Compression Heating of Selected Polymers During High-Pressure Processing

Pressure-assisted thermal processing is an innovative technology, which has been discussed for sterilization of low-acid foods for over a decade. This approach can result in improved product quality from rapid temperature increase during pressurization and temperature decrease during depressurization. Five plastics (high-density polyethylene, low-density polyethylene, polyamide, polypropylene and polystyrene) were evaluated for their compression heating characteristics from initial temperatures of 25, 50 and 75 degrees C. Samples were pressurized to 600 MPa with a laboratory-scale high pressure kinetic testing unit. Temperature and pressure were recorded as a function of time. Samples showed temperature increases as a function of pressure and initial temperature. For initial temperatures of 25, 50 and 75 degrees C, low-density polyethylene showed transient temperature increases of 5.650.03 degrees C, 6.44 +/- 0.07 degrees C and 7.41 +/- 0.07 degrees C per 100 MPa pressure increase, respectively. Polystyrene had the least transient temperature increase of 4.31 +/- 0.02 degrees C, 4.39 +/- 0.02 degrees C and 4.53 +/- 0.02 degrees C per 100 MPa pressure increase, respectively. All polymers tested generally have greater heat of compression than water, protein and carbohydrates, but lower than those of fatty food substances. Practical ApplicationsThermal gradients in pressure vessels can cause nonuniformity in the microbial and enzyme inactivation, as well as nutrient and organoleptic quality retention, achieved in foods during pressure assisted thermal processing. Packaging materials, insulation and carrier baskets may be made of plastic polymers. Only limited studies documented heat of compression values of different polymers as a function of pressure and heat. Knowledge gained from this study will help to improve temperature uniformity within a pressure vessel. Improved understanding compression heating behavior of plastic polymers will aid in pressure vessel design and implementing and validating pressure processes.