Modeling heat and mass transfer during air impingement thawing of frozen foods.

摘要

Current methods used for thawing of frozen foods are typically undesirably slow or are expensive and cause uneven thawing. One possible method for improved, rapid thawing that has not been investigated is the use of air impingement technology. The goal of this research is to develop a better understanding of the thawing process when frozen food is impinged with jets of air. To accomplish this goal, a mathematical model was developed, based on fundamental principles, that predicts the heat and moisture transport during impingement thawing; effective moisture diffusivities and heat transfer coefficients were determined experimentally for use in the mathematical model; the model was validated using laboratory experiments; and the influence of properties and process conditions on the rate of thawing was studied using a sensitivity analysis.; Moisture diffusivities for Tylose were determined using a drying method and a concentration-distance method. The variation of diffusivity with temperature was fit well using an Arrhenius relationship. Effective heat transfer coefficients for impingement thawing of frozen foods were determined using an inverse method and found to vary both with position and with changes in surface temperature. Heat transfer coefficients were significantly lower at temperatures below 0°C than at higher temperatures due to the frost formation. Temperatures matched well between predictions and experiments. Differences between the two can be partly attributed to different effective heat transfer coefficients resulting from variable humidity during the experiments and possible differences in frost formation. Virtually all mass transfer occurred in the top few millimeters of the samples. The model gave good results for some cases, but tended to under predict moisture contents of the samples because the added moisture due to frost and condensation was not included in the model. Impingement jets reduce thawing times by over 75% for both the Tylose samples and the individually wrapped bratwurst packages. The sensitivity analysis found that product thickness, air temperature, and heat transfer coefficient have the greatest impact on thawing times. Using an average heat transfer coefficient rather than one that varies with position or surface temperature can cause substantial variation in the temperature versus time curves.