pears with sugar and water in a tin lined pan does not show any reddening, which lead to think that
this culinary precision is wrong. However a study in our laboratory showed that red pear jams can be
obtained not through tin, but through pH adjustment: pears turn red when cooked in acidic
environment, whether tin ions are present or not. It is to be guessed that cooks of the past were able
to see red jams, and mistakenly interpreted that tin was responsible of the effect. In fact, the reddening
is due to some polyphenols molecules naturally present in pears, such as quercitine, who absorb in
the visible spectrum when under a special form called “flavylium”.
Some chemistry of flavour
We are now on the “chemical way”. Let’s stay on it, because it is important in the kitchen, as chemical
processes are responsible of much appreciated flavours.
For example, grilled meat has a very strong flavour due, in particular but not exclusively, to Maillard
processes. These chemical reactions involve sugars such as glucose, and amino acids. The full
description is quite complex, and many odorant molecules are produced, as well as brown products
called melanoidins. It is useful to know that these Maillard processes are not the same with or without
fat, that contribute to the general spectrum of odorant molecules.
As many chemical reactions, Maillard processes occur faster when the temperature increases. More
precisely, any increase by 10°C doubles the speed of the reaction. This explains why people having
diabetes get blind after one life (at 37°C) but why meat turns brown in minutes at 180°C. Contrary to a
wrong idea, this browning occurs at any temperature, and even in boiling water: if glycine and glucose
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are dissolved in water, and the solution is heated, the liquid stays clear for about 30 minutes, and later
only turns yellow, and then brown.
This browning has nothing to do with caramelization, as it is sometimes said by cooks. Caramelization
was a mystery for long, but it was recognized in the last decade that heated sucrose dissociates into a