Types of Starter Cultures Used in Dairy Fermentation

Every fermented dairy product you enjoy – from creamy yogurt to tangy buttermilk to aged cheddar – owes its flavour and texture to tiny microorganisms called starter cultures. These carefully chosen strains of bacteria are added to milk to kick off and control the fermentation process. But not all starter cultures are the same. They differ in composition, complexity, and the kind of control they offer to dairy manufacturers. Understanding the types of starter cultures – single strain, mixed strain, and multiple strain – is essential for anyone studying or working in dairy technology.

What are starter cultures? 🔗

Starter cultures are preparations of specific microorganisms, mainly lactic acid bacteria (LAB), that are introduced into milk to initiate fermentation. Their primary job is to convert lactose (milk sugar) into lactic acid. This acidification does several things at once: it preserves the milk, develops characteristic flavours and aromas, improves texture, and can even enhance nutritional value.

Without starter cultures, the natural microflora in milk would produce unpredictable and inconsistent results. Heat treatments like pasteurisation destroy most native bacteria in milk, making it necessary to add well-defined starter cultures that can deliver reliable outcomes every time. Beyond lactic acid production, these cultures also contribute to flavour and aroma development, proteolytic and lipolytic activity, and the inhibition of spoilage and pathogenic organisms.

Classification of starter cultures based on composition 🔗

In dairy science, starter cultures are broadly classified into three types based on how many strains they contain and how those strains are combined. Each type serves different production needs, from small artisanal batches to large-scale industrial manufacturing.

Single strain starters 🔗

A single strain starter consists of a pure culture of just one type (strain) of lactic acid bacterium. This is the simplest form of starter culture. Because there is only one organism at work, the fermentation behaviour is highly predictable and easy to control.

Here are the key characteristics of single strain starters:

High consistency: Since only one organism is present, the rate of acid production, the flavour profile, and the texture of the final product remain uniform from batch to batch. This makes single strain starters ideal for situations where absolute repeatability is required.

Ease of management: With only one strain to monitor, quality control is straightforward. Dairy operators can quickly detect any deviation in the culture’s performance. This simplicity makes single strain starters well-suited for small-scale or artisanal dairy production.

Limited flavour complexity: The downside is that a single organism can only produce a limited range of metabolic by-products. This means the resulting product may lack the depth of flavour and complexity that comes from multiple organisms working together.

Vulnerability to bacteriophage attack: This is perhaps the biggest risk. Bacteriophages (phages) are viruses that specifically target and destroy bacteria. Since a single strain starter contains only one type of bacterium, a phage attack against that specific strain can completely halt fermentation – a situation dairy manufacturers call a “dead vat.”

A common example of a single strain starter is Lactococcus lactis subsp. lactis, used in the production of certain fresh cheeses. Another well-known single strain application is Lactobacillus delbrueckii subsp. bulgaricus, which can be used individually for specific acid development in fermented milks.

Mixed strain starters 🔗

A mixed strain starter (also called a compound or undefined starter) contains two or more strains or species of lactic acid bacteria. These strains may not all be individually identified or present in precisely known ratios – the culture exists as a complex microbial community.

Mixed strain starters have been used traditionally in many dairy-producing regions for centuries. The Dutch dairy industry, for instance, traditionally relies on complex mixed strain cultures for making butter, buttermilk, and Gouda cheese. These cultures typically contain a majority of Lactococcus lactis subsp. cremoris strains as acid producers, along with a smaller proportion of aroma bacteria such as Leuconostoc species and citrate-utilising Lactococcus strains.

Key features of mixed strain starters include:

Greater phage resistance: This is the most significant advantage. Because the culture contains multiple strains, a phage attack against one strain does not destroy the entire culture. Bacteriophages are highly strain-specific, so if one strain is affected, the remaining strains continue to ferment. This built-in redundancy provides a strong safety net against fermentation failure.

Richer flavour profiles: Different strains contribute different metabolic by-products – lactic acid, diacetyl (a buttery compound), carbon dioxide, acetaldehyde, and various volatile aroma compounds. This metabolic diversity produces more complex and desirable sensory characteristics in the finished product.

Less predictable behaviour: Since the exact composition and ratio of strains in a mixed culture can shift over time – especially with changes in temperature, incubation time, or growth medium – the product quality may show some batch-to-batch variation. Managing mixed strain starters requires careful attention to propagation conditions.

Self-regulating microbial balance: In well-established mixed cultures, the various strains tend to maintain a relatively stable ecological balance. Research shows that in traditional Dutch starters, the proportion of citrate-utilising aroma bacteria remains naturally constant at about 10-20% of the total culture.

Mixed strain starters are commonly used in the production of sour cream, cultured butter, buttermilk, Gouda cheese, Edam cheese, and certain white-brined cheeses.

Multiple strain starters 🔗

A multiple strain starter (also known as a defined multiple strain starter) consists of a known number of carefully selected, compatible strains. Unlike mixed strain starters where the exact composition may be undefined, every strain in a multiple strain starter is individually identified, tested, and chosen for specific characteristics.

This type represents the most advanced approach to starter culture technology and is the standard in modern industrial dairy production. According to ScienceDirect’s overview of starter cultures, the modern large-scale production of fermented foods demands consistent quality and predictable schedules – requirements that multiple strain starters are specifically designed to meet.

Here is what sets multiple strain starters apart:

Carefully selected compatibility: Each strain is tested to ensure it works well alongside the others. There should be no antagonistic interactions – instead, the strains should complement one another. For example, one strain may be a strong acid producer while another contributes primarily to flavour development.

Known phage relationships: Because each strain is individually characterised, their phage sensitivities are mapped in advance. The strains are selected so that they are non-phage-related – meaning a phage that attacks one strain will not attack the others. This provides systematic protection against fermentation disruptions.

Consistent product quality: Since the composition of the starter is precisely defined and reproducible, the dairy manufacturer gets uniform results across batches and production runs. This is critical for large brands that need to maintain identical product characteristics across multiple factories and over long periods.

Extended usage: Multiple strain starters are engineered for sustained use. By rotating specific strains in and out of the production schedule – a practice known as strain rotation – manufacturers can keep using the same fundamental starter system for extended periods without accumulating phage problems.

A typical example is the combination of several defined strains of Lactococcus lactis (including subsp. lactis and subsp. cremoris) used in Cheddar cheese production. In yogurt manufacturing, the defined pairing of Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus is another classic example of a multiple strain approach, where both organisms work in a well-documented symbiotic relationship.

The role of temperature in starter culture classification 🔗

Beyond the number of strains, starter cultures are also classified based on the temperature at which they grow best. This is important because different dairy products require fermentation at different temperatures.

Mesophilic cultures grow optimally between 20°C and 30°C. Common mesophilic species include Lactococcus lactis subsp. lactis, Lactococcus lactis subsp. cremoris, Lactococcus lactis subsp. lactis biovar diacetylactis, and Leuconostoc mesenteroides subsp. cremoris. These are widely used in the manufacture of cheese varieties like Cheddar, Gouda, and cottage cheese, as well as cultured butter and buttermilk.

Thermophilic cultures prefer temperatures in the range of 37°C to 45°C. Key thermophilic species include Streptococcus thermophilus, Lactobacillus delbrueckii subsp. bulgaricus, Lactobacillus helveticus, and Lactobacillus acidophilus. These cultures are essential for yogurt production, acidophilus milk, and high-temperature scalded cheeses such as Swiss cheese and Mozzarella.

Some dairy products even use a combination of mesophilic and thermophilic organisms to achieve specific fermentation profiles.

Why bacteriophage protection matters 🔗

Bacteriophage infection is one of the most serious threats to dairy fermentation. Phages are among the most persistent challenges in large-scale commercial dairy operations. These viruses specifically target lactic acid bacteria, and when they strike, they can slow or completely stop acid production – leading to failed batches and significant economic losses.

The dairy industry is considered the sector most affected by phage problems, given the enormous volumes of bacteria cultivated daily. Even though phage concentrations in raw milk are typically low, a population of vulnerable bacteria can allow phage numbers to explode rapidly during fermentation.

This is exactly why the choice between single strain, mixed strain, and multiple strain starters is so consequential. Single strain starters offer no backup if phage strikes. Mixed strain starters provide a natural buffer because their diverse composition means at least some strains will survive an attack. Multiple strain starters offer the most controlled approach – strains are pre-screened for phage resistance and can be rotated strategically.

Modern strategies to combat phage include proper sanitation, use of phage-inhibitory media, strain rotation programmes, and the development of phage-resistant bacterial mutants. Some manufacturers also use Direct Vat Set (DVS) cultures that bypass the bulk starter stage entirely, reducing the opportunity for phage contamination.

Commercial forms of starter cultures 🔗

Regardless of whether a starter is single, mixed, or multiple strain, it needs to be delivered to the dairy in a form that preserves viability and activity. Today, commercial starter cultures are available in three main forms:

Frozen concentrated cultures: These are stored at very low temperatures (typically -45°C or below) and come as deep-frozen pellets or granules. They can be added directly to the fermentation vat (DVS/DVI cultures) or used to prepare a bulk starter.

Freeze-dried (lyophilised) cultures: These are dried powders that can be stored at refrigeration temperatures and have a longer shelf life during transport. They are reconstituted before use.

Liquid cultures: These are the traditional form, prepared by growing the bacteria in sterilised skim milk. While still used in some operations, liquid cultures carry a higher risk of contamination and strain degradation compared to frozen or dried formats.

Major culture companies invest heavily in research to compose specific culture blends tailored for individual products – ensuring that dairies can access cultures with defined properties for texture, flavour, viscosity, and phage resistance.

Choosing the right starter type for your product 🔗

The decision of which starter type to use depends on several factors:

Scale of production: Small-scale or artisanal producers may find single strain starters sufficient for their needs, especially when producing simple products under tightly controlled conditions. Large-scale industrial operations almost always use multiple strain starters for their reliability and phage protection.

Product complexity: Products that require complex flavour profiles – such as Gouda, Swiss cheese, or cultured butter – benefit from mixed or multiple strain starters where different organisms contribute different metabolic characteristics.

Risk tolerance: If a manufacturer cannot afford fermentation failures (and few can), mixed or multiple strain starters are the safer choice due to their inherent phage resistance.

Regulatory and market requirements: Certain products have legally defined culture requirements. Yogurt, for example, must contain Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus in many countries to be labelled as yogurt.

Summing it up 🔗

Starter cultures are the foundation of dairy fermentation. Single strain starters offer simplicity and consistency but come with the risk of phage vulnerability and limited flavour complexity. Mixed strain starters bring microbial diversity and natural phage protection, though their behaviour can be harder to predict. Multiple strain starters combine the best of both worlds – defined composition, systematic phage management, and consistent industrial-scale performance.

As dairy technology continues to advance, the trend is clearly moving toward precisely defined, phage-resistant multiple strain systems that can deliver reliable quality day after day. But traditional mixed strain cultures still hold value, especially in artisanal and regional dairy traditions where their complexity contributes to the unique character of local products.

What do you think? If you were setting up a small dairy operation, would you prioritise the simplicity of a single strain starter or the safety of a mixed or multiple strain approach? And how do you think emerging technologies like genomics and CRISPR-based phage resistance will reshape starter culture design in the coming years?

References

1. https://books.lib.uoguelph.ca/dairyscienceandtechnologyebook/chapter/starter-cultures/
2. https://dairyprocessinghandbook.tetrapak.com/chapter/cultures-and-starter-manufacture
3. https://foodsafety.institute/food-biotechnology/importance-starter-cultures-food-fermentation/
4. https://www.sciencedirect.com/topics/food-science/multiple-strain-starter
5. https://www.sciencedirect.com/topics/food-science/cultured-milk-starters
6. https://www.sciencedirect.com/topics/food-science/starter-culture
7. http://dairy-technology.blogspot.com/2014/01/types-of-starters.html
8. https://journals.asm.org/doi/10.1128/aem.59.2.365-372.1993
9. https://www.annualreviews.org/content/journals/10.1146/annurev-food-060721-015928
10. https://microbialcellfactories.biomedcentral.com/articles/10.1186/1475-2859-10-S1-S20
11. https://pmc.ncbi.nlm.nih.gov/articles/PMC3530524/