Effects of pH and the Gel State on the Mechanical Properties, Moisture Contents, and Glass Transition Temperatures of Whey Protein Films

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Abstract
The mechanical properties, moisture contents (MC), and glass transition temperature (Tg) of whey protein isolate (WPI) films were studied at various pH values using sorbitol (S) as a plasticizer. The films were cast from heated aqueous solutions and dried in a climate chamber at 23 °C and 50% relative humidity (RH) for 16 h. The critical gel concentrations (cg) for the cooled aqueous solutions were found to be 11.7, 12.1, and 11.3% (w/w) WPI for pH 7, 8, and 9, respectively. The cooling rate influenced the cg, in that a lower amount of WPI was needed for gelation when a slower cooling rate was applied. Both cooling rates used in this study showed a maximum in the cg at pH 8. The influence of the polymer network on the film properties was elucidated by varying the concentration of WPI over and under the cg. Strain at break (εb) showed a maximum at the cg for all pH values, thus implying that the most favorable structure regarding the ability of the films to stretch is formed at this concentration. Young's modulus (E) and stress at break (σb) showed a maximum at cg for pH 7 and 8. The MC and εb increased when pH increased from 7 to 9, whereas Tg decreased. Hence, Tg values were −17, −18, and −21 °C for pH 7, 8, and 9, respectively. E and σb decreased and εb and thickness increased when the surrounding RH increased. The thickness of the WPI films also increased with the concentration of WPI. Keywords: Whey protein; critical gel concentration; mechanical properties; moisture content; glass transition temperature; dynamic mechanical analysis