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Maturity and Storage of "Ambrosia" Apples

The past year was the final season of a 3-year collaborative project that focused on the maturity and storage of 'Ambrosia' apples. This project was supported by the Ontario apple industry as well as British Columbia. The following is a brief summary of the 2008-09 results and some final project conclusions.

'Ambrosia' produces very little ethylene during maturation on the tree. As starch is degraded and color changes, there continues to be only trace amounts of internal ethylene present (<1 ppm). This makes internal ethylene a difficult indicator of fruit maturity for 'Ambrosia' (but at the same time makes 'Ambrosia' an ideal candidate for SmartFresh (1-MCP) treatment). Overall, the starch index has been the most consistent indicator of 'Ambrosia' fruit maturity, with starch values of 2.5 - 4.0 optimal for fruit storage. It is important not to wait until the background color is yellow to start harvest, as the fruit will be over-mature for storage. Background color has more to do with nitrogen levels in the fruit than maturity and background color usually changes in storage.

Optimum harvest of 'Ambrosia' (from Norfolk County, ON) for storage in 2008 occurred within the last week of September. At this time, fruit firmness was ~17-19 lb, starch index ~2 to 4, and ground color yellow 2 to 3 on the BC color chart. Apples harvested later than this were softer and developed significantly more disorders and greasiness during storage.

'Ambrosia' apples treated with 1-MCP were firmer and had less internal ethylene than those not treated after 3 months of air storage at 0oC. In addition, 1-MCP-treated fruit had higher malic acid content and reduced greasiness after an additional 7 days at 21oC. Controlled atmosphere (CA) storage also improved fruit firmness and acidity retention, with no greasiness development.

Soft scald and hard skin bronzing developed in 'Ambrosia' during storage (Photo 1). However, DPA dramatically reduced the incidence of these two disorders. Soft scald was also reduced by warmer storage temperatures (3oC vs. 0oC) and CA with ultra-low O2 (0.7% O2 + 1.0% CO2). Otherwise there was no consistent pattern of soft scald development among various CA regimes and 1-MCP treatment. Storing 'Ambrosia' in CA with no CO2 did not reduce the incidence of hard skin bronzing, suggesting it is not a true type of external CO2 injury. Higher incidence of storage rots tended to develop in regimes with more soft scald and/or hard skin bronzing. Therefore, it is likely that many of the rots were due to secondary infections into damaged skin tissue.

Some vascular breakdown, internal browning, and senescent breakdown developed in fruit held in air storage at 0.5oC for 3 months this past season, but the latter two disorders only appeared after an additional 7 days at 21oC. No vascular or senescent breakdown developed in CA-stored apples, but large amounts of internal browning were observed after 6 months. The 2008-09 storage season was a high risk year for chilling-related disorders, as noted in the Orchard Network Newsletter in Dec. 2008 (DeEll, p. 3).

General Conclusions

• Starch index around 2 to 4 consistently related to optimum harvest maturity.
• IEC was not a good indicator of fruit maturity and determining optimum harvest time.
• Later harvested fruit developed more storage disorders and rots, and softened faster.
• Ambrosia' is a low producer of ethylene, making it very responsive to 1-MCP treatment.
• CA storage was beneficial for maintaining 'Ambrosia' fruit quality.
• 1-MCP treatment delayed fruit ripening
• Ambrosia' is susceptible to soft scald, but incidence can vary with orchard.
• Air storage at 3oC vs. 0oC reduced soft scald incidence.
• Hard skin bronzing may develop and this is not a type of external CO2 injury.
• DPA drench substantially reduced soft scald incidence and hard skin bronzing.

Photo 1: Soft scald with secondary rot (left) and hard skin bronzing (right) in "Ambrosia'