Box 2 Wettability

The wettability of a solid by a liquid is determined by the balance between adhesive forces (work of adhesion, Wa) of the liquid on the solid and cohesive forces (work of cohesion, Wc) of the liquid. Adhesive forces cause the liquid to spread over the solid surface while cohesive forces cause it to shrink:

The contact angle of a liquid drop on a solid surface is defined by the mechanical equilibrium of the drop under the action of three interfacial tensions: solid-vapor (gSV), solid-liquid (gSL) and liquid-vapor (gLV). This equilibrium relation is known as Young's equation (Rulon and Robert, 1993):

When a solid comes into contact with a liquid in the presence of vapor, the liquid will adhere well on the solid surface if the total free energy required for the creation of the new interface decreases. The physical significance of this energy change is the work needed to separate the solid and liquid from the solid/liquid interface. The equilibrium spreading coefficient (Ws) is defined by equation [16.7] (Rulon and Robert, 1993) and it can only be negative or zero:

angles on watermelon, melon, tomato and mandarin were also from 87.5° to 90.0° (Table 16.2). The critical surface tension (gC) of apple skin was 18.7 dynecm-1 (R2 = 0.99). Hershko and Nussinovitch (1998) observed that the critical surface tension of garlic skin was 18.3 dynecm-1. Hagenmaier and Baker (1993) found that the critical surface tension was about 23 dyne cm-1 for grapefruit (Table 16.2). These results indicate that the skin of most fruit covered with a layer of wax has a low surface energy. The layer with low surface energy interacts with liquids primarily through dispersion (van der Waals) forces (Rulon and Robert, 1993). The contact angle of the chitosan coating solution on the pear skin was 8.5° (Table 16.2), an unusually low value. It may have been caused by the surface characteristics of pears which have a coarser surface than that of other fruits. Park and

Table 16.2 Surface tension, contact angle and critical surface tension of fruits and vegetables

Coating

Surface

Fruits

Critical

Contact

Spreading

emulsion

tension

surface

angle

coefficient

(dyne cm-1)

tension

(degrees)

G

(dyne cm-1)

(dyne cm-1)

Chitosan

emulsion

Chitosana

61.5

Apple

18.7

88.9

-66.8

Without

emulsifiera

Tomato

-

90.0

-

Melon

-

88.0

-

Tangerine

-

87.5

-

Pear

-

8.5

-

Other

emulsions

Alginate

(2.0%)b,

51.5

Garlica

18.3

81

-43.0

without

emulsifier

Alginate

(2.0%)b,

31.1

Garlica

18.3

51

-11.2

added

b-sitosterol

(2000 ppm)

Shellacc

33.4

Grapefruitb

23.0

53

Polyethylene

waxc

33.4

Grapefruitb

23.0

56

Carnauba

waxc

28.8

Orangeb

20.0

46

Resinc

35.6

Orangeb

20.0

46

a Choi et al. (2001). b Hershko and Nussinovitch (1998). c Hagenmaier and Baker (1993).

a Choi et al. (2001). b Hershko and Nussinovitch (1998). c Hagenmaier and Baker (1993).

others (1996) reported that pear surface wasmoreevenlycoated bycorn-zeinand Semperfresh™ solutions.

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