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264 FOOD PACKAGING TECHNOLOGY Store Heat Eat Figure 8.16 PET lined paperboard trays (courtesy of Iggesund Paperboard). two stages. This type of tray can be used for a ready meal for frozen or chilled distribution and reheating at up to 200Â°C in either a microwave or a radiant heat oven. It could have a peelable heat sealable printed paperboard or plastic lid and be packed in a paperboard sleeve or carton (Fig. 8.16). The early developments in microwave foods provided convenience and rapid heating. It was not suitable for products where browning or a degree of crispness was expected. Developments in microwave food formulations, which improved their performance in the microwave oven, and the use of susceptors in the packs has widened the range of foods suitable for this application. Susceptors absorb microwave energy and heat food rapidly, mainly by contact, and induce crispness and localised browning. Susceptors usually comprise aluminium metallised plastic film, such as PET, laminated to paper or paperboard. Inconel, nickel/chromium, susceptors can be used to induce even higher temperatures. The use of susceptors in this way is an example of active packaging. Cartons may be lined by the carton maker with a flat tube of a flexible barrier material which is inserted during carton manufacture. The flexible material is usually heat sealable â examples include paper/aluminium foil/PE and laminations of plastic films. The lined cartons are supplied folded flat to the packing/filling machine. One end of the liner is heat sealed and after filling, the other end is sealed and the carton flaps closed. This type of carton is used for ground coffee, dry foods and liquids. A lined carton may be fitted with a plastic hinged lid incorporating a tamper evident diaphragm. In another type of lined carton, flat carton blanks and a roll of the material to be used as the liner, frequently bleached kraft paper, are supplied to the packing machine. Firstly, the paper is formed around a solid mandrel. The side seam and base is either heat sealed or glued with adhesive, depending on the specification. The carton is then wrapped around the liner with the side seam and base sealed with adhesive. The product is filled and both liner and carton sealed/closed. This type of pack is suitable for the vertical filling of powders, granules and products such as loose filled tea. Folding cartons can have windows or plastic panels for product display, e.g. spirits.