Impingement jet freezing of biomaterials

Impingement jet systems have been identified as an alternative to conventio nal freezing methods, given their high turbulence characteristics, which en hance heat transfer and therefore quality product. Surface heat transfer co efficients and freezing times in agar particles with similar water content to vegetable food, carrot cylinders, and jack mackerel spheres were determi ned under several operation conditions (velocity and air temperature, parti cle geometry, and material load to freezer) in an impingement freezer. Heat transfer coefficients were estimated in the 70-250 W/m(2) K range, whereas freezing times were within 2-11 min. Freezing time prediction models devel oped by Nagoaka, Plank, and Pham adequately describe the experimental resul ts obtained here. Freezing time was significantly influenced by air tempera ture and particle diameter. Results indicate that a decrease in air tempera ture and velocity, and an increase in particle diameter, improves frozen fo ods quality (texture). Precooling or scalding of raw materials was also fou nd to increase the final quality of frozen foods. (C) 2001 Elsevier Science Ltd. All rights reserved.