Apple Maturity and Harvest
Knowing when to harvest apples is critical not only for planning a labor schedule, but also for making decisions about whether the fruit will be sent to fresh markets or stored. Fruit harvested too early may have insufficient skin coloration, be too firm, have low SSC, poor flavor or be prone to storage scalds and bitter pit. Apple varieties mature at different times during the season. For the same variety, maturity can vary within the same orchard block and even within the same tree. For pre-harvest planning, days after full bloom (DAFB) can be used to estimate harvest dates. DAFB is only a general reference point and there may be a 5 to 20 day spread between the average harvest date and the optimal harvest date for a particular cultivar. To use DAFB as a guide, record the date of full bloom by block and cultivar in the spring, since full bloom may vary annually from one site on your farm to another.
There are several precise methods used to determine apple maturity. Maturity testing begins two weeks or more before estimated time of harvest and continues each week until harvest. The most important indicators driving harvest timing are firmness, starch content and background color. These are somewhat correlated to sugar content, acidity, flavor, aroma, texture, internal ethylene content (IEC), and potential storage life. Fruit skin color does not change significantly during storage, so if a fruit is picked without the correct color, marketability decreases. Also, if the fruit hasn’t achieved the correct background color, that fruit is most likely going to be too firm, starchy and be generally immature. In general, a combination of the presence of the desired background color, starch conversion of 25–35 percent, and firmness above 15 pounds will qualify for a good storage candidate. Candidate fruit for immediate fresh market sales would be those with the desired background color, starch ranging from 4.5 to 6.0 on the starch-iodine scale, sugar content above 13%, and firmness greater than 13 pounds. Keep in mind that there are set standards for specific varieties and markets. Links for the Washington State and the U.S. apple Standards webpages are found below:
- Standards for apples marketed within the state of Washington (Accessed: 1/19/17).
- United States standards for grades of apples (Accessed: 1/19/17).
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.
Testing for IEC involves the use of a gas chromatograph, making it a more difficult method of maturity testing that is normally performed by a specially trained laboratory technician. However, the other parameters already noted are more easily tested. Some testing methods are listed below. While some methods are older, they are still used and very reliable. Just as technology advances, so do the tools and methods used to determine fruit maturity.
Listed below are some of the primary changes in apple physiology that will occur as fruit become more mature:
- Starch is converted to sugar
- Seeds turn a darker color
- Acidity decreases
- Chlorophyll levels decrease
- Respiration increases
- Cell walls weaken, fruit begins to soften
- Ethylene production increases
Starch Test: The stage of apple fruit maturity can be assessed by performing a simple starch-iodine test. Make up a solution of iodine using 10 grams iodine crystals + 25 grams potassium iodide in 1 liter of water. Avoid contact with the solution and its ingredients and be cautious when mixing and applying iodine solution. Cut an apple horizontally; apply iodine solution to the cut surface, draining off any excess solution. Rate the fruit’s reaction to the stain after 2 minutes. Portions of the flesh that contain starch will turn a blue-black color. In contrast, the cells that contain sugar will remain unchanged. The reaction to the iodine is temperature-dependent. Under cold conditions, the reaction will take longer. The pattern of starch disappearance is specific for each apple variety. For example, Delicious loses its starch in a fairly even ring, while Golden Delicious shows an uneven pattern. The common rating system is on a scale of 1 to 6 as follows:
1 = full starch (all blue-black)
2 = clear of stain in seed cavity and halfway to vascular area
3 = clear through the area including vascular bundles
4 = half of flesh clear
5 = starch just under skin
6 = free of starch (no stain)
However, not all apples are scored according to this scale. Researchers studying WSU’s new release, Cosmic Crisp™ WA38, use the Cornell maturity scale (ranging from 1-8). For more information on the maturity and storage of WA 38 apples, read the Good Fruit Grower article How to harvest and store WA 38 Cosmic Crisp™ apples. More information regarding apple maturity can be found in the following links:
- Fruit testing in the WSU apple breeding program (Video)
- Predicting harvest date windows for apples, (Cornell Starch-Iodine maturity scale), G. D. Blanpied and J. Silsby, Cornell Extension Publication 221,1992. (Accessed: 1/19/17).
- A starch index chart specifically for Honeycrisp, developed by Dr. Ines Hanrahan, Washington Tree Fruit Research Commission. (Accessed: 1/19/17).
Pressure Test: Fruit firmness is another measurement reflecting the level of maturity. As fruit mature the flesh becomes softer. This is an important test because it also determines the fruit’s short or long-term storage capabilities without compromising the quality. Factors such as the presence of watercore or fruit size can affect the pressure readings. A fruit with watercore will give erroneously high readings. Any fruit with watercore should not be included in the test sample. Larger apples are usually softer than smaller ones from the same lot. Therefore, fruit samples for evaluation should consist of fruit relatively uniform in size and that are representative of the orchard block. Pressure tests are performed using a penetrometer. Although there are several devices currently on the market, the basic procedure is the same. For consistency, the same technician should perform tests for each fruit lot. To test, a disc of skin is removed along the circumference of the fruit from both the blush side and nonblush side to expose the underlying flesh. The penetrometer is plunged directly into the exposed fruit flesh (not through the skin) using even pressure lasting about 2 seconds. For apples, use the 11 mm tip supplied with the device and penetrate to a depth of 7.9 mm as marked on the plunger. Proper speed is the most critical part of this test. Applying pressure too fast is the most common way of getting an inaccurate reading. Finally, average the two readings to determine the single fruit value.
Soluble solids concentration (SSC): A refractometer is used to measure the soluble solids concentration of an extracted juice sample from the fruit. This is expressed as degrees or percent Brix and it is related to the fruit sugar content. As the fruit matures, starch is converted to sugar, and readings from this test will increase. The ideal readings will vary depending on the variety tested. There are several devices available including both manual and digital instruments. Measurements are made by squeezing a small amount of juice onto the device’s prism. If using a handheld device, hold the instrument up towards the light, look through the lens and read the percentage of soluble solids from the scale. Automatic instruments have an internal light source and sensor that delivers a digital value. The prism surface should be rinsed and carefully wiped off after each juice sample to prevent cross-contamination of juice from one sample to another. Temperature can also affect readings, therefore all samples should be at the same temperature when tested. Instruments can be calibrated by zeroing with distilled water and then using a 10% sugar solution (10 g of sucrose in 10 g of water).
Many factors can affect the SSC levels in fruit samples. Samples from heavily cropped trees will commonly have lower readings than those from lightly cropped trees. SSC values will be higher in years of reduced moisture availability, high temperatures, and high sunlight. SSC values will also vary between fruit from different locations on the tree. Fruit located in sun-exposed areas, where considerable photosynthesis is taking place, have higher SSC values while fruits in heavily shaded areas located inside the tree or on weak spurs have lower values.
Acidity: As fruits mature, their acid content decreases. Malic acid is the major acid in apple juice, and it plays a major role in flavor attributes. There are no state or federal guidelines for maturity based on acidity level. The amount of acid present is related to the variety and its stage of maturity. A drop in acidity is an indicator of advancing maturity. Measuring acidity is somewhat cumbersome and involves the use of common laboratory instruments such as a titrator or a buret. For best use as a maturity indicator, acid level should be recorded over a number of harvests to develop patterns and guidelines.
Color measurements: Color requirements differ depending on whether the variety is solid red, striped or partial red, red cheeked or blushed or whether the variety is green or yellow. Generally, for the varieties with red coloration, the commercial grade relates to the percent of the surface of the fruit that has a good shade of red color (more color, better grade). Red skin coloration is related to sunlight exposure.
Ethylene measurement: This type of test is very accurate, but requires a gas chromatograph and laboratory. Using a small gauge needle, the gas is extracted from the core of the apple to determine the maturity level. Before apples ripen and mature, ethylene is hard to detect, but as the apple starts to mature, ethylene levels begin to rise.
DA Meter: A new instrument developed by scientists at the University of Bologna, Italy, called the DA meter is used to measure the chlorophyll content in the fruit flesh. For each variety of apple, a characteristic DA index can be developed which includes a range of values related to the maturity of the fruit. As fruit mature, the DA value decreases. DA values are related to fruit maturity and this instrument may be used for nondestructive fruit maturity determination as compared to the destructive measurement needed with the starch-iodine test. It can be used while fruit are still attached to the tree or also on a commercial packing line. For more information on the DA meter, read the Good Fruit Grower article An easy way to test maturity.
Dr. E. Kupferman noted that apple quality and maturity must be monitored throughout the storage duration to assure that apples of the highest quality reach the marketplace. (Excerpt from Delicious Harvest Maturity and Storage, 2010, WSU Postharvest Information Network webpage.)
Once fruit is determined to be at the desired level of maturity, harvesting begins. Excellent harvesting practices are an essential part of the industry. Pickers must be well trained to ensure the quality of the apple is preserved from the tree to the warehouse. Rough handling is the most common way apples become damaged, and quality is compromised. Impact and compression injuries cause bruising. Picking wet or cold fruit increases the chance of bruising. Although all apples are subject to bruising, some varieties are more susceptible than others.
Automated technology is being developed to help preserve quality, and to address the problems of labor shortages and cost. Although a number of machines have been developed in recent years to aid workers and improve efficiency, the industry still depends on human workers to pick apples off the tree. Some of the new technologies include: Self-propelled platforms and remote controlled Bin-Dog for in-orchard bin handling. For more information on the status of mechanical apple harvesting technology, a good synopsis can be found in this recent Western Fruit Grower article The state of mechanical apple harvesting.
Once fruit is picked, and taken to a warehouse, additional sampling of lots may occur to determine if fruit will be packed immediately or stored. The goal is to provide consumers with optimal fruit quality year round. For this reason, storage capabilities will be conducive to the apple variety. 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. Visit our Storage link page for further details on storage.
- Fruit testing in the WSU apple breeding program, K. Evans, 2014.
- Standards for apples marketed within the state of Washington, webpage. (Accessed: 1/19/17).
- United States standards for grades of apples, webpage. (Accessed: 1/19/17).
- Predicting harvest date windows for apples, G. D. Blanpied and K. J. Silsby, Cornell Cooperative Extension, Information Bulletin 221, 1992. (Accessed: 1/19/17).
- Starch-iodine index chart for Honeycrisp, I. Hanrahan, Washington Tree Fruit Research Commission, 2012. (Accessed: 1/19/17).
- How to harvest and store WA38 Cosmic Crisp™ apples, I. Hanrahan et al. Good Fruit Grower, December, 2014.
- An easy way to test maturity, (DA-meter) G. Warner, Good Fruit Grower, August, 2014.
- The state of mechanical apple harvesting, R. Jones, Growing Produce, February, 2015.