The suitability of glass jars for heating in microwave ovens was studied. The aim of the study was to determine the effects of the type of foodstuff, the shape and the volume of the jars and the heating conditions (e.g. the type of microwave oven and the initial temperature) on the functionality and durability of glass jars heated in microwave ovens and on the heating result. Moreover, the behaviour of glass jars was compared with that of paperboard and plastic containers.
A total of 243 glass jars were heated during the the tests. None of the jars were broken, even when the contents were initially frozen or when all the jars were placed on a cold surface immediately after heating. However, splashing of the contents and/or jumping of the jar with a resulting loud clatter represented a major problem. Jumping of the jars and splashing of the contents depended on both the recipe and the consistency of the food and the shape of the jar, but less on the packaging material, the initial temperature or the heating time. The plastic and board containers also jumped and food splashed out of them. If glass jars or cylindrical/jar-like packages are to be used for the packaging of microwaveable foods, both the recipe of the foodstuffs and the shape of the jars need to be developed further.