824—JOURNAL OF FOOD SCIENCE—Volume 61, No. 4, 1996 Milk Protein-based Edible Film Mechanical Strength Changes due to Ultrasound Process R. BANERJEE, H. CHEN, and J. WU ABSTRACT The effects of ultrasound frequency, acoustic power, and exposure time on the functional properties of whey protein concentrate and sodium caseinate ﬁlms were examined. Average tensile strength of the ultra- sound treated caseinate ﬁlms was 224% higher than that of the control. The ultrasonic process was more effective on sodium caseinate than whey protein concentrate ﬁlm. Resistance to puncture was improved for both types of ﬁlms treated at an acoustic power of 5.22W. Increased exposure time resulted in stronger ﬁlms. Elongation at break, water vapor permeability, and moisture content of ﬁlms were not affected by the treatment. Ultrasound showed potential for improving mechanical strengths of milk protein ﬁlms. Key Words : ultrasound, milk proteins, edible ﬁlms, mechanical strength, water vapor permeability INTRODUCTION ULTRASOUND TECHNOLOGY has been widely applied in medical ﬁeld for diagnostic, therapeutic and surgical uses (Fry, 1978). Ultrasound has also found applications in food science, ranging from determining rheological properties (Lima and Sastry, 1990) to monitoring microbial growth in aseptic packages (Gestrelius et al., 1993). The use of ultrasonic homogenization to produce oil in water emulsions in the dairy, pharmaceutical, cosmetic and mineral oil industries was reported (Brown and Goodman, 1965). Ultrasound waves can reduce particle size. Some hypotheses suggest a relationship between ultrasonic wavelength and par- ticle size after ultrasound treatment, i.e., smaller wavelengths yield smaller particle sizes (Brown and Goodman, 1965). Others hypothesize that acoustic cavitation (bubble activity caused by ultrasound) is the most important and pertinent mechanism in the application of high power ultrasound (Suslick, 1988). A comprehensive report has been published on the basic theories of ultrasound by the U.S. Department of Health and Human Services (Anonymous, 1982). The potential use of milk protein products to form edible ﬁlms has been reported and their barrier and mechanical properties were studied (Krochta, 1992; McHugh and Krochta, 1994; Chen et al., 1993b; Banerjee and Chen, 1995a, 1995b). Some milk protein ﬁlms showed similar or lower mechanical strengths than ﬁlms from wheat and soy proteins, and cellulose based ﬁlms (Chen et al., 1993a; Banerjee and Chen, 1995a; Gennadios and Weller, 1992). Banerjee and Chen (1995b) discovered that microﬂuidization, an ultra-homogenization process, could improve the mechanical strength and resistance to water vapor transmission of milk pro- tein ﬁlms by reducing particle size and therefore enhancing mo- lecular crosslinking. Although microﬂuidization processing improved ﬁlm strengths, the technique was not aseptic because of the mechanical parts of the equipment in contact with ﬁlm- forming liquid. Authors Banerjee and Chen are afﬁliated with the Northeast Dairy Foods Research Center, Dept. of Animal & Food Sciences, and author Wu is with the Dept. of Physics, The Univ. of Vermont, Burlington, VT 05405. Direct inquiries to Dr. H. Chen. Ultrasonic homogenization can produce uniform solutions and dispersions with reduced particle size without mechanical con- tact with the ﬁlm-forming liquid. The suitability of such ﬁlms for edible applications would predominantly depend on micro- bial and chemical quality. Thus, the ﬁlms produced should be sterile. We hypothesized that ultrasonically reduced particle size in a ﬁlm-forming solution could result in increased molecular interaction and produce a ﬁlm with greater rigidity and com- pactness. Our study explored the potential application of the ultrasonic process in improving functional properties of protein- based edible ﬁlms. Speciﬁc objectives were (1) to develop methodology to apply ultrasound waves to ﬁlm making; (2) to study the effects of ultrasound treatment (frequency, acoustic power and exposure time) on mechanical properties and water vapor permeability of milk protein ﬁlms; (3) to examine changes in ﬁlm microstructure due to ultrasound treatment. MATERIALS & METHODS Materials Whey protein concentrate (WPC, 76.6% protein) was obtained from Vermont Whey Co., (St. Albans, VT), and sodium caseinate (SC, Alan- ate 191, 90.7% protein) from New Zealand Milk Products, Inc. (Santa Rosa, CA). Glycerin (OPTIM, 99%), purchased from Dow Chemical Co., Inc., (Midland, MI), was used as a plasticizer. Methods Film solution. The method of Strange and Konstance (1991) was adapted for preparation of 10% WPC and SC dispersions. Measured quantities of protein products (adjusted to percentage dry solids) were added into an ice-water (1:1) mixture previously ground for 1 s in a blender (Hamilton Beach Model 554 CK, Washington, NC). Protein and ice-water mixtures were then ground for 15 sec. The solution was hand mixed for 30 sec after which it was reprocessed for 15 sec. Solutions were mechanically stirred for 8 min and degassed by vacuum. The WPC solution was thermally processed at 75°C for 0.5 hr after pH adjustment to 6.6 using 2M NaOH (Banerjee and Chen, 1995a). Glycerin was mixed into the solutions at 1:2 (w/w) ratio (glycerin: protein). Film casting Control. Film forming solutions were poured onto a 9.1  10 -2 m diameter polystyrene petri dish at 1.2g solids/plate. They were dried at room temperature (23°C) for 18 hr. These ﬁlms were designated as controls. Ultrasound treatment. A laboratory ultrasound system consisting of a Hewlett Packard synthesizer, ENI power ampliﬁer, and a PZT trans- ducer of 7.2  10 -2 m diameter was used for treating the edible ﬁlm solutions. The milk protein solutions were poured onto a 9.1  10 -2 m petri dish at 1.2g solids/plate and placed on the ultrasound transducer. The assembly was placed in a large tank ﬁlled with 40L distilled water to dissipate thermal energy generated by ultrasound waves. Water also served as coupling between the ultrasound transducer and the petri dish. Since the diameter of the petri dish was greater than that of the trans- ducer, we expected that most of the ultrasound energy generated by the transducer was coupled into the solution through the petri dish. An ex- periment was designed to study the effects of frequency (168 kHz and 520 kHz), acoustic power (low, medium, and high), and exposure time (0.5 and 1 hr) on mechanical properties and water vapor permeability of the ﬁlms. To further assess the effects of ultrasound frequency, 860 kHz