8.1 Milk and Milk Products
Objectives
- To illustrate coagulation and ways of coagulating the different milk proteins.
- To demonstrate some of the basic concepts of protein cookery.
- To compare the appearance, texture, and flavor as well as compositional differences among various types of milk products other than cheese.
Laboratory Problems
- Observe the effect of heat on the curd and whey of coagulated milk:
- Effect of heat on sour milk,
- Effect of heat on rennin-clotted milk.
- Observe the effect of rennin on milk.
- Observe the effect of heat and acid on milk.
- Prepare cream of tomato soup.
- Taste a variety of milks and milk products.
Terms
- Denaturation
- Coagulation–cheese is made when milk is coagulated with rennin (enzyme) or acid
- Casein Protein
- Whey Protein
- Isoelectric Point (ph 4.6 isoelectric point of casein protein)
Observe the effect of heat on the curd and whey of coagulated milk:
Coagulate Casein with Acid:
- In small saucepan, combine 1 tbsp. distilled vinegar or lemon juice to 1 cup milk. Stir just to blend.
- Allow mixture to stand until milk has thickened, about 10 minutes.
- Heat slowly, over low heat, to temperature specified in table on following page. Remove from heat.
- After 5 minutes, separate curd from whey by straining mixture into a glass measuring cup through small squares of cheesecloth in a strainer.
- Roll curd gently on the cloth to remove the whey.
- Display the whey in measuring cup and the curd on the cheesecloth.
- Record observation of firmness and texture of curd, and appearance and amount of whey.
| Procedure | Curd | Whey | Explanation |
|---|---|---|---|
| A. Sour milk heated to 60°C | |||
| B. Sour milk heated to 100°C |
Coagulate Casein with Rennin.
- Dissolve 1 rennet tablet in 1 tbsp. water.
- Heat 1 cup milk to 45oC in a small saucepan; remove from heat.
- Stir dissolved rennet into heated milk. Allow mixture to set, approximately 20 minutes.
- Heat slowly on low heat without stirring to specified temperature. Remove from heat.
- After 5 minutes, cut through the curd in each direction so that the surface area is cut into 1-inch squares.
- Separate curd from whey by straining mixture into a glass measuring cup through a double thickness of cheesecloth lining a strainer.
- Roll the curd gently on the cloth to remove the whey.
- Display the whey in measuring cup and the curd on the cheesecloth.
- Record observations of firmness and texture of curd, and appearance and amount of whey.
| Procedure | Curd | Whey | Explanation |
|---|---|---|---|
| C. Rennin clotted milk heated to 60°C | |||
| D. Rennin clotted milk heated to boiling |
Observe the Effect of Rennin on Milk:
- Dissolve 1 rennet tablet in 4 tsp. water. Use 1 tsp. of this solution with each ½ cup milk treated as specified below.
- Prepare milk as directed, stir rennet in quickly, pour at once into custard cup, and set cup in a warm place.
- Allow at least 20 minutes for gel to set. Test by carefully tipping container. Record observations and conclusions on the effect of temperature and dilution of milk on the activity of the enzyme, rennin.
| Procedure | Firmness of gel | Results and explanation |
|---|---|---|
| 1. Heat milk slowly to 45°C | ||
| 2. Boil milk, cool to 45°C | ||
| 3. Boil milk, add rennin immediately | ||
| 4. Use ¼ cup milk and ¼ water. Heat to 45°C |
Observe the effect of heat and acid on milk.
Use ¼ cup fresh milk and ¼ cup tomato juice for each method of combining acid and milk described in the chart on next page. Stir to blend.
| Method | Immediate Results | Results after 15 minutes | Explanation |
|---|---|---|---|
| A. Cold tomato juice added to cold milk | |||
| B. Hot (180°F) tomato juice added to hot (180°F) milk | |||
| C. Hot (180°F) milk added to hot (180°F) tomato juice |
Prepare Cream of Tomato Soup
Cream of Tomato Soup
| Tomato Sauce | |
| ½ cup tomato juice | ½ tsp. sugar |
| dash pepper | small piece of bay leaf |
| White Sauce | |
| 1 tsp. celery, chopped fine | 1 tsp. onion, chopped fine |
| ½ tsp. green pepper, chopped fine | 2 tsp. butter |
| 2 tsp. flour | dash salt |
| ½ cup milk |
Simmer tomato juice, sugar, pepper and bay leaf for five minutes. Remove bay leaf. In a separate pan, sauté celery, onion, and green pepper in butter. Blend in flour and salt; remove from heat. Add milk, stirring to blend, return to heat and stir gently while heating to a full boil. Slowly stir hot tomato juice into hot white sauce. Serve immediately.
Characteristics of a Standard Product for Cream of Tomato Soup:
| Appearance | Texture and Consistency | Flavor |
|---|---|---|
|
Typical color No skin or fat film |
Smooth — neither curdled nor lumpy Definite body; neither watery nor too thick |
Well seasoned Appealing blend of flavors |
Evaluation:
| Product | Appearance | Texture | Flavor |
|---|---|---|---|
| Cream of Tomato Soup |
Questions for Cream of Tomato Soup:
- What textural differences are there between acid and rennin coagulated cheeses?
- What is the pH of milk, the pH of tomato juice, and the pH of cream of tomato soup?
- What is the isoelectric point of casein in milk? When making cream of tomato soup, do we want to aim for or avoid the isoelectric point (Ip) of milk? Why?
- Therefore, what is the general rule for combining milk with acid ingredients such as tomato juice?
- In the tomato soup recipe, we want to avoid the isoelectric point of casein, but in some food products we aim for the isoelectric point of casein. Give an example of this type of food.
Describe the distinguishing sensory properties of each of the following dairy products:
| Name of Product | % Fat Content, etc. | Processing | Sensory Characteristics |
|---|---|---|---|
| Nonfat milk (skim milk) | 0.0-0.5% fat; >8.25% milk solids | ||
| Lowfat milk | 0.5-1.0% fat; >8.25% milk solids | ||
| Reduced fat milk | 1.0-2.0% fat; >8.25% milk solids | ||
| Whole milk | At least 3.25%; >8.25% milk solids | Homogenized | |
| Half-and-half | 10-12% fat | ||
| Whipped cream | Light: 30-36% fat
Heavy: 36-40% fat |
||
| Butter | 80% milk fat | ||
| Margarine | 80% vegetable or animal fat | ||
| Nonfat dry milk | 0.0-0.5% fat | ||
| Evaporated milk | At least 7.5% protein; >25% milk solids (skim version is also available) | Homogenized, sterilized | |
| Sweetened condensed milk | At least 8.5% protein; >28.5% milk solids; 44% added sugar (lowfat version is also available) | Condensed | |
| Cultured buttermilk | 0.0-1.0% fat; >8.25% milk solids | Cultured (streptococcus lactis and/or Leuconostoc citrovorum) | |
| Yogurt | Varies by type, 0.0-3.5% fat | Cultured (streptococcus thermophilus and/or Lactobacillus acidophilus or Lactobacillus bulgaricus) | |
| Sour cream | 18-20% fat | Cultured (streptococcus lactis and/or Leuconostoc citrovorum) |
