rennet Ca++ casein -------> para-casein (aq.) --------> para-casein (ppt)The precipitate is soft at this point and can be separated from the whey by the use of cheese cloth. Filtration does not work very well; filter paper clogging is a recurrent problem.
There is no standard method of cheese making; limitless variations exist for all stages of the process: pre-ripening, curdling, addition of artificial ingredients and salt for flavor, and aging. This variation in processing accounts for the wide range of cheeses commercially available, differing in texture and flavor. The curd can also be processed with other techniques to make a variety of desserts. However, all processes have one thing in common: the separation of the curd from the whey.
Three ways of preparing milk for curdling will be introduced in this experiment. Industrially, the lactic acid level in the milk is increased by adding a starter culture of Streptococci or Lactobacilli to the milk and fermenting at 32ºC for 10 to 75 minutes. In addition to biologically converting the lactose present in the milk to lactic acid, these strains of microorganisms also greatly affect the flavor of the final product. Thus, the selection of a suitable strain, the amount of starter culture, and the length of pre-ripening, is of the utmost importance in creating the subtle differences in the final color and aroma that distinguishes an expensive cheese from a cheap one.
If one has not yet acquired a keen palate for cheeses, the second approach should suffice. In this approach, one takes advantage of the existing Lactobacillus culture in buttermilk and uses it as the starting culture. One ml of buttermilk is added per 100 ml of milk, and the mixture is then fermented at room temperature for 4-12 hours. At the end of fermentation, the temperature of the mixture is raised to 32ºC, and artificial coloring is added to the mixture prior to curdling.
The third way to prepare the milk in a short time frame is to add acid (HCl) and to heat to 32ºC. Of course, there leaves much to be desired in this method if you are a cheese connoisseur.
After rennet is added to the pre-cured milk, the coagulation process is started. In cheese making, as coagulation comes to completion, the temperature is gradually raised to about 38ºC. This slightly elevated temperature facilitates the separation of the curd from the whey. A higher temperature also hardens the curd. The curd may be hardened further by cooking it for a longer period of time, either with or without the whey.
After the curd is separated from the whey, salt, seasoning, and other curing and flavoring ingredients are added. The curd is wrapped in cheese cloth and pressed for 12 to 18 hours to remove the additional whey soaked in the curd. The curd hardens and forms a cheese block in the shape of the press as the whey is squeezed out. Finally, the cheese block is dried for 6 hours.
It is now ready for consumption, or it may be left to age in a controlled cool environment (2-13ºC). Although a higher temperature promotes faster curing, there is also a higher chance of spoilage due to undesirable microbial activities at elevated temperatures. Prior to aging, the cheese block is usually wrapped tightly to exclude air and microbial contaminants from entering and spoiling the cheese. One way to accomplish this is to dip the cheese block in a pot of melted wax. During the aging process, many complicated microbial and chemical actions continue to take place in the cheese block. Thousands of techniques exist to develop various distinctive flavors. These reactions are not well characterized; thus, cheese making is still an art rather than a science. Depending on the technique employed, this final aging process takes any where from 2 weeks to 6 months.
| A | 0 hr | B | 2 hr | C | 4 hr | D | 6 hr | E | 8 hr | F | 10 hr | G | 12 hr | +------------------------------------- NUMBER OF HOURS INCUBATED
Chemical engineering is the study of how to make a large quantity of chemicals in an economical fashion. For example, frequently a chemical engineer must devise a process to mass-produce a polymer that is totally different from the one originally used by an organic chemist working with small test tubes and beakers in a laboratory. Biochemical engineering, being a sub-field of chemical engineering, also deals with the same kinds of problems facing chemical engineering, except that the chemicals are not synthetic (manmade) but biological (naturally existing) in nature. Although rennet is naturally excreted from a calf's stomach lining, extracting it from its natural source is not economical. Other proteases can also convert casein to para-casein, but their action does not stop there. They further degrade the curd to soluble subunits. Fortunately, large quantities of rennet of consistent quality can now be produced easier and cheaper in a well controlled environment by microbial fermentation.
A word of caution is in order here. Enzymes as a class of chemicals are not generally considered as dangerous, toxic, nor poisonous; they do not cause skin irritations or burns as acids or bases. Some exceptions are proteases that catalyze the breakdown of protein molecules to amino acids components. Because meat is mainly protein, protease can digest the soft moist sections of the skin. We can all imagine how that is going to feel. Thus, you should exercise the same caution with enzymes as you do with any other chemicals.