You can sterilize empty glass ware and metal objects in a simple oven at 220 oC for 3 hours. But unfortunately for media steam sterilization in a pressure cooker or an autoclave is the only way. Media has to get to a temperature of 121 oC for atleast 15 to 20 minutes to be completely sterilized and without without pressure that temperature cannot be attained. One other way which I have personally used is the use of a microwave oven. First steam the microwave by boiling some water in it for about 5 to 10 minutes. Then I added media (150 ml) to my bottles and then microwaved them on high (900 w) for 3 minutes. It worked for my dendrobium and phal bud cultures, but the test was performed on a statistically insignificant sample size (8 bottles) so the results do not infer much. Hope it helps.