Starter Culture Growth Inhibitors

[Kev]

Dairy starter cultures are applicable as single strains, in pairs, or as a mixture. Whatever the case may be, it is important to consider the starter culture growth inhibitors that may impede the activity of the starter culture bacteria.

Mesophilic lactic starter cultures (whose optimum temperatures range between 20-30ºC) are widely used to make many fermented dairy products. In the cheese industry, they are found in three categories (as single, multiple or mixed strains).

Thermophilic LABs (whose optimum temperatures range between 37-45ºC) are used to manufacture yoghurt, acidophilus milk, and high temperature scalded cheese (e.g. Swiss varieties). These bacteria include Streptococcus thermophilus and all Lactobacillus spp.

Single Strain and Multiple Strain Cultures​

In theory, a single strain starter should consist of only one type of organism but this is very rare in practice. However, you can pair up single strain cultures to safeguard against bacteriophage attack, intolerance of salt or cooking temperature, and to vary in the quality of the end product.

Multiple strain starter cultures consist of known numbers of single strains developed for lengthy use during the cheese-making season. These mixed strain starters consist of a combination of Streptococcus lactis, Streptococcus cremoris, and other gas and aroma producing mesophilic LABs.

The aroma producing lactic starters are essential for the production of buttermilk, sour cream, cultured butter, and some fermented milk products.

The starter culture growth inhibitors​

There are many factors that can cause inhibition or reduction of the activity of a starter culture. The resultant effect would be poor quality fermented dairy products reaching the consumer and financial loss to the producer. These factors include:

1. Antibiotics​

Residues of antibiotics in milk result from mastitis therapy in dairy cows. Some unscrupulous milk traders intentionally add penicillin and other antibiotics in milk to preserve its quality. Starter cultures are susceptible to very low concentrations of the antibiotic residue.

2. Bacteriophages​

Some viruses (also known as phages) can attack bacteria and destroy starter cultures. The result is a failure to produce lactic acid after inoculation. The lactic streptococci and lactobacilli are the most vulnerable microorganisms in the dairy starter cultures.

You can reduce the effect of the phages in the dairy industry by:​

• Propagating starter cultures in very aseptic conditions i.e. adopt aseptic technique in handling dairy products and processes
• Proper heat treatment (temperature/time combination) of bulk starter milk to destroy the viruses in milk
• Daily rotation of phage-resistant strains
• Effective filtration of air in the starter room
• Proper sanitation of the equipment and premise
• Location of starter room far away from production area
• Personnel, especially those from cheese room should NOT enter the starter room
• Propagate starter culture in phage inhibitory medium
• Develop phage-resistant strains
• Use mixed strain starter cultures.

3. Detergent and disinfectant residues​

Detergents and disinfectants for cleaning and sanitization in the dairy plant may cause contamination. The residues of these compounds (alkaline detergents, chlorine-based materials, iodophors, quaternary ammonium compounds and ampholytes) do affect the activity of the starter culture.

Yoghurt cultures are more tolerant to the activities of these residues at the inhibitory levels (mg/l) of culture compounds. Contamination of starter milk with these compounds is majorly due to human error, or malfunction of the automatic chain cycle.

4. Miscellaneous starter culture growth inhibitors​

Natural antibodies (such as lactelins/agglutinins) that are present in milk can inhibit the growth of the starter cultures. These antibodies are heat sensitive, and heat treatment of bulk starter milk ensures their destruction. Leucocytes in mastitis milk can cause phagocytosis of the starter microorganisms. Thiocyanates present in late lactation milk may also inhibit the growth of starters. Heating of the starter results in no significant improvement of the end product

You can attribute other inhibitors to environmental pollution factors, such as insecticides, volatile and non-volatile compounds. Such volatile compounds may include fatty acids, formic acid, formaldehyde, acetonitrile, chloroform, and ether. When their concentrations reach 100ppm, they will inhibit growth of Streptococcus spp. and Lactobacillus cremoris.

Growth Inhibitors: The Effects of Adulterants on Milk Curdling​

Adulteration of milk, as we have discussed, takes place through many ways, some of which can be intentional while others are non-intended. Growth inhibitors are very critical when it comes to milk contamination since they will impede the growth of culture bacteria needed 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.

An experiment to investigate the effect of inhibitory substances on milk curdling was done and the findings have been shared below.

The experiment​

• 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​

Samples	A	B	C	D	E
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.

The conclusion​

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.