In the days before detergents and detergent-containing cleaning aids, there would be a bottle of ammonia solution under the sink.
It was some years before I learned how it worked as a cleaning agent (it’s apparently/allegedly to do with creating alkaline conditions that turn fat and oil into soap – it’s a ‘saponifying agent’). Do you really think it would have been used if exposure to ammonia fumes caused latent yellow coloration of fabrics etc?
Just think: starch used to be used for shirt collars etc, indeed entire items like table napkins? What would the man of the house have said if his collars turned yellow because missus had been using ammonia at the sink?
If ammonia reacted appreciably with starch or other linen constituents to make white go yellow -or worse still brown- methinks our parents and grandparents would have quickly cottoned on – no pun intended – and the domestic market for ammonia would have fallen flat on its face.
The thing that protects reducing sugars – whether from broken down starch or other sources – is the relatively high temperature needed to get a Maillard reaction going – at least 50 degrees C, but preferably higher – much higher. Superheated steam is used to make the brown coloring agent in Coke from corn sugar and ammonia – a typical Maillard product: typical room temperature, or even the heat of the kitchen, is/was simply not high enough to produce visible amounts of those yellow polymeric substances that we lump together with toasted food under the heading of ‘Maillard reaction products’.
Rogers’ Maillard hypothesis was based on a fantasy – allied with wishful thinking – namely that a particular length of fabric was impregnated and indeed a rich source of reducing sugars, and that the latter are an easy target for gaseous amines.
For a thermochemist to have proposed that a Maillard reaction could produce an iconic image at environmental temperatures, without an obvious source of heat ( except a brief hypothesised spell of post-mortem thermogenesis to 42 degrees max) was remarkable to say the least. Maybe he was too focused on finding chemistry that would provide a ‘low-temperature’ scorch-like image – to avoid having to think about high-temperature scorches that did not fit with the scenario of a 1st century tomb. High-temperature scorching can be easily accommodated, needless to say, into a medieval narrative – with or without intentional forgery as the first step- the Mark 1 image, ‘negative’ and seemingly ‘branded-on’, being simply that of a naked un-Christ-like unbloodied man – like the one on the Lirey Pilgrim’s badge (you know, that artefact marking the first recorded display of the Shroud in western Europe, circa 1355 the one that Shroudologists studiously ignore for the most part, much preferring to play “hunt the Mandylion“).
PS: this comment (click on hash 10 below) has just appeared from Jos Verhulst on The Other Site:
August 13, 2012 at 2:50 am | #10
“Again, it should be noted that the gist of the argument offered by Colin Berry is this one:
“However, one can always fall back on first principles, and write the Rogers reaction as follows:
Putrefaction amines (gases) + reducing sugars from starch etc (solid) —-> Maillard reaction products (solid)
Straightaway one sees that the entropy change is in principle unfavourable, since it involves a disordered gas reacting to form a more ordered solid.”
This seems to be wrong, as far as I can see, because there will be small gaseous products that should appear on the right side of this reaction scheme.”
Not in the initial step, Jos, which we know from the early model studies of Lea and Hannan (1949)almost certainly has a very high Q10 (see yesterday’s post) which means it – the reaction between gaseous amine and reducing sugar – is exceedingly reluctant to ‘go’ in the normal range of environmental temperatures, say 0 degrees to 40 degrees C (as might have existed in a 1st century tomb).
The splitting off of small molecules comes later in the dehydration, condensation and maybe final polymerisation reactions to produce the yellow or brown colour, but if the first step in the reaction is hindered for any reason, kinetic or thermodynamic- there is nothing to feed into that long, complex sequence of steps we call a Maillard reaction.
(Did you see the post I addressed to you earlier? Perhaps not, since Dan Porter deleted the link I placed on his site, which according to him was ‘bad netiquette’. How bizarre that someone who shamelessly pirates my copy, sometimes most if not all of it including images, usually within an hour or two, even minutes of me posting it here on MY blog, should be lecturing me on ‘netiquette’).
PS: I have just been refreshing my memory on Schiff base formation, often described as the first step in the Maillard reaction between an amine and a reducing sugar. In fact it is two steps, the first to form a hemiaminal by nucleophilic addition, which then dehydrates in the second step to form the imine with loss of a molecule of water. So there’s a sense in which we are both right – it either does or does not involve splitting off of water, depending on whether you consider both steps, or just the first addition step in isolation. If you are measuring loss of amino nitrogen (see Lea and Hannan) as distinct from Schiff base formation, then the first addition step alone is presumably sufficient to record loss of the -NH2 functional group.
Afterthought: Tuesday 14 Aug Even if the “first”step is deemed to involve loss of a small molecule, and declared to be thermodynamically more feasible (more arrangements, more randomness, more entropy) it overlooks a certain factor. Water at normal temperature and pressure is an unusually ordered liquid, due to extensive intermolecular hydrogen bonding. The thermodynamic dividend from splitting off water might not be realized unless or until the water were in the vapour state. I do wonder in fact, looking at Lea and Hannan’s relative humidity data, if evaporation of water is not a key thermodynamic driving force in the early stages involving one or more dehydration reactions – contributing to the enormous effect of temperature on reaction rates in the environmental range of temperature. Could rates of dehydration of intermediates be the rate-limiting factor?