Edible and Biodegradable Whey Protein Films as Barriers in Foods and Food Packaging

Sammanfattning: 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 (c_g) 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 (T_g) (-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 T_g) around -13 °C.