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atomic scale to produce small whole number ratios of atoms of the various elements Mixtures can be either homogenous or heterogeneous If a substance is a Heat the milk in the 500 mL beaker to 88-90°C (190°F) using the Bunsen burner

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C C C h h h e e e m m m i i i s s s t t t r r r y y y I I I

Name ______________________________________

© 2005 Van Der Sluys Witness initials ________________________1

Title: Preparation of Cottage Cheese

Background: Materials can be classified as either pure substances or mixtures. Pure substances can either be elements or compounds. Elements consist of all the same type of atoms, whereas compounds consist of more than one type of atom, combined on the atomic scale to produce small whole number ratios of atoms of the various elements. Mixtures can be either homogenous or heterogeneous. If a substance is a homogeneous mixture, it should be possible to separate the component pure substances from one another by various physical or chemical methods, such as filtration, distillation, precipitation, etc. In this experiment you will investigate milk to determine if it is a pure substance or a mixture by preparing a fresh soft cheese, often referred to as "Cottage Cheese". Materials: Bunsen burner, source of natural gas, 450 mL of 2% pasteurized milk, stir rod, thermometer, 15-20 mL of white vinegar, two 500 mL beakers, a large filter funnel, sodium chloride (optional), plastic spoon or fork and filter paper (coffee filters will do).

Procedure:

1

1. Heat the milk in the 500 mL beaker to 88-90°C (190°F) using the Bunsen burner.

Continuously stir to prevent scorching the milk. It is best to use a thermometer to monitor the temperature, but if a thermometer is not available, simply turn off the heat just before the milk begins to boil.

2. Add the vinegar and allow the mixture to cool back to near room temperature.

3. When cool, pour the mixture, (which now consists of curds and whey as in the

Miss Muffet nursery rhyme) into a filter paper and funnel and separate the curds from the whey. The whey is the liquid that passes through the filter paper. Your teacher will demonstrate how the set up the filter funnel and paper.

4. Pour the curds into a bowl and sprinkle on the sodium chloride (salt) and mix

well. You may wish to use less salt or more, it is simply a matter of your taste preference for salt.

5. Using the plastic spoon or fork, taste your cottage cheese and answer the

questions in the data analysis section. 1 Modified from the recipe given by http://schmidling.com/making.htm C C C h h h e e e m m m i i i s s s t t t r r r y y y I I I

Name ______________________________________

© 2005 Van Der Sluys Witness initials ________________________2

Data Analysis

1. Briefly describe the color and texture of the Cottage Cheese that you have

prepared.

2. Describe the taste of your cottage cheese. How does it compare with store bought

cottage cheese? What compounds in the milk or vinegar might cause the flavor?

3. Speculate if the Cottage Cheese is a pure substance or a mixture. Briefly explain

your reasoning

4. Describe the color and appearance of the whey and identify at least one

compound in this mixture.

5. Is milk a pure substance or a homogeneous mixture? How does this experiment

support your conclusion? Hint: reread the background section. C C C h h h e e e m m m i i i s s s t t t r r r y y y I I I

Name ______________________________________

© 2005 Van Der Sluys Witness initials ________________________3

For the Teacher

If possible, it is suggested that the teacher obtain some raw, un-homogenized milk to show the students. The cream in raw milk should readily separate from the milk and thereby, re-enforce the concept that milk is a mixture. Some discussion of the homogenization and pasteurization processes is probably appropriate in a discussion. Milk is an oil-in-water emulsion, with the fat globules dispersed in a continuous skimmilk phase. If raw milk were left to stand, however, the fat would rise and form a cream layer. Homogenization is a mechanical treatment of the fat globules in milk brought about by passing milk under high pressure through a tiny orifice, which results in a decrease in the average diameter and an increase in number and surface area, of the fat globules. The net result, from a practical view, is a much reduced tendency for creaming of fat globules. Three factors contribute to this enhanced stability of homogenized milk: a decrease in the mean diameter of the fat globules (a factor in Stokes Law), a decrease in the size distribution of the fat globules (causing the speed of rise to be similar for the majority of globules such that they don't tend to cluster during creaming), and an increase in density of the globules (bringing them closer to the continuous phase) oweing to the adsorption of a protein membrane. In addition, heat pasteurization breaks down the cryo- globulin complex, which tends to cluster fat globules causing them to rise. 2

Milk is a

complex mixture of fats, proteins (caseins and whey proteins), sugar (lactose), minerals, vitamins and water, all of which can be separated from one another to produce various dairy products. 3 Note that lactose intolerant students do not have enough of the enzyme necessary to digest the monosaccharide and should avoid tasting the cheese. The addition of vinegar causes a denaturing of the caseins, and the precipitation of the curds by protonating amino groups and disrupting hydrogen bonding interactions of the proteins. A number of state standards are addressed by this experiment. 4

As specifically stated in

the descriptor for section 3.4 Physical Science, Chemistry and Physics, "Physics and chemistry involve the study of objects and their properties. Students examine changes to materials during mixing, freezing, heating and dissolving and then learn how to observe and measure results. In chemistry, students study the relationship between matter, atomic structure and its activity. Laboratory investigations of the properties of substances and their changes through a range of chemical interactions provide a basis for students to understand atomic theory and a variety of reaction types and their applications in business, agriculture and medicine." Also, under the heading of 3.4.10, "Apply knowledge of mixtures to appropriate separation techniques."quotesdbs_dbs7.pdfusesText_5