Graduate chemist Alfred Toepel started lecturing at the School of Dairy Farming in Halberstadt in 1960. In 1992 he took over the Department of Training in Oranienburg. He is also author of the textbook The Chemistry and Physics of Milk.
The butyrometric determination of fat content in milk was developed by Dr. N. Gerber
in 1892 and incorporated into official regulations as a sulphuric acid process in 1935.
The rapid testing method appears both in German standards (e.g. DIN 10479) and
international standards (e.g. ISO 2446).
Determination of fat content according to Gerber is a rapid testing method and is still
used today despite the introduction of automated methods for determining fat content
in dairy laboratories. The advantages of the Gerber method over modern rapid test-
ing methods are:
The disadvantage is the use of very corrosive, concentrated sulphuric acid, which
necessitates the observation of special precautions and the disposal of the sulphuric
acid mixture in an environmentally suitable way.
The determination of fat content according to Gerber involves running off the fat into
a special measuring vessel, the butyrometer, and determining its volume as a percent-
age by mass. The fat is present in the milk in the form of small globules of various dia-
meters, from 0.1 to 10 micrometers. The globules of fat form a consistent emulsion
with the milk liquid. All globules of fat are surrounded by a protective coating, a fat
globule membrane which is made up of phospholipids, a fat globule coat protein and
hydrate water. This protein coating the fat globules prevents them from coalescing and
stabilizes the emulsified state.
In order to completely isolate the fat, the protection coating around the fat globules
must be destroyed. This is done with concentrated sulphuric acid of 90-91 % by mass.
The sulphuric acid oxidizes and hydrolyzes the organic components in the protective
coating around the fat globules, the lactoprotein fractions and the lactose. This pro-
duces a high heat of reaction in addition to the heat of dissolution. The butyrometer
gets quite hot. The oxidation products turn the resulting solution brown. The released
fat is then isolated by centrifuging, whereby the addition of amyl alcohol facilitates phase
separation and a sharp delineation is produced between the fat and the acid solution.
The fat content of the milk can be read off as a mass percent content on the butyrometer
scale.
This process can be used for untreated and pasteurized milk with a fat content of 0-16 %
for milk which contains a suitable preservative as well as for homogenized milk.
| Requirements | hazard rating |
|---|---|
| C2 R 35 S 2 - 26 -30 |
Please note:
| Requirements | Hazard rating |
|---|---|
| Xn R 10-20 S 24/25 VBF A II |
Please note:
Contamination with other isomers of amyl alcohol, particularly with the
tertiary amyl alcohol 2-methylbutane-2-ol, produces false results.
The obtained fat content result is too high.
Pipette DIN 10283-p for milk or pipette DIN 12837-A for milk
Tempering devices for butyrometers, e.g. a (65 ± 2)°C water bath
With a heated centrifuge, a centrifuge bushing can be used to attach the butyrometer
into the water bath. The read-off temperature must be (65 ± 2)°C.
The milk in the specimen bottle is heated up to 20°C and thoroughly mixed through
gentle shaking. This is done to bring about an even distribution of fat and to prevent
frothing and the formation of butter.
Milk fat is lighter than water and creams if it is left standing. A fat-rich layer accumu-
lates on the surface. Stirring and careful shaking restore the original distribution
If the layer of cream cannot be evenly distributed in this way, the milk should be slowly
heated to 35-40°C and gently swirled around until a homogenous fat distribution is
achieved. The milk is then cooled to 20°C before being drawn into the pipette.
Foam breaks the fat globule coating. The milk may begin to turn into butter when stirred
and uniform distribution of fat is no longer possible.
The fat liquefies at 35-40° and the distribution process is speeded up.
After the temperature has been set, the milk can stand for 3 to 4 minutes to allow any
pockets of air to disperse.
The volumeters are calibrated at 20°C. Any variations in temperature will influence the
volume. Air pockets reduce the density and hence the mass of milk measured.
Place 2 butyrometers into a clamp (butyrometer stand).
With the aid of a measuring tap, introduce 10 ml of sulphuric acid
into the butyrometer without wetting the neck of the butyrometer
(see Fig. 1).
Carefully turn the bottle with the milk specimen upside down
three or four times. Then immediately pipette 10.75 ml of milk
into the butyrometer in such a way that the butyrometer neck
does not get wet and the milk does not mix with the sulphuric
acid. To do so, lean the tip of the pipette laterally as deeply
as possible against the wall of the butyrometer so that the milk
forms a layer on top of the sulphuric acid (see Fig. 2).
When the Gerber method was first introduced, 11.0 ml of milk were used.
By reducing the quantity of milk 10.75 ml, the determined fat content more
closely matches the results of the reference method. If the butyrometer
neck is wetted with milk, residues may cling to it.
A clear dividing line between the acid and the milk, without a brownish-
coloured edge, is the sign of good layering.
1 ml of amyl alcohol is pipetted onto the milk, or introduced by
means of a measuring tap.
Due to the low density of amyl alcohol, the two liquids do not mix.
The butyrometer is sealed with a stopper without mixing the
liquids.
As a rule, the lower end of the stopper comes into contact with the liquid.
The butyrometer is placed in the butyrometer tube with the bulb
pointing downwards. Shake the butyrometer vigorously until the
two liquids are thoroughly mixed. Keep your thumb firmly pressed
down on the butyrometer stopper while doing so. Turn the buty-
rometer up and down several times in order to enable the sulphu-
ric acid remaining in the bulb to disperse (see Fig. 3).
When the liquids are mixed, a considerable amount of heat is
given off. The gas built up in this way can cause the stopper to
shoot out, or the butyrometer to break.
The butyrometer tube is used as a safety precaution. Instead of
using a butyrometer tube, the butyrometer can be wrapped in
a cloth.
Too lax shaking of the butyrometer or holding it unnecessarily
in a slanted position inhibits quick mixing and therefore the
rapid oxidation of the en-tire specimen, thus ruining the careful
work done layering the liquid.
. Immediately after the mixture has been shaken and turned upside
down a few times, the butyrometers, still hot and with the stop-
pers pointing downwards, are placed in buckets inside the heated
Gerber centrifuge (see Fig. 4), whereby the butyrometers must be
placed exactly opposite each other.
Beforehand, the column of fat should be set at the height of the
expected level of fat content by turning the stopper.
After setting the time on the centrifuge, the centrifuge is started.
After reaching a centrifugal force of (350 ± 50) g, which is reached
as a rule after 1 minute, the corresponding speed of (1100 ± 50) rpm
should be maintained for 4 minutes.
The centrifuge must be fitted with an interlocking lid. After the time
set for the centrifuge has been reached, the rotor brake is automati-
cally applied.
The butyrometers are now removed from the centrifuge, taking
care not to tilt them, and are placed with the stoppers facing down-
wards for 5 minutes in a water bath heated to 65°C (see Fig. 5).
It is important to maintain an exact temperature so as to obtain accurate
results. Only a read-off at 65°C will ensure an exact result. If the
temperature is too low, the volume of the column is reduced and a fat
content reading that is too low will be indicated.
After removing the butyrometer from the water bath, it should be held vertically
at a height where the meniscus of the column of fat is at eye level. With the help
of the stopper, mark the demarcation line between the residual mixture and fat
on a whole subdivision of the butyrometer scale and read off the height of the
fat column at the lowest point of the meniscus. If the reading takes too long, the
butyrometer must be placed back into the water bath (see Figs. 6 and 7).
If the meniscus of the fat column is not at eye level, a parallax error results.
The result should be read off to half a scale point, i.e. to 0.05 %. It is not possible to
obtain a more accurate result with whole milk butyrometers. If the meniscus touches
the graduation mark, then the result is to be recorded as such (Fig. 7a).
If the meniscus intersects the graduation mark, then the lower value is taken
(Fig. 7b).
The difference between the readings from the two butyrometers must not be greater
than 0.10 %, i.e. the reproducibility amounts to 0.10 %.
When recording the result, you must add the note “fat content according to the Gerber
method”.
If the two specimens differ by 0.1 %, the mean value of both readings is taken.
Specimen 1: 4.20 % | Specimen 2: 4.30 % | Result: 4.25 % fat content
However, if the two readings are 4.20 % and 4.25 % fat content, the lower value of
4.20 % is recorded – on the principle that it is better to err on the side of caution.
