The Science of Starch
What does the smoothest confectioner’s custard you have ever tasted have in common with the creamiest mashed potatoes? Both are equally delicious, of course, but the reason why they are so good can be summed up in one magical word: starch. But what is starch and how can we use it at the stoves? Today we shall learn how to dose the quantity of starch in our dishes. Here it is science that comes to our aid in the kitchen! We shall start by saying that starch is a sugar, made up of two types of molecules called amylose and amylopectin, as explained in this article[1]. Just think that potatoes contain from 16 to 22% of starch according to the variety. A 6% difference between one type of potato and another will radically alter the result, according to the dish you intend to make. For instance, a high quantity of starch is fine for mashed potatoes but would make chips rubbery. So, how can we tackle this problem?
To answer this question, let us see what happens when we boil potatoes. Once they reach a temperature of around 60°C, the starch granules start to absorb water and go on swelling until they reach a temperature of around 70°C. On exceeding this temperature, the granules rupture and fill up with water, which causes an amylose gel to be expelled. At this point, the amylose is dispersed within the potato cells. At a temperature of 80°C, pectin, one of the main substances of which the cell walls are made, starts to disintegrate. This causes the potato cells holding the amylose gel to burst in their turn, and the potato mass will become rather gluey. It is, after all, a gel, isn’t it? If there is a lot of this gel, the mashed potatoes will be excellent. We can say that, if we are making mashed potatoes, we need not worry about how much starch the potatoes contain: the more the better!
It is a different matter, however, with all the other cooking methods: fried, boiled or roast potatoes. In this case, it is preferable to eliminate as much starch as possible. How? By cutting the slices as finely as possible and rinsing them several times in salty water. And what if we wanted to increase the quantity of starch for our mashed potatoes and its various offshoots (such as potato croquettes)? Once the potatoes are boiled, we put them in the food processor and blend them for a few seconds at a high speed. In fact, it is useful to know that many starch granules remain intact and preserve their amylose gel, even when cooked at high temperatures. By processing the potatoes at high speed, we cause these granules to rupture and release the amylose our dish requires.
But the surprising thing is that amylose is not only useful in potato-based recipes but also in those containing… eggs. It all stems from the fact that egg is made up of many proteins, that is to say long interconnected chains of amino acids. As we apply heat to the egg its proteins gradually bind together to form a densely interconnected network. It is thanks to this process of coagulation that we have boiled eggs as we know them. If we add starch to raw egg, once the mixture reaches a temperature of approximately 80°C, as we have already learned, the cells rupture and release amylose. The amylose interferes with the egg protein network and raises the temperature necessary for coagulation to take place. You may wonder what possible use this magic trick could be put to. Think of all the egg-based custards and sauces, for a start. When they are cooked, we often find lumps in them, owing to the presence of eggs. By adding a little bit of starch (better still if it is in the form of cornflour), our creamy sauces will be perfectly smooth. Would you like to try this recipe for yourself? Half a litre of fresh milk, three egg yolks, 180 grams sugar, 60 grams cornflour, some vanilla flavouring. Blend the egg yolks and sugar in a bowl with a wooden spoon until the mixture is perfectly smooth. Dissolve the cornflour in a little milk and add to the mixture. Then gradually add the rest of the milk and the vanilla flavouring, stirring as you go. Put the mixture into a saucepan on a low heat and continue to stir until it has reached the required density. Too good to be true!
References
- ^ as explained in this article (www.finedininglovers.com)