Bags allow colostrum to be pasteurised (optional), frozen (up to one year or refrigerated for 2 days) and quickly thawed to feed a calf within the critical time frame.  The tube or teat can be applied directly to the bag to allow most efficient and hygienic feeding.  Colostrum can be stored in bottles but this results in a very slow thawing process where the thawing time is longer than the critical time to feed the calf and normally requires the transfer of colostrum to another feeding vessel.