Food preservatives play a crucial role in keeping the quality of food for a later use. However, such preservatives cause contamination/adulteration in milk and other food products. Milk adulteration is a big problem in many countries.
Many milk dealers prefer to add preservatives in milk to prolong the shelf life of their product given that raw milk is a product with a very short shelf life.
Driven by greed for quick money, many have been driven to lower depths of moral carelessness as to add dangerous chemicals into the product to avoid incurring refrigeration costs.
As a result of this, many dealers add chemicals such as formaldehyde, hydrogen peroxide, boric acid, and antibiotics into the milk.
They do this to prevent the milk from going bad so that they can sell the whole batch without incurring further business costs.
The list below contains food preservatives added (illegally) to milk and how you can test for their presence.
How to Detect Formaldehyde/Formalin in milk
Formalin reacts with iron in the presence of concentrated sulphuric acid to produce a violet colour.
Take 2 ml of the sample to be tested, add 2 ml of 90% H2SO4 with traces of ferric chloride. The acid should be added slowly on the side of the test tube.
When you see a purple ring being formed at the interface if the two ingredients, then formaldehyde is present.
The HPLC method will then be used to quantify the amount of formalin present.
How to Detect Hydrogen Peroxide in Milk
Use a hydrogen peroxide strip to check for the formation of blue colour, which indicates the presence of hydrogen peroxide.
The intensity of the blue colour will depend on the concentration of hydrogen peroxide present.
Alternatively, dissolve 1g of Vanadium oxide in 100 ml of 6% H2SO4. To 10 ml of the milk sample, add 10 -20 drops of the reagent and mix thoroughly.
Check for the formation of pink or red colour, which indicates the presence of hydrogen peroxide.
How to Detect Boric Acid in Milk
Dip a turmeric paper into the milk sample containing sulphuric acid with a pH of between 1 – 2 and then allow the paper to dry.
If the boric acid is present, the paper will turn orange to red. Then dip the paper into concentrated sodium hydroxide. The paper will turn green-black if boric acid is present.
Quantify the amount of boric acid present by using a chromatographic method.
Other food preservatives: Determination of Neutralizers in Milk
Rosalic Acid Test
This is a simple and rapid method that aims at testing for the traces of carbonates in milk.
Take 10 ml of milk, add equal amount of 95% alcohol to it followed by a few drops of rosalic acid. If the carbonates are present, the milk will turn to a rose red colour.
Pure milk will give a brown colour.
Alkalinity of Ash Test
Alkalinity and the ash content of the milk increases when neutralized using caustic soda (NaHCO3).
Ash 20 ml of milk and then dissolve the ash in 10 ml of distilled water. Titrate the resultant solution with 0.1 N HCl.
A titre value of more than 1.2 ml indicates the presence of neutralizer in milk.
Detection of Hypochlorites in Milk
Hypoclorites result from the reagents used for cleaning the milk handling equipment.
Take 5 ml of milk sample in a test tube and add 1.5 ml if 7% potassium iodide (KI). Mix the two and observe for the formation of a brown colour.
In case the brown colour is not formed, add 4 ml of HCl and note the colour of the curd. Heat the curd in a waterbath at 85°C for 10 minutes.
The curd will rise to the surface of the test tube; both the curd and the liquid will have a deep yellowish brown colour. Add 0.5 to 1.0 ml of starch solution to the liquid below the curd.
Formation of a blue colour will indicate the presence of hypochlorites in milk.
How to Detect Penicillin/Antibiotic in Milk
Most antibiotics are tested using dye marking or a microbial assay. The aim of the test is to check for growth inhibition.
Incubate the sample with the microorganism, especially Streptococcus thermophillus, then add 1% Triphenyltetrazolium chloride (TTC). Check for the growth of the microorganism that has been inoculated.
Inhibition is indicated by the failure of the TTC to change from colourless to red colour. The concentration of penicillin is inversely proportional to the intensity of the red colour.
Testing for Pesticides in Milk
First extract the oil/butterfat using an organic solvent and then purify the oil and conduct HPLC analysis to determine the pesticides present.
How to Detect Mycotoxins in Milk
The most notorious of the mycotoxin is the afflatoxin. Perform chromatography and mass spectroscopy to identify and quantify the present mycotoxin in milk.