Sterilise-jars-microwave-oven, pre-heat the oven to 280 degrees fahrenheit or 160 degrees celsius. the hot oven will be used to dry the jars once they have been sterilized in the microwave. fill jars halfway with cold water. place the jars in the microwave.. Arrange the jars on the oven shelf, making sure the jars are not touching each other. close the oven door and sterilize the jars for a minimum of 20 minutes. using thick oven mitts, remove each jar from the oven as needed onto a heatproof mat or heat pad, making sure you fill while the jam or preserve is as hot as the jar.