When I see a recipe that says —cook at 139 degrees for 26-30 hours, does that mean the meat is ready after 26 hours, but can leave in to the 30 mark before changes start in the texture of the meat?
Yes. Granted, there may be subtle differences in cooking recommendations. I’ve seen recipes where you have a 10 hour window, and red goes for the same cut and size from the same source with a 4 hour window. I’d use threw recipe more as a guideline rather than a mandatory operation.
When I am trying a new recipe I start in the middle of the recommended time. I have seen some where the range is from 24-72 hours. I tried short ribs at various times and the changes were subtle. They were more tender at 72 hours but not by much.
NB, the change in texture of meat during SV cooking is linear from start to finish.
In your example recipe the meat’s texture was judged acceptable by the recipe writer between 26 hours and 30 hours of cook time. After 30 hours the texture was judged less acceptable and would be increasingly so with longer cook times.
We all have different opinions of quality so it’s useful to journalize a detailed record of each SV cook noting the outcome and making any recommended adjustments for future use of the recipe.