Osmo-dehydration of fruits

Osmo-Dehydration of Fruits: a thermodynamic approach via Knudsen
Thermogravimetry
Dimitrios Fessas, Marco Signorelli, Alberto Schiraldi
DISTAM, Università di MilanoVia Celoria 2, 20133 Milano, Italy (dimitrios.fessas@unimi.it)
ABSTRACT
The osmo-dehydration of a fruit pulp can be described with a thermodynamic model that takes into account
the heterogeneity of the system. The experimental data are those relevant to the isothermal desorption trends
across the swept RH range for the original (peeled) and partially osmo-dehydrated fruit pulp, and the
hypertonic sugar syrup used. These trend allow evaluation of the mass of intra- and extra-cellular phases of
the system at any given RH. The mass change of the intra- cellular aqueous phase allows a rough evaluation
of the shrinkage that the fruit cells undergo and the correlation between shrinkage and RH, which may be
applied to any kind of dehydration treatment, provided it does not affect the cell walls.
Keywords: osmo-dehydratio, Knudse thermo-gravimetry; thermodynamics
INTRODUCTION
Water in fruits and vegetables has a thermodynamic activity, a W , close to unity, which makes easy their
degradation. This is why a previous dehydration is demanded to avoid non enzymatic browning and
preservation of bioactive components (mainly vitamins and aromatic compounds).
In osmo-dehydration the fruit is poured into a hypertonic solution of simple sugars and the intracellular
moisture is drained out because of the sugar concentration gradient. The final product is still rather soft and
microbiologically unstable, as part of its moisture is retained. A final mild heat treatment (e.g., low
temperature convection drying, freeze drying etc.) enhances the stability.
It must be noticed that fruit cell walls are not actually semipermeable. Some fruit solutes (sugars, organic
acids, minerals, salts, etc.) are therefore leached into the hypertonic solution while some sugar of this can
migrate into the intra-cellular spaces (usually, even simple sugars are not able to trespass the cell walls) and
the whole system shrinks as the fruit cells change size and shape. Dehydration extent and composition of the
hypertonic syrup can affect the quality of the final product [1, 2].
In order to assess the best operating practice models for the water migration and location during the process
have been so far searched. A number of models [3,4] were proposed, where, in order to exclude any change
of the hypertonic syrup during the process, its mass was supposed to be much larger than the fruit poured in.
All these models assume a fickian behaviour for the diffusion, although they can substantially differ from one
another for the number of the fitting equations and adjustable parameters. Unfortunately, they do not
satisfactorily represent the actual system and the real operating conditions for a number of reasons: (i) a
substantial change of the hypertonic syrup the mass of which actually does not exceed the 4:1 ratio versus
that of the fruit; (ii) the sample is referred to as a continuum with a given macroscopic geometry that
undergoes shrinkage as a whole; (iii) no general fundament is assumed that may be directly applied to any
kind of fruit or vegetable (kind of cultivar, age after harvest, storing conditions, etc., are presented as the
reason for the model limit).
A basic statement is indeed lacking in all these models, namely, that the moisture content of biological
materials is partitioned in different fractions and states, depending on the kind of tissue compartment and the
solutes therein hosted. Because of the presence of extracellular biopolymers, cell membranes and other
physical barriers, water molecules of the various moisture fractions have different mobility, as evidenced by
means of NMR relaxometry [5], and are perpetually involved in exchanges between different states.
The present work proposes a thermodynamic approach that defines boundary conditions that must be anyway
obeyed, no matter the kind of fruit to be dehydrated and the hypertonic medium used in the process. It makes
use of the aW changes both in fruit pulp and hypertonic syrup during a real osmo-dehydration process.
To this aim, the isothermal desorption trends of: (i) non-treated fruit, (ii) syrup used for the osmodehydration,
(iii) a partially osmo-dehydrate fruit, have to be determined. The experimental method proposed
is the Knudsen Thermogravimetry, which was so far exploited to determine the desorption isotherms of

aqueous solutions of salts and sugars, and food systems, like dough of various cereal flours [6,7]. The kind of
equipment used, namely, a TG-DSC combined instrument, allows extention of the investigation to various
temperatures.
MATERIALS & METHODS
The Knudsen TG, designed by Schiraldi and Fessas [6], allows the direct determination of water activity. The
thermo-balance cells are replaced with Knudsen cells, namely closed cells the cover of which has a 20 m
orifice through which water molecules are pumped out under high dynamic vacuum (pext ≈ 0). In isothermal
conditions the flux of water through the orifice, J, is proportional to the water partial pressure, p W , within the
cell,
dmW
J = ∝ ( pW
− pext
) ≈ K ⋅ pW
(1)
dt
where the constant K depends on the temperature and the size of the orifice and the flux J corresponds to the
record of the time derivative of the thermo-gravimetric (DTG) trace.
A separate experiment (at the same temperature and vacuum conditions) with a pure water sample produces a
flux J* = K p W *, where p W * is the vapour pressure of pure water. The p W /p W * ratio is a good approximation
for the value of water activity, a W , (or relative humidity, RH) of the sample considered. Because of the
sample dehydration that takes place during the KTG run, the p W /p W * ratio decreases, while the mass drop is
simultaneously recorded: this allows the direct evaluation of the residual sample moisture content versus RH,
namely, the desorption isotherm.
A single run therefore allows to draw the whole desorption trend, plotted in the plane (m W /m dm , RH), where
the subscripts “W” and “dm” stand for water and dry matter, respectively, with reference to the trace of pure
water recorded immediately before [6].
These desorption trends are to be considered within the RH range actually experienced during an osmodehydration
process, i.e. RH ≥ 0.8.
The instrument used was a SETARAM TG-DSC111, Lyon, France, which allowed the simultaneous output
of TG trace, namely mass loss-vs-time, its time derivative, DTG, and the relevant thermal effect. Thanks to
such a coupling, the mass loss can be reliably attributed to the release of water if the mass-loss-rate/heat flux
ratio has a value of about 2.2 J g-1, namely, the vaporization enthalpy of water [6], which is expected to hold
also for water rich systems, like salt and sugar solutions, vegetables and fruits.
Fuji cultivar apples, peeled and cut into spheres (d about 1.5 cm), were osmo-dehydrated at room temperature
(~25°C) with three different sugar syrups of given water activity (a W = 0.89). The hypertonic syrup was
continuously re-circulated through a peristaltic pump and the fruit-syrup mass ratio was kept at 1/8 (w/w);
composition and water activity of the syrup underwent some detectable changes. Different treatment times
were exploited (up to 6 h) in order to obtain samples at different dehydration levels. After the osmotic
treatment, the samples were carefully and rapidly washed with cold water (details are reported in [8]). 40 mg
mass samples were used for the KTG investigations.
RESULTS & DISCUSSION
In a osmo-dehydration process the dry matter of the sample increases, the RH of the fruit decreases and that
of the hypertonic syrup increases, since the removal of water from the fruit is more than counterbalanced by
the migration of sugars from the syrup toward the apple pulp. The process ends (after about 4 hour, in the
present case) when fruit and syrup attain the same RH value.
Since the intracellular water is sucked out of the fruit cells and displaced toward the intercellular regions and
the surrounding medium, the RH of this is indeed a little higher at the end of the treatment.
The starting samples of a Knudsen experiment must be in a steady state, i.e., with no neat water displacement
between fruit compartments. During the isothermal dehydration the water fraction with the easiest access to
the head space of the Knudsen cell determines the actual vapor pressure and therefore the RH detected. This
means that RH gradients are soon formed between different fruit compartments. The relevant Knudsen trend
therefore reflects quasi equilibrium states only when the effusion through the cell orifice is as slow as the
displacement of water from one compartment to another.
The desorption isotherm of non-treat fruit deals with a system the dry matter of which does not change. The
release of water from the fruit is in this case limited by the diffusion of water through the cell walls. The fact

that partially heat-dehydrated apple pulp shows a Knudsen desorption trend perfectly overlapped to that
obtained from the original non treated sample (Figure 1) indicates that the Knudsen desorption goes through
the states that are actually met during a heat drying process.
The desorption isotherm of a partially osmo-dehydrated fruit instead reflects the removal of water from the
intercellular regions where it is more mobile: the desorption trend therefore concerns the partially diluted
(because of the water sucked from the apple cells) syrup that remains entrapped in the fruit pulp. If a partially
osmo-dehydrated fruit is further dried with a mild heat treatment, the removal of water still deals with the
intercellular moisture. The desorption isotherm of such a sample is indeed overlapped to that obtained from a
sample that did not undergo the further heat drying (Figure 1).
Figure 1. Desorption isotherms drawn from KTG traces. Apple pulp samples: non treated, partially heat
dried, partially osmo-dehydrated and heat dried after a partial osmo-dehydration.
The desorption trend of a partially osmo-dehydrated fruit is shifted toward larger RH with respect to those of
the non-treated fruit and the hypertonic syrup (Figure 2). It has to be noticed that any fruit pulp is are multiphase
system. In a rough approximation, it may referred to as a two phase system, namely, the intra- and the
extra-cellular aqueous regions. A true thermodynamic equilibrium would imply that each compound that can
be exchanged between these phase may attain the same thermodynamic activity in either phase. The partially
osmo-dehydrated samples considered for the isothermal Knudsen desorption had been removed from the
hypertonic syrup and let at rest for some hours to allow any neat displacement of water and sugars between
phases to vanish. The composition of either phase therefore reflects the balance of the relevant chemical
potentials and the amount of each phase can be evaluated with the simple lever rule.
This means that the desorption trends recorded for the non-treated fruit pulp and the sugar syrup reflect the
composition of the intra and extra-cellular phase, respectively (Figure 2). Any partial dehydration state must
therefore remain between these trends (Figures 2) and, at any given RH, the mass of intra- and extra-cellular
phase of the partially osmo-dehydrated sample can be easily evaluated [8].
The decrease of the intracellular water produces shrinkage of the fruit cells, namely, a reduction of their
volume. The volume drop is mainly related to the release of water: therefore a rough evaluation of such cell
shrinkage may be:
VW
,0
−VW
shrinkage ≈ (2)
V
W ,0

Figure 2. Knudsen desorption isotherms collected from non- treated, partially (one hour treatment) osmodehydrated
apple pulp, and the hypertonic sugar syrup used in the process. mW and mdm stand for water and
dry matter mass, respectively.
Since these quantities can be directly correlated to the RH through the relevant desorption isotherm, one may
describe the shrinkage extent versus RH (Figure 3), which therefore applies to every partial dehydration
level, no matter the way it has been achieved (through osmo-dehydration, or heat drying, or combined
treatment).
Figure 3. Shrinkage of the cells of apple pulp evaluated according to equation 2 (see text). Data from the
Knudsen desorption trend of non- treated samples.

CONCLUSION
The osmo-dehydration of a fruit pulp can be described with a thermodynamic model that takes into account
the heterogeneity of the system. The experimental data are those relevant to the isothermal desorption trends
across the swept RH range for the original (peeled) and partially osmo-dehydrated fruit pulp, and the
hypertonic sugar syrup used. These trend allow evaluation of the mass of intra- and extra-cellular phases of
the system at any given RH. The mass change of the intra- cellular aqueous phase allows a rough evaluation
of the shrinkage that the fruit cells undergo and the correlation between shrinkage and RH, which may be
applied to any kind of dehydration treatment, provided it does not affect the cell walls.
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