Modelling and validation of robust partial thawing of frozen convenience foods during distribution in the cold chain

In collaboration with two commercial distributors we have tested a new concept for distribution, where convenience products for the food service industry are prepared, frozen and packed in cardboard boxes, but distributed in the chill chain at +5°C instead of in the frost chain. This will lead to a partial thawing before reaching the end user. The high thermal buffering capacity of the product under thawing ensures that the product temperature remains stable at subzero temperatures, also in cases of temperature abuse for some hours. To investigate and validate the concept, cardboard boxes packed with small blocks of a frozen model food (23 pct. Tylose® gel) and quipped with temperature loggers were distributed by trucks operating in the cold chain. In addition, controlled storage and temperature abuse experiments were conducted. To predict the product temperature–time relationship we developed a new thawing model for the slow thawing of food pieces packed in a box. The model is based on enthalpy balances under quasi-stationary conditions; a condition that is fulfilled in the practical distribution experiments. The experiments confirmed the model predictions that the products were still partially frozen even after two days or more of distribution at +5oC, and that the temperatures inside the product and in the middle of the box were quite stable against the normal oscillations of the ambient temperature in the cold chain. The product temperature was also robust against temperature abuse, as predicted. The new distribution method not only has the potential of prolonging product shelf life but also gives the distributors an increased flexibility in the distribution chain.