by dougal » Sat Aug 26, 2006 10:36 am
I have corresponded with the Parson on this.
The graph above 10 lb comes from the same ancient MAFF publication that Mrs Grigson drew upon.
Below 10 lb, Parson quotes the same conventional wisdom as Mrs Grigson.
HOWEVER, he believes that the optional additional times are not required, and therefore although he refers to them in his text, he doesn't apply them when drawing the graph.
I understand him to agree that for practical application one should smooth the line (rather than treat the switchback as gospel) and anyway take the timings as indicative.
My presumption is that these times would be based on the meat starting at a cool ambient temperature, since the advice pre-dates home refrigeration.
A more modern, and precise, approach would be to monitor the internal temperature of the ham.