Slide 1

Hii, Law - Quality Evolution During Drying of FVFs

Table 6.3. Examples of food shrinkage model

Type of model Geometry Reference

Empirical:

Vr=aX+b Cylinder

Sphere

Ratti (1994)

Mclaughlin & Magee (1998)

Vr=a+bX+cX^2 +dX^3 Cylinder Ratti (1994)

Vr=aexp(

bX

Xo

) Cylinder & slab^ Mayor & Sereno (2004)^

Fundamental:

A

Ao

=�

V

Vo

�

2

3

Cube Suzuki et al. (1976)

Vr=

1

( 1 −ε)

� 1 +

ρo(x-xo)

ρw( 1 +xo)

−εo�

Cylinder Mayor & Sereno (2004)

where Vr = volume ratio, A = area (m2), V = volume (m3), X = moisture content (dry ba-
sis), ρ = density (kg m-3) and ε = porosity.

6.2.1.4. Porosity

Porosity is a measurement of pore or empty spaces of a material to that of the total
volume or simply it can be defined as the volume fraction of air in the food product (Eq-
uation 6. 7).

t

a

V

ε= 1 −V (6.7)

where Va = volume of air (m^3 ) and Vt = total volume (m^3 ).
Transport, mechanical and textural properties are affected by porosity (Hussain et
al., 2002; Chen, 2008). Therefore, the formation of pores can be categorized into one
with an inversion point and another without an inversion point based on experimental
evidence (Rahman, 2000). Figure 6. 2 (a) shows pores collapsed at a critical value but
further drying causes formation of pores (smooth line) and vice versa (dotted line)
while Figure 6. 2 (b) shows pore sizes increased (dotted line) or decreased (smooth line)
with moisture content.

Figure 6.2. Change of porosity as function of moisture content (Rahman, 2000)

Glass transition theory is one of the concepts that is used to explain changes in po-
rosity during drying. Generally, structural collapse is negligible (more pores) if the food

Water content

Porosity

Water content

Porosity

a. With inversion b. Without inversion