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:
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.
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.