February 26,2023 –

The main purpose of formulation in the context of microbial products is to preserve and protect the sensitive live active ingredient from the unfavorable conditions during storage and application. One shoe doesn’t fit all – there is an abundance of microbial species and application methods in agriculture, so the formulation approach needs to be tailored according to the specifics of the microbe(s) and the targeted application method. The relevance of microbial properties in formulation development was extensively covered in the first part of the blog, while the primary focus of this segment are the technical considerations in the context of application.

Before the microbial product is ready to go to the field, it needs to go through two key formulation development phases – technical formulation and final formulation. Let’s dive into the intricacies of both phases to learn what it takes to achieve the stable product quality of microbes from bioreactor to application.

Technical Formulation

The goal of the technical formulation phase is to prepare the selected microbes and equip them with the right tools to remain stable and effective on their long road to the field. The two basic approaches to achieving this goal include liquid and solid formulations. Both approaches can yield outstanding products, but the how’s are a bit different. However, before we delve into the differences, let’s address the main similarity between liquid and solid formulations, and the main challenge in technical formulation development – active water stabilization.

Liquid formulations

Liquid formulations are very favored in agriculture due to their straightforward use and compatibility with different application equipment. They are either water or oil-based. Most liquid formulations are water-based, providing a conducive environment for microbial survival and activity. However, water (or oil) on its own cannot provide the desired stability and shelf-life, support the microbes in delivering the desired effects, or provide the physical qualities suitable for targeted application route. That’s why liquid formulations require specific additives, including stabilizing agents and carriers that support the viability, stability, and efficient application of beneficial microorganisms. These agents can be roughly generalized into two groups – sugars and salts.

Sugars

In liquid formulations, sugars act as osmoprotectants, helping microorganisms endure environmental stresses such as drought and temperature extremes. Furthermore, sugars like sucrose can serve as carbon sources for microbial metabolism, sustaining their vitality during storage and application. The amount and the type of added sugar strikes a balance between providing nutrients for microbial growth and enhancing the shelf life of the product.

Salts

The inclusion of salts in liquid formulations aims to modulate the osmotic environment, fostering microbial resilience. Salts bind water, and as such can help maintain water activity levels, preventing loss. This is especially crucial in preventing desiccation during storage and application. However, since microbes are sensitive to pH changes, it is essential to choose the right amount and the right type of salt for the formulation. The judicious use of salts contributes to the stability of liquid formulations, ensuring the viability of microbial populations.

Solid formulations

Solid formulations of microbial products involve creating stable, dry, and often granular or powdered formulations of beneficial microorganisms. These formulations are easily handled and stored, and are often characterized by slow-release action, and superior shelf life compared to liquid formulations.

Drying

Most solid formulations start out as liquid mixtures of microbial cells and necessary additives (adjuvants, stabilizers, protective agents, etc.), that go through a specific drying process. The drying process needs to be fine-tuned to remove enough water to ensure optimal stability, without jeopardizing the vitality of microbial cells or letting them desiccate. Similar to liquid formulations, addition of sugars and salts modulates the water-binding capacity and stability. There are different approaches to drying microbial formulations, but the three main ones include spray drying, freeze-drying, and fluid beds.

Spray Drying

Spray drying is a prominent technique for creating solid microbial formulations with enhanced stability. In this process, a liquid microbial suspension is atomized into fine droplets and rapidly dried using hot air, resulting in micro-sized particles. The powder produced through spray drying offers improved shelf life, ease of handling, and enhanced dispersibility. Formulators can fine-tune the formulation parameters, such as drying temperature and protective agents, to optimize the survival of microbes during the process.

Freeze-drying

Freeze-drying, also known as lyophilization, is a drying method commonly used in preservation of microbial products. This process involves freezing the material and then removing the ice by sublimation, resulting in a dry, stable product. Freeze-drying is particularly advantageous for preserving the viability of microbial cells and maintaining the integrity of sensitive compounds, as it removes water without damaging cell structures.

Fluid beds

This drying technique involves suspending the solid particles in a stream of air or gas, creating a fluidized bed. The fluidized bed facilitates efficient heat transfer and moisture removal, resulting in a dried and stable product.

Encapsulation

Encapsulation involves enclosing microbial cells within protective matrices, shielding them from environmental stresses. These protective matrices can be made of different materials – biopolymers, solid matrixes, gels. Their job is to guard the microbial cells against moisture and oxygen ingress, preserving their viability. Encapsulated microbes excel in maintaining the integrity of their cells during storage and transportation, ensuring high activity and efficiency of the product in the field.

Final formulation

The final formulation ensures that the formulated microbes are compatible with application equipment and are able to able to provide desired effects upon application. Both of these factors are affected by the selected application route – is the product intended to be applied onto the growing plant, onto the seeds before planting, or into the soil? The environments at the end of these application routes vastly differ in terms of temperature, humidity, pH, and UV light exposure – all of which influence microbial growth and activity.

Foliar application

Foliar application includes spraying aerial parts of growing plants (foliage, stems) with active ingredients dissolved or suspended in water. It is the most common way of applying plant protection products. Formulations that are compatible with foliar application need to meet the following constraints:

• Be water-soluble or create stable suspensions
• Have particle size that can pass the mesh of the sprayer
• Be compatible with the other components in the tank mix
• Stick to plant’s foliage and stems
• Have thermal and photostability

In the context of foliar application of microbials, it is important to note that microbial cells are solid particles, and as such have a tendency to sediment in the tank. Additives that enhance suspensibility and proper mixing of the tank components during foliar application are essential to ensure even distribution of microbial products.

Seed application

Seed application includes directly distributing the formulated product onto the surface of the seeds. This application route is very effective, as it ensures early contact of microbes and growing plants, and high level of quality control – being performed in controlled facilities, by automated tools, under the prying eye of experienced technicians. Formulations for seeds are commonly applied as a coating, whose thickness is measured in µm. More challenging but more favorable for microbes is the pelleting approach. In any case, the technical constraints that seed-applied microbial formulations need to meet include:

• Being compatible with seed application equipment and having stability during the seed application process
• Adhering to the seed’s surface
• Having a long-lasting action to support the seed as it germinates and grows
• Being adapted to the pH value, organic matter content, and moisture levels in the soil

Soil application

Soil application is the most common approach for delivering fertilizers, biostimulants, conditioners and pesticides. Soil application is a rather versatile approach that can incorporate liquid as well as solid formulations. However, its often considered as less effective, as the application is not very precise. In-furrow application tries to address the precision problem by incorporating microbial products locally, into the furrow or trench where the crop will be planted. As a result, microbial cell concentration in these products must be carefully calibrated to provide an effective dose without adverse effects on seed germination or early plant growth.

Being applied broadly or in-furrow, microbial formulations intended for soil application have to:

• Adhere to soil particles
• Be adapted to the pH value, organic matter content, and moisture levels in the soil
• Include microbial strains that are adapted to growing and thriving in the soil

The meticulous balance between viability, stability, and targeted delivery of microbial products must be met to maintain their quality on the long, rocky road to the fields. The symbiotic relationship between cutting-edge technology, microbial science, and formulation development is paving those roads for a more seamless transition from bioreactor to the fields, fostering sustainable practices and optimal yield outcomes. In the growing demand for microbial products, formulation excellence provides the strong foundation for product quality and efficacy necessary to yield healthy, vigorous crops for generations to come.

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