Cooking time for small lamb breasts

masplin

Hi. I've found a receipe in that says 1.3-1.5Kg deboned lamb breast  rolled up takes 6-7 hours at 110C. we cant get lamb breasts that big at this time of year in UK so I've sourced 3 x 500g breasts.  I can't work out how long they need to cook?  I'm guess not a 1/3rd of the time i.e. 2-2.5hrs seems to short.    Is it the difference in the ratio of surface area/volume i.e. a joint 1/3rd the length same diameter has a surface area 1.3x bigger so cooking time 4.5-5.5 hours?

Be really useful to know how time changes with size as often cant buy a large a joint as US receipe.

Appreciate any advice
Mike