COLLECTION AND RECEPTION OF MILK
The milk is brought from the farm (or collecting centre) to the dairy for processing. All kinds of receptacles have been used, and are still in use, throughout the world, from 2 – 3 litre calabashes and pottery to modern bulk-cooling farm tanks for thousands of litres of milk.
Formerly, when dairies were small, collection was confined to nearby farms. The microorganisms in the milk could be kept under control with a minimum of chilling, as the distances were short and the milk was collected daily. Today the trend is towards progressively larger dairy units. There is a demand for a higher production and increased quality of the finished product. Milk must be brought from farther away and this means that daily collection is generally out of the question. Nowadays, collection usually takes place every other day, but the interval can sometimes be three days and even four.
Keeping the milk cool
The milk should be chilled to + 4 °C immediately after milking and be kept at this temperature all the way to the dairy.
If the cold chain is broken somewhere along the way, e.g. during transportation, the microorganisms in the milk will start to multiply. This
will result in the development of various metabolic products and enzymes. Subsequent chilling will arrest this development, but the damage will already
be done. The bacteria count is higher and the milk contains substances that will affect the quality of the end product.
DESIGN OF FARM DAIRY PREMISES
The first steps in preserving the quality of milk must be taken at the farm. Milking conditions must be as hygienic as possible; the milking system designed to avoid aeration, the cooling equipment correctly dimensioned. To meet the hygienic requirements, dairy farms have special rooms for refrigerated storage. Bulk cooling tanks are also becoming more common. These tanks (Figure 5.1) with a capacity of 300 to 30 000 litres, are fitted with an agitator and cooling equipment to meet certain stipulations – for example
that all the milk in the tank should be chilled to +4 °C within two hours after milking. Larger farms, producing large quantities of milk, often install separate plate coolers for chilling the milk before it enters the tank (Figure 5.2). This saves mixing warm milk from the cow with the already chilled contents of the tank. The milk room should also contain equipment for cleaning and disinfecting the utensils, pipe system and bulk cooling tank.
DELIVERY TO THE DAIRY
The raw milk arrives at the dairy in insulated road tankers. The milk must be kept well chilled, free from air and treated as gently as possible. For example, tanks should be well filled to prevent the milk from sloshing around in the container.
When milk is collected by the tanker, it must be possible to drive all the way to the farm milk room. The loading hose from the tanker is connected to the outlet valve on the farm cooling tank (Fig. 5.3). The tanker is usually fitted with a flow meter and pump so that the volume is automatically recorded. Otherwise, the volume is measured by recording the level difference which, for the size of the tank in question, represents a certain volume. In many cases, the tanker is equipped with an air-eliminator. Pumping is stopped as soon as the cooling tank has been emptied. This prevents air from being mixed into the milk. The tank of the bulk collection vehicle is divided into a number of compartments to prevent the milk from sloshing around during transportation. Each compartment is filled in turn, and when the tanker has completed its scheduled round, it delivers the milk to the dairy.
Testing milk for quality
Milk from sick animals and milk which contains antibiotics or sediment must not be accepted by the dairy (Fig. 5.4). Even traces of antibiotics in milk can render it unsuitable for the manufacture of products which are acidified by the addition of bacteria cultures, e.g. yoghurt and cheese. Normally, only a general assessment of the milk quality is made at the farm. The composition and hygienic quality is usually determined in a number of tests on arrival at the dairy. The outcome of some of these tests has a direct bearing on the compensation to the farmer. The most common tests carried out on milk supplies are detailed below.
Taste and smell
In the case of bulk collection, the driver takes a sample of the milk at the farm for testing at the dairy. Milk that deviates in taste and smell from normal milk receives a lower quality rating. This affects the payment to the farmer. Milk with significant deviations in taste and smell should be rejected by the dairy.
The inside surfaces of farm tanks are carefully inspected. Any milk residue is evidence of inefficient cleaning and will result in a deduction in accordance with a quality payment scheme.
Hygiene or Resazurin Tests
The bacteria content of the milk is a measure of its hygienic quality. The Resazurin Tests are used frequently. Resazurin is a blue dye which becomes colourless when it is chemically reduced by the removal of oxygen. When it is added to the milk sample, the metabolic activity of the bacteria present has the effect of changing the colour of the dye at a rate which bears a direct relationship to the number of bacteria in the sample.
Two hygiene tests use this principle. One is a quick-screening test, which may form the basis for the rejection of a bad churn supply. If the sample starts to change shade immediately, the consignment is considered unfit for human consumption.
The other test is a routine test and involves storage of the sample in a refrigerator overnight, before a Resazurin solution is added. The sample is then incubated in a water bath and held at 37,5 °C for two hours.
Somatic cell count
A large number (more than 500,000 per ml of milk) of somatic cells in the milk indicates that the cows are suffering from udder diseases. The cell content is determined with specially designed particle counters (e.g. a Coulter counter).
- Taste and smell
- Somatic cell count
- Bacteria count
- Protein content
- Fat content
- Freezing point
A simplified form of bacteria count can also be used to assess the bacteria content. In this, the Leesment method, the bacteria are cultivated at 30 °C for 72 hours in a 0.001 ml milk sample with a nutritive substrate. The bacteria count is determined with a special screen.
Many dairies pay farmers according to the protein content of the milk. This is analysed by means of instruments operating with infrared rays. Up to 300 analyses per hour can be performed.
Various methods can be used to determine the butterfat content. The Gerber test is the most widely used method for whole milk.
Many dairies check the freezing point of the milk to determine whether or not it has been diluted with water. Milk of normal composition has a freezing point of -0.54 to -0.59 °C. The freezing point will rise if water is added to the milk. Special instruments are used for this check.
Dairies have special reception departments to handle the milk brought in from the farms. The first thing done at reception is to determine the quantity of the milk. The quantity is recorded and entered into the weighing system that the dairy uses to weigh the intake and compare it with the output.
The quantity of the intake can be measured by volume or by weight.
Tankers arriving at the dairy drive straight into a reception hall, often large enough to accommodate several vehicles.
The milk is measured either by volume or by weight.
Measuring by volume
This method uses a flowmeter. It registers the air in the milk as well as the milk, so the results are not always reliable. It is important to prevent air from entering with the milk. Measuring can be improved by fitting an air-eliminator before the flowmeter (Figure 5.7).
The tanker outlet valve is connected to an air-eliminator and from this the milk – free from air – is pumped through the flowmeter, which continuously indicates the total flow. When all the milk has been delivered, a card is placed in the meter for recording the total volume.
The pump is started by the control equipment, which senses when the milk in the air-eliminator has reached the preset level for preventing air from being sucked into the line. The pump is stopped as soon as the milk level drops below a certain level.
After measuring, the milk is pumped to a storage (silo) tank.
Measuring by weight
Bulk-collected milk can be measured in in two ways:
- Weighing the tanker before and after unloading and then subtracting one value from the other (Figure 5.8).
- Using special weighing tanks with load cells in the feet (Figure 5.9). In the first alternative, the tanker is driven onto a weighbridge at the dairy.
Operation may be manual or automatic. If manual, the operator records the weight against the driver’s code number. Where operation is automatic, the necessary data are recorded when the driver places a card in a card scanner. Before being weighed the tanker normally passes a vehicle washing station. This is of special importance when the weather is bad.
When the gross weight of the tanker has been recorded, the milk is delivered into the dairy. This may take place in line with a de-aerator but not a flowmeter. When empty, the tanker is weighed again and the tare weight is deducted from the previously recorded gross weight.
When the weighing-tank method is used, the milk is pumped from the tanker into a special tank with load cells built into the feet. The cells supply an electric signal that is always proportional to the weight of the tank. The strength of the signal increases with the weight of the tank as the milk enters the tank. The weight of the contents in the tank can be recorded when all the milk has been delivered. After this the milk is pumped to a silo tank.
Tankers are cleaned every day, as a rule at the end of a collection round. If the tanker makes several rounds a day, cleaning should take place after each round. Cleaning can be carried out by connecting the tanker to a cleaning system while in the reception area, or by driving it to a special cleaning station.
Many dairies also clean the outside of their tankers every day so that they always look clean when they are on the road. In more and more countries new rules are introduced about disinfection of tankers to avoid spreading animal diseases.
Chilling the incoming milk
Normally, a temperature increase to slightly above + 4 °C is unavoidable during transportation. Therefore, the milk is usually cooled to below + 4 °C in a plate heat exchanger, before being stored in a silo tank to await processing.
Raw milk storage
The untreated raw milk – whole milk – is stored in large vertical tanks – silo tanks – which have capacities from about 100,000 litres up to 500,000 litres. Smaller silo tanks are often located indoors while the larger tanks are placed outdoors to reduce building costs. Outdoor silo tanks are of double-wall construction, with insulation between the walls. The inner tank is of stainless steel, polished on the inside, and the outer wall is usually of welded sheet metal.
Agitation in silo tanks
These large tanks must have some form of agitation arrangement to prevent cream separation by gravity. The agitation must be very smooth. Extreme agitation causes aeration of the milk and fat globule disintegration. This exposes the fat to attack from the lipase enzymes in the milk. Gentle agitation is therefore a basic rule in the treatment of milk. The tank in Figure 5.10 has a propeller agitator, often used with good results in silo tanks. In very high tanks it may be necessary to fit two agitators at different levels to obtain the required effect.
Outdoor silo tanks have a panel for ancillary equipment (Fig. 5.11). The panels on the tanks all face inwards towards a covered central control station.
Tank temperature indication
The temperature in the tank is indicated on the tank control panel. Usually, an ordinary thermometer is used, but it is becoming more common to use an electric transmitter, which transmits signals to a central monitoring station.
There are various methods available for measuring the milk level in a tank. The pneumatic level indicator measures the static pressure represented by the head of liquid in the tank. The higher the pressure, the higher the level in the tank. The indicator transmits readings to an instrument.
All agitation of milk must be gentle. The agitator must therefore not be started before it is covered with milk. An electrode is often fitted in the tank wall at the level required for starting the agitator. The agitator stops if the level in the tank drops below the electrode. This electrode is known as the low-level indicator (LL).
A high-level electrode (HL) is fitted at the top of the tank to prevent overfilling. This electrode closes the inlet valve when the tank is full, and the milk supply is switched to the next tank.
Empty tank indication
During an emptying operation, it is important to know when the tank is completely empty. Otherwise, any milk remaining when the outlet valve has closed will be rinsed out and lost during the subsequent cleaning procedure. The other risk is that air will be sucked into the line if emptying continues after the tank is dry. This will interfere with later treatment. Consequently an electrode, lowest low level, (LLL) is often located in the drainage line to indicate when the last of the milk has left the tank. The signal from this electrode is used to switch to another tank or to stop emptying.