FAQs

Appearance

Off‑white to pale yellow powder/flakes, uniform, free of visible impurities.

Visual inspection.

Ensures physical purity; avoids foreign matter that could interfere with media clarity or introduce contaminants.

Odor

Odorless, no solvent or chemical odor.

Organoleptic evaluation.

Indicates absence of residual processing solvents or degradation products.

Moisture Content

≤ 12% (typically 8–12%).

Gravimetric loss on drying (105 °C to constant weight; USP/Ph.Eur./GB).

Prevents caking, ensures accurate weighing and consistent gel strength.

Total Ash

≤ 1.0% (often 0.2–1.0%).

Incineration at 550 °C (USP/Ph.Eur.).

Reflects total inorganic impurity level; lower ash indicates higher purity.

Sulfated Ash

≤ 0.5% (or as per supplier spec).

Sulfation followed by ignition (USP/Ph.Eur.).

More sensitive indicator of inorganic residues, especially sulfates and related ions.

Total Nitrogen (Protein/Amine)

≤ 0.5% (or supplier limit).

Kjeldahl or equivalent.

Controls organic impurities that could act as unintended nutrients or microbial inhibitors.

Heavy Metals (Pb, Cd, Hg, As)

Each ≤ 1 mg/kg (may be stricter).

ICP‑MS or AAS.

Prevents metal‑ion toxicity that could inhibit microbial growth or affect susceptibility testing.

Soluble Ions (Na⁺, K⁺, Cl⁻, SO₄²⁻)

Reported on CoA; limits as agreed.

Ion chromatography / titration.

Ensures ionic composition does not interfere with medium osmolarity or specific biochemical tests.

Re‑melting (Re‑solution)

Complete re‑melting of gelled agar upon heating (85–95 °C) with no persistent lumps or residues.

Visual inspection after heating 1.5–2.0% gel.

Confirms thermal reversibility; essential for media re‑melting and repouring.

Turbidity (Clarity)

≤ 8 NTU (1.5–2.0% solution, 25 °C).

Prepare 1.5% agar in DI water, dissolve completely, cool to 25 °C, measure with calibrated turbidimeter (ISO 7027/EPA 2130B).

High clarity is critical for colony observation, automated imaging, and avoiding false‑positive contamination readings.

Phosphate‑Precipitation Test

No visible precipitate or haze.

Dissolve agar in phosphate‑buffered solution (e.g., 10 mM phosphate, pH 7.0), heat, cool, and observe.

Detects free polyvalent cations (e.g., Ca²⁺, Mg²⁺) that can form insoluble phosphates, causing media cloudiness or precipitation.

Alkali‑Precipitation Test

No significant precipitate.

Treat agar solution with 0.1–0.5 M NaOH, heat, cool, and assess for insolubles.

Identifies impurities that precipitate under alkaline conditions (e.g., in some selective media).

Gel Strength

≥ 800–1200 g/cm² (1.5% gel, 20 °C).

GB/T, USP, or supplier method (e.g., Texture Analyzer with 1 cm² plunger).

Determines plate firmness; affects colony morphology, ease of streaking, and susceptibility‑test readability.

Melting Point

85–95 °C.

Visual or differential‑scanning method.

Indicates temperature required for complete liquefaction; must be above typical incubation temperatures.

Gelling (Setting) Point

30–40 °C.

Monitor liquid‑to‑gel transition upon cooling.

Defines temperature window for pouring plates; should be safely below incubation temperature to prevent re‑melting.

Thermal Hysteresis

Typically 45–55 °C (m.p. – g.p.).

Calculated from melting and gelling points.

Large hysteresis ensures gel stability at incubation temperatures (e.g., 37 °C).

Growth‑Promotion / Inhibition Test

No inhibition of reference strains.

Prepare media with test agar; inoculate with E. coli ATCC 25922, S. aureus ATCC 25923, etc.; compare growth to reference‑agar control.

Verifies absence of residual antimicrobials or growth‑inhibiting substances.

Total Aerobic Microbial Count

≤ 100 CFU/g.

Plate‑count method (e.g., USP).

Preerts carry‑over of contaminating microbes that could compromise sterility or interfere with low‑bioburden assays.

Specified Pathogens

Absent in 10 g (E. coli, Salmonella, S. aureus, P. aeruginosa).

Enrichment and selective plating (USP/EP/GB).

Ensures raw material does not introduce hazardous microorganisms.