Engineering Properties of Biological Materials and Food Quality 3(2+1)

Lesson 15. Concept of colour in food quality

Colour of agri-food products such as fruit and vegetables is derived from natural pigments, many of which change as the plant proceeds through maturation and ripening. The primary pigments imparting colour quality are the fat-soluble

chlorophylls (green), carotenoids (yellow, orange, and red), water-soluble anthocyanins (red, blue), flavonoids (yellow) and betalains (red). Colour features can be used to detect defects in food products, such as those on the surface of apples, or to classify products having different qualities (Leemans et al. 1998). The product should look fresh, have normal size and colour associated with the particular fruit or vegetable, and be without blemishes or signs of decay. The absence of blemishes or signs of decay is also of utmost importance.

Optical properties of food are those which govern the way food materials respond to absorption of electromagnetic radiation in the optical wavelength and frequencies. Includes visible light and color, reflection and refraction

 

Fig. Electromagnetic spectrum

How do we see colours?

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. The colour of a material becomes

visible only when light from a luminous object or source illuminates or strikes the surface.

Electromagnetic spectrum	wavelength
Ultra violet	< 400 nm
Violet	400-450nm
Blue	450-500 nm
Green	500-570 nm
Yellow	570-590 nm
Orange	590-620 nm
Red	620-760 nm
Infrared	>760 nm

Human eye can see the objects in the visible range.

Cones and rods:

Table 1 : Relationship between absorption and visual colour

Wavelength absorbed (nm)	Colour absorbed	visual colour
400-430	violet	Yellow-green
435-480	Blue	Yellow
480-490	Green-blue	orange
490-500	Blue-green	red
500-560	Green	purple
560-580	Yellow-green	Violet
580-595	Yellow	blue
595-605	Orange	Green-blue
605-750	Red	Blue-green

 

Terms in colourimetry:

Hue: is the attribute described by colour name such as red, green, blue etc

Saturation: is the colorfulness judged in proportion to its brightness

Chroma: is the colorfulness relative to the brightness of its surrounding

Lightness: is the relative brightness unaffected by luminance, as it is the proportion of light reflected

Brightness: depends on the luminance

Colourfulness: is the visual sensation according to which an area appears to exhibit more or less chromatic colour

Gloss: is a measure of reflected light. It is a visual aspect of quality that depends on the ability of a surface to reflect light.

Gloss on the outside of the whole fruit tends to be a desirable attribute for whole fruits. Products that are freshly harvested often have a bright, glossy surface, and this appearance factor can be greatly reduced with weight loss and other postharvest handling conditions. Freshly cut fruits and vegetables must appear to be fresh, generally indicated by the brightness of colour and the absence of visual defects or drip. Sheen on the outside of most cut fruits is preferred to a dried appearance.

Sahin S. & Sumnu, S. G. 2006. Physical Properties of Foods. Springer, USA