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It is important that the consistency and quality of milk does not change during storage. In order to store milk and maintain high milk quality, proper cooling equipment is essential. When determining the suitable type of cooling equipment, the following questions must be answered:
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What is the daily milk volume?
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What is the number of milkings for storage (total storage capacity)?
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What cooling capacity is needed?
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What is the ambient temperature?
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Which are the suitable options to ensure efficient cooling?
Critical factors here are the number of milkings, ambient temperature and milk cooling time.
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The numeral (2) designates a tank for two milkings
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The numeral (4) designates a tank for four milkings
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The numeral (6) designates a tank for six milkings
| Classification temp. |
Performance temp. (PT) in °C |
Safe operating temp. (SOT) in °C |
|
A
B
C |
38
32
25 |
43
38
32 |
PT, Performance Temperature – ambient temperature to be used when measuring the milk cooling temperature.
SOT, Safe Operating Temperature – highest limit of the range of ambient temperatures at which the equipment is required to function.
| Milk cooling time (from 35°C to 4°C) |
| Classification |
Hours |
|
0
I
II
III |
2
2.5
3
3.5 | |
0 – maximum acceptable cooling time of 2 hours
I – maximum acceptable cooling time of 2.5 hours
II – maximum acceptable cooling time of 3 hours
III – maximum acceptable cooling time of 2.5 hours
For example, cooling equipment with the code 2BII is designed for two milkings, with calculated cooling capacity at an ambient temperature of 32ºC. The cooling time (35 ºC – 4 ºC) for each milking will take less than three hours.
In practice, the required cooling capacity becomes lower as the number of milkings becomes larger. This is because the relative added milk volume is smaller.
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If a tank for two milkings is empty, or contains 50 % of its rated volume of milk at 4 °C and then 50 % of volume is added in one batch at 35 °C, all of the milk should be cooled to 4 °C no longer than the specific cooling time.
With four milkings, the respective tank stages are: empty, 25 %, 50 %, 75 % and 100 % respectively. With six milkings: empty, 16.7 %, 33.3 %, 50 %, 66.7%, 83.3 % and 100 % of its rate volume for the same temperatures.
Temperature course of milk in a storage tank (72-hour period).
Different materials can be used to construct a milk cooling tank, each of which has its advantages and disadvantages.
| Material |
Advantage |
Disadvantage |
| Stainless steel |
- free from rust
- easy to clean
- easy to build
- scratch proof
- shock proof
- acid proof
|
|
| Synthetic |
- lightweight
- easy to build
- shock proof
- acid proof
|
- easy to scratch
- difficult to clean
- more expensive
- difficult to adapt
|
| Enamelled steel |
- free from rust
- easy to clean
- scratch proof |
- most expensive
- not shock proof
- difficult to repair |
Advantages and disadvantages of materials used for the construction of cooling tanks.
Materials in contact with cleaning water and chemicals must be resistant to cleaning and disinfecting agents, in normal conditions of dosage and temperature. This is to avoid tainting the milk.
The chief alloying element in stainless steels is chromium (CR), which in concentrations above 12 – 13 % forms a passive layer on the metal. Increasing chromium content leads to a stronger passivity and thus a higher corrosion resistance. Although chromium makes the steel stainless, it cannot resist certain more aggressive environments. Other elements are therefore added to modify the structure, mechanical properties and corrosion resistance. These elements are Nickel (NI), Molybdenum (Mo), Nitrogen (N) and Copper (Cu). Stainless steel is available in many different quantities. Most milk tanks meet the quality grade AISI 304, and in special cases AISI 316.
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Cleaning can not be passed over. The careful cleaning of a milk cooling system provides the chance to avoid infections, while cooling delays micro bacterial growth and chemical processes. Avoiding bacterial growth by quick cooling, and good cleaning clearly pays good return on any extra cost that might be incurred.
Because of the nature of the product, milk, it is necessary to clean the milking equipment after every milk turn is complete. This means that the total installation must be free from any remainders of milk, one reason being that the most important life condition for bacteria, the presence of food, is taken away. By using high temperatures and thoroughly disinfecting the installation, most bacteria will be killed. A holistic look at why and how to clean can be found in Efficient Cleaning from DeLaval.
External hygiene
- Clean the tank with soapy water of a special cleansing agent
- Pay attention to the lid and rubber seals.
- Clean valve with sweeper and check the condition of the rubber seals.
Condensing unit hygiene
- Ensure sufficient fresh air supply
- Remove dust, hay, cobwebs, etc.
Areas that need to be checked when cleaning the cooling equipment
- Inner surfaces must be smooth and clean
- Dark places, and where water has been mixed with fat and stays in drops.
- The agitator wing.
- The tank interior. If necessary, climb in the tank and clean with a brush.
For milk cooling, mainly halogenic cooling agents are used. These are indicated by the letter ‘R’ (standing for refrigerant), followed by a code. This code gives the following proportions in R of
- Carbon [C]
- Hydrogen [H]
- Fluorine [F]
- Chlorine [Cl]
Halogenic cooling agents are described by the following items
- In the vapour phase they are odourless and non-irritating
- They are not poisonous (except by open fire)
- They cause no corrosion
- They are neither inflammable nor explosive.
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R12
The first widely used artificial refrigerant. Yet because of effects on the ozone layer, and the negative influence of greenhouse gases, it is not longer allowed. Production has therefore been stopped. Boiling point = [1 x 105 Pa] (°C) – 30.%
R22
Presently the most widely used artificial refrigerant. Disadvantage is that it still has some effect on the ozone layer (5 % of R12). R22 is no longer allowed for installation in Europe. It can only be used to repair an installed cooling machine. Boiling point = [1 x 105 Pa] (°C) – 40 %
R134a
Replacement for R12, with no ozone and only a slight greenhouse effect. Disadvantages are that it requires special oil and that it is rather difficult to change an existing R12 installation to R134a.
Boiling point = [1 x 105 Pa] (°C) – 26.5 %
R404a
Replacement for R22, with no ozone and only a slight greenhouse effect. Disadvantages are that it requires special oil and that it is rather difficult to change an existing R22 installation to R404a.
Boiling point = [1 x 105 Pa] (°C) – 46.4 %
R407c
Replacement for R22, with no ozone and only a slight greenhouse effect. Disadvantages are that it requires special oil and that it is rather difficult to change an existing R22 installation to R407c.
Boiling point = [1 x 105 Pa] (°C) – 44.%
R507
Replacement for R22, with no ozone and only a slight greenhouse effect. Disadvantages are that it requires special oil and that it is rather difficult to change an existing R22 installation to R507.
Boiling point = [1 x 105 Pa] (°C) – 46.5 %.
Requirements and norms for milk cooling tanks:
LIDS
The opening, closing and locking operations require a positive action. Accidental opening, closing and locking shall not be possible.
AGITATORS
No hazardous part of the agitator shall come into contact with the operator. Non-protected parts shall be present on the agitator shaft, with the exception of the agitator blades and accessories for the cleaning system.
STABILITY
The tank should be constructed in a way that, under normal operating conditions, it shall not tilt or move when subjected to an external force of 750N applied at any accessible points.
THERMAL INSULATION
The tank should be provided with thermal insulation so that milk at 4 °C shall not exceed 7 °C within 12 hours when the rated volume is allowed to stand undisturbed, without refrigeration.
FREEZING OF MILK
When the tank is in use, ice shall not form under the milk surface during cooling or storage.
AGITATION OF MILK
Operation of the agitator shall not cause milk to overflow when the tank contains any volume of milk up to 100 % of its rated volume. The agitator shall be capable of producing a uniform distribution of fat throughout the milk in an operating time of not more than 2 minutes and, after that, when the milk is allowed to stand non-agitated for 1 hour.
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