Maturity and Storage of Apple Varieties New to Washington State-1992
The Washington Tree Fruit Research Commission has funded a number of research projects on new varieties. Scientists working on the maturity of new varieties include: Bob Norton (WSU, Mount Vernon); Bob Stebbins (OSU, Corvallis); Braeburn--Eric Curry (USDA-ARS, Wenatchee), Anne Plotto (OSU student); Fuji--Jim Mattheis (USDA-ARS, Wenatchee), Max Patterson (WSU, Pullman); Xuetong Fan (WSU student); Anne Plotto; Gala--Gene Kupferman (WSU, Wenatchee); Terri Boylston (WSU, Pullman); Anne Plotto; Granny Smith--Max Patterson; Jonagold--Sam Lau (BC, Summerland). Thanks to these scientists who have graciously reviewed this article for completeness and accuracy.
Summarized below is what we currently know about the maturation process and storage potential of these new apple varieties. This information is preliminary and new studies will undoubtedly improve the information presently available. I would appreciate feedback from the industry regarding experiences with these new varieties.
New Zealand growers produce the largest volume of Braeburn. They rely on ground color to determine maturity and they harvest fruit from each tree 2-3 times per season. Quality and storage behavior of fruit borne on 1-year wood may not be comparable to fruit on spurs from older wood.
Red color develops in Washington only late in the season and is highly dependent upon nitrogen level and light penetration within the canopy. Harvest maturity should be determined by ground color rather than red skin color. Timing harvest on the basis of red skin color is not effective for determining edible fruit quality.
Washington State growers have noticed that Braeburn apples size rapidly during the last month of the growing season.
There is very little experience in storing Braeburns. The New Zealand industry markets them shortly after harvest, having little experience with intermediate or long-term CA storage. Indications suggest that it stores well in regular or CA storage. It does not appear to be chilling-sensitive, so storage at 32°F might be suitable.
Potential Storage Problems
Bitterpit--This can be a serious problem with Braeburns, making calcium sprays important. Balanced tree growth, fertility and water management help reduce this problem. Braeburns are also susceptible to the type of bitter pit called lenticel blotch pit. Other internal disorders may develop due to mineral imbalances. Nitrogen nutrition appears to be important with this variety.
Internal breakdown--Overmature fruits are highly susceptible to internal breakdown. Breakdown occurs internally during storage but it cannot be seen from the skin of affected fruit.
Scald--As in other varieties, if Braeburns are harvested too early, scald will develop if stored beyond 6 months in regular storage.
Fuji appearance is variable. Its skin color goes from a dull muddy brown to a clear bright red. Fuji apples growing on light crop trees mature earlier than on well-cropped trees by as much as 10 days. Fujis grown on weak fruiting wood or late flowering young spurs mature later than fruit borne on older wood. Fruit on young and vigorous trees has the dull muddy appearance while fruit on older trees is usually more attractive.
This variety requires several pickings. In Tasmania, Australia, it is recommended that the first picking include all light crop fruit. The main picking would include normally cropped trees except fruit on 1-year wood. The third and final picking includes fruit on 1-year wood, which is usually of lower quality.
Fuji produces less ethylene than other apple varieties so it is possible that the harvest season can be extended.
Soluble solids concentration has not been a useful guide since seasonal variation and cropping levels cause large variations. For example, soluble solids can range from 12% in one year to 16% the next. Washington-grown Fujis have been tested at 17-19% soluble solids. Flesh firmness is also not a suitable harvest indicator.
Some people are harvesting at the first sign of watercore in the most mature fruit on the tree. Selection of fruit to harvest should be determined by ground color. By the time the fruit reach a golden yellow ground color, most apples will have severe watercore. Tests in Washington State have shown that these apples are ready for harvest when they start to lose their green background color.
Fujis have a reputation as keeping well in long-term storage, even regular storage. However, in one test, soluble solids did not rise during storage and firmness declined slowly over time. Washington Fujis stored in regular storage at 32-33°F until June remained above 15 lbs firmness.
Fuji apples have been stored in CA (2% oxygen with 2% carbon dioxide) with results depending upon length of storage and the maturity of the fruit. In one test, flesh firmness after CA storage was almost 3 lbs above that in regular storage. There was no difference in soluble solids. In other preliminary tests, Fujis have responded well to very low oxygen storage (0.5- 1.0%) with carbon dioxide at less than 1%.
Potential Storage Disorders
Scald--Fujis are susceptible to storage scald and DPA is needed to minimize scald. Very low oxygen levels or initial oxygen stress might be helpful in reducing scald.
Rotting--Fuji apples sometimes develop storage rots. Their thin skin makes them very susceptible. A prestorage fungicide should help reduce rot.
Watercore--Severe watercore can develop in Fuji very early in the harvest season. There are reports that slight to moderate watercore in Fujis may disappear if not too severe.
Coreflush--Internal core browning has been reported to be a problem with Washington-grown Fujis. Browning begins around the core line and may even be present in some fruits at harvest.
Moldy core--In one test in Australia, 5% of the stored Fuji apples developed moldy core.
Shrivel--Since Fuji has a thin skin and produces very little natural wax it is prone to shrivel, especially in storages with low humidity.
Gala apples size rapidly during the harvest season. This can be an important factor in determining financial returns of this small-fruited variety. Red skin color, firmness, soluble solids and acidity do not relate well to internal maturity. Although red color develops differently on red clones, these clones still require selective picking. Red-striped strains are prone to reversion to less highly colored fruit.
New Zealand growers harvest Gala apples 4-5 times, picking fruit on the basis of ground color. It appears that Washington growers are harvesting fruit of excellent edible quality when the ground color is changing from green to yellow (in the white). For example, in 1990 Royal Gala harvested with white ground color were 17.5 lbs firmness, 13.3% soluble solids and 0.376% acidity. The starch rating was 3.6 on a 1-5 scale. Starch has been used by some researchers and orchardists to time harvest. Fruit sampled for starch analysis should be of uniform ground color. In Orondo, WA, these fruit were harvested between August 29 and September 12. Fruit destined for storage should be harvested slightly on the green side. Multiple picking appears to be essential in order to have fruit of good size, color, edible quality and uniformity for storage.
Gala apples in storage for more than 4 months lost flavor and texture even in CA. Rapid marketing of Gala will result in a product superior to that out of storage.
Fruit stored in CA tests has had similar firmness (by at least 1 pound) and higher acidity than those stored in regular storage. Galas have not yet been subjected to different CA regimes. They respond well to 1% oxygen and 1% carbon dioxide at 32°F.
Potential Storage Problems
Rot--Late-harvested Galas appear to be more susceptible to fruit rotting fungi than those harvested at white ground color.
Firmness and Acidity Loss--Softening can occur and acidity be lost if the fruit is not placed in CA directly after harvest. It is not possible to achieve good results if the CA is delayed. For example, Galas harvested in early September should not be held out of CA until enough Goldens are harvested in late September in order to fill a room. Galas must be placed in CA immediately.
Granny Smith originated as a chance seedling near Sydney, Australia, in the 1860s. Selections of Granny Smith have been widely planted in Washingtonboth nonspur and spur types (Greenspur and Granspur). Virus diseases that created symptoms on fruit have been a problem in non-heat-treated nursery stock.
Granny Smith apples are the third largest volume apple variety shipped from Washington State, behind Red and Golden Delicious. Debate rages about the value of marketing solidly (Emerald) green skin Grannies vs those with the "kiss of the sun." Scientists and others state that Grannies allowed to develop full eating potential are superior to fruit harvested too early in order to retain green skin color. However, the worldwide Granny Smith market is oriented to solidly green color Grannies. This has led to the processing of good quality fruit. One year, more than 50% of the Granny Smith apples grown in New Zealand were diverted simply because they were not fully green.
Judging Harvest Date
Year-to-year variation in fruit firmness, soluble solids and titratable acidity indicates that these factors are more indicative of fruit quality than of maturity. New Zealand researchers have determined that the starch iodine test best determines fruit maturity. Starch clearing shows less variation between orchards and changes more uniformly over the season than either soluble solids or flesh firmness. The New Zealand industry uses the average starch rating from 20 apples to determine harvest maturity. This agrees with work done in Australia, South Africa and South America. They have determined that when all areas within the core line are white, a quarter of the cortex is white and the remainder is blue, the fruit is ready to harvest.
In Washington State, Max Patterson has studied Granny Smith maturity and its condition after storage. He has sampled Granny Smith fruit in different years and found great difference in firmness, starch, soluble solids and titratable acidity. He suggests that the fruit not be picked until it is physiologically mature, when both edible quality and storage longevity will be optimized. Unfortunately, he is not able to suggest a field operation that can determine physiological maturity other than by measuring ethylene evolution and/or increased respiration rate. Physiological maturity on fruit from a single Granny Smith orchard ranged from October 14 in 1985 and 1987 to October 28 in 1986.
In California, Gordon Mitchell determined that the starch index was the single best maturity indicator. His recommendation is to start harvest when about 30% of the cortex is white. He has observed that this occurs at about 170 days after bloom with fruit at 16 lbs firmness, 12% soluble solids and 0.75 titratable acidity.
In South Africa extensive research has been done by A. B. Truter to determine optimum harvest date for long-term storage of Granny Smith. He has developed a program similar to that used in Washington State for Delicious, based on noting an accelerated or decelerated rate of change of many maturity indices. He rejects the concept that maturity can be determined through specific values due to year-to-year and orchard-to-orchard variation. During different seasons the range in soluble solids at optimum harvest was 11.4-12.5%, firmness 14-17.5 lbs, acidity 0.6-0.8% and starch 20-38% unstained cortex.
These studies have shown that there is a picking window of 3-4.5 weeks. The Washington industry is currently considering using a certain starch clearance level as an indicator of minimum quality.
Patterson has subjected Granny Smith apples to regular and CA storages at different atmospheres. He has determined that Washington grown Granny Smith apples do best at oxygen levels of 0.75-1.0% with carbon dioxide at less than 0.75% at 31°F. His research has shown that DPA treated fruit had less storage scald, lower ethylene levels in storage and greater firmness than nontreated fruit. His tests in regular storage have shown that 31°F, compared with 34° and 38°F, resulted in firmer fruit with greener color and less scald, coreflush and core browning. Patterson has stored Granny Smith apples in CA for a full year at low oxygen and low temperatures without negative effects.
Research from New Zealand, Australia and South Africa has suggested that rapid temperature reduction to 32°F or below has led to internal browning of the fruit. Several researchers recommend CA and rapid temperature reduction to 34°F for the first month and then lowering the temperature to 31-32°F for the remainder of the storage period. Most researchers recommend holding the oxygen at 1-1.5% and CO2 below 1%.
Potential Storage Problems
Sunburn and delayed sunburn--Sunburn and delayed sunburn are serious problems in Granny Smith apples. Delayed sunburn appears only after the fruit is in storage, and sunburn appears while the fruit is still on the tree. Sunscald appears in storage on fruit that has been exposed to the sun while in the bin awaiting transport to the packinghouse. In New Zealand, bins are held under shade cloth before transport to the packinghouse. Various chemicals are sold that are claimed to reduce sunburn. Whitening agents and orchard shade cloths have been tested on trees in California without success. Modification of training and pruning is helpful in reducing sunburn. Sunscald can be reduced by holding bins of fruit under shade and covering straddle carriers to avoid direct sunlight. A DPA drench also helps retard this fruit browning.
Superficial scald--Superficial (storage) scald is a serious problem with Granny Smith apples in CA. Fruit from warmer districts is more susceptible than fruit from cooler districts at the same level of maturity. Proper harvest maturity is essential. DPA treatment is imperative with this variety if it is to be stored. DPA damage can increase on fruit with sun-bleached skin. "Initial oxygen stress" used experimentally in Australia has led to reduced scald in laboratory tests with reduced amounts of DPA. When oxygen stress was initially used, coreflush and alcoholic tainting increased.
Bitter pit--Granny Smith apples are very susceptible to bitter pit both on the tree and in storage. Lenticel blotch pit (a form of bitter pit) has been observed on Washington-grown fruit stored in CA. Regular season-long calcium spray programs help reduce bitter pit. Minimizing stress through proper pruning, water management and fertilizing can be very effective. Postharvest applications alone can increase fruit calcium (by less than 5%) but are not as effective as repeated orchard sprays. Application of calcium may help retain green color.
Coreflush--Fruit is susceptible to coreflush if it is picked too early or too late. Large fruit, especially from low cropping trees, is more likely to develop coreflush than moderate size fruit. Fruit from the inner shaded parts of the tree and fruit from warmer districts are more susceptible. Stepwise lowering of temperature has helped minimize coreflush. Coreflush increases when carbon dioxide levels are held above 1%. Low oxygen storage helps reduce coreflush. DPA has also been shown to help reduce coreflush.
Jonagold is a cross of Golden Delicious x Jonathan. There are a great many Jonagold strains. It is juicy with a fine texture. Even after storage the fruit retains its acidity. It is a triploid, so the fruit is large. Bitter pit and sunburn can be problems, especially on fruit grown under stressful conditions.
Jonagold fruit is striped red with yellow ground color. Under Washington conditions, red color does not develop early in the season. Delaying harvest to promote red color development can lead to serious watercore. In storage, this watercore can turn into flesh browning and storage breakdown. In addition, late-harvested fruit quickly loses firmness in storage.
In tests performed by Sam Lau between September 19 and October 21, 1985, in British Columbia, it was found that as Jonagold matures the fruit weight increases by 7 grams/week. The ground color yellows during this period, red skin color develops and soluble solids increase (0.37%/week). After October 10, the ethylene increased dramatically, starch hydrolyzed and flesh firmness decreased by 0.35 lb/week.
In tests performed by Lau since 1985, he determined that flesh firmness, soluble solids, acidity, watercore and red skin color are not good harvest predictors since they exhibit large seasonal and orchard variations. For example, well-exposed Jonagold fruit harvested at the first acceptable harvest date (10 October) were 50% red in 1984 and 1986 and 80% red in 1985.
Lau suggests that the change in the starch index was fairly linear with time and correlated internal ethylene concentration and watercore. Thus, starch index can be used to predict optimal harvest timing. The change of skin ground color from green to green-yellow has been proposed as an additional harvest aid for Jonagold.
For single harvested fruit, the optimum maturity for long-term CA-stored, B.C.-grown Jonagold is as follows (in order of importance):
- Starch 6.5-7.5 starch index (0-9 Canadian scale)
- Flesh firmness 16.0-17.0 lbs
- Watercore-under 12% of the fruit are affected
- Yellow ground color 4.0-4.5 on the Golden Delicious scale
- Internal ethylene 0.1- 1.5 ppm or 0-40% of the fruit with internal ethylene conc. at >1 ppm
- Titratable acidity >0.6-0.7% malate
- Soluble solids >12.5-14.5%
Multiple harvests might be considered to optimize red color and maturity. Bruising is also a problem.
Air Storage Potential
Harvest maturity has a strong effect on titratable acidity, watercore and breakdown and fruitiness of Jonagold held in air storage. Fruit harvested within one week of the first acceptable picking date kept well in air at 32°F until February. Fruit held longer than 4 months was soft, bland and prone to breakdown and coreflush.
CA Storage Potential
The harvest window for long-term CA storage of Jonagold can be as long as 10 days, since picking within 10 days of the first acceptable harvest date does not significantly increase fruit disorders. Fruit picked at optimal maturity can be held at 1.5% O2 + 1.5% CO2 at 32°F until June without breakdown or injury. CA-stored fruit was 25% firmer, 47% higher in titratable acidity and had less coreflush and higher flavor rating than air-stored fruit.
Potential Postharvest Problems
Bitterpit--Jonagold is prone to bitter pit due to large fruit size and genetic susceptibility. Avoid growing conditions that promote stress. Apply calcium sprays (at least 4-6) throughout the growing season.
Watercore--Harvest at the proper maturity. Do not store severely watercored fruit.
Loss of firmness--Harvest at the proper maturity. Cool rapidly and store properly, preferably in CA.
Flesh browning and breakdown--Avoid storing moderately or seriously watercored fruit. Store in CA at the proper temperature.
Tables 1 & 2
Table 1. Range of harvest periods for new apple varieties. These are very rough estimates of harvest dates. Season, orchard location, fertility, and rootstock play a role in determining maturity.
|Gala||August 15-September 25|
|Jonagold||September 15-October 15|
|Braeburn||October 10-October 20|
|Granny Smith||October 15-November 10|
|Fuji||October 15-November 10|
Table 2. Maximum storage periods for new varieties picked at optimum maturity.
|Variety||Regular storage||CA storage|
|Gala||2-3 months||3-4 months|
|Jonagold||4 months||9 months|
|Granny Smith||6-9 months||12 months|
|Fuji||9 months||12 months|
Dr. Eugene Kupferman, Postharvest Specialist
WSU Tree Fruit Research and Extension Center
1100 N. Western Ave., Wenatchee, WA 98801
Tree Fruit Postharvest Journal 3(1):9-16