I was mooching around on brewing blog's yesterday and came across this http://poppylandbrewer.blogspot.co.uk/2016/01/making-yeast-ring.html It's something I've never even heard of until yesterday "If you brew only occasionally - say every few months or even just once a year - how do you keep your yeast until it is needed? It won't keep indefinitely in the fridge. Most of us will just order some fresh or dried yeast from the supplier every time but there are some yeasts that are unobtainable that way, while in the past that simply wasn't an option. In Norway, they have utilised the natural property of yeast cells that when they dry out they go into suspended animation until woken up again by re-hydration. It happens all the time in nature and it is a godsend for the occasional brewer. Traditional Norwegian brewers have found several ways of achieving the goal of drying a reasonable quantity of yeast, keeping it safe and then re-hydrating it for later use."