Ong, Law - Hygrothermal Properties of FVFs2.3.3. Shrinkage and Volume Change
Shrinkage of foodstuff during drying is unavoidable because heating and removal of
water from the food matrix may cause stresses in the cellular structure, hence leading to
structural collapse, changes in volume, shape deformation and capillaries contraction
(Mayor and Sereno, 2004). Ideally, it can be considered that the shrinkage of the materi-
al is equal to the volume of the removed water. Therefore, a parametric relationship can
be obtained that relates the volume shrinkage to the moisture content of the material.
Drying temperature alone is not a significant variable that influences shrinkage es-
pecially when the material is in rubbery state at high moisture content level. The influ-
ence of temperature can be neglected i.e. experimental results from banana drying has
reported percentage error of less than 4% by not taking into account drying tempera-
ture (Talla et al., 2004). However, at low moisture content the glass transition tempera-
ture increases and this causes phase transition from rubbery to glassy state. As a result,
the rate of shrinkage reduces significantly due to the rigidity of the material. In addition,
temperature affects the rate of moisture removal and hence influences the rate of shrin-
kage during drying. The effective diffusivity of the material is affected due to the shorter
distance that moisture needs to travel before evaporates to the surrounding. Published
results have indicated better estimation in the diffusivity values by taking into account
the shrinkage effect during drying (Katekawa and Silva, 2006). Some articles have been
published on the effect of drying on product shrinkage (Katekawa and Silva, 2006; Koç
et al., 2008; Mayor and Sereno, 2004; Talla et al., 2004; Yan et al., 2008). Readers may
refer to these papers for additional information.
Principally, shrinkage of food material increases with the amount of water being re-
moved since the more the water is removed, the more contraction stresses are exerted
on the material. Error! Reference source not found.(a-d) shows some selected examples
of shrinkage characteristics of food materials during drying. It can be seen that shrin-
kage increases (reduction in volume ratio) with decreasing moisture content in general,
but shrinkage characteristics vary among drying products and drying methods. This is
probably due to the unique biopolymer structure of individual agricultural product and
the combined effect of process conditions that determines the type and extent of shrin-
kage.