Protein networks are responsible for the structure and mechanical properties of many foods such as eggs and meat. Even bread gets its chewy texture from the formation of springy gluten protein networks. As we will see in this recipe, protein network formation is vital for the successful production of cheese. Milk, the primary component of cheese, is made up of fat globules, the sugar lactose, and a variety of proteins that can be divided into two categories: whey proteins and curd proteins. The most prevalent proteins in milk are curd proteins called caseins, comprising approximately 80% of the total milk protein. Casein proteins are not particularly well structured [1] and gather together into large, calcium-rich assemblies called “micelles.” Casein proteins at the surface of each micelle expose short negatively charged regions that improve micelle solubility and prevent multiple micelles from clumping together [2,3].
Caseins are remarkably heat stable, even at temperatures as high as 140˚C, and therefore require additional methods to promote curd formation. Milk curds can be formed by using special enzymes or by lowering the pH of the milk. Chymosin (a digestive enzyme found in rennet) selectively removes charged regions from the surface casein micelles; altering the exposed charge of the casein micelles by causes them to clump together to form curds. The addition of acid neutralizes the negative charges on the surface of casein micelles, causing them to clump together. Acid can also solubilize the calcium phosphate “glue” holding micelles together and eventually cause casein micelles to disassemble completely [2,5]. This recipe is a fast and simple way to make cheese at home. Ricotta (“re-cooked” in Italian) is traditionally made from whey, the liquid that remains when curds have been formed and separated during a previous round of cheese-making. A true Italian ricotta therefore contains very little casein and is instead made by coagulating leftover whey proteins. Because this recipe starts with milk instead of whey, it produces a soft, spreadable cheese reminiscent of ricotta but more similar to Indian paneer or Mexican queso blanco.
Structure of a casein micelle. (A) The exact structure of casein micelles remains unknown, but they are thought to be made of many smaller “submicelles” held together by a calcium phosphate. (B) Short negatively charged regions of casein proteins (“protruding chains”) are exposed all over the surface of the micelle. (C) An electron micrograph of a single casein micelle; scale bar represents 100nm. Figures A and B are taken from [2]; Figure C is from [4].
