A smoother product can be obtained by modifying its milk composition and fabrication process.
HOW DOES ONE MAKE A GOOD YOGURT? The question is poorly posed, for some like their yogurt runny and others like it firm. Ideally what one would want to be able to do, then, is to balance the composition of milk and the method of fabrication in a way that will yield a specific texture and taste. Achieving this objective will take some time, but already Anne Tomas and Denis Paquet of the Danone Group, together with Jean-Louis Courthaudon and Denis Lorient of the École Agro-Alimentaire in Dijon, have shown that the texture of yogurt depends on the microstructure of the milk, which varies according to the concentration of proteins and fats.
The best way to understand the difficulty of the problem will be to make some yogurt ourselves. Put a tablespoon of commercial yogurt in some milk and heat it gently for a few hours. The milk sets—or, as a physical chemist would say, a gel has formed.
Milk is an emulsion, which is to say a dispersion of fat globules and aggregates of casein (protein) molecules in water, in which various molecules such as lactose are dissolved. When one adds yogurt to this emulsion, one is sowing it with bacteria—Lactobacillus bulgaricus and Streptococcus thermophilus—that transform the lactose into lactic acid. This process of fermentation acidifies the liquid environment and triggers the aggregation of casein micelles in a network that traps the water, fat globules, and microorganisms, which in the meantime have proliferated.
Textures to Order
Unless a great many precautions are taken, yogurt produced by this method is disappointing. A bit of home experimentation will show why. Sowing two identical samples of milk with different yogurts yields different textures and tastes. Similarly, when the milk is curdled with the aid of glucono-delta-lactone, a molecule that progressively acidifies the environment in which it is placed, a still different result is obtained. What is more, causing the milk to curdle at two different temperatures gives different results as well.
Entertaining though they may be, these experiments are not enough to satisfy the needs of the food processing industry, which is obliged to make consistently good products—hence the crucial question posed at the outset. For want of an answer that would put an end to further research, the chemists at Danone and in Dijon restricted their attention to the problem of composition: Given that commercial producers make yogurt from milk that is fortified by the addition of powdered milk, condensed milk, and various milk constituents, how does the composition of this milk determine its microstructure and therefore that of the yogurt made from it?
Because of the variability of these products, the chemists examined emulsions of fixed composition that were prepared by processing a mixture of milk fats and skimmed milk in a microfluidizer (which injects the mixture under pressure into a clear small-diameter tube). Analysis of the light diffused by the various emulsions indicated the size of the fatty droplets.
A Sufficiency of Proteins
Contrary to what prior studies had suggested, the size of the fatty globules did not change with the concentration of fat and protein; what had appeared to be an increase in the size of the droplets, when the proportion of fat is raised, turned out to be only an aggregation of globules of the same size. Naturally the number of droplets grows when the concentration of fatty matter increases, but the proteins are always sufficiently numerous to coat the fatty globules and emulsify them.
Because proteins are not the only molecules that are tensioactive—that is, capable of adhering to the surface of fatty droplets, so that one part is in contact with the fat and the other with the water—the Danone and Dijon chemists studied the changes produced by adding other kinds of tensioactive molecule to emulsions that had already been formed and to mixtures that were subsequently emulsified with the aid of the microfluidizer.
It was expected that molecules with the greatest affinity for fat and water would preferentially attach themselves to the surface of the fatty globules, but experiments showed that this is not the case as long as the tensioactive molecules are put into the mixture before it is emulsified. When the tensioactive molecules are added to an already constituted emulsion, the milk proteins that coat the fatty droplets are not disturbed by these molecules, and their degree of aggregation is unchanged. By contrast, when the tensioactive molecules are introduced at the outset of emulsification, the distribution of the proteins is altered and the degree of aggregation is reduced.
As a result of this research, the prospect of creating new kinds of diet yogurt with the same smooth texture as high-fat ones no longer seems quite so remote.