From Milk to Yogurt -an Electron Microscope Story
~ October 1997 No.20 ~
Yogurt found its way to North America some 30 years ago and now is part of the dairy counter even in the smallest grocery stores. It is a cultured milk product, i.e., a product made through the action of microorganisms. It has been traditionally produced and consumed in the Balkans and the Middle East. Compared to other milk products such as cheese, ice cream, or butter, yogurt contains all milk proteins and salts whereas the milk sugar (lactose) is mostly converted into lactic acid which gives yogurt a pleasant acidic flavour.
Reduction of the lactose content allows yogurt to be consumed even by people suffering from mild lactose intolerance, i.e., their inability to digest the milk sugar. Live lactic acid bacteria in the yogurt give the product a long shelf life. The fat content in yogurt is in some products reduced for dietetic reasons.
Yogurt is very interesting from the structural viewpoint, because if has a high water content and yet is solid. Electron microscopy reveals some peculiarites in the development of yogurt structure. The changes which take place in milk when yogurt is formed, involve particles so small that they cannot be seen by light microscopy. Milk placed under an optical microscope reveals only fat globules - but nothing else. Electron microscopy, however, shows the prevalent milk proteins, i.e., casein, as minute globules, about 100 nm in diameter (1 nanometer = 1x10-9 m). They are called ‘casein micelles’. Evenly dispersed in the milk and separated from each other by a distance of three micelle diameters in fresh milk, they are subject to Brownian motion and thus they do not settle at the bottom of the container. Electron microscopy makes it possible even to study the surface of the casein micelles. In fresh or boiled milk, the surfaces of the casein micelles are non-reactive. When two casein micelles collide, they ricochet and stay intact. This situation changes, however, when the casein micelles are destabilized. Lactic acid produced by bacteria such as Lactobacillus delbrueckii susp. bulgaricus, and Streptococcus salivarius subsp. thermophilus increases the acidity of the milk and destabilizes the micelles. After a certain acidity is reached, the micelles stick together. Observed by naked eye, the milk is seen to ‘coagulate’ or ‘curdle’.
Milk may be coagulated by other means, e.g., by proteolytic enzymes, used in the production of cheese. Yogurt and cheese, of course, have quite different structures. It is interesting to note that the way in which milk coagulates, is controlled by heating.
Traditionally, yogurt has been made in the Balkans and the Middle East by heating the milk to partially remove water. When the temperature of the milk reaches 85°C, one particular micellar protein (kappa-casein) at the surface of the casein micelles reacts with one particular whey protein (beta-lactoglobulin). This interaction produces minute ‘bumps’ on the casein micelle surfaces (frame A in the figure on the left). The beta-lactoglobulin-kappa-casein complex later prevents other casein micelles from getting attached at these sites. ‘Later’ means when yogurt bacteria metabolize lactose and produce lactic acid and the milk starts to coagulate.
The surfaces of the heated casein micelles are partially blocked, so only a few micelles can interact. This leads to the formation of short branched micellar chains. When the coagulation is complete, the milk has changed into a gel. Under an electron microscope, the gel looks like a sponge with small pores filled with the whey (scanning electron microscopy on the right, diagram below it).
Milk that has not been heated consists of casein micelles with smooth surfaces (frame B below frame A on the left). This milk is used to make cheese. Casein micelle surfaces interact with other casein micelles and form large micellar clusters from which whey separates easily (diagram lower on the right). The casein micelles become compacted to form curd which is then processed into one of the cheeses varieties. Cheeses have a markedly lower water content than yogurt.
The unique microstructure of yogurt means that all the liquid (whey) is immobilized within its body. Of course, no consumer would like to buy yogurt from which whey separates easily. This would be a sign that the yogurt is susceptible to ‘syneresis’, and that there was something wrong in the yogurt manufacture. If, for example, the milk is not heated at about 90°C for a time long enough (about 15 min), larger pores may develop in the yogurt body in some areas and larger clusters of casein micelles may develop in other areas. The whey then starts showing in the containers during storage. To be on the safe side, some yogurt manufacturers use small additions of various ‘thickening agents’ such as starch gel, various plant gums or pectin to the milk to improve the retention of water in yogurt. Water may also be retained in the yogurt by increasing the amount of milk solids but in this case the reduction of the pore sizes changes the overall mouthfeel of the yogurt and is not desirable.
The preceding statements about the solid nature of yogurt are correct considering the so-called set-style yogurt. There is also a liquid yogurt variety on the market. It is made in the same way as set-style yogurt and then is pumped into retail containers, whereby the body of the yogurt is broken into small particles, which allow the yogurt to be poured.
Illustrations (including lactic acid bacteria) may be obtained at cost from the author in the form of high-resolution 35-mm colour slides
Yogurt: Electron Microscopy - Dr. Miloš Kaláb