Effective Maintenance and Preservation of Starter Cultures

Every batch of yogurt, cheese, or buttermilk you enjoy owes its consistent taste and texture to one critical factor – the starter culture used during fermentation. These carefully selected strains of lactic acid bacteria are the engine behind dairy fermentation, converting lactose into lactic acid and giving each product its signature flavour, aroma, and body. But here’s the challenge: these living microorganisms are delicate. Without proper maintenance and preservation, they lose viability, become contaminated, or simply stop performing. That’s why dairy technologists invest significant effort in keeping starter cultures alive, pure, and active – ready to deliver reliable results every single time.

Why maintenance and preservation of starters matter 🔗

Starter cultures are living organisms with specific environmental needs. They require the right nutrients, temperature, and pH to function properly. Left unmanaged, bacteria multiply rapidly, exhaust their nutrients, produce excess waste products, and eventually die. The entire quality of a fermented dairy product – its acidity, flavour, texture, and safety – depends directly on the purity and activity of the starter culture used during manufacturing.

Contamination is an equally serious concern. Even trace amounts of unwanted bacteria, yeasts, or molds can compromise entire production batches, leading to off-flavours, poor texture, or even unsafe products. This makes rigorous preservation protocols and quality control measures non-negotiable in any dairy operation.

The basic principle behind preservation 🔗

At its core, preserving a starter culture means slowing down or completely halting the metabolic activity of bacteria – without killing them. The goal is to place these organisms into a dormant or “hibernation” state where they remain viable but inactive until they are needed for production. Two main factors are manipulated to achieve this: temperature and moisture content. By lowering temperature, reducing available water, or both, dairy scientists can extend the shelf life of starter cultures from days to months or even years.

Method 1: Refrigeration 🔗

Refrigeration is the simplest and most widely used approach for short-term starter preservation. Cultures are stored at temperatures between 2-8°C, which dramatically slows bacterial metabolism and extends viability from a few days to several weeks.

This method is popular because it requires no specialized equipment beyond a standard refrigeration unit. However, it has limitations. Even at low temperatures, some metabolic activity continues, and cultures gradually lose their potency over time. Maintaining a consistent cold chain can also be difficult during transportation or in regions with unreliable electricity. For these reasons, refrigeration is best suited for cultures that will be used within a short timeframe – typically for day-to-day or week-to-week production schedules.

Liquid culture maintenance under refrigeration 🔗

Working cultures in liquid form are commonly maintained through regular sub-culturing. The culture is transferred to fresh sterile skim milk (typically reconstituted to 10-12% total solids) at regular intervals – daily or weekly – to keep the organisms active. According to FAO guidelines on small-scale dairy farming, mother cultures should be inoculated at approximately 0.5% into treated milk on a regular schedule. Without timely sub-culturing, cultures weaken progressively and eventually become unusable.

Certain additives can also extend liquid culture shelf life under refrigeration. Research has shown that cultures preserved with sodium citrate or potassium phosphate buffer solutions at 4°C can remain active for up to 12 months. Similarly, adding calcium carbonate (CaCO₃) to the growth medium has been found to maintain yogurt culture activity for extended periods at cold storage temperatures.

Method 2: Concentration and separation 🔗

Before cultures can be preserved through freezing or drying, they usually need to be concentrated. The aim is to pack as many viable bacterial cells as possible into a small volume. This is achieved through high-cell-density fermentation followed by concentration techniques such as continuous centrifugation or cross-flow membrane filtration.

During concentration, a washing step may be included to remove spent growth medium. The resulting cell concentrates can reach densities as high as 10¹¹ to 10¹² cells per gram – an extraordinary level of concentration that makes it possible for a single small container to inoculate thousands of kilograms of milk. These super-concentrated preparations form the basis of Direct Vat Set (DVS) or Direct Vat Inoculation (DVI) cultures, which have increasingly replaced traditional bulk starter systems in modern dairy plants.

Advantages of DVS/DVI cultures 🔗

DVS cultures eliminate the labour-intensive process of preparing bulk starters at the factory level. Since the culture goes directly from the package into the fermentation vat, there is far less opportunity for contamination. This also reduces the threat of bacteriophage (phage) infection – a major concern in dairy fermentation where viruses that attack lactic acid bacteria can cause catastrophic production failures. Additionally, DVS cultures provide greater consistency and convenience, making them especially valuable for small and medium-sized dairy plants that may lack dedicated starter preparation facilities.

Method 3: Freezing 🔗

Freezing is a widely used preservation method that offers different levels of effectiveness depending on the temperature employed. There are broadly three temperature ranges used in practice:

Standard freezing (-18 to -20°C): This is the most accessible option. Liquid starters and mother cultures can be preserved for several months at these temperatures. However, bacterial viability declines gradually because ice crystal formation can damage cell membranes.

Deep freezing (-40 to -45°C): Cultures stored at this range retain viability for longer periods and are commonly used in commercial frozen culture production.

Ultra-low temperature freezing (-80°C or below): This provides the highest level of long-term preservation, keeping cultures viable for years. Some commercial frozen concentrates are even stored in liquid nitrogen at -196°C for maximum stability.

The role of cryoprotectants 🔗

The key challenge in freezing is preventing ice crystal damage to bacterial cells. When water inside and around cells freezes, the resulting ice crystals can rupture cell membranes, killing the organisms. To counter this, cryoprotective agents are added before freezing. Common cryoprotectants include glycerol, sucrose, sodium citrate, and sodium β-glycerophosphate. These substances help maintain cell membrane integrity by reducing the concentration of damaging electrolytes and preventing excessive ice crystal growth during the freezing and thawing process.

The rate of freezing also matters significantly. Rapid freezing generally produces smaller ice crystals that cause less cellular damage compared to slow freezing. Commercial frozen DVS cultures are typically available as pellets – small frozen droplets that can be thawed quickly and added directly to the fermentation vat.

Method 4: Freeze-drying (lyophilization) 🔗

Freeze-drying is widely regarded as the gold standard for long-term preservation of starter cultures. The process involves two stages: first, the culture is frozen, and then the ice is removed through sublimation under vacuum – meaning the ice converts directly from solid to vapour without passing through a liquid phase. This gentle removal of moisture preserves the cellular structure of bacteria far better than other drying methods.

The advantages of freeze-drying are significant. Properly freeze-dried cultures can remain viable for several years when stored appropriately (ideally at 4°C, though many strains tolerate room temperature storage for months). The dried product is lightweight, stable, and easy to transport – a major advantage for global supply chains. Research published in the International Dairy Journal has demonstrated that certain strains maintained full viability and metabolic activity after 14 months of storage at 4°C following freeze-drying.

The main drawback is cost. Freeze-drying requires expensive specialized equipment and consumes significant energy, making it more practical for high-value cultures, long-term culture banking, and commercial DVS production rather than routine day-to-day preservation.

Protective agents in freeze-drying 🔗

The survival of bacteria during freeze-drying depends heavily on the protective agents used. Common protectants include skim milk, sucrose, maltodextrin, and lactose. Studies have shown that maltodextrin performs well as a protective agent across different storage temperatures, maintaining high cell viability even after extended periods. The choice of protectant is often strain-specific – what works well for one bacterial species may not be optimal for another.

Method 5: Spray-drying 🔗

Spray-drying is a faster and more economical alternative to freeze-drying. In this process, the liquid culture is atomized into fine droplets and exposed to a stream of hot air, which rapidly evaporates the moisture. The result is a dry powder that can be stored and transported easily.

The primary advantage of spray-drying is its lower production cost and continuous processing capability. It is significantly more energy-efficient than freeze-drying and can handle large volumes quickly. However, the exposure to high temperatures during drying places considerable thermal stress on bacterial cells, which can substantially reduce survival rates.

Recent advances have improved spray-drying outcomes considerably. The use of optimized inlet and outlet temperatures, protective compounds (such as gum acacia and maltodextrin), and pre-adaptation strategies – where bacteria are exposed to mild stress before drying to build tolerance – have all helped increase the viability of spray-dried cultures. A review in Trends in Food Science & Technology notes that spray-drying has been widely studied as an alternative industrial preservation method, though achieving consistently high survival rates remains an ongoing area of research.

Choosing the right preservation method 🔗

No single preservation method is ideal for every situation. The choice depends on several practical factors:

Intended shelf life: Short-term needs (days to weeks) can be met with refrigeration. Medium-term storage (months) may call for freezing. Long-term preservation (years) requires freeze-drying or ultra-low temperature freezing.

Budget and infrastructure: Refrigeration and standard freezing are the most affordable options. Freeze-drying demands significant capital investment. Spray-drying falls somewhere in between.

Transportation and distribution: Freeze-dried cultures are the easiest to ship since they don’t require cold chain logistics. Frozen cultures need unbroken cold chain maintenance throughout distribution.

Strain characteristics: Different bacterial strains respond differently to preservation stresses. Some tolerate freeze-drying well but perform poorly after spray-drying, and vice versa. Resistance to preservation processes is strain-dependent, meaning each culture needs to be evaluated individually.

Many large-scale dairy operations use a multi-tiered approach – refrigeration for immediate production, freeze-drying for medium-term storage and distribution, and ultra-low temperature freezing for long-term culture banking. This layered strategy ensures production continuity even if one preservation system encounters problems.

Maintaining purity and activity: quality control essentials 🔗

Preservation is only half the equation. Equally important is ensuring that preserved cultures remain pure and functionally active. A robust quality control program for starter cultures typically includes the following:

Viability testing: Regular plate counts to determine the number of living bacteria in preserved cultures. A culture that has dropped below acceptable cell counts will not perform adequately during fermentation.

Activity testing: Measuring how quickly a culture acidifies milk after reactivation. A culture may have acceptable cell counts but still show sluggish acid production – a sign of metabolic impairment.

Purity testing: Checking for the presence of contaminants such as unwanted bacteria, yeasts, or molds. Even low-level contamination can multiply during fermentation and ruin the final product.

Phage monitoring: Bacteriophages are one of the most serious threats to dairy fermentation. Regular whey testing and prospective phage monitoring help detect infections before they cause production failures.

Aseptic handling practices 🔗

Throughout every stage of maintenance and preservation, strict aseptic technique is essential. This includes sterilizing all equipment that comes in contact with the culture, using sterile packaging materials, and working in clean environments – ideally under positive air pressure with filtered air. As the FAO’s dairy farming manual emphasizes, any dirt or chemical residue in utensils can alter the behaviour of the starter culture, leading to unpredictable results.

Antibiotic residues in milk are another serious concern. Even trace amounts of antibiotics can inhibit or kill starter bacteria, causing slow or failed fermentations. All milk used for culture propagation must be rigorously tested and confirmed free of antibiotic contamination.

Modern trends in starter culture preservation 🔗

The field continues to evolve. Encapsulation technology – where bacterial cells are coated in protective matrices before drying – is gaining popularity as a way to improve survival rates during both spray-drying and freeze-drying. Materials such as alginate, chitosan, and various food-grade polymers are being explored as encapsulation agents.

There is also growing interest in developing cultures that maintain their functionality at higher storage temperatures, reducing the energy costs associated with cold storage. Research into stress pre-adaptation – exposing cultures to sub-lethal heat, osmotic, or oxidative stress before preservation to “toughen” them – has shown promise in improving post-drying survival and storage stability.

Automated inoculation systems, such as the AISY system developed through collaboration between culture companies and equipment manufacturers, are also streamlining how concentrated cultures are handled at the plant level, further reducing contamination risks and improving process efficiency.

What do you think? Given the trade-offs between cost, shelf life, and bacterial survival, which preservation method do you think offers the best balance for a dairy operation in a developing region with limited cold chain infrastructure? And as encapsulation and stress-adaptation technologies advance, could spray-drying eventually replace freeze-drying as the industry standard?

References

1. https://dairyprocessinghandbook.tetrapak.com/chapter/cultures-and-starter-manufacture
2. https://juniperpublishers.com/nfsij/NFSIJ.MS.ID.555756.php
3. https://www.fao.org/4/t1265e/t1265e12.htm
4. https://www.sciencedirect.com/topics/food-science/cultured-milk-starters
5. https://www.dairyscience.info/cheese-starters/108-starter-concentrates.html
6. https://pubmed.ncbi.nlm.nih.gov/17305363/
7. https://www.sciencedirect.com/science/article/abs/pii/S0958694623001826
8. https://www.tandfonline.com/doi/full/10.1080/09168451.2020.1770572
9. https://www.sciencedirect.com/science/article/abs/pii/S0924224411000100