How to make it spreadable.
BUTTER IS A STRANGE SOLID: When one takes it out of the refrigerator it is sometimes necessary to wait as long as fifteen minutes before it can be spread easily. Would it be possible to make a butter that is spreadable immediately after being removed from the refrigerator?
The question has been around since 1988, when legislation in France granted a butter appellation to products that, like butter, consist of droplets of water in milk fats, on the condition that such products have been separated by physical methods. Thus one could imagine selling butters having various properties, prepared by mixing together various ingredients that were first isolated from butter.
How would one go about separating and recombining these ingredients? In 1992 the research department of the dairy manufacturer Arilait hired several laboratories to analyze milk fats and to identify the physical principles governing spreadable butters. The analysis was complicated by the fact that the molecules that make up milk fats are various and polymorphous; that is, each type of molecule crystallizes in several ways, depending on the sort of processing it has undergone, and the crystals assume their equilibrium form only after a long resting period.
In milk the fatty matter assumes the form of droplets dispersed in water. Each droplet is a few micrometers in diameter and coated with casein micelles, each micelle being a collection of several proteins cemented together by calcium phosphate. Aromatic molecules are dissolved in the fatty matter of the droplets, and other molecules (vitamins, sugars such as lactose, mineral salts, proteins) are dissolved in the water of the milk.
A team sponsored by Arilait first studied this fatty matter, whose composition changes even with the seasons. But there is unity as well as diversity: The fatty matter in milk is made up of triglyceride molecules, composed of a glycerol molecule to which three fatty acid molecules are bound (although, of course, the glycerol and fatty acid molecules lose their identities, as when oxygen and hydrogen molecules combine to form water). On the other hand, milk contains more than 500 such fatty acids, and because each acid can bind with any carbon atom in glycerol, the number of possible triglycerides exceeds several thousand.
Strange Fusion of Butter
One of the chief consequences of this diversity is the strange behavior of butter at the point of fusion. Unlike a pure body such as water, which melts at a fixed temperature (0°C [32°F]), the fusion of butter begins at –50°C (–58°F) and ends at about 40°c (104°F). The various triglycerides in milk melt in three principal stages involving homogeneous chemical families: From –50°C (–58°F) to 10°C (50°F) one observes the fusion of molecules whose fatty acids are short and composed of double chemical bonds among carbon atoms; between 10°C (50°F) and 20°c (68°F) one sees the fusion of molecules containing a single double bond or a short chain; finally, between 20°c (68°F) and 40°c (104°F) one sees the fusion of molecules that contain three fatty acids and have only single chemical bonds between carbon atoms.
Instead, physical chemists Frédéric Lavigne, Michel Ollivon, and their colleagues in the faculty of pharmacy at Chatenay-Malabry used melted butter to study the opposite of fusion: crystallization. To separate the various parts they therefore performed a split crystallization, slowly cooling the liquid and isolating crystals of the same molecular type that appear at the same temperature.
New Butters
Having thus isolated these similar parts, Lavigne, Ollivon, and their colleagues next looked for a way to form mixtures that would be spreadable straight out of the refrigerator. They hit on the idea of mixing high-fusion temperature triglycerides, which remain solid at room temperature, with a suitable proportion of low-fusion temperature triglycerides, which are liquid at room temperature.
In this way one obtains an apparently solid body that, like traditional butter, contains a proportion of molecules in liquid form (even in milk the fatty droplets are partially solid, the proportion of solid matter reaching 70% at 4°C [39°F] but only 10% at 30°C [86°F]). Enrichment by low–fusion temperature molecules makes the mixture easier to spread. The parts that fuse at high temperatures are used to make pastry (still under the name butter because the law permits it), particularly puff pastry.
Why, then, does a solid that contains liquid appear to be solid? Because of the crystals that increase with cooling and interlock with one another: Scraped with a knife, butter seems to soften, not because it is heated but because the crystals are separated.
To have an idea how these discoveries can be used in cooking, try testing split crystallization yourself. Melt the butter and skim off the solids as they form, just as the physical chemists did. You will then be able to manufacture your own butters by mixing proportions of solids and liquids and in this way obtain the specific texture appropriate to a particular dish.