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.
Common Sources of Contamination in Milk
Intoxicating substances in milk present a very big challenge when dealing with food poisoning incidences. Given that milk is a complete food with a stable proportion of both macronutrients and micronutrients, it is an ideal vehicle for the pathogens that produce toxins.
Food toxicology is the study of these toxins and their effects.
The toxic compounds can originate from:
- Natural sources as metabolites of plants and animal metabolism
- Biological or chemical contaminants from the air/water/soil
- Intentionally introduced food additives
- Toxicants from the food processing e.g. acrylamines
Sources of food poisoning in milk:
i) Microorganisms and their toxins
Milk and milk products harbor microorganisms that metabolize the milk for their normal metabolic needs producing metabolites in the process.
These metabolites/byproducts can produce both desirable and undesirable effects in the product. Desirable effects include flavouring and texture while undesirable effects may include food poisoning.
Sources of microorganism contamination include the following:
Dairy animal: - health of the animal is an important consideration because the animal can transmit the diseases to humans through the milk e.g. brucellosis. They can get into the milk directly from the udder or indirectly through body discharges.
Human handlers: - sick people can transmit diseases like TB and brucellosis to other humans through the milk.
Environment: - the general cleanliness of the cowshed and the milking parlour has a greater bearing on the quality of the milk produced from the farm.
Antibiotics: - the residues of the antibiotics that remain after treating the cow for mastitis or any other bacterial infection. These residues will find their way into the milk and cause contamination or even food poisoning.
ii) Toxic chemicals
These contaminants include the following:
Insecticides: - such as those used on the cowshed to control ectoparasites, which the animal may partially absorb through the skin and release into the milk later. Foods containing organophosphate pesticides can cause food poisoning by inhibiting the acetylcholinesterase enzyme.
Preservatives and disinfectants: - added to increase the storage time of the milk e.g. hydrogen peroxide, boric acid, and formalin.
Formalin and boric acid are highly toxic in small quantities. Some compounds such as the hypochlorites remain in the milk containers after cleaning.
iii) Radionuclides and heavy metals
Radionuclides including iodine 131 and Strontium 90 may be present in contaminated milk. Iodine 131 causes malfunction of the thyroid gland while Strontium 90 is lodged in the bones.
Heavy metals such as Zinc, Copper, and Iron gets into the milk through corroded metals.
Serious food poisoning by these metals is rare due to their effect on colour and taste; one will easily identify them due to their offensive smell and taste and therefore, choose not to drink the milk.
Relatively large amount of heavy metals is required to cause toxicity. However, these metals have a cumulative effect in the body and their effect will multiply over time.
iv) Plant toxins
This type of contamination is prevalent during dry season when animals eat a lot of plant matter. There is no significant evidence to show that toxic compounds are present in milk in quantities enough to cause effect on human consumers.
v) Flavor and drugs excreted in milk
Flavours act as extraneous substances that occur when cows consume certain plants that impart flavor in milk rendering the milk unwholesome.
A number of therapeutic drugs might be excreted in milk in minute quantities. This might affect heavy consumers of milk, especially children whose immunity is yet to be fully developed.
Do not use the milk for at least 96 hours after the last administration of drugs.
vi) Dirt and other abnormal bacterial load
The common adulteration methods include watering and skimming or addition of foreign fat.
To determine the amount of dirt present in a given batch of milk, sample half a liter of the milk, warm and pass through filter paper.
Compare the dirt on the filter paper with a sediment comparison chart. Warming makes the milk fat liquid to allow it to pass through the filter paper.
To test for foreign fat in any given milk sample, extract the fat using Rose Gotlieb method and then use a Butrorefractor at 40°C to take the readings.
The difference in reading from the standard value will show the presence of foreign fat. The readings of the Butrorefractor range from 1 – 100.
The normal refractive index of milk ranges between 1.422 and 1.895.
You can also employ the Gas-Liquid Chromatography (GLC) method to check the fatty acids present in the milk sample.