I don't translate food and drink, but got interested in the word association. I found that some sources use
affranchi as if it meant "cut lengthwise":<p>
l’affranchi consiste à séparer, au sein d’un même muscle, les parties tendres et les parties dures. (
https://www.agrireseau.net/bovinsboucherie/documents/qualite... ) Could breastbone mean "meat on the breastbone" here or could this be some general meaning?<p>
Rond de tranche affranchi. Rond de tranche paré et épluché, « nerf » central retiré, affranchi côté rotule. Can't copy the link for this one, but just copy the text in your search engine. It has a lot of gory pictures and instructions about how to cut things lengthwise.<p>And this one mentions the same length as your source text:
affranchi à l’extrémité côté rotule sur 3 cm (
https://www.agrireseau.net/bovinsboucherie/documents/qualite... )<p>There is also a Wikipedia page saying that
affranchi is about a type of meat that is less fat than another one, but I don't think it's relevant.<p>And why would anyone want to cut 3cm off the length of a breastbone is beyond me.