these types of radiation determines the appearance of the object.  A material is said to be transparent when the light impinging on it passes through with a minimum reflection and absorption. The opposite of transparency is opacity. That is, an object that does not allow any transmission of light but absorbs and/or reflects all the light striking is termed opaque.  In opaque surfaces, certain specific wavelengths of light are absorbed and the others are reflected. As a result, colour is formed. If all the visible light is absorbed, the object appears black. If both reflection and transmission occur, the material is said to be translucent

8. If I 0 is the radiant energy striking the object and I ref is the amount of energy reflected from the object, the total reflectance R is defined as: Reflection R = I ref / I 0

9. If I 0 is the incident energy, I 1 is the energy entering the object, I 2 is the energy striking the opposite face after transmission, and I out is the energy leaving the opposite phase, transmittance ( T ) and absorbance ( A ) are defined as: T = I out / I 0 A = log( I 1 / I 2 ) Optical density (OD) is a measure of the relative amount of incident energy transmitted through the object. OD = log( I 0 / I out ) = log(1 /T )

10. Two types of reflection: diffuse reflection and specular reflection. (a)In diffuse reflection, radiation is reflected equally in all directions. When a surface is rough, the incident light will bounce around and will rise to a greater amount of diffuse light. Opaque surfaces reflect light diffusely. (b)Specular reflection is highly directional instead of diffuse. The angle of reflection is equal to the angle of incidence of the radiation beam. It is mainly responsible for the glossy or shiny(mirror-like) appearance of the material. If the object is smooth, such as a mirror, most of the light will be reflected in this way Diffuse reflection Specular reflection

11. Two different types of transmission: diffuse transmission and rectilinear transmission. (a)Diffuse transmission occurs when light penetrates an object, scatters, and emerges diffusely on the other side. Transmitted light leaves the object surface in all directions. It is seen visually as cloudiness, haze, or translucency (b)Rectilinear transmission refers to the light passing through an object without diffusion. Rectilinear transmission measurements are widely used in the chemical analysis and colour measurements of liquids Rectilinear transmission Diffuse transmission

12. C OLOUR : C OLOUR MEASUREMENT METHODS AND COLOUR ORDER SYSTEMS

13. COLOUR  Colour is one of the important quality attributes in foods.  Although it does not necessarily reflect nutritional, flavour, or functional values, it determines the acceptability of a product by consumers.  Sometimes, instead of chemical analysis, colour measurement may be used if a correlation is present between the presence of the coloured component and the chemical in the food since colour measurement is simpler and quicker than chemical analysis. For example, total carotenoid content of squash can be determined from colour measurements without performing a chemical analysis because there is a correlation between total carotenoid content and colour for squash  It may be desirable to follow the changes in colour of a product during storage, maturation, processing, and so forth. Colour is often used to determine the ripeness of fruits. Colour of potato chips is largely controlled by the reducing sugar content, storage conditions of the potatoes, and subsequent processing.

14.  Colour is a perceptual phenomenon that depends on the observer and the conditions in which the colour is observed. It is a characteristic of light, which is measurable in terms of intensity and wavelength. Colour of a material becomes visible only when light from a luminous object or source illuminates or strikes the surface.\  The selective absorption of different amounts of the wavelengths within the visible region determines the colour of the object. Wavelengths not absorbed but reflected by or transmitted through an object are visible to observers.  For example, a blue object reflects the blue light spectrum but absorbs red, orange, yellow, green, and violet. If all radiant energy in the visible region is reflected from an opaque surface, the object appears white. If it is almost completely absorbed, the object is black.

15.  Physically, the colour of an object is measured and represented by spectrophotometric curves, which are plots of fractions of incident light (reflected or transmitted) as a function of wavelength throughout the visible spectrum.

16.  Light sources and illuminants are often confused. A light source can be turned on and off and can be used to view an object. An illuminant, on the other hand, is a mathematical description of a light source.  For detection of differences in colour under diffuse illumination, both natural daylight and artificial simulated daylight are commonly used.  However, natural daylight varies greatly in spectral quality with  direction of view,  time of day and year,  weather, and  geographical location.  Therefore, simulated daylight is commonly used in industrial testing. Artificial light sources can be standardized and remain stable in quality. Presence of Color Object Light Source/ illuminant Observer

17. Creating Light (ways) IncandescenceLuminescenceFluorescencePhosphorescence  The human eye is the defining observer of colour. The eye can determine approximately 10 million colours. People who believe that the eye is the most important observer of colour. However, everyone’s perception of colour differs. Thus, instrumental methods for colour measurement have been developed.

18. Color Measuring Equipments (A)Spectrophotometers (B)Colorimeters

19. Spectrophotometers  Early instrumental methods for colour measurement were based on transmission or reflection spectrophotometry.  In spectrophotometers, three projectors each with red, green, or blue filters in front of the lens are required. Red, green, or blue light beams are focused on a screen such that they overlap over half a circle. The other half is illuminated by another projector or by spectrally pure light from a prism or grating.  The observer can see both halves of the circle on the screen simultaneously. Each projector is equipped with a rheostat to vary the amount of light from each of the red, green, and blue sources. The observer can determine the amounts of red, green, and blue required to match almost any spectral colour by varying the amount of light.  Spectral colour can be defined in terms of the amounts of red, green, and blue. Pure Light RGB

20.  A special set of mathematical functions X, Y, and Z to replace red, green, and blue lights, respectively. The colour matching functions for X, Y, and Z lights are all positive numbers and are labeled as x, y, z.  The colour can be matched using the appropriate amounts of X, Y, and Z light. The amount of X, Y, and Z light required to match a colour are called the colour's tristimulus.  The red, green, and blue data for spectral colours are taken and transformed into X, Y, and Z coordinates. Then, the response of the human eye against wavelength was plotted. These standardized curves are called the CIE x, y, z standard observer curves.

21.  In a spectrophotometer, light is usually split into a spectrum by a prism or a diffraction grating before each wavelength band is selected for measurement.  The spectral resolution of the instrument depends on the narrowness of the bands utilized for each successive measurement. Spectrophotometers contain monochromators and photoiodes that measure the reflectance curve of a product’s colour every 10 nm or less. The analysis generates typically 30 or more data points with which a precise colour composition can be calculated.  Spectrophotometers measure the reflectance for each wavelength and allow calculation of tristimulus values.  Advantage:  obtain adequate information to calculate colour values for any illuminant and metamerism (difference in colour, lighting and at different angles )  Disadvantages:  are very expensive and  measurements take longer time.

22. Example of Spectrophotometers

23. Colorimeters  A tristimulus colorimeter has three main components:  Source of illumination  Combination of filters used to modify the energy distribution of the incident/reflected light  Photoelectric detector that converts the reflected light into an electrical output  Each colour has a fingerprint reflectance pattern in spectrum. The colorimeter measures colour through three wide-band filters corresponding to the spectral sensitivity curves.  Measurements made on a tristimulus colorimeter are normally comparative. It is necessary to use calibrated standards of similar colour to the materials to be measured to achieve the most accurate measurements.  Each colour has its own tristimulus values that distinguish it from any other colour. These values can be measured to determine if a colour match is accurate.  They can also be used to determine the direction and amount of any colour difference.

24. Example of Colorimeters

25. Colour Order Systems (A) Munsell Colour System (B) CIE Colour System (C) CIE L ∗ a ∗ b ∗ (CIELAB) Colour Spaces (D) Hunter Lab Colour Space (E) Lovibond System

26. (A) Munsell Colour System  Developed by American artist and teacher, Albert Munsell, 1898. Thousands of colours could be described by using  hue,  value (lightness), and  chroma (saturation)  Hue is a quality by which one colour is distinguished from another.  Perception of colour: absorption of radiant energy at various wavelengths.  425 to 490 nm : blue.  medium wavelength: range of green or yellow  640 to 740 nm: red objects.  Hue is defined on the Munsell colour tree in the circumferential direction by five principal and five intermediate hues.  main hues are R (red), Y (yellow), G (green), B (blue), P (purple)  intermediates YR (yellow-red), GY (green-yellow), BG (blue- green), PB (purple-blue), and RP (red-purple).

27.  Munsell defined value as the quality by which light colours can be differentiated from the dark ones.  Value is a neutral axis that refers to the grey level of the colour ranging from white(10) to black (0). It is also called lightness, luminous intensity, and sometimes brightness.  “Chroma” is the quality that distinguishes a pure hue from a grey shade. The chroma (saturation or purity) axis extends from the value (lightness) axis toward the pure hue.  A pastel tint has low saturation while pure colour is said to have high saturation. Two objects having the same lightness and hue can be differentiated from their saturation.  In the Munsell system, chroma is defined by 10 or more steps (up to 16 steps) in the radial direction.  6.5R 2.6 / 2.2 indicates a product with red hue of 6.5, a value of 2.6, and a chroma of 2.2.

29. (B) CIE Color System  This is a trichromatic system, that is, any colour can be matched by a suitable mix of the three primary colours—red, green, and blue which are represented by X, Y, and Z, respectively.  The application of the weighting to a reflectance curve gives the tristimulus values, which are denoted by the capital letters X, Y, and Z. These values are then used to calculate the chromaticity coordinates, designated by lowercase letters x (red), y (green), and z (blue). The value for x can be calculated by:  The values for y and z can be calculated by replacing X with Y and Z, respectively, in the numerator. Since the sum of the chromaticity coordinates ( x, y, and z ) is always unity, the values x and y alone can be used to describe a colour.

30.  When x and y are plotted, a chromaticity diagram is obtained.  The third dimension of lightness is defined by the Y tristimulus value. Colour is defined by its chromaticity coordinates x and y and its lightness, the tristimulus Y value. All real colours lie within the horse shoeshaped locus marked with the wavelengths of the spectrum colours.

31. Chromaticity diagram of the CIE system

32. (C) CIE L ∗ a ∗ b ∗ (CIELAB) Color Spaces  The CIELAB colour measurement is more uniform and based on more useful and accepted colours describing a theory of opposing colours.  The location of any colour in the CIELAB colour space is determined by its colour coordinates: L ∗, a ∗, and b ∗.  L ∗ represents the difference between light ( L ∗ = 100) and dark ( L ∗ = 0).  a ∗ represents the difference between green (− a ∗ ) and red (+ a ∗ ) and  b ∗ represents the difference between blue (−b ∗ ) and yellow (+b ∗ ).  If the L ∗, a ∗, and b ∗ coordinates are known, then the colour is not only described, but also located in the space.

33.  Formula for in CIELAB L*, a* and b*  Cylindrical coordinates in CIELAB color space  L* -the lightness co-ordinates  C* -the chroma co-ordinate, the perpendicular distance from the lightness axis  h* -the hue angle, expressed in degrees, with 0° being a location on +a* axis, continuing to 90° for the +b* axis, 180° for -a*, 270° for -b* and back to 360°

34. (D) Hunter Lab Colour Space  This system is based on L, a, and b measurements.  The L value represents lightness and changes from 0 (black) to 100 (white). The a value changes from − a (greenness) to + a (redness) while the b value is from − b (blueness) to + b (yellowness).  Like the CIE system, the Hunter scale is also derived from X, Y, Z values.  In the human eye, there is an intermediate signal switching stage between the light receptors in the retina and the optic nerve taking colour signals to the brain. In this switching stage, red responses are compared with green to generate a red-to-green colour dimension.

35. (E) Lovibond System  The Munsell space is developed by an entirely different principle, namely, a systematic sampling of the colour space, but when the spinning disc technique is used, it creates in principle an additive colorimeter.  The Lovibond scale is based on the opposite principle. It starts with white and then by the use of red, yellow, and blue filters, colours are subtracted from the original white to achieve the desired match with the sample.  Lovibond is used for the measurement of beer colour, in the oil industry, in the honey industry.  The instrument has a set of permanent glass colour filters in the three primary colours: red, yellow, and blue. The sample is placed in a glass cell and the filters are introduced into the optical system until a colour match is obtained under specified conditions of illumination and viewing. The colour of the sample is measured with transmitted light.

36. The Lovibond measurements of different colours is shown in following table

37. Colour Differences  CIE L ∗ a ∗ b ∗ (CIELAB) total colour difference (∆ E ∗ ) is the distance between the colour locations in CIE space. This distance can be expressed as: where ∆ L ∗ is the lightness difference: L ∗ sample − L ∗ standard ∆a ∗ is the red/green difference: a ∗ sample − a ∗ standard, ∆ b ∗ is the yellow/blue difference: b ∗ sample − b ∗ standard

38. ∆E ∗ is an equally weighted combination of the coordinate ( L ∗, a ∗, b ∗ ) differences. It represents the magnitude of the difference in colour but does not indicate the direction of the colour difference. Colour difference can also be described specifying L ∗, C ∗, and h ∗ coordinates as: The chroma difference between sample and standard is given as: ∆C ∗ = C ∗ sample − C ∗ standard The metric hue difference, H ∗, is not calculated by subtracting hue angles. It is expressed as:

39. Exercise: Colorimetric properties of potato slices during microwave frying in sunflower oil are studied in terms of a CIE scale. As a standard, a BaSO 4 plate with L ∗, a ∗, and b ∗ values of 96.9, 0.0, and 7.2, respectively was used. L ∗, a ∗, and b ∗ values of potato slices are given in Table as below. Determine E ∗ values of the potato slices during frying and discuss the results.

40. Solution: Subtracting the standard colour values from those of french fries; By using the colour difference equation: For frying time of 2 min:

41. As frying time increases, E ∗ value increases showing that colours of potatoes become darker

42. D IELECTRIC P ROPERTIES OF F OOD