Effect of pulsed electric field pretreatment on shrinkage, rehydration capacity and texture of freeze-dried plant materials. (March 2020) - Fauster, T.
Abstract: For this study, strawberries and red bell peppers were pre-treated with pulsed electric fields (PEF) to reduce the negative effects on physical properties of the products after freeze-drying. PEF treatment was carried out at constant electric field strength of E = 1.0 kV/cm and specific energy input was varied between 0.3 and 6.0 kJ/kg (treatment time 2.0–28.6 ms). Additionally, the impact of different pre-freezing temperatures (−4 and −40 °C) on the final product quality was described. Investigations showed that due to PEF treatment a significant reduction of the shrinkage phenomena for both bell peppers and strawberries was detected compared to untreated samples with 30% and 50% lower volume losses, respectively. The rehydration capacity of PEF pre-treated freeze-dried samples increased for both matrices up to 50%. Furthermore, the mechanical properties of the final product were improved for both matrices with a significant firmness reduction up to 60%. The results of this study suggest that PEF can be an effective pretreatment with low energy requirements to improve the quality of freeze-dried fruits and vegetables. Highlights: Quantitative determination of shrinkage phenomena was performed by innovative 3D scanning method. PEF treatment significantly improved the freeze-drying process due to the enhanced mass transfer. Volume loss and firmness were reduced and rehydration capacity increased for strawberries and bell pepper. PEF effects are more pronounced at lower freezing temperatures as usually applied during industrial quick freezing processes. The impact of PEF on selected physical parameters is already detectable at low energy inputs of 0.3 kJ/kg.