BS 81-3:2001 pdf download – Milk — Determination of fat content — Gravimetric method (Reference method)
6.4Drying oven，electrically heated，with ventilation port(s) fully open,capable of being maintained at atemperature of 102C±2 C throughout its working space.
The oven shall be fitted with a suitable thermometer.
6.5 Water bath , capable of being maintained at a temperature of between 35 °C and 40 ℃.6.6Mojonnier-type fat-extraction flasks, as specified in lSO 3889.
NOTE lt is also possible to use fat-extraction tubes, with siphon or wash-bottle fitings, but then the procedure is different.The alternative procedure is given in annex B.
The fat-extraction flasks shall be provided with good quality bark corks or stoppers of other material [e.g. siliconerubber or polytetrafluoroethylene (PTFE)] unaffected by the reagents used. Bark corks shall be extracted with thediethyl ether (5.4), kept in water at a temperature of 60 C or more for at least 15 min, and shall then be allowed tocool in the water so that they are saturated when used.
6.7Rack, for holding the fat-extraction flasks (or tubes)(6.6).6.8Wash bottle, suitable for use with the mixed solvent (5.6).A plastics wash bottle shall not be used.
6.9Fat-collecting vessels, such as boiling flasks (flat-bottomed), of capacities 125 ml to 250 ml, conical flasks,of capacity 250 ml, or metal dishes.
lf metal dishes are used, they shall be of stainless steel,flat-bottomed with a diameter of 80 mm to 100 mm and aheight of approximately 50 mm.
6.10 Boiling aids, fat-free, of non-porous porcelain or silicon carbide (optional when metal dishes are used).6.11 Measuring cylinders, of capacities 5 ml and 25 ml.
6.12 Pipettes, graduated, of capacity 10 ml.
6.13 Tongs, made of metal, for holding flasks, beakers or dishes.6.14 Volumetric flask, one-mark, of capacity 100 ml.
Sampling is not part of the method specified in this International Standard. A recommended sampling method isgiven in iso 707.
lt is important that the laboratory receive a sample which is truly representative and has not been damaged orchanged during transport or storage.
Store the samples at a temperature of between 2 °C and 6 C from the time of sampling.
8Preparation of test sample
Warm the test sample to a temperature of between 35 C and 40 ·C by means of the water bath (6.5). Gently mix the test sample thoroughly by repeatedly inverting the sample bottle without causing frothing or churning. Cool the test sample quickly to approximately 20 °C. Do not cool test samples of churned milk as these samples have to be weighed at a temperature of between 30 °C and 40 °C (see 9.1 ).
lf a homogeneous test sample can be obtained without prewarming to between 35 °C and 40 C(e.g. for samplesof skimmed milk), the following procedure should be followed.
Bring the test sample to a temperature of 20 °C.Mix thoroughly to ensure a homogeneous mixture of the fatthroughout the test sample. Do not agitate so vigorously as to cause frothing of the milk or churning of the butterfat.NOTEA reliable value for the fat content cannot be expected:
a)if the milk is churned;
b)when a distinct smell of free fatty acids is perceptible;
c)if during or after preparation of the test sample, white particles are visible on the walls of the sample bottle or fat droplets
float on the surface of the sample.
NOTE1 lf it is required to check whether the repeatability limit (11.2) is met,carry out two single determinations inaccordance with 9.1 to 9.4.
NOTE2 An alternative procedure using fat-extraction tubes with siphon or wash-botle fitings (see note in 6.6) is given inannex B.
Mix the test sample (clause 8) by gently inverting the bottle three or four times. Immediately weigh, to the nearest1 mg, 10 g to 11 g of the test sample, directly or by difference, in a fat-extraction flask(6.6).
Transfer the test portion as completely as possible into the lower (small) bulb of the fat-extraction flask.
9.2.1 Blank test for method
Carry out a blank test simultaneously with the determination using the same procedure and same reagents, butreplacing the test portion in 9.4.1 by 10 ml of water (see A.2).
lf the value obtained in the blank test regularly exceeds 1,0 mg, check the reagents if this has not been recentlydone (9.2.2).Corrections of more than 2,5 mg should be mentioned in the test report.