"Chemical composition and physical
characteristics of milk determine the nutritive value and its suitability
for the processing of different dairy products."|
(J C LaJaouen in "Goat
Production" ed. C Gall)
The statement made above may seem obvious, but how much do we really know
about the chemistry and physics of goats milk, and about its advantages
and disadvantages compared with the milks produced by other mammals?
The International Dairy Federation seminar on the Production and Utilisation
of Ewe's and Goat's Milk, held in Athens in September 1985, sought the
answer to these questions.
Chemically speaking cream is fat. Fats by definition are substances which are insoluble in
water but which dissolve in solvents such as chloroform or ether.
How does the fat content of goats milk compare
with other milks? (Le Jaouen, 1974)
|Species||Fat(gms per kg)|
The range of fat content found in different samples of goats milk is similar to that found in
samples of cows milk, ie 2% to 8%, with a similar frequency distribution.
The content of fat varies more than the content of any other ingredient
in milk: it is influenced by genetic factors, level of production, stage
of lactation, health, climate, milking method, and feeding.
There is an obvious visual difference between goat and cow cream: the former is white and the latter yellow. The yellow colour of cow cream is due to the presence of carotenoid pigments. These are the precursors of vitamin
A, which is colourless. In goats milk, the pigments have already been converted into vitamin A before the milk is secreted.
Milk contains many substances in solution in water: but the fat is in the form of spherical globules, in emulsion. These globules are not all of the same size, and have been measured in both goats and cows milk, using
an electron microscope. The milk of both species contains globules of the
same range of sizes: 1 to 10 microns in diameter. However, in goats milk,
small globules are more numerous: 65% of the globules are less than 3
microns in diameter in goats milk, while only 43% of the globules in cows
milk are less than 3 microns in diameter. It is thought, though perhaps not
proven, that this feature makes goats milk easier to digest than cows milk.
The reason for the formation of a greater number of globules in goats
milk does not seem to be known.
Each globule is surrounded by a membrane containing fat and protein molecules,
which have a special arrangement and complex functions. Suffice it to say
here that the situation appears to be similar in cows milk and goats milk.
It is well known that cream rises to the top of goats milk on standing less
markedly than it does in cows milk. This has been thought to be due to the
smaller fat globules of goats milk, but apparently this is not the whole answer.
In cows milk, the rapid rise of cream to the surface is due to the fat
globules clustering together. This in turn is due to the presence in cows
milk of dissolved simple protein molecules which are adsorbed onto the fat
globules, causing them to agglutinate, It would seem that the same agglutinating proteins are
present only to a much smaller extent in goats milk, since goats cream, mixed
into skimmed cows milk, rises rapidly to the surface, whereas cows cream
mixed into skimmed goats milk does not.
Heating goats milk to pasteurise it (61' C for 30 minutes) increases the
size of the fat globules by 12% and hence reduces their number.
What happens when goats milk is put through a centrifugal cream separator
depends on the speed: a speed of 3,000 rpm leaves only 1.7% fat in the
skimmed milk, in both goats and cows milk. At a lower speed of 1,200 rpm,
however, 6.5% of the fat was left in the skimmed milk in goats milk, but
only 3% in cows milk. This is thought to be due to the larger number of
small fat globules in goats milk. If milk is separated after standing for 1-2
days at 4' C, more fats remain in the skimmed milk,presumably due to
damage to the fat globules.
Further differences between cows and goats cream lie in their chemical composition. Animal bodies,and the substances such as milk which they produce, are made up of particular groupings of atoms such as carbon,
hydrogen, and oxygen, sometimes with nitrogen, and sometimes with atoms of other elements as well.
Carbon atoms are often arranged in rows, or chains (the more carbon atoms,
the longer the chain), with oxygen and hydrogen atoms grouped around them.
Many chemical reactions involve taking away some of these hydrogen and oxygen atoms and putting
other carbon compounds in their place, to form a completely different substance.
The starting point for 'building' a molecule of fat is a molecule of glycerine,
which consists of a row of 3 carbon atoms with a particular arrangement
of hydrogen and oxygen atoms surrounding them. Also involved are substances
called fatty acids, which also consist of a chain of carbon atoms with
mostly hydrogen atoms surrounding them. The addition of a each extra
carbon atom to the chain creates a different fatty acid.
A fat is formed by joining a fatty acid onto each of the 3 carbon atoms
on the glycerine molecule. There are a number of different fatty acids and they
may be joined to the glycerine in many assortments. This therefore gives
a big range of possible different types of fat, cream or oil. The
arrangements found are mainly alike in goats and cows milk.
We hear a lot nowadays about 'unsaturated' and 'saturated' fats. A saturated
fat contains all the hydrogen atoms that its carbon atoms are able to
'hang on' to. An unsaturated fat could take up more hydrogen under suitable
conditions. 'Polyunsaturates' are fats or oils which have many places on
their fatty acid carbon chains which could take up more hydrogen atoms.
An unsaturated fatty acid has different properties from the saturated one
which has the same length of carbon chain: this gives further possibilities
for different types of fat, cream or oil to exist.
The reason why cream is so disastrously fattening also lurks within its molecules.
The large amount of hydrogen present creates a huge energy store which 'burns
up' with added oxygen to release large numbers of calories.
The fatty acids which compose the fat of milk mostly have chains of even numbers of carbon
atoms and are named according to that number:
|C18 :1||oleic acid (1 unsaturated link)|
|C18 :2||linoleic(2 unsaturated links)|
|C18 :3||linolenic (3 unsaturated links)|
These acids all occur in both goats milk and cows milk, but in differing proportions. It is typical of the milk fat of ruminants that it contains high proportions of the C4 - C12 fatty acids. Goats and ewes milk, however, are higher in their content of C6 - C12 fatty acids than cows milk, particularly in C10, capric acid. Fatty acids are produced from a combining of the acetic acid molecules manufactured in the rumen, and there appears to be a difference between cows and goats in the amount of this polymerisation.
Various tests are used to elucidate the chemical composition of milk fat:
The Reichert-Meissl value is a measure of the water-soluble fatty acids
(C4 - C8) and is obtained by neutralising the free acids with an alkaline
solution. The value is considerably lower for goats than cows (18-25 as against 26 - 30).
The Polenske value is a measure of the water-insoluble fatty acids (C10 - C18),
and is higher for goats than cows milk (5-12 for goats and 1.7 - 4
for cows). Incidentally, human milk has been found to give a figure of
around 2 for both of these values.
The ratios of the contents of the different fatty acids
to each other are therefore different in goats and
cows milk, and this is the basis of a method of
detecting the presence of cows milk as a
contaminant in goats milk. The method of gas
liquid chromatography is used to identify the
individual fatty acids present in milk samples.
Physical Properties of Fatty Acids
Some fatty acids are solid, some are liquid. Some are tasteless, others are not. Their relative proportions therefore influence the nature of the milk, cream and butter. Studies have been made of the changes in the fatty acid content of goats milk during the lactation and during the year. Such changes do occur, some fatty acids increasing and some decreasing.
The fat content of the diet has a considerable influence on the fat content of the milk, as does the ratio of forage concentrates. It is well known amongst goat keepers that to increase the forage and decrease the concentrates in the diet (provided the energy level of the diet stays the same) will increase the butterfats - due to increased formation of acetate, the precursor to C4 - C16 fatty acids. C18 acids are formed when more concentrates and less hay are fed.
It is said that goats producing fat richer in long-chain fatty acids (genetically) have a
noticeably higher fat content in their milk and mobilise their fat reserves, especially at the start of lactation.
The iodine number of a fat sample is an indication of how many unsaturated fatty acids are present, since iodine can be taken up in place of hydrogen. In one study the iodine number of goats milk was lower (31.07) than cows milk (36.51), but further work is needed to establish this point. Human milk, incidentally, contains a great deal of unsaturated fatty acid, having an iodine number of 45.66.
We have said that the fat in goats milk is mostly in the form of fatty acids combined with glycerine, to form 'triglycerides'. This combination can be split up, however, leaving free fatty acids in the milk. It is thought that this event is responsible for a 'goaty' flavour developing, though other chemical
constituents have also been implicated in the flavour. (Potassium chloride and various cresols). The goaty flavour has been found to be strongest in milk samples with a low content of fat, protein and lactose.
Storage time increases the strength of the flavour. The enzyme lipase is responsible for the breakdown of thiglycerides, leaving free fatty acids in the milk. This subject has been studied in Norway where cheese where cheese with a strong goaty flavour is liked by some people. Goats were selected over five generations for the production of goaty tasting
milk, and as a control, another line of goats in the same herd were selected for milk with low flavour intensity. After the eight years of this trial it was found that the 'high flavour' milk did indeed contain more free fatty acids than the 'low flavour' milk.
Also, the percentage of lauric acid had gone down and that of palmitic acid had gone up in the high flavour milk.
Mid-lactation milk is stronger flavoured than early or late lactation milk. Trials indicate that grazing versus stall-feeding has an effect on milk flavour, but the results appear to be inconsistent.
The goaty flavour is not the same as the 'rancid milk defect' and the 'oxidation flavour' found in cows milk. Also, it is not due to unhygienic dairying or the presence of a male goat. Unfortunately for us in the UK, who do not regard goaty flavoured milk as desirable, the enzyme lipase is distributed in goats milk in a very different way to that in cows milk, and this, in addition to the smaller fat globules, which are more easily broken up, makes for easy and marked splitting of the fats to give free fatty acids and a goaty flavour. This splitting (lipolysis) can happen spontaneously. More needs to be known of factors, such as agitation, which can produce this unfortunate effect. Lipase is destroyed by heating the milk to 56' C.
We have said that fatty acids contain chains of
carbon atoms. It is possivble for fatty acids to occur in which these chains are branched. Such branched acids, which seem more prevalent in goats than in cows milk, are implicated in a different type of off-flavour in goats milk, the sort that is sometimes cured by administration of vitamin B12, which is needed to prevent these branched-chain acids building up. An infestation of intestinal worms can rob the goat of her supplies of B12 (made in the rumen in the presence of adequate dietary cobalt), and this in turn can lead to this form of milk taint, which disappears when the goat has been treated to get rid of her worms.
It is necessary for man to consume certain fatty
acids in the diet: the so-called 'essential fatty acids', eg linoleic acid, an unsaturated C18 acid. Though this is not a major constituent of either cows or goats milk, both apparently contain sufficient for the human diet.
A postulated reason for the greated ease with which goats cream is digested, is the greater amount of short to medium chain length fatty acids present in goats milk. Lipase activity is a necessary part of digestion and it is stated that the enzyme can attack triglycerides containing short-chain acids more readily than those containing long-chain acids.
Goats of several European breeds (British Alpine, Anglo Nubian and Saanen) produce milk of lower fat content in tropical than temperate zones.
The consumption by goats, while browsing, of plants containing alkaloids (pharmacologically
active plant substances) may cause tham to secrete milk of changed amount or composition, due to the presence of these alkaloids.
Reprinted from NZ Dairy Goat News Oct & Nov 1987.
"Goats Milk - The Natural Alternative" Tinsley Beck BA MEd
On this Site:
What's Wrong with Your Baby? by Margaret Beck
Goat's Milk for Infants by JB Tracey MB
Why Goat Milk by G F W Haenlein PhD
Differences Between Cow and Goat Milk by G F W Haenlein PhD and R Caccese