Why is it important to measure extraneous water in milk?
Milking equipment on the farm, transportation and storage vessels are all “wet cleaned” leaving ample possibility for the accidental addition of water to milk.
Consumers who buy milk are entitled to receive milk and not milk diluted with water, it is therefore illegal to sell milk containing extraneous water. In underdeveloped countries, where milk is not heat treated prior to consumption, there is a risk of water borne pathogens such as Cholera being carried over into milk. Additionally there are very strong economic arguments for minimising the amount of extraneous water, which is allowed to find access to milk. If a processor allowed 1% water into milk he would incur additional 1% transport costs from farm to process centre, he would incur additional storage and processing costs.
He would also be breaking the law and could be prosecuted.
These extra costs and the risk of prosecution can be minimised by regular routing monitoring of milk for extraneous water.
What are the common sources of extraneous water in milk?
Although purposeful adulteration of milk is believed not to be common, it does occur from time to time.
Careless milk drainage of equipment involved in milking, milk storage, transportation and processing which are all “wet cleaned” can lead to water finding access to milk.
Inadequate drainage of plant, leaking equipment, sweeping through milk with water to clear pipelines and switching automatic cleaning on too early are common sources of contamination.
How does the freezing point of milk relate to extraneous water content?
The freezing point of water is 0°C. If salts are added to water the freezing point is lowered below zero – the greater the amount of salt added the lower the freezing point of the mixture. This is why seawater freezes at about minus 1 to 2°C and why salt is added to roads in the winter.
How does the freezing point of milk remain reasonably constant when milk composition changes?
The osmotic pressure, and hence the freezing point of cows blood remains fairly constant. The cow has to balance the osmotic pressure of its milk with that of its blood hence the freezing point of milk, as a biological, is maintained fairly constant.
When the composition of milk changes, for example if lactose levels drop due to mastitis, the cow adds a little salt in order to bring the osmotic pressure of its milk back in line with that of its blood.
How constant is the freezing point of milk?
As milk is a biological fluid there is a small variation in freezing points and this has to be taken into account when using the freezing point test to assess whether or not water has been added to milk.
The freezing point of genuine milk from individual cows varies with the breed of cow, its feed, the season, time of lactation and the climate. However as milk is pooled in a farm tank, then into tankers, silos and into commercial pasteurised milk these variations tend to be averaged out.
Is the freezing point test a recognised reference test for milk?
Yes. The freezing point test has been used for estimation of water in milk for almost 100 years. The early reference test (The Hortvet method) has now been replaced by the thermistor cryoscope.
Freezing point, measured by thermistor cryoscope, has been widely studied and is THE international reference standard method (ISO/DIS 5764).
Who establishes the “actionable” level for the freezing point of milk in assessing extraneous water?
The standard method only specifies the test method for freezing point – it does not deal with the use of the freezing point to determine whether or not extraneous water is present in milk. The interpretation of the freezing point to establish whether or not milk contains extraneous water is left to individual National Authorities. Each National Authority has historic data on the freezing point on milk known to be free from extraneous water and this is taken as a reference point for genuine water free milk for that country.
What is the effect of processing on the freezing point of milk?
Heat treatments, providing they do not involve vacuum treatment (which strips out gases which affect the freezing point) have little effect on the freezing point of milk. Hence the International Standard applies to pasteurised, UHT and sterilised whole, partially skimmed and skimmed milks.
Treatments involving freezing will not affect the freezing point providing the total milk is thawed and mixed well before a sample is taken.
Processes involving changes to the chemical composition of milk (such as culturing milk to give lactic acid) cause a significant change in the freezing point.
I find the units of measurement, working from zero to –0.510°C confusing, is there a simpler way of expressing them?
Yes, to simplify matters the fractions of a degree may be expressed in millidegrees centigrage. Thus 0.510°C is expressed as 510 millidegrees. It can also be confusing to talk of a freezing point being increased from -0.510°C to -0.500°C. To overcome this confusion these changes are sometimes referred to as freezing point depressions. Thus the freezing point depression is the amount by which the freezing point of milk is depressed below zero i.e. a freezing point of -0.510°C may be expressed as a freezing point depression of 510 millidegrees.
Does the milk from other species behave in a manner similar to cows milk?
Yes the same arguments apply to milk from other species (e.g. goat, sheep, buffalo etc.) as to milk from the cow. However the reference value for the genuine milk will be slightly different from species to species. As with cow’s milk the reference value for genuine goat, sheep and buffalo milk will vary from country to country. However, the freezing point of goat (–0.560 °C), and ewe
(-0.575 °C) milk is generally lower than cows milk and the freezing point of buffalo milk is similar to that of cows milk (-0.525 °C).
The figures in brackets are approximate values and will differ from country to country and within breeds.
Are there other means of measuring extraneous water?
There is an infra red method but this calculates the freezing point and is a standard or reference method.
Are cryoscopes only used for the measurement of extraneous water In milk?
Cryoscopes can be used to measure any change in the osmotic pressure or salt balance of solutions. For example changes to the salt balance occur when milk is cultured and lactose is converted to lactic acid also when lactose is hydrolysed to give glucose and galactose.
Where farmers are paid for their milk based on fat and protein content is it still important to test for extraneous water?
Yes. Unless you sample and test for payment purposes every supply a farmer consigns there is always the possibility that consignments not included in the payment program might contain extraneous water.
Even if every consignment is sampled and tested for composition the farmer is normally paid for the kilograms of fat and protein he consigns irrespective of how “dilute” they are. More dilute fat and protein milks cost more to transport, store and process –regular checking for the absence of extraneous water makes good commercial sense.