Key Factors Influencing Starter Culture Fermentation

The production of yogurt, cheese, buttermilk, and other fermented dairy products depends almost entirely on the performance of starter cultures – carefully selected strains of lactic acid bacteria (LAB) that convert lactose into lactic acid. But getting these bacteria to do their job well isn’t automatic. A range of environmental and biological factors can speed up, slow down, or completely derail the fermentation process. Understanding and controlling these factors is essential for producing consistent, high-quality fermented dairy products.

Temperature: the primary driver of bacterial growth 🔗

Temperature is one of the most influential factors in starter culture fermentation. Every strain of LAB has a specific temperature range where it grows and produces acid most efficiently. Mesophilic bacteria, such as Lactococcus lactis subsp. lactis and Lactococcus lactis subsp. cremoris, perform best at 20-30°C. Thermophilic species, including Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus, prefer higher temperatures in the range of 37-45°C. Leuconostoc species, which contribute aroma and flavour to products like butter and cheese, grow optimally between 20-30°C.

When fermentation temperature drifts outside the ideal range for the culture being used, the consequences are significant. Too low a temperature leads to sluggish acid production and extended fermentation time. Too high a temperature can injure or kill sensitive strains. In mixed-strain cultures, even small temperature shifts can alter which strain dominates, changing the flavour, texture, and acidity of the final product. This is why precise temperature control during incubation is non-negotiable in dairy processing.

pH: keeping acidity in check 🔗

As starter cultures grow, they produce lactic acid, which progressively lowers the pH of the milk. While a certain level of acidity is the goal of fermentation, excessive acid accumulation can become harmful to the very bacteria producing it. When pH drops below approximately 5.0, significant cell injury can occur, reducing the viability and performance of the starter organisms.

In commercial bulk starter production, pH management is critical. Externally pH-controlled media use insoluble buffers to neutralise lactic acid as it is produced, keeping conditions favourable for continued bacterial growth. Internally buffered systems work on a similar principle. Some advanced approaches even involve the use of pH-sensitive genetic promoters that regulate acid production genes within the bacteria themselves. Without effective pH control, cultures become over-ripened, leading to weak starters and inconsistent product quality.

Strain compatibility in mixed cultures 🔗

Many fermented dairy products rely on mixed-strain or multiple-strain starters, where two or more species of LAB work together. The success of these cultures depends on how well the component strains cooperate. In a well-designed culture, the strains function in symbiosis rather than competition, with complementary characteristics like acid production rates, flavour compound generation, and temperature tolerance.

However, maintaining this balance is challenging. Repeated subculturing of mixed-strain starters can gradually shift the population, with one strain eventually outcompeting and replacing the others. Differences in growth rates, acid sensitivity, and the production of bacteriocins (antimicrobial peptides produced by some LAB strains) all contribute to this strain dominance problem. This is one reason why many modern dairy operations now prefer defined-strain starters – commercially prepared cultures with precisely known composition – over traditional mixed cultures that must be maintained through repeated transfers.

Growth medium: what the bacteria feed on 🔗

The composition of the growth medium directly affects how well starter cultures perform. Milk is the natural medium for dairy LAB, but not all milk is equally suitable. Seasonal variations in milk composition can significantly affect fermentation. Milk from late lactation or winter months tends to be lower in growth-stimulating nutrients, requiring the addition of supplements to achieve normal acid production rates.

For commercial starter production, reconstituted skim milk with 10-12% solids-not-fat is a commonly used medium. The addition of growth factors such as manganese (Mn²⁺) can promote the growth of specific species like Leuconostoc mesenteroides subsp. cremoris. Yeast extract, added at about 0.5%, is another useful supplement that can eliminate extended lag phases in certain cultures. Some facilities use casein hydrolysates or milk protein concentrates to improve the growth of probiotic strains that are otherwise slow growers in plain milk.

Inhibitory substances: antibiotics, sanitisers, and natural inhibitors 🔗

The presence of inhibitory substances in milk is one of the most common causes of slow or failed fermentation. Antibiotics like penicillin and streptomycin can enter the milk supply when they are used to treat mastitis or other udder infections in dairy cattle. Even trace residues can severely inhibit the growth of sensitive LAB strains. Milk must be rigorously tested for antibiotic residues before starter cultures are added – methods like the Charm test and immunological assays are widely used for this purpose.

Sanitiser residues from improperly rinsed equipment pose a similar threat. Chlorine-based sanitisers, iodophors, and quaternary ammonium compounds can all suppress LAB activity if they carry over into the milk or culture medium. Strict adherence to cleaning-in-place (CIP) protocols and thorough rinsing is essential.

Milk also contains natural antimicrobial systems. The lactoperoxidase-thiocyanate-hydrogen peroxide system, naturally present in raw milk, can inhibit LAB growth, especially in fresh milk that has not been heat-treated. Some wild strains of Lactococcus lactis found in raw milk can produce bacteriocins that inhibit the starter cultures added during processing.

How to minimise the impact of inhibitors 🔗

Preventing inhibitor-related fermentation problems requires a multi-pronged approach. Dairy farms must observe proper withdrawal periods after antibiotic treatment before milk is shipped. At the processing plant, incoming milk should be screened using rapid antibiotic detection kits. Equipment must be cleaned following validated CIP procedures with adequate final rinses. Heat treatment of the milk (discussed below) also helps inactivate some natural inhibitory systems.

Bacteriophages: the biggest biological threat 🔗

Bacteriophages – viruses that specifically infect and destroy bacteria – are widely considered the single most significant biological threat to dairy fermentation. Despite decades of effort, phage infection of starter LAB cultures remains the most common cause of slow or incomplete fermentation in the dairy industry.

The mechanism is straightforward but devastating. A phage particle attaches to a susceptible bacterial cell, injects its DNA, hijacks the cell’s machinery to produce hundreds of new phage particles, and then lyses (bursts) the cell, releasing those new phages to infect neighbouring cells. In a fermentation vat containing billions of LAB cells, a small initial phage population can multiply explosively, causing acid production to slow dramatically or stop entirely. The consequences range from reduced product quality to complete batch loss.

Where do phages come from? 🔗

Phages can enter the dairy environment through raw milk, airborne contamination, recycled whey products, and improperly cleaned equipment. They are remarkably resilient – some dairy phages can survive standard pasteurisation conditions, meaning they pass through heat treatment and enter fermentation vats via contaminated milk. Whey by-products that are recycled back into processing can also concentrate phage populations if not properly treated.

Strategies for phage control 🔗

The dairy industry employs multiple strategies to manage phage risk. Culture rotation – regularly switching between different starter strains – prevents any single phage population from building up to dangerous levels. The use of phage-resistant starter strains, developed through natural selection or genetic techniques like CRISPR-Cas systems, has become increasingly common. Phage-inhibitory media (PIM), which contain chelating agents that bind calcium ions required for phage attachment, provide another layer of protection during bulk starter preparation. Factory-level measures include positive air pressure systems, strict sanitation protocols, and segregation of raw milk handling areas from fermentation zones.

Incubation period: timing matters 🔗

The duration of incubation directly affects the growth phase and metabolic activity of starter cultures. Most LAB cultures reach their maximum cell density within 16-24 hours when incubated at their optimal temperatures. The higher the temperature (within the permissible range), the faster the culture passes through its growth phases.

Under-incubation produces a weak starter with insufficient cell numbers and low acid-producing capacity. Over-incubation, on the other hand, allows excessive acid to accumulate, which can injure cells and reduce their viability. Cultures that have been over-ripened and then stored for extended periods show noticeably reduced activity. For this reason, timing the incubation precisely and cooling the culture promptly once the target acidity is reached are both critical steps in starter preparation.

Heat treatment of milk 🔗

Heat-treating milk before using it as a starter culture medium generally improves its suitability for LAB growth. Pasteurisation (typically 72°C for 15 seconds or equivalent) eliminates competing microorganisms and ensures the milk provides a favourable environment for the starter culture. Heat treatment also partially denatures whey proteins, which can actually improve the growth of some LAB species by making amino acids and peptides more accessible.

However, the intensity of heat treatment must be carefully calibrated. Excessive heating – such as prolonged autoclaving at 121°C – can denature proteins to the point where they become less available as nutrients, and can produce off-flavours (scorched taste) in the culture. Standard practice for starter culture propagation involves heating reconstituted skim milk at 90°C for about 30-60 minutes, or steaming for 30 minutes, which balances microbial safety with nutrient preservation.

Degree of aeration: managing oxygen levels 🔗

Lactic acid bacteria are generally classified as microaerophilic or facultatively anaerobic organisms – they can tolerate some oxygen but do not require it for growth. In fact, excessive aeration during fermentation can be problematic.

Research has shown that high dissolved oxygen levels negatively affect both the acidification rate and the reducing capacity of LAB in milk. When oxygen levels are high, the bacteria must first consume the dissolved oxygen before efficient lactic acid fermentation can begin, leading to extended lag phases. Some LAB strains produce hydrogen peroxide in the presence of oxygen, which can further inhibit their own growth and that of neighbouring cells.

On the other hand, a minimum concentration of carbon dioxide (approximately 0.2-2.3%) is actually necessary for bacterial growth to initiate. Complete removal of CO₂ from the medium can prolong the lag phase until the bacteria produce enough CO₂ on their own to support normal development. Managing aeration means avoiding excessive agitation or air incorporation during fermentation, while ensuring that trace levels of CO₂ are present in the medium.

Storage conditions: preserving starter viability 🔗

How starter cultures are stored before and after propagation significantly affects their performance. The rate of cooling after incubation, the level of acidity at the end of incubation, and the temperature and duration of storage all influence starter culture activity.

Once a starter culture reaches the desired level of acidity, it must be cooled promptly – typically to 4-5°C – to halt further metabolic activity and prevent over-acidification. Storing ripened starters at refrigeration temperatures for up to about 18 hours does not significantly affect their activity. However, over-ripened cultures that are stored for prolonged periods show marked decline in performance.

For commercial cultures, the two main preservation formats are deep-frozen concentrates (stored at -45°C or below) and freeze-dried (lyophilised) powders. Both formats maintain high cell viability over extended storage periods when handled correctly. The frozen format is particularly popular for Direct Vat Inoculation (DVI) systems, where the concentrated culture is added directly to the product milk without intermediate propagation steps. Maintaining the cold chain from the culture supplier through to the dairy is essential – any break in temperature control can reduce cell viability and fermentation performance.

Bringing it all together 🔗

Successful dairy fermentation is not about controlling just one or two variables – it requires managing an interconnected web of factors simultaneously. Temperature must be matched to the specific culture being used. pH must be monitored and controlled throughout propagation. The milk medium must be free from antibiotics, sanitiser residues, and excessive oxygen. Phage contamination must be prevented through rigorous factory hygiene and smart culture rotation strategies. And storage conditions must preserve the viability and activity of the culture right up to the point of inoculation.

When all of these factors are properly controlled, the result is a vigorous, consistent fermentation that produces dairy products with the desired flavour, texture, acidity, and safety. When even one factor is neglected, the consequences can range from minor quality defects to complete batch failure – and significant financial losses.

What do you think? Which of these fermentation factors do you believe poses the greatest challenge for small-scale or artisanal dairy producers who may lack the sophisticated monitoring equipment of large commercial plants? And how might traditional fermentation practices in different regions have evolved to manage some of these challenges without modern technology?

References

1. https://dairyprocessinghandbook.tetrapak.com/chapter/cultures-and-starter-manufacture
2. https://biologynotesonline.com/factors-affecting-fermentation-characteristics-of-starter-cultures/
3. https://www.journalofdairyscience.org/article/S0022-0302(02)74330-0/fulltext
4. https://pmc.ncbi.nlm.nih.gov/articles/PMC3530524/
5. https://qualitru.com/impact-of-bacteriophages-in-dairy-processing-part-1/
6. https://pmc.ncbi.nlm.nih.gov/articles/PMC3426687/
7. https://www.intechopen.com/chapters/42330
8. https://www.sciencedirect.com/topics/food-science/cultured-milk-starters
9. https://www.researchgate.net/publication/228661559_Effect_of_process_parameters_on_the_production_of_lactic_acid_bacteria_in_batch_fermentation
10. https://www.sciencedirect.com/science/article/pii/S0022030215000272