Density Measurement in Dairy Industry

Application Note Density Measurement in Dairy Industry »

Introduction Milk is a very complex food with over 100.000 different molecular species found. There are many factors that affect the composition of raw milk such as breed, age and physical state of the cow and seasonal variations. Therefore only an approximate milk composition of 87-88 % water and 12-13 % total solids can 1 be given . The total solids consist of approx. 4 % fat and 9 % solids-not-fat solids-not-f at (SNF) (proteins, lactose, minerals, vitamins, ...). Milk is an emulsion of milk fat globules in fat-free milk liquid (mainly water). There are up to 4.6 billions fat globules in one milliliter milk. Density of Milk The density of raw milk is in the range of 1.026 3 1 1.034 g/cm at 20 °C. The density of milk is given by the densities of its components components according to t heir mass content. The following densities of milk components (at 1 20 °C) mainly contribute to milk density: •

Water

0.998 g/cm

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Fat

0.931 g/cm

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Proteins

1.451 g/cm (average)

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Lactose

1.545 g/cm (average)

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Salts

3.000 g/cm

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Composition

Temperature

The density of milk decreases, decreases, if the temperature increases. The higher the fat content of the milk, the bigger the density changes with temperature. The reason is that the volume of fat changes much more with temperature than the volume of water. The temperature coefficient of whole milk is in the range of 0.00027 to 0.00031 g/cm³/K and the one of skim milk is in the range of 0.00023 to 2 0.00028 g/cm³/K.

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Included air is "trapped" in viscous dairy products like yogurt and escapes only very slowly or not at all. Inclusion of air gives wrong density results with bad repeatability. repeatability. »

Milk-Specific Phenomenon

Milk density increases slowly after milking (by up 3 to 0.001 g/cm ) and reaches a constant value after approx. six hours. This phenomenon was discovered by Recknagel. The density increase is due to escaping air and slow solidification of milk fat. »

"Temperature Product

History"

of

the

Dairy

Depending on the "temperature history" of the milk or the dairy product small differences in density are observed. These are caused by different solidified states of the milk fat. For example, different densities are found for the same milk sample, if it is at 40 °C or at 20 °C before filling it into the measuring cell (at 20 °C).

The density measurement of raw milk is used for quality control. By density measurement, deviations of milk composition due to addition of water (watering) and skimming are revealed.

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See table 1 on page 3. »

The inclusion of air strongly influences the density of milk and dairy products.

Application of Density Measurement

The density of milk and dairy products depends on: »

Inclusion of Air

Milk density in combination with the fat content can be used to calculate the total milk solids. The density measurement of milk plays a more important role for the conversion of volume to weight and for process control (milk standardization). Considering the relatively big, natural variations of milk density an uncertainty of measurement of 3 0.001 g/cm (DMA 35, DMA 38) is sufficient for routine measurements. For investigations at university institutes and national dairy institutes more accurate density meters like DMA 4100 M, DMA 4500 M and DMA 5000 M are used. Quality Control of Milk Since milk is a multi-component system it is not possible to determine the concentration of one component only by density measurement.

Created on 3/12/2009 4:30 Created by JEK/JUG

Xdlia45d_dairy_ind.doc Page 1 of 4

Application Note Yet, milk density measurement quickly indicates deviations from the normal milk composition due to e.g. watering. In some countries density measurement is therefore used for the quality control of the delivered raw milk. If 10 % of water is added to the milk, the density decreases by 3 approx. 0.003 g/cm . Due to the relatively big, natural variations of milk, density watering of milk can only be detected, if at least 15 % water have been added.

Calculation of Total Milk Solids As mentioned above, raw milk contains approx. 12-13 % total solids that consist of approx. 4 % fat and 9 % solids-not-fat (proteins, lactose, minerals, vitamins, ...). There is a direct relationship between milk density, fat content and solids-not-fat. The fat content of milk is routinely determined in dairies. In combination with milk density it can be used for the calculation of the total milk solids 4 according to the Fleischmann formula:

If skimming (smaller fat content gives higher density) and watering (higher water content decreases the density) are done at the same time, a "normal" milk density can be observed. For this reason density measurement alone is no absolute proof of watering. However as a quick control method it indicates deviations from normal milk composition. The portable DMA 35 is applicable for intake control of the delivered raw milk. Conversion of Volume to Weight

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The volume of milk is determined with volume meters during delivery. However, in different countries the milk is paid by weight. For the conversion of volume to weight the milk density can be used as conversion factor:

t ... f ... d ...

total solids in mass percent fat in mass percent relative density at 15 °C

The fat content of milk can be determined in different ways e.g. according to the Rose Gottlieb method or the Gerber method with butyrometer (hydrometer). Principle: The content of milk fat can be quickly determined using the Gerber method. The organic substance is partly destroyed upon addition of sulfuric acid; the fat is separated and is volumetrically determined. The result is reported in mass percent and is rounded off to 0.05 %. In combination with milk density that is measured with the DMA 35 (or DMA 38) the total milk solids can be calculated.

Mass = Density x Volume

Milk density is influenced by different factors, one of them is inclusion of air during delivery. For this reason and because its natural variations, milk density must be measured periodically on the spot to calculate an average density or conversion factor. Investigations have shown that milk density, if used as conversion factor, has to be determined under "real" conditions during delivery. Experimental results that are obtained in the laboratory - under different conditions - cannot be compared with on-the-spot measurements. Determination of density as a conversion factor from volume to weight is another application for the portable DMA 35, since it measures milk density with high accuracy on the spot.


t = 1.2 x f + 266.5 x (d - 1)/d

Features and Benefits of the Anton Paar Instruments DMA 35: •

Compact, light weight instrument

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Easy operation

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Large, easy to read digital display with backlight Automatic temperature correction Small sample volume of 2 to 5 mL 3

Accuracy 0.001 g/cm

Memory for 1024 results Memory for up to 20 measuring methods and up to 100 sample IDs



Application Note •

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IrDA-interface for wireless data transfer to computer or printer

Besides density, freezing point determination is commonly used to determine watering of milk.

Robust design

The freezing point of milk varies only a little, in the range of -0.53 to -0.55 °C and is mainly influenced by the water content. A freezing point above -0.53 °C indicates that water has been added to the milk.

Product version DMA 35 Tag&Log with additional RFID interface allowing to read RFID tags for easy sample identification and change of measuring method

DMA 38: •

Especially for viscous dairy products like yogurt, creams, ...

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Ease of use

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Robust design

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Easy to read digital display

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Minimal sample volume of approx. 1 mL

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In modern dairy labs the classical methods to determine the concentration of various milk components are more and more replaced by infrared spectroscopy (IR). With IR analyzers (e.g. MilkoScan by Foss) different milk parameters like fat, proteins, total solids, ... can be determined simultaneously. Previous MilkoScan instruments that determine the fat and protein content were combined with density measurement. The measuring results of MilkoScan and density meter were transferred to a PC program for the determination of the total milk solids.

Sample temperature between 15 °C and 40 °C 3

Accuracy 0.001 g/cm

On-line Density Measurement

Built-in Peltier thermostat, therefore no temperature compensation necessary

On-line density measurement is successfully used for process control (milk standardization) in dairy industry.

Interface for data transfer to computer or printer For automated measurements optional connection of sample changers

Competitive Methods

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For density measurement in dairy industry hydrometers (lactodensimeter, milk tester according to Gerber, milk tester according to Quevenne, ...) or pycnometer are still used.

During milk standardization milk fat is added to the skim milk. The densities of skim milk and milk fat are known. The density change upon addition of fat is used to monitor and control the fat content of the standardized milk e.g. 4.5 % fat.

Table 1: Density of various dairy products as a function of fat and solids-not-fat (SNF) content. Product

Composition

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Density [g/cm³] at

Fat [%]

SNF [%]

4.4 °C

10 °C

20 °C

38.9 °C

Skim milk

0.02 0.02

8.9 10.15

1.036 1.041

1.035 1.040

1.033 1.038

1.026 1.031

Whole milk

4.0 3.6 12.25 11.3

8.95 8.6 7.75 8.9

1.035 1.033 1.027 1.031

1.033 1.032 1.025 1.030

1.030 1.029 1.020 1.024

1.023 1.022 1.010 1.014

Cream

20.0 36.6

7.2 5.55

1.021 1.008

1.018 1.005

1.012 0.994

1.000 0.978




Application Note References

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"Chemie und Physik der Milch", A. Töpel, VEB Fachbuchverlag Leipzig, 2. Auflage, 1981 "Einfluss der Temperatur auf die Dichte von Milch zwischen 15 °C und 25 °C", M. Rüegg, U. Moor, Schweiz. Milchw. Forschung, 1985, 14(3), 7 "Zur Umrechnung des Milchvolumens in Gewicht", M.Rüegg, E. Flückinger, Schweiz. Milchztg., 1987, 113(7), 7 "Milk and Dairy Products", Ullmann's Encyclopedia of Industrial Chemistry, 589 (a) AOAC Official Method 925.22, Specific gravity of milk, (b) AOAC Official Method 925.23, Solids (total) in milk, (c) AOAC Official Method 990.22, Freezing point of milk, (d) AOAC Official Method 980.15,Water (added) in milk Goff, H.D., Hill A.R., "Dairy Chemistry and Physics", Dairy Science and Technology Handbook, VCH Publishers, 1993, Vol.1




Density Measurement in Dairy Industry

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