Energy efficient and high quality thawing of cod in large scale
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Thawing of block frozen cod is the main challenge of the overall industrial process due to the large variation in temperature after processing. The geometry makes the heat transfer slow and further difficult to conduct a precise thawing process. Controlled thawing, however, can give higher yield due to a cleaner cut along the backbone caused by a level of firmness at temperatures just below the initial freezing point. Industrial thawing aim to result in quality comparable to fresh fish for further processing. Yield and energy use influence the profitability whereas bacterial growth determine the shelf life of the resulting product. Hence, the aim is to operate the thawing process in an optimal way with respect to yield, bacterial quality and energy use.In this work, thawing as a part of industrial processing is introduced. The preliminaries include basic principles of thawing methods and the literature survey of different thawing procedures. The complexity of the block split is yet only investigated for the water thawing process. The investigations on other techniques were mostly conducted with fillet pieces and show a possible thawing time reduction of up to 80%. The producers offer process solutions in various size or in modular build up. Thus, all introduced techniques are availability for large scale thawing. A two-dimensional model was developed to simulate to thawing process. By a Kirchhoff- and enthalpy transformation, a partial differential system with two mutually related dependent variables was solved to deal with the abrupt changes of the thermophysical material properties. The influence of the process temperature on bacterial quality of the product was evaluated by integration of the temperature distribution. Basic investigation on single fish thawing delivered a guideline for thawing regarding time, temperature and bacterial growth. The temperature distribution during thawing shows that the surface temperature is reduced within 30 minutes after the transfer to the chill media. The core temperature rises during the chill time due to the enthalpy equalization. Thus, the final core temperature depends on the transferred energy during the bath time. The split times were compared with the time frame to temper the core into the latent heat zone. The results revealed adverse characteristics of high thawing media temperatures at expected split times.The block thawing model was used to simulate the expected split times at particular temperatures. The temperature range between 7 and 11˚C showed the best performance due to the balance between fast and precise thawing. The split time was varied to investigate the effects of enhancing the block split. The positive effect of the block split acceleration is especially strong for higher temperatures. Thus, fast splitting shifted the recommendations towards higher process temperatures. The quality evaluation showed no significant difference between the different split times and that thawing above 7˚C isn t increasing the bacterial growth.