CA Storage Ethylene Control: Chemistry, 1-MCP and Scrubbers

By the Bulky Business Agronomy & Quality Desk · Cold-Chain Engineering

Controlled-atmosphere (CA) storage is often described as low oxygen and high carbon dioxide. For apples held through the winter, though, the real target is ethylene: how much the fruit makes, how much stays in the room, and how strongly the fruit responds to it. This note covers the chemistry behind ethylene control and the practical tools a storage operator or buyer should understand. Storage programmes often start with Kinnaur storage apples.

How fruit makes ethylene

Ethylene is synthesised from the amino acid methionine in three steps:

1. Methionine → S-adenosyl-L-methionine (SAM), catalysed by SAM synthetase.
2. SAM → 1-aminocyclopropane-1-carboxylic acid (ACC), catalysed by ACC synthase. This is usually the rate-limiting step. The by-product, methylthioadenosine, is recycled back to methionine through the Yang cycle, so the fruit can keep making ethylene without running short of methionine.
3. ACC → ethylene, catalysed by ACC oxidase. This step needs molecular oxygen, and carbon dioxide activates the enzyme.

Climacteric fruit such as apples, pears, bananas, tomatoes and mangoes switch from a low basal ethylene output (“System 1”) to an autocatalytic burst (“System 2”), in which ethylene stimulates more ACC synthase and ACC oxidase. Once System 2 is under way, ripening is very hard to stop. Most ethylene control therefore aims to delay that switch or to block the fruit’s response to ethylene.

What low O2 and high CO2 actually do

Low oxygen acts at several points. It slows respiration in general, it reduces ACC oxidase activity because O2 is a substrate, and it lowers the tissue’s sensitivity to ethylene. Elevated CO2 also opposes ethylene action and slows ripening-related changes, although it activates ACC oxidase in vitro. The overall result in a well-run CA room is less ethylene produced and less effect from what is produced, but not zero.

There are limits on both gases. If O2 falls below the fruit’s lower tolerance, respiration turns fermentative and produces ethanol and acetaldehyde, off-flavours and tissue damage. Too much CO2 causes internal browning and cavities in sensitive cultivars. Typical apple CA regimes run at about 1.5–3% O2 and 1–3% CO2 depending on cultivar. Delicious-type apples tolerate low O2 well, and our desk specification for apples is 1.5% O2 / 1.0% CO2, held at about −0.5 to 1 °C and 90–95% RH.

1-MCP: blocking the receptor

1-Methylcyclopropene (1-MCP, sold as SmartFresh among other brands) is a gas that binds ethylene receptors with much higher affinity than ethylene itself. With the receptors occupied, the fruit cannot sense ethylene, even when ethylene is present. It is applied once, usually soon after harvest and after cooling, in a sealed room at a low concentration (often in the region of 1 ppm or below) for about a day, following the product label and local registration.

In apples, 1-MCP slows softening and loss of acidity and suppresses superficial scald, a disorder to which Delicious-type fruit is prone. It works best on fruit harvested at the right maturity. Fruit already in System 2 responds poorly, because new receptors and high internal ethylene overwhelm the treatment. Used together, 1-MCP and CA give better results than either alone.

Removing ethylene from the room

In ordinary cold storage, ventilation with fresh air is the simplest way to dilute ethylene. In CA rooms that is not an option, because fresh air brings back oxygen. Ethylene has to be removed chemically or catalytically instead.

Method	Chemistry	Strengths	Limitations
Potassium permanganate (KMnO4) scrubbers	Oxidises ethylene, ultimately to CO2 and water; KMnO4 is reduced to manganese dioxide	Simple, no power needed for sachets; colour change from purple to brown shows exhaustion	Finite capacity; media must be replaced; must never contact produce directly
Catalytic oxidation	Room air passed over a heated catalyst (commonly platinum-based) that oxidises ethylene to CO2 and water	Continuous, high capacity, suitable for large CA rooms	Energy use and heat load; capital cost
Photocatalytic oxidation	UV light activates a titanium dioxide surface that oxidises ethylene at low temperature	Low-temperature operation; compact units	Performance depends on airflow and catalyst condition
Ventilation	Dilution with outside air	Cheap in regular cold rooms and reefers	Not compatible with CA; brings in warm or humid air

Ethylene thresholds by commodity

Sensitivity varies by orders of magnitude, and published thresholds vary with temperature, exposure time and maturity. The figures below are indicative ranges, not specifications.

Commodity	Ethylene production	Sensitivity	Indicative practical aim
Kiwifruit	Low when unripe	Extreme	Tens of parts per billion or lower in storage
Apples in long-term CA	High once climacteric	Moderate to high	Below about 1 ppm; lower is better for long storage
Green bananas	Low before ripening	High	As low as practical; ripening rooms deliberately use around 100–150 ppm under control
Leafy greens, cucumbers, broccoli	Very low	High	Keep away from producers; yellowing at low ppm
Carrots	Very low	High	Ethylene causes bitterness
Onions, potatoes	Very low	Low to moderate	Effects on sprouting depend on dose and timing

Mixed-load compatibility

Storage and transit decisions combine temperature with ethylene. Apples are high producers and sit at −0.5 to 1 °C. Bananas (about 13–14 °C) and mature-green tomatoes (not below about 10–12 °C) are chilling-sensitive, so they cannot share a room or reefer with apples on temperature grounds. Kiwifruit, carrots, cucumbers and leafy greens are compatible with apples on temperature but not on ethylene. Onions need lower humidity than apples, and processing potatoes such as Chipsona-1 store warmer, at about 8–12 °C, to limit cold-induced sweetening. In practice, consolidated loads out of the hills should keep apples in their own compartment or vehicle. The same rules apply to mandi cold rooms. Our wider cold-chain logistics notes cover reefer set-points and pre-cooling. For sourcing from the apple belt, see the Himachal Pradesh wholesale produce hub.

What buyers should check

• Room O2 and CO2 logs throughout storage, not just at sealing.
• Whether 1-MCP was applied, when, and on fruit of what maturity.
• The ethylene removal method and the maintenance or replacement records for scrubbers.
• Firmness and soluble solids at room opening, compared with values at harvest.

If CA-stored apples are part of your requirement, state the storage history you expect in your request for quotation.

Working with our desk

Our Agronomy & Quality Desk can share the CA set-points and room-opening quality checks that apply to the apple lots we offer. For details or a quote, message us on WhatsApp at +91-9015316162. To book storage-grade fruit, talk to our wholesale apple mandi supplier desk.

Frequently asked questions