Effect of heating temperature on cell impedance properties and water distribution in apple tissue

Abstract: China has a large amount of apple production and a wide planting area. But a small number of apples have been processed into concentrated juice. There is a very large development space for juice processing industry. To increase the yield of juice, it is necessary for juice processing industry to pretreat apples. A traditional method of pretreatment is to heat the apples to make the cell wall broken, change the intercellular adhesion force, enhance the mobility of water in apple tissues and soften the cells. Up to now, the traditional chemical methods and the modern techniques, such as microtechnic and image processing techniques, have been adopted to analyze the influence of temperature on fruit tissues. It is difficult for these approaches to achieve fast and cheap measurements. Electrical impedance spectroscopy (EIS) is a fast developing method in analyzing materials' characteristics. When the currents with different frequencies flow through the biological tissue, the cytomembrane, the extracellular and intracellular fluid would show various electrical properties. Therefore, EIS has the potential to be used for detecting the change of apple cell microstructure during the heating. To develop a quick, convenient and cheap method for exploring the effect of temperature on apple tissue cells, EIS technique was used to analyze the change of biological tissue at cellular level and water distribution during the heating process by using an equivalent electrical circuit. Hayden model was chosen to describe or predict the change of cell structure in apple tissue during the heating process, which is composed of extracellular resistance, intracellular resistance and constant phase element representing the cytomembrane. Complex nonlinear least square (CNLS) method was used for fitting the EIS data and determining the parameters of Hayden model. The results indicated that the influence of temperature on apple tissue cells could be divided into 3 stages, <60℃, 60-70℃, and >70℃. The impedance of apple tissues decreased with the frequency increasing, and it decreased significantly when temperature increased to 65℃. When the temperature was 65℃, it was the critical factor to break the cells, although holding time could increase the permeability of cell membrane. And it was obvious that when the temperature was higher than 65℃, impedance magnitude tended to be stable at different frequencies. The phase angles of different temperatures increased sharply and then decreased quickly with the increase of frequency, and reached the maximum values at 12 kHz when the temperature was lower than 65℃. Cole-Cole plots of apple tissues presented the semicircles with different radii, which represented the characteristics of typical biological tissue. The radius decreased obviously when the temperature was 65℃. So it was assumed that the thermal damage temperature of apple tissues was 65℃ which could also be seen from the scanning electron microscope (SEM) pictures. And the extracellular and intracellular resistance, and cell membrane impedance began to change at 65℃. Also the volume percentage of intracellular fluid accounting for the total water started to decrease from 90% to 10% from 60 to 80℃. EIS can be used to determine the thermal damage temperature and calculate the water distribution in apple tissues, and provide the theoretical basis for improving the thermal pretreatment efficacy of fruits and vegetables.