ORIGINAL PAPER Influence of Ultrasound on Freezing Rate of Immersion-frozen Apples Adriana E. Delgado & Liyun Zheng & Da-Wen Sun Received: 30 November 2007 / Accepted: 17 June 2008 / Published online: 9 July 2008 # Springer Science + Business Media, LLC 2008 Abstract The use of power ultrasound within the food industry is an innovative subject. Application of sound to monitor a process or product is common, e.g. in quality assurance. However, the use of ultrasound to directly improve processes and products is less popular in food manufacturing. In the present work, ultrasound-assisted immersion freezing was investigated on apple samples. Because the apple parenchyma is mechanically anisotropic, the influence of applying ultrasound on radial or tangential orientated samples was also examined. Apple cylinders were immersed in an ultrasonic bath system, which operates at 40 kHz frequency. Experiments were carried out at a power level of 131.3 W (0.23 W/cm 2 ), and ultrasound was applied intermittently for different times from temperatures below and close to the initial freezing point. Results showed that ultrasound application at 0°C or -1°C for 120 s in total, with 30 s intervals, significantly improved the freezing rate represented by the characteristic freezing time up to 8% (P <0.05), compared to immersion freezing without ultrasound. Results of the effect of ultrasound waves applied on radial or tangential cut samples sonicated for 120 s from -1°C and/or 0°C indicated that at the power level considered there were no significant differences among the ultrasonic radial or tangential irradiated samples of these treatments, though the freezing rates were enhanced and different (P <0.05) from the control treat- ment. Some evidence of the influence of ultrasound to induce primary nucleation was also observed. Keywords Apples . Crystallisation . Freezing . Freezing rate . Immersion freezing . Nucleation . Primary nucleation . Ultrasound Introduction Freezing is a well-known preservation method widely used in the food industry. The freezing process combines the favourable effect of low temperatures with the conversion of water into ice. The water–ice transition has the advantage of fixing the tissue structure and separating the water fraction in the form of ice crystals in such a way that water is not available either as solvent or reactive component (Delgado and Sun 2001). However, the size and location of the ice crystals may damage cell membranes and break down the physical structure. Thus, the cause of the undesirable physico-chemical modifications during freezing is the crystallisation of water and sometimes solutes (Delgado and Sun 2001). It is well-known that the crystallisation of ice has two steps: the formation of nuclei and the later growth of the nuclei to a specific crystal size, the final crystal size being a function of the rates of nucleation and crystal growth, and also of the final temperature (Martino et al. 1998). Slow freezing generally leads to large ice crystals formed exclusively in extracellular areas that could damage cell structure and have an effect on the thaw behaviour as well as on the sensory properties and nutritional value of foodstuffs, while high freezing rates produce small crystals evenly distributed all over the tissue. Therefore, extended research has been carried out to control the crystal size. Conventional cooling methods such as air blast, plate contact, circulating brine and liquid nitrogen (ordered in increasing values of the heat transfer coefficient) are the Food Bioprocess Technol (2009) 2:263–270 DOI 10.1007/s11947-008-0111-9 A. E. Delgado : L. Zheng : D.-W. Sun (*) FRCFT Group, School of Agriculture, Food Science and Veterinary Medicine, University College Dublin, National University of Ireland, Belfield, Dublin 4, Ireland e-mail: dawen.sun@ucd.ie URL: www.ucd.ie/refrig