The process of milk curdling or clotting requires the action of enzymes. An enzyme is a “globular protein which acts as a biological catalyst by speeding up the rate of a chemical reaction.” (Cornell) Rennin or chymosin is the active enzyme in rennet, a proteolytic enzyme (protease) found in Zingiber officinale which causes milk to curdle. Rennin is also used to curdle milk and is found in the stomach of young mammals.
These enzymes are composed of amino acid chains and they lower the activation energy. By increasing the temperature, their kinetic energy also increases hence the rate would increase. However, at extremely high temperatures, the enzyme becomes denatured where the secondary and tertiary structures of the enzyme is altered and disrupted. An enzyme’s tertiary structure determines its function and so a disruption to the structure also disrupts its activity. High temperature cause the hydrogen bonds and nonpolar hydrophobic interactions in these structures to be disrupted and broken, unfolding the polypeptide chains (Ophardt). This means that its active site is altered as well and hence the substrate cannot bind to it, and so the reaction slows down or is inhibited due to the denaturation of the enzyme.
Milk clotting activity is a result of the interaction between renin and casein proteins found in milk. Casein proteins bind to produce ball like structures called micelles that are held intact by calcium ions. ?-casein coat the outer surface of the micelles. It consists of a hydrophilic portion, the macropeptide or glycomacropeptide, and a hydrophobic portion, the para-?-casein (Dalgleish). The hydrophilic proteins “migrates to the surface and leaves the micelles covered by a ‘hairy’ layer” (shown in the second stage in figure 3.), allowing for the micelles to be soluble in water and preventing them from binding to one another and aggregating (Lersch).
Hence, due to the “hairy” layer, milk remains stable and liquid. By adding rennin (or chymosin), the hydrophilic coating is hydrolyzed and so ?-casein loses its water-soluble tail and therefore the micelles are able to combine, and with the calcium ions, result in the production of aggregates, creating a gel like substance (Lersch). This process is illustrated in figure 3.