I just bought a food Saver from Walmart. Is it necessary to freeze food ( pulled pork) before you vacuum seal? It seems redundant to have to freeze the food then vacuum and seal and refreeze. Also I am having a little trouble getting a seal every time. It will vacuum fine and seem to seal but then loses it's seal. I know it's simple but my technique needs improving. At first I thought there was something wrong with the machine, but I think it is me. I would welcome any advice.