edited by: MICHAELR. S L A B A U ~ H HELEN J. JAMES Weber Stale College Ogden. Ulah 84408
Curdling Chemistry-Coagulated Milk Products Craig J. Oberg Department of Microbiology, Weber State College, Ogden, UT 84408
As early as 2000 B.C. writings from the East mentioned cheese, including the fact that i t was used in trade. The Greeks offered cheese to their gods and the Romans consumed it as a delicacy. Crusaders in the 11th to 13th centuries brought new varieties of cheese from the East to Europe, where cheese was already a well-established food ( I ) . Since then manv new varieties of cheese have been produced and cheese has become an increasingly important product. In 1983.4.8 billion lhs of cheese were produced in the United states, including 2.75 billion lhs of American (Cheddar) cheese (2) Hovine (COW) milk is a l~i~~logiral fluid with smnr uniqw ch:lr:wt(,ri.;~:csthat result from thearran~ementofthe w l ~ d i suspended in it. On the average, milkcontains only 87% water. This is considerably less water than is found in celery, ereen beans. or lettuce. Milk also contains 3.5-3.7% fat. of which 98% are triglycerides synthesized in the lacteal glahds of the udder. Historically, the fat content of milk has been the basis of payment to the dairy farmer, making fat the most valuable component in milk. The fat is suspended in milk in the form of spherical nlobules. 1-20 um in diameter. Each globule is surrounded 6y a membrane containing a high conc&ration of phospholipids. Homogenization is a process in which the fat globules aie forced through a seriei of very small orificesat high pressure. The globules are ruptured into small pieces. A membrane quickly reforms, stabilizing the fat globules a t the smaller size. While most fluid milk is homogenized, the milk used to manufacture cheese is rarelv homogenized. " Lactose (milk sugar) is found a t a constant concentration of 4.9% in milk. Its concentration is used to maintain osmotic equilihriulo Iretween tlw milk and the hlwd. I'r~teinomtent idmilk is amruaimatel\~:MCirbs u'eiahr~ which includes the caseins a$ the whe; proteins (alGhalactalhumen and beta-lactoelohulin), along. with a small amount of immunoglobulins. Protein levels vary according to time of year, type of feed, and the period of lactation of the cow. ~ l ~ h a - l a c t a l h u m ehas n some &king similarities to a protein found in chicken eggs called eggwhite albumen. Much of their amino acid seouence is identical. and their structural conformation and' molecular weights are the same. They also both react with certain types of sugar molecules. Aoproximatelv 80% of the protein in milk is the caseins. Cas& is the fractionthat precipitates (settles out) of milk a t pH 4.6 and 20 "C. There are three maior tvpes of casein proteins. Alpha casein comprises 50-55%;f the total casein. This protein has a hydrophilic (water-loving) center surrounded hy hydrophobic (water-hating) ends and is extremely sensitive to calcium. Beta casein, 30-35% of the total casein, is also very hydrophobic, but is less sensitive to calcium. Kappa casein comprises only about 15% of the total hut plays an integral role in the stahilizationof the micelle and in the enzymic coagulation of milk. It has a heavily glycosylat770
Journal of Chemical Education
ed (sugar molecules attached to the amino acids) end that is very hydrophilic. It is also insensitive to calcium. The caseins are assembled into small spherical structures called micelles. Casein micelles have an average diameter of 50-300 nm. much smaller than the fat elohules (3). Thev im11.m thf: white color I
