Correlation between rheological and textural properties of apple tissue under moderate electric field treatment

Fruits and vegetables pretreatment can often be required for cost saving, due to the high energy consumption in the subsequent deep processing. Among them, the moderate electric field (MEF) can induce both electrical and thermal effects, and then tailor the microstructure of fruits and vegetables. The resulting rheological properties can further change the textural properties. This study aims to clarify the correlation between the rheological and texture properties of apple tissues under the MEF. A systematic investigation was also carried out to determine the creep, dynamic viscoelasticity, and TPA texture properties of apple tissues. An analysis was finally made to explore the effects of electric field strengths (15 ~ 90 V/cm) and temperature (30 ~ 70 ℃) on the viscoelasticity and TPA parameters of apple tissues. The results showed that the MEF treatment was performed better to rapidly soften the apple tissues, compared with the water bath treatment at the same temperature. The hardness, brittleness, and chewiness of apple tissues decreased linearly with the increase of the electric field strengths and temperature. There was no variation in the mechanical properties of apples that were dominated by elastic characteristics after MEF treatment. The viscoelasticity of apples decreased with the increase of the temperature and strengths of electric field treatment. Instant elastic modulus (E0) was significantly positively correlated with the storage modulus (G') (P<0.05). The correlation coefficient of 069 indicated the variation in the elasticity of apple tissue during pretreatment. Therefore, E0 and G' were taken as the elastic factors. The coefficient of viscosity factor (η0) was significantly positively correlated with the loss modulus (G") (P<0.05) with a correlation coefficient of 0.94, indicating the variation in the viscosity during pretreatment. Therefore, η0 and G" were used as the viscosity factors; There was a significant positive correlation between damping factor (tanδ) and delay time (T1) (P<0.05), where the correlation coefficient was 0.88. As such, the tanδ and T1 indexes were used to measure the proportion of viscoelastic change in the pretreatment. The parameters E0, G", and T1 were selected as the evaluation indexes for the rheological properties of apple tissues using the Pearson correlation coefficient. The principal component analysis showed that the hardness, brittleness, and Chewiness were significantly positively correlated with the viscoelastic factors (P<0.05), while there was no significant correlation between the resilience, cohesiveness, and viscoelasticity (P>0.05). Especially, the cohesiveness and resilience of apples were closely related to the fracture and deformation caused by nonlinear deformation. There were some differences from the viscoelastic parameter properties in the linear range. It was also observed that there was an intact cell structure of untreated apple tissue. At the electric field strengths of 30 V/cm, the cell structure was complete, and a few cells showed a tendency to curl up, which was little different from that in the water bath treatment at 40 ℃, indicating no significant electrical effect. At the electric field strengths of 45 V/cm, the cells also shared a complete structure, and some cells appeared to curl up at the edge under the joint action of the electrical effect and electric heating effect. A large range of cell tissue collapse often occurred at the electric field strengths of 60~75 V/cm. As such, the cell tissue basically collapsed and lost its support at 90 V/cm. When the electric field intensity was 45 V/cm, the apple tissue cells gradually collapsed with the increase in temperature (40~60 ℃). Even the cells were severely deformed and lost their support at 70 ℃. Under the condition of 45 V/cm and below, there was no variation in the influence of the electric field with temperature. When the field strength was greater than 45 V/cm, even if the action time was short, the texture and viscoelasticity of apples were greatly affected at a lower temperature, where the hardness was reduced. There was no significant difference in the cohesion, indicating a short time of high field strength suitable for the pretreatment of canned products. The finding can provide theoretical support to improve the technological conditions of fruit and vegetable pretreatment by medium voltage electric field.