A thing or two about prawns Prawns in New Zealand are imported raw as we have no prawn fishery. They are snap frozen at sea and can be easily thawed at home, so never buy thawed prawns because you don’t know how long they have been thawed for. Maybe it’s only been a couple of hours, but maybe it’s longer. Why risk it? If they’re snap frozen at sea and you thaw them just prior to cooking, they will be as fresh as possible. Prawn size and weight If you have bought prawns before, you may have noticed a special numbering system in place. It seems counter-intuitive, but the smaller the number, the larger the prawn. Less is more! Well, less is big. U10 or U20 means under 10 or 20 prawns per kilo. These are the biggest prawns and also the most expensive. Handy guide to prawn sizes (per kilo) Extra large 10/20 also displayed as U10 or U20 Under 10 and Under 20 prawns per kilo Large 21/30 Under 30 prawns per kilo Medium …