Osmo-dehydration of fruits

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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
Page 1: Osmo-Dehydration of Fruits: a therm
Page 5: CONCLUSION The osmo-dehydration of

Osmo-dehydration of fruits

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