Saccharomyces siccum Veterinary Grade API is a dried viable yeast active substance derived from Saccharomyces cerevisiae, standardised for direct incorporation into veterinary tablets, capsules, powders, granules, premixes, and oral liquid preparations. The material is not a single-molecule chemical entity; the active principle is the viable whole-cell population, and batch potency is expressed as colony-forming units per gram rather than as mass purity. Model designation SsVet-API-200 identifies the standard veterinary grade, while manufacturer batch records document lot-specific viable count, moisture, water activity, and elemental impurity data. In contrast to baker’s yeast, brewing yeast, or feed-additive yeast, this grade is produced under pharmaceutical quality systems with defined bioburden, species identification, and drying controls. The dried form is intended for oral mucosal and gastrointestinal delivery in monogastric and ruminant veterinary species; the exact dose is species-specific and must be established through veterinary clinical studies rather than transferred from feed additive data.

Specification Profile and Pharmacopoeial Alignment

Release testing combines pharmacopoeial general methods with validated internal procedures because no single monograph establishes all relevant acceptance criteria for viable yeast used as a veterinary API. Table 1 summarises a representative release specification for SsVet-API-200.

Viable count is determined by serial dilution in buffered peptone water and pour plating on Sabouraud dextrose agar, incubated at 30°C for 48–72 h. Results are calculated according to ISO 7954 and expressed as CFU per gram of dry material. The method is validated for repeatability and reproducibility; the acceptance criterion for analytical repeatability is a log standard deviation below 0.15.

Stepwise geometric dilution is required in dry powder and premix operations because direct addition of the API to a full ribbon mixer charge creates active ingredient segregation and poor dose uniformity. A 300 L horizontal ploughshare mixer with a 1:10 first dilution into dextrose monohydrate, followed by a second 1:10 dilution into the bulk carrier, yields a bulk premix with coefficient of variation ≤ 5.0% when ten-point sampling is assayed by viable count. Mixing time is maintained between 10 min and 15 min at 20–25°C; extended mixing beyond 20 min does not improve homogeneity and may reduce viable count through mechanical attrition of yeast cells. In feed premix operations, the API is diluted with calcium carbonate or lactose to a calculated concentration; the actual inclusion rate is target-species specific and must be established by the marketing authorisation holder. Cross-contamination control requires validated cleaning of mixers, elevators, and surge bins because yeast cells are sticky when hydrated and can accumulate at dead zones.

Powder sachet filling for direct oral administration requires low-humidity filling suites. The filling line is operated at 25°C and 30–35% RH; exposure time from drum opening to final sealing is controlled to 4 h. Desiccant sachets are inserted when the powder fill exceeds 500 mg and the package moisture vapour transmission rate is above 0.1 g/m²/day. The moisture ingress risk is assessed by near-infrared moisture mapping or water activity measurement after sealing. Batch-to-batch variance in the API density can shift the fill weight on volumetric auger fillers; therefore, fill depth is adjusted using bulk density feedback and fill weight is monitored at 10-min intervals.

For granule dosage forms, fluid-bed top-spray granulation with an aqueous binder solution containing povidone K30 at 2–4% w/w is preferred over high-shear wet granulation because lower impeller shear reduces cell rupture. Inlet air temperature is set to 35–40°C, product temperature to 28–32°C, and spray rate to 10–30 g/min/kg of batch, with final loss on drying ≤ 3.5%. Granule size between 150 µm and 710 µm is suitable for sachets and oral pastes; oversize material above 1 mm is milled through a cone mill at 500 rpm to avoid excessive cell disruption. Granule moisture above 4.0% at the end of drying is a critical process deviation because residual water raises water activity and shortens the viability stability profile.

Why Does Viable Cell Recovery Decline Under High-Shear Granulation?

Granulation imposes simultaneous moisture, thermal, and shear stress on the viable cell population. In high-shear wet granulation, the impeller tip speed, water addition rate, and wet massing time determine the extent of viability loss. Production-scale observations on a 600 L high-shear granulator with impeller tip speed 2.5 m/s and wet massing time 5 min show that granule temperatures above 35°C produce a measurable decline in recoverable CFU. Wet mass temperature should therefore be maintained at 25–32°C. After granulation, drying in a fluid-bed dryer with inlet air temperature 35–40°C and final granule moisture ≤ 3.5% is preferable to tray drying at 50°C, which can reduce the viable count by more than 1 log at the granule surface. The exact loss profile is formulation-specific; published data for this exact configuration is limited, so process validation with viability assay at three granule size fractions is required before scale-up.

Direct compression and capsule filling are less destructive than wet granulation when the formulation has adequate flow. For tablets, the API is pre-blended with microcrystalline cellulose and crospovidone, then compressed on a rotary tablet press at a main compression force below 15 kN; higher force may crack yeast cells but the measured viability loss depends on excipient deformation characteristics. Tablet hardness should be controlled between 30 N and 60 N for oral veterinary tablets; hardness above 80 N can extend disintegration time and expose the yeast cells to prolonged hydration in the gastrointestinal tract, which may delay release. Uniformity of mass is assessed according to Ph. Eur. 2.9.5, and disintegration is assessed according to Ph. Eur. 2.9.1.

For hard gelatin capsule filling, the API particle size distribution affects flow and fill weight variation. The API is sieved through 500 µm mesh to break soft agglomerates; mean particle size determined by laser diffraction typically falls between 80 µm and 150 µm, with the fines fraction below 45 µm not exceeding 20%. On a dosator-type capsule machine running at 24,000 capsules/h, powder bed temperatures remain below 28°C if the tamping pin setting is adjusted to a compression ratio of 1:1.2. Higher compression reduces air permeability and increases frictional heat, causing viability loss. Fill weight is controlled to ±5% of target; weight variation outside this range affects dose uniformity. Capsule mass uniformity is assessed according to Ph. Eur. 2.9.5.

Oral paste formulations for horses or companion animals incorporate the API into a non-aqueous vehicle of medium-chain triglycerides and fumed silica. The vehicle has water activity below 0.35, which keeps the yeast cells in a dry state and avoids the in-use stability loss observed in aqueous suspensions. Homogenisation is performed under vacuum at 25–30°C for 15 min; high-shear rotor-stator mixing above 10,000 rpm is avoided because local temperature spikes may exceed 40°C.

When the API Is Dispersed in Aqueous Vehicles for Oral Solutions or Drenches

When the API is dispersed in aqueous vehicles for oral solutions, drenches, or drinking-water additives, the dosage form is a suspension rather than a true solution, because viable Saccharomyces cerevisiae cells remain particulate. The aqueous vehicle should be buffered to pH 4.5–6.5 and maintained at 15–25°C. Viability declines once the API is hydrated; therefore, reconstituted oral suspensions should be assigned a maximum in-use period of 24 h unless preservative and real-time microbiological challenge data support longer storage. Hard water containing more than 250 mg/L calcium carbonate equivalent may reduce dispersibility and should be chelated or replaced with purified water. The addition of sucrose at 5–10% w/v can provide short-term osmotic protection, but this effect is not equivalent to lyophilisation and should not be used to extend shelf life. Oral drenches for ruminants may require viscous suspending agents such as xanthan gum at 0.1–0.3% w/v; shear during mixing should be kept below 500 s⁻¹ to avoid cell damage.

For parenteral formulations, viable whole-cell Saccharomyces siccum is not considered suitable as a live organism due to particulate load, potential immunogenicity, and the absence of established sterile processing parameters. If the API is specified for injection, the formulation developer should process it as an inactivated antigen or clarified extract after cell disruption and filtration; published data for this specific configuration is limited, and each batch must be validated for sterility according to Ph. Eur. 2.6.1 and absence of bacterial endotoxins according to Ph. Eur. 2.6.14. The untreated dried API should not be reconstituted with sterile water and administered parenterally, because whole yeast cells are not removed by 0.2 µm sterilising-grade filtration.

Stability studies for the API follow VICH GL18 and are conducted in aluminium-laminated foil bags at 25°C/60% RH and 40°C/75% RH. Viable count, water activity, and moisture are monitored at 0, 3, 6, 9, 12, 18, 24 months. Data from production-scale batches show that water activity is the main driver of viability loss; batches with release water activity above 0.35 exhibit faster decline under the 40°C/75% RH condition. This is why the water activity limit is considered critical and why secondary packaging must be sealed immediately after sampling.

Comparative Behaviour Against Saccharomyces boulardii and Inactivated Yeast Fractions

The API is distinguished from other yeast-based ingredients by viable cell count, metabolic activity, and intended pharmacological effect. Table 2 summarises key differences that affect formulation choice.

Veterinary API grade Saccharomyces siccum differs from feed-additive yeast preparations in traceability, bioburden control, elemental impurity control, and documentation. A feed-grade product may carry enterobacteria and may not be controlled for species identity or viability after storage. The API grade is assigned a retest date based on real-time stability data rather than a fixed expiry; when stored in unopened aluminium-laminated bags at 25°C and 60% RH, the viable count is expected to remain above the release limit for the claimed storage period, but this expectation must be confirmed by real-time stability data. Storage below 8°C is not required but can reduce viability loss in high-humidity climates. Once opened, the material should be used within 30 days and protected from moisture ingress because hydration initiates metabolic activity and accelerates cell death. Co-formulation with polyene antifungals, such as nystatin or amphotericin B, should be avoided because these agents are fungicidal against Saccharomyces cerevisiae; compatibility with acidifiers, organic acids, and essential oils should be tested in the finished formulation because the viable count may decline in combinations that reduce pH below 2.0 or increase ethanol above 5% v/v.