Drying of alcohol precipitated pectin gel with low pressure superheated alcohol vapor.

LOZANO, JORGE E.; URBICAIN, MARTÍN J.; GENOVESE, DIEGO B.

Copenhague

Congreso; European Congress of Chemical Engineering; 2007

The use of superheated steam at moderate temperatures and pressures below the equilibrium ones, as the heating agent in drying of foodstuffs instead of hot air, has been reported elsewhere by these authors among others, as a successful technique to obtain dry products of much better quality than those dried by the conventional hot air procedure.In this work, this technique has been applied to the recovery of pectins extracted from apple pomace produced in juice concentrate manufacturing plants. Pectins are precipitated from the water solution by successive washing with ethanol yielding a gel, saturated with a water-alcohol mixture, which requires to be dried before further processing. This drying is performed in a closed chamber, where the gel sample is brought into contact with an alcohol superheated vapors stream.Vaporization of the retained alcohol is provoked by the heat transferred to the wet gel from the gas stream, at a prevailing pressure lower than the atmospheric one, such that the liquid phase boils at a relatively low temperature. The vapors removed from the gel are mixed with the superheated vapor and recycled after being reheated again by a controlled electric resistance. An equivalent mass is simultaneously removed from the camera by a vacuum pump and condensed in a film condenser. In this particular case, the liquid obtained is almost pure ethanol.Drying is produced in a thin boundary within the gel, as the evaporation takes place in the liquid-vapor interface, making the boundary to move inwards. The required latent vaporization heat is conducted through the dry matter, while the evaporated alcohol diffuses through it outwards.A typical superheated alcohol temperature is in the order of 60o C, while the wet zone remains at the temperature in equilibrium with the prevailing pressure (order of 40o C), building a suitable temperatures difference for the heat transfer to take place with no risk of damaging the product.Conventional drying was carried out on the same samples in a typical hot air drier at 60o C, and all samples were ground and screened, to obtain a dry powder with a particle size smaller than 250. Comparative results indicated that air drying took 2 hours and the product measured 158 °USA-SAG and 85.50 ED% while those dried with superheated alcohol required only 1 hour to yield pectin with 165 °USA-SAG and 88.21 ED%.It can be concluded that the proposed method renders a better product in a shorter time and offers a technology worth to be further explored.