Thermal transitions, extrusion, and heat-sealing of whey protein edible films

Edible films provide the opportunity to effectively control mass transfer among different components in a food or between the food and its surrounding environment, resulting in higher food quality and increased shelf life. Methods for production of edible films include solution-casting, compression-molding, and extrusion. The latter is widely used for the production of synthetic packaging films.; It was hypothesized that mixtures of whey protein isolate (WPI) powder and glycerol plasticizer could be extruded into flexible, transparent, heat-sealable films, if the appropriate formulations and processing conditions were used. Differential Scanning Calorimetry (DSC) suggested that increasing glycerol and/or moisture content in WPI-glycerol mixtures could result in lower processing temperatures for extrusion of WPI-glycerol-water mixtures into films. Whey protein-based sheets containing 45.8, 48.8, and 51.9% glycerol in dry basis (db) were obtained using a Haake-Leistritz co-rotating twin-screw extruder and a slit die. The extruder had a length-to-diameter ratio of 30:1. The extruder barrel temperature profile at the time of sheet formation was 20-20-20-80-110-130°C for sections 1--6, respectively. The die was also set at 130°C. However, due to viscous heat dissipation, the product temperature at the die was 143--150°C. The screw speed used to form films with different glycerol contents was 250 rpm.; Whey protein-based sheets containing 48.8% glycerol db were also obtained at screw speeds of 200, 225, 275, and 300 rpm. Feed rates were proportionally decreased or increased depending on the screw speed. Extruded sheets with 48.8% glycerol db, obtained at a screw speed of 250 rpm, were compression-molded into thinner films that could be heat-sealed using an impulse heat-sealer, at an effective jaw pressure of 293.31 kPa, a voltage of 15 V, and a cooling time of 4 seconds. A heat-seal could be obtained with an impulse time between 2 and 2.5 seconds, which corresponded to sealing temperatures between 164.6+/-5.1 and 204.0+/-5.4°C, as determined by a thermocouple.; Manufacture of whey protein-based edible films via extrusion would increase commercial potential and interest of the food and packaging industries for large-scale production of films. These films could be used as food wraps or heat-sealed to form pouches for dry foods and ingredients.