Mould and warm/cold stratifying seeds

I've got seeds that need stratifying before they'll germinate. Some need warm and then cold stratification. I had Canadian wild ginger (Asarum canadense) and Meum ( Meum athamanticum) seeds in damp vermiculite in freezer bags in a warm dark spot in the house. All the seeds went mouldy within a week and I assume are now dead. What can I do to prevent mould growth other than using clean fresh vermiculite from a newly opened packet?