Hii, Law - Quality Evolution During Drying of FVFsmaterial is processed below glass transition temperature (T’g). The higher the difference
between the process temperature and the glass transition temperature, the higher is the
collapse (Rahman, 2001). This explains why freeze dried material (T < T’g) is generally
more porous with negligible shrinkage as compared to hot air dried material. However,
this is not applicable when the drying product experiences case hardening and crust
formation as these will impede porosity development. In addition, surface tension,
structure, environment pressure and mechanisms of moisture transport equally play
significant roles in porosity development. Correlations that relates porosity with mois-
ture, density and shrinkage based on fundamental relationship was developed by Mar-
tynenko (2008). The correlations are given in the following equations (6. 8-6. 10).
Porosity as function of shrinkage and moisture content:
⋅
+
= − +
β ξ
ε α β^1
1 X
1 1 X
o(6.8)Porosity as a function of shrinkage and density:
⋅
+
−
+
= −
ξ
αβ
ρ
ρ
ε
1
1 X
( 1 )
1
l o(6.9)Porosity as function of moisture content and density:
+
+
= − ⋅
1 X
1 X
1
sβ
ρ
ρ
ε^ (6.10)^
Where α, β = density ratio coefficient, X = moisture content (kg kg-1 dry basis), ξ =
bulk shrinkage, ρ = bulk density (kg m-3) and subscript 0 = initial, s = solid and l = liquid.
These equations allow porosity to be calculated based on one of the two measurea-
ble variables (moisture content, shrinkage and density). No additional information is
needed about the kinetics or spatial distribution of these variables.
6.2.1.5. Rehydration
Most dried food products are rehydrated prior to consumption. It is considered as
the percentage of the original weight gained by dried samples during rehydration for a
given time at a given temperature in water which depends greatly on the porosity of the
material (Perera, 2005). Rehydration of dried products typically composed of the imbi-
bition of water into dried product, swelling of the rehydrated product and leaching of
solubles (Lewicki, 1998). It is generally accepted that the degree of rehydration is de-
pendent on the degree of cellular and structural disruption (Krokida and Philippopoulus,
2005 ).
A first order kinetic model was used by Krokida and Marinos-Kouris (2003) to mod-
el the rehydration kinetics of various dried fruits and vegetables (Equation 6.11). The
water temperature was found to influence the rehydration rates and the equilibrium
moisture content in a positive way.
k(X-X)
dt
dX
− = r e (6.11)
where X = moisture content and kr = rehydration rate (s-1).