Impact of material structure on the course of freezing and freeze-drying and on the properties of dried substance, as exemplified by celery

• The internal structure of the plant tissue influences the temperature pattern of the sample.
• Blanching caused slower rate of the following freezing process.
• Manipulation with the freezing conditions can be used to control the freeze-drying time.

The aim of this study was to analyze the influence of plant material structure on the course of freezing, freeze-drying and on the properties of the dried material, as exemplified by celery. The structure of tissue before further processing remained untreated or was altered by blanching or mechanically disintegrated. Afterwards, the samples were frozen by the means of natural convection at −20 °C or forced convection at −40 °C and freeze-dried at different (20–50 °C) shelf temperatures. Products were analyzed regarding their reconstitution, hygroscopic properties and microstructure. Blanching caused freezing to progress more rapidly both in the case of natural and forced convection method, whereas the enhancement of freezing rate by the means of mechanical disintegration was observed only in the case of forced convection freezing. Only in the case of slow freezing the internal structure influenced the temperature pattern of the samples. Mechanically disintegrated samples were characterized by the longest mean sublimation time (702–1282 min). In comparison, in the case of the untreated, freeze-dried material this parameter varied from 328 to 826 min. Both blanching and mechanical fragmentation of the structure improved hygroscopicity of the material whereas the final water content of blanched samples after rehydration was close to the content of water in fresh material.