Ultrasonic effect on physicochemical and functional properties of a-lactalbumin Anet Re  zek Jambrak a, * , Timothy J. Mason b , Vesna Lelas a , Greta Kres ˇic ´ c a Faculty of Food Technology and Biotechnology, University of Zagreb, 10000 Zagreb, Pierottijeva 6, Croatia b Sonochemistry Centre, Faculty of Health and Life Sciences, Coventry University, Priory Street, Coventry CV1 5FB, UK c Faculty of Tourism and Hospitality Management, Department of Food and Nutrition, University of Rijeka, Primorska 42, P.O. Box .97, 51410 Opatija, Croatia article info Article history: Received 15 December 2008 Received in revised form 24 August 2009 Accepted 1 September 2009 Keywords: Ultrasound treatment a-Lactalbumin Physicochemical properties Functional properties Particle size Molecular weight abstract Ultrasound is the sound whose frequency is too high for humans to hear which is within the frequency range of 20 Hz–20 kHz, and the frequency of ultrasound is above 20 kHz. The aim of this study was to observe the effect of ultrasound and sonication on a-lactalbumin (a-LA) with a view to improving its physicochemical and functional properties. In this work both low-intensity ultrasound (500 kHz bath) and the high-intensity ultrasound (20 kHz probe and 40 kHz bath) were used. Ten per cent wt (g g 1 dry matter) protein model suspensions of a-lactalbumin (a-LA) were treated with ultrasound probe (20 kHz for 15 and 30 min) and ultrasound baths (40 kHz and 500 kHz for 15 and 30 min). Changes in pH values, electrical conductivity, solubility measurements, foaming properties, as well as rheological and freezing-thawing properties have been examined. The protein fractions of a-lactalbumin were analyzed before and after ultrasound treatment by SDS-PAGE (sodium dodecyl sulfate-poly- acrylamide gel electrophoresis). The result showed that pH did not change significantly upon ultrasound however conductivities increased significantly after 20 kHz sonication. Electrical conductivity decreased significantly for ultra- sound treatments in baths at 40 kHz and 500 kHz for all samples. Solubility increased significantly for all samples at 20 kHz. Foam capacities and foam stabilities were improved after ultrasound treatments for both 20 kHz and 40 kHz treatments. Foaming properties were not improved for protein model suspensions for 500 kHz treatments. The molecular weight of the protein decreased significantly after ultrasound treatments both using a 20 kHz probe and 40 kHz bath. The flow behaviour of a-lactalbumin was observed to be shear-thickening after all treatments. Apparent viscosity data calculated with power law equation (R 2 ¼ 0.983–0.999) have not been changed significantly after all treatments. A remarkable decrease of initial freezing point was obtained after 20 kHz treatments. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Application of the low frequency high-energy power ultrasound (10–1000 W cm 2 , with the frequency range from 20 to 100 kHz) in the food industry is relatively new and has not yet been explored until recent years (Mason, 1998; McClements, 1995). Various areas have been identified with great potential for future development, e.g. freezing (Aparicio, Otero, Guignon, Molina-Garcı ´a, & Sanz, 2008), drying (Jambrak, Mason, Lelas, Herceg, & Herceg, 2008), extraction (Vilkhu, Mawson, Simons, & Bates, 2008), sterilization (Cameron, McMaster, & Britz, 2008) etc. For these reasons high- intensity ultrasound is considered to be a potential unit operation for the non-thermal processing of food. In broader applications ultrasound is also used in emulsification and dispersion, to improve chemical reactions and surface chemistry (sonochemistry) and to influence crystallization processes (Knorr, Ade-Omowaye, & Heinz, 2002). Ultrasound is able to produce these effects through the phys- ical, mechanical and chemical results of acoustic cavitation a process which involves the formation, growth and violent collapse of small bubbles in liquid as a result of acoustic pressure fluctuation. Cavitation can accelerate chemical reactions, increase diffusion rates, disperse aggregates, break down small particles and polymeric materials such as enzymes and destroy microor- ganisms. The lower frequency range of ultrasound is normally used to produce cavitation because at very high frequencies, i.e. above 1 MHz, cavitation becomes more difficult and above 2.5 MHz cavitation does not occur (Sala, Burgos, Condon, Lopez, & Raso, 1995). These higher frequencies (5–10 MHz) are of use as an analytical technique for quality assurance, process control and non-destruc- tive inspection and have been applied to determine food properties, * Corresponding author. Tel.: þ385 1 4605 035; fax: þ385 1 4605 072. E-mail address: arezek@pbf.hr (A.R. Jambrak). Contents lists available at ScienceDirect LWT - Food Science and Technology journal homepage: www.elsevier.com/locate/lwt 0023-6438/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2009.09.001 LWT - Food Science and Technology 43 (2010) 254–262