Process efficiency in polymer extrusion: Correlation between the energy demand and melt thermal stability

Thermal stability is of major importance in polymer extrusion, where product quality is dependent upon the level of melt homogeneity achieved by the extruder screw. Extrusion is an energy intensive process and optimisation of process energy usage while maintaining melt stability is necessary in order to produce good quality product at low unit cost. Optimisation of process energy usage is timely as world energy prices have increased rapidly over the last few years. In the first part of this study, a general discussion was made on the efficiency of an extruder. Then, an attempt was made to explore correlations between melt thermal stability and energy demand in polymer extrusion under different process settings and screw geometries. A commodity grade of polystyrene was extruded using a highly instrumented single screw extruder, equipped with energy consumption and melt temperature field measurement. Moreover, the melt viscosity of the experimental material was observed by using an off-line rheometer. Results showed that specific energy demand of the extruder (i.e. energy for processing of unit mass of polymer) decreased with increasing throughput whilst fluctuation in energy demand also reduced. However, the relationship between melt temperature and extruder throughput was found to be complex, with temperature varying with radial position across the melt flow. Moreover, the melt thermal stability deteriorated as throughput was increased, meaning that a greater efficiency was achieved at the detriment of melt consistency. Extruder screw design also had a significant effect on the relationship between energy consumption and melt consistency. Overall, the relationship between the process energy demand and thermal stability seemed to be negatively correlated and also it was shown to be highly complex in nature. Moreover, the level of process understanding achieved here can help to inform selection of equipment and setting of operating conditions to optimise both energy and thermal efficiencies in parallel. 

Combined effect of high pressure processing and antimicrobial packaging to control the growth of Listeria monocytogenes on ready-to-eat chicken breast

In the European Union, food is considered safe with regard to Listeria monocytogenes if its numbers do not exceed 100 cfu/g throughout the shelf-life of the food. Therefore, it is important to determine if a food supports growth of L. monocytogenes. Challenge tests are laboratory-based studies that measure the growth of L. monocytogenes on artificially contaminated food stored under foreseeable conditions of transportation, distribution and storage. The aim of this study was to elaborate and optimize a user-friendly protocol to perform challenge tests on food and to apply it to determine whether growth of L. monocytogenes is supported during the production and distribution of a potentially risky food i.e. mushrooms. A three-strain mixture of L. monocytogenes was inoculated onto three independent batches of whole mushrooms, sliced mushrooms, mushroom casing and mushroom substrate at a concentration of about 100 -1000 cfu/g. The batches were incubated at potential abuse temperatures, as a worst case scenario, and at intervals during storage L. monocytogenes numbers, % moisture and pH were determined. The results showed that the sliced and whole mushrooms supported growth of L. monocytogenes while mushroom casing allowed survival but did not support growth. Mushroom substrate showed a rich background microflora able of growing in Listeria selective media which hindered enumeration of L. monocytogenes. Combase predictions were not always accurate, indicating that challenge tests are a necessary part of growth determination of L. monocytogenes.