Time and again we see a trotting out on The Other Site of some highly questionable thermochemistry that was first mooted by (none other than) STURP’s Raymond N.Rogers. (STURP for the uninitiated is/was the Shroud of Turin Research Project, mainly US-populated in the late 70s/early 80s).
I have kept silent so far, knowing that Ray Rogers was a thermochemist, knowing that he was founder of the Thermochimica Acta journal and its Chief Editor for some years. Fools rush in…
When I first read Rogers arguing that a temperature of 40 degrees Celsius, easily reached by a recently deceased cadaver he maintained (like the Man in the Shroud, say in the first few hours after supposed crucifixion and consignment to a tomb) would be sufficient to kick-start his hypothesised Maillard reaction, creating that negative sepia image we see today, my first thought was: “This guy (sadly now deceased) must know something about Maillard (browning) reactions that I don’t.” That’s despite the fact that I was once Head of Nutrition and Food Safety at a food RA and had picked up a fair bit along the way about Maillard reactions (essentially reactions between amino groups in proteins or other sources of amines and reducing sugars to form yellow or brown products of chemical condensation and polymerisation).
Why, I asked myself, should a modest rise in temperature of a mere 10 or 20 degrees Celsius kick start a browning reaction? When did you ever see bread toast itself in a warm kitchen? Sure, a 10 degree rise in temperature can typically double or sometimes treble the rate of a chemical reaction, but if the chemical reaction is not THERMODYNAMICALLY FEASIBLE in that range of temperature, then raising the temperature 10 degrees will not have the desired effect. In point of fact it will have NO EFFECT, because chemical kinetics, which is about rates of reaction, are totally irrelevant if the reaction is not thermodynamically feasible.
To cite an everday example, you can quickly get bread to go golden-brown in an electric toaster (the classical Maillard browning reaction), but if your toaster was broken, then sitting bread on the lid of the electric kettle, and heating it to 100 degrees will not produce toast, no matter how long you wait – hours, days, weeks etc. Not even slightly toasted. The Maillard reaction is simply not THERMODYNAMICALLY FEASIBLE at 100 degrees Celsius, even if other chemical reactions are occuring at multiples of their normal room-temperature rate. Maillard reactions generally do not get going until the temperature is 150 degrees and above, and there are sound thermodynamic reasons for that which have nothing, I repeat NOTHING, to do with chemical kinetics, the latter being a separate subject from chemical thermodynamics, though interfacing with it as discussed later. Thermodynmaics is about whether reactions are feasible in principle, i.e. go of their own accord. Kinetics is about the rates of reaction that are feasible. Some feasible reactions will not proceed at measurable rate, due to a minuscule rate of reaction, but a non-feasible reaction will not respond to a modest rise in temperature unless that rise (improbably) brings it into the range of thermodynamic feasibility.
So how could Rogers have got it so wrong, despite his impressive, some might say impeccable credentials in the area of thermochemistry? Looking through his oft-quoted comments I am struck by a glaring omission. His comments on how the Shroud sepia image may or may not have been formed (NOT formed by man-made scorching, he insisted, but probably (he insisted) by a natural albeit complex Maillard reaction) appear to be based ENTIRELY on consideration of chemical KINETICS. I have not seen a single mention to chemical THERMODYNAMICS. But if you were a chemist (or like me, a biochemist) and wished to know if a particular hypothesised chemical reaction were feasible or not, the first thing you would have to look at is the THERMODYNAMICS. The latter is not about whether the reaction will go in practice. It is more fundamental than that. It is about whether the reaction will go in PRINCIPLE. If the reaction stands no chance of going in principle, either because the energetics are wrong (it is too “uphill” on the potential energy scale) or because the temperature is wrong then a 10 degree rise is unlikely to make it feasible, since doubling the rate of a reaction that does not go is assuredly guaranteed to make it not go twice as much.
So how does one tell if a chemical reaction is thermodynamically feasible or not in a given temperature range?
It all comes down to a fundamental equation in chemical thermodynamics. Don’t let it intimidate, since the the underlying concepts can be quickly grasped, even if it takes a lifetime to grasp all the subtleties (and believe me, as someone who once taught the subject to London University Chemistry A-Level there is a minefield of subtleties where chemical thermodynamics is concerned).
Delta G, the Gibbs free energy change (Delta being Greek letter D, shorthand for difference or change in) has to be NEGATIVE for a reaction to be feasible – not necessarily practical mark – it may be too slow to be useful – simply FEASIBLE.
Delta G, as the equation shows, is the difference between two other thermodynamic quantities – Delta H, the enthalpy change (which is a posh name for the thermal energy – what you and I call heat – given out or taken in) and the product of T, the absolute temperature, and Delta S, the ENTROPY change.
Note first of all the appearance of a TEMPERATURE term in the thermodynamic equation which has ABSOLUTELY NOTHING whatsoever to do with with chemical kinetics, i.e. rates of reaction. Kinetics are a secondary consideration at this stage. We shall come to kinetics later, even if they were strangely the up-front consideration of Rogers’ pitch in favour of his Maillard reaction ( and also deployed, incomprehensibly I thought in his attempt to dismiss scorching – though a separate issue). It is those ENTROPY changes which are the fundamental driving force for s0-called spontaneous chemical reaction. Entropy is about disorder, randomness, chaos. Natural processes, unless coupled up in precise ways (like in lifeforms) tend towards greater disorder, greater randomness, i.e. greater ENTROPY. (Even in carbon-based life forms, such as ourselves, there is always an entropy increase – the trick is to dump the entropy on the surroundings, while steadily building order in the life form). Other things being equal, a chemical reaction that results in more entropy is more likely to go of its own accord than one that does not. But it’s not just the entropy term, note, but the PRODUCT of entropy and temperature. The latter is not just a factor to be considered in Rogers’ chemical kinetics but in the THERMODYNAMICS too on which, as I say, he remained strangely silent.
To be continued (writing these chemical spiels can be tiring, but you dear reader will I hope see where this blog posting is heading, possibly even the take-away message). I shall try to complete in the next hour or two.
So what are the ways in which one can stack the odds in favour of an increase in total entropy (system + surroundings) thereby making one’s desired chemical reaction “go”? There are two main ways. The first is having a reaction that is exothermic (liberating heat energy to the surroundings). The more exothermic the better, though it’s a factor that is not easily under one’s control, except by careful choice of co-reactants. So why should an exothermic reaction result in greater entropy? Because the heat dissipates into the surroundings, provided the system is OPEN ( always a trap for unwary students who mistake closed for open systems and vice versa). Heat that dissipates though increasing thermal energy of surrounding air makes that heat energy less concentrated, less able to return to the system via the back door. Result: the reaction becomes for all intents and purposes thermodynamically ‘irreversible’. Instead of reaching an equilibrium, it just keeps on going until all the reactants are used up, with no possibility of a reverse reaction.
A second factor are changes of physical state. Any reaction that results in an ordered state (solid, and to a slightly less degree – a liquid) changing to form gases that can escape invariably introduces a large or largeish positive entropy change, i.e. a driving force, favouring the reaction, because the nett change is from a relatively ordered to a more disordered state. But the reverse is also true – like Rogers’ Maillard reaction – where a chance mix of a surface contaminant (starch) and wandering gaseous component (putrefaction amines) is forced to react at close to normal environmental temperatures (contrary to everyday experience re browning reactions) to become a static more ordered Maillard reaction product. Nope, it simply does not make sense in thermodynamic, i.e. entropy terms, or even everyday experience. It’s what in those boring everyday terms we call a non-starter.
So burning barbecue charcoal (carbon) in air to CO2 is a reaction that goes spontaneously (provided it is given a big kick start in the form of lighter fluid) not only because it is highly exothermic, but because the solid carbon combines with gaseous oxygen to form a product (CO2) with stronger bonds than those in the reactants, and because the single product is entirely a disordered high-entropy gas.
So the forward reaction goes spontaneously – in principle – but needs, as I say, that big kick start – the energy of activation – which is where chemical kinetics finally enter the equation (thank you for your patience dear reader).
So is the reverse reaction NOT feasible, i.e dissociation of CO2 into its contsituent elemnents? In practice – yes, because of the unfavourable entropy change and energetics. Is it non-feasible in principle? No. It could occur at the temperature of the Sun, where all chemical bonds are broken. But as soon as the reaction mixture cools down, the entropy changes are then favourable to recombination, so it would be impossible to capture one’s high temperature products (unless quenching with cold liquid nitrogen to instantly “freeze” the system). Note the difference between feasibility and practicality. Note that the breaking of chemical bonds at high temperature is more about thermodynamics than kinetics, although in that situation the kinetics assists . An interplay, indeed synergism, between thermodynamics and kinetics is possible, but only when the thermodynamics are permissive.
Enough of the science tutorial: back to the Shroud, back to Rogers’ largely-untested Maillard reaction hypothesis that so dominates discussion on The Other Site.
First, a confession. I do not know the thermodynamic parameters (enthalpy, entropy) etc f0r Rogers’ proposed reaction (did he?). In fact I do not even know them for the simplest model Maillard reactions, say between ammonia and glucose, having obtained no promising returns as yet with search engines (though I shall persevere).
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. Straightaway one has a thermodynamic rationale for appreciating why that reaction needs an elevated temperature, not just for fast reaction rates, but to make it THERMODYNAMICALLY FEASIBLE (sorry to keep banging on. And we are not talking about a mere 10 or 20 degrees but more like 100 or 150. Again, sorry to keep repeating myself, but the distinction between a small temperature rise as a kinetic kick start and a LARGE temperature rise to bring a proposed reaction into the realm of thermodynamic feasibility is ABSOLUTELY FUNDAMENTAL to any discussion re a proposed hypothesised chemical reaction. But you would not know that from reading Ray Rogers or his posthumous disciples…
(ed: I’m stumped at the moment as to why a higher temperature would help the above Maillard reaction with its unfavourable negative entropy gas to solid conversion . It can’t be via the Tdelta S term, since that would make it larger and more positive, and more conducive to a positive delta G, i.e. non-feasible. There must be a reason since the literature is choc-a-bloc with references to browning reactions being more likely as one raises the temperatures – in line of course with everyday experience But as I said earlier, thermodynamics is full of subtleties).
There’s more I could say – on the role of chemical kinetics – on why I think that a Maillard reaction is a complete non-starter, not just in thermodynamic terms, but on a host of other grounds- but writing this has been draining. So if you don’t mind, dear reader, I think I shall call it a day. Comments welcome, but I consign to the bin those that I perceive as ad hom attacks (and believe me, a Shroud sceptic is assured of a constant barrage of the latter).
Update: Saturday 11 Aug: here is a screen grab of part of Dan Porter’s response on The Other Site.
So what’s the ammonia reacting with, and how do we know the yellow colour is a Maillard reaction product? It may have been a Maillard product, but the chemical dice were loaded, first by using “dextrin”, to ensure the presence of some reducing sugar, dextrin being an imprecise term for partially-degraded starch (a strange choice for a scientific experiment, if indeed one can so dignify the above “cookbook chemistry”) and second by tossing in some saponins for good measure (just to be on the safe side?). Why use starch, correction degraded starch and saponins? Oh yes, I remember now. Pliny said they were used to coat the yarn in 1st century linen. How handy to be able to quote Pliny if you are a 20th century chemist looking for a source, or potential source, of reducing sugar in your entirely hypothetical Maillard reaction, and it does save having to run tedious tests for starch and saponins on the TS which either give atypical colours or no result (respectively).
Incidentally, Rogers seemed to have a blind spot for the primary cell wall and its chemically reactive hemicelluloses and pentose sugars. Or there again Rogers knew they were there, and their potential for producing yellow or brown colours merely by caramelisation if the temperature was high enough. But we would not want that queering the pitch for our Maillard reaction story now, would we? So no more mention of caramelisation thank you, which only requires heat (look, no amines needed – no Maillard reaction).
Sorry Dan, but that so-called experiment was truly appalling. As I say – cookbook chemistry – indeed a cooked result.
PS: at the very least the experiment should have been done with pure starch or pure glucose alone (or run as controls) and, importantly, in a closed vessel with a before and after analysis of ammonia. How else does one know if the ammonia has reacted or not, whether with reducing sugar or something else? For all we know the ammonia simply adsorbed onto the film of moisture that accounts for up to 20% of the weight of “dry” linen, creating alkali conditions that then produced a yellow colour by something other than a Maillard reaction. In a typical Maillard browning reaction, there is no alkalinisation to cause competing chemical reactions, since the source of the amine is solid-state proteins and their constituent amino acids.
Second update: This comment from “anoxie”has just appeared on The Other Site
Reply to anoxie: somehow I get the impression that you have based your view on the extract that Dan Porter chose for his posting, and have not been to this site to read my entire posting. Had you done so you would have seen that the sentence you quote was not a blanket statement regarding all Maillard reactions – it was in the context of the browning of toast etc. I did in fact consider the simplest model system that was studied in that paper you cite – and there you will find that I was circumspect about the thermodynamics – highly so in fact- freely admitting that I did not know the enthalpy or entropy values. What I did was to say that a reaction that involves a gas (e.g. ammonia) becoming a solid was thermodynamically less favourable compared to one that does not.
Now let’s look at the paper you cite, of which only the first page is available online, but shows that you, anoxie, should maybe read carefully the papers you cite before using them as fly swats.
Notice anything? The reactant was not ammonia gas, but ammonium hydroxide, i.e. ammonia dissolved in water (hardly relevant the Shroud, unless the water is a thin adsorbed layer – see below). The thermodynamics of ammonium in solution will be different (and almost certainly more favourable) than those with ammonia gas, and there is also the additional benefit, which the authors note, of having hydroxide ion present to raise the pH. That may well offer the possibility of alternative chemical pathways that are thermodynamically more favourable, especially in aqueous solution.. In fact I mentioned alkalinisation as a factor in my previous comment.
Incidentally, elevated temperatures WERE used routinely in that work, to “approximate conditions of the normal cooking of food”.
But without the full paper we are left ignorant of rates of reaction at room temperature, or even 40 degrees. Maybe “anoxie” could enlighten if anoxie has the full paper to hand. But as I say, they are irrelevant to the thermodynamics of the Rogers hypothesis, the focus of this blog, given the aqueous reaction medium.
Water, whether as bulk water or an adsorbed layer on linen may or may not influence the chemistry, but is unlikely one would have thought to help in creating a sharp image by Rogers’ Maillard hypothesis. On top of all the randomness in getting gaseous amines from body to cloth, especially where there are air gaps, you then have the randomness created by ammonia dissolving or hydrogen-bonding to water adsorbed onto linen, and immediately diffusing or desorbing before it has had time to react which is hardly likely to create a sharp image.
Enough said. I’m not enamoured of the Maillard hypothesis, basically because it is weighed down with too many qualifying assumptions. There is a better theory for how the image was formed – by contact, scorching and caramelisation of the more reactive hemicelluloses of PCW of flax linen fibres, possibly even a starch coating too.
Footnote: I have just refreshed my memory on the ammonia-caramels that are used to colour cola drinks (still the subject of cancer scare stories, as they were years ago when I was involved with food safety). They are made by Maillard reactions between corn syrup, the source of reducing sugar, together with ammonia (or sulphite) and may use superheated steam and pressure to get the temperature as high as possible ( in excess of the normal boiling point of water at 100 degrees C, or 212 degrees F. The idea that Maillard reactions occur at or close to room temperature with ammonia fumes on “dry” linen is at best conjectural – unless you are a Shroudologist who is desperate for Rogers’ amine hypothesis to be true… As I said earlier, that experiment that Dan Porter quotes offers no evidence that the yellow colour was a Maillard reaction product, the aim being merely to produce a yellow colour from a Pliny recipe without bothering with detailed chemical analysis, despite being a “model system”. That I regret to say was all too typical of what can pass for “science” in the slapdash world of Shroudology investigation.
Btw: those patents in the above link refer to the ammonia as a “catalyst”. Inasmuch as catalysts are by definition not used up at the end of a reaction, being used merely to speed up the reaction, and the description of the product as a caramel (“burnt sugar”) one begins to wonder if the brown colour is really due to Maillard reaction. Now that would be interesting – to show that ammonia can make reducing sugars go yellow or brown without forming Maillard products – but by speeding up the kinds of reaction that require heat alone without incorporation of nitrogen. What price that Rogers Pliny-inspired recipe with the linen, ammonia, dextrin and saponin – if the yellow colour was NOT a Maillard product?
Hooray: confirmation of thermodynamic rather than kinetic control comes from discovering a oft-cited mid-20th century paper
If a typical Maillard reaction were under kinetic control one would expect each 10 degree rise in temperature to double reaction rate (a generally-recognized rule of thumb). So raising the temperatures from 0 degrees C to 80 degrees C should increase reaction rates by some 256 times ( 2 raised to the power of 8). In fact Lea and Hannan in 1949 showed that the rates of browning in a casein-glucose system increased 40,000 fold in going from 0 to 80 degrees! That is clear evidence that this reaction at least is under more than kinetic control. It might be under thermodynamic control, as I have argued here. Alternatively, the explanation may lie in the complexity of reaction pathways that lead to final coloured end-products involving sequential series of precursor-product relationships, with the rate-limiting reactions being those at higher temperatures, maybe with large energies of activation. (Yes, I know I risk being accused of contradicting myself here, but that 40,000 factor is extraordinary, and may need extraordinary combinations of thermodynamics and kinetics to account for it).
The takeaway message is this: Rogers attempted to argue that a relatively small rise in cadaver temperature might be sufficient to make his Maillard reaction proceed at a sufficiently fast rate to produce an image, citing the 10 degree-doubling (or even tripling) of rate. But if, as one suspects from the Lea and Hannan finding, the relationship is exponential, with a higher power than 2, for whatever reason, whether kinetic or thermodynamic, then the change from say 30 to 40 degrees will be tiny compared, say, with that from 70 to 80 degrees. In other words, Maillard reactions, for whatever reason are almost certainly sluggish in the temperature range that existed in a tomb or shroud. Much higher temperatures – tens of degrees higher – would be needed to get a Maillard reaction properly underway, ones that are impossibly high to accommodate within the Rogers hypothesis.