BS 3095-1 2-1988 Methods for determination of the freezing-point depression of milk - Methods - Hortvet method《牛奶冻结点下降值的测定方法 霍尔特维特法》.pdf

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1、BRITISH STANDARD BS3095-1.2: 1988 Methods for Determination of the freezing-point depression of milk Part1: Methods Section1.2 Hortvet method UDC 637.12.04/.05:536.421.4.632.2BS3095-1.2:1988 This British Standard, having been prepared under the directionof the Dairying Standards Committee, was publi

2、shed under the authority ofthe Board of BSI and comes intoeffect on 29February1988 BSI 10-1999 First published as BS3095-1 March1959 First revision as BS3095-1 January1980 Second revision as BS3095-1.2 February1988 The following BSI references relate to the work on this standard: Committee reference

3、 DAC/2 Draft for comment86/53328 DC ISBN 0 580 16387 3 Committees responsible for this British Standard The preparation of this British Standard was entrusted by the Dairying Standards Committee (DAC/-) to Technical Committee DAC/2 upon which the following bodies were represented: AFRC Institute of

4、Food Research, Reading Laboratory Association of Public Analysts Association of Public Analysts of Scotland Dairy Trade Federation Department of Trade and Industry (Laboratory of the Government Chemist) Department of Trade and Industry (National Physical Laboratory) GAMBICA (BEAMA Ltd.) Institute of

5、 Trading Standards Administration Joint Committee of the Milk Marketing Board and the Dairy Trade Federation Milk Marketing Board Milk Marketing Board for Northern Ireland National Farmers Union Royal Association of British Dairy Farmers Scottish Milk Marketing Board Society of Dairy Technology Amen

6、dments issued since publication Amd. No. Date of issue CommentsBS3095-1.2:1988 BSI 10-1999 i Contents Page Committees responsible Inside front cover Foreword ii 1 Scope 1 2 Definition 1 3 Principle 1 4 Hortvet apparatus 1 5 Calibration of the Hortvet apparatus 5 6 Sampling, preservation and preparat

7、ion of the sample 7 7 Test method 7 8 Calculation of the FPD of the milk 9 9 Test report 10 10 Precision of the test method 10 Appendix A Method for calculating means of duplicate values 11 Appendix B Method for the calculation of “true” FPD 11 Figure 1 Schematic diagram of a mechanical cooling syst

8、em for the Hortvet cryoscope 1 Figure 2 Detail of the Hortvet apparatus (freezing-point tube and fittings) 2 Figure 3 Example of a Hortvet calibration graph 9 Table 1 Thermometric glasses approved by the National Physical Laboratory(1974) 3 Table 2 Details of the Hortvet thermometer 4 Table 3 Compos

9、ition of standard solutions for calibrating the Hortvet apparatus 6 Table 4 Example of the calculation of the correction to be added to an uncorrected FPD of a standard solution 7 Table 5 Example of the calculation of the FPD of a milk sample using a calibration graph 9 Table 6 Example of the calibr

10、ation of the FPD of a milk sample using a single standard solution 10 Table 8 Calculation of “true” FPD 11 Publications referred to Inside back coverBS3095-1.2:1988 ii BSI 10-1999 Foreword This Section of BS3095 has been prepared under the direction of the Dairying Standards Committee, and together

11、with Section1.1 is a revision of BS3095-1:1980. This Section of BS3095 together with Section1.1 supersedes BS3095-1:1980, which is withdrawn. This Section of BS3095 describes the method for determination of the freezing-point depression of milk based on the work of J. Hortvet first described in1921

12、1)and1922 2)with subsequent modifications. A standard mercury-in-glass thermometer is used in the procedure. Section1.1 describes a method using a thermistor cryoscope, in which the temperature sensor is a semiconductor. Experimental work carried out at the National Institute for Research in Dairyin

13、g (now the AFRC Institute of Food Research, Reading Laboratory) Shinfield has shown the results obtained by the two methods are comparable. Part2 provides some guidance on the interpretation of results of the freezing-point depression of herd milk. Part3 describes the preservation of milk samples du

14、ring transport to the laboratory and during subsequent short-term or long-term storage prior to testing for freezing-point depression. The freezing-point depression of genuine fresh milk is the extent to which the milk solutes cause the freezing point to be lower than that of water. It is virtually

15、a biological constant: its determination provides a test for the presence or absence of extraneous water. The values for the freezing-point depression of genuine milk, however, vary naturally over a narrow range and are influenced by developed acidity and, consequently, this variation and the titrat

16、able acidity have to be taken into account in the interpretation of results. In the methods described in Sections1.1 and1.2 the apparatus requires calibration. This is carried out using solutions of fixed composition. The freezing-point depressions of several standard solutions have been established

17、 by experimental programmes undertaken in both the UK and USA. In these studies the compositions of the standard solutions were based on the addition of an exact mass of solute to1kg of water. In practice, it is sufficiently accurate if these standard solutions are prepared to contain weighed amount

18、s of solute in a known volume of solution. The compositions given in this standard are equivalent to the proper exact mass ratios if the solute is weighed in air using brass or stainless steel weights, and the solution is prepared at20 C. The precision values quoted in clause10 are based on a collab

19、orative study involving eight laboratories using a Hortvet apparatus calibrated by the graphical sodium chloride procedure. It is to be noted that the reproducibility values quoted are calculated on the mean results of two determinations carried out in each laboratory. The extent of the study satisf

20、ied the requirements set out in the draft IUPAC/ISO guidelines for the development of standard methods of collaborative study. In addition seven other laboratories took part in the collaborative study using Hortvet apparatus calibrated using a graphical sucrose procedure or a single sodium chloride

21、or sucrose solution procedure. In view of the limited numbers of participants involved for each type of calibration procedure, it was not possible to draw firm conclusions about the repeatability or reproducibility of any of them. However, as the results obtained when the apparatus was calibrated us

22、ing the single sodium chloride solution procedure were sufficiently different from results obtained using the other procedures, the procedure for calibrating the Hortvet apparatus using a single sodium chloride solution has been omitted from this revision of the method. Appendix A describes the calc

23、ulation of means of duplicate values. 1) Hortvet J.J. Ind. Eng. Chem.,1921,13. 2) Hortvet J.J. Ass. Off. Agric. Chem.,1922,5.BS3095-1.2:1988 BSI 10-1999 iii A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their

24、 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 inside front cover, pagesi toiv, pages1to12, an inside back cover and a back cover. This standard has been updated (see copyr

25、ight date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover.iv blankBS3095-1.2:1988 BSI 10-1999 1 1 Scope This Section of BS3095 describes a method for the determination of the freezing-point depression of milk based upon the original

26、method and apparatus of Hortvet. The method is applicable to whole milk, partly skimmed or skimmed milk, either raw, pasteurised, sterilized or UHT, whether or not homogenised, provided that the titratable acidity of the milk is also determined. NOTEThe titles of the publications referred to in this

27、 standard are listed on the inside back cover. 2 Definition For the purposes of this Section of BS3095, the following definition applies. freezing-point depression (FPD) of milk the extent to which the freezing-point of milk is lower than that of pure water when both freezing-points are measured on

28、the Celsius scale in the same Hortvet apparatus as described in BS3095-1.2, or when the FPD is determined directly in a thermistor cryoscope as described in BS3095-1.1 the FPD of a sample of milk is expressed in thousandths of a degree Celsius, i.e.as a whole number without positive or negative sign

29、s, e.g.545m C, and, to avoid confusion, is reported asm C (Hortvet) NOTEIt is important to avoid confusing freezing-point depressions measured using the Hortvet procedure with “true” freezing-point depressions (seeAppendix B). 3 Principle A sample of milk is supercooled below its freezing point by p

30、lacing it in a refrigerated bath. When the temperature falls below a predetermined value, freezing is initiated. The release of heat as ice forms causes the temperature to rise until it reaches a maximum value. The temperature remains more or less constant at this value for several seconds before it

31、 falls again as cooling predominates. The maximum temperature is recorded as the freezing-point of the milk. The FPD is then derived by reference either to the freezing-point of distilled water and a calibration graph, or to the freezing-point of a standard solution of known FPD. 4 Hortvet apparatus

32、 (seeFigure 1 andFigure 2) 4.1 Hortvet thermometer 4.1.1 The Hortvet thermometer shall be of the mercury-in-glass type with a solid stem. It shall be graduated in accordance with the Celsius scale as defined in the current definition of the International Practical Temperature Scale as adopted by the

33、 General Conference of Weights and Measures and in accordance with the International System of Units (SI). For the purpose of manufacture and calibration, the thermometer shall be graduated for175mm immersion. Figure 1 Schematic diagram of a mechanical cooling system for the Hortvet cryoscopeBS3095-

34、1.2:1988 2 BSI 10-1999 Figure 2 Detail of the Hortvet apparatus (freezing-point tube and fittings)BS3095-1.2:1988 BSI 10-1999 3 4.1.2 The stem shall be of glass with an enamel back extending over the graduated portion. The bulb shall be manufactured from one of the thermometric glasses approved by t

35、he National Physical Laboratory, and listed in Table 1. NOTE 1The glass should be selected and processed so that the finished thermometer shows the following characteristics. a) The stress in the glass should be reduced to a level sufficient to minimize the possibility of fracture due to thermal or

36、mechanical shock. b) The legibility of the reading should be unimpaired by devitrification or clouding. c) The image of the meniscus should be distorted as little as possible by defects or impurities in the glass. NOTE 2The thermometer should be constructed to comply with the following. a) It should

37、 be filled with dry inert gas at a suitable pressure. b) The thermometer should be straight and its external cross section should be approximately circular. c) The bulb should be cylindrical and in line with the stem; its diameter should not exceed the diameter of the stem. d) The stem should have a

38、 plain top or button finish. e) The stem should incorporate an enamel backing positioned so that it lies behind the mercury column, when the column is viewed in alignment with the right-hand end of the shortest scale lines. f) The bore of the capillary should be such as to reduce sticking of the men

39、iscus to a minimum and the meniscus should be free of local sticking between1.70 C and+0.60 C. In order to prevent excessive sticking of the meniscus, the bore should be as large as possible, therefore the scale length should not unduly exceed the specified minimum (seeTable 2). g) An expansion cham

40、ber of sufficient capacity to allow the thermometer to be heated to50 C without damage should be provided at the upper end of the bore. h) No enlargement of the bore should be within10mm of a scale line, and no contraction chamber should be more than10mm above the top of the bulb. i) The shapes of t

41、he expansion chamber, bulb and contraction chamber should be such as not to entrap small quantities of mercury or gas, e.g.the expansion chamber should not have a pointed end. 4.1.3 The thermometer shall be stabilized by suitable heat treatment before it is graduated. The thermometer shall be marked

42、 as follows. a) A ring shall be etched round the stem of the thermometer175 5mm from the bottom of the bulb. b) The scale lines shall be clearly etched and of uniform thickness not exceeding0.05mm. The lines shall be in planes at right angles to the axis of the thermometer. When the thermometer is v

43、iewed from the front and held in a vertical position the left-hand end of the lines shall be in a vertical line. The0.1 C and0.05 C scale lines shall be extended on the right-hand side, but the shorter lines shall not extend across the bore. c) The figures shall be to the right of the scale lines an

44、d upright when the thermometer is held in a vertical position, and shall be so placed that they would be intersected by an extension of the lines to which they refer. The fractional lines shall be lightly figured with small numerals giving the decimal fraction of a degree only, and the degree lines

45、shall be figured with slightly larger and, preferably, heavier figures. 4.1.4 The following inscriptions shall be durably and legibly marked on the thermometer: a) “C” to indicate that the Celsius scale is being used; b) “175mm immersion” close to the ring indicating the immersion level; c) an ident

46、ification number; d) vendors and/or manufacturers name or other readily identifiable mark; e) the number of this British Standard, a reference to the Hortvet method and the scale range, i.e.“BS3095 Hortvet2 C to1 C”. 4.1.5 The characteristics of the thermometer shall be as given in Table 2. 4.1.6 Th

47、e repeatability of a reading on the thermometer shall be such that the correction at any point on the scale can be determined with a maximum uncertainty of2m C. NOTEIt is desirable that the thermometer be calibrated by the National Physical Laboratory or other appropriate organization to ensure comp

48、liance with the correction requirements given in Table 2. The true temperatures at which calibration is carried out should be close to the following: +0.050 C,0 C,0.500 C,0.520 C,0.540 C, 0.560 C and0.580 C. Table 1 Thermometric glasses approved by the National Physical Laboratory(1974) Glass Identi

49、fication stripe or approved abbreviation Normal glass, made by Whitefriars Glass Ltd. Normal glass, Dial, made by Plowden and Thompson Ltd. Normal glass, Schott-N16, made by Jenaer Glaswerk Schott and Genossen, Mainz Normal glass,7560, made by Corning Glass Company Corning borosilicate glass, made by Corning Glass Company Thermometric glass, Schott-2954, made by Jenaer Glaswerk Schott and Genossen, Mainz Borosilicate glass, made by Whitefriars Glass Ltd. Corning glass,1720, made by Corning Glass Company Schott-Supremax R