Impact of shelf temperature on a grapefruit puree temperature evolution during freeze-drying

[EN] The high nutritional and functional value of fruits justifies the use of freeze-drying to obtain high value-added products. As to identify in-line the drying end point, the usefulness of temperature evolution of a grapefruit puree, added or not with gum Arabic and bamboo fibre, when applying 20 or 40 oC as shelf temperature has been investigated. The obtained results point for the monitoring of the product's heating rate as to identify the end of sublimation in the sample, coinciding with the time this rate starts to decrease. On the other hand, the end point of the drying step of the freeze-drying process, optimised in terms of duration, is proposed to be related to the sample reaching a certain target temperature which does not necessarily match with the temperature of the chamber. In the concrete conditions of this study, it seems to be between 20 and 40 oC. As for the variables considered, the biopolymers added to the grapefruit puree did not affect the duration of the process and to increase the freeze-drier shelves temperature made it possible to reduce processing time by 40%.