The effect of blanching on the quality of dehydrated broccoli florets

Different blanching treatments were applied to broccoli florets prior to dehydration in order to improve product quality. The pretreatments used were a conventional blanching in water at 99 +/-1 degreesC, and a stepwise blanching (a blanching at low temperature followed by a short blanching at high temperature after a holding time) using different temperatures for the first step (50, 55, 60, 65 and 70 degreesC). Five rehydration temperatures were used (25, 40, 55, 65 and 80 degreesC). Rehydration rate (D-eff and W-e), chlorophyll content and texture of the rehydrated product were evaluated. Rehydration was modelled according to Fick's diffusion equation. A good agreement between the model and the experimental data was obtained when D-eff and W-e values were identified for each temperature (average explained variance 96.9%). Samples stepwise blanched at 60 degreesC showed, on the average, the lowest W-e values. Stepwise blanching at 60-65 degreesC and rehydration at 25-55 degreesC were the combinations that gave the firmest product. Stepwise blanching at 50 degreesC together with the conventional blanching (99 +/-1 degreesC) and rehydration between 25 and 65 degreesC were the combinations that allowed preservation of the highest chlorophyll content. From these results, it seems difficult to obtain firm samples with a high chlorophyll content without additives.