Edibles storage - How to keep fresh

Storage technologies have improved the quality of apples on the supermarket shelf but more research is needed, says Brian Lovelidge.

Eating quality improves after treatment with Smartfresh. Image: Brian Lovelidge
Eating quality improves after treatment with Smartfresh. Image: Brian Lovelidge

Considerable developments in apple storage in recent years have led to markedly improved quality of English fruit being available to consumers over a longer period. This particularly applies to two of our three main varieties - Cox and Bramley's Seedling.

Worldwide Fruit Qualytech storage specialist Dr Martin Luton believes that the now widespread use of the ethylene inhibitor Smartfresh on Cox (it is also widely used on Bramleys) "could be the saviour" of the variety. Furthermore, combined with the almost universal use of the five per cent carbon dioxide, one per cent oxygen regime for supermarket supplies - known as five and one - it has "revolutionised" Bramley storage.

"Looking back over the past five years or so, storage has changed (improved) dramatically," he maintains. "It means that for Bramleys, the bitter pit is almost a thing of the past and good scald control is possible without the use of a post-harvest drench (diphenylamine, or DPA, now withdrawn).

"Smartfresh on Cox has greatly improved its firmness," adds Luton. "Its average pressure (in late January) was 7.5kg, which is unheard of. The firmness of untreated Cox was pretty marginal (for supermarket sale) at 6-6.5kg."

However, he reckons that the product is not of much benefit for Gala, the UK's third main variety, because when it is stored in five and one at 1 -1.5 degC it will retain good texture and flavour until early April. As a result, very few Gala stores have been treated.

There are potential drawbacks to using Smartfresh on Cox, though. This includes the increased risk of core flush if the fruit is stored later than January and the browning of stalk end russet in early picked fruit. But neither condition has yet proved a problem.

But Luton warns that unexpected problems could arise with Smartfresh treatment in the long-term, in similar way to when the widespread use of triazole sprays caused boggy bank, or diffuse browning disorder, in Cox. Research is needed to identify possible disorders induced by Smartfresh so that ways can be found to prevent them arising, he says.

Norman Collett technical director Nigel Jenner is even more upbeat about the advantages of using Smartfresh on Cox. He reckons that if the resulting "massive improvement" in the variety's edibility over the past year or two is sustained it might even lead to a turnaround in its demand - something that has been declining, along with its orchard area, for the past 10 years or more.

Collett's growers have been treating all of their Cox and Bramleys stored beyond early November with Smartfresh. Jenner's monthly monitoring of stored fruit is demonstrating just how much its firmness has improved since pre-Smartfresh days. In late January, for example, the variety's pressure readings were 7-7.5kg, while those for fruit destined for March sale were over 8kg - 1-2kg higher than before and well within supermarkets' 6kg threshold.

"The Cox we're marketing now is (still) crisp and crunchy and that's what consumers want," Jenner claims. "The fruit I've eaten has also had excellent flavour and a robust shelf life. We will go with it into April because we always try to meet up with the New Zealand crop.

"Our marketing team has actually been asking me where the next store is rather than me saying 'come on chaps, we've got to move these stores within the next month' because the fruit is getting too soft," he adds.

Further Horticultural Development Company (HDC)-funded work is beginning this autumn to address other pressing storage problems including Bramley's propensity to carbon dioxide injury in five and one storage. To avoid this, growers have been delaying establishing five and one conditions for Smartfresh-treated fruit for three weeks.

However, this procedure has the disadvantage of slowing establishment of the five and one regime. Many growers try to get round the shortcoming by sealing their stores immediately after filling followed by carbon dioxide scrubbing, allowing the oxygen level to drop to 10 per cent for 21 days and then creating the five and one regime.

But Natural Resources Institute project leader Dr Richard Colgan states that an optimum carbon dioxide injury prevention strategy has yet to be found. The HDC project will seek to develop one while maintaining the fruit's nice, green background colour, good firmness and other benefits of five and one long-term storage.

Another new line of work beginning this autumn is aimed at overcoming the frequent failure of Smartfresh-treated Conference pears to ripen properly after removal from storage. Earlier trials have shown that by adjusting the storage temperature or, in the case of long-term storage, reducing the Smartfresh rate, the fruit retains its ripening ability.

The project will determine the affect on ripening of storing Conference long-term at higher than the usual temperature of between -1 degC and -0.1 degC, and exposure of the fruit to low levels of ethylene in conjunction with Smartfresh treatment.


Further advances are on the cards thanks to Horticultural Development Company (HDC)-funded work being carried out by East Malling Research (EMR) and Natural Resources Institute (NRI) scientists. The project is aimed at reducing the amount of in-store Nectria rotting in Bramley's Seedling and Cox and enhancing the quality of Cox coming out of store after Christmas.

- Experimenting with storage temperatures

The researchers involved - the NRI's Dr Debbie Rees and EMR's Dr Angela Berrie - explain that the variety's recommended storage temperature of 3.5-4 degsC has remained unchanged for 40 years or so. But this fails to curb softening and rotting sufficiently in fruit stored beyond Christmas.

To try to tackle these problems, fruit was subjected to eight different 180-day storage regimes comprising 1.2 per cent oxygen and less than one per cent carbon dioxide and one or two temperatures - 3.5-4 degsC and 1.5-2 degsC. The research included 180 days at the higher or lower temperature and various 60-day and 120-day permutations of both. The affect of Smartfresh on the fruit's texture and rotting incidence was monitored.

The researchers point out, however, that while reducing the storage temperature helped improve firmness, it could also lead to the development of low temperature breakdown (LTB). This condition occurred in five per cent of samples, whether treated with Smartfresh or not, after 120 days (until January) of storage at 1.5-2 degsC.

On the positive side, the same fruit without Smartfresh treatment was approximately 1kg firmer than that stored at 3.5-4 degsC. But samples treated with Smartfresh were 2kg firmer than the non-treated ones and this reading was not affected by store temperature.

Rees and Berrie also discovered that the storage of Cox at 1.5-2 degsC for the entire 180 days (until March) resulted in much higher levels of LTB and it was worse in the Smartfresh fruit. Although treatment reduced the rate of fruit softening, the high LTB incidence ruled out continuous 1.5-2 degsC storage beyond January for the project's next stage. The same applied to the 60-day low temperature and 60-day normal temperature regime because it didn't prevent significant LTB incidence.

Storage at 1.5-2 degsC for the whole 180 days proved a case of swings and roundabouts because - despite its much greater LTB risk and improved firmness - the fruit had the lowest level of rots. Smartfresh treatment did not significantly affect the incidence of rots even when the fruit was stored at 3.5-4 degsC for 180 days.

In the project's second year (2010-2011), the 180-day low temperature regime and some of the low/high temperature permutations were dropped. Instead, five two-stage storage regimes were used and the fruit was picked on three dates - 7, 14 and 21 September - to see what affect these had on firmness and rotting.

- Ethylene scrubbing

The potentially serious problem of Nectria rotting in long-term stored Bramley's Seedling sometimes causes serious loss of income. It has been exacerbated by the five and one storage regime, which has become widely used due to its good control of bitter pit and scald and general improvement in fruit quality.

But, as Berrie maintains, it is difficult to grow the variety without latent infection of the fruit because the fungus is so common in orchards in the form of lesions on the wood that are almost impossible to totally eliminate.

That is why it is so important to discover how best to reduce the level of in-store rotting - the aim of the HDC-funded project. The research involves assessing the effect of reducing the concentration of ethylene within the fruit or in the store itself because ethylene is thought to influence rot development.

Smartfresh treatment is being used to depress the fruit's ethylene production and catalytic or potassium permanganate scrubbers are being used to remove the gas from the stores. Some of the fruit has been inoculated with Nectria spores to help ensure valid results. The work is being carried out in commercial stores and special controlled atmosphere cabinets at EMR.

In the project's first year (2009-10) ethylene scrubbing by both methods proved very effective, reducing ethylene concentrations to below 100ppb. There was no difference in the incidence of rots between the ethylene-scrubbed and Smartfresh-treated stores, but in the inoculated fruit they were reduced slightly more by ethylene scrubbing than by Smartfresh treatment.

Rees says that the latter difference was not unexpected because ethylene is involved in activating the fruit's natural defence system against pathogens.

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