CO₂ in Apple and Pear Storage

Robert Prange
Adjunct Professor, Faculty of Agriculture, Dalhousie University, Canada
South Africa Hortgro CA Webinar, 3 November 2020

Acknowledgement: Dr. Barbara Daniels-Lake for editorial and graphical assistance.

Introduction

There have been no recent breakthroughs in reliable methods to determine the acceptable CO₂ concentration, so one must rely on an indirect approach to choose the correct CO₂ concentration.

This presentation examines the utility of selecting the CO₂ concentration in controlled atmosphere (CA) storage as a proportion of the recommended O₂ (as defined by global use), i.e., using a CO₂/O₂ ratio.

Why Discuss CO₂/O₂ Ratio?

• Very fast cooling to <1°C can cause a big increase in the CO₂/O₂ ratio.

Example Table: StoreRoom With and Without Disorder

CO₂ Damage on Golden Delicious

Last summary of CA storage recommendations for apples and pears was at the 8th International CA Conference in 2001:
Kupferman, E. 2003. Controlled Atmosphere Storage of Apples and Pears. Acta Horticulturae 600: 729-735.

Concept of CO₂/O₂ Ratio

• The concept is generally accepted but not well-defined.
• Example (Kupferman, 2003):
  “In CO₂-intolerant cultivars, the CO₂ should remain well below the O₂ level at all times.”

Cultivar Tolerance (Kupferman, 2003):

• CO₂ tolerant: Gala, Golden Delicious
• Somewhat CO₂ tolerant: Red Delicious
• CO₂ intolerant: Fuji, Braeburn, Rome

Other factors affecting CO₂ tolerance:

• Storage temperature
• Time
• O₂ concentration
• 1-MCP
• Temperature: Chilling injury (<1°C) increases CO₂-associated disorders.
• Time: More low-temperature-induced CO₂ (high CO₂/O₂) damage at the beginning of storage.
• O₂ concentration: Lowering O₂ increases risk of CO₂ damage; CO₂ must also be lowered, maintaining a specific CO₂/O₂ ratio.
• 1-MCP: Can increase CO₂-induced disorders in CA.

Example: Drop in Recommended CO₂ with Drop in O₂ (Bramley’s Seedling, UK)

• DCA (typically <1% O₂): Further proportional lowering of CO₂ concentrations is assumed but not extensively studied.

Methodology

• Database compiled from published and unpublished sources (2000–2020), including Kupferman (2003).
• 116 apple and 41 European/Asian pear cultivars from ~16 growing regions globally.
• Each recommendation included:
  Cultivar, country/region, year, optimum refrigerated air (RA) storage temperature, optimum CA storage temperature, O₂ concentration, CO₂ concentration, storage duration, notes.

Results and Discussion

• For each cultivar:
  • Determined mean O₂ and CO₂ recommendations.
  • Calculated CO₂/O₂ ratio using mean CO₂/mean O₂ for apples and pears.
  • Cultivars arranged from highest to lowest CO₂/O₂ ratio, divided into 7 categories:

CO₂/O₂ Ratio Categories

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

CO₂/O₂ Ratios for Major Apple Cultivars in South Africa

1) CO₂/O₂ ratio = 1.5 or more (Very high CO₂ tolerance)

2) CO₂/O₂ ratio = 1.25–1.49 (Moderately high CO₂ tolerance)

These are global records.

4) CO₂/O₂ ratio = 0.75–0.99 (Medium CO₂ tolerance)

5) CO₂/O₂ ratio = 0.50–0.74 (Slightly low CO₂ tolerance)