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-Experimental bubble size distributions and bubble mean diameters were obtained by means of a photographic technique for a direct-contact evaporator operating in the quasi-steady-state regime. Four gas superficial velocities and three different spargers were analysed for the air-water system. In order to assure the statistical significance of the determined size distributions, a minimum number of 450 bubbles was analysed for each experimental condition. Some runs were also conducted with an aqueous solution of sucrose to study the solute effect on bubble size distribution. For the lowest gas superficial velocity considered, at which the homogeneous bubbling regime is observed, the size distribution was log-normal and depended on the orifice diameter in the sparger. As the gas superficial velocity was increased, the size distribution progressively acquired a bimodal shape, regardless of the sparger employed. The presence of sucrose in the continuous phase led to coalescence hindrance.

An alternative rote is proposed for fruit juice processing, according to which aroma compounds are extracted by gas stripping in a bubble column and then recovered by vapor permeation, in a first step, the aroma-depleted juice being subsequently concentrated by direct-contact evaporation. To illustrate the adequacy of the proposed process, experiments were conducted with a synthetic juice, an aqueous solution of sucrose and two esters. In these tests, an aroma recovery degree of about 90% was obtained. Even though the operating pressure was slightly higher than 101 kPa, the liquid temperature did not exceed 66 °C and sucrose concentrations up to 77 wt % were achieved without any reduction in the evaporation rate. Additionally, pervaporation experiments were conducted with an aqueous solution of the esters and the flux results were compared with those related to the vapor permeation tests, revealing that the latter technique is less susceptible to concentration polarization and hence more appropriate for aroma recovery.

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