A number of websites are spreading a myth that apples continue to "ripen" after being harvested, that is being removed from the tree. This is actually pretty bad advice, as it is only true under very unusual conditions. For one thing, apples will soften after being removed from the tree, but few people want a soft apple.
One exception to improvement in quality after harvest appears to be "late apples" which are reported by researchers to improve in flavor in storage. Reference 1. But that exception aside, let's examine this.
We use the term "ripe" to define when an a fruit or vegetable is at it's best quality for eating. For an apple that means:
Strength and length of sunlight reaching the apple while it is still on the tree is a primary factor in sugar content and ripening on the tree.
Apples ripen from the outside of the tree towards the center, so the apples out the outside of the tree will ripen first.
Apples on the sunny side, usually the southern side, of the tree ripen first.
Ethylene gas is produced inside the apple during maturing while on the tree.
Washington State University (Reference 2) tells us:
As an apple starts to mature and ripen, it produces ethylene gas, known as the 'ripening hormone.’ Testing for internal ethylene content (IEC) is crucial, because at some point during maturity, respiration will increase, and a surge of ethylene is produced. This is known as the respiratory climacteric. After the surge of ethylene is released, the apple will begin to senesce, losing quality, softening, and beginning to deteriorate. Essentially, there is a 7 to 11 day (+/- a few days) window of opportunity that exists in harvesting fruit of optimal quality, and that window needs to occur before the climacteric stage begins, as fruit quality will start to decline afterwards.
Note that the researchers found that fruit quality declines during storage due to senesce. Storage, under cold, humid conditions helps to slow the degradation (discussed below) . NC State Extension (see below) tells us that exposure to ethylene gas, post harvest, degrades apple quality.
The University of Maine's Extension elaborates on the role of ethylene in the ripening process in apples:
Most fruits produce a gaseous compound called ethylene that starts the ripening process. Its level in under-ripe fruit is very low, but as fruit develop, they produce larger amounts that speed up the ripening process or the stage of ripening known as the “climacteric.” The level of ethylene and rate of ripening is a variety-dependent process. Some apple varieties such as McIntosh, produce prodigious amounts of ethylene and are difficult to store once this occurs. When harvested after the rapid rise in ethylene, they quickly soften and senesce in storage. Other varieties have a slower rise in ethylene and slower ripening rate. For apples that will be stored longer than two months, it is imperative to harvest them before the level of ethylene begins its rapid increase.
Ethylene, post harvest, results in softening and degradation of quality, but no researcher found any evidence that it substantially changed starch into sugars.
Post harvest storage conditions for apples are designed to preserve the quality of apples at the time of harvest... not improve it (except in a few cases of certain late apples).
"Senescent breakdown" the term used to describe the aging disorder that refers to the degradation of apple quality during storage. The University of Maine's Cooperative extension tells us:
Some varieties, such as Macoun, develop it after a few months in cold storage whereas other varieties, such as Red Delicious, can be stored several months without an occurrence. However, all varieties will eventually develop senescent breakdown if stored long enough. Harvest at the right maturity for the storage duration, cold temperatures, and controlled atmosphere storage are the most effective methods of delaying its occurrence. Application of 1-MCP is also effective in delaying it.
Some of the folks asserting that apples improve during storage are using data collected from research into specially controlled conditions in designed warehouses, where humidity, temperature, oxygen and other gases are tightly controlled. Obviously, we cannot do this at home, so this is like comparing, well, between apples and oranges!
NC University Extension (Reference 5) tells us that the optimal storage conditions for most apples are:
For most of use, the fruit and vegetable draw of a refrigerator, which maintains cold (but not freezing) temperatures and humid conditions is the best to maintain apple quality. A second best alterative is a root cellar, basement or cold garage. Gala and Delicious have short storage lives and quickly become soft, while late apples, like Fuji maintain firmness and quality for much longer periods of time.
Utah State University advises consumers to:
Cool apples for storage as quickly as possible and keep them in the refrigerator between 34° and 40°F. Colder temperatures will allow apples to be stored longer, however, don’t allow the fruit to freeze. Keep the fruit in plastic bags with small holes. The holes allow gases to leave the bags but keep in most of the moisture while letting enough escape so water does not condense on the apple skins.
Aside from market supply and demand, many apples survive long-term storage without compromising the quality, however certain varieties and cultivars are better suited for short-term storage.
In a specially designed warehouse, called controlled atmosphere (CA) storage. NC University Extension (Reference 5) tells us:
Controlled atmosphere storage prolongs marketable life by lowering the oxygen concentration and increasing the carbon dioxide concentration in the storage atmosphere. CA storage facilities are specially constructed, airtight cold storage rooms with auxiliary equipment to monitor and maintain specific gaseous atmospheres. Oxygen concentrations from 2 to 3 percent and carbon dioxide concentrations from 1 to 5 percent below that achieved by low temperatures alone. These concentrations of oxygen and carbon dioxide also reduce the ability of the ethylene produced by ripening apples to further accelerate fruit ripening.
Although ethylene activity is reduced in CA storage, it has taken many years to recognize that ethylene levels of more than 1,000 parts per million, which commonly accumulate in conventional CA storage facilities, can still have a detrimental effect on storage life. Oxygen, carbon dioxide, and ethylene levels should be monitored daily and controlled within narrow limits. Concentrations of carbon dioxide greater than 10 percent can injure apples, whereas oxygen concentrations of less than 1 percent can cause storage disorders.
Recommendations for controlled atmosphere storage conditions are changing as a result of ongoing research. Optimum conditions depend on several factors, including variety and growing conditions. The current recommendations for many apple varieties is an atmosphere containing 5 percent carbon dioxide and 3 percent oxygen at a temperature of 32°F. However, these recommendations are being reexamined and replaced in some areas by recommended concentrations of 1.5 percent oxygen and less than 0.5 percent carbon dioxide. Specially selected and treated apples can be commercially stored for more than 10 months in these new controlled environments.
The University of Maine's Extension tells us that apple producers are working to design storage warehouses and treatments to HALT changes during storage; in other words, quality (and ripeness) does not improve during storage, but rather, the goal is to maintain the quality at harvest.
1-Methylcyclopropene (1-MCP) is a relatively new postharvest chemical that interferes with ethylene, the naturally occurring plant hormone that stimulates ripening in many fruits. 1-methlycyclopropene allows storage operators to more tightly control the progression of ripening so that fruit quality can be maintained for a longer duration. When combined with cold storage or controlled atmosphere storage, it can prevent quality loss and disorders for a longer duration than cold storage alone. The commercial formulation, SmartFresh®, is currently widely used on stored apples.
The publication cite above goes on to say that some varieties of apples produce high quantities of ethylene soon after harvest, which degrade quality, again emphasizing that post harvest, ethylene harms apple quality rather than improving it.
For more information, see "Predicting Harvest Date Windows for Apples" by G. D. Blanpied and K. J.
Silsby, Information Bulletin 221, Cornell Cooperative Extension
Harvest tables. Finally, Maturity dates, that is, the usual date that a variety ripens in a given geographic area is usually know by a state's apple association, local county extension offices, university extension offices, and the orchards themselves. Below are tables of typical harvest dates for apple varieties in some of the common apple growing states: