CHAPTER 2 LITERATURE REVIEW
2.1 Honey and composition of honey
2.1.2 Physical properties of honey
Honey is a viscous liquid mainly consisting of glucose and fructose. However, its physical properties are different from an invert sugar solution with the same moisture content. Thus, the physical characteristics of honey are largely determined by the types and concentrations of sugars and other compounds in honey.
1) Viscosity of honey
The important property or parameter in the handling and processing of honey is viscosity. The flow properties depend on the composition, moisture content, and
temperature of honey. Normally, honey with a low viscosity is present from it has high moisture content, furthermore, the composition of individual sugars and the size and amount of the colloids in honey are influence with honey viscosity (Bhandari et al., 1999). Honey is newtonian fluid, also the viscosity is independent of the shear rate and previous shear history, depends only on composition and temperature (D'Arcy, 2007).
2) Colour
Colour is the first attractive attribute of honey, and as such is very important for commercialization. It is important parameter in the quality, acceptance and preference of consumers. The color of honey depends on the floral source and its mineral content. A dark color in honey may develop during storage temperature and the composition of the honey (Da Silva et al., 2016). Nevertheless, colour is an important characteristics of upon which honey is classified by honey producers, packers and end-users (Rahima, 2014).
Honey’s colour is a temperature sensitive parameter, and honey can become darker as a result of different storage conditions. Honey’s colour form a continuous range from very pale yellow through ambers to a darkish red amber to nearly black.
The variations are almost entirely due to the plant source of the honey, although climate may modify the colour somewhat through the darkening action of heat (White and Doner, 1980). However, the colour of honey could be assessed by the method adopted by the Association of Official Analytical Chemists uses a Lovibond 2000 visual comparator (Subba Rao, 1990). Studied have also shown that honey colour can be assesses by the CIE-1931 or the more recent CIE-1976 (L*a*b*) or CIELAB methods (Rahima, 2014). According to CIE concepts, the human eye has three colour receptors; red, green and blue; and all colours are combinations of those. The amount of all needed to form any particular colour are called the tristimulus values and are denoted X, Y and Z, respectively. It uses the chromaticity diagram to designate various colours as shown in Figure 2.
Figure 2 CIE chromaticity diagram Source: Pathare et al. (2013)
Primary Y, known as luminous reflectance or transmittance, contains the entire lightness stimulus. The application of the weighting to a reflectance curve gives the tristimulus values, which are denoted by the capital letters X, Y and Z. The Hunter L a b developed in 1948 for photoelectric measurement and the CIE L* a* b*
colour space devised in 1976 provide more uniform colour differences in relation to human perception of differences as shown in Figure 3.
Figure 3 CIELAB colour space Source: Pathare et al. (2013)
The parameter a* takes positive values for reddish colours and negative values for the greenish ones, whereas b* takes positive values for yellowish colours and negative values for the bluish ones. L* is an approximate measurement of luminosity, which is the property according to which each colour can be considered as equivalent to member of the greyscale, between black and white. Chroma (C*), considered the quantitative attribute of colourfulness, is used to determine the degree of difference of a hue in comparison to a grey colour with the same lightness.
The higher the chroma values, the higher is the colour intensity of samples perceived by humans. Chroma was calculated using Eq. (1).
* *2 *2
C = a +b Eq. (1)
Hue angle (h0), considered the qualitative attribute of colour, is the attribute according to which colours have been traditionally defined as reddish, greenish, etc., and it is used to define the difference of a certain colour with reference to grey colour with the same lightness. This attribute is related to the difference in absorbance at different wavelengths. A higher hue angle represents a lesser yellow character in the assays as shown in Eq. (2) – Eq. (4) (Briones and Aguilera, 2005; Costa et al., 2015).
0 tan 1 *
b *
h a
−
= a* > 0 and b* > 0 Eq. (2)
0 tan 1 * 180
b *
h a
−
= + a* < 0 Eq. (3)
0 tan 1 * 360
b *
h a
−
= + a* > 0 and b* < 0 Eq. (4)
The meaning of h0values were including;
0 - 45 = purple-red to orange-red 180 - 225 = green to green-blue
45 - 90 = orange-red to yellow 225 - 270 = dark blue-green to dark blue 90 - 135 = yellow to green 270 - 315 = dark blue to purple
135 - 180 = yellow-green to green 315 - 360 = purple to purple-red
closely with consumers’ preferences for white colours. It mathematically combines lightness and yellow-blue into a single term. WI indicates the degree of whiteness as shown in Eq. (5).
(
100 *2)
*2 *2WI = −L +a +b Eq. (5)
3) Specific heat of honey
Specific heat is determined as a heat related property of honey, and has received very little attention in the past. Helvey (1954) reported that the specific heat of honey containing 17.4% moisture content is 0.54 at 20C but that is varies depending on the moisture content (Table 6). He also presented the results obtained by MacNaughton for a temperature range of 29C to 48C, where the specific heat ranged between 0.56 – 0.73.
Table 5 Specific heat of honey
Moisture content (%) Specific heat
20.4 0.60
19.8 0.62
18.8 0.64
17.6 0.62
15.8 0.60
14.5 0.56
Coarsely granulated 0.64
Finely granulated 0.73
Source: adapted from D’Arcy (2007)