How is very white wool achieved?

I understand that 100% wool can not be bleached. Therefore, I assume that white wool is made from the most white of fleeces available. But, even there I would think that the fleece would have a cream or slight yellow appearance to it. So how do companies manage to make their wool so white? I have not been able to find anything on line about this.

http://www.pburch.net/dyeing/dyelog/B1063361308/C1691090068/E47264978/

http://www.prochemical.com/directions/BleachingWool.htm

Google Books directions from 1883

http://books.google.com/books?id=gIsAAAAAMAAJ&printsec=frontcover&dq=how+to+bleach+wool+white&source=gbs_similarbooks_s&cad=1#v=onepage&q=&f=false