Typical storage time 6 to 12 months
Harvested August to November, the fruit is placed in bins submerged in water so the apples float out.
At this ‘pre-sort’ stage, defective ‘culls’ are spotted and removed. Soap and chlorine are applied and rinsed off with hot water. Brushes clean and dry the fruit.
In the US, wax is applied to improve sheen and seal in moisture, after which the apples are hot-air dried. They pass along a conveyor belt and are photographed by two cameras; the images are analysed by computer for consistency of colour, shape and size.
The fruit is graded and arranged in trays (Class 1) or bagged (apples for processing) – both by machine – then put in boxes and weighed.
Loaded on pallets, the fruit goes into cold storage – a sealed room where its respiration rate is slowed. Fruit for imminent consumption (November to January) is chilled to 0C. In longer-term controlled atmosphere (CA) storage, the oxygen level is addition ally lowered from 21 per cent-1.2 per cent, putting the apples to sleep for 6-12 months.
The apples are taken by lorry to a central depot, then the supermarket.
Typical storage time 1 to 4 weeks
Lettuce is best harvested in warm weather, but this is also when demand for salad peaks, resulting in shortages at the start of a fine spell. Weather forecasts are used to predict surges in demand – but if these are wrong, the lettuces may be refrigerated for several days. In the US, lettuce for shipping abroad is refrigerated for anything up to 4 weeks.
Usually harvested by hand, heads of lettuce are packed in the field to avoid deterioration and mechanical damage.
Wrapped or bagged in plastic, they are packed in boxes, transported to the cold store and cooled to 0C.
Lettuce for salad packs is cut, washed in cold water, dried by centrifuge and mixed with other leaves. It may be treated with a chlorine-based compound, antioxidant or preservative to prolong shelf life. It is placed in ‘pillow packs’, or Modified Atmosphere Pack aging (MAP), in which levels of O2 and CO2 have been altered to slow deterioration. Then it is stored.
The optimum controlled atmosphere storage temperature is 0-5C, with O2 levels reduced from 21 per cent-3 per cent; though CO2 can damage it, lettuce keeps for a month with CO2 levels raised.
Typically, lettuce is sold in supermarkets 3-4 days after packing, which can be extended to 10 days.
Typical storage time 14 days
In the Caribbean, say, the bananas are washed in water, packed in cardboard boxes and loaded on to refrigerated ships at 14C within 36 hours. This inhibits ripening. The journey by sea to Britain typically takes 6 days.
Collected from the docks by refrigerated lorry, the bananas – which are bullet-hard and emerald green – are loaded into ripening rooms when their pulp temperature is 11C.
The room is sealed and heated for 12-16 hours until the pulp temperature reaches 17C.
Ethylene gas is discharged from cylinders or cartridges, catalysing the hormonal process of ripening. The room is kept closed for 24 hours.
The room is ventilated with extractor fans to clear the ethylene, then resealed for a further 3-4 days at 17C. The temperature inside the fruit reaches twice that, and large volumes of C02 and other gases are produced as the bananas ripen.
The ripe fruit is removed and transported to the supermarket.
Typical storage time 1 to 6 weeks
In Britain, greenhouse tomatoes are harvested from March to November, with imports from Spain, Portugal and the Canary Islands plugging gaps at other times of the year.
After harvesting, they are chilled to take the ‘field heat’ out of them.
When ripening, temperature is critical: too high, and the intense red colour will fade; too low (10C instead of 12C), and the tomatoes will lose flavour and aroma, ripen slowly and suffer chilling damage, like frostbite.
Shelf life in the UK for softer, fruitier varieties is five days, rising to seven days for thick-walled, ribbed types.
In America, mature green tomatoes in CA storage (low oxygen, high nitrogen) have been kept successfully for up to six weeks.
Typical storage time 2 to 12 months
Most potatoes will keep for up to a year if they are first ‘cured’, by storage at moderate temperatures (20C) and high humidity (80-100 per cent) for 1-2 weeks.
Those harvested in a British winter are often colder than this, so they have to be warmed up to cure them – the process known as ‘sweating’. The temperature is then lowered to a maintenance level of 2-3C.
In potatoes intended for long-term storage, a chemical sprout-inhibitor spray may be applied in the field, during grading or in the cold store.
Typical storage time 1 to 9 months
Immediate washing and cooling are essential to maintain the carrots’ crispness. Often, they are cooled in chlorinated water before packing.
Storage just above 0C inhibits sprouting and decay, while raised humidity prevents desiccation.
In these conditions, mature topped carrots will last 7-9 months, though 5-6 months is more typical. Cellophane packs of baby carrots will last two to three weeks. Bunched carrots are highly perishable due to leaf being present; they will store for 8-12 days if stored and shipped with shaved ice.