Adulteration of milk, as we have discussed, takes place through many ways. Some of these ways can be intentional while others are unprecedented and non-intended. Growth inhibitors are very critical when it comes to milk contamination since they will impede the growth of fermentative bacteria necessary for fermentation.
Common adulterants include water added to increase volume (baptizing the milk), preservatives added to improve the keeping quality of the milk (such as hydrogen peroxide, antibiotics, and sodium hydroxide).
Sometimes, detergents may accidentally find their way into the milk and being bacteriostatic; they will inhibit bacterial activity in the milk and increase the keeping quality of the milk.
Antibiotics are used to treat mastitis and other common bacterial infections in lactating cows and the residue may find its way into the milk. They have a longer lasting effect on the milk.
Antibiotics will kill the lactic acid bacteria (LABs) in milk; therefore, milk fails to curdle when you inoculate it with a starter culture.
I carried out an experiment to find out the effect of inhibitory substances on milk curdling. The process and the findings is described here below.
For this experiment, we partitioned five liters of heat-treated milk into five beakers and subjected to acidity and pH tests. We recorded the results we obtained from the tests.
After partitioning of the samples, we added adulterants including sodium hydroxide, antibiotic, detergent, and water into the beakers containing the milk samples.
We labelled the samples as A, B, C, D, and E in the order of the adulterants we added. We did not adulterate sample B with any substance; it was the control sample for the experiment.
Immediately after adding the adulterants, we determined the acidity and recorded the observations.
We then incubated the beakers containing the samples at 37ºC and measured the pH of the samples after every 15 minutes. We recorded all the observations.
The results we obtained from the experiment to determine the effects of growth inhibitors in milk
|Titratable Acidity (%)||0.18||0.19||0.2||0.17||0.17|
|pH (on addition)||6.4||6.37||6.33||3.21||6.51|
|pH (after 15 minutes)||6.27||6.22||6.12||6.06||6.34|
|pH (after 30 minutes)||6.43||6.35||6.35||6.17||6.76|
|pH (after 45 minutes)||6.46||6.37||6.33||6.29||6.53|
Discussion of the results
The acidity of the samples ranged fairly the same, indicating developing lactic acid in the sample. Addition of the adulterants seemed to have an effect on the acid development of the samples as the acidity of the adulterated sample seemed to stagnate showing no developed acidity.
The inhibitors affected the bacterial activity in the samples, impeding production of lactic acid in the milk samples during incubation. Some samples (B and E) did not show the downward trend in acid development observed in other samples.
The samples were left overnight and observed the following day. Only the control sample and samples B and E coagulated. Sample B formed a firm curd while E had curd with separated whey.
What we can conclude from the experiment on the effects of growth inhibitors on milk curdling
Sample B formed a firm curd because we did not add any adulterants into it. Its curd did not show any signs of syneresis.
We adulterated sample E with water, which explains whey separation.
All the other samples contained inhibitory substances that inhibited bacterial activity in the milk and prevented acid development in the samples. As a result, we did not observe any curd formation in these samples on the following day.