Development of a time–temperature indicator (TTI) label by rotary printing technologies

A novel continuous production process is presented for smart labels specifically designed as time–temperature indicators (TTI). Printing and converting technologies are set out on a traditional rotary press that works at high web speed with a great repeatability of the printed product. TTI labels are based on a functional water-based ink that impregnates some areas of an absorbent paper whose edges are waterproofed by a UV graphic ink. A rotary screen printing process is used for the deposition of the inks and a plastic layer is applied by lamination on both sides of the substrate to ensure optimum wetness of the functional ink. The formulation of the ink includes a chromogenic growth medium with a starter culture; the metabolism of the bacteria acidifies the ink, hence changing the pH indicator colour. A model based on the Gompertz equation has been developed to simulate the colour change of the labels for different starter culture loads and temperatures, with an accurate match between predictions and experimental data. This model can be a useful tool in the design of new features of the tags that can be a straightforward scale-up to a pre-industrial level. The rotary printing process allows massive production of low cost smart tags.