Mold Resistance Evaluation of Packaging Materials: Standards, Test Methods, and Laboratory Assessmen

Mold resistance evaluation of packaging materials is a critical laboratory assessment that determines how susceptible packaging—such as films, paperboard, foams, and wooden crates—is to fungal growth under high-humidity conditions. By following internationally recognized standards such as ISO 846, ASTM G21, ASTM D2020, and GB/T 24128, manufacturers and accredited third-party laboratories can verify antifungal performance, prevent product contamination during storage and transport, and meet buyer requirements in humid climates.

Technical Overview

Packaging materials are frequently exposed to warm, humid environments during production, warehousing, and long-distance transport, particularly in maritime shipping and tropical climates. Under such conditions, mold spores present in the air and on material surfaces can germinate and colonize packaging, leading to visible staining, unpleasant odors, loss of mechanical strength, corrosion of enclosed products, and serious customer complaints or rejected shipments. Mold resistance evaluation is therefore an essential quality control step for packaging suppliers serving the food, electronics, pharmaceutical, leather, and export industries.

Mold resistance evaluation of packaging materials refers to a set of accelerated laboratory tests in which specimens are inoculated with standardized fungal spores and incubated under controlled temperature and humidity conditions that favor fungal growth. After a defined exposure period, the specimens are examined for visible growth and rated on a standardized scale. These tests can also quantify the fungistatic or fungicidal effectiveness of antifungal treatments, coatings, and additives incorporated into packaging materials. The most widely applied standards include ISO 846 (plastics — evaluation of the action of microorganisms), ASTM G21 (resistance of synthetic polymeric materials to fungi), ASTM D2020 (mildew resistance of paper and paperboard), GB/T 24128, GB/T 2423.16, and JIS Z 2911.

Testing Samples

Practically all common packaging material categories can be evaluated for mold resistance. Specimens are typically cut to standard dimensions, conditioned according to defined procedures, and tested in both treated and untreated states where antifungal performance must be demonstrated. Typical sample types submitted to accredited third-party laboratories include:

Plastic Films and Laminates

Single-layer and multilayer flexible films such as PE, PP, PET, PVC, PA, and aluminum-plastic composite laminates used for bags, pouches, and wrapping films, including films incorporating antifungal masterbatches.

Paper and Paperboard

Corrugated fiberboard boxes, kraft paper, carton board, molded pulp trays, and coated or impregnated paper grades used for shipping containers and retail packaging.

Cushioning and Foam Materials

Expanded polyethylene (EPE), expanded polystyrene (EPS), polyurethane foams, and air-cushion films used to protect goods against mechanical shock, which can harbor moisture and support fungal growth.

Wooden Packaging Components

Wooden pallets, crates, boxes, and veneer packaging, which are naturally susceptible to mold colonization and often require antifungal treatment before export shipment.

Textile and Nonwoven Packaging

Jute and woven polypropylene bags, nonwoven fabric wraps, and desiccant sachet materials used in moisture-sensitive cargo packaging.

Auxiliary Packaging Materials

Adhesives, pressure-sensitive tapes, labels, printing inks, and sealing compounds, all of which may contain organic ingredients that serve as nutrient sources for fungi.

Testing Items

Depending on the material type, intended application, and buyer specification, mold resistance evaluation of packaging materials may include the following test items:

• Fungal growth resistance rating: visual assessment of mold colonization on the specimen surface after incubation, reported on a 0–4 intensity scale.
• Fungistatic efficacy: the ability of a material or antifungal treatment to inhibit fungal growth, expressed as a percentage of growth reduction compared with an untreated control.
• Fungicidal efficacy: the ability of a treated material to kill fungal spores, verified by recovery and germination testing after contact.
• Long-term mold resistance under cyclic humidity and temperature conditions simulating extended storage or multi-week sea transport.
• Spore germination inhibition on treated surfaces.
• Retention of material performance after fungal exposure, such as tensile strength, bursting strength, or seal integrity of packaging after the mold resistance test.
• Supporting moisture characterization, including moisture content and water vapor transmission rate, since moisture availability is the dominant factor controlling mold growth on packaging.

Testing Methods

Agar Plate Method (ISO 846 / ASTM G21 / GB/T 24128)

The specimen is placed on the surface of a mineral-salts agar medium that contains no carbon source other than the test material itself. The specimen is then inoculated with a mixed spore suspension of standard test fungi and incubated, typically at 28 ± 1 °C with relative humidity of at least 90%, for 28 days. Growth observed on the specimen indicates that the material itself supports fungal development. ISO 846 defines complementary procedures: Method A assesses intrinsic resistance to fungal growth, Method B determines fungistatic action, and Method C determines fungicidal action.

Humidity Chamber Method (GB/T 2423.16 / JIS Z 2911)

Specimens are suspended or placed inside a chamber maintained at high humidity (typically above 90% RH) and inoculated by spraying or brushing with a spore suspension. This approach simulates real-world storage conditions more closely than direct agar contact and is commonly used for complete packaging assemblies, wooden crates, and oversized components. Incubation periods of 28 days are standard, with extended durations of up to 84 days for long-term storage qualification.

Paper and Paperboard Mildew Test (ASTM D2020)

Paper-based packaging is tested by exposing conditioned specimens to fungal inoculum under controlled humid conditions suited to cellulosic substrates, evaluating both untreated susceptibility and the effectiveness of antimicrobial treatments applied to paper, board, and coated grades.

Standard Test Fungi

Test programs commonly employ a consortium of well-characterized fungal strains selected for their aggressive growth on polymeric and cellulosic substrates, including:

• Aspergillus niger
• Penicillium funiculosum
• Trichoderma viride
• Chaetomium globosum
• Paecilomyces variotii
• Aureobasidium pullulans

Growth Rating and Acceptance Criteria

Following incubation, specimens are inspected visually and, where necessary, under magnification, then assigned a growth intensity rating consistent with ISO 846 and ASTM G21:

• Rating 0 — No visible growth on the specimen surface.
• Rating 1 — Trace growth, covering less than 10% of the surface.
• Rating 2 — Light growth, covering 10% to 30% of the surface.
• Rating 3 — Moderate growth, covering 30% to 60% of the surface.
• Rating 4 — Heavy growth, covering more than 60% of the surface.

For most commercial packaging applications, a rating of 0 or 1 is considered acceptable, meaning no growth or only trace growth attributable to nutrient carryover rather than degradation of the material itself. Procurement specifications for export packaging frequently require a rating of 0 on all replicate specimens.

Test Conditions and Duration

Standard incubation conditions are 28 ± 1 °C (ASTM G21 specifies 28–30 °C) at 85% relative humidity or higher. The routine test cycle is 28 days; extended 56-day and 84-day exposures are used for qualification of long-term storage or repeated ocean shipments. Positive and negative controls are included in every run to confirm spore viability and to verify that observed effects are attributable to the test material or its antifungal treatment.

Applications

Mold resistance evaluation supports quality assurance and regulatory compliance across a broad range of industries:

• Export and marine transport packaging: verifying that cartons, pallets, and wraps will resist mold during multi-week voyages through humid tropical climates.
• Food and beverage packaging: confirming that direct-contact and secondary packaging materials do not support fungal growth that could contaminate products.
• Electronics and precision instruments: preventing mold-related staining, corrosion of contacts, and degradation of moisture-barrier bags and cushioning materials.
• Pharmaceutical and medical device packaging: demonstrating microbial robustness of outer packaging used in long distribution chains.
• Leather goods, footwear, and textile packaging: controlling mold growth on organic substrates and their wrapping papers and boxes.
• Antifungal product development: comparing candidate antifungal agents, coatings, and masterbatches to optimize formulation and dosage.
• Supplier qualification and dispute resolution: providing objective, standards-based evidence of packaging performance for buyers, freight forwarders, and insurers.

FAQ

What is mold resistance evaluation of packaging materials?

It is a laboratory test in which packaging specimens are inoculated with standardized fungal spores and incubated under warm, high-humidity conditions for a defined period, after which fungal growth is visually rated. The test determines whether the material supports mold growth and, when antifungal treatments are used, how effective those treatments are.

Which standards govern this testing?

The principal standards are ISO 846 and GB/T 24128 for polymeric materials, ASTM G21 for synthetic polymeric materials, ASTM D2020 for paper and paperboard, GB/T 2423.16 for environmental fungus tests, and JIS Z 2911 for general fungus resistance testing. The choice of standard depends on the material type and the market requirements.

What fungi are used in the test?

A mixed inoculum of five or six strains, typically including Aspergillus niger, Penicillium funiculosum, Trichoderma viride, Chaetomium globosum, Paecilomyces variotii, and Aureobasidium pullulans, is applied to represent the diversity of molds encountered in real storage and transport environments.

How long does the test take?

The standard incubation period is 28 days at 28 ± 1 °C and high relative humidity. Extended tests of 56 or 84 days are available for long-term storage qualification. Including sample preparation, conditioning, and final evaluation, a routine project typically requires five to six weeks.

What result is considered a pass?

Most specifications require a growth intensity rating of 0 (no growth) or 1 (trace growth) on all replicates. For antifungal-treated materials, an efficacy of well above 50% growth reduction relative to the untreated control is generally expected, with many buyers requiring 90% or greater.

How can the mold resistance of packaging be improved?

Effective measures include incorporating antifungal agents or coatings into films, paper, and board; reducing the residual moisture content of paper and wood before packing; improving moisture-barrier layers and using adequate desiccants; avoiding organic contaminants on material surfaces; and maintaining dry, ventilated storage conditions throughout the supply chain.

Where should this testing be performed?

Testing should be carried out by accredited third-party laboratories with microbiological competence and appropriate incubation facilities, ensuring that results are traceable to recognized standards and acceptable to international buyers and regulatory authorities.