Azarpazhooh, Ramaswamy - Osmotic Dehydration
4.2.1. Osmotic pressure
Water as the main constituent of most foods affects food stability. During osmotic dehy-
dration, water in solution is in an interaction with solute. This interaction is characte-
rized by the thermodynamic state of water. Energetic state of each substance can be de-
fined as its internal energy which is called chemical potential. Chemical potential is a
function of concentration, temperature, and pressure, however under isothermal condi-
tions; it is just determined by concentration and pressure. The chemical potential can be
defined according to the following relationship:
μw=μ 0 w+RT ln aw (4.1)
a water activity coefficentR- gas constantabsolute temperaturechemical potential in a sandard statechemical potential of waterw−−−−Tww
μ 0μThe energy is exchanged during the interaction of two systems with different energy
state until reaching the equilibrium state. Under isothermal conditions, chemical poten-
tials of two systems are the same, and can be achieved by the change of either concen-
tration or pressure. Osmotic pressure is the excess pressure that pushes the system to
reach the state of equilibrium between pure solvent and a solution and is expressed by
the formula:
V^ ln aw
RT
Π=-
(4.2)Where ∏ is the osmotic pressure and V is molar volume of water.
Osmotic dewatering of fruits and vegetables utilized by the difference in osmotic pres-
sure between two systems results in mass transfer (Lewicki and Lenart, 2007).
4.2.2.The plant tissue structure
Plant tissue as a living material plays an important role during osmotic dehydration
(Marcotte and LeMaguer, 1991). Although different parts of a plant such as roots, stems,
shoots, leaves, flowers, fruits and seeds can be used during osmotic dehydration, all of
them consist of cells that are highly specialized and are called tissues. Tissues consist of
epidermal tissue which forms the outermost layer of cells which are thick and covered
with a waxy substance known as cutin. Parnchymatous tissue, the main parts of organ,
which has the ability to produce and store nutritional substances; and the vascular tis-
sue which can carry the solution of minerals and nutritional substances in a plant
(Rahman and Perera, 1999).
A fresh plant tissue is composed of cells connected to each other by the middle la-
mella, and the protoplast. The cell wall consists of three independent materials; cellulose
microfibrils, hemicelluloses and pectin substance (Carpita, 1996). Hemicelluloses with
branched polymers (xyloglucans, glucomannans) link with cellulose and pectin by hy-
drogen bonds. Generally, the rigidity of a dried product comes from the cellulose whe-