3985

Fat Content Determination during Milk Standardization using Density

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Кулинария и общественное питание

Process Application Note Fat Content Determination during Milk Standardization using Density 1. 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 m...

Английский

2012-11-10

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23 чел.

Process Application Note

Fat Content Determination

during Milk Standardization

using Density

1.

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 be given

(references [1], [2]).

The total solids consist of approx. 4% fat and

9% solids-not-fat (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.

2.

Density of milk

The density of raw milk varies between approx.

1.026 g/cm3 and 1.034 g/cm3 at 20°C [1].

Milk density is derived from the densities of its

components, according to their content.

The following densities of milk components (at 20 °C)

strongly influence the density of milk [1]:

• Water

0.998 g/cm3

• Fat

0.931 g/cm3

• Proteins

1.451 g/cm3 (average)

• Lactose

1.545 g/cm3 (average)

• Minerals

3.000 g/cm3.

The higher the fat content of the milk, the bigger

the density change with temperature. The reason is

that the volume of fat changes much more with

temperature than the volume of water.

• "Temperature history" of the dairy product

Depending on the "temperature history" small

differences in density are observed. These are

caused by different solidified states of the milk fat.

2.2. Density measurement of milk

Since milk is a multi-component mixture and the

density variations of raw milk are very high, it is not

possible to determine the concentration of single milk

components, like fat, directly from density

measurement.

Yet the fat content of milk can be determined from the

density difference between standardized milk and skim

milk.

The DPRn on-line transducers by Anton Paar are used

for the accurate determination and control of the milk

fat content during milk standardization.

Density measurement with laboratory digital density

meters by Anton Paar [3] is used for

• quality control of milk (watering or skimming)

• conversion of volume to weight

• calculation of total milk solids.

3.

Measuring principle

The following figure illustrates the principle of density

measurement used in the DPRn density transducers

by Anton Paar.

2.1. Influences on milk density

The density of milk and dairy products depends on:

• Composition

The density of milk increases with increasing

content of proteins, lactose or minerals and it

decreases with increasing fat content.

See table 1 on page 4.

• Temperature

The density of milk decreases, if the temperature

increases.

Anton Paar High-precision Instruments

www.anton-paar.com

Fig. 1: DPRn density transducer

The U-tube, a mechanical oscillator system, is excited.

It is kept oscillating at its resonant frequency by two

coils and an electronic circuit.

Tel.: +43 (0)316 257-0

E-mail: info@anton-paar.com

Xdpia31d-Milk-Fat-Content-Determ.doc

Page 1 of 4


Process Application Note

Oscillation period and sample temperature are

measured and transferred to a mPDS evaluation unit

for data processing and control purposes.

The accuracy of DPRn density transducers typically

amounts to ± 1 x 10-4 g/cm3.

4.

DPRn

Typical installation sites

mPDS 2000V3

Dairy products must have a defined and constant

composition.

During milk standardization the fat content of milk is

adjusted to a legally defined value.

Figure 2 shows a simplified scheme of the milk

standardization process.

In the first step the raw milk is skimmed in a centrifuge

to obtain cream and skim milk.

The cream is again added to the skim milk to obtain a

"standardized" fat content of e.g. 4.5 % fat.

The fat content of the standardized milk is determined

by the density difference to skim milk upon addition of

cream.

Raw milk

Skim milk

FT

Centrifuge

FT

Cream

FT

DPRn

FT

Fig. 2 Scheme of the milk standardization process

Density measurement can be performed with a

DPRn transducer in combination with a mPDS 2000V3

evaluation unit.

The DPRn density transducer is mounted in a bypass

to the main line.

Anton Paar High-precision Instruments

www.anton-paar.com

Bypass

Fig. 3: Bypass installation of the DPRn

The mPDS 2000V3 uses the density difference

between skim milk and standardized milk to determine

the fat content.

There are two ways to determine the fat content of the

standardized milk with DPRn transducers:

(1) The densities of skim milk and standardized milk

are continuously measured with two DPRn

transducers and mPDS 2000V3 evaluation units.

The density result of skim milk is automatically

transferred to the evaluation unit of the transducer

for standardized milk and the fat content is

determined from the density difference between

skimmed and standardized milk.

(2) Only the density of standardized milk is

continuously measured with one DPRn

transducer and mPDS 2000V3 evaluation unit.

The density of skim milk is determined before the

production start:

• Skim milk is measured with the DPRn

transducer later to be used for standardized

milk and the density result is stored in the

evaluation unit.

• The density of skim milk is determined in the

laboratory and the result is manually entered

in the evaluation unit.

Again the fat content is determined from the

density difference between skimmed and

standardized milk.

Tel.: +43 (0)316 257-0

E-mail: info@anton-paar.com

Xdpia31d-Milk-Fat-Content-Determ.doc

Page 2 of 4


Process Application Note

5.

Measuring range

Temperature:

Concentration:

6.

4 to 70 °C

0 to 40 % fat content

Measuring results - accuracy

Following parameters influence the measuring result:

• Density accuracy: 1 x 10-4 g/cm3

Corresponding variations in fat content

concentration: 0.1 % fat content (typical).

Repeatability: typically ± 0.01% fat content.

• Temperature influences are compensated by the

built-in temperature measurement.

Total error:

Variations of all process conditions lead to a total

error of 0.1 % fat content.

7.

Fig. 4 Classical bypass installation

The system can be disconnected from the main line

with the 2 installed valves.

Benefits

• With the DPRn density transducers the fat content

of the milk is continuously monitored during milk

standardization and milk quality is assured with

high precision.

• The precise control of the fat content in

standardized milk is important for economic (fat is

an expensive component of milk) and legal reasons

(a fat content which is too low is considered to be

customer deception).

Fig. 5 Bypass with In-line-fitting

8. DPRn 427S

With the new Stainless Steel housing cell more

installation possibilities are open. Depending on the

process situation the bypass can be created in the

following way:

No pump is required; the flow speed in the main line

have to be at lease 1 m/s.

Fig. 6 Bypass with pump

Anton Paar High-precision Instruments

www.anton-paar.com

Tel.: +43 (0)316 257-0

E-mail: info@anton-paar.com

Xdpia31d-Milk-Fat-Content-Determ.doc

Page 3 of 4


Process Application Note

The measurement is independent from the flow speed

in the main line.

9.

Summary

On-line density measurement with the Anton Paar

transducers DPRn offers accurate determination and

control of the milk fat content during milk

standardization.

Table 1: Density of various dairy products as a function of fat and solids-not-fat (SNF) content [4].

Product

Composition

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

Temperature coefficients: whole milk 2.7 to 3.1 x 10-4 g/cm³/°C; skim milk 2.3 to 2.8 x 10-4 g/cm³/°C [5].

10.

References

[1] "Chemie und Physik der Milch", A. Töpel, VEB

Fachbuchverlag Leipzig, 2. Auflage, 1981

[2] "Milk and Dairy Products", Ullmann's Encyclopedia

of Industrial Chemistry, 589

[3] Anton Paar Lab Application Note, "Density

Measurement in Dairy Industry"

[4] Goff, H.D., Hill A.R., "Dairy Chemistry and

Physics", Dairy Science and Technology

Handbook, VCH Publishers, 1993, Vol.1

[5] "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

Anton Paar High-precision Instruments

www.anton-paar.com

Tel.: +43 (0)316 257-0

E-mail: info@anton-paar.com

Xdpia31d-Milk-Fat-Content-Determ.doc

Page 4 of 4



 

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