Formulation: Stable Product Quality From Bioreactor to Application (Part 1)

January 11,2024

The surging relevance of sustainable food production inevitably leads to an increasing commercial relevance of microbial products. Farmers all over the world are demanding novel active ingredients that will effectively guard their crops against pesky organisms and replenish soil fertility, all without causing ecological damage or health hazards. And microbes are simply the best candidates for this job, offering solutions as diverse as they are.

However, novel products come with novel challenges. Chemical pesticides and mineral fertilizers work very well since they are very stable. However, this stability is also the reason why they tend to accumulate in the environment, causing a myriad of undesired effects. In contrast, microbes are quite sensitive to the environment. Microbial products need to survive the long road from the production facility to the field while maintaining their effectiveness, stability and purity. Solving this engineering challenge is paramount to establishing microbes as reliable tools for farmers, and subsequently for the wide adoption of microbial ag inputs.

Formulating microbes – biology meets technology

Technological advancements in biotechnology and fermentation engineering paved the road for microbes to the agricultural industry by enabling their large-scale production. However, the primary product of the fermentation process is raw microbial biomass that is not fit to survive harsh conditions outside the cozy bioreactors and production facilities. Not to mention being applied with current agricultural application technologies. So, before the microbes start their journey to the end user, they need to be adequately formulated.

To put it in formal terms, formulation refers to the process of stabilizing the active ingredients (microbes in this case) while enabling compatibility with the various intended application methods. In the context of microbial products, formulation directly impacts their viability, efficacy, and practical applicability.

Biological considerations

Most microbial products are either based on fungi or bacteria (or their consortia). Although similar in some aspects, the biology of these organisms is very different, even between members of the same family. There are millions of bacterial and fungal species, and each comes with its unique combination of traits. From the perspective of formulation, there are three crucial biological aspects of microbes that influence development:

  1. Cell wall structure & spore formation
    Depending on their cell wall structure, all bacteria are either classified as Gram-positive (G+) or Gram-negative (G-). The main structural component of both G+ and G- bacterial cell walls is peptidoglycan, but its thickness is the main difference between these two bacterial types. Compared to G+ bacteria, G- bacteria have much thinner cell walls that make them more sensitive to environmental changes and thus more challenging to formulate. Instead of peptidoglycan, fungi have chitin as the main component of their cell walls. In terms of stability, they are more similar to G+ bacteria, having more tolerance to environmental changes.

    Spore formation is one of the central steps in the life cycle of fungi, while bacteria only use spores as a way to go dormant and survive unfavorable conditions. This aspect holds the most significant difference between fungal and bacterial formulation approaches.
  2. Nutritional requirements
    Both fungi and bacteria require sources of carbon, nitrogen, phosphorus and various trace elements for proper growth and development, but the optimal amount and ratio of these nutrients can highly differ. Additionally, fungi have a more diverse set of nutritional needs compared to bacteria.
    Small or big, nutritional needs have to be met to ensure the efficacy and viability of microbes over time. Depending on the targeted microbes, formulations may include specific substrates or compounds, tailored to the needs of specific bacteria or fungi.
  3. Microbe size
    Size matters in the formulation of microbial cells, as it directly affects stability, application, and overall product performance. Smaller cells, like bacterial cells, display better suspension stability (preventing settling or aggregation in the formulation), lower viscosity, and create homogenous solutions, which makes them easier to encapsulate and formulate into liquid forms. Larger cells, like those of fungi, tend to aggregate, have higher viscosity, and are more robust but more sensitive to formulation ingredients. As a result, the encapsulation of fungal cells is very challenging, as well as formulating them into liquid products for spray application.

Technical considerations

Once we know all the ins and outs of the targeted species’ biology and behavior, we are ready to go into the technicalities – how to prepare the microbe for the application process and ensure the best effect in the field? We can answer this question by looking at the key considerations in application, active water stabilization, and interrelation with fermentation.

Application

The primary considerations in the application of microbial products include target product concentration, shelf life and application technology constraints. Let’s get a bit technical and explain these concepts.

  1. Target product concentration – Formulation engineers must find the optimal balance to achieve a concentration of fungal or bacterial cells that enhances microbial effectiveness without compromising the stability of the product. Products with low concentrations of microbial cells are not likely to be effective in the field, while overly concentrated products tend to be unstable.
  2. Shelf life – Microbes age, as all living things tend to do. Sitting on the shelf at room temperature, microbial products lose viability over time. To ensure product efficiency even after months of storage, formulation engineers need to process the microbes in a particular way or/and add longevity-promoting agents. However, even with the help of these tools, microbial aging cannot be fully stopped (for now), so formulation engineers also need to determine the acceptable level of loss in microbial viability or activity over time to ensure the product remains effective until its expiration date.
  3. Application technology constraints – Formulating microbial products must take into account compatibility with other ag input ingredients and the desired application technologies. The three basic ways of applying any ag input product include spray (foliar), in-furrow (soil), and seed applications, and they require different formulation approaches. For example, spray application requires a liquid product that can stick to leaves and other aerial parts of the plant, while also withstanding the high exposure to environmental changes (temperature, moisture, UV light, etc.). In contrast, microbial products applied in-furrow are not limited to liquid formulations and are not as exposed to the elements, but they need to be compatible with specific soil conditions, like pH value and the presence of certain chemicals.

Active water stabilization

Maintaining the appropriate water content within the formulation is paramount to ensuring the stability and effectiveness of the microbial product. Too much water and the product will “spoil” quickly; too little water and the microorganisms will die. There are various strategies to stabilize active water content in formulations, including the use of protective agents, stabilizers, and encapsulation techniques. The choice of the right strategy ensures a longer shelf life and enhances the product’s resilience during storage and transportation.

Interrelation with fermentation

The interrelation between formulation development and fermentation is significant. The conditions under which microbes are cultured during fermentation directly impact their characteristics and performance in the final product. The fermentation process can trigger various metabolic processes, and, as such, the production of secondary metabolites that affect the efficacy and stability of the formulation. Also, the choice of the fermentation substrate will dictate the level of residue in the end product – the more residue the product contains, the higher the risk of contamination.

If you’d like to learn more, stay tuned for Part 2, where we’ll explore specific formulation approaches, types of microbial formulations, application technologies, and other important considerations.

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