A kinetic model for turgor loss in red cabbage cells during mild heat treatment

Turgor changes in red cabbage leaves (Brassica oleracea L. covar. Capitata Alef. var. rubra DC.) due to mild heat treatments were measured in the epidermal and first subepidermal cell layers with the pressure probe technique. After an equilibration time in a sorbitol solution of equal water potential, cabbage discs were immersed in solutions heated to 53 °C and 65 °C for different time intervals. Turgor loss at both temperatures could be described by a first order degradation model. This could be due to the increased permeability of membranes at higher temperatures, resulting in passive efflux of solutes and an irreversible reduction of cell turgor well before membranes are completely destroyed. The residual turgor (at t = ∞) for the 53 °C treatment was significantly different from zero. Assuming an Arrhenius type model for the temperature dependency of the rate constant k, the activation energy (Ea = 41 kJ · mol-1) and reference rate constant (kref = 1.53 min-1 for a reference temperature Tref = 323 K) were calculated for the turgor loss model in the epidermal cell layer. In the second cell layer, a time shift of the commencement of turgor decrease could be observed at the lower processing temperature. Temperature profiles at different locations in the cabbage disc were calculated by use of a finite element model and related to turgor loss observations.