Optical properties Flashcards
Application
= physical properties of the food
= Food composition: moisture, sugar, salt
= consumer judgment on food appearance and produce some kind of visual effect.
determined by the way they interact with electromagnetic radiation
Visible
infrared
Different
Transmittion Reflection
Refraction Diffraction
Absorption Scattering
Colorimetry
- Colour measurement
- Organoleptic perception of properties
- Perception
Gloss
light specularly reflected from a plain smooth surface.
represents intensity of light reflected at the surface at different angles of incidence and viewing.
Translucency
an opaque-to-transparent scale.
In liquid foods, light is ____ when interacting with
suspended particles. Most foods are neither fully opaque nor fully transparent, but are _______.
scattered
translucent
Color
a beam of light composed of irregularly distributed energy emitted at different wavelengths
Objects modify light colorants in the food selectivity ___________________
absorb some wavelengths of incident light while reflecting or transmitting others
Rayleigh’s theory
describes scatter by particles which are much smaller than the wavelength of light
Diffuse reflectance occurs where light is reflected from a
particles surface which is greater than the wavelength of light impinging on it
The particles primarily responsible for scattering in milk are
fat globules and casein micelles.
Syneresis
Cheese Manufacture
Colour wavelength
~ 400- 750
~ 400 - 435
Purple - comple yellowish green
Blue
435- 480 comple yellow
Green
500-560 comple red
580 - 595
Yellow comple blue
610 - 750
Red comple green
Infrared wavelength
> 750
color perceived when the eye views a food is related to the factors
= the spectral composition of the light source
= the chemical and physical characteristics of the food
= and the spectral sensitivity properties of the eye
To evaluate the colorimetric properties of a food, which factors need to be standardise
= instrumental sensor
= Visual colorimeters facilitate visual comparisons and eliminate differences in interpretation between operators
Colour: 3 measures
Hue, Lightness and Saturation
Hue
- Used for the classification of colour
- Mix hues
- continuum of these hues results in the color wheel
Lightness
Separate bright and dark colours
Saturation
Vivid vs dull
color saturation or vividness
Numerical Expression of Colour
description of color for purchase specifications of food
commodities or packaging materials involves color
tolerances, which are defined in one, two, or three
dimensions in color space to avoid variability of the human eye
color analysis systems
Munsell CIE Hunter
Munsell
Colours described by: Hue/ Value (Lightness) / Chroma (purity)
hue scale based on ten hues distributed on a circumference (scaled 1 to 10)
lightness ranges from black to white (0 to 10) and is
distributed on a perpendicular line
Chroma is of irregular length beginning with 0 for the
central gray to the limit of purity obtainable by available
pigments in the Munsell book of color.
Munsell Renotation System
Any given color is expressed as a letter/number combination
H V/C) in terms of its hue (H), value (V), and chroma (C
CIE
Commission Internationale de l’Eclairage (CIE)
Yxy system - tristimulus values XYZ defined by CIE (1931)
Lab* system - provide more uniform color differences in relation to visual differences (1976)
XYZ tristimulus \ Yxy
- XYZ tristimulus values
- Yxy color space form
- foundation of present CIE color spaces
- based on the three-component theory of color vision
eye possesses receptors for three primary colors (red, green, and blue) and that all colors are seen as mixtures of these three primary colors. - graphing colour into two dimensions independent of lightness [ Y is the lightness]
- x and y are the chromaticity coordinates calculated from the tristimulus value XYZ
Lab* color space or CIELAB
- one of the most popular colour spaces
- Overcome original Yxy space problems
- L* indicates lightness
- a* and b* are chromaticity coordinates
Lab*: Colour Difference
Delta values associated with this color scale
ΔL, Δa, and Δb* indicate how much a standard and sample differ from one another or individual samples differ from eachother in L, a, and b*.
The total color difference, ΔE*
a single value which takes into account the differences
between the L, a, and b* of the sample and standard.
Will not indicate if L, a, and/or b* are out of tolerances
May be misleading
Hunter Lab
- Overcome original Yxy space problems
-Similar to Lab* color space but calculated differently
Colour measurement
Tristimulus colorimeters - measure only some parameters related to sensory colors
Spectrophotometers .- measures intensity of light through the completely visible spectrum
Colorimeters are very useful in
the quality control of foods, and give results normally correlated with visual measurements.
Colorimeter mainly used in
production and inspection applications for color difference measurements and color chart measurement
Spectrometer required for
more complex/precise applications - e.g. Research
Colorimeter features
- Built in light source
- Data memory
- Data communication
- Data display
- Constant illumination/ viewing angles
- Constant ‘observer’
- Elimination of area effect and contrast effect
- Colour- difference measurment
Colour measurement - application
- Perform quality control and continuous inspection during processing operations
- Fruit ripeness
- Milk or cream discoloration during sterilization
- Degree of roasting of coffee grains
- Browning of apples slices during storage
Continuous color measurements are used in
in tasks involving color sorting (or “electronic sorting”) by using in-line systems
Color sorting is used for
for a very wide range of food materials in screening defects.
