Microstructural change of potato tissues frozen by ultrasound-assisted immersion freezing
Da-Wen Sun *, Bing Li
FRCFT Group, Department of Agricultural and Food Engineering, University College Dublin, National University of Ireland, Earlsfort Terrace, Dublin 2, Ireland Received 22 March 2002; accepted 17 August2002
Abstract Power ultrasound has proved to be very useful in controlling crystallisation processes since sonication can enhance both the nucleation rate and crystal growth rate by producing fresh and/or more nucleation sites. Therefore, in this study, power ultrasound was applied to assist the freezing process. The results showed that the freezing rate of potato sample was improved with theapplication of ultrasound, compared to that without ultrasound. Higher output power and longer exposure time to ultrasound would help to enhance the freezing rate. However, the heat produced when ultrasound passes through the medium limited the power applied and the exposure time. In this work, the freezing rate was fastest with an output power of 15.85 W and a treatment time of 2 min. The analysesconducted on the microstructure of potato tissue using cryo-scanning electron microscopy technique showed that the plant tissue exhibited a better cellular structure under ultrasonic power of 15.85 W. Much less intercellular void and cell disruption was observed. This was attributed to high freezing rate obtained under high ultrasonic level and thus the domination of intracellular small icecrystals. Ó 2003 Elsevier Science Ltd. All rights reserved.
Keywords: Crystallisation; Freezing; Freezing rate; Food; Frozen potato; Ice crystal; Rapid freezing; Ultrasonic; Ultrasound
1. Introduction Freezing is an excellent method for long preservation of vegetables and fruit (Delgado & Sun, 2001). The quality of frozen foods depends on the size of ice crystals (Li & Sun, 2002a,b). Presence oflarge ice crystals within the frozen food tissue which could result in mechanical damage, drip loss and thus reduction in product quality. The size and location of ice crystals are closely related to the rate of freezing (Delgado & Sun, 2001; Martino, Otero, Sanz, & Zaritzky, 1998; Ngapo, Babare, Reynolds, & Mawson, 1999). Rapid freezing produces small intracellular ice, while slow freezing produceslarge ice crystals. Therefore, rate of freezing is recognised as critical in quality of frozen product. In the food industry, currently air blast, plate contact, ﬂuidised-bed freezing and cryogenic freezing are the
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most common methods used toobtain optimum freezing rate for food products. However, the high freezing rate achievable by these methods is controlled by the thermal conductivity of food, which has a low value of about 0.5– 1.5 W/m K. Therefore, new methods are being proposed and developed (Li & Sun, 2002a), e.g. the high-pressureassisted freezing (Kalichevsky, Knorr, & Lillford, 1995; Otero, Martino, Zaritzky, Solas, & Sanz,2000; Sanz, Otero, Elvira, & Carrasco, 1997), or dehydrofreezing (Spiazzi, Raggio, Bignone, & Mascheroni, 1998). Power ultrasound, which is the ultrasound with frequency in the range from 20 to 100 kHz and high power, has proved to be useful in the formation of ice crystals during the freezing of water (Li & Sun, 2002b). Under the inﬂuence of power ultrasound, a much more rapid and even seedingoccurs and this leads to shorten the time between the initiation of crystallisation and the complete formation of ice, and reduce cell damage. This may be mostly due to acoustic cavitation, which consists of the formation, growth and violent collapse of small bubbles or voids in liquids (Simal, Benkito, Sanchez, & Rosello, 1998), the cavitation bubbles acting as nuclei
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