Barrier Plastics, Processing of Wheat Gluten and A New Method to Measure Permeability

Sammanfattning: One way to extend the shelf life for foodstuff is to usemodified atmosphere packaging (MAP). It is important that MAPare gas tight or in some rare cases permselective to preservethe right gas mix. To be able to determine how the gas contentchanges it is essential to know the gas permeability of thewhole packaging with effects from seals, folds and possibleplasticisation effects from the products.In the first part of the study a new technique fordetermining oxygen and carbon dioxide permeability of flexiblepackaging was developed. The method was tested on very-lowdensity, low density and high density polyethylene pouches. Thepouch head-space and pouch volume of carbon-dioxide-filledpouches were measured as a function of time until anequilibrium pouch gas composition was established. Permeabilitywere obtained from the rates of carbondioxide loss and oxygenuptake. HSP-method permeability values were in good agreementwith those obtained from traditional flat-film permeabilitytechniques. The HSP-method was found to be a valuable tool fordetermining and quantifying changes in carbon dioxide andoxygen pouch barrier properties due, for example, to theexposure to olive oil or due to the existence of poorwelds.When use of MAP are growing the demands for suitable andcheap gas barrier materials are increasing. Films made fromwheat gluten, which is a by-product from the manufacture ofethanol and sweeteners, has low oxygen permeability propertiesin dry conditions and that makes it interesting as a packagingbarrier. Several studies have been performed on solution castedwheat gluten films, but to get a new material on the market itis of great importance that the material is adapted to thepresent process equipment, such as plastic extruders. Thesecond part of the study deals with a way to expand theprocessing window of wheat gluten plastisazed withglycerol.The temperature -processing window of glycerol-plasticizedwheat gluten was increased by the use of a well-known scorchretarder, salicylic acid. It was possible to extrude 30 wt.%glycerol-wheat gluten films with a die-head temperature as highas 135°C, rather than 95°C, by incorporating only 1%salicylic acid. Small effects of shear induced heating duringextrusion suggested that the acid acted as a lubricator.Chemilumiscence, in turn, showed that it also had an effect onthe heat-induced chemical changes of gluten. Tensile tests onextrudates revealed that the substantially enhanced processingtemperature was achieved to the expense of a slightly reducedductility. Tensile tests were obtained on compression-moldedsamples containing 30wt% glycerol and an additional 2.5-5% ofglycerol, salicylic acid or sulfonamide. The tensile tests,performed during the first 30 min after molding, did not revealany scorch retarding effects. However, the retarders did reducethe level of aggregation and crosslinking as compared toglycerol, observed by that fact that the fracture stress wasconsistently lower for salicylic acid and sulfonamide comparedto glycerol. The complex shearmodulus increased more slowlyabove 110-120°C with increasing salicylic acid content,indicating that it did have a scorch retarding effect.