Ong, Law - Hygrothermal Properties of FVFs
2 .1. INTRODUCTION
Hygrothermal changes of material properties occur due to moisture absorption and
temperature variation; they are very important in thermal processing of foods. As dry-
ing is a process involving simultaneous moisture and heat transport phenomena, appli-
cation of the term hygrothermal in the context of food drying can be referred as changes
in the drying medium and food material properties induced by moisture and tempera-
ture. At present, thermodynamic properties of drying air are very well documented. Re-
liable data for hygrothermal properties of the drying medium can be found in many en-
gineering handbooks. Hence, no attempt is made to include this information in this chap-
ter. On the other hand, hygrothermal data for diverse food materials is necessarily mas-
sive due to the vast variety of food products and the assorted parameters that are used
to assess the product quality. Compilation of those data is very challenging. Therefore,
for convenient and quick reference, only the fundamental knowledge on the effects of
hygrothermal behavior to quality of food materials is reviewed and discussed here.
Selected literature is cited to illustrate the effects of moisture content and tempera-
ture changes in dehydrated product to the alteration of physical, chemistry and bioche-
mistry properties of the food material. Appropriate mathematical descriptions applied
in estimation of the hygrothermal properties are provided while details of measurement
and experimental methodology can be found elsewhere. Overview of some major dete-
riorative reactions in foods is presented and the discussion includes both the physical
properties (effective moisture diffusivity, thermal properties, shrinkage and volume
change) and chemistry and biochemistry properties (non-enzymatic browning, vitamin
degradation, protein denaturation and enzymatic reaction).
2 .2. HYGROTHERMAL EFFECTS ON CHEMICAL AND BIOCHEMI-
CAL PROPERTIES
2 .2.1. Food Chemistry
Temperature, moisture content and water activity are important physical factors in
influencing the chemistry and biochemistry properties of food product during drying
and storage. Water is not only an important medium for heat transfer and heat storage
but also takes part in various biochemical reactions in food. A water molecule provides
protons (H+), hydroxide ions (OH-), hydrogen atoms (H), oxygen atoms (O) and radicals
(H•, •OH). Hence, water may act as a solvent, reactant or dispersing agent in the food
matrix (Chieh, 2006). As such, condition of the water presents in dehydrated foods is
very important as it affects several deterioration reactions in food such as non-
enzymatic browning, lipid oxidation, vitamin degradation, enzyme activity, microbial
activity and pigment stability (Osuna-Garcia and Wall, 1998). Moreover, dissolved spe-
cies in food matrix are concentrated as water is removed during drying. As a general
rule of thumb, reaction rate increases with temperature and reactant concentration.
Therefore, with the simultaneous concentration of dissolved solutes and elevated tem-
perature during drying, reaction between species can be accelerated and thus increases
the destruction rate of nutritional value (Labuza and Tannenbaum, 1972).