(A) Fluid Milk :
Quantification of fat in milk can be done by the following dstems :
(a) Direct
determination by extrction with ether as adopted in the Adam’s coil method.
(b) Separation
of fat by acid/alkali treatnent and its subse-quent removal by ether. This
system finds application in the Werner-Schimdt and Gottleib method.
(c) Separation
of fat by chemical means followed by centri-fuging. This technique is employed
in the Gerber method as well as in Lefrnann-Beam method.
Adams’ paper coil
method is the most accluate hence is employed in samples having fat percentage
either of doubtful accuracy or bordering on the minimum statutory limit. This
method is suitable for fresh milk only.
Special fat-free
strips of paper are needed in this method besides routine apparatus and
chemicals. These strips can be prepared in the laboratory as well, from thick
filter paper defatted through ether extaction and subsequenct drying.
Method’s theory is
that a fat free strip is suspended over a hot plate to dry but not scorch so
that 5 ml of the sample may be spotted on this strip carefully by delivering it
from a pipette. The weight of this 5 ml of milk is detemrined by weighing separately.
Another similar 5 ml of the same milk sample is weighed in a platinum dish. The
paper strip is dried completely lest any moisture be left behind, retained and later
on carry water-soluble substances which dissolve in ether subsequently. At the same
time, the thorough and/or quick drying must also be avoided lest the strip becomes
flaky. This is accomplished by fitting a small fibre filter in the side tube of
Soxhlet’s extractor. The dried paper strip is rolled up loosely and transferred
to the Soxhlet’s extractor for extraction with ether for about four hours. The
top of the condenser is plugged with cotton wool. This helps to partly feed
back the escaping few vapours of ether but mostly to keep away moisture as done
in non-volatile ether extract in groud spices. The rest is the same.
The salient points
of the Lefmann-Beam process are that the protein is precipitated first and
dissolved later on by the acid to free fat globules. Amyl alcohol produces a great
difference in surface tensions between that of fat globules and their
supporting liquids, thus causing an aggregation and separation of the fat.
Gerber’s method differs, though
slightly, in approach from the Beam’s method. In he Gerber’s method the proteins and mineral
matter is dissolved in H2SO4 and a solution is obtained. Fat is dissolved
in hot amyl alcohol. Its seperation from the heavy acid solution is effected by
subjecting the mixture to centrifuging. Since this is mostly applied on a routine
basis in the Indian milk plants, therefore it has been dealt here in detail.
10 ml of conc. H2SO4
is run into the Gerber tube which is returned to the stand. I ml. of fat free
amyl alcohol is added avoiding the threaded neck of tube.The milk sample is
well mixed. 10.94 ml of this milk sample is pipetted out, the exterior of the
pipette wiped clean of milk. The tip of the pipette is inserted into the
slightly tilted Gerber tube without touching the neck. Rapid discharge of milk
is scrupulously avoided to keep off milk which would result in obscuring the
junction of the fat column with the acid after centrifuging, therefore, the
milk release is controlled to keep the three liquids (acid, alcohol and milk as
separate layers in the tube which is held firmly by the stem and the contents
(alcohol and milk) are shaken briskly. Finally the acid lying in the stem is
also mixed with the milk + alcohol solution by inverting the Gerber tube twice
and brisk shaking after a few minutes. Appreciable heat is generated due to action
of the acid on water, the main constituent of milk.
The
tube is cntrifuged for 3 minutes at 1000 rpm (approx). The tube is then placed
in a hot water bath, keeping the stem upwards to a temperature indicated on the
Gerber tube for exact volume reading, reconciling the volume changes due to
glass expansion. The column of the separated fat is read off the graduated stem.
The high rpm
centrifuge drives the fat towards the narrow end of the tube. Some tips for accurate operation and
result are :
(i) Separation
by centrifugal force need to be hastened.
(ii) It is essential to increase the difference
in the specific gravity of milk plasma and its fat By increasing the sp. gravity of plasma the
separation of its fat is accelerated.
(iii) Casein is a disturbing
factor because it appears to clasp the fat globules and unless casein is not made to divorce this characteristic,
separation of fat would be slow
and little.
(iv) Temperature rise also helps in fat
separation.
All these
objectives are achieved simply by the addition of conc. H2SO4
which dissolves proteins to effect sp. gravity differences, brings casein
solution to release fat globules. Further, during dilution of H2SO4
through water present in the milk, heat, sufficient for the purpose is released.
By adding a trace of FeCl3 to the H2SO4 used,
a violet colour is produced if the milk contains formaldehyde.
The test is
simple, quick and accurate. Correctness of the results have a considerable
bearing on the financial and operational administration of a milk plant where
lakhs of liters of milk are handled every day. The pricing of milk supplied to the
plant depends upon its fat content besides other constituents. The total fat
accrued through the supply of fluid milk has to. be accounted for its
distribution in the various products daily manufactured. Therefore, these
precautions deserve to be always borne in mind.
(i)
Proper strength of the H2SO4. Too strong acid chars the
reaction products rendering
difficult to distinguish the fat column from the rest of the liquids present. Too weak acid will not
dissolve all the coagulated proteins, leading to haziness and not yielding a sharp demarcation between the
fat and the acid layers. A strength
between 90 to 9l% is alright. Commercial H2SO4(sp. gr. between 1.820 to 1.825) can be used. Generally
H2O : H2SO4 : : 2 : 25 gives suitable acid for this purpose.
(ii)
Graduation of the Gerber tube (0 to 8%) fat level must be accurate.
(iii) Amyl
alcohol must be free from fat and petroleum, should have specific gravity 0.8165 at 15ºC and its boiling
point should lie between 51º to 55ºC.
Fat separates out in the form of an amber coloured solution.
Numerous
reagents have been proposed for replacing the sulphuric acid used in the Gerber
test by less corrosive liquids. Those which incorporate synthetic detergents appear
to be the most promising. Such a method due to Macdonald (1955) appears to be
convenient
Macdonald’s
neutral reagent can be prepared by using 5% w/v trisodium citrate, 5% w/v sod. salicylate,
l% w/v EDTA (di-sod salt), l .l% w/v Tween
85, 3% v/v n-butyl alcohol and 25% v/v methylated spirit (over proof 66) in
water. It is inverted before use.
Procedure : 10
ml of Macdonald’s reagent is pipetted out into a milk butyrometer (Gerber
method) and 10.94 ml of milk also delivered. The contents are stoppered,
inverted twice and the tube placed in water bath at 65ºC for 5 minutes. The tube
is centrifuged at 1100 rpm for 5 minutes, the butyrometer tube returned to the water
bath at 65ºC for atleast 3 minutes and the fat percentage read off from the scale.The
tube is again returned to 65ºC water bath’ recentrifuged and read again. All
determinations are required to be carried out in duplicate. The method is applicable
to other dairy products also (Macdonald 1959a)
The
gravimetric Warner-Schmidt method depends upon fat separation through strong
HCI which factually first precipitates the casein and then dissolves it thereby
lowering the surface tension. This helps in the separation of fat and subsequent.
dissolution in ether.
A weighed
quantity of milk and conc. Hydrochloric acid are heated in the Stoke’s tube for
the complete precipitation and redissolution of casein followed by thorough
separation of fat which is dissolved by ether in the tube itself (total volume 40 ml). An aliquot of ether layer
is pipetted out accurately in a tared flask. Ether is evaporated and fat percentage
determined. The method is more suitable for sour milks.
In the
gravimetric Gottleib-Rose method., casein is precipitated by alcohol which also
lowers the surface tension between fat and removes the fat as before. The
method is prescribed as the reference method for determining fat in milk
because it gives very accurate results provided due attention is Paid to
details.
It has been
observed during court trials that after sometime say > 4/5 months, the fat
content percentage in milks, preserved even with adequate dose of formalin, is
found less due to hydrolysis of butyrate and caproate etc. converting them to
free butyric and caproic acid respectively. These being water soluble are
neither recoverable as total solids nor as fat. It is the fat in the glyceride
form that on drying becomes a residue and recoverable in these aforesaid tests.
(B) Chhanna/Paneer
shall not contain more than 70% moisture
and the milk fat content
shall not be less than 50% of the dry
matter.
In several
cases, the reports emanating from central Food Laboratories of the PFA deptt.
mention fat content on dry weight basis.
Same is the
position in cases involving cheese hard (A. 11.02.07) and processed cheese (A.
11.02.07.01). However in processed cheese spread (A.11.02.07.02) it is on dry weight basis.
These two
positions appear to be same but in reality are not so, Determination of fat
content on dry matter basis means that the material which has been dried, only that
dried remanant shall be employed for determining the fat content and the calculations.
on the other hand, in the event of dry weight basis two separate pieces are
used, one for finding the moisture content (by any method), the other for
finding the fat content and the results are calculated on
Wt. of
fat × 100
______________________ All
these products are not
fat % =
100 – moisture percentage
homogeneous hence the results may not be reproducible
creating problems while finding the true position. Besides, during heating in
the oven to drive out moisture till constant weight, many volatiles may also evaporate.
This may not be the position when two separate materials are analysed
separately for each test on the results calculated as above.
It is a
wrong notion that by adding water to the chhanna/parurer, the vendor can escape
from the law as clear from the following example where the original
coustituents of a sample are
Moisture
%
Required
fat %
40
= 30 but has 24
Add 20 ml. water ie. Has 6% less fat
(120-60)
=
= 50%
25%, but in a total volume of
100
+ 20 = 120 ml. the fat
distribution
shall be
i.e.
20% hence
has
5% less fat.
Specific
Gravity
Specific
gravity in milk is determined by using specific gravity bottle which should be
deemed as the reference method. The other method is by using lactometer a reliable
instrument in routine testing.
Lactometer
registers sp. gravity between 1015 to
1040. The range of readings on the instrument may be from 26 to 36. Lactometers
are graduated for a definite temperature [60ºF (15.6ºC)]. For temperature
correction, 0.0001 is added or subtracted for every 1ºF recorded temperature
above or below 60ºF respectively.
