Irreversible thermochromic ink in the identification of over- and under-processed product segments in microwave-assisted freeze drying

The formation of over- and under-processed product regions is an inherent drawback of microwave-assisted freeze drying (MWFD) of food attributed to the inhomogeneity of microwave energy distribution. To overcome limitations in the investigation of inhomogeneity effects, the scope of this study was the development and validation of a new method using an irreversible thermochromic colour-component (ITC) with a particular colour intensity profile as a function of temperature (40–70 °C). For validation, thermal images and spatially-resolved biomarker inactivation was compared to the colour intensity pattern resulting from the temperature-induced colour reaction of ITC in MWFD. The ITC method provides advantages over existing methods, as it visualises the spatial distribution of maximum processing temperature independently of post-treatment effects and existing dryer design. This easy-to-use method has the potential to eliminate or reduce preconceptions due to poorly known inhomogeneity of microwave applications and pave the way for energy-efficient and fast MWFD of food.