Impingement jet freezing of biomaterials

Impingement jet systems have been identified as an alternative to conventional freezing methods, given their high turbulence characteristics, which enhance heat transfer and therefore quality product. Surface heat transfer coefficients and freezing times in agar particles with similar water content to vegetable food, carrot cylinders, and jack mackerel spheres were determined under several operation conditions (velocity and air temperature, particle 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 developed by Nagoaka, Plank, and Pham adequately describe the experimental results obtained here. Freezing time was significantly influenced by air temperature and particle diameter. Results indicate that a decrease in air temperature and velocity, and an increase in particle diameter, improves frozen foods quality (texture). Precooling or scalding of raw materials was also found to increase the final quality of frozen foods. (C) 2001 Elsevier Science Ltd. All rights reserved.