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Eat up your crusts !

Eat up your crusts!

Me and my wife were going to cook a nice joint of roast beef and show you how to do this roast dinner but sadly I overcooked the joint a little. So as a result I am going to tell you something about the chemistry of how meat cooks. During cooking a complex set of chemical reactions alter the food creating a complex palette of new chemicals which contribute to the sensual experience of eating the food.

One of the important reactions which occurs during cooking is the Maillard reaction; this is a reaction which involves carbohydrates and the amino acids from the protein in meat. Firstly the aldehyde form of a sugar such as glucose reacts with the amino group of an amino acid to form a thing called an imine. This imine then can undergo a series of reactions; please do not be afraid if it looks complex.

Figure 1. Formation of the imine followed by the Amadori reaction.

The reactions forming the imine are closely related to what we teach first years in Chemical and Biological Engineering. I hold the view that most organic chemistry can be understood after a relatively small number of reactions are understood, what a person needs to do is to understand the organic reactions, recognise them when they attempt to go incognito and understand how to combine them into sequences rather than just memorizing thousands of examples of reactions.

My advice right now to anyone about to start the organic part of the first year course and older students who are nervous of organic chemistry is “Don’t Panic!”. When you see some scary looking organic chemistry it is important to take a deep breath, relax for a moment and see if it is some variation on an existing reaction you have seen.

The ketosamine then can do a wide range of things, for instance it can form on acid treatment 5-hydroxymethylfuran-2-carboxaldehyde (HMF) with great ease, the HMF is an important substance which can react further. It is also important for bee keepers to ensure that the sugar they feed to their bees has a very low level of furans, this is because the furans are very toxic to bees. Here is the reaction scheme showing how HMF forms under acidic conditions from the ketosamine.

Figure 2. Formation of hydroxymethyl furan aldehyde from an Amadori product

Now you might ask why I am telling you about the Maillard chemistry so close to Christmas, the reason is that many of my favourite English Christmas foods are the way that they are because of the Maillard chemistry, for example a Christmas pudding is cooked with rasins. A paper by F. Karadeniz, R.W. Durst and R.E. Wrolstad, Journal of Agricultural and Food Chemistry, 2000, 48, 5343 explains how Maillard chemistry occurs during the sun drying of grapes into raisins. On a more everyday note I am about to do something which will make British children hate me, I am about to become worse than the Child Catcher in Chitty Chitty Bang Bang (Most British children hate eating their crusts). Here is a slice of bread.

Figure 3. A slice of Bread

I have a new reason for telling children to eat up their crusts! Because of the Maillard chemistry the crust on bread contains a chemical (Pronyl-L-lysine) which protects your body against cancer. For details see M. Lindenmeier and T. Hofmann, Journal of Agricultural and Food Chemistry, 2004, 52, 350. This paper was interesting to me, while it is known that Maillard chemistry can form some carcinogens in food, here was a substance which protects the lower end of the digestive system from cancer. An amino acid named lysine has a primary amine in the side group, this can react with the carbohydrates to form an exotic amino acid named pronyl-lysine which is able to react with carcinogenic hydrazines to form a very stable looking pyrazole which should inactivate the hydrazines.

Figure 4. The hydrazine catching reaction of the pronyl-lystine

I suspect thatthe chemistry is rather similar to that which I used years ago in Aberdeen when I used to make to make pyrazoles; I used to add with care hydrazine to solutions of 1,3-diketones in alcohol. I say with care are it was always a very exothermic reaction which sometimes made my alcohol reach boiling point. Here is one of the beauties of organic chemistry; the same reactions appear again and again but in different contexts. We can work backwards in our brains from the pronyl group to find that we need the sugar based fragment known as 3,4-dihydroxyhex-e-ene-2,5-dione (Acetylformoin). Below I have shown how it can form from glucose.

Figure 5. The formation of acetylformoin from the Amadori product

What happens next is that it reacts with the dangling amino group of the lysine to form a five membered heterocycle. What can happen is that the open chain form of acetylformoin can react with the amino group to form an imine. This imine can then close the ring to form the wonderful pronyl group.

Now I hope that some of you have started to think a little more about organic chemistry, I would like to stress that all the chemistry I have written about can be explained with first and second year courses taught at Chalmers. While thinking about the chemistry of my Christmas dinner I have introduced to you some carbonyl chemistry and some heterocyclic chemistry, I hope I have given you a taste or flavour of what organic chemistry is about. After having done all this organic chemistry I have worked up quite an appetite which brings me onto the subject of what I have for you. The movement of electricity in electrical circuits canbe described mathematically using the laws of physics. In organic chemistry we have electrons whose movement can be explained with the curved arrow. This mechanistic tool helps us to make sense of the vast world of organic chemistry; I want to set before you a banquet of organic chemistry.


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