The Biochemistry of Milk

1. The Biochemistry of Milk

2. The Complex Nature of Milk • Vital to health - nutritious • Versatile for cooking – 87% water (a solvent for over 250 chemical compounds, the milk sugar lactose, water-soluble vitamins, trace minerals and mineral salts) • Milk is a solution, a colloidal dispersion, and an emulsion

3. World use of milk • Around the world, people use milk from many different kinds of animals, but cow’s milk is the most common type consumed. • Milk contains all major nutrients: water, protein, carbohydrates, vitamins and minerals… • The exact composition depends on the cow’s breed, health and environment.

4. Proteins • 80% of milk protein is casein • Casein molecules associate with each other in tiny, raspberry-like structures called micelles – an aggregation, or cluster of molecules often found in colloidal dispersions • As long as milk keeps its normal acidity, level (pH - 6.6), casein remains stable…if an acid is added, the casein coagulates to form curds. When cooking, you can prevent curdling by using starch to thicken the milk. The starch surrounds the casein and prevents coagulation.

5. Proteins • 20% of milk protein is whey • Whey is the protein found in the liquid that remains after fat and casein have been removed from milk • Whey is also called serum protein. Heat will easily coagulate whey, although heat has little effect on coagulating casein.

6. Proteins • In addition to the proteins, casein and whey, milk contains a number of enzymes that are also protein molecules. Most of these are denatured during pasteurization. • Another way to coagulate milk is with the enzyme rennin, which is used to clot milk in the first stage of cheese making.

7. Fats • Milk is an emulsion because small globules of fat are dispersed throughout the water. Around each fat globule is a thin membrane containing proteins, phospholipids and bound water, suspending clusters of fat in droplets. • Creaming – fat droplets form larger clusters that rise and float to the top. • Cream is milk that is extra rich in emulsified fat droplets

8. Fats • The fats in cow’s milk are the most complex lipids known. The lipids include over 400 different fatty acids. • Milk fat contains very little cholesterol compared to egg yolk. • The fats in milk influence the flavor, texture and price – the more fat, the higher the price.

9. Carbohydrate • Milk sugar – Lactose • Lactase – enzyme that breaks lactose into glucose and galactose • Lactose intolerance – no lactase in intestines – lactose remains undigested, then certain intestinal bacteria attack and ferment the lactose, producing acids and gas – this causes discomfort • Cultured milk products (yogurt, buttermilk)

10. Vitamins and Minerals • Minerals – in the form of salts – chlorides, phosphates, citrates of potassium, calcium, sodium, and magnesium (salts prevent milk from curdling) • Trace elements – cobalt, copper, iodine, iron, magnesium, nickel, molybdenum • Vitamins: • Riboflavin – breaks down when exposed to light – this is why milk cartons are opaque • Thiamin • Niacin • Vitamin A

11. Processing Milk • Pasteurization – heated to a high temperature for a short time – destroys pathogenic bacteria • Homogenization – eliminates creaming – under pressure, milk is forced through small openings to break down fat and keep the fat molecules permanently separated • Fortification – Vitamin D is added (helps the body absorb calcium and phosphorus), and Vitamin A (a fat-soluble vitamin) is added back to low-fat milk products (by law)

12. Types of Milk Products • Fluid milk • Ultrahigh-Temperature Milk (UHT) – in sealed box – long shelf-life (3 months or more if unopened) • Concentrated milk – Evaporated – carrageenin is added to stabilize the casein – off white color is caused by chemical reaction of lactose and amino acids • Sweetened condensed – 50% of water is removed, 44% sugar is added – used in dessert recipes

13. Types of Milk Products • Cream • Half and Half – 10.5% to 18% fat • Light cream or Coffee cream – 18% to 30% fat • Light whipping cream – 30% to 36% fat • Heavy whipping cream – at least 36% fat • Whipping cream is often UHT pasteurized to lengthen shelf life

14. Types of Milk Products • Dry milk – dried into powder form – milk solids can be reconstituted by adding water • Non-fat dry milk has longer shelf life – no fat to oxidize and spoil the product • Cultured milk products • Fermented – buttermilk, yogurt, cheese • Process – pasteurized, lactic-acid bacteria breaks down lactose (lowers the pH), inoculation (starter is added), incubation, cooling

15. Cheese • Made by coagulating the casein protein • Ripening – aged – makes the flavor and texture distinct • Moisture content varies • Very hard – Parmesan, Romano • Hard – Cheddar, Colby, provolone • Semisoft – Muenster, Roquefort • Soft – Brie, Camembert, mozzarella • Unripened – cottage cheese, cream cheese, mozzarella

16. Storing Milk Products • Keep it COLD!! • Use closed, opaque containers – riboflavin is light-sensitive • Refrigerate cans or UHT boxes after opening • Store non-fat dry milk at room temperature – keep moisture out • Keep cheese cold, although it tastes better when served at room temperature – wrap carefully to keep it from drying out

17. Cooking with Milk • Making foams • More fat = higher viscosity (thickness) = better whipped cream • Cold temperatures increase viscosity – whip cream when it’s cold!! Use cold mixing bowls and beaters • Whip small amounts • Adding sugar decreases the volume and stiffness of the foam, so add sugar after whipping the desired amount of air into the foam

18. Cooking with Milk • Heating Milk • Milk scorches – it’s highly sensitive and highly reactive to heat – heat denatures and coagulates the whey proteins of fresh milk, causing them to precipitate, then it settles to the bottom, and scorches – use a DOUBLE BOILER to prevent this • When heated till scalding in open containers, evaporation causes a skin to form on top (marshmallow or whipped cream prevents this) • Curdling can occur at high temperatures – use low temperatures and non-acid ingredients to prevent this

19. Cooking with Milk • Enzyme action • Some desserts use rennet to make clotted milk - use moderate temperatures so enzyme is not inactivated • Acids • Causes milk to curdle (like when you add fruit to your milk and cereal or lemon juice to tea and milk)