Experiment 3 - Extraction and Characterization of Casein from Milk and Test of Its Other Nutrients

Milk

The most nutritionally complete food found in nature

• Vitamins

• Minerals

• Proteins

• Carbohydrates

• Lipids

All kinds of milk, human or animal, contain:

• Thiamine

• Riboflavin

• Pantothenic acid

• Vitamin A

• Vitamin B12

• Vitamin D

Vitamins:

• Calcium

• Potassium

• Sodium

• Phosphorus

• Trace metals

Minerals:

• Principally lactose

Carbohydrates:

• Fats

Lipids:

Cows’ milk & Goats’ milk

Almost identical in every respect

Human milk

Contains less than half of the proteins and minerals of cows’ or goats’ milk, but almost 1.5 times as much sugar

• Iron

• Vitamin C

Only important nutrients lacking in milk:

Casein, Lactalbumin, and Lactoglobulin

3 Kinds of Protein in Milk:

Casein, Lactalbumin, and Lactoglobulin

They are globular proteins

Casein, Lactalbumin, and Lactoglobulin

Tend to fold back on themselves into compact

Casein, Lactalbumin, and Lactoglobulin

Nearly spheroidal units

Casein, Lactalbumin, and Lactoglobulin

More easily solubilized in water as colloidal suspensions than fibrous proteins

Casein, Lactalbumin, and Lactoglobulin

• “complete proteins”

• Contain all amino acids essential for building blood and tissue

Casein, Lactalbumin, and Lactoglobulin

• Sustain life and provide normal growth

• Not only contain more amino acids than plant proteins, but contain greater amounts of amino acids than the proteins in egg and meats

Casein

Main protein in milk

Casein

Phosphoprotein, phosphate groups are attached to the hydroxyl groups of some of the amino acide side-chains

Casein

Mixture of at least three similar proteins which differ primarily in molecular weight and the amount of phosphorous groups they contain

• α-casein

• β-casein

• κ-casein

Types of Casein:

α-casein and β-casein

Both insoluble in water

κ-casein

Responsible for solubilizing the other two caseins in water by promoting the formation of micelles

Calcium caseinate

Casein exists in milk as the _________

Calcium caseinate

Has an isoelectric point of pH 4.6

Calcium caseinate

Insoluble in solutions of pH less than 4.6

Calcium caseinate

Casein has a negative charge at this pH and is solubilized as a salt

pH 6.6

pH of milk is about

Neutralized (by protonation of phosphate group)

If acid is added to the milk, the negative charges on the outer surface of the casein micelles are __________

Ca-caseinate + 2H+ → casein↓ + Ca2+

Neutral protein precipitates, with the calcium ions remaining in solution:

Souring of Milk

An intricate process started by the action of microorganisms on the principal carbohydrate in milk, lactose

Glucose and galactose

The microorganims hydrolyze the lactose into

Lactobacilli

Once galactose has been formed, __________ a strain of bacteria present in milk, convert it to the sour-tasting lactic acid

Precipitation of casein

Since the production of the lactic acid also lowers the pH of the milk, the milk clots when it sours due to the

Lactalbumin

Second most abundant protein types in milk

Lactalbumin

Is a globular protein that is soluble in water and in dilute salt solutions

Lactalbumin

Denatured and coagulated by heat

Heating the mixture to precipitate them

Once the caseins have been removed, and the solution has been made acidic, the lactalbumin can be isolated by

Lactoglobulins

A third type of protein in milk

Lactoglobulins

• present in smaller amounts than the albumins

• Denature and precipitate under the same conditions as the albumins

Lactoglobulins

• Carry immunological properties of milk

• Protect young mammal until its own immune system has developed