mardi 28 juin 2016

Low temperature cooking of vegetables

Being asked about  "low temperature cooking of vegetable", here is an analysis :

What happens when vegetables are cooked at low temperature?

Indeed for the sake of the discussion, it's good to consider that plant tissues are made of cells, which are soft object, full of water and limited with a fragile membrane. They are cemented with the cell wall, i.e. pectins and celluloses: one can regard cellulose molecules as strong, chemically inert pillars, and pectins as ropes that are entangled around the pillars, so that they keep the cells together.

When the plant tissues are boiled, a chemical process called beta elimination of pectins occur: the pectins "ropes" are cut, piece after piece, so that the pillars can separate, and the cells also. This  is why the plant tissue becomes soft.
# Of course, some others phenomena occur, such as starch gelatinization, above about 80 Celsius degrees.
This can occur in water or outside water, but of course outside water some evaporation occurs as well, which means the formation of a crust. And when the temperature + time is enough, various chemical processes can turn the plant tissue brown, such as hexoses degradation, Maillard reactions (but few), oxidation, pyrolysis, caramelization (remember that plant tissues are full of the three sugars glucose, fructose, sucrose), etc.

Another important piece of information is needed when low temperature cooking is considered: at low temperature, some particular enzymes called pectin methyl esterase are activated, so that the provoke the leaking of calcium from cells... and this makes the plant tissues harder because the calcium ions link to pectins and prevent further pectin disruption. This can be seen by a wonderfully simple experiment of heating first carrot slices in water at 50 °C : the carrots don't change apparently. Then when you  boil the carrots, they don't soften as they would do during boiling.
And this effect occurs frequently during low temperature cooking, with different temperature threshold depending on the particular tissues that you consider. Cooking in "hard" water, with a lot of calcium ions, can also have this effect: in certain waters, lentils don't soften, even after hours of boiling.

If you can find on line the PhD document by my former student Anne Cazor, some more information is given. But the most important is given here.

Finally, I would say that I don't really see the interest of low temperature cooking of vegetables... because the goal of low temperature cooking was limiting the protein coagulation of meats, avoiding them being hard, and the slow dissolution of collagenic tissue, so that hard meat can be transformed into tender meat. For plant tissue, the issue is often to make them soft when they are hard, and there is no harmful effect of high temperature, only the process is accelerated.
Only for particular processes low temperature is helpful, for example when you  want to make non soft gherkins, for pickles, for example. Or when too  soft plant tissues have to keep some hardness (some varieties of potatoes, for example), etc.

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