Factors Affecting Internal Browning in Apples During Storage

Advancements in controlled atmosphere (CA) storage technology and the advent of postharvest treatments with 1‑methylcyclopropene (1‑MCP) have enabled apples to be stored longer than ever before.  Unfortunately keeping apples longer has also allowed for certain storage disorders to become more prevalent.  Internal browning is one such problem for many apples.

Radial flesh browning near the stem-end (shoulder) of the apple is usually the first sign of internal browning in ‘Gala’ apples (Figure 1). This can progress towards the calyx end of the fruit, becoming the more common internal browning that can found be when cutting the apple horizontally across the equator.  Full internal browning develops more quickly in other apples, such as ‘Ambrosia’ (Figure 2).

High incidence of internal browning is typically found when apples are harvested late, or at advanced maturity stages.  Fruit maturity at harvest time is a major factor affecting this disorder and it is something that can be easily corrected.  Our research has shown a main effect of harvest time, across several years, with much higher incidence of internal browning in fruit from later harvests (4+ on Cornell starch chart). 

For example, ‘Ambrosia’ apples from two orchards were harvested twice, 6 days apart. Apples were cooled overnight to 0.5^oC and treated with 1‑MCP (SmartFresh^TM) the following day. CA storage was established 4 days after harvest, as 1.7% O₂ + 1.2% CO₂ at 0.5^oC. Apples were evaluated for quality after 8 months of storage, plus during 14 days at room temperature. ‘Ambrosia’ from the first harvest had significantly less internal browning, compared to those from the later harvest (31 vs 63%, respectively). This is very important to note, as harvesting only 6 days later resulted in over double the amount of browning.  Similarly, ‘Ambrosia’ apples were stored in ambient air storage at 0.5^oC for 4 months and fruit from the first harvest had significantly less browning than those from the later harvest (8 vs 37%, respectively).

Preharvest 1-MCP (Harvista^TM orchard spray) can be very effective at reducing internal browning, as it delays advanced maturity (ripening) at harvest time.  Rate and timing of application are critical for its efficacy.  In contrast, postharvest 1-MCP treatment does not have consistent effects on internal browning and has been shown to decrease, increase, or have no effect on browning. 

Ultra-low oxygen storage or dynamic CA with less than 1% oxygen reduces the development of internal browning.  ‘Gala’ held in oxygen as low as 0.4% using SafePod technology to monitor fruit respiration (Storage Control Systems Inc., Michigan) had zero internal or stem-end browning, compared to 17% incidence of browning in 2.5% O2 + 2% CO2 and 6% incidence in 1.5% O2 + 1.1% CO2.  Similarly, ‘Ambrosia’ held in oxygen as low as 0.7% using SafePod technology had <1% internal browning, compared to 20% incidence in 2.5% O2 + 2% CO2 and 8% incidence in 1.7% O2 + 1.2% CO2. 

Incidence of internal browning increases with longer storage durations.  For example, ‘Gala’ stored in air at 0.5^oC had no internal browning after 2 months, 12% after 4 months, and 18% after 6 months.  Similarly, ‘Gala’ held in CA of 2.5% O₂ + 2.0% CO₂ at ~1^oC had little internal browning through 5 months of storage, but then the disorder increased significantly with storage time thereafter (up to 59% after 9 months).  Furthermore, internal browning increased with subsequent holding at room temperature.

Slow cooling tends to reduce the development of internal browning.  Rapid cooling to 3^oC within 24 hours after harvest resulted in more internal browning in ‘Gala’ apples, compared to slow cooling over 7 days (DeEll, The Grower, May 2017).  ‘Gala’ cooled rapidly and stored in CA for 8 months at 1^oC had up to 59% browning incidence, compared to zero to 11% in fruit cooled slowly.  In another study, ‘Ambrosia’ apples were held at 10^oC for 1 week after harvest or cooled immediately to 0.5^oC. After 7 months of CA storage, internal browning was significantly less in apples held at 10^oC initially, with 1.5% incidence compared to 12.6% in those cooled immediately to 0.5^oC. There were no significant differences in fruit firmness.

Overall, it is important to remember that there are many factors that influence the development of internal browning in apples.  These can range from orchard management, fruit maturity at harvest time, postharvest treatments, to storage conditions and duration.   

Acknowledgements:

Thanks to the Ontario Apple Growers, Norfolk Fruit Growers’ Association, Apple Marketers’ Association of Ontario, AgroFresh Inc., Pommes Philip Cassidy Inc., GRB Ag. Technologies Inc., and Storage Control Systems Inc., for their continuous support; as well as Sky Lesage and Younes Mostofi for their technical assistance.