Yep, pasturising is easy, as Zeddy says. It's a real shame we can't get unpasturised any more. My mum used to leave unpasturised milk in bottles to go really sour - sludgy stuff at the top, clear at the bottom. Strained it through muslin, added a little salt, twisted the muslin tight, and hung it on the washing line for about a week, Result? Lovely crumbly cream cheese from the mycella organisms picked up from the air. Try that with pasturised, and the milk just rots in the bottle.