Advances in Induction and Microwave Heating of Mineral and Organic Materials

Freeze-drying is a discontinuous process mainly used in the food and pharmaceutical industry to preserve the initial properties of raw products after dehydration. To date, the freeze-drying process has been monitored according to time-dependent empirical rules. Several authors have attempted to develop measuring systems for the on-line determination of the dehydration rate (freeze-drying kinetics) and the sublimation end-point. Of the various measurement systems tested, the measurement of the partial pressure of water vapour by a moisture sensor appears to be the best. Nevertheless, no theoretical background has been advanced to explain the relationship existing between the moisture measurement and the freeze-drying kinetics. This can be achieved using the standard laws of mass transfer corresponding to either a diffusion-driven process or diffusion through a stagnant medium. Moreover, estimation of the sublimation end-point is rigorously possible by referring to these mass-transfer laws. A time-dependent function requiring only the measurement of the operating conditions, and particularly the partial pressure of water vapour inside the apparatus, has been established.

A method for on-line determination of residual water content and sublimation end-point during freeze-drying

The consumer demand for healthy convenience meals with ‘near fresh’ properties challenges researchers and industry to develop new or improved conservation procedures for food products. However, this recent food trend towards fresh image on one ride and convenience on the other side often conflict. In most cases the fresh quality is negatively affected by the processing procedure. Therefore nowadays efforts ars focussed on extending the shelf life of fresh products. However, sophisticated energy demanding facilities are required for storage and transporation, whereas thc use of ecologically unsound cooling agents is a major drawback. The development of a dehydration process on the basic of electromagnetic energy (EME) may bring about a major breakthrough with respect to the retention of product quality and improved rehydration characteristics. Due to the tenfold weight reduction established in the dehydration process transport and storage costs are minimised thus reducing energy consumption. In co...

Research needs and opportunities in the dry conservation of fruits and vegetables

Issues in Freeze Drying of Aqueous Solutions

An NMR method was developed for detection of the sublimation period endpoint during the freeze-drying of potato pieces. Longitudinal (T1 and transverse (T2) relaxation times were estimated using rapid NMR methods. Samples were immediately examined to determine presence of a frozen core. The plot of T1 vs T2 showed an abrupt increase at a point corresponding to the complete elimination of the frozen core. Similar Tj vs T2 curves were observed when monitoring freeze-drying of a single potato piece inside the NMR probe-head. The detection of an anomalous freeze-drying is possible by monitoring the T1 the T2 or the free induction decay signal.

Monitoring Freeze‐Drying by Low Resolution Pulse NMR: Determination of Sublimation Endpoint

Freeze-drying experiments were carried out for milk and water in a vial, and the influence of the heating plate temperature was examined. A simple simulation based on a 'uniformly retreating ice front' type model was performed. A numerical model enables the changes in both water content and temperature of these products to be described satisfactorily during the experiment. Three transfer parameters (water vapour diffusivity in the dry layer, the external mass transfer coefficient, and the contact resistance to the heat transfer) were determined. The contact resistance between the product and the heating plate is the most important resistance to heat transfer, even controlling the dehydration kinetics.

H. Gibert

Papers

Vacuum freeze-drying kinetics and modelling of a liquid in a vial