Refrigerated Storage of Apples and Pears–a Practical Guide
The factor which as the greatest effect in this direction is temperature. Apples keep about two and a half times a long at 40-degrees as they do at 60-degrees F. Pears keep twice as long at 28-degrees as at 34-degrees F. (Page 3)
The period represented by a “time unit” is of variable length, depending on the cultivar, being longer for late-crop-apples or pears than for early cultivars. (Page 3)
Apples: Cox’s Orange Pippen & Bramley’s Seedling keep longest at a temperature around 37-degrees F. to 40-degrees F when stored in air. (Page 3)
For pears and early apples the grower should use the starch test.
As the fruit grows, its starch content increases, but near to the right time for harvest it begins to fall. Starch stains blue with iodine, so that if a solution of iodine is applied to a cut surface of the fruit the area of blue coloration is a measure of the starch content. In practice several fruits are gathered some two to three weeks before the expected picking date. These are cut equatorially with a sharp knife and a 1% solution of iodine in 4% potassium iodide is applied to the cut surface. This must be done as soon as the fruits are picked. If this procedure is repeated two to three times a week, it will be noted that the starch content attains a maximum value, and afterwards begins to decline. When it has fallen to 2/3 of the maximum amount, that is the optimum time to pick… (Page 4)
Apples should, whenever possible, be put into store on the day they are picked, or at the latest on the following morning, having been left in the orchard overnight. The store must have an efficient cooling system. The recommended rate of cooling is from an initial temperature of 70F, down to 42F overnight (assuming that one quarter of the store is filled each day), and to 38F to 39F by the third day after completion of loading. If the chamber is stored in a single day the temperature should be down to 38F to 39F by the fifth day. (Page 8)
THE STORAGE AND RIPENING OF PEARS
Most pear cultivars keep longest in air at a temperature just above the freezing point. Examples of exceptions to this rule are English-grown Beurre Hardy, which develops low temperature breakdown below 32F, and Bristol Cross, which is prone to breakdown at any temperature, especially at very low temperatures. (Page 14)
Assessing the storage life of pears is not as straightforward business of keeping them under refrigeration and seeing how long they last. They do not ripen at low temperatures, but only after being removed from the store and kept for some days at a temperature which can range from 50F to 70F. (Page 15)
’Conference’ pears, moved to 65F after different periods at 34F (show) as the period of cold storage is prolonged, the time taken to ripen at the higher temperature decreases. Up to about the 100th day, ripeness is succeeded by core breakdown. (Page 15)
’Conference’ pears, 12 weeks at 34F, could be extended to 18 weeks at 32F and to about 24 weeks at 30F. (Page 16) One important difference between a cold storage for pears and one for apples is that in an apple store the temperature of the fruit is recorded. This may be done in a pear store, but since the average freezing point of a pear is 28.2F care must be taken that none of the fruit is exposed to air at this temperature any considerable length of time. Thus it is the temperature of the air delivered from the cooler that is important; this must not be allowed to fall below 29F. The thermometer to measure this must be as close to the cold air delivery point as possible. The mean temperature of pears in a store of this type is 30F. (Page 16)
Pears are commonly ripened at 65F in air in the U.K., but it has been shown that Fertility, Conference, Comice, Beurre Hardy, Packham’s Triumph and Winter Nelis pears ripen to a better quality at 50F to 60F than at 65F. Subsequently it was found that the optimum temperature for Conference pears is 50F. At 50F ripening takes two to three times as long as at 65F; this could be an important commercial consideration, since much larger ripening rooms would be needed to provide a constant supply of ripe or partly-ripe pears. (Page 17)
“Partly ripe”; this is the operative phrase in practice. A fully ripe pear cannot be transported from the ripening room to the point of retail sale without serious bruising. But a partly ripe pear will tolerate transport and will continue to ripen satisfactorily, even at low temperature. (Page 17)
Brownheart Disorder: ‘a presence of too much C02 in storage’; ‘smaller coolers should be opened somewhat frequently to replenish air.’
There is no evidence that the liability to develop brownheart is affected by the date on which apples are harvested. Pears, however, are much more susceptible to injury by C02 as their maturity advances. Furthermore, pears picked at the correct stage of maturity, but kept out of store for two or three days, or put into store at once but cooled very slowly, may develop the disorder. (Page 2)
CORE BREAKDOWN OF PEARS
It has earlier been stated that if pears have not been kept for too long a time at low temperature, they do not develop core breakdown until after they have become ripe. However, if storage is inordinately prolonged, core breakdown will occur when the pears are moved to a higher temperature, before they ripen; in fact, they do not ripen normally. This type of breakdown is often referred to as “sleepiness”; the pears are mealy externally, and the core is wet and mushy. (Pages 29-30)
HUMIDITY
Excessive loss of weight by evaporation of water from stored fruit is a major cause of deterioration. It is therefore essential that fruit stores should with a high air relative humidity if evaporation losses are to be kept to a low level. (Page 49)
OPERATION (OF A STORE)
A fruit store should be loaded as expeditiously as possible. The stowing should be neat with no large gaps between the tiers of boxes or bins. Irregular stowing can result in uneven fruit temperatures, apart from the loss of valuable space. The bottom layer of the containers should be 6 in. above the floor and the level of the top layer should not be above the air trunking, where it could restrict the flow of air to or from the cooler unit. The required conditions of temperature and gas concentrations must be established with the minimum of delay after harvesting; boxes of fruit should not be left standing about in orchard or packhouse. (Page 58)
The refrigeration plant is generally designed with sufficient capacity to cool the daily intake of fruit from 65F to 40F overnight, and on completion of loading to reduce the fruit to the holding temperature within 2-3 days. The sensing elements of the temperature indicator should be placed in such a position that they indicate fruit temperature and not air temperature. Probably the best position for the elements (usually two per 2,500-bushel store) is one in a box of fruit next to the top layer and half the distance from the cooler to the wall, and the other in a similar position but in a box of fruit next to the bottom layer. An exception to this arrangement is needed when pears are stored at 29F, which is near their freezing point. It is then advisable to have the sensing elements where they indicate air temperature rather than fruit temperature. One of the elements should therefore be in the delivery air and the other in the return air. Temperatures should be recorded at least twice daily, i.e., in the morning and in the evening. (Page 58)
When a store is which the temperature is the only factor to be controlled is closed up, the thermostat controls the operation of the plant, maintaining temperature at a preset level. The siting in the store of the thermostat sensing element is important. It should be in such a position that it is ventilated but not directly in the stream of air coming off the cooler; also it should not be attached to a store wall, particularly a south wall where it could be influenced by wall surface temperature as well as store air temperature. (Page 58)
Once a cold store is closed it should require little attention except perhaps to run the store fan in reverse for a few hours to even up the fruit temperatures if the top and bottom temperatures differ by more than about 1-degree F, or to defrost the cooler if store temperatures tend to rise even thought the refrigeration unit is running for long periods. (Page 58)
The relative humidity in a well designed fruit store should be about 90-92% to avoid excessive weight loss by evaporation. (Page 60)
There are about 20 pages dedicated on how to build a cold store that I did not include in these notes.
