Slide 1

Hii, Law - Quality Evolution During Drying of FVFs

Table 6.1. Factors contributing to colour changes during drying

Components Compounds Effects of drying

Pigments Chlorophylls Changes from green to yellow or to red
colours
Carotenoids Oxidation of carotenoid pigments by oxy-
gen in air
Anthocyans Quite stable during processing at low pH
Betalaines Very sensitive to pH, degraded to brown
compound at neutral pH
Reactions
Maillard reactions Reducing su-
gars, amino
acids, proteins

Formation of brown or black pigments,

melanoidins and other scented compounds

Enzymatic browning Phenolics Transformation of phenolics compounds to
brown or black polymers

The following parameters (Equations 6.1-6. 3) can be further defined for the purpose of
colour evaluation:









= *

* *

a

Hue angle (H) arctan b (6.1)

Chroma (C*) = a*2+b*2 (6.2)

* 2

ref

* 2 *

ref

* 2 *

ref

Total colour change (ΔΔE = (L*−L ) +(a −a ) +(b −b ) (6.3)

High value of hue angle indicates less browning and vice versa while chroma is a
measurement of the strength of colour such as the intensity or saturation. Table 6.2
shows the descriptive levels of colour change as determined from the ∆E values (Chen,
2008 ). In most cases a large difference in ∆E is not desirable as consumers tend to prefer
product that resemble the colour of the fresh product before drying. A very large ∆E val-
ue could indicate higher degree of browning which could be unattractive in appearance.

Table 6.2. Descriptive levels of colour change

Level (^) ∆E range
Trace level difference 0 – 0.5
Slight different 0.5 – 1.5
Noticeable difference 1.5 – 3.0
Appreciable difference 3.0 – 6.0
Large difference 6.0 – 12.0
Very obvious difference > 12.0