Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. The manufacturers suggest that 2–3 g of chymosin powder will curd 100 L of milk under the right conditions. This kit contains 2 g of chymosin, which could theoretically coagulate more than 100 L of milk. For each class it is recommended that a fresh batch be mixed by dissolving 0.3 g of chymosin in 100 mL of bottled water (pH tends to be 7.0).
2. You will need to devise a way to hang the bags of curding cheese in a refrigerator so that the liquid can drain. Some refrigerators have wire shelves and the strings can be tied from them with drip pans below. If this is not possible, perhaps a shelf can be removed and the ring stands placed in the refrigerator. Be creative with the equipment available to you.

Safety Precautions

Real cheese is made in controlled food preparation environments to assure quality and human safety. The cheese you produce should not be eaten. Be sure to wash thoroughly after handling the cheese materials and products. Be careful around sources of heat such as hot plates or stoves. Follow all other normal laboratory safety rules.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. All materials can be disposed of following your normal disposal procedures for food items.

Teacher Tips

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. This laboratory requires about 30 minutes for the initial setup and parts of two other class periods to complete the laboratory. Additional class time can be spent if other experimentation is done.

• This lab provides a perfect opportunity for class discussions of advances in biotechnology, such as cloning and gene therapy, that make it possible to produce bioengineered enzymes. What are the advantages and disadvantages?
• If room temperature is cooler that 25 °C, it may take longer to obtain the initial curd from the milk/chymosin mixture.
• Keep the chymosin solutions refrigerated when not in use.
• Chymosin is an excellent enzyme for basic studies of enzyme chemistry. It is safe, can be observed with the naked eye, and experiments can be done quickly with simple equipment.
• The small cheese ball produced by students will not have the consistency of commercial cheese. Achieving commercial cheese requires months of careful aging and techniques not used in this lab. The cheese will likewise not be colored like some commercial cheese, nor flavored. Their cheese will smell and taste like cheese, however.

Further Extensions

Further topics for students to discuss and research:

1. How could cheese be made faster?
2. How do cheese-makers make different varieties of cheese?
3. What precautions must be taken when making cheese?
4. What is spoiled cheese? How do you know if it is spoiled?
5. What is added to cheese to make it more marketable?
6. Are there any medical problems associated with eating or making cheese?
7. What happens to goat’s milk, buttermilk, whole milk, skim milk, cream, etc., when cheese is made from these starting materials?
8. How do pH, temperature and bacterial type affect the cheese-making process?

Introduction

Cheese is a food product made from milk. The many forms and tastes of cheeses result from the varieties of bacteria used, the type of milk used and various other “secret” ingredients and procedures. Let’s experiment with making cheese.

Concepts

• Enzyme activity/chymosin

• Curds/whey
• Biotechnology

Background

Milk is often considered to be one of nature’s most “complete” foods with its mixture of fats, proteins and other nutrients. Milk is an emulsion, a mixture of two normally immiscible liquids, fat and water. The fat and water are held together by a natural emulsifying agent, called casein, the name for a group of proteins. If the protein (casein) is destroyed, the emulsion breaks down into its component parts and the insoluble calcium caseinate and fat precipitate out of the suspension. The “solid” portion that precipitates out of milk is called the curd. The “liquid” portion of the milk that remains after the curding process is called the whey.

Fermentation is a process in which microorganisms, such as bacteria or yeast, grow in a food and produce acids or alcohol and carbon dioxide. When milk is allowed to ferment, lactic acid is produced. The lactic acid curdles the milk protein, forming a solid curd and clear liquid whey. The whey is drained from the curd and then the curd is allowed to age.

The process of separating milk into curds and whey has been used for centuries. The process of forming curds has several advantages for preserving and using milk as a food source. Milk has a very short shelf life and spoils very easily. (Bacteria like it too!) Turning milk into curds does several things to help keep milk protein unspoiled. In solid form, the milk curds can be dried and treated in a variety of ways to prevent bacterial growth and spoilage. In addition, the curdling process concentrates the food value into a small, solid material that is easier to store. Further treating the curds and turning it into cheese gives a whole new set of food textures and tastes. Different curing techniques, subtle variations in additives, varietal cultures of bacteria/fungi and processing stages produce the differences in taste, texture, color, and aroma of cheeses. However, most cheeses begin using the same coagulation procedure used in this laboratory activity.

The consistency and amount of curd produced from a batch of milk depends upon the availability and concentration of chymosin, the enzyme that converts liquid milk to curds and whey. Historically, the enzyme source used by cheesemakers has been from extracts taken from the stomachs of calves while they are still nursing their mothers’ milk. The stomach extract, called rennet, provides a high concentration of enzyme and works very quickly to curd milk. The chore of maintaining calves on a milk diet and extracting the stomach enzymes has prompted the continual exploration for alternative sources of the enzymes for making cheese.

With recent advances in DNA biotechnology, a new source of curdling enzymes has been developed. The first step was to identify the gene for the curdling enzyme, chymosin. Once the gene was isolated, recombinant DNA technology was used to splice the gene into a host bacterial plasmid. The “recombinant” plasmid was then introduced into the bacterium, which began producing the enzyme. The enzyme is then extracted, dried and stored until needed. Chymosin was one of the first products produced by this method that was approved by the Food and Drug Administration.

Materials

Safety Precautions

Real cheese is made in controlled food preparation environments to assure quality and human safety. The cheese you produce should not be eaten. Be sure to wash thoroughly after handling the cheese materials and products. Be careful around sources of heat such as hot plates or stoves. Follow all other normal laboratory safety rules.

Procedure

1. Mix 50 g of powdered milk in 500 mL of water and heat using a hot plate or stove until the milk is 37 °C. Do not heat too quickly or the milk will scald. Stir continuously and remove from the heat when the milk reaches 37 °C.
2. Add 3 mL of chymosin enzyme solution to the milk and stir thoroughly.
3. Cover the container with plastic wrap or other loose fitting cover. Let the mixture sit at room temperature until a firm curd has separated from the whey. The time it takes to curd will depend upon the room temperature and the fluctuation in the room temperature. Check the chymosin/milk mixture at 24-hour intervals until a firm curd is obtained. This will likely occur within 24 to 48 hours.
4. Cut a large square of cheesecloth and fold it two or three layers thick. Ask a partner to hold the cheesecloth over a 600–mL beaker like a loose net. Pour the curd/whey mixture slowly into the cheesecloth sack. Gather the edges together to form a small cheesecloth bag. Tie the top of the bag together with string, leaving enough string to hang the bag.
5. Hang the bag from a ring support (or from other devices as directed by your instructor) and allow the whey to drain from the bottom of the bag into the 600-mL beaker.
6. Transfer the hanging bag and collection beaker to a refrigerated area as directed by your instructor. Allow the whey to drain for 48 hours.
7. After aging for 48 hours, inspect your bag of cheese. How does the amount of whey compare to the original amount of milk? How much cheese is obtained from 500 mL of milk? Untie the bag and inspect your cheese. Observe its color, odor, texture and other physical properties. You might experiment further with cheese-making and try answering questions of interest to you. What happens if cheese is aged longer? What effect will temperature have on aging? What effect would using different milk types have on the cheese? What about cream?