Non-equilibrium States and Glass Transitions in Fruits and Vegetables

Glass transition is a second-order time-temperature and moisture dependent transition since there is no latent heat involved in this state change. It is also considered as super-cooled liquid. It is usually observed by a characteristic discontinuity (i.e., a shift or change in slope) in the physical, mechanical, electrical, thermal, mass transfer, and other properties of a material. This transition is most commonly identified as a shift in the thermogram line as measured by differential scanning calorimetry (DSC). The glass transition in foods could be considered as a nature of second-order, since each measurement technique monitors change in a specific property within a certain temperature range rather at a specific temperature. A perfect second-order transition occurs at a specific temperature. In foods, glass transition may occur even within 50°C (T_gi-T_ge = 50°C). In addition, the sensitivity of a specific property at glass transition may widely vary based on the types of foods and their compositions. For example, the change of mechanical property at glass transition is usually more sensitive as compared to the thermal property. In addition, measurement conditions and sample preparations significantly affect the measured values. For example, heating rate, frequency (for dynamic mechanical analysis), sample size and shape could affect the measured value. Therefore, in order to characterize this state change, numbers of terminologies are defined for foods when foods experience complex compositional and structural changes during processing and storage. This chapter covers the characteristics of the glass transition in the case of fruits and vegetables.