Ethylene gas is produced by apples during the process of ripening either on the tree or during storage. Ethylene, the ‘ripening’ hormone, triggers and co-ordinates many of the ripening changes that occur in stored apples such as softening and aroma development.
Storage under refrigeration and in controlled atmospheres reduces the rate at which these ripening changes take place. More effective control of ripening and a consequent improvement in fruit quality may be expected by further suppression of ethylene production by the fruit.
Ethylene removal is generally only worthwhile with unripe fruit therefore apples need to be harvested pre-climacteric i.e. before ethylene production rate of fruit on the tree increases.
Ideally all fruits should contain less than 0.1 ml l-1 of ethylene. It is easier to meet these requirements for Bramley than for Cox (or other dessert varieties). With the latter, picking fruit that is too immature may affect adversely its eating quality.
Bramley
The use of an ethylene converter is sufficient to control ethylene production in CA-stored Bramley apples without additional measures to inhibit production of ethylene. This is related to the immature state of the fruit at commercial harvest when compared with Cox and other dessert cultivars rather than an innately low production rate. Additionally the high concentration of CO2 (5-10%) used for the storage of Bramley suppresses ethylene production.
Beneficial effects of ethylene removal from Bramley stores include delayed development of superficial scald, firmness retention and reduced susceptibility to core flush.
Ethylene scrubbing has no effect on chlorophyll loss (greenness) and does not affect acidity or sugar content of the fruit. There were no adverse effects on fruit quality resulting from the removal of ethylene from commercial stores of Bramley.
However, in other trials ethylene removal increased susceptibility of fruit to external CO2 injury. Consequently it is important to delay the establishment of CA conditions.
Requirements for effective ethylene scrubbing
- To achieve scald control in Bramley apples, maintain an ethylene concentration of less than 1 ml l-1 and ideally less than 0.1 ml l-1in the storage atmosphere for most of the storage period.
- Ethylene removal systems for Bramley should be capable of maintaining 1ml l-1 of ethylene in the storage atmosphere at an assumed rate of ethylene production of 1ml kg-1 h-1.
- Fruit should not be late picked otherwise the ability to control ethylene production will be compromised. This may not be evident from measurements of ethylene concentration in the store atmosphere. Once the rapid phase of ethylene production is underway then removal of ethylene has little effect in slowing down ripening processes.
- The removal of ethylene from the storage atmosphere using heated catalyst scrubbers is an alternative to the use of SmartFresh for controlling superficial scald.
- Growers can achieve control of scald for 6 months by attaching an ethylene scrubber to storage chambers where the carbon dioxide level is maintained at 8‑10% by ventilation with outside air.
- Longer storage without scald development is possible where ethylene scrubbing is combined with low oxygen storage (5% CO2 + 1%O2).
- In commercial trials no scald was evident in fruit stored in 5% CO2 + 1%O2 with continuous removal of ethylene when the trials were concluded after 10 months.
- Despite the effectiveness of ethylene scrubbing in controlling scald the use of heated catalyst systems may not be justified due to the high capital and running costs.
- Currently there are a number of alternative ethylene removal systems being tested on UK stores. These include low temperature catalyst systems, ozone generators and ethylene absorbents.
- Whilst growers will be interested in comparing the costs of these various ethylene removal technologies it is the ethylene removal efficiency that is of paramount importance.
- There is a need to compare the efficacy of current ethylene control technologies (chemical absorption, destruction by ozone or by a low temperature (photo-induced) catalyst) to enable growers to adopt the most appropriate method for their particular situation.
Cox’s Orange Pippin
Early harvesting plus effective ethylene removal can only achieve a delay in the onset of rapid production of ethylene but this can produce worthwhile benefits in the quality of fruit from store. However ethylene scrubbing without additional treatments to inhibit ethylene production will not slow the rate of softening during storage.
Inhibition of ethylene can be achieved by subjecting fruit to 15% CO2 for 15 days prior to CA storage although this was found to induce the development of low temperature breakdown in the fruit late in the storage period. Subsequently 5% CO2 for 15 days was found to reduce the risk of storage problems without affecting the beneficial response to ethylene removal.
The main benefit of ethylene scrubbing of Cox stores is reduced softening of the fruit although greater retention of acidity is also likely. There have been no effects of ethylene removal on the change in background colour.
- Although there are major quality benefits that can be achieved in Cox apples by removing the ethylene for CA stores the effects are inconsistent from year to year and between consignments.
- Ethylene production in Cox apples is higher than in Bramleys due to the fact that they are harvested closer to the point of ripening (initiation of rapid ethylene production) on the tree.
- For Cox stores the size of scrubber that is required and consequently the investment and running costs are increased by a factor of about 4 when compared to Bramley stores.
- Additional ‘conditioning’ techniques are required to reduce ethylene production in order to make ethylene removal effective which adds to the complexity of managing the crop during the early period of storage.
- Some ‘conditioning’ treatments such as pre-treatment with high levels of carbon dioxide may have deleterious effects on the fruit late in the storage period.
- Further developments in ethylene removal technology may improve the efficacy or cost-effectiveness of ethylene removal for Cox and may make it viable commercially
Gala
Most of the work on ethylene removal at East Malling was done on Cox and Bramley apples. There are recent reports from Brazil that removal of ethylene from CA stores (3% CO2, 1% O2 at 1oC) containing Gala apples improves the firmness of fruits stored for up to 8.5 months. Low ethylene also maintained the appearance, background colour, crispness, juiciness and taste of stored fruit.