Hii, Law - Quality Evolution During Drying of FVFskFn
dtdF=± (6.6)where F is the textural attribute, t is the time, k is the rate constant and n is the ki-
netic order of textural changes.
The value of n can be determined from linear regression analyses by assuming a ze-
ro order, half order, first order and second order kinetics (Lau et al., 2000). A more
comprehensive model can also be used that takes into account the specific disappear-
ance rate of the raw and soft tissue (Figure 2) analogous to two irreversible series reac-
tions (Troncoso and Pedreschi, 2007).
Raw tissue→k^1 Soft tissue→k^2 Hard tissue6.2.1.3. Shrinkage
When moisture is removed within the solid network of a food product during drying,
a pressure unbalance is produced between the inner and external part of the food ma-
terial, generating contracting stresses that lead to material shrinkage, changes in shape
and sometimes product cracking (Mayor and Sereno, 2004). Visual appeal is affected too
should severe shrinkage is experienced by the product. Typically, shrinkage increases
proportionally with the volume of water removed during drying. Under ideal shrinkage
condition the volume of water removed is equivalent to the reduction in sample volume.
Sometimes, this could be untrue especially at low moisture content where transition
from the rubbery to the glass state occurs. At this stage the rate and extension of shrin-
kage decreases significantly.
Mayor and Sereno (2004) and Ratti (1994) gave an extensive review of shrinkage of
food materials during drying. Shrinkage of food products during drying can be modeled
using empirical or fundamental relationships. In empirical modeling, shrinkage (volume,
area or thickness) is a function of moisture content (or moisture ratio) in most models.
Empirical models are suitable for materials with very low porosity or materials with
uniform porosity development during drying. In contrast, the fundamental models are
derived based on mass balance, density and porosity. Table 6.1 shows some examples
of empirical and fundamental models used in modeling food shrinkage.
The measurement of shrinkage is carried out based on the displacement methods to
determine its apparent volume. Comparison of shrinkage measurement methods during
drying of banana, pineapple and mango slices was reported by Yan et al. (2008) using (i)
displacement with glass beads, (ii) liquid displacement, (iii) liquid pycnometer and (iv)
Archimedes principle. The Archimedes method using solvent n-heptane was recom-
mended based on the lowest coefficient of variation. The greater difference between the
density of the solvent and sample enables n-heptane to distinguish the finer difference
of sample’s weight between in the air and solvent.