Listeria isn't an issue as it's pasteurised, bacteria is the problem. I have had the misfortune (or fortune depending on how you look at it) of seeing inside the soft serve machines when they've needed repairing. It's not pretty. And by that I mean it's disgusting. Mould and I guess old milky sludge build up inside the tubes that run from where the ice cream is to the dispenser nozzle at the front. They top it up with new ice cream mix regularly but they don't flush out the tubes or clean inside the dispenser nozzle every time they fill the machine up. Same goes for the soft drink dispensers, except to me that's not as bad as a dairy machine. Not saying they all don't clean their machines regularly but I'd be surprised if many do. I've been scarred for life.