Keeping tea fresh in transit: moisture and condensation

Keeping tea fresh in transit starts with dry tea: China’s national standards cap moisture at 7.0% for gunpowder, chunmee, black tea and most oolong, 8.5% for jasmine and white tea and 6.5% for Wuyi rock tea. In a closed container, the tea and its packing then set the humidity of the air. The CTU Code of the IMO, the ILO and UNECE puts the mould threshold at 75% relative humidity, asks for organic cargo to be packed “container-dry”, in balance with about 60%, and explains how daily temperature swings make water condense under the roof and drip onto the cargo. For the warehouse, China’s tea storage standard, GB/T 30375-2013, sets the rules: stacks at least 200 mm from walls, on dunnage at least 150 mm high, checked every month.

Moisture limits for Chinese tea by type

Each Chinese grade standard caps the moisture of the tea as a percentage of its mass, with one cap for all its grades. Most cite test method GB/T 8304 (withdrawn on 2 March 2017; the current method is GB 5009.3-2016), and the gunpowder, chunmee, congou, souchong, oolong, jasmine and white tea standards check moisture on every batch before release.

Maximum moisture in China’s national tea standards (% by mass; sources: the standards’ official records)

The Wuyi rock tea standard gives way to GB/T 18745-2026 on 1 April 2027. The international standards for green, black and oolong tea, ISO 11287:2011, ISO 3720:2011 and ISO 20716:2022, set no moisture limit for tea as received, so a buyer who wants one has to write it into the contract, with the test method.

These caps are ceilings, not targets. The cargo handbook of the German Insurance Association (GDV) calls black tea strongly hygroscopic: at 4–6% water it is in balance with very low relative humidities, below 2% it turns hay-like and above 9% it tends to grow mould and turn musty. China’s 7.0% cap for black tea thus sits 2 percentage points below that mould line (The Tea Trade’s comparison).

Why moisture rises during a sea voyage

Tea does not have to get wet to take up water. In a closed container, says Annex 3 of the CTU Code, the packing code of the International Maritime Organization (IMO), the International Labour Organization (ILO) and the UN Economic Commission for Europe (UNECE), the air quickly settles at a relative humidity set by the water in the cargo, its packaging and the dunnage. Below 100% nothing condenses and below 75% no mould grows, but only as long as the temperature does not change. The Code’s 2014 edition was still the current one in October 2026; a revised edition, finalised by an ILO, IMO and UNECE group of experts in December 2025 with Annex 3 unchanged in substance, was before the three organisations for adoption.

The water sits in the cargo, not in the air. At the 7.0% cap, the 12.5–16.7 tonnes of leaf tea that fit in a 40 ft box in the worked example of the guide to tea container capacity hold about 880–1,170 kg of water (12.5–16.7 t × 7.0%), while the 65.7 m³ of air in the empty box hold at most about 2.0 kg of water vapour at 30 °C (65.7 m³ × 30.3 g/m³, the CTU Code’s saturation value; The Tea Trade’s calculations).

Wood and paper bring water too. The Code names the cargo itself, timber bracing, pallets, porous packaging and packing in rain, snow or hot, humid weather as the usual sources, and estimates dry dunnage, pallets and packing material at 12% to 15% moisture, enough on its own to set the air in a closed box at about 60% to 75% relative humidity. GDV wants the wood of tea chests at 10–12% water, equivalent to 60–70% relative humidity, and says the same of wooden container floors.

For a margin against temperature swings, the Code asks for organic cargo, foodstuffs included, to be packed “container-dry”: with a moisture content in balance with about 60% relative humidity, below the 75% at which the risk of mould rises. GDV lists 50–60% relative humidity for tea.

Sources: IMO/ILO/UNECE CTU Code (2014 edition), Annex 3; German Insurance Association (GDV), Transport Information Service, Tea

Container sweat and cargo sweat

Temperature swings are common on long voyages, and their size depends mainly on stowage: on top of the deck stow, daily variations can exceed 25 °C, while in the hold they may be marginal (CTU Code). The Code describes the daily cycle:

• Morning: the box warms, its relative humidity falls and the tea, cartons and wood release water vapour into the warm upper part of the box.
• Late afternoon: the roof cools first; the air touching it reaches its dew point and large drops fall onto the cargo, while condensate runs down the walls: container sweat.
• Day after day: some of the water evaporates again, but over repeated cycles the water set free can be considerable.

The Code’s figures show how little cooling it takes. Air at 30 °C and 57% relative humidity holds 17.3 g of water per cubic metre, the most air can hold at 20 °C, so its dew point is 20 °C. Cooled to 10 °C, where saturation is 9.4 g/m³, it sheds 7.9 g per cubic metre: about 0.5 kg in the air of an empty 40 ft box (65.7 m³ × 7.9 g/m³ = 519 g; The Tea Trade’s calculation), and the cargo releases more vapour the next morning. GDV therefore requires tea in containers to be stowed below deck, out of reach of rain, seawater, heating by day and cooling at night, and away from sources of heat, which also dry tea out.

Arrival brings two more risks (CTU Code):

• Warm cargo in a cold port: a box of warm, hygroscopic cargo left closed for some days in a cold climate soaks its cargo with water from the roof. Such goods should be unpacked right after unloading; drying then outruns mould, provided the packages are well ventilated.
• Cargo sweat: goods packed in a cold climate and unpacked at once in warm, humid air are cold enough for that air to condense on them. The Code advises waiting until they have warmed up, which can take a day or more, unless they are under vapour-tight plastic sheeting with enough desiccant, left on until the cargo has acclimatised.

The longer the voyage, the more cycles the tea goes through; the guide to China–Africa shipping times gives the transit times.

What the CTU Code asks before and during packing

Checking the box (chapter 8 of the 2014 edition). A closed container should be clean, dry and free of residues and persistent odours from previous cargo; if a stain comes off on a gloved hand, the box should be replaced. It should be weatherproof: rusty streaks below patches suggest leaks, and light entering the closed box reveals leaks. GDV adds that containers for tea must be watertight and uncontaminated, and that a box whose floor smells, is contaminated or is too damp should be rejected.

Packing (chapter 3 and Annexes 3 and 7).

• Avoid packing wet or damp goods, and do not use wet timber, pallets or packaging.
• Keep odorous or moisture-giving cargo away from sensitive cargo, and dusty or dirty cargo away from foodstuffs in porous packaging.
• Use dunnage against condensed water: timber planks against water on the floor, gunny cloth, paperboard or natural-fibre mats against drops from the ceiling, planks or plywood against sweat on the walls, and fleece over very sensitive cargo.
• If desiccants are used, calculate the absorption capacity needed. The Code pairs them, or active drying of dunnage and packing, with a sealed plastic wrapping, but says they generally fail with hygroscopic cargo that is not container-dry. Plastic sheeting stops drips but may promote mildew if the box is too moist overall.

Ventilation and temperature (chapters 5 to 7). The vents of a standard container only equalise pressure and do not measurably exchange air; ventilated containers have weatherproof grills in their rails, and the Code says ventilation can control condensation damage to cocoa and other agricultural produce. For tea, GDV says no ventilation is needed at “shipping dryness”, notes that tea mostly travels in standard containers and gives 5–25 °C as a favourable travel temperature; the CTU Code says a temperature-controlled unit should be considered whenever heat or cold could damage the cargo.

Odours. Tea absorbs any foreign odour at once, says GDV. Many freight contracts carry a special tea clause that keeps goods such as sugar, rubber, hides, turmeric and cassia out of its compartment, and the handbook also keeps tea away from copra, tar, camphor, tobacco, pepper, essential oils, strong-smelling chemicals and dusty or oily cargo.

Packaging and loading that keep tea dry

The pack is the first barrier; formats and liners are compared in the guide to tea export packaging formats. China’s gunpowder, chunmee and jasmine tea standards require transport that is clean, dry, odour-free and uncontaminated, with protection from rain, damp and strong sun. Tea bags are covered in our guide to tea bag materials and standards.

Morocco’s Decree 2-13-711 of 4 March 2015 requires tea to be packed in closed, clean and dry containers, as ISO 11287 does for green tea.

GB/T 30375: what China’s tea storage standard requires

Tea also waits in warehouses before and after the voyage. China’s standard for that is GB/T 30375-2013, Tea storage (茶叶贮存), a recommended national standard proposed by the All China Federation of Supply and Marketing Cooperatives and managed by the national technical committee for tea standardisation (SAC/TC 339). It was published on 31 December 2013 and took effect on 22 June 2014. An official English version followed on 14 December 2020; the project record for that translation, planned in January 2018, noted that there was then no international standard on tea storage. The gunpowder, chunmee, jasmine and white tea standards refer storage to it. Matcha storage is covered in our guide to Chinese matcha.

Its main requirements:

• Receipt: tea packed and put into store promptly, recorded (type, grade, quantity, origin, production date) and labelled; each type kept in its own zone so that odours do not transfer; packages firm, intact, damp-proof, undamaged, clean and odour-free.
• Stacking: by grade and batch, never against pillars, at least 200 mm from walls, on dunnage at least 150 mm high.
• Inspections: changes in moisture at the bottom and on the surface of each stack; mustiness, foreign odours, contamination or other faults in the packages; any heating inside the stacks; and the warehouse’s temperature, relative humidity and ventilation. Checks are monthly, at least twice a month in hot, rainy seasons, and recorded.
• Climate and housekeeping: ventilation and heat dissipation with a thermometer, dehumidification with a hygrometer, both set for the type of tea; nothing else stored in the warehouse; protection against fire and theft.

The standard is under revision. A review on 17 May 2021 concluded that it should be revised, and plan 20251082-T-442, issued on 30 April 2025 with a 12-month schedule, is to replace the 2013 edition in full and take effect six months after publication. In October 2026 the official record still listed the plan as awaiting approval and the 2013 edition as current.

A moisture checklist for each tea shipment

• Specification: maximum moisture at loading, test method (GB 5009.3-2016, which replaced the GB/T 8304 that most Chinese standards cite), a test on arrival and a retained sample.
• Packing: tea within its limit, sealed in a moisture-barrier liner, in dry, odour-free packs and cartons.
• The box: clean, dry, odour-free and weatherproof, with dry pallets and dunnage; reject a damp or smelly floor.
• Desiccants and drips: desiccant sized by calculation, as a complement to dry cargo; fleece, mats or paperboard over the top layer.
• Loading and stowage: under cover, never in rain, with no odorous, dusty, oily or moisture-giving cargo in the box; below deck and away from heat.
• Unpacking: at once in a cold climate; in a warm, humid one, only once the cargo has warmed up.
• Arrival check: the moisture and smell of the top and bottom layers, where GB/T 30375 has warehouses check each stack.

This is general information, checked in October 2026, not technical or legal advice: agree moisture limits, tests and packing with your buyer or supplier before each shipment.