b'SENSE, NONSENSE AND SCIENCEBY: JOE SCHWARCZSAY CHEESE!C heese producers were cheesed off.of casein molecules which are insoluble inResearchers also discovered that cer-People were just not eating enoughwater and aggregate into tiny spheres calledtain fungi, Mucor miehei being a prime veal. Slaughterhouses were run- micelles. Since their density is compara- example, could produce enzymes that ning short of calf stomachs and the cheeseble to the surrounding solution, micelleswould cleave proteins much like chymosin. industry was feeling the pinch. There wasremain suspended. There are three kindsThis meant that cheese could be produced not enough rennet to meet the demands ofof casein molecules, referred to as alpha,without using any animal rennet at all. This cheese lovers around the world. beta and kappa-casein. Within the micelle,addressed the rennet shortage and met the Rennet is critical to the cheese-makingthe alpha and beta caseins are curled upneeds of vegetarians. Such cheese was also process. At least if you want to indulge inlike a ball of string and are held togetherkosher because there was no mixing of milk something that is a little more exciting thanby kappa casein, which functions muchand meat. cottage cheese. Traditionally it has beenlike an elastic band. The job of chymosinThen genetic engineering entered the made by washing, drying, macerating andis to break the band and allow the caseinpicture and essentially solved the chymo-brining the lining taken from the fourthmolecules to stretch out and form a long,sin problem. The bit of DNA, the gene, stomach of calves. This leads to a producttangled network of protein molecules thatthat gives the instructions for the forma-that is a mixture of two enzymes, chymo- settles out of the solution. Fats and mineralstion of chymosin was isolated from calf sin and bovine pepsin, both of which canget snared in this protein net, and presto,cells and was copied, or "cloned." It was coagulate milk and convert it into cheese.we have cheese! then successfully inserted into the genetic The stomach lining of mammals containsChymosin catalyzes this process. In anmachinery of certain bacteria (E. coli), these enzymes, because they are needed foracidic environment, it snips kappa-caseinyeasts (Kluyveromyces lactis) or fungi proper digestion. If milk did not coagulatespecifically, allowing the other caseins to(Aspergillus niger) which dutifully churned to some extent in the stomach, it would flowunwind. In the stomach, the acidic envi- out pure chymosin. Approved in 1990 by through the digestive tract too quickly andronment is created by cells that secretethe Food and Drug Administration in the its proteins would not be sufficiently brokenhydrochloric acid, whereas in cheese-mak- U.S., chymosin became the first product down into absorbable amino acids. ing a bacterial starter culture that convertsof genetic engineering in our food supply. Chymosin and bovine pepsin are pro- lactose into lactic acid is added before theIt is 100% identical to that found in calf teases, which means that they catalyze therennet. Bovine pepsin is not quite as suit- stomach, but because it does not come from breaking down of proteins, a task that isable because it has a more general proteaseanimals it is acceptable to consumers who central to the milk coagulation process.activity, snipping caseins in a variety ofdo not want meat products in their cheese. Milk consists of about 87% water, 3.5%ways. This weakens the protein networkExtraordinaryprecautionswere fat, 5% lactose, 1% minerals and 3.5% pro- needed to trap fat and results in a lowertaken before chymosin, made by recom-tein. The protein content consists mostlyyield of cheese. On top of that, some of thebinant DNA technology, was marketed. protein fragments it produces have a bitterRegulators ensured that no toxins of any taste and subtly alter a cheese\'s flavourkind had been introduced and that no profile. live recombinant organisms were present. By the 1960s the short- Indeed, the product contained nothing age of rennet was becom- but pure chymosin. Cheese made with it ing critical. The stomachsis completely indistinguishable from that of older animals wereproduced with animal rennet. In any case, pressed into service,chymosin itself is degraded during cheese but the resulting rennetmaking and none is left in the finished was not suitable. As anproduct. Today, in North America, over animal ages, chymosin80% of all cheese is made using chymo-production goes downsin produced by recombinant DNA tech-and pepsin productionnology. Cheese makers no longer have to increases. Scientistsworry about a shortage of calf stomachs tried anion exchangeand turophiles can satisfy their critical chromatographytotastebuds. Thanks to biotechnology they separate pure chymo- can "say cheese" and smile.sin from the stomach extracts of older ani- Joe Schwarcz is author, sessional instructor mals, but the processat McGill University and the director of was complicated and notMcGill\'s Office for Science and Society.cheap.26ISEED WORLD EUROPEISEEDWORLD.COM/EUROPE'