Dr. Hurley’s Magical Milk Show
Walter Hurley
Crystal Allen
Professor
Animal Sciences
PhD Student
Animal Sciences
The Main Milk Components
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•
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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
For the pig, the milk is usually no more than an hour and a half old
from synthesis to digestion:
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
Dr. Hurley’s Magical Milk Show
These experiments will demonstrate:
You will conduct a
series of experiments
with milk that
demonstrate the
various fractions and
phases of milk.
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:
Milk viewed through a microscope
– 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
Experiment A: Observe and
compare thickness of the
cream layer in the large tubes.
What is the composition of
each layer?
Cream
Milk fat,
some
protein
Skim
Proteins,
lactose,
water
Processing of milk
from the store:
Raw
Milk
Cool
to 4 C
Cream
40% fat
Cream
12% fat
Centrifuge
60 C
Skim
Homogenize
Standardized
milk
Pasteurize
Packaging
Pasteurization:
Ensures safety and enhances shelf life of the milk.
Time & Temperature
“low” pasteurization: 15 seconds at 72 C
or 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 bacteria
Inactivates 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.
2.
3.
4.
Observe consistency of cream
in the container cream.
Vigorously shake the
container until butter is
formed.
Observe consistency of the
butter compared with that for
cream (step 1).
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:
Milk viewed through a microscope
– 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 Composition
Breed Variability
• Milk fat concentration varies among the
breeds of dairy cattle
– lowest % fat - Holstein
– highest % fat - Jersey and Guernsey
Species Variability
Cream Cheese =
44% fat, 6% protein, 45% water
Fat percentage is
the most variable
component
among species.
Fat %
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?
Whey
Curds
Skim
milk
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?
Pepsin
digested
Rennin
digested
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 phosphorous
PO3-
PO3-
Ca++
PO3-
Ca++ PO3-
Casein protein
scanning EM of casein micelle
Casein micelle
Perhaps >1000 varieties of cheese
Started >8000 years ago
Micelle
structure
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