The most superficial part of the linen fibre is the PCW, and that comprises hemicellulose as a major constituent. Hemicellulose has a lot of pentose sugars, which are chemically reactive, more so than the hexose sugars of starch and cellulose, and known to enter freely into Maillard reactions. Maybe the linen provided the sugar for the Maillard reaction.
… on the shroud (or misnoma-shroud). Colin Berry teases it out a bit for us:
This blogger has already been accused of plagiarizing Rogers’ ideas (in seeing a role for Maillard reaction products, albeit between reducing sugars and proteins of white flour, and needing an exceedingly hot iron to get the colour). Well, I’m about to make things even worse for myself – by narrowing the gap between my medieval model and the pro-authenticity 1st century tomb scenario of Rogers. It involves volatile amines, those fishy smelling things with the general formula R-NH2 (primary amine) where R is an alkyl group, e.g. CH3, C2H5, or, if a secondary amine, R-NH-R’, or a tertiary amine, R-N(R’)-R”. What you may ask! We know where the amines are implicated in the Rogers’ model (putrefaction of a corpse). How can amines be implicated in a white-flour model?
Well, it’s a long shot, but here we go. The yellow-brown image has been described here as a Maillard reaction product, formed between reducing sugars and proteins. But there’s a problem. The “Shroud” image was tested by Adler et al for protein – none were found. But my image appears to have two components – an outer one that looks and feels thick, and can be reduced by washing, brushing etc, and a more resistant one that survives those treatments, and seems more like an intrinsic part of the linen fibres. What might have happened to produce the latter. Well, there’s a little protein in linen fibres, and one might propose that had reacted with reducing sugar, and that the Maillard product formed had failed to react as protein. But one instinctively dislikes qualiofying assumptions. Might there be an alternative explanation? Yes, there is. The most superficial part of the linen fibre is the PCW, and that comprises hemicellulose as a major constituent. Hemicellulose has a lot of pentose sugars, which are chemically reactive, more so than the hexose sugars of starch and cellulose, and known to enter freely into Maillard reactions. Maybe the linen provided the sugar for the Maillard reaction. But where did the amine come from? It might have been the protein of the flour or linen, especially the epsilon amino group of lysine (not involved in peptide bond formation). But there’s an intriguing alternative. Enter volatile amines. When one adds cold limewater to white flour there’s an immediate strong fishly odour. So there’s an amine precursor there that is easily released by alkali. Maybe it’s released by heat also, even at lower pH closer to neutrality. Maybe it’s that amine that reacts with the pentose sugars of the linen PCW to produce the ‘resistant’ image that survives washing etc, and that does NOT test positive for protein.
What might be the source of the free amine? Am not sure. It might be glutamine, with terminal -CONH2. It might be polar secondary or tertiary amine groups of phospholipids (lecithin, phosphatidylethanolamine etc). Much food for thought (maybe a few experiments can help reduce the search options).