What is the Correct Carbon Dioxide Concentration?

Or watch Dr Robert Prange’s carbon dioxide presentation on YouTube here:

Senior Research Scientist (Retired), Agriculture and Agri-Food Canada

Introduction

There have been no recent breakthroughs in reliable methods to determine the acceptable CO₂ concentration, so one must rely on an indirect approach. This presentation examines the utility of selecting the CO₂ concentration in Controlled Atmosphere (CA) storage as a proportion of the recommended O₂, i.e., using a CO₂/O₂ ratio.

CO₂ Damage

Too much CO₂ can cause both external and internal damage:

• External CO₂ damage: e.g., Golden Delicious (R. Prange)
• Internal CO₂ damage: (Rudell, 2023)

Factors Affecting CO₂ Damage

• Cultivar
• Oxygen concentration
• Stage in storage season
• Storage temperature
• Storage humidity
• Use of post-harvest chemicals (1-MCP, DPA)

The CO₂/O₂ Ratio Concept

The CO₂/O₂ ratio is accepted in a general sense but is not well-defined.

“In CO₂-intolerant cultivars, the CO₂ should remain well below the O₂ level at all times.”
— Kupferman, 2003

CO₂/O₂ Ratio Categories

Using a postharvest cultivar database, 7 CO₂/O₂ ratio categories were created:

1. 1.5 or more (Very high CO₂ tolerance)
2. 1.25 to 1.49 (Moderately high CO₂ tolerance)
3. 1.00 to 1.24 (Slightly high CO₂ tolerance)
4. 0.75 to 0.99 (Medium CO₂ tolerance)
5. 0.50 to 0.74 (Slightly low CO₂ tolerance)
6. 0.25 to 0.49 (Moderately low CO₂ tolerance)
7. 0 to 0.24 (Very low CO₂ tolerance)

Mean CO₂/O₂ Ratios for Major Apple Cultivars

1) Very High CO₂ Tolerance (≥1.5)

2) Moderately High CO₂ Tolerance (1.25–1.49)

4) Medium CO₂ Tolerance (0.75–0.99)

5) Slightly Low CO₂ Tolerance (0.50–0.74)

Mean CO₂/O₂ Ratios for Major Pear Cultivars

• Group 3 (1.00–1.24): Packham’s Triumph – 1.03 (17)
• Group 4 (0.75–0.99): Forelle – 0.83 (9), Beurré Bosc – 0.70 (8)
• Group 5 (0.50–0.74): Williams (Bartlett) – 0.73 (13), Doyenné du Comice* – 0.71 (8), Beurré D’Anjou – 0.51 (2)
• Group 6 (0.25–0.49): Conference – 0.38 (11), Abaté Fetel – 0.25 (4)

Additional Observations

• Increasing CO₂/O₂ ratio with declining O₂ appears mainly below 1.0–2.0 kPa O₂ (e.g., Gala).
• High CO₂ at the beginning of storage is more damaging than later in storage.

Examples of Initial Low CO₂ Before Increasing for Long-Term CA

Temperature and Humidity Effects

• Declining CO₂/O₂ ratio with chilling temperature occurs mainly in high CO₂-tolerant cultivars (e.g., Golden Delicious).
• Lowering temperature increases the respiration quotient (RQ) of apples.
• Lowering storage humidity can reduce CO₂-related disorders.

Postharvest Chemicals

• 1-MCP: The CO₂/O₂ ratio in CA is lower if 1-MCP is used.
• DPA: Can reduce CO₂ damage; still allowed in some countries.

Example: Empire (US)

• Not DPA-treated: 1.5–2.0% CO₂ for first 30 days
• DPA-treated: 2–3% CO₂ from day 0 in storage

Example: McIntosh – First 6 Weeks

• CA alone: 2.5% CO₂ (US), 2.0% CO₂ (Canada)
• +1-MCP: 0.5% CO₂ (US and Canada)
• +1-MCP + DPA: 2.5% CO₂ (US), 2.0% CO₂ (Canada)

CO₂ Final Thoughts

• If no history of CO₂ damage, do not consider changes.
• If there is a history of CO₂ damage, consider:
  • Scrubbing CO₂ during loading and cooling (e.g., activated carbon, lime)
  • Scrubbing CO₂ during O₂ pull-down
  • Controlling CO₂ levels according to the CO₂/O₂ ratio for a specific cultivar
  • Scrubbing CO₂ for the first few weeks of CA (e.g., 4 weeks)
  • Note: Cultivars that tolerate high CO₂/O₂ ratios may lose tolerance at chilling temperatures (<1.0°C)
  • In DCA (<1 kPa O₂), the CO₂/O₂ ratio can be higher
  • If 1-MCP is applied at the beginning of storage, reduce CO₂ by ~50%
  • If DPA is allowed, it may reduce risk of CO₂ damage

Key Risk Factors

1. Beginning of storage
2. Cooling too quickly
3. Sudden increase in CO₂/O₂

References

• Controlled Atmosphere Storage of Apples and Pears, R.K. Prange (ISBN 978-1-7776828-0-4)
• Rudell, 2023
• Köpcke, 2024
• Fidler and North, 1967
• De Wild, 2021

For more information, contact rkprange@gmail.com or see the digital version on Amazon Kindle.