Ethylene Treatment Systems: Techniques and Considerations

Every banana, mango, or avocado you pick up at a grocery store has likely spent time inside a carefully controlled ripening room. Getting fruit to reach consumers at the perfect stage of ripeness is not left to chance – it is managed through ethylene treatment systems. These systems apply ethylene, a naturally occurring plant hormone, to trigger and regulate the ripening of climacteric fruits in enclosed chambers. The two most widely used approaches are the shot system and the trickle (flow-through) system, each with distinct mechanics, advantages, and requirements. Understanding how they work – and what can go wrong – is essential for anyone involved in post-harvest management.

Why ethylene matters in post-harvest ripening 🔗

Ethylene (C₂H₄) is a simple two-carbon gas that acts as a signalling molecule in plants. In climacteric fruits – such as bananas, tomatoes, mangoes, avocados, and papayas – a natural surge in ethylene production triggers a cascade of physiological changes: chlorophyll breaks down, new pigments develop, starches convert to sugars, and cell walls soften. This is what turns a hard, green banana into the soft, yellow fruit we eat.

The problem is that fruits are often harvested before they are fully ripe so they can survive long-distance transport. Once they arrive at distribution centres, ripening must be initiated artificially using ethylene gas in sealed ripening rooms. The USDA’s National Organic Program permits ethylene for post-harvest ripening of tropical fruits and degreening of citrus, confirming its status as a safe and accepted practice in commercial agriculture.

The U.S. FDA also classifies ethylene as Generally Recognized as Safe (GRAS), and at the concentrations used in ripening rooms (typically 10-150 ppm), it poses no direct toxicity risk to humans. The real challenges lie in managing the broader environment inside the ripening chamber – temperature, humidity, CO₂ build-up, oxygen levels, and the risk of gas accumulation.

The shot system: periodic ethylene delivery 🔗

The shot system (also called shot injection) delivers a concentrated dose of ethylene gas into a sealed ripening room at set intervals. The gas is typically administered every six to eight hours to bring ethylene concentration up to the required level, usually around 100 ppm or below. Between doses, the room remains sealed so the fruit absorbs the ethylene and the ripening process advances.

How it works in practice 🔗

A batch of fruit – say, green bananas – is loaded into the ripening chamber. The room is sealed, and an initial ethylene dose is administered, either from a compressed gas cylinder or a catalytic ethylene generator. Temperature is set to the target range (for bananas, typically 15-20°C), and relative humidity is maintained at 90-95%. After the specified interval, the room is vented to release accumulated CO₂ and then another ethylene dose is applied. This cycle repeats throughout the ripening programme, which may last four to six days depending on the fruit type and desired ripeness stage.

Advantages and limitations 🔗

The shot system is well suited for fruits that need to be ripened quickly and uniformly, and it is popular in banana and tomato ripening facilities. It allows operators to control exactly when and how much ethylene is introduced. However, it requires manual intervention at regular intervals – someone needs to open the doors for venting and then reseal and re-dose the room. This hands-on approach can be labour-intensive, and there is always a risk of inconsistent dosing if timing lapses or concentrations are misjudged.

The trickle (flow-through) system: continuous ethylene supply 🔗

The trickle system, also known as the flow-through system, takes a different approach. Instead of periodic high-concentration doses, ethylene is supplied continuously at a low, steady concentration. At the same time, inside air is constantly being released and replaced with fresh air from outside.

How it works in practice 🔗

Two openings are placed on opposite ends of the ripening room. An air intake fan is mounted in one opening, while the other serves as an exhaust. Ethylene is fed into the incoming air stream at a controlled rate. This setup ensures that the atmosphere inside the room is constantly being exchanged – stale, CO₂-rich air exits while fresh, ethylene-enriched air enters. The result is a steady ripening environment without the peaks and troughs associated with periodic dosing.

Advantages and limitations 🔗

The trickle system is particularly effective for fruits that benefit from a more gradual ripening process, such as avocados and kiwis. Because ventilation is continuous, CO₂ never builds up to problematic levels, and oxygen is constantly replenished. This reduces the risk of ripening delays or quality defects caused by excess carbon dioxide. The system is commonly used in large-scale tomato ripening rooms and is increasingly adopted for other commodities. The trade-off is that it requires more infrastructure – dedicated air openings, properly sized fans, and a reliable ethylene delivery mechanism – and uses more ethylene overall since some gas is continuously lost through the exhaust.

Other ethylene delivery methods 🔗

Beyond shot and trickle systems, two additional approaches are worth noting. Catalytic generation uses a device that converts a liquid ethylene concentrate (such as ethanol-based solutions) into ethylene gas through a heated catalytic process. These generators are dosed every six to eight hours, similar to the shot method, and are popular because they eliminate the need to handle pressurised gas cylinders. Controlled monitoring systems represent the most automated option – gas is supplied only when sensors detect that ethylene levels have dropped below the target threshold (typically below 100 ppm). This on-demand approach minimises waste and reduces the risk of overexposure.

Critical environmental parameters 🔗

Regardless of whether you use a shot or trickle system, several environmental factors must be tightly controlled for successful ripening.

Temperature 🔗

Temperature is the single most important variable influencing ripening speed. Higher temperatures accelerate the process; lower temperatures slow it down. Each fruit has an optimal ripening temperature range. For example, bananas are typically ripened at 15-20°C, while avocados with a dry matter content above 26% may need temperatures as low as 16°C. Ripening rooms must have adequate refrigeration capacity to remove the metabolic heat that climacteric fruits generate as they ripen. Trying to speed up the process by raising temperatures beyond the recommended range often results in uneven ripening, poor flavour development, and shortened shelf life.

Relative humidity 🔗

Most fruits ripen best at a relative humidity of 90-95%. If humidity drops too low, fruits lose moisture, leading to weight loss, shrivelling, and poor visual appeal. If it is too high, condensation can form on fruit surfaces and promote fungal growth. Humidifiers can be installed in ripening rooms, but only if the water source is clean – contaminated water can introduce pathogens that cause more losses than dehydration would.

Ethylene concentration 🔗

For most fruits, ethylene levels in the ripening room should be maintained below 100 ppm. For citrus degreening, much lower concentrations of 3-5 ppm are used. Overexposure to ethylene can cause premature senescence, off-flavours, and accelerated decay. Underexposure leads to incomplete or uneven ripening. Gas analysers and dedicated sensors are used to continuously measure and adjust ethylene levels in real time.

The role of ventilation and CO₂ management 🔗

As fruits ripen, they consume oxygen and release carbon dioxide through respiration. If CO₂ is allowed to accumulate in a sealed room, it slows down the ripening process, reduces the effectiveness of applied ethylene, and can cause quality problems. CO₂ concentrations above 1% (10,000 ppm) are known to retard ripening and lead to uneven results.

In the shot system, venting is typically done by opening the room doors for about 20 minutes every 12 hours, starting 24 hours after the first ethylene application. Fans are left running during this period to disperse the CO₂ and draw in fresh air. Many modern facilities have upgraded to automatic venting triggered by CO₂ sensors – when CO₂ reaches a preset threshold, the ventilation system activates without manual intervention.

In the trickle system, ventilation is inherently continuous, which is one of its key advantages. The constant exchange of air keeps CO₂ levels well below the 1% threshold and ensures that oxygen is always available for the fruit’s respiratory processes.

Beyond ripening performance, ventilation is also a worker safety issue. CO₂ concentrations in the range of 3-8% can cause headaches and loss of consciousness in people. No worker should enter a ripening room without confirming that oxygen and CO₂ levels are within safe limits.

Safety considerations in ethylene treatment 🔗

While ethylene is not toxic at the concentrations used in ripening, it is a highly flammable gas. Its lower explosive limit (LEL) is 27,000 ppm (2.7% by volume in air). The typical ripening room operates at around 100-1,000 ppm – well below the danger zone. However, real-world accidents have occurred when gas cylinders were left open or leaked, allowing ethylene to accumulate to explosive concentrations. In at least one documented incident, an ethylene cylinder believed to be empty continued releasing gas into a room, exceeding the LEL and resulting in an explosion when a spark was generated.

Key safety measures 🔗

To minimise risk, facilities should implement the following practices:

Gas detection systems: Install ethylene and CO₂ detectors in every ripening room and in any area where gas cylinders are stored. These systems should be capable of triggering alarms and activating ventilation when concentrations approach unsafe levels.

Proper cylinder storage: Compressed ethylene cylinders should be stored in well-ventilated areas away from ignition sources. Staff must be trained in proper handling procedures, including leak detection and valve management.

Elimination of ignition sources: Ripening rooms should be free of open flames, sparking equipment, and unshielded electrical components. Even the act of opening a metal door can potentially generate a spark if ethylene has accumulated beyond safe levels.

Dual O₂/CO₂ monitoring: Before workers enter a ripening chamber, oxygen and carbon dioxide levels should be checked to confirm a safe breathing environment. Depleted oxygen or elevated CO₂ can cause dizziness, respiratory distress, or unconsciousness.

Staff training: Every person working in or around ripening rooms should understand the properties of ethylene, the function of monitoring equipment, emergency procedures, and the importance of regular venting schedules.

Preventing overexposure and ensuring uniform ripening 🔗

One of the most common problems in ethylene treatment is uneven ripening within a single batch. This typically happens when ethylene gas does not distribute evenly throughout the chamber. Poor air circulation means that fruit near the gas inlet gets more exposure than fruit in far corners.

To prevent this, ripening rooms must have properly sized fans and carefully planned stacking arrangements. Boxes should be positioned to allow air to circulate both around and through the packaging. Many facilities use forced-air systems with pressure walls or baffles to direct airflow uniformly across the entire load. The way pallets are arranged in the room matters just as much as the ethylene delivery method.

Overexposure is the opposite problem – too much ethylene for too long can push fruit past its optimal ripeness stage, leading to mushiness, off-flavours, and increased vulnerability to fungal pathogens. This is why monitoring is not optional. Gas analysers should be calibrated regularly, and ripening protocols should specify exact concentration targets, exposure durations, and venting schedules for each commodity.

Choosing the right system 🔗

The choice between a shot and trickle system depends on several factors: the type of fruit being ripened, the scale of the operation, labour availability, and budget. For small to mid-sized operations handling fruits like bananas, the shot system with catalytic generators is often the most practical and cost-effective solution. For large-scale facilities processing tomatoes or handling mixed loads with longer ripening cycles, the trickle system offers superior environmental control with less manual intervention.

Increasingly, automated monitoring systems that combine real-time gas sensing with programmable ethylene delivery are becoming the standard in modern ripening facilities. These systems allow operators to pre-set ripening “recipes” – customised programmes for each fruit type that automatically manage ethylene concentration, temperature, humidity, and ventilation throughout the cycle.

What do you think? If you were managing a ripening facility handling multiple fruit types, would you invest in a single flexible system or dedicate separate rooms with specialised setups for each commodity? And how important do you think automation will become in replacing manual ripening protocols in the coming years?

References

1. https://felixinstruments.com/blog/the-ripening-room-resource/
2. https://www.ams.usda.gov/sites/default/files/media/2023Technical_Report_Ethylene_Handling.pdf
3. https://www.catalyticgenerators.com/faqs
4. https://www.catalyticgenerators.com/blog/venting-rooms-an-overlooked-necessity
5. https://www.inspirafarms.com/ask-expert-important-elements-ripening-program/
6. https://sensorsandtransmitters.com/sensors-for-ripening-climacteric-fruits-in-a-ripening-chamber/
7. https://www.critical-environment.com/applications/ripening-rooms
8. https://www.catalyticgenerators.com/ethylene-dangers
9. https://www.critical-environment.com/news/cet-blog/expert-insights/monitoring-ethylene-c2h4-and-carbon-dioxide-co2-in-ripening-rooms
10. https://www.pureairemonitoring.com/from-farm-to-market-fruit-ripening/