Introduction

The rate of ripening and senescence in apples is retarded primarily by the use of refrigeration and controlled atmosphere (CA) storage. Growers are familiar with the benefits of these techniques in terms of season extension and for maintaining the visual and eating quality of apples, particularly those harvested mid- to late season.

However, control of post-harvest temperature has marked benefits in maintaining the quality of early season apples during distribution and marketing. This is particularly important for cultivars such as Discovery that ripen rapidly and have an inherent short shelf-life and are harvested when high ambient temperatures normally prevail.

Control of post-storage temperature for the main storage cultivars is just as important as for the marketing of early apples. Months of effort in maintaining appropriate storage conditions can be wasted by lack of consideration of the time and temperature that the apples experience after removal from CA and low temperatures.

As apples ripen slowly during storage there will be a corresponding reduction in the expected shelf-life i.e. the time to reach an unacceptable quality. Research has shown that the loss of quality after storage is predictable and this allows a logistics approach to ensure that consumers are provided with consistent quality.

Use of refrigeration

Discovery and other early season apples

Control of post-harvest temperature is just one part of a package of measures that have been devised to regulate the supply of fruit to the market and to ensure that produce reaches the consumer with minimal loss of quality.

• It is necessary to harvest fruit at the correct stage of maturity and to ensure rapid removal of field heat.
• To obtain significant benefits fruit temperature must continue to be controlled during distribution of the fruit to the supermarket depot or wholesale market.
• The rate of softening can be more than halved by holding Discovery apples at 10^oC rather than 20^oC throughout a 10-day shelf-life period.

Cox’s Orange Pippin

• There was a lot of research done on the effects of post-storage temperature on Cox in order to address the problem of inconsistent eating quality in consignments of apples reaching the market.
• In particular the texture of the fruit was often inadequate to satisfy consumers with a known preference for firm, juicy apples.
• As in the case of Discovery apples, quality loss in Cox at 10^oC was approximately half that at 18^oC (Table 29). Quality was measured in terms of weight loss, background colour and firmness.
• A more complete account of the affects of temperature on post-storage quality changes in Cox is provided elsewhere.

 

Table 29 – Average losses (per day) in Cox quality after removal from CA storage

 

Temp. (oC)	Weight Loss (%)	Greenness (Worldwide fruit chart units)	Firmness (N) (kg in brackets)	Days from 6.5 kg to 6 kg firmness (projected)
3.5	0.02	0.02	0.28 (0.03)	18
10	0.13	0.05	0.45 (0.05)	11
18	0.31	0.10	0.71 (0.07)	7

 

Use of modified atmosphere (MA) storage

Packaging of apples in semi-permeable films has the effect of modifying the atmosphere surrounding the produce with respect to the concentration of oxygen, carbon dioxide, nitrogen and other volatiles such as ethylene.

• The effects of elevated carbon dioxide and lowered oxygen in modified atmosphere (MA) packs in slowing product deterioration are essentially similar to those achieved using CA storage.
• However, atmosphere conditions generated in MA packs are generally less well controlled than in a CA store. Nevertheless, an MA pack appropriately designed to accommodate respiration differences due to cultivar and variable ambient temperature affords worthwhile improvements in post-harvest or post-storage life.

Retail packs for Discovery

• In packs of Discovery apples sealed with low density polyethylene films (LDPE) equilibrated atmospheres containing 3-5% carbon dioxide and 5-6% oxygen developed within 1-2 days at 20^oC.
• Softening and yellowing of the fruit was markedly retarded. Subsequent work showed that the benefit of MA packaging for Discovery apples was reduced when the technique was used for late-picked fruit and the risk of adverse effects such as the development of off-flavours was increased.
• Discovery apples that benefit most from MA packaging are those that are just beginning to ripen at the point of harvest. Discovery apples harvested earlier are endowed with a longer shelf-life and may not benefit from MA packaging over the period allowed for marketing.
• Late-picked Discovery fruits are unlikely to benefit from MA technology and may develop off-flavours unless accompanied by cool chain marketing.
• Further work is required to identify markers of respiration rate that would enable growers to select fruit that would be most suited to MA packaging.

Retail packs for Bramley’s Seedling, Cox’s Orange Pippin, Egremont Russet and Spartan

• Beneficial effects on the post-storage quality of Bramley, Egremont Russet and Spartan apples were achieved by sealing fruit in LDPE bags of 30 mm thickness for 2‑4 weeks at 15^oC.
• In Cox, the best results over 2 weeks at 15^oC were achieved using a 30 mm ethylene vinyl acetate (EVA) film.
• Another development in MA technology was a bulk pack consisting of a 18 kg (40 lb) case of fruit with a sealed permeable liner. LDPE (30 mm) liners produced atmospheres of 7-10% carbon dioxide and 5-7% oxygen for Bramley apples during a 4-week period at ambient (10-20^oC) temperatures.
• Micro-perforated LDPE liners were most suitable for Cox apples. Shelf-life improvements with the bulk packs were similar to those achieved using retail (1 kg) packs.