The Biochemistry of Milk

The Complex Nature of MilkVital to health - nutritiousVersatile 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

World use of milkAround 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.

Proteins80% 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.

Proteins20% 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.

ProteinsIn 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.

FatsMilk 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

FatsThe 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.

CarbohydrateMilk sugar – LactoseLactase – enzyme that breaks lactose into

glucose and galactoseLactose 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)

Vitamins and MineralsMinerals – 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 opaqueThiamin NiacinVitamin A

Processing MilkPasteurization – 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)

Types of Milk ProductsFluid milkUltrahigh-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

Types of Milk ProductsCream

Half and Half – 10.5% to 18% fatLight cream or Coffee cream – 18% to 30% fatLight whipping cream – 30% to 36% fatHeavy whipping cream – at least 36% fat

Whipping cream is often UHT pasteurized to lengthen shelf life

Types of Milk ProductsDry milk – dried into powder form – milk

solids can be reconstituted by adding waterNon-fat dry milk has longer shelf life – no fat

to oxidize and spoil the productCultured milk products

Fermented – buttermilk, yogurt, cheeseProcess – pasteurized, lactic-acid bacteria

breaks down lactose (lowers the pH), inoculation (starter is added), incubation, cooling

CheeseMade by coagulating the casein proteinRipening – aged – makes the flavor and

texture distinctMoisture content varies

Very hard – Parmesan, RomanoHard – Cheddar, Colby, provoloneSemisoft – Muenster, RoquefortSoft – Brie, Camembert, mozzarella

Unripened – cottage cheese, cream cheese, mozzarella

Storing Milk ProductsKeep it COLD!!Use closed, opaque containers – riboflavin is

light-sensitiveRefrigerate cans or UHT boxes after opening Store non-fat dry milk at room temperature –

keep moisture outKeep cheese cold, although it tastes better

when served at room temperature – wrap carefully to keep it from drying out

Cooking with MilkMaking 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

Cooking with MilkHeating 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

Cooking with MilkEnzyme action

Some desserts use rennet to make clotted milk - use moderate temperatures so enzyme is not inactivated

AcidsCauses milk to curdle (like when you add fruit

to your milk and cereal or lemon juice to tea and milk)