Edible and Biodegradable Whey Protein Films as Barriers in Foods and Food Packaging
Doctoral thesis, 2000
This thesis focuses on the characterization of whey protein films. The films were cast from heated aqueous solutions and dried in a climate room at 23 °C and 50% relative humidity for 16 h. The microstructure of the films was found to be dependent on the protein concentration, the plasticizers, and the pH. When the concentration increased, a more aggregated structure was formed, with a denser protein network and larger pores. This resulted in increased water vapor permeability (WVP) and decreased oxygen permeability (OP). The mechanical properties were measured and the strain at break showed a maximum at the critical gel concentration (cg) for pH 7-9, thus implying that the most favorable network structure regarding the ability of the films to stretch is formed at this concentration. When glycerol was used as a plasticizer instead of sorbitol, the microstructure was different, and the moisture content (MC) and WVP approximately doubled. When the pH increased from 7 to 9, a denser protein structure was formed, the strain at break increased, and the OP decreased.
The barrier against water vapor was improved by adding a lipid and making laminate and emulsion films. The laminate whey protein-lipid films decreased the WVP 70 times and the WVP value was in the vicinity of that observed for synthetic polymeric films such as ethylene-vinyl alcohol copolymer and low-density polyethylene. The WVP of emulsion whey protein-lipid films was half that of the pure whey protein films and was not affected by changes in lipid concentration, whereas increased homogenization led to a slight reduction in the WVP. The mechanical properties showed that the lipid functioned as a plasticizer for the emulsion films, and this effect increased with homogenization. The maximum strain at break was 117% compared with 50% for the less-homogenized emulsion films and 20% for the pure whey protein films. Phase-separated emulsion films were produced with a concentration gradient of fat through the film, but pure bilayer films were not formed.
The whey protein-sorbitol films were more stable during storage compared with whey protein-glycerol films. The MC of the latter decreased from 22% (2 days) to 15% (45 days) and was thereafter constant at 15% (up to 120 days). This caused an increased stress at break, a decreased strain at break, and an increased glass transition temperature (Tg) (-56 to -45 °C). The barrier properties were however unaffected. The MC of the whey protein-sorbitol films was constant at ~9%, which resulted in unchanged film properties and a Tg) around -13 °C.
water vapor permeability
critical gel concentration
glass transition temperature