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The freezing point of authentic and original farm bulk tank milk in The Netherlands
International Dairy Journal 11 (2001) 121–126
The freezing point of authentic and original farm bulk tank milk in The Netherlands Betsie A. Slaghuis* Research Institute for Animal Husbandry, P.O. Box 2176, 8203 AD Lelystad, Netherlands Received 14 December 2000; accepted 13 March 2001
Abstract On 99 farms, geographically spread over The Netherlands, authentic freezing points of bulk tank milk samples were determined during the winter and summer of 1990. Authentic samples were taken under the supervision of an adviser, who ascertained that no avoidable water was added. Results were compared with routine quality control freezing points of samples from 73 farms still in operation in 1997. The mean freezing point of 338 samples was @0.52091C with a standard deviation of 0.00471C. Influences of farm, year of sampling, season (winter or summer) and region were small and/or not consistent. Evening milk had a lower freezing point than authentic milk from one evening and one morning milking together. In another study on ten experimental farms, differences between samples from original evening and morning milk were also found. Original milk samples are definitely not mixed with water. Variation in freezing points of authentic and original samples was similar, indicating more natural variation in freezing points of milk. Results are discussed in relation to previous surveys. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Freezing point; Raw milk; Bulk tank
1. Introduction The freezing point of cows’ milk is rather constant and the freezing point of bulk tank milk is therefore used to determine whether water has been added. However, since the introduction of the determination of the freezing point of milk an increase was observed for cow can and bulk tank milk (Eisses a Zee, 1980; Schukken, Fulton, a Leslie, 1992; Coveney, 1993). Sometimes a decrease was found (Packard a Ginn, 1979, 1990a). Different sampling methods have been used. Original or genuine samples are known to contain no added water at all. However, some confusion may be due to different definitions being used. In the UK, Harding (1995) and Coveney (1993) define authentic sampling as that with no water added to milk, because milking equipment must be rinsed with milk before authentic sampling. In our definition this is original sampling. The definition of authentic sampling is that samples are to be taken under supervision, while checks for added water *Corresponding author. Tel.:+31-320-293-211; fax:+31-320241-584. E-mail address: [email protected] (B.A. Slaghuis).
are performed. Therefore, there may be some unavoidable water present in the milk samples, due to milking equipment and bulk tanks not being completely dry after cleaning and disinfection procedures (Anonymous, 1992; Zee, Drogt, a Giessen, 1982). Differences between different analytical procedures may also contribute to the confusion surrounding appropriate freezing point measurement. Especially, temperature units and values adjudged to calibration solutions may differ. Brouwer and Dijkman (1989) give values adjudged to calibration solutions. Brouwer (1981) gives equations to convert Hortvet-freezing points, being used in USA, Canada and UK, into degrees Celsius, according to ISO 5764 (Anonymous,1987). About 20 years ago, an extensive study was performed on the freezing point of authentic farm can milk and farm tank milk in The Netherlands (Zee et al., 1982). After correction according to Brouwer and Dijkman (1989), a mean value of @0.5231C for the freezing point of bulk tank milk can be derived. In other countries the levels found ranged from @0.5291C to @0.5201C (Buchberger, 1986; Mitchell, 1986; Coveney, 1993, Packard a Ginn, 1990a,b), based on calculations according to Brouwer (1981).
0958-6946/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 5 8 - 6 9 4 6 ( 0 1 ) 0 0 0 4 3 - 7
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In original cow milk, Van der Velden, Brouwer, Hartog, Jansen, and Nooitgedagt (1984) found a mean freezing point of @0.52541C. In other countries the levels found ranged from @0.5311C to @0.52341C (Buchberger, 1986; Rohm, Pleschberger, a Foissy, 1991; Coveney, 1993). Differences between morning and evening milkings have also been reported (Eisses a Zee, 1980; Van der Velden et al., 1984; Zee et al., 1982; Mitchell, 1986; Buchberger, 1986; Rohm et al., 1991). Other factors affecting the freezing point of genuine or original cows’ milk are: breed of cows, stage of lactation, season, feed, water intake, climate, mastitis and storage time and temperature (Br(athen, 1983). As developments in dairy cattle husbandry progressed and quota were introduced, questions were raised about the level of the authentic freezing point in farm tank milk. Packard and Ginn (1990a, b) found a decrease, while Eisses and Zee (1980) found an increase from the beginning of this century. Therefore, a survey was conducted to determine the authentic freezing point of farm tank milk. The authentic freezing points were compared with routine quality control freezing points determined in routine quality testing for payment purposes of the same farms seven years later to evaluate possible changes. Further, on experimental farms original samples were taken to evaluate levels.
2. Material and methods
2.2. Sampling 2.2.1. Hundred farms In 1990, milking advisers of the dairy companies were asked to visit the selected farms and to collect milk samples. For one authentic sampling, the adviser visited the farm four times. During the first visit, a bulk tank milk sample was taken, after which, during the second and third visit, the actual authentic sampling took place. Before the evening milking, the milking equipment and the empty bulk tank were visually inspected. It was ensured that, besides the unavoidable water, no extra water was added to the milk. Milking was done under supervision of the adviser. From the bulk tank a sample of the evening milk was taken. The third visit, the next morning, included visual inspection of milking equipment and again milking under supervision. After milking, the authentic milk sample was taken from the tank which is a mixture of one evening and one morning milking. Although control on added water was ensured, there may have been some unavoidable water present in milk samples, due to milking equipment and bulk tanks not being completely dry after cleaning and disinfection procedures. About one week after authentic sampling, during the fourth visit, bulk tank milk was sampled without precautions for comparison with the authentic samples. In 1997, values of freezing points of bulk tank milk from the 73 remaining farms obtained from the Milk Control Station were used for analysis. Those freezing point values were determined in routine raw milk quality control for payment purposes.
2.1. Selection of farms A random selection of 100 farms spread over The Netherlands was assumed to give a good estimate of the mean freezing point of farm tank milk. In order to obtain a proper representation, stratification according to farm size was applied and the country was divided into four regions. The selected farms were sampled twice, in winter and in summer. On the actual 99 farms (one farm stopped milking after selection), the average number of cows was 50 (varying from 7 to 250 cows per farm; standard deviation 33), being a little higher than the mean of 47 on all farms participating in herd testing. The mean number of milking clusters per farm was 8, varying from 2 to 32. The mean amount of milk in the bulk tank after authentic sampling was approximately 1000 L (minimum 120 L and maximum 5500 L after two milkings) and the mean amount of milk per cow was about 20 L per day. Ten farms (also spread over the country) were used for an extended study with both original and authentic sampling. These farms were sampled four times, two times both in winter and in summer.
2.2.2. Ten farms In addition to the authentic sampling, sampling on ten farms was extended by taking samples of the first milk coming out of the milking equipment, which is a mixture of milk and unavoidable water from the milking equipment. Also, original samples were taken from the milk pumped into the bulk tank (four times each during evening and morning milking). Care was taken to ensure that the whole equipment was rinsed with milk before sampling. From four original samples freezing points were determined and a mean original freezing point was calculated. Finally, from the bulk tank milk after milking in the evening and from the same milk in the morning just before the morning milking, a sample was taken to study storage effects. All samples were cooled in ice water and stored at 41C until analysis within 36 h. 2.3. Analysis Samples were analysed the day after sampling according to ISO 5764 (Anonymous, 1987) with a
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thermistor cryoscope (Advanced Instruments model 4DII, Massachusetts, USA) with a plateau seeking modification and a plateau time of 22 s. The freezing points of the quality control samples of 1997 were obtained from the Milk Control Station. Here, analyses were also performed according to ISO 5764 (Anonymous, 1987) with thermistor cryoscopes (Advanced Instruments model 4CII, Massachusetts, USA) also with a plateau seeking modification and a plateau time of 22 s. Freezing points are expressed in degrees Celsius.
ences between summer and winter within a year and within farms Fixed effect YRk SEm REn
eijl ; eij
fixed effect of year of sampling k (k=1990 or 1997) fixed effect of season m ( m=1 or 2, being winter or summer) fixed effect of region n (n ¼ 1; y; 4, being west, north, east, or south) residual random error
2.4. Statistical analysis Freezing points of the milk samples were analysed using the Restricted Maximum Likelihood procedure (REML) in Genstat (Genstat 5 Committee, 1994). This is a procedure for analysis by linear mixed models, including estimation of variance components. To examine factors that affect the authentic freezing points, the following models were fitted. Yijkl ¼ m þfarmi þfarmðyearÞij þfarmðyearðseasonÞÞijl þYRk* SEm* REn þ eijl ;
ð1Þ
Yijm ¼ mþfarmi þfarmðperiodÞij þSEm þ eij ;
ð2Þ
where, Response variable Yijkl , Yijm m Random effect farmi
Model (1) is used for milk samples from the complete sample set of farms in 1990 and 1997 and model (2) for the extended sampling on ten farms in 1990, including authentic and original sampling, sampling for storage effects and amount of water in the first milk from the milking equipment. The significance of the year, season and the region and their interactions in model (1) was assessed by Wald tests; using the procedure VWALD (Buist a Engel, 1994). The Student t-test was used to analyse paired samples in model (2) comparing different samples (original evening and morning milk, the same milk in the evening and in the morning after storage, original and authentic, authentic and before and authentic and after sampling).
freezing point overall mean
farm(year)ij
farm(period)ij
farm (year(season))ijl
random effect of farm i (i ¼ 1; y; 99 for (1) and 1,y,10 for (2)) representing differences between farms random year of sampling j within farm i (j=1990 or 1997) representing differences between year of sampling within farm random period of sampling j within farm i (j ¼ 1; y; 4) representing differences between period of sampling within farm season l in year j within farm i (l ¼ 1; 2) representing differ-
3. Results In Table 1 the results are given for the farms sampled in 1990 and 1997. In Fig. 1 the frequency distribution of the samples are given for the two years under study. Despite some variation, the distribution of freezing points is normal for both years. Thus, no extra water appears to be present in these milk samples. In Table 2 results of three samplings are given; authentic sampling and before and after authentic sampling. The last two samplings do not differ from the authentic sampling. The mean freezing points of the evening authentic sample (@0.5222 with a standard deviation of 0.00561C) are significantly lower (P o 0.05) than the mean freezing points in the mixed samples (Table 2). The results from 1990 and 1997 were analysed
Table 1 Mean freezing points and standard deviations of a mixture of authentic evening and morning farm tank milk in 1990 and of farm tank milk in 1997
1990 survey 1997 Average
Number of farms
Number of samples
Freezing point (1C)
Standard deviation (1C)
99 73
194 144 338
@0.5204 @0.5216 @0.5209
0.0049 0.0044 0.0047
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B.A. Slaghuis / International Dairy Journal 11 (2001) 121–126
Fig. 1. Frequency distribution of freezing points of authentic farm tank milk (one evening and one morning milking together) of 99 farms in 1990 and from routine quality control (four to six milkings) of 73 farms still in operation in 1997.
Table 2 Mean freezing points and standard deviations (between brackets) of farm tank milk in 1990 before, during and after authentic samplinga
Mean freezing point (1C) Min (1C) Max (1C) Number of samples a
Before authentic sampling
Authentic sampling
After authentic sampling
@0.5197(0.0058) @0.533 @0.490 191
@0.5204(0.0049) @0.535 @0.505 194
@0.5196(0.0050) @0.533 @0.503 187
No significant differences in Student t-test (Po0.05).
Table 3 Mean freezing points and standard deviations of different samples collected on ten experimental farms visited four times in one yeara Sample Before authentic sampling Authentic samples After authentic sampling First milk from equipment Evening milk in tank Same milk in the morning Original evening milk Original morning milk a
n
Mean freezing point
Standard deviation
40 39 38 40 40 40 40 40
@0.5191 @0.5198e @0.5186e @0.5057 @0.5229a @0.5203b @0.5257c @0.5219d
0.0061 0.0052 0.0064 0.0233 0.0054 0.0060 0.0049 0.0052
e
Different superscripts within the same section indicate significance (Po0.05) in Student t-test.
together. Although the sampling in 1997 was not authentic, combined analysis is possible, because in 1990 no differences between the authentic sampling and sampling before or after this sampling were found. In 1990, a season by region interaction was found (data not shown). Interactions between region and season were not observed in 1997. Statistical differences
for year and season were found, but these showed inconsistent interactions. Effect of farms and interactions with year and season were small and not consistent either. In Table 3 results of the original sampling carried out on ten farms are given. Differences existed between evening and morning sampling, between evening and
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B.A. Slaghuis / International Dairy Journal 11 (2001) 121–126 Table 4 Freezing points in authentic farm tank milk samples, reported in two surveys in The Netherlands
Mean authentic freezing point (1C) Standard deviation (1C) Number of farms Number of samples
Survey 1978/79 (Zee et al., 1982)
Survey 1990 (this study)
@0.5230 0.0040 70 255
@0.5204 0.0049 99 194
morning sampling of the same bulk tank milk and between original and authentic sampling. No effects of season were found for the different samples. Estimated variance components showed small effects of farms compared to the total variance and to the standard error. From the results of Table 3 an increase in freezing point, possibly due to unavoidable water, was calculated (according to Zee et al., 1982) to be about 0.00241C, after correction for storage and unequal milk amounts. Standard deviations of authentic and original freezing points are comparable, indicating that this variation may not be due to addition of water but to natural variation in milk composition.
4. Discussion The freezing point of bulk tank milk in The Netherlands has increased from @0.52331C (Eisses a Zee, 1980; Zee et al., 1982) in the seventies to @0.52091C in the nineties. The increase found earlier (Eisses a Zee, 1980) was confirmed in the present study, but the level remained about the same from 1990 until 1997. Eisses and Zee (1980) calculated an increase in the mean freezing point of 0.0151C since 1920 for The Netherlands. In Table 4 the results of the 1978/1979 survey (Zee et al., 1982) and from the 1990 survey of authentic milk are shown. The survey methods appear to be comparable. Over a ten-year period, an increase in authentic freezing points of 0.00261C occurred. Schukken et al. (1992) found an increase in freezing point of 0.0051C in seven years in Ontario (Canada) in . bulk milk. Foissy, Rohm, Pleschberger, Godl, and Kollmann (1990) reported an increase of 0.0031C in about 20 years in Austria in can milk. Packard and Ginn (1990a) however, found a decrease of 0.00271C comparing results of two 4-year periods (1979–83 and 1984–88) in Minnesota (USA) in bulk milk. In the present study, from 1990 to 1997 a slight non-significant decrease (@0.00121C) was found. Here, authentic freezing points are compared to farm bulk tank freezing points, but from Tables 2 and 3 (first section) hardly any difference between the authentic sampling and the bulk milk sampling before and after this authentic sampling can be observed. As discussed before, in most cases an increase can be seen over time, which can be ascribed to changes in
milking method (from hand to machine). Also, storage of milk (from can to bulk tank) and higher productions levels (from 6000 kg milk per cow per year in 1990 to about 7000 kg per year in 1997 in The Netherlands) may have contributed to these changes. Finally, changes in feed management (extra maize silage in summer for example) and changes in milking equipment and cleaning methods (wider milking lines needing more water to be cleaned) may partly explain the increase in freezing points. Some of these changes also affected milk composition and the variation in authentic freezing point can be mainly explained by these changes in milk composition. Compared to the literature, the levels of the freezing points in this study are within the same ranges. But one ought to be careful comparing levels, because of the difference in analytical equipment and procedures (Rohm a Foissy, 1994; Rohm, 1993). Differences between authentic evening milk and authentic milk (mixture evening and morning milk) were significant (Table 3). Also, original evening milk has a lower freezing point than the original morning milk (Table 4), again indicating differences in milk composition. In Eisses and Zee (1980), Van der Velden et al. (1984) and Zee et al. (1982) differences were higher. Buchberger (1986) and Wiedemann, Buchberger, and Klostermeyer (1993) found similar differences, Rohm et al. (1991) found lower differences. Mitchell (1986) in New Zealand found lower freezing points in the morning milk. Although the levels of differences vary, a tendency to smaller differences at this time seems to occur. Mitchell (1986) in New Zealand indicated that a longer milking interval is associated with a lower freezing point, while in European countries the opposite is found (shorter intervals, lower freezing points of milk).
5. Conclusions The freezing point of farm bulk tank milk in The Netherlands, being @0.52091C, has increased since the last survey in 1978/1979, but remained rather constant from 1990 to 1997. Still differences in freezing points between morning and evening milkings (which are smaller than in 1978/79) and between original and authentic milk samples (which are a little higher than in
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B.A. Slaghuis / International Dairy Journal 11 (2001) 121–126
1978/79) are found. Some unavoidable water is present in authentic and bulk tank milk, but as the variation is comparable to the variation of original milk, this amount of water seems rather constant.
Acknowledgements The authors would like to thank H!el"ene Olijslagers, Harrie Schippers and the late Jack Brouwer for their technical support, all farmers for their co-operation and milking advisers for sampling milk. Also, help from the Milk Control Station in Zutphen with supplying information is highly appreciated. Geert Andr!e is thanked for his statistical support and Henk Hogeveen, Albert Meijering, Jacques Stadhouders and Kerst Stelwagen, for critically reading the manuscript.
References Anonymous (1987). MilkFDetermination of freezing pointFThermistor cryoscope method. ISO 5764. Anonymous (1992). Council directive 92/46/EEC of 16 July 1992. Laying down the health rules for the production and placing on the market of raw milk, heat-treated milk and milk based products. Br(athen, G. (1983). Factors affecting the freezing point of genuine cows’ milk. In Measurement of extraneaous water by the freezing point test. International Dairy Federation Bulletin Document 154, Brussels (pp. 6–11). Brouwer, T. (1981). Calculations concerning the determination of the freezing-point depression of milk. Netherlands Milk and Dairy Journal, 35, 159–175. Brouwer, T., a Dijkman, A. J. (1989). New Dutch standards for the determination of milk: a switch to non-arbitrary calibration values. Netherlands Milk and Dairy Journal, 43, 79–83. Buist, W. G., a Engel, B. (1994). Procedure VWALD. In P. W. Goedhart a J. T. N. M. Thissen (Eds.), Genstat 5 GLW-DLO procedure library manual release 3[1], Report LWA-94-17, Wageningen, The Netherlands: DLOFAgricultural Mathematics Group. Buchberger, J. (1986). Untersuchungen zum Gefrierpunkt der Milch. I. Teil: Rohmilch. Deutsche Molkerei Zeitung, 10, 244–252.
Coveney, L. (1993). The freezing point depression of authenticated and bulk vat milk: results of surveys 1989–1991. Journal of the Society of Dairy Technology, 46, 43–46. Eisses, J., a Zee, B. (1980). The freezing point of authentic cow’s milk and farm tank milk in The Netherlands. Netherlands Milk and Dairy Journal, 34, 162–180. . Foissy, H., Rohm, H., Pleschberger, C., Godl, L., a Kollmann, K. . (1990). Zum Gefrierpunkt der osterreichischen Rohmilch. Milchwirtschaftliche Berichte, 105, 234–239. Genstat 5 Committee (1994). Genstat 5 Release 3. Reference Manual. Oxford: Clarendon Press. Harding, F. (1995). In F. Harding (Ed.), Milk quality (pp. 60–74). Glasgow: Blackie Academic a Professional, Chapmann a Hall. Mitchell, G. E. (1986). Studies on the freezing point of milk produced in south-east Queensland. Australian Journal of Dairy Technology, 41, 57–62. Packard, V., a Ginn, R. (1979). Need for a working-factor in evaluating freezing point results on raw milk samples. Journal of Food Protection, 42, 110–112. Packard, V., a Ginn, R. (1990a). An evaluation of freezing point changes in raw milk analyzed by Dairy Quality Control Institute Inc. over ten years, 1979–1988. Dairy, Food and Environmental Sanitation, 10, 347–351. Packard, V., a Ginn, R. (1990b). A 12-month study of freezing point of regional raw milk supplies within the state of minnesota. Dairy, Food and Environmental Sanitation, 10, 597–600. Rohm, H. (1993). Determination of the freezing point of milkFan . Lebensmittel- Untersuchung undanalytical myth? Zeitschrift fur Forschung, 197, 558–561. Rohm, H., a Foissy, H. (1994). Gefrierpunktsbestimmung in Milch. . Osterreichische Milchwirtschaft, 14, 12–14. Rohm, H., Pleschberger, C., a Foissy, H. (1991). Der Gefrierpunkt . osterreichischer Rohmilch. 1. Einflussfaktoren auf den origin.aren . Gefrierpunkt. Ernahrung, 15, 333–337. Schukken, Y. H., Fulton, C. D., a Leslie, K. E. (1992). Freezing point of bulk milk in ontarioFan observational study. Journal of Food Protection, 55, 995–998. Van der Velden, H., Brouwer, T., Hartog, B. J., Jansen, J. T., a Nooitgedagt, A. J. (1984). Study of the freezing point of cow’s milk free from extraneous water. Netherlands Milk and Dairy Journal, 38, 91–106. Wiedemann, M., Buchberger, J., a Klostermeyer, H. (1993). Ursache . anomale Gefrierpunkte der Rohmilch. DMZ- Lebensmittelinfur dustrie und Milchwirtschaft, 114, 634–644. Zee, B., Drogt, J., a Giessen, T. J. J (1982). The freezing point of authentic farm tank milk in The Netherlands. Netherlands Milk and Dairy Journal, 36, 291–303.
The freezing point of authentic and original farm bulk tank milk in The Netherlands Betsie A. Slaghuis* Research Institute for Animal Husbandry, P.O. Box 2176, 8203 AD Lelystad, Netherlands Received 14 December 2000; accepted 13 March 2001
Abstract On 99 farms, geographically spread over The Netherlands, authentic freezing points of bulk tank milk samples were determined during the winter and summer of 1990. Authentic samples were taken under the supervision of an adviser, who ascertained that no avoidable water was added. Results were compared with routine quality control freezing points of samples from 73 farms still in operation in 1997. The mean freezing point of 338 samples was @0.52091C with a standard deviation of 0.00471C. Influences of farm, year of sampling, season (winter or summer) and region were small and/or not consistent. Evening milk had a lower freezing point than authentic milk from one evening and one morning milking together. In another study on ten experimental farms, differences between samples from original evening and morning milk were also found. Original milk samples are definitely not mixed with water. Variation in freezing points of authentic and original samples was similar, indicating more natural variation in freezing points of milk. Results are discussed in relation to previous surveys. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Freezing point; Raw milk; Bulk tank
1. Introduction The freezing point of cows’ milk is rather constant and the freezing point of bulk tank milk is therefore used to determine whether water has been added. However, since the introduction of the determination of the freezing point of milk an increase was observed for cow can and bulk tank milk (Eisses a Zee, 1980; Schukken, Fulton, a Leslie, 1992; Coveney, 1993). Sometimes a decrease was found (Packard a Ginn, 1979, 1990a). Different sampling methods have been used. Original or genuine samples are known to contain no added water at all. However, some confusion may be due to different definitions being used. In the UK, Harding (1995) and Coveney (1993) define authentic sampling as that with no water added to milk, because milking equipment must be rinsed with milk before authentic sampling. In our definition this is original sampling. The definition of authentic sampling is that samples are to be taken under supervision, while checks for added water *Corresponding author. Tel.:+31-320-293-211; fax:+31-320241-584. E-mail address: [email protected] (B.A. Slaghuis).
are performed. Therefore, there may be some unavoidable water present in the milk samples, due to milking equipment and bulk tanks not being completely dry after cleaning and disinfection procedures (Anonymous, 1992; Zee, Drogt, a Giessen, 1982). Differences between different analytical procedures may also contribute to the confusion surrounding appropriate freezing point measurement. Especially, temperature units and values adjudged to calibration solutions may differ. Brouwer and Dijkman (1989) give values adjudged to calibration solutions. Brouwer (1981) gives equations to convert Hortvet-freezing points, being used in USA, Canada and UK, into degrees Celsius, according to ISO 5764 (Anonymous,1987). About 20 years ago, an extensive study was performed on the freezing point of authentic farm can milk and farm tank milk in The Netherlands (Zee et al., 1982). After correction according to Brouwer and Dijkman (1989), a mean value of @0.5231C for the freezing point of bulk tank milk can be derived. In other countries the levels found ranged from @0.5291C to @0.5201C (Buchberger, 1986; Mitchell, 1986; Coveney, 1993, Packard a Ginn, 1990a,b), based on calculations according to Brouwer (1981).
0958-6946/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 5 8 - 6 9 4 6 ( 0 1 ) 0 0 0 4 3 - 7
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In original cow milk, Van der Velden, Brouwer, Hartog, Jansen, and Nooitgedagt (1984) found a mean freezing point of @0.52541C. In other countries the levels found ranged from @0.5311C to @0.52341C (Buchberger, 1986; Rohm, Pleschberger, a Foissy, 1991; Coveney, 1993). Differences between morning and evening milkings have also been reported (Eisses a Zee, 1980; Van der Velden et al., 1984; Zee et al., 1982; Mitchell, 1986; Buchberger, 1986; Rohm et al., 1991). Other factors affecting the freezing point of genuine or original cows’ milk are: breed of cows, stage of lactation, season, feed, water intake, climate, mastitis and storage time and temperature (Br(athen, 1983). As developments in dairy cattle husbandry progressed and quota were introduced, questions were raised about the level of the authentic freezing point in farm tank milk. Packard and Ginn (1990a, b) found a decrease, while Eisses and Zee (1980) found an increase from the beginning of this century. Therefore, a survey was conducted to determine the authentic freezing point of farm tank milk. The authentic freezing points were compared with routine quality control freezing points determined in routine quality testing for payment purposes of the same farms seven years later to evaluate possible changes. Further, on experimental farms original samples were taken to evaluate levels.
2. Material and methods
2.2. Sampling 2.2.1. Hundred farms In 1990, milking advisers of the dairy companies were asked to visit the selected farms and to collect milk samples. For one authentic sampling, the adviser visited the farm four times. During the first visit, a bulk tank milk sample was taken, after which, during the second and third visit, the actual authentic sampling took place. Before the evening milking, the milking equipment and the empty bulk tank were visually inspected. It was ensured that, besides the unavoidable water, no extra water was added to the milk. Milking was done under supervision of the adviser. From the bulk tank a sample of the evening milk was taken. The third visit, the next morning, included visual inspection of milking equipment and again milking under supervision. After milking, the authentic milk sample was taken from the tank which is a mixture of one evening and one morning milking. Although control on added water was ensured, there may have been some unavoidable water present in milk samples, due to milking equipment and bulk tanks not being completely dry after cleaning and disinfection procedures. About one week after authentic sampling, during the fourth visit, bulk tank milk was sampled without precautions for comparison with the authentic samples. In 1997, values of freezing points of bulk tank milk from the 73 remaining farms obtained from the Milk Control Station were used for analysis. Those freezing point values were determined in routine raw milk quality control for payment purposes.
2.1. Selection of farms A random selection of 100 farms spread over The Netherlands was assumed to give a good estimate of the mean freezing point of farm tank milk. In order to obtain a proper representation, stratification according to farm size was applied and the country was divided into four regions. The selected farms were sampled twice, in winter and in summer. On the actual 99 farms (one farm stopped milking after selection), the average number of cows was 50 (varying from 7 to 250 cows per farm; standard deviation 33), being a little higher than the mean of 47 on all farms participating in herd testing. The mean number of milking clusters per farm was 8, varying from 2 to 32. The mean amount of milk in the bulk tank after authentic sampling was approximately 1000 L (minimum 120 L and maximum 5500 L after two milkings) and the mean amount of milk per cow was about 20 L per day. Ten farms (also spread over the country) were used for an extended study with both original and authentic sampling. These farms were sampled four times, two times both in winter and in summer.
2.2.2. Ten farms In addition to the authentic sampling, sampling on ten farms was extended by taking samples of the first milk coming out of the milking equipment, which is a mixture of milk and unavoidable water from the milking equipment. Also, original samples were taken from the milk pumped into the bulk tank (four times each during evening and morning milking). Care was taken to ensure that the whole equipment was rinsed with milk before sampling. From four original samples freezing points were determined and a mean original freezing point was calculated. Finally, from the bulk tank milk after milking in the evening and from the same milk in the morning just before the morning milking, a sample was taken to study storage effects. All samples were cooled in ice water and stored at 41C until analysis within 36 h. 2.3. Analysis Samples were analysed the day after sampling according to ISO 5764 (Anonymous, 1987) with a
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B.A. Slaghuis / International Dairy Journal 11 (2001) 121–126
thermistor cryoscope (Advanced Instruments model 4DII, Massachusetts, USA) with a plateau seeking modification and a plateau time of 22 s. The freezing points of the quality control samples of 1997 were obtained from the Milk Control Station. Here, analyses were also performed according to ISO 5764 (Anonymous, 1987) with thermistor cryoscopes (Advanced Instruments model 4CII, Massachusetts, USA) also with a plateau seeking modification and a plateau time of 22 s. Freezing points are expressed in degrees Celsius.
ences between summer and winter within a year and within farms Fixed effect YRk SEm REn
eijl ; eij
fixed effect of year of sampling k (k=1990 or 1997) fixed effect of season m ( m=1 or 2, being winter or summer) fixed effect of region n (n ¼ 1; y; 4, being west, north, east, or south) residual random error
2.4. Statistical analysis Freezing points of the milk samples were analysed using the Restricted Maximum Likelihood procedure (REML) in Genstat (Genstat 5 Committee, 1994). This is a procedure for analysis by linear mixed models, including estimation of variance components. To examine factors that affect the authentic freezing points, the following models were fitted. Yijkl ¼ m þfarmi þfarmðyearÞij þfarmðyearðseasonÞÞijl þYRk* SEm* REn þ eijl ;
ð1Þ
Yijm ¼ mþfarmi þfarmðperiodÞij þSEm þ eij ;
ð2Þ
where, Response variable Yijkl , Yijm m Random effect farmi
Model (1) is used for milk samples from the complete sample set of farms in 1990 and 1997 and model (2) for the extended sampling on ten farms in 1990, including authentic and original sampling, sampling for storage effects and amount of water in the first milk from the milking equipment. The significance of the year, season and the region and their interactions in model (1) was assessed by Wald tests; using the procedure VWALD (Buist a Engel, 1994). The Student t-test was used to analyse paired samples in model (2) comparing different samples (original evening and morning milk, the same milk in the evening and in the morning after storage, original and authentic, authentic and before and authentic and after sampling).
freezing point overall mean
farm(year)ij
farm(period)ij
farm (year(season))ijl
random effect of farm i (i ¼ 1; y; 99 for (1) and 1,y,10 for (2)) representing differences between farms random year of sampling j within farm i (j=1990 or 1997) representing differences between year of sampling within farm random period of sampling j within farm i (j ¼ 1; y; 4) representing differences between period of sampling within farm season l in year j within farm i (l ¼ 1; 2) representing differ-
3. Results In Table 1 the results are given for the farms sampled in 1990 and 1997. In Fig. 1 the frequency distribution of the samples are given for the two years under study. Despite some variation, the distribution of freezing points is normal for both years. Thus, no extra water appears to be present in these milk samples. In Table 2 results of three samplings are given; authentic sampling and before and after authentic sampling. The last two samplings do not differ from the authentic sampling. The mean freezing points of the evening authentic sample (@0.5222 with a standard deviation of 0.00561C) are significantly lower (P o 0.05) than the mean freezing points in the mixed samples (Table 2). The results from 1990 and 1997 were analysed
Table 1 Mean freezing points and standard deviations of a mixture of authentic evening and morning farm tank milk in 1990 and of farm tank milk in 1997
1990 survey 1997 Average
Number of farms
Number of samples
Freezing point (1C)
Standard deviation (1C)
99 73
194 144 338
@0.5204 @0.5216 @0.5209
0.0049 0.0044 0.0047
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B.A. Slaghuis / International Dairy Journal 11 (2001) 121–126
Fig. 1. Frequency distribution of freezing points of authentic farm tank milk (one evening and one morning milking together) of 99 farms in 1990 and from routine quality control (four to six milkings) of 73 farms still in operation in 1997.
Table 2 Mean freezing points and standard deviations (between brackets) of farm tank milk in 1990 before, during and after authentic samplinga
Mean freezing point (1C) Min (1C) Max (1C) Number of samples a
Before authentic sampling
Authentic sampling
After authentic sampling
@0.5197(0.0058) @0.533 @0.490 191
@0.5204(0.0049) @0.535 @0.505 194
@0.5196(0.0050) @0.533 @0.503 187
No significant differences in Student t-test (Po0.05).
Table 3 Mean freezing points and standard deviations of different samples collected on ten experimental farms visited four times in one yeara Sample Before authentic sampling Authentic samples After authentic sampling First milk from equipment Evening milk in tank Same milk in the morning Original evening milk Original morning milk a
n
Mean freezing point
Standard deviation
40 39 38 40 40 40 40 40
@0.5191 @0.5198e @0.5186e @0.5057 @0.5229a @0.5203b @0.5257c @0.5219d
0.0061 0.0052 0.0064 0.0233 0.0054 0.0060 0.0049 0.0052
e
Different superscripts within the same section indicate significance (Po0.05) in Student t-test.
together. Although the sampling in 1997 was not authentic, combined analysis is possible, because in 1990 no differences between the authentic sampling and sampling before or after this sampling were found. In 1990, a season by region interaction was found (data not shown). Interactions between region and season were not observed in 1997. Statistical differences
for year and season were found, but these showed inconsistent interactions. Effect of farms and interactions with year and season were small and not consistent either. In Table 3 results of the original sampling carried out on ten farms are given. Differences existed between evening and morning sampling, between evening and
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B.A. Slaghuis / International Dairy Journal 11 (2001) 121–126 Table 4 Freezing points in authentic farm tank milk samples, reported in two surveys in The Netherlands
Mean authentic freezing point (1C) Standard deviation (1C) Number of farms Number of samples
Survey 1978/79 (Zee et al., 1982)
Survey 1990 (this study)
@0.5230 0.0040 70 255
@0.5204 0.0049 99 194
morning sampling of the same bulk tank milk and between original and authentic sampling. No effects of season were found for the different samples. Estimated variance components showed small effects of farms compared to the total variance and to the standard error. From the results of Table 3 an increase in freezing point, possibly due to unavoidable water, was calculated (according to Zee et al., 1982) to be about 0.00241C, after correction for storage and unequal milk amounts. Standard deviations of authentic and original freezing points are comparable, indicating that this variation may not be due to addition of water but to natural variation in milk composition.
4. Discussion The freezing point of bulk tank milk in The Netherlands has increased from @0.52331C (Eisses a Zee, 1980; Zee et al., 1982) in the seventies to @0.52091C in the nineties. The increase found earlier (Eisses a Zee, 1980) was confirmed in the present study, but the level remained about the same from 1990 until 1997. Eisses and Zee (1980) calculated an increase in the mean freezing point of 0.0151C since 1920 for The Netherlands. In Table 4 the results of the 1978/1979 survey (Zee et al., 1982) and from the 1990 survey of authentic milk are shown. The survey methods appear to be comparable. Over a ten-year period, an increase in authentic freezing points of 0.00261C occurred. Schukken et al. (1992) found an increase in freezing point of 0.0051C in seven years in Ontario (Canada) in . bulk milk. Foissy, Rohm, Pleschberger, Godl, and Kollmann (1990) reported an increase of 0.0031C in about 20 years in Austria in can milk. Packard and Ginn (1990a) however, found a decrease of 0.00271C comparing results of two 4-year periods (1979–83 and 1984–88) in Minnesota (USA) in bulk milk. In the present study, from 1990 to 1997 a slight non-significant decrease (@0.00121C) was found. Here, authentic freezing points are compared to farm bulk tank freezing points, but from Tables 2 and 3 (first section) hardly any difference between the authentic sampling and the bulk milk sampling before and after this authentic sampling can be observed. As discussed before, in most cases an increase can be seen over time, which can be ascribed to changes in
milking method (from hand to machine). Also, storage of milk (from can to bulk tank) and higher productions levels (from 6000 kg milk per cow per year in 1990 to about 7000 kg per year in 1997 in The Netherlands) may have contributed to these changes. Finally, changes in feed management (extra maize silage in summer for example) and changes in milking equipment and cleaning methods (wider milking lines needing more water to be cleaned) may partly explain the increase in freezing points. Some of these changes also affected milk composition and the variation in authentic freezing point can be mainly explained by these changes in milk composition. Compared to the literature, the levels of the freezing points in this study are within the same ranges. But one ought to be careful comparing levels, because of the difference in analytical equipment and procedures (Rohm a Foissy, 1994; Rohm, 1993). Differences between authentic evening milk and authentic milk (mixture evening and morning milk) were significant (Table 3). Also, original evening milk has a lower freezing point than the original morning milk (Table 4), again indicating differences in milk composition. In Eisses and Zee (1980), Van der Velden et al. (1984) and Zee et al. (1982) differences were higher. Buchberger (1986) and Wiedemann, Buchberger, and Klostermeyer (1993) found similar differences, Rohm et al. (1991) found lower differences. Mitchell (1986) in New Zealand found lower freezing points in the morning milk. Although the levels of differences vary, a tendency to smaller differences at this time seems to occur. Mitchell (1986) in New Zealand indicated that a longer milking interval is associated with a lower freezing point, while in European countries the opposite is found (shorter intervals, lower freezing points of milk).
5. Conclusions The freezing point of farm bulk tank milk in The Netherlands, being @0.52091C, has increased since the last survey in 1978/1979, but remained rather constant from 1990 to 1997. Still differences in freezing points between morning and evening milkings (which are smaller than in 1978/79) and between original and authentic milk samples (which are a little higher than in
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1978/79) are found. Some unavoidable water is present in authentic and bulk tank milk, but as the variation is comparable to the variation of original milk, this amount of water seems rather constant.
Acknowledgements The authors would like to thank H!el"ene Olijslagers, Harrie Schippers and the late Jack Brouwer for their technical support, all farmers for their co-operation and milking advisers for sampling milk. Also, help from the Milk Control Station in Zutphen with supplying information is highly appreciated. Geert Andr!e is thanked for his statistical support and Henk Hogeveen, Albert Meijering, Jacques Stadhouders and Kerst Stelwagen, for critically reading the manuscript.
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