Food freezing and thawing time prediction with new simple calculation formulas application

There were developed new methods for freezing and thawing time prediction analysing the effects of water behaviour and product properties. Introduction of a characteristic freezing temperature resulted in the elimination of cryoscopic temperature out of the calculations. Hence, both models got more universality. The models had been comprehensively verified and their high accuracy and full practicability was confirmed. Nomenclature Indices A area, [m] c centre, end Bi Biot number kr cryoscopic c specific heat capacity, [J/kg K] n unfrozen d characteristic dimension, [m] o medium E prediction error, [%] p initial h heat transfer coefficient, [W/m2 K] rz experimental H enthalpy difference, [J/m3] z frozen k heat conductivity, [W/m K] L latent heat of freezing, [J/kg] R correlation coefficient t time, [s, h] T temperature, [K, C] V volume, [m] density, [kg/m] degree of water freezing out, % Introduction Particularly significant effects observed at food freezing-thawing processes are freezing (crystallization) water melting in a product structure. The general character of water freezing out kinetics is most often described by logarithmic dependencies (Fig.1) [1]. Regarding the complexity of the changes from both, physical and biochemical aspect as well as their determination of process course (freezing, thawing) and product quality, the effects were and still have been the objects under investigation in the food technology and engineering and some fields related with them (i.e. cryobiology, cryosurgery). From an engineering and technological aspect the most significant fact is a way in which a process of water freezing outice melting in a product and product properties formed due to these and other factors operation affect the very process and product quality following the treatment. Thawing is an ultimate operation in a cooling chain performed directly before further industrial treatment or product relay for market. Technological Methods to Improve Food Quality 303 0,4 0,5 0,6 0,7 0,8 0,9 1 -45 -40 -35 -30 -25 -20 -15 -10 -5 0 Tem perature [ C] D eg re e o f w at er f re ez in g o u t [ ] Figure 1: Dependency of water freezing out degree ( ) on product temperature (T). This process aims at bringing the products to a state suitable for further use, in a maximal degree approaching a product natural state. Possibility for freezing and thawing time prediction allows, to form advantageous process course, to facilitate fixing effects as well as to enable a refrigerating machine design. Considering ice crystallisation and melting processes in a problem fundamental for food freezing and thawing, it was comprehensively examined to work out a new analytical and empirical methods those they would let predict effective freezing and thawing time. So, the objective of the investigation was to work out, simplified models for freezing and thawing time calculation for foods of regular dimensionality. Development of the models Some possibility to obtain the new simple freezing time prediction model [7] was found in Mott's method [1]. Calculations acc. to this method consist in the application of criterion numbers: d Q T h t = G , 2 d h = B , V d A = G 1 + B = S z z