How to gelatinize milk without destabilizing it.
SLOWLY, OVER CENTURIES, COOKS LEARNED to make solid foods from liquid milk. Cheeses are milk “preserves,” made by destabilizing milk and eliminating the water it contains in the form of whey. Yogurt is obtained by heating milk fermented by the bacteria Lactobacillus bulgaricus and Streptococcus thermophilus. These microorganisms transform the principal sugar in milk, lactose, into lactic acid, which in turn acidifies its liquid environment and causes a network to form throughout the liquid, creating a gel.
In recent years fermentation and curdling methods have been improved, and the texture of yogurt is now known to be determined by the particular procedure used to solidify its milk constituents. This discovery has made it possible to create new milk products. With the aid of gelatinizing and thickening compounds used to make sauces, for example, milk-based desserts have been invented. But unexplained accidents have occurred: When gelatin is added to hot milk, for example, the result often is a disagreeable lumpiness. Jean-Louis Doublier, Sophie Bourriot, and Catherine Garnier, at the Institut National de la Recherche Agronomique station in Nantes, have shown that excessive concentrations of thickening and gelatinizing agents of all kinds have the effect of destabilizing milk.
At first sight this seems a surprising result, considering the varied character of these agents. Gelatin is an extract of animal bones, starches are present in grains and tubers, carrageenans and alginates are derived from algae, galactomannans (guar and carob gums) come from seeds, pectins come from plants, and xanthan gum is obtained from fermented starch.
Destabilizing Sugars
The chemical analysis of these different compounds revealed common features. With the exception of gelatin, all of them are polyosides, compounds of the same chemical family as the sugars; the numerous hydroxyl (–OH) groups found in these molecules are responsible for the thickening of solutions by bonding with water molecules.
But in milk polyosides interact with various dissolved proteins in addition to the casein proteins, which are gathered in bundles called micelles. These bundles are either dispersed in the milk or attached to the surface of the fatty droplets. Why, then, should destabilization be observed when attractive forces seemingly ought to bind the polyosides to the micelles and the fatty droplets?
Doublier and his colleagues explored this question with the aid of laser scanning confocal microscopy, which reveals internal structures without any need for a thinly sliced sample. Using both casein and polyoside labels, they sought to identify areas that were rich in polyosides and proteins, even in mixtures where no destabilization was visible to the naked eye. They discovered that high concentrations of each of the polyosides produce a phase separation: The polyosides come together in certain areas of the solution and the casein proteins in others. In the case of some polyosides, however, this separation sometimes occurs at a very low concentration, although not immediately, because the more viscous the environment the slower the phase separation. Commercial producers who ignore this phenomenon therefore run the risk that their products will separate between the time they are made and the time they are consumed.
Instabilities Arising from a Tendency to Equilibrium
Two Japanese physical chemists, S. Asakura and F. Oozawa, have identified a mechanism known as depletion–flocculation, which occurs in particle suspensions and seems to take place in milk. Particles are in equilibrium when they repel one other (by electrostatic forces between the electrically charged molecules on their surface) more than they attract. Nonetheless, this equilibrium may be disturbed by the presence of a polymer so large that it cannot insert itself between adjacent particles; the polymer concentration is said to be null in this space, which is called a depletion zone. In solution, as a consequence of molecular diffusion, the concentrations of each type of molecule tend to become equal. Because the polymers cannot migrate to the depletion zone, its concentration there is always null, with the result that water leaves this zone in order to reduce the polymer concentration outside it. When the water diffuses in this way the particles are moved closer together. In milk, the casein particles thus form a flocculent mass, forming the dreaded lumps.
This very general phenomenon explains why one finds the same instabilities in milk to which various polyosides have been added. The only way to avoid them is to use as few polyosides as possible. Sailors are fond of saying that a boat can never be too strong; the hull, the rigging, the mast, and the spars should all be reinforced as far as possible in order to avoid rupture. When it comes to food processing, however, this is worthless advice.