Effects of CA Storage, SmartFresh (1-MCP) and Cooling Delays on 'Honeycrisp' Apples

The following is a brief summary of one experiment from the 2-year project Optimum Postharvest Handling and Storage of 'Honeycrisp' Apples, which is supported by the Ontario Apple Growers through the CORD funding program and the Agricultural Adaptation Council. 

'Honeycrisp' apples were harvested three times (September 11, 15 and 25^th) from a commercial orchard in Norfolk County, Ontario. Each harvest consisted of 27 boxes taken randomly from several 7-year-old trees on M7 rootstocks. Three boxes (reps) from each harvest were handled and stored as follows: 

1. held for 5 days at 3^oC and then placed into CA-1 (2.5% O₂ + 2.5% CO₂)
2. held for 5 days at 3^oC and then placed into CA‑2 (1.7% O₂ + 1% CO₂)
3. held for 5 days at 10^oC and then placed into CA-1 (2.5% O₂ + 2.5% CO₂)
4. held for 5 days at 10^oC and then placed into CA‑2 (1.7% O₂ + 1% CO₂)
5. treated with 1-MCP at 3^oC and stored in air at 3^oC
6. no 1-MCP and stored in air at 3^oC
7. held for 5 days at 10^oC, then treated with 1-MCP at 3^oC and stored in air at 3^oC
8. held for 5 days at 10^oC, no 1-MCP and stored in air at 3^oC
9. treated with 1-MCP at 10^oC, held for 5 days at 10^oC, and stored in air at 3^oC

All 1-MCP (SmartFresh^TM) treatments consisted of 1 ppm for 24 hours. Fruit were held for 4 and 6 months in either controlled atmosphere (CA) or air storage as noted above.

Table 1 - Maturity of 'Honeycrisp' apples at harvest

* Each value is the average of 10 fruit
IEC = internal ethylene concentration, SSC = soluble solids concentration 

Overall, 'Honeycrisp' apples stored in CA tended to be slightly firmer (1-2 lb) and have lower internal ethylene (IEC) and higher soluble solids concentration (SSC) than those stored in air. However, upon removal to room temperature, IEC of the CA-stored apples quickly increased and became similar or higher than the levels found in fruit from air storage. CA-stored fruit did not appear to soften much during storage nor during the post-storage 7-day ripening period at 22^oC. However, in air-stored apples there was some flesh softening during storage and ripening, especially in fruit from harvest 3. There was little difference between apples from the two CA regimes.

Bitter pit and lenticel breakdown were found in CA-stored fruit, with higher incidences in apples from the first harvest (up to 36 and 14%, respectively). Fruit that did not receive a cooling delay of 5 days at 10^oC tended to have less lenticel breakdown than those subjected to the cooling delay. Some external CO₂ injury developed in CA-stored 'Honeycrisp' from the first harvest (up to 13%), but this was likely due to an unstable CA regime during the first week of storage. It is worth noting that the incidence was highest in apples that did not receive a cooling delay and were held in CA with 2.5% CO₂. Soft scald developed on a few apples in CA, but there was no apparent association with any specific treatment.

Some low temperature breakdown developed in apples stored in CA and this was more prominent in fruit from the first harvest (up to 23%). However, it is important to note again that the CA regime was not stable for fruit from the first harvest for the first week of storage. There appeared to be greater incidence of low temperature breakdown in apples that did not receive a cooling delay of 5 days at 10^oC, but this needs to be investigated further with more steady CA regimes.

In general, 'Honeycrisp' treated postharvest with gaseous SmartFresh (1-MCP) had lower IEC than those not treated. However, this effect was less evident with more time in storage and in later harvests. The lowest levels of IEC were found in fruit treated with SmartFresh and not subjected to a cooling delay (<2 ppm). There was little difference in fruit firmness and SSC among apples from the various treatments.

Bitter pit also developed in air-stored fruit, with the lowest incidence in fruit treated with SmartFresh and not subjected to a cooling delay. Lenticel breakdown occurred in a few apples held in air storage, but the incidence was much lower compared to CA-stored fruit.

Core breakdown developed in air-stored 'Honeycrisp' and there tended to be more in SmartFresh-treated apples with no cooling delay (up to 47%). There was no core breakdown in CA-stored fruit, whereas there was no low temperature breakdown in air-stored apples. These types of disorders need to be investigated further before any conclusions can be made.