The Biochemistry of Milk

1. The Biochemistry of Milk Ch. 23

2. How do you milk a cow? • Video • http://www.moomilk.com/faq.htm#To%20make

3. The Complex Nature of Milk • Milk supplies the calcium for building bones and teeth, especially during adolescence • Milk is 87% water • However that’s a solvent for over 250 chemical compounds, milk sugar (what is the name for milk sugar_____?____) water-soluble vitamins and trace minerals and salts

4. Protein • Two main proteins make up milk: • 80% Casein = milk protein • 20% Whey = milk protein • Micelles = an aggregation or cluster, of molecules, often found in colloidal dispersions • The light reflecting from micelles makes milk white • As long as milk keeps its normal acidity level, about pH 6.6, casein remains stable

5. Protein Continued • When the pH level decreases the milk forms lumps or coagulates. • Curds = The casein clumps that separate from the liquid when milk coagulates • When cooking with milk you can prevent curdling by using starch to thicken the milk • If you want to coagulate milk, you can use the enzyme rennin or vinegar. • Whey = protein found in the liquid that remains after fat and casein have been removed from milk. • Whey is also called serum protein

6. Fats • Milk is an emulsion because small globules of fat (lipid) are dispersed throughout the water • Creaming = process in which some of the fat droplets come together in larger clusters that rise and float to the top of the milk • These clusters rise to the surface because fat is less dense than the watery portion of milk • Cream = simply milk that is extra rich in emulsified fat droplets • The fat in cow’s milk are most complex lipids, over 400 different fatty acids • Click For Milk Experiment

7. Carbohydrate • Lactose or milk sugar is the main carbohydrate • Lactose provides food energy • Also adds body to milk as well as a sweet flavor • When milk is heated, lactose reacts with amino acids in the protein (giving cooked milk a slightly caramel flavor) • When you consume milk, lactose becomes available to the body because of the enzyme lactase. • This enzyme breaks lactose down into galactose and glucose, which is used in the body for fermentation during digestion.

8. Carbohydrate Continued • Do you know someone who cannot drink milk??? • People can be lactose intolerance = the inability to digest milk due to the absence of the enzyme lactase in the intestines • People who are lactose intolerant cannot break down lactose in their body. When they drink the milk they can have bloating, stomach cramps and diarrhea.!!! -NOT FUN

9. Minerals and Vitamins • Milk has salt that occurs naturally • Salts in milk include chlorides and phosphates (in very low amounts) • Salt in milk helps prevent curdling • If an acid is added to milk (what would be an example of an acid), the pH lowers which removes the calcium ions from the casein, making the milk curdle – forming _____and _______ • Four vitamins found naturally in milk, riboflavin is found in the largest quantity. • Breaks down when exposed to light

10. Pasteurization • Shelf Life = time a food product can be stored before deteriorating. • When pasteurizing milk, its heated to high temperatures for a short time to destroy harmful bacteria • Pasteurization denatures enzymes that cause milk to spoil • Pasteurization has a small effect on milk’s nutritional value, creating a slight loss of thiamin, vitamin B12, vitamin C • DO YOU THIN THAT DRINKING UNPASTURIZED MILK IS BETTER FOR YOU?

11. Homogenization • Under pressure of 2,000 to 2,500 pounds per square inch, milk is forced through small openings in a machine called a homogenizer • The fat particles break down, an emulsifier surrounds the fat particle, which keeps the particles permanently separated • As a result the milk you buy does not have cream on the top.

12. Fortification • Fortification = the addition of a nutrient to a food • Ex. What nutrient is milk fortified with___?___ • Without vitamin D, your body can’t absorb calcium and phosphorus very well. • To fortify milk with vitamin D, it may be exposed to ultraviolet light, which convert some of the milk fat components to vitamin d

13. Fluid Milk/ Ultrahigh-Temperature Milk (UHT) • Fluid milk is categorized by fat content • Whole milk – 8g fat • Reduce-fat milk (2%) – 5g fat • Low-fat milk (1%) - 2.5g fat • Fat-free milk – trace fat • Ultrahigh-Temperature Milk (UHT) • With special processing milk can be stored without refrigeration for three months • This is called UHT because it’s prepared by heating for a short time at ultrahigh temperatures. • Placed in special sealed packages • Flavor improves over time

14. Concentrated Milk • Evaporated milk – is whole milk that has been heated at low pressures, causing up to 60% if the water present to evaporate at temperatures well below milks normal boiling point. • Carrageenin = a vegetable gum, is often added to evaporated milk before processing to stabilize the casein proteins • Sweetened condensed milk – canned product in which 50% of the water has been removed and sugar added. • Sweetened condensed milk is 44% sugar

15. Cream/Dry Milk • Half-and-half = is 10.5-18% fat • Light cream = 18-30% fat • Light whipping cream = 30-36% fat • Heavy whipping cream = contains at least 36% fat • Dry Milk – water is removed, leaving a dry solid material • Milk Solids = proteins, carbohydrates, fat, minerals, and vitamins that were dissolved in the liquid portion of the milk • Nonfat milk has a longer shelf life because it has the least amount of fat (used for baking an enriching products)

16. Cultured Milk Products • Culture = controlled bacterial population that is added to milk • The starter bacteria produce acid and flavors that are characteristic of each fermented milk product. • Commercial producers use lactic-acid bacteria to break down lactose • Inoculation = adding the starter culture to milk during the fermentation process that produces different cultured milk products • Incubation period = time needed for bacteria to grow and ferment the milk during the process of culturing milk to make various products – finally it’s cooled down to stop or slow bacterial growth What are some examples of cultured foods???

17. Buttermilk/Sour Cream/Yogurt • Buttermilk – used to be the fluid left over from producing butter, today it’s a cultured product prepared from low-fat milk • Sour Cream – culture of streptococcus lactis is added to cream. • Yogurt – eastern European food, yogurt contains lactose, lactic acid, B vitamins and high concentrations of protein • Yogurt is prepared by adding a bacterial culture to milk that has been heated • Bacteria used to making yogurt must be incubated between 41°C-45°C for around 8 hours

18. Cheese • Cheese is made by coagulating the casein protein in milk. • Once the curds have formed, the whey is drained away. (heating and cutting the whey helps this drain) • The curd is then treated and allowed to ripen • During ripening, the cheese is hung for at least 60 days, some take months • Four main categories of cheese are based on moisture content: • Very hard cheese – parmesan and Romano • Hard cheese – cheddar, Colby and provolone • Semi-Soft cheese – Munster, Roquefort, and stilton • Soft cheese – brie, camembert, and mozzarella Unripened Cheese – cottage cheese and cream cheese

19. Storing and Cooking with Milk • How many of you leave milk on your table when eating dinner?? • Is that safe?? – Even if it’s for ½ hour? • Milk retains it’s quality for 1-3 weeks when properly stored • Extra credit for someone who can tell me the correct temperature • Remember riboflavin is sensitive to light • Would whole milk or low fat milk spoil faster or would they spoil at the same time?

20. Making Foams • Several factors affect the ability of cream to make a foam. • Fat content – higher the fat level, the better the foam • Temperature – cold temperature increase the viscosity , including the beaters and bowl • Amount of cream – whipping small amounts of cream usually gives better results than whipping large amounts • Sugar content – adding sugar decreases both the volume and stiffness of the foam, its best to add sugar after the cream has reached the desired consistency

21. Heating Milk • Milk is sensitive and highly reactive to heat • Heat denatures and coagulates the whey proteins of fresh milk, causing them to precipitate • Precipitate = to cause a solid substance to separate from a solution • To prevent milk from scorching put milk in a double boiler or heat over VERY LOW TEMPERATURE • What happens to milk when you heat milk in a sauce pan that is not stirred?!?? • Skin develops, if you try to remove the skin it will regenerate and you lose milk solids • What is curdling and how does it happen

22. Questions • How does casein react to acids? • How are casein and why alike and different? • What makes milk a solution, a colloidal dispersion, and an emulsion? • Why is lactose a valuable nutrient in milk? • How do salts prevent milk from curdling? • How is milk processed? What is the purpose of each type of processing?

23. Questions Continued • What does the phrase “whole milk” mean? How does whole milk compare to other classifications of milk? • What is the difference between sweetened condensed milk and evaporated milk? • How are cultured milk products made? • Explain what ripening is and why it’s needed to make cheese. • To make a cream foam, would you get better results by using light or heavy whipping cream? WHY? • What might happen if you leave an open pan of milk over high heat without stirring?