1、BRITISH STANDARD BS734-2:1959 Incorporating amendment issued October, 1963 (PD5041) Density hydrometers for use in milk Part 2: MethodsBS734-2:1959 This British Standard, having been approved by the Dairying Industry Standards Committee and endorsed by the Chairman ofthe Chemical Divisional Council,
2、 was published under theauthorityof the GeneralCouncil on 31December1959 BSI09-1999 First published May1937 First revision March1955 Second revision December1959 The following BSI references relate to the work on this standard: Committee reference DAC/3, DAC/3/1, and DAC/3/1/1 Draft for comment A (D
3、AC)775 ISBN 0 580 35526 8 Co-operating organizations The Dairying Industry Standards Committee, under whose supervision this British Standard was prepared, consists of representatives from the following Government departments and scientific and industrial organizations: Agricultural Research Council
4、 British Dairy Farmers Association* Central Milk Distributive Committee* Chemical Society Dairy Engineers Association Dairy and Ice Cream Equipment Association Department of Agriculture for Scotland D.S.I.R.Laboratory of the Government Chemist* Hannah Dairy Research Institute* Milk Marketing Board*
5、Ministry of Agriculture, Fisheries and Food* Ministry of Health National Association of Creamery Proprietors and Wholesale Dairymen* National Farmers Union National Institute for Research in Dairying* National Physical Laboratory (D.S.I.R.) Office of the High Commissioner for Australia* Office of th
6、e High Commissioner for Canada Office of the High Commissioner for New Zealand* Office of the High Commissioners for the Union of South Africa Royal Society for the Promotion of Health Scottish Milk Marketing Board* Scottish Milk Trade Federation Society for Analytical Chemistry* Society of Chemical
7、 Industry Society of Dairy Technology The Government departments, and scientific and industrial organizations marked with an asterisk in the above list, together with the following, were directly represented on the committees entrusted with the preparation of this standard: Association of British an
8、d Dominion Condensed Milk Manufacturers British Food Manufacturing Industries Research Association Coated Paper and Board Makers Association Department of Agriculture for Canada Government of Northern Ireland (Ministry of Agriculture) Ice Cream Alliance Ltd. London Provision Exchange University of R
9、eading Amendments issued since publication Amd. No. Date of issue Comments 5041 October 1963 Indicated by a sideline in the marginBS734-2:1959 BSI 09-1999 i Contents Page Co-operating organizations Inside front cover Foreword ii 1 Scope 1 2 Determination of density 1 3 Corrections to observed hydrom
10、eter readings 1 4 Determination of S.N.F. content 2 Appendix A Derivation of the equations 3 Appendix B Determination of corrections for scale errors 4 Table 1 Temperature corrections to hydrometer reading 2 Table 2 Percentage of solids-not-fat (S.N.F.) (with milk fat in liquid state) 6 Table 3 Perc
11、entage of solids-not-fat (S.N.F.) (with milk fat in solid state) 14BS734-2:1959 ii BSI 09-1999 Foreword This standard makes reference to the following British Standards: BS696, Gerber method for the determination of fat in milk and milk products. BS1741, Methods for the chemical analysis of liquid m
12、ilk. Part 2 of this British Standard, published under the authority of the Dairying Industry Standards Committee, specifies two procedures for determining the density of milk so that it can be measured either with the milk fat wholly in the solid state or wholly in the liquid state, and provides tab
13、les relating the known fat content and measured density of a milk with its solids-not-fat (S.N.F.) content. The density of milk held at20 C or less after leaving the cow slowly increases to a maximum (Recknagel phenomenon), due mainly to the gradual solidification ofthe fat. The density of a milk wi
14、ll therefore vary with the duration and temperature of storage. This variation can be overcome by ensuring that the fatiseither completely in the liquid state or completely in the solid state. This British Standard was first published in1937 and revised in1955. It had as its theoretical basis the eq
15、uation established in1895 by H.DroopRichmond which relates the total solids content of milk to fat content and specific gravity (measured at15.5 C with the fat in the solid state). In these editions the prescribed hydrometers gave the density of milk with the fat in the liquid state in grammes (mass
16、) per millilitre at20 C. Richmonds formula was modified to allow for substitution of density at20 C for specific gravity at15.5 C. No allowance was made for the change in state of the fat, although the density of a milk is less when the fat is in the liquid state than when in the solid state. This o
17、mission wasdeliberate since it was known that when Richmonds formula is applied to density readings obtained with the fat in the solid state it gives higher figures fortotal solids than those obtained by modern gravimetric methods 1) . However, subsequent investigations showed that the use of the mo
18、dified formula with thefat in the liquid state over-compensates for the high figures obtained by Richmonds method and usually gives lower results for total solids than those obtained gravimetrically. Other investigations have been carried out in which thedensity was determined with fat in the solid
19、state. An amendment to BS734 was published in1957 which took into account the results of these investigations and also the reduction in the capacity of the milk pipette specified in BS696 and used in the determination of the fat content ofmilk 2) . The present edition of this standard incorporates t
20、he amendment and hence provides two formulae (Appendix A), one for use when the fat is solid and one for use when the fat is liquid. These formulae result in total solids values that are, on average, in close agreement with those obtained by gravimetric methods. As in the previous editions of this s
21、tandard, hydrometers of three sizes are provided to meet the need of the dairy industry to deal with samples of differing volume. The construction of the Nos.2 and2A hydrometers, a size in common use, has been altered to make them more robust. The hydrometers are used in cylinders of which details a
22、re given in Appendix A of Part1 of this standard. Recent observations have indicated the desirability of providing greater clearance between the side of the cylinders and the bulb of the hydrometers, and this has been done by increasing the internal diameter of the cylinders. The basis of calibratio
23、n of the hydrometers adopted in1937, namely density ingrammes (mass) per millilitre at20 C in a liquid with a surface tension of46dyn/cm, has become established in the dairy industry and is retained in this standard. The value46dyn/cm is an average for the surface tension at20 C of freshly formed su
24、rfaces of milk, i.e.the surface obtained by pouring milk into a vessel until some has overflowed. The surface tension of such milk surfaces may vary by about 4dyn/cm from the mean value of46dyn/cm. If the milk is not allowed to overflow, the surface has a slightly lower and more variable surface ten
25、sion. 1) BS1741, “Methods for the chemical analysis of liquid milk”. 2) BS696, “Gerber method for the determination of fat in milk and milk products”.BS734-2:1959 BSI 09-1999 iii An important innovation in this standard is that the hydrometers are calibrated to be read at the top of the milk meniscu
26、s and not at the level of the milk surface. It is essential that the hydrometers should comply with the specifications set out in Part1 of this standard. The accuracy of the calibration of the hydrometers should be tested by comparison with hydrometers tested at the National Physical Laboratory, as
27、described in Appendix B. It should be stressed that methods employing hydrometers for the determination of the total solids content of milk, although having the advantage of speed, yield results less accurate than gravimetric methods. The accuracy of the value of the total solids content calculated
28、from the equations is dependent on the density of the fat and of the density of the S.N.F. Only when these densities approximate to the average values implicit in the equations will the results be in close agreement with those obtained gravimetrically; it is known that these densities may vary appre
29、ciably in milk from individual cows and to a lesser extent even in bulked milks. As it is usual to express the composition of milk in terms of fat and S.N.F. content, this standard provides tables relating fat content and density to S.N.F. content and not to total solids content. A British Standard
30、does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Summary of pages This document comprises a front cover, an i
31、nside front cover, pagesi toiv, pages1to22, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover.iv blankBS734-2:1959 BSI 09-1999 1 1 Scope Part2 of
32、 this British Standard specifies procedures for determining the density of milk with the fat either wholly in the liquid state or wholly in the solid state and provides tables relating the known fat content and measured density to percentage ofS.N.F. in the milk under test. Of the hydrometers specif
33、ied in Part1 of this standard the Nos.1 and1A hydrometers are the most accurate;225ml(8ft.oz) of milk is ample for these hydrometers. The Nos.2 and2A hydrometers can beused with150ml(5fl.oz) of milk while the Nos.3 and3A hydrometers may be used with as little as90ml (3fl.oz) of milk but the use of s
34、uch a small sample should be avoided whenever possible. 2 Determination of density The density of milk may be determined with the milk fat wholly in the liquid state or wholly in the solid state by the procedures set out below. Method 1. Milk with fat in liquid state. The milk is warmed to a tempera
35、ture of4045 C and maintained within this range for5minutes, during which time the contents of the bottle are adequately mixed. Care must be taken to avoid the formation of air bubbles or froth when mixing the sample. The sample is then water-cooled to20 2 C and held within this range until the densi
36、ty reading is taken. The sample must not be so held for more than1 hour. The reading is then taken according to the following procedure: The sample bottle is gently inverted two or three times. The milk is then poured down the side of the cylinder so as to avoid the formation of air bubbles. Suffici
37、ent milk must be poured into the cylinder to ensure that some overflows when the hydrometer is inserted. The hydrometer, held by the top of the stem, is inserted in the sample and released when approximately in its position of equilibrium thus avoiding wetting more than a very short length, of the s
38、tem above the milk surface. As soon as the hydrometer is at rest, the scale reading corresponding to the top of the meniscus of the milk is noted. The eye must be level with the top of the meniscus when the reading is taken. The cylinder must be vertical and the bulb of the hydrometer must not touch
39、 the side. It is advisable to repeat the reading after depressing the hydrometer about3mm and allowing it to come to rest. The temperature of the milk is recorded immediately after taking the hydrometer reading. Method 2. Milk with fat in solid state. The milk iscooled quickly to a temperature of35
40、C in chilled waterand maintained within this range for at least15hours. The temperature of the sample is then raised to1820 C in a water bath in any part of which the temperature must not exceed21 C at any time. During this time the contents of the bottle are adequately mixed by rotary motion of the
41、 bottle followed by inversion at least six times. Care must be taken to avoid the formation of air bubbles or froth when mixing the sample. The density reading is then taken within1 hour according to the procedure given under Method1. 3 Corrections to observed hydrometer readings When the observed h
42、ydrometer reading has been obtained it must be corrected before it can be used in the calculation of the S.N.F. content of the sample. Two corrections are necessary. The first is the correction (see Appendix B) for any scale error and the second is the correction (see Table 1) for the error introduc
43、ed when the reading is made at a temperature other than20 C. The method of applying these corrections is shown in examples1 and2 of Clause4. BS734-2:1959 2 BSI 09-1999 Table 1 Temperature corrections to hydrometer reading 4 Determination of S.N.F. content The corrected hydrometer reading is used in
44、conjunction with the known fat content to obtain the S.N.F. content of the sample, either by substitution in the appropriate formula or from the appropriate table. Fat in liquid state Fat in solid state F =percentage (by weight) of fat D =(corrected hydrometer reading1) 1000 Example 1. Fat in liquid
45、 state. Fat content3.65percent The part of Table 2 which includes the column headed1.029 and the row opposite3.65percent of fat is consulted. The S.N.F. figure corresponding to a density of1.0292g/ml and a fat content of3.65percent is8.82percent. Example 2. Fat in solid state. Fat content,3.60percen
46、t The part of Table 3 which includes the column headed1.029 and the row opposite3.60percent of fat is consulted. The S.N.F. figure corresponding to a density of1.0297g/ml and a fat content of3.60percent is8.77percent (by interpolation). Temperature ( C) Fat in liquid state Fat in solid state 22.0 21
47、.5 21.0 20.5 20.0 19.5 19.0 18.5 18.0 +0.0005 +0.0004 +0.0003 +0.0001 0 0.0001 0.0003 0.0004 0.0005 0 0.0001 0.0003 0.0004 0.0005 NOTEThese corrections apply to milk containing4percent of fat but they can be used for all milks(010percent fat) as the maximum error that may be so introduced is only 0.
48、0001 in the corrected hydrometer reading. Formula: Percentage (by weight) of S.N.F.=0.25D+0.22 F +0.72 Table: Use Table 2 Formula: Percentage (by weight) of S.N.F.=0.25D+0.22F +0.55 Table: Use Table 3 Observed hydrometer reading 1.0294g/ml Temperature of sample 18.5 C Correction for scale error +0.0
49、002g/ml Reading corrected for scale error 1.0294+0.0002=1.0296g/ml Correction for temperature from Table 1 0.0004g/ml Reading corrected for scale error and temperature 1.02960.0004=1.0292g/ml Observed hydrometer reading 1.0301g/ml Temperature of sample 19.5 C Correction for scale error 0.0003g/ml Reading corrected for scale error 1.03010.0003=1.0298g/ml Correction for temperature from Table 1 0.0001g/ml Reading corrected for scale error and temperature 1.02980.0001=1.0297g/mlBS734-2:1959 BSI 09-1999 3 Appendix A De
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