What is the secret behind a creamy ice cream? And why, on the contrary, we often taste a grainy ice cream?
Have you ever met anybody who dislikes ice cream? Maybe you have, but you may agree with me: it is very rare, especially in the summertime, when almost everybody would like to grab an ice cream.
You may like chocolate flavour or strawberry, but the most appreciated element in the ice cream, believed to make it really good, is its creaminess: the easy way with which it gets melted in your mouth.
But a bad ice cream experience may have occurred to you too: having tasted an ice cream full of ice granules. Your tongue cannot melt them, and they have no flavour, not to mention the moment when you chew a big piece of it and you get an instant headache, just like you dove in a lake at the North Pole.
But which is the secret to the ice cream creaminess?
Creaminess is related to the ice crystals dimension composing the ice cream: small crystals, creamier ice cream, big crystals, granular ice cream; easy.
Creating a creamy ice cream is relatively easy, especially if we prepare ice cream with a high quality machine.
But the significant problem is: how to maintain the creaminess of ice cream after the passing of time.
Let’s suppose that, enthusiastic about this marvelous technology, we decide to prepare 30 ice cream kgs with the machine and to store it in the refrigerator: we may be convinced to have a 5-year stock.
Bad news, though: after a few days, your ice cream will be transformed in ice. Not a food stock, but a destruction weapon.
Let’s try to figure out why small ice crystals tend to thicken and transform in bigger ones, when left alone in the refrigerator though melted in your ice cream machine.
The answer is not very different from: “they get closer because they’re cold”. It is possible to give a scientific explanation of that, so we can try to process it.
Wilhelm Ostwald, a physicist and chemist from Riga, was the first scientist who studied this phenomenon in 1896.
We might consider the crystal structure of ice, as in the image below.
The crystal structure composing the ice cream may be a little more complicated than that, but we can understand the main principle just looking at ice.
We immediately notice that the little balls on the outside, the ones on the walls, have less bonds (the black sticks) with the other balls than the ones on the inside.
This bond shortage makes the external area of the structure very instable. That happens because the chemically ideal quantity of bonds is the one in the internal area. Given that, an external small ball would like to form at least another bond to be more stable.
Due to this reason, small dimension crystals tend to break and to tie in bigger crystals, minimizing the exposed surface and maximizing the bonds formed on average by the small balls.
Even though the amount of small balls does not change, the configuration with the minimal amount of particles on the external wall is the one where the small balls belong to the same crystal.
This process also happens, for examples, in this: the increasing or reduction of the exposed surface dividing or merging a system elements is reflected in a tree maximizing its green surface dividing its structure in lots of small leaves, so it can optimize the light absorption.
The common denominator is that nature wants to obtain the maximum result with the minimum effort: lazy.
So, we figured out why our creamy ice cream tends to get granular if we leave it in the freezer for some days.
This is a problem for us, because we have just prepared 300 ice cream kgs. And it is a problem for big ice cream producers.
To remedy this, chemical additives are used, which slow this process forming big crystals.
Some of these anti-freezing proteins are developed by some fishes living in the polar habitat, evolved to survive the low temperatures, in order to avoid to get frozen.
So, when you eat ice cream: remember to thank polar fishes to give us the idea!
Would you like to know more about ice cream? Don’t miss this article: “N-ice cream: ice cream liquid nitrogen, explained scientifically”!
Matteo Biagetti | PhD Student @ Department de Physique Théorique, Université de Genève.