I have baked a 2.5kg smoked gammon ham as per Nigella's Treacle Ham recipe. I've used a meat thermometer but it never reached 76C despite being in oven for 21 hours. I've read on other sites that internal temp of 71C is OK (mine reached 72c) on 80C fan, so took it out as afraid it would overcook. I'm wonderng what advice you could give for future years in terms of knowing it is definitely cooked?
Nigella's Slow Cooked Black Treacle Ham (from SIMPLY NIGELLA) cooks a gammon (a cured but uncooked ham) in a very low oven for between 12 and 24 hours. The ham is given an inital 30 minutes at a high heat to help to kick start the cooking process. The low temperature combined with a sealed foil package plus the fat in the ham means that the ham remains moist despite the long cooking time.
Most food-related pathogens are killed at temperatures above 75c (165F), so ideally you should cook a ham to an internal temperature of at least 75c. A temperature probe is the easiest way to confirm this. If you are worried then we would suggest turning the oven back to 200c/180c fan/400F for the last 30 minutes, as this shlould bring the ham close to 75c and if you rest the ham for a further 30 minutes after this second blast of high heat then the internal temperature should rise to 75c. If you have a meat thermometer that can remain in the gammon during cooking, then it is worth noting that the UK Food Standards Agency website states that as long as the internal temperature has been sustained at 70c or above for at least 2 minutes then this should ensure that food-related bacteria have been eradicated. If you don't have a meat thermometer then you can insert a metal skewer into the centre of the ham, leave it there for 30 seconds and then remove it. Very carefully test the tip of the skewer, it should feel very hot to the touch, but be careful not to burn yourself doing this. However, this is not as reliable as using a meat thermometer.