Method of and equipment for controlling the content of fat in milk

A method of and equipment for controlling the content of fat in milk. Whole milk is separated into skim milk. Some of the cream is returned to the skim milk to create a standardized milk. The standardized milk is employed to derive a parameter that dictates how much cream is to be added. The fat contents detected in the standardized milk are exploited to vary the amount of added cream. The density of the skim milk is measured at intervals and the results are stored. The density of the standardized milk is measured and more or less cream is added until a prescribed difference between the skim milk and the standardized milk, corresponding to the desired fat content in the standardized milk, is obtained. The density of the skim milk and that of the standardized milk are measured in the same sensor. The result of each measurement of the density of the skim milk is compared with the stored result. When there is a discrepancy between the two results, the most recently measured result is stored instead of the previously stored result. How much cream is added is controlled in accordance with a density difference that varies with the discrepancy.

BACKGROUND OF THE INVENTION 
The invention concerns a method of controlling the content of fat in milk. 
Whole milk is separated into skim milk and cream. Some of the cream is 
returned to the skim milk to create a standardized milk. The standardized 
milk is then employed to derive a parameter that dictates how much cream 
is to be added. The fat contents detected in the standardized milk are 
exploited to vary the amount of added cream. The density of the skim milk, 
however, is also measured at intervals and the results are stored. The 
density of the standardized milk is then measured and more or less cream 
is added until a prescribed difference between the skim milk and the 
standardized milk, corresponding to the desired fat content in the 
standardized milk, is obtained. 
A method of this type is known from German OS 2 531 141. The densities of 
the skim milk and cream leaving a centrifuge are detected by two separate 
sensors. Some of each liquid flows through the sensors. The two currents 
subsequently rejoin the mainstream. How much cream to add to the skim milk 
in order to obtain a particular fat content in the standardized milk 
accordingly depends on the empirical difference between the densities of 
the two liquids. 
Density, however, depends not only on fat content but also on such non-fat 
solids as proteins, sugar, minerals etc. and on temperature. Measuring the 
density of the skim milk and that of the standardized milk and calculating 
the difference between them is intended in this known process to eliminate 
all factors other than fat content such that the difference between the 
results will depend only on the difference between the fat content of the 
skim milk and that of the standardized milk. Since the same product is 
theoretically flowing through both sensors, it is assumed that 
establishing a difference in density that represents the desired content 
of fat in the standardized milk will also maintain that content constant. 
Using two sensors, however, necessarily entails two basic sources of error. 
One source comprises manufacturing tolerances that lead to discrepancies 
between the results obtained from the two sensors even when the products 
flowing through them are equally dense. The other source is the difference 
in on-site fluidics that derives from the alignment between the two 
sensors, necessitating a pressure difference to ensure flow. When the 
products, as is common with milk, contain air, the air will lead to 
different density readings in the event of pressure differences, even 
though the fat content does not differ. 
Still other errors can derive from variations in the fat content of the 
skim milk throughout an operating cycle. Such variations can occur for 
example as the result of reductions in the centrifuge's separating 
efficiency, of the presence of homogenized milk in the milk being skimmed, 
and of different percentages of air in the whole milk. The difference in 
density detected by the first and the second sensor will no longer ensure 
the desired fat content in the standardized milk. 
SUMMARY OF THE INVENTION 
The object of the present invention is a more precise method of controlling 
the fat content of the standardized milk at less expenditure of equipment 
and controls. 
This object is attained as will now be described. The density of the skim 
milk and that of the standardized milk are measured in the same sensor. 
The result of each measurement of the density of the skim milk is compared 
with the stored result. When there is a discrepancy between the two 
results, the most recently measured result is stored instead of the 
previously stored result. How much cream is added is now controlled in 
accordance with a density difference that varies with the discrepancy. 
Since the method in accordance with the invention necessitates only one 
sensor to measure the density of the skim milk and that of the 
standardized milk, the equipment will be less expensive, and manufacturing 
tolerances and differences in fluidics will be similarly oriented, 
generating no errors in the differences between the two results. 
Furthermore, no constant difference between the density of the skim milk 
and that of the standardized milk is exploited, but every measurement 
detects whether the difference still corresponds with the previously 
measured skim-milk difference. An appropriate correction in the computer 
allows the method to considerably increase the requisite precision in 
establishing the fat content of the standardized milk. 
One advantageous embodiment of the method is characterized in that, in 
controlling how much cream is added, a correction factor is taken into 
consideration that compensates for any alteration in the protein content 
of the standardized milk at various fat contents. 
Restoring some of the cream removed from the whole milk to the skim milk 
displaces the solids in the standardized milk in that cream also includes 
milk constituents to a percentage that depends on the concentration of 
fat. In producing standardized milk with different fat contents, different 
amounts of cream are diverted, meaning that different protein contents are 
also extracted. Different percentages of protein are accordingly added to 
the standardized milk along with the cream. Since the percentage of 
protein in the skim milk is no longer equal to that of the protein in the 
standardized milk, there will be errors in measuring the fat content of 
the standardized milk on the basis of measured density. These errors will 
be compensated for by the correction factor. 
The equipment for carrying out the method includes a centrifuge with an 
intake line for whole milk, an outlet line for skim milk, and an outlet 
line for cream. The equipment also includes a line between the 
cream-outlet line and the skim-milk outlet line. It is characterized in 
accordance with the invention as follows. A bypass line that accommodates 
a shut-off valve branches off from the skim-milk outlet line. Another 
bypass line that accommodates a shut-off valve branches off from a line 
that conveys standardized milk. Both bypass lines communicate with a 
recirculation line. The recirculation line accommodates a single sensor 
that the measures the density of both the skim milk and the standardized 
milk. 
Further advantageous embodiments are recited in the subsidiary claims. 
One embodiment of the invention will now be specified with reference to the 
accompanying drawing, wherein

DETAILED DESCRIPTION OF THE INVENTION 
The centrifuge 1 illustrated in FIG. 1 has an intake line 2 for whole milk, 
an outlet line 3 for skim milk, and an outlet line 4 for cream. Branohing 
off of cream-outlet line 4 is a mixing line 5. Mixing line 5 communicates 
in conjunction with skim-milk outlet line 3 with a line 6 for standardized 
milk. A bypass line 7 extends out of skim-milk outlet line 3. Another 
bypass line 8 extends out of standardized-milk line 6. Lines 7 and 8 
extend into a recirculation line 9. Recirculation line 9 communicates with 
whole-milk intake line 2. Recirculation line 9 accommodates a sensor 10 
connected to controls 11. Controls 11 are in turn connected to a valve 12 
in cream-outlet line 4. Valves 13 to 20 vary and interrupt the flow of 
liquid through their associated lines. Devices 21 through 25 maintain 
constant flow and pressure. 
Whole milk is supplied to centrifuge 1 through intake line 2 and separated 
into skim milk and cream. The centrifuge's separating precision is 
adjusted by way of valves 13 and 24. Once controls 11 have been activated, 
they open valve 14, diverting some of the skim milk to recirculation line 
9 through first bypass line 7. The density of the skim milk is measured by 
sensor 10. The result is stored in controls 11 and a difference associated 
with it. Controls 11 now close the valve 14 in first bypass line 7 and 
open valve 15 in second bypass line 8, diverting some standardized milk 
out of line 6 and into sensor 10 by way of recirculation line 9. 
Controls 11 now activate the valve 12 in cream-outlet line 4, adding enough 
cream to the skim milk in line 3 through mixing line 5 to ensure that the 
density of the standardized milk in line 6 will equal the aforesaid 
difference. The valve will be maintained in that state for a prescribed 
interval, subsequent to which controls 11 will reverse valves 14 and 15 
and reinstitute measurement of the skim milk. 
If there is no change in the skim milk's density, the stored result will 
continue as a basis for adjusting valve 12. If a discrepancy from the 
previously measured density is detected, however, the new result will be 
stored in association with a different density difference and controls 11 
will activate valve 12 to generate the new difference. 
Since the density of the skim milk is verified only discontinuously, brief 
and insignificant malfunctions at the skim-milk end will not directly 
occasion any errors. 
The density of the liquids flowing through the sensor is measured 
independent of any oscillations in pressure in downstream equipment 
because recirculation line 9 extends into whole-milk intake line 2, which 
is subject to constant pressure. Recirculation line 9 can on the other 
hand extend into an unillustrated storage tank upstream of the centrifuge. 
Standardized-milk line 6 accommodates a blender 26 that ensures thorough 
mixture of the skim milk with the cream arriving through mixing line 5. 
Recirculation line 9 accommodates a vacuum pump 27 upstream of sensor 10. 
Vacuum pump 27 comes into operation when the stored result is exceeded to 
too great and extent. 
Controls 11 activate an alarm when certain thresholds are exceeded. The 
alarm can for example be occasioned by deposits on probes in sensor 10. 
Sensor 10 can then, once valves 17 and 18 have closed and the valves 19 
and 20 in lines 28 and 29 opened, be rinsed out with a rinse. The system 
does not need to be stopped for this operation. 
It will be evident from FIG. 2 that the content E of protein in 
standardized milk S will vary, specifically in accordance with the 
concentration K of cream and with the fat content F of the milk. The 
resulting errors are compensated for with an appropriate correction factor 
in controls 11. Skim milk M acts as a reference.