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Babcock Institute for InternationalDairy Research and
Development Dairy
EssentialsUniversity of Wisconsin-Madison
240 Agriculture Hall , 1450 Linden Dr., Madison, WI 53706 USA,
phone: 608-265-4169, [email protected] 73
11119999))))MMMMIIIILLLLKKKKCCCCOOOOMMMMPPPPOOOOSSSSIIIITTTTIIIIOOOONNNNAAAANNNNDDDDNNNNUUUUTTTTRRRRIIIITTTTIIIIOOOONNNNAAAALLLLVVVVAAAALLLLUUUUEEEEMichel
A. Wattiaux
Babcock Institute
WHAT IS MILK?
Milk is the normal product of mammarygland secretion. The
average composition ofmilk from cows and buffalo is presented
inTable 1. Milk is a complex, nutritiousproduct that contains more
than 100
substances that are either in solution,suspension or emulsion in
water. Forexample:
Casein, the major protein of milk, isdispersed as a great number
of solidparticles so tiny that they do not settle,but remain in
suspension. Theseparticles are called micelles, and thedispersion
of the micelles in the milk isreferred to as a colloidal
suspension;
The fat and fat soluble vitamins in the
milk are in the form of an emulsion; thatis, a suspension of
small liquid globulesthat do not mix with the water in milk;
Lactose (milk sugar), some proteins(whey protien), mineral salts
and othersubstances are soluble; that is, they areentirely
dissolved in the water in milk.
The casein micelles and the fat globulesgive milk most of its
physical characteristics,and give taste and flavor to dairy
products
such as butter, cheese, yogurt, etc. Thecomposition of milk
varies considerably withthe breed of cow, stage of lactation,
feed,season of the year, and many other factorsHowever, some
relationships betweenconstituents are very stable and can be
used
to indicate whether any tampering with themilk composition has
occurred. For examplea milk of normal composition has a
specificgravity that varies normally from 1.023 to1.040 (at 20oC)
and a freezing point thavaries from -0.518 to -0.534oC.
Anyalteration, by addition of water for examplecan be easily
identified because thesecharacteristics of milk will no longer be
in thenormal range.
Milk is a highly perishable product tha
should be cooled to about 4o
C as soon aspossible after collection. Extremes otemperature,
acidity (pH) or contaminationby microorganisms can rapidly decrease
itsquality.
MILK AS FOOD FOR HUMANS
Water
The nutritional value of milk as a whole isgreater than the
value of its individuanutrients because of its unique
nutritionabalance. The amount of water in milk reflectthat balance.
In all animals, water is thenutrient required in the greatest
amount andmilk does supply a great amount of wateritcontains
approximately 90% water.
The amount of water in milk is regulated bythe amount of lactose
synthesized by thesecretory cells of the mammary gland. Thewater
that goes into the milk is delivered to
Table 1: Composition of milk from differentspecies (amount per
100 g)Nutrient Cow Buffalo HumanWater, g 88.0 84.0 87.5Energy, kcal
61.0 97.0 70.0Protein, g 3.2 3.7 1.0Fat, g 3.4 6.9 4.4Lactose, g
4.7 5.2 6.9Minerals, g 0.72 0.79 0.20
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Dairy Essentials - Lactation and Milking
74 The Babcock I nstitute
the mammary gland by the blood. Milkproduction is very rapidly
affected by ashortage of water and drops the same daydrinking water
is limited or unavailable.This is one reason why the cow should
havefree access to a plentiful supply of drinkingwater at all
times.
Carbohydrates
The principal carbohydrate in milk islactose (Figure 1).
Although it is a sugar,lactose is not noticeably sweet to taste.
Theconcentration of lactose in the milk isrelatively constant and
averages about 5%(4.8-5.2%). As opposed to the concentrationof fat
in milk, lactose concentration is similarin all dairy breeds and
cannot be alteredeasily by feeding practices. The moleculesfrom
which lactose is made are found in
much lower concentrations in milk: glucose(14 mg/100g) and
galactose (12 mg/100g).In a significant portion of the human
population, the deficiency of the enzymelactase in the digestive
tract results in theinability to digest lactose. Most
individualswith low lactase activity develop symptomsof intolerance
to large doses of lactose, butthe majority can consume moderate
amountsof milk without discomfort. Not all dairyproducts contain
similar proportions of
lactose. The fermentation of lactose duringprocessing lowers its
concentration in manydairy products, especially in yogurts
andcheeses. In addition, milk pre-treated withlactase, which
minimizes the problemsassociated with lactose intolerance, is
nowavailable.
Proteins
Most of the nitrogen in themilk is found in the form ofprotein
(Figure 2). Thebuilding blocks of all proteinsare the amino acids.
There are
20 amino acids that arecommonly found in proteins.The order of
the amino acids in
a protein, which is determined by the geneticcode, gives the
protein a uniqueconformation. In turn, the spatialconformation of
the protein gives it a specificfunction.
The concentration of protein in milk variesfrom 3.0 to 4.0%
(30-40 grams per liter). Thepercentage varies with the breed of the
cow
and in proportion to the amount of fat in themilk. There is a
close relationship betweenthe amount of fat and the amount of
proteinin milkthe higher the fat, the higher theprotein.
The protein falls into two major groups:caseins (80%) and whey
proteins (20%).Historically, this classification followed
theprocess of cheesemaking, which consists ofseparating the casein
curd from the wheyafter the milk has clotted under the action
of
rennin or rennet (a digestive enzymecollected from the stomach
of calves).
The behavior of the different types ofcaseins (, and ) in milk
when treated withheat, different pH (acidity) and different
saltconcentrations provide the characteristics ofcheeses, fermented
milk products anddifferent forms of milk (condensed,dried,
etc.).
O OH
H
OH
HOH
H
CH2OH
H
H
HO
OH
OHOH
OHOCH2OH
H
H
H
HOHOH
OH OH
OH
O OCH2OH CH2OH
H
H
H
H
H
HH H
HO
O+ =
Galactose Glucose Lactose
Figure 1: Lactose is synthesized in the udder from glucose
andgalactose.
CC
H
O
R1
H N3
HH
CC
O
R2
N
HH
CC
O
R3
N
HH
CC
O
O
Rn
N -+
Amino AcidPeptide
bond
Figure 2: Structure of proteins (R1, R2, etc., areradicals
specific to each amino acid. The numberof amino acids in the
caseins of milk varies from199 to 209).
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19 - Milk Composition and Nutrit ional Value
University of Wisconsin-Madison 75
Occasionally, infants or young children areallergic to milk
because their bodies developa reaction to the proteins in the milk.
Theallergy causes rash, asthma, and/orgastrointestinal disorders
(colic, diarrhea,etc.). In cases of allergies, goat milk is
often
used as a substitute; however, sometimeshydrolyzed casein milks
must be used.
Fat
Normally, fat (or lipid) makes up from 3.5to 6.0% of milk,
varying between breeds ofcattle and with feeding practices. A
rationtoo rich in concentrates that do not elicitrumination in the
cow may result in milkwith a depressed percentage of fat (2.0
to2.5%).
Fat is present in milk in small globulessuspended in water. Each
globule issurrounded by a layer of phospholipids,which prevents the
globules from clumpingtogether by repelling other fat globules
andattracting water. As long as this structure isintact, the milk
fat remains as an emulsion.
The majority of milk fat is in the form oftriglycerides formed
by the linking ofglycerol and fatty acids (Figure 3).
Theproportions of fatty acids of different lengths
determine the melting point of fat and thusthe consistency of
the butter derived from it.Milk fat contains predominantly
short-chainfatty acids (chains of less than eight carbonatoms)
built from acetic acid units derivedfrom fermentation in the rumen.
This is aunique feature of milk fat compared withother kinds of
animal and plant fats. Thelong chain fatty acids in milk are
primarily
the unsaturated (hydrogen deficient) acidswith the predominant
one being oleic(18-carbon chain), and polyunsaturatedlinoleic and
linolenic acids.
Minerals and vitamins
Milk is an excellent source of mostminerals required for the
growth of theyoung. The digestibility of calcium andphosphorus are
unusually high, in parbecause they are found in association
with
the casein of the milk. As a result, milk is thebest source of
calcium for skeletal growth inthe young and maintenance of bone
integrityin adults. Another mineral of interest in themilk is iron.
The low iron concentration inmilk cannot meet the needs of the
young, but
this low level turns out to have a positiveaspect because it
limits bacterial growth inmilkiron is essential for the growth
omany bacteria.
Table 2: Mineral and vitamin concentrationsin milk (mg/100
ml)MINERALS mg/100 ml VITAMINS g/100 ml1
Potassium 138 Vit. A 30.0Calcium 125 Vit. D 0.06Chloride 103
Vit. E 88.0Phosphorus 96 Vit. K 17.0
Sodium 58 Vit. B1 37.0Sulfur 30 Vit. B2 180.0Magnesium 12 Vit.
B6 46.0Trace minerals2 < 0.1 Vit. B12 0.42
Vit. C 1.71 g = 0.001 gram2 Includes cobalt, copper, iron,
manganese,
molybdenum, zinc, selenium, iodine and others.
MILK AS FOOD FOR THE CALF
Immune components
Milk contains prote ins cal ledimmunoglobulins that are one of
the calfsprincipal defenses against infectiousorganisms (viruses,
bacteria, etc.)Concentrations of immunoglobulins areespecially high
in the colostrum, the milkproduced immediately at the onset
oflactation.
TriglyceridesGlycerol Fatty Acids
C OHH2
C OHH
C OHH2
CO
CO
CO
R1
R2
R3
OH
OH
OH
C OH2 CO
C OH CO
C OH2 CO
R1
R2
R3
+
Figure 3: Structure of triglycerides (R1, R2, R3represent the
fatty acid carbon chains that givetriglycerides their individual
characteristics.)
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Dairy Essentials - Lactation and Milking
76 The Babcock I nstitute
Immunoglobulins are not produced in themammary tissue but are
transferred directlyfrom the blood serum into the milk. The calfcan
best adsorb the immunoglobulinsimmediately after birth, with the
ability toabsorb decreasing to near zero by 36 hours of
age. This is because, in the first 12 hours oflife, the calf
does not produce appreciableamounts of hydrochloric acid in its
stomachso the immunoglobulins are not damaged.
Colostrum should be given to the calf assoon after birth as
possible. This will at leastdouble the young calfs chances of
survival.Colostral immunoglobulins are stable in thecalfs
bloodstream for 60 days, providingprotection until its own immune
system isfunctional.
Not only is colostrum of vital importance tothe newborn calf, it
also has no commercialvalue as it is not acceptable for
commercialmilk collection for human consumption. So,the milk from a
cow that has freshly calvedmust not be included in the milk for
sale forthree to four days. Colostrum can be frozenand stored for
administration to other calves.
COMPONENTS INFLUENCINGMILK QUALITY
Cells in milk
Somatic cells in milk do not affect thenutritional quality per
se. They are only
significant as indicators of other processesthat may be taking
place in the mammarytissue, including inflammation. Whensomatic
cells are present at rates of over halfa million per milliliter,
there is reason tosuspect mastitis.
Undesirable components in milk
Milk and milk products are perishablefoods. High standards of
quality throughoutthe entire dairy processing industry arerequired
to enhance and/or maintain theconsumers confidence, and to make
themdecide to buy dairy products. The milkleaving the farm must be
of highestnutritional qualityunaltered anduncontaminated. Here is a
partial list of the
most common undesirable substances foundin milk: Additional
water; Detergents and disinfectants; Antibiotics; Pesticides or
insecticides; Bacteria.
The vigilance of producers in followinginstructions for the use
of chemicals, as wellas good milking, cleaning and storage
procedures are not only essential to their ownsuccess, but also
to the success of the dairyindustry as a whole.
