BS 3095-2-1988 Methods for determination of the freezing-point depression of milk - Recommendations for the interpretation of the freezing-point depression of herd milk《牛奶冻结点下降值测定方.pdf

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1、BRITISH STANDARD BS 3095-2: 1988 Methods for Determination of the freezing-point depression of milk Part 2: Recommendations for the interpretation of the freezing-point depression of herd milk UDC 637.12.053:536.421.4.032.2BS3095-2:1988 This British Standard, having been prepared under the direction

2、of the Dairying Standards Committee, was published under the authority ofthe Board of BSI and comes intoeffect on 29 February 1988 BSI 07-1999 First published December 1981 First revision February 1988 The following BSI references relate to the work on this standard: Committee reference DAC/2 Draft

3、for comment 86/53329 DC ISBN 0 580 16398 9 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 Food Researc

4、h, 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 Trading Sta

5、ndards 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 Amendments issue

6、d since publication Amd. No. Date of issue CommentsBS3095-2:1988 BSI 07-1999 i Contents Page Committees responsible Inside front cover Foreword ii 0 Introduction 1 1 Scope 1 2 Definitions 1 3 Survey of herds 1 4 Interpretation of the FPD of samples of herd milk 2 5 Estimation of extraneous water 2 6

7、 Interpretation of the FPD of samples of milk other than herd milk 3 Publications referred to Inside back coverBS3095-2:1988 ii BSI 07-1999 Foreword This revision of this Part of BS 3095 has been prepared under the direction of the Dairying Standards Committee. It supersedes BS 3095-2:1980, which is

8、 withdrawn. Section 1.1 of BS 3095 describes a method using a thermistor cryoscope, in which the temperature sensor is a semi-conductor. Section 1.2 of BS 3095 describes the method for determination of the freezing-point depression of milk based on the work of J. Hortvet first described in 1921 1)an

9、d 1922 2)with subsequent modifications. A standard mercury-in-glass thermometer is used in the procedure. Experimental work carried out at the National Institute for Research in Dairying, Shinfield (now the AFRC Institute of Food Research, Reading Laboratory) has shown the results obtained by the tw

10、o methods are comparable. Part 2 provides some guidance on the interpretation of results of the freezing-point depression of herd milk. Part 3 describes the preservation of milk samples during transport to the laboratory and during subsequent short-term or long-term storage prior to testing for free

11、zing-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 a biological constant: its determination provides a test for the presence or absence of extraneous water. The va

12、lues 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 titratable acidity have to be taken into account in the interpretation of results. In the methods described in Section

13、s 1.1 and 1.2 the apparatus requires calibration. This is carried out using solutions of fixed composition. The freezing-point depressions of several standard solutions have been established by experimental programmes undertaken in both the UK and USA. In these studies the compositions of the standa

14、rd solutions were based on the addition of an exact mass of solute to 1 kg of water. In practice, it is sufficiently accurate if these standard solutions are prepared to contain weighed amounts of solute in a known volume of solution. The compositions given in this standard are equivalent to the pro

15、per exact mass ratios if the solute is weighed in air using brass or stainless steel weights, and the solution is prepared at 20 C. A collaborative study was carried out to determine the precision of the test methods. Details of the extent of the study and the values for precision found are given in

16、 Sections 1.1 and 1.2. A British Standard 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 T

17、his document comprises a front cover, an inside front cover, pages i and ii, pages1 to 4, 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. 1) H

18、ortvet J.J. Ind. Eng. Chem., 1921, 13. 2) Hortvet J.J. Ass. Off. Agric. Chem., 1922, 5.BS3095-2:1988 BSI 07-1999 1 0 Introduction The addition of water to genuine milk raises its freezing-point, thus decreasing its freezing-point depression (FPD). The determination of FPD has been used therefore for

19、 over 50 years to detect the presence of extraneous water in milk. However, in order to interpret the results of FPD determinations it is essential to have accurate information on the natural variation in the FPD of genuine milk. Any acidity developed by milk on storage influences test results by in

20、creasing the FPD. Although data on which to base the interpretation of test results have been available for many years, it was decided in 1977 that an experimental programme was needed to dispel doubts about the validity of the accepted values. This Part of BS 3095 gives details of this survey and g

21、ives recommendations for the interpretation of the results of the survey and other experimental work undertaken for the Technical Committee preparing this standard. 1 Scope This Part of BS 3095 gives recommendations for the interpretation of the freezing-point depression (FPD) of herd milk. NOTEThe

22、titles of the publications referred to in this standard are listed on the inside back cover. 2 Definitions For the purposes of this Part of BS 3095, the following definitions apply. 2.1 authenticated milk samples of milk representative of a whole milking and taken under supervised conditions to ensu

23、re no pick-up of extraneous water 2.2 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 the Celsius scale in the same Hortvet apparatus as described in BS 3095-1.2, or when the FPD is dete

24、rmined directly in a thermistor cryoscope as described in BS 3095-1.1 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 signs, e.g.545m C, and, to avoid confusion, is reported asm C (Hortvet) NOTEIt is important to avoid

25、confusing freezing-point depressions measured using the Hortvet procedure with “true” freezing-point depressions (see Appendix B to BS 3095-1.1:1988 or BS 3095-1.2:1988). 3 Survey of herds To study the effects on FPD of the natural variations in milk composition, including any influence of season an

26、d herd size, a survey was carried out in which the milk from 400 herds in England and Wales was tested. In each quarter of the year,100herds were sampled. A contemporary survey of 48 herds was made in Scotland, the results of which were in general agreement with those of the larger survey. For each

27、herd, a sample of authenticated milk was taken at both the evening and the morning milking, each sample being collected throughout the milking. In addition, a sample was taken from the farm bulk tank containing the total yield from the two milkings. From the FPDs of the authenticated evening and mor

28、ning samples, the FPD of the authenticated herd milk (evening plus morning yields) was calculated by weighting the results according to the evening and morning volumes of milk. The conclusions given in a) to f) are based on the statistical analysis of the FPD results obtained in the survey by public

29、 analysts using the Hortvet methods described in BS 3095-1.1 and BS 3095-1.2, and the method described in BS 1741 for determining titratable acidity. NOTEThe variability in the FPD of milk observed in the survey includes that inherent in the measurement as well as that caused by differences in compo

30、sition of the milk. a) The average FPD of the samples of authenticated milk was 543.3 m C (Hortvet) with a standard deviation of 4.6 m C, assuming a statistically normal distribution. This indicates that 95 % of samples of authenticated herd milk can be expected to have an FPD of between534m C (Hort

31、vet) and 552.3 m C (Hortvet). b) The average FPD of the milk samples taken from the farm bulk tanks, i.e. combined evening and morning milk, was 541.9 m C (Hortvet). This indicates that even under the carefully supervised milking procedures, a small amount of extraneous water was present in the samp

32、les. The amount of extraneous water was variable but the average was 0.2 % (m/m). c) In about two herds out of three, the samples from the evening milking had a larger FPD than those taken in the morning. The difference varied greatly from herd to herd to give a mean for all herds of + 2.2 m C with

33、a standard deviation of5.0 m C.BS3095-2:1988 2 BSI 07-1999 d) When repeat authenticated samples were taken from the same herd within 48 h, the overall mean of the differences between the two samples was not significant. However, this difference varied greatly from herd to herd and in 95 % of the cas

34、es in which the repeat authenticated sample had the larger FPD, the differences did not exceed 6.2 m C. e) The average FPD varied from season to season, being greatest in October and least in February. The difference between the two extreme average values was 3.1 m C for authenticated milk and2.8 m

35、C for samples taken from the bulk tank. f) There was no significant relationship between the total daily volume of milk produced by a herd and the FPD of the milk. g) There was no significant relationship between the titratable acidity and the FPD of the fresh milk samples. The value of the titratab

36、le acidity of the samples tested ranged from 0.12 g to 0.18 g of lactic acid per 100 mL of milk. 4 Interpretation of the FPD of samples of herd milk The findings listed in clause 3 show that there is no unique FPD of genuine herd milk. It is therefore not possible to establish a standard value again

37、st which to compare an unknown sample. However, it is possible to assess the probability of the presence of extraneous water in herd milk as indicated by the following. The Technical Committee has reached the following conclusions on statistically sound data and has based its recommendations for the

38、 interpretation of an FPD on its own judgement of what is practical and reasonable. a) If the FPD of a sample of herd milk is 535 m C (Hortvet) or greater it can be accepted as being free from extraneous water. b) Although an individual sample of herd milk whose FPD is 534 m C (Hortvet) to 530 m C (

39、Hortvet) may contain extraneous water, the FPD alone is insufficient justification to presume it contains extraneous water. c) It is likely that herd milks having an FPD of529 m C (Hortvet) or less contain extraneous water. The lower the FPD value the greater the likelihood that the herd milk contai

40、ns extraneous water. d) Herd milk with an FPD of less than 525 m C (Hortvet) almost certainly contains extraneous water. e) Whilst repeat sampling (“appeal to herd” procedure) is applicable in all cases for which it is practicable, it is strongly recommended for samples with an FPD of 529 m C (Hortv

41、et) to525m C (Hortvet). If an authenticated repeat sample has been taken from a herd within 48 hours, the original sample of milk should not be considered to have contained extraneous water unless the FPD of the repeat sample is at least 7 m C greater than that of the original sample. f) If the titr

42、atable acidity of a sample of herd milk, determined at approximately the same time as its FPD, does not exceed 0.18 g of lactic acid per100mL of milk, the FPD of the milk should be accepted as valid even though some development of acidity may have occurred. If the titratable acidity exceeds 0.18 g,

43、but does not exceed 0.30 g, an approximate estimate of the FPD may be obtained from the following equation: where The estimated FPD is only an approximation of the sample when fresh and, therefore, it has to be interpreted with caution. 5 Estimation of extraneous water 5.1 If the FPD of the herd mil

44、k free from extraneous water is known, or an assumption of its value can be made, the percentage of extraneous water in a suspect sample may be estimated from the following equation: where A is the estimated FPD in m C (Hortvet); B is the determined FPD in m C (Hortvet); T is the titratable acidity,

45、 determined in accordance with BS 1741, (in grams of lactic acid per 100 mL of milk). W is the percentage (m/m) of extraneous water; C is the actual or assumed FPD of the genuine milk in m C (Hortvet); D is the FPD of the suspect milk in m C (Hortvet); S is the percentage (m/m) of total solids in th

46、e suspect milk, determined in accordance with BS 1741. AB340 T 0.18 () = W CD () 100S () C - =BS3095-2:1988 BSI 07-1999 3 5.2 A report of an estimated percentage of extraneous water should be accompanied by: a) the FPD of the suspect sample; b) the titratable acidity of the suspect sample and whethe

47、r the FPD has been adjusted for it; c) the total solids content of the suspect sample; d) the actual FPD of the genuine milk sample and how this sample was obtained, or the assumed value of the FPD and the basis of its choice; e) the date of sampling and the date of testing. 6 Interpretation of the

48、FPD of samples of milk other than herd milk It is appreciated that the mixing of milk from different herds would be expected to cause the FPD of the mixed milk progressively to approach the national average, but when milk from a small number of herds all within restricted zone is bulked, it would be

49、 unwise to carry this assumption too far. Evidence of the degree of mixing that would justify the use of a narrower range of FPDs in the estimation of extraneous water in bulked milk is not available. Theoretical calculations of the survey data did however confirm that the FPDs of unadulterated bulk herd milk followed a nornal distribution around the same estimated mean as herd milks but had a smaller standard deviation. Hence it is unlikely that milk obtained by the bulking of milk from a num