Why didn't it tenderize?

I'm not sure this makes a difference... 125F sounds a bit low, I would have tried 55 Celsius (131 F) instead. <br />Also, note that according to <a href="http://en.wikipedia.org/wiki/London_broil">http://en.wikipedia.org/wiki/London_broil</a> there is some confusion as to what cut of beef the London Broil is, if the cut you got was really Beef Top Round, the charts say 55C for 1 to 3 days for Medium rare.

I recently cooked brisket at 55 Celsius, which is 131 Fahrenheit, for 72 hours. A lot of juice did come out of the meat, but I wouldn't call the result dry by any means, and the result was a nice medium-rare, extremely tender dish. I didn't brine it at all as Baldwin recommends though, but I will be trying that next time. <br /> <br />Have you checked your temperature with a calibrated thermometer?

The key with "London Broil" is how you cut it. Cut in thin slices against the grain, that'll make all the difference.

First of all, I think it should be emphasized that the minimum temperature for long cooking times to prevent spoilage is 131 degrees F. And 131 degrees F should only be attempted if one is assured that temperatures are accurately controlled. <br /> <br />Round steak is challenging for sous vide. It is such a tough cut, and is unforgiving. I have never been pleased with the 72-hour recipes for round steak sous vide. After 72 hours it has always tasted dry and chalky to me. I would prefer 24 hours at 131 degrees F for round steak. <br /> <br />Cooking temperatures below 131 F should only be attempted when the cooking time is less than the time it takes the meat to spoil.

I agree that the term "london broil" is a description of a cooking method. However, in my case it is is meat product sold as a very lean cut of meat with virtually no visible fat. I will try a higher temp one time and see what happens. Has no one addressed this issue or am I a discoverer? thanx

In this case it is not a matter of how the meat was cut. I cut it across the grain and on a bias so it was not even straight up and down. The muscle should have only fractions of an inch of " in line" fibers. So it was 90 degrees across/against the grain and then 45 degrees from the vertical for the second cut. It can't get much more against the muscle fiber than that!

From memory, collagen doesn't start to convert to gelatin until 130F (and then slowly), so I think your temperature was just a touch low to allow that conversion to take place. The absence of fat in the cut would make the result potentially dry unless you add additional fat, but I think that's a different issue.

Another attempt but this time using a bottom round roast. Basically the same results as before. I cooked at 140 degrees for 40+ hours and the meat was moderately tender but also dry. The vacuum bag was quite full of juice and therefore the reason for the dryness. I made a simple gravy and put sauteed onions in that to salvage the meat. It was good but once again, nothing to brag about. I am becoming convinced that you must have a juicy and sort of tender cut from the start to make this work. I am now going to try the same cut at 130 degrees and see what happens. I will let both of my loyal readers know what happens!

I just concluded a 12 hour cook of another london broil at 140 degrees. It is somewhat more tender but now it is pink inside instead of the red that I prefer. Perhaps a longer cook would have tenderized it more and there was enough gain to warrant another longer cook time at this temp. I will keep you posted.

I ran the SVS all day to be certain of a stable temperature. My thermapen and the temp shown on the SVS were within two degrees for what that is worth. I don't think a two degree swing, which by the way was up and down would cause an overcook situation. Do you?

I would try to marinade the london broil first, then cook it at a desire temp it will be quite tender

This site says 140 degrees F is the magic number <br /><a href="http://forums.egullet.org/index.php?/topic/99923-collagen-%26gt%3B-gelatin-temperature/">http://forums.egullet.org/index.php?/topic/99923-collagen-%26gt%3B-gelatin-temperature/</a> <br />that would also negate one reason to sous vide as the meat will be cooked to an overdone state. However, I will give it a try. Thanks for the tip as I never considered looking for the right temp and just assumed the SVS would do all the legwork.