Osmo-dehydration of fruits

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