Dr. Hurley’s Magical Milk Show

Walter HurleyProfessor

Animal Sciences

Crystal AllenPhD Student

Animal Sciences

The Main Milk Components• Water

• Lactose

• Lipid

• Proteins

• Minerals

• Vitamins

• Others

Each component can be viewed as to its :

• Biochemical & physicochemical properties

• Mechanisms of component synthesis

• Importance to the nursing young

• Importance as a food source for humans

• Factors that affect component variability

Milk is suppose to be easily digested – perishable

Gland is emptied by piglet, starts refilling with milk (milk synthesis & secretion)

Milk accumulates in the gland for ~45 min (suckling interval)

Milk is ingested by piglet, ~15-20 seconds (milk ejection)

Milk digested by piglets stomach and intestine

Piglet suckles again 45 min later

For the pig, the milk is usually no more than an hour and a half old from synthesis to digestion:

Dr. Hurley’s Magical Milk Show

You will conduct a series of experiments with milk that demonstrate the various fractions and phases of milk.

These experiments will demonstrate:

A. Milk phases.

B. Preparation of butter from cream.

C. Precipitation of milk protein with acid.

D. Enzymatic hydrolysis of milk protein.

Physicochemical Properties of Milk• Milk is:

– an emulsion of fat globules floating in the plasma phase of milk (skim milk),

– which in turn is a suspension of casein micelles suspended in the milk serum phase (whey),

– which in turn is composed of all the water soluble components (lactose, whey proteins, etc)

Lower magnification Higher magnification

Milk viewed through a microscope

Experiment A: Observe and compare thickness of the cream layer in the large tubes. What is the composition of each layer?

Cream

Skim

Milk fat, some protein

Proteins, lactose, water

Raw Milk

Coolto 4 C

Centrifuge60 C

Skim

Standardized milk

Pasteurize

Packaging

Cream40% fat

Cream12% fat

Homogenize

Processing of milk from the store:

Pasteurization:

Ensures safety and enhances shelf life of the milk.

Time & Temperature

“low” pasteurization: 15 seconds at 72 Cor 15 sec at 75 C for homogenized milk

“high” pasteurization: 15 sec at 85 C“ultra-high temp” [UHT milk]: 2 sec at 140 C or 3 sec at 135 C

Consequences of pasteurization:Kills bacteriaInactivates some enzymes (from bacteria or from the milk)

Homogenization:

Prevents the formation of cream layer in stored milk.

High pressure forces liquid through a narrow opening

Fat globules are deformed and disrupted

Must be proper ratio of fat and milk protein so that the protein covers the surface of the smaller droplets so that they will not re-form larger droplets

Experiment B: Butter from cream

1. Observe consistency of cream in the container cream.

2. Vigorously shake the container until butter is formed.

3. Observe consistency of the butter compared with that for cream (step 1).

4. What is the consistency of the cream initially and after butter is formed. What happened to the milk components for the butter to form?

Butter

Butter “milk”

Physicochemical Properties of Milk• Milk is:

– an emulsion of fat globules floating in the plasma phase of milk (skim milk),

– which in turn is a suspension of casein micelles suspended in the milk serum phase (whey),

– which in turn is composed of all the water soluble components (lactose, whey proteins, etc)

Lower magnification Higher magnification

Milk viewed through a microscope

Milk CompositionBreed Variability

• Milk fat concentration varies among the breeds of dairy cattle– lowest % fat - Holstein

– highest % fat - Jersey and Guernsey

Species Variability

Fat percentage is the most variable component among species.

Fat %

Cream Cheese = 44% fat, 6% protein, 45% water

Experiment C: Precipitation of casein with acid

1. Swirl the milk in the flask marked Skim and note the consistency of the film of milk that stays on the wall of the flask.

2. Slowly add an acidic solution drop-wise to flask. Swirl the flask after each drop. Count the drops.

3. Observe when a precipitate forms (on the sides of the tube).

4. Allow precipitate to settle.

What are the two layers called that are formed by this process? What milk component is precipitated under these conditions? Why does this component precipitate under these conditions?

Curds

WheySkimmilk

What are the components of the two layers?

Experiment D: Enzymatic hydrolysis of milk protein (casein) to form a curd.

1. Gently shake contents of the tube marked Whole to remix the milk phases.

2. Dump the contents of one tube marked Whole into the tube marked Rennin, and dump the other tube marked Whole into the tube marked Pepsin.

3. Seal the tubes with parafilm, mix and hold in your hand to warm.

4. Observe each tube every few minutes for hardening of the curd. Turn the tubes upside down to determine when the curd has formed.

What is the consistency of each curd? What happened to the milk for the curds to form?

Pepsindigested

Rennindigested

Casein is the major protein in cow milk

Total milk protein = 80% casein + 20% whey proteins

Casein is secreted as a micelle (granular) structure

The casein micelle also contains:the casein proteins

calcium and phosphorousPO3

-

PO3-

PO3-

PO3-Ca++

Ca++

Casein protein

scanning EM of casein micelle

Casein micelle

Micellestructure

Perhaps >1000 varieties of cheese

Started >8000 years ago

Cheese Making• Heating of milk-promotes bacterial growth • Inoculation- buttermilk/yogurt contains bacteria

which acidifies (ferments) the milk• Rennin-enzyme digests casein (milk protein),

causing coagulation• Curds & Whey-liquid whey separates from

coagulated milk• Pressing-gives shape, squeezes out whey• Curing-bacteria acts upon curds, changing flavor

and texture of cheese