Response to Jos Verhulst: delving in the literature confirms my hunch that the Maillard browning reaction is EXQUISITELY sensitive to temperature – especially in the environmental range of interest to Shroudologists…

Here’s my immediate reaction to a generally supportive comment that appeared late yesterday from Jos Verhulst on the Other Site.  (ed: I placed a link there to alert him, but it is no longer showing. 

Newsflash: In Despot Dan’s own words, after my trying a second time: “I don’t allow link-only comments”)

To other more important matters: Jos’s  comment first, which I reproduce in full, with the tail-end section that I address here in my bold (click on the #3  below to be taken to the other site)

August 11, 2012 at 4:57 pm | #3

I do not believe in Rogers’ Maillard hypothesis, because of the unicity of the image on the Shroud of Turin. Rogers’ theory implies that, in the case of the Shroud, there was a very unusual set of physico-chemical conditions occurring together by mere chance and allowing the development of this very special image. However, it is extremely improbabe that (a) the only known case of such an extraordinary image is also an unbelievably complete (frontal & dorsal!) one (for every single complete image, we should expect the occurrence of a plethora of partial images – for instance, from a single hand or leg), and (b) it is improbable to the extreme that the only image thus produced also pops up in connection with the very special man who also happens to have the largest impact ever on human history (one expects that this single image, if produced by mere chance, should be connected with some random burial, and not with Golgotha).

This being said, I think that Colin Berry should develop his argument starting from a more complete global representation of the Maillard reaction. For instance, during the different steps gaseous products (water and carbon dioxide) are produced and these should be taken into account. Already during the first condensation step, water is split off, thereby increasing the entropy of the reaction products & pulling the reaction towards the right.

I doubt that the approach you suggest, Jos, laudable though it might be, will ever provide any quick answers, or even slow ones (unfavourable conceptual kinetics?)  given the complexity of the chemical reactions that finally produce coloured end-products. However, I especially like your observation that splitting off small molecules makes reactions more likely to to “go” from a thermodynamic standpoint. I shall be using that same argument in a future post shortly in which I focus on  that very same principle maybe assisting late-stage polymerisation. Reactions that have unfavourable negative entropy terms in the Gibbs equation (like polymerisation) can be made to go by linking them up like horse and cart to ones that ARE favourable… Thermodynamics is solely about the initial v final states -being independent of reaction pathway, mechanism, reaction intermediates, presence or absence of catalysts –  at least that’s what I learned at school and college and have in turn passed on to my pupils and students…

I came across a paper yesterday describing how state-of-the art technology had discovered over 4000 compounds contributing to the colour and flavour of Maillard products in food.  So you’ll maybe understand why I’m not keen to go too far down that long and winding road…

I guess what one could do is try and identify the crucial steps in the sequence – either under kinetic or thermodynamic control – that are either rate limiting, or tending to make them more or less spontaneous by  increasing or decreasing entropy (late stage polymerisation?) and attempt to understand the role of temperature, catalysts or, thermodynamically,  factors that promote entropy increase.

At present I am content to see a proposed Maillard reaction in operational rather than theoretical terms. One starts with a mix of chemicals, one ends with a visible yellow or brown colour.  How likely is that reaction at room temperature or at the temperatures that may have existed in a tomb or Shroud?

The first thing that has to be asked is whether a given yellow colour is a Maillard product of not. It is not something one can assume, especially if an ill-defined mix chosen to represent a 1st century AD Pliny-described recipe is briefly exposed to ammonia, like the one in that so-called “experiment” that Dan Porter quoted   – the one with no attempt to characterize either reactants or products in chemical terms – see my previous comments yesterday.

If one is certain it is a Maillard product, if only by demonstrating that ammonia has been consumed, as distinct from merely making alkaline conditions, the crucial aspect is the way the reaction rates or yield respond to relatively small increases in temperature, say between 20 and 40 degrees C – the latter being approx the highest temp that, according to Rogers, a recently deceased person might reach (Why? Post mortem effect of stress hormones?  Glycolysis? Other anaerobic metabolism? Did Rogers ever give a reason?)

My gut instinct yesterday was that a typical Maillard reaction is one that needs a high temperature, not just to get going via a kinetic kick- start to supply a large Arrhenius activation energy (the well-known energy hump) but perhaps more likely because one of more of the crucial steps was under thermodynamic rather than kinetic control (again, I think especially now of those later polymerisation stages that produce the yellow or brown colour).

But nothing could have prepared me for my discovering late last night the remarkably well-documented, some might say ahead-of their- time  1949 Lea and Hannan  paper (link to abstract only), describing a 40,000 fold increase in reaction rate between 0 and 80 degrees C.  That is astonishing, truly astonishing, but while bearing out everything I have said so far – based mainly on gut-feeling – i.e.  that temperature in the normal environmental range plays a crucial role,  never for one moment  did I imagine  it would be quite so crucial!!!  One wonders what kind of kinetics or thermodynamics or combination of the two could create so steep an increase in colour over so relatively small range of temperature,even the highest being well short of boiling. (OK, so 80degrees C  is not relevant to the Rogers hypothesis, but it places a question mark over whether anything significant happens between 20 and 40: is all the action, so to speak, between 40 and 80, or even between 60 and 80, given what looks like an exponential relationship , e.g. with exponential e raised to some POWER of T?

Let’s think about that 40,000 factor some more. Suppose it were due to each 10 degree rise in temp increasing reaction rates by a certain multiple? How does that compare with the oft-quoted doubling (Q subscript 10 = 2 to use the jargon) that is the general rule of thumb?  In fact, a 40,000 fold increase over 80 degrees represent a Q10 almost TWICE as great, i.e. 3.76 approx!!!  Nobody ever learned that in chemistry at school or college.

Now let’s imagine that one gets a certain strong Maillard colour at 80 degrees C, one that has arisen from a progressive 3.76-fold increase in rate each 10 degrees C.  Let’s call the max colour intensity at 80 degrees  “100” and then work back to see what colour is present at each 10 degree increment in temperature for the same arbitrarily fixed time in our thermally supersensitive system.

Here’s the expected table of values:

Temp(C)             Colour intensity

80                     ( 100)  arbitrary

70                       26.6

60                         7.07

50                         1.88

40                        0.500

30                        0.133

20                        0.035

10                        0.009

0                          0.0025

Yes, it works in reverse too, obviously, simply by entering 0.0025 etc into your calculator, and raising 3.76 fold for each 10 degree rise in temperature, to return to 100 (with rounding error) at 80 degrees C. Yup, I have checked.

So while there may be a decent colour at 80 degrees C, the colour at 20 degrees is a mere 0.035, and raising that 3.76-fold takes it first to 0.133 (30 degrees) and then to 0.50 (40 degrees). The latter is just half of 1 percent of the colour at 80 degrees!!!!

This shows dramatically the effect of a system in which the change with temperature is an exponential function of temperature, e.g. exponential e raised to the power of T, or even some multiple or power of T (why????). Thank you Lea and Hannan (from some 60 years ago) for pre-confirming my gut feeling that Maillard reaction rates are indeed EXQUISITELY sensitive to temperature in the environmental range of interest to Shroudologists. I need all the help I can get right now, looking at the hostile response to these ideas building up on  The Other Site.

Yup, I feel another post coming on, one in which I compare Maillard browning with caramelisation, with the latter requiring heat only ( look, no external source of amino nitrogen!)  with the accent on that (nod, nod,wink wink)  late stage polymerisation and the negative entropy changes that have to be compensated elsewhere …

Publish and be damned. (Yes, I see the distinguished Thibault Heimbuger MD is onto my case, but I shall wait to see that second instalment he has promised (threatened) before replying to him individually here, on my own site. His opinion on my table above would be interesting, especially in view of his claim that  a reaction that goes at 60 will also go at a lower temperature if you are willing to wait long enough, regardless of the exponential term with T in the kinetics or thermodynamics. How long Thibault? Weeks? Months? Years??


About Colin Berry

Retired science bod, previous research interests: phototherapy of neonatal jaundice, membrane influences on microsomal UDP-glucuronyltransferase, defective bilirubin and xenobiotic conjugation and hepatic excretion, dietary fibre and resistant starch.
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