Gala and Fuji stored longer as volumes increase

Put Gala into storage immediately, but don't be in a rush to put Fuji into CA.

As volumes of Gala and Fuji apples produced in Washington State increase, it becomes more important to know how to store them long term, Dr. Jim Mattheis said at the Washington Tree Fruit Postharvest Conference.

Mattheis said shippers will no longer be able to wait to see what market conditions are like at the beginning of the season before putting Gala into storage.

"If you delay at the beginning, you have no flexibility at the end," said Mattheis, a plant physiologist with the U.S. Department of Agriculture in Wenatchee, Washington.

Gala is easy to store in some respects. It is not prone to many of the cosmetic disorders that affect other varieties during storage, nor does it develop watercore. Gas exchange is good, so it responds well to rapid CA and does not develop internal browning.

However, Gala ripens and loses firmness quickly. Mattheis said it soon reaches a point where firmness readings do not give an indication of textural changes in the fruit. "If you're eating this fruit, you can still detect differences in texture although we can't measure it using a penetrometer," he explained.

It also tends to lose flavor in storage. Sugar levels increase while acidity goes down. A Gala apple can lose a third or more of its acidity in storage, which makes it taste flat, even if it still has a lot of sugar and is producing aroma compounds.

A low-oxygen atmosphere--about one percent--can help sustain firmness during long-term storage. However, loss of acid is not dependent on low oxygen, and low oxygen affects the ability of the fruit to produce aroma esters when it is brought out of storage.

Gala apples stored for up to 60 days will produce flavor compounds quite well after they have had a week or so to recover, but this is not the case for fruit stored longer.

Fuji

Mattheis said Gala and Fuji represent the two extremes. Fuji ripens slowly, retains firmness well, and does not lose acid very quickly, but presents other problems in storage.

It tends to develop watercore, and is susceptible to an array of disorders, both internal and external. A particularly perplexing one is the skin browning known as stain, which usually shows up on fruit in storage and occasionally at harvest. Typically, it is a dark brown marking with a well defined border, although it can be lighter and less defined. It is more likely to affect fruit that is harvested late. It cannot be controlled with the antioxidant DPA (diphenylamine), but bagging in the field can reduce the incidence and severity.

Mattheis said there appears to be a link between light exposure in the orchard and development of the disorder in storage. It often affects the blush side of the fruit and is found on the margin of sunburned areas on the red part of the apple.

Staining has been seen on apples in Japan, where it is known as "sunnyside scald." Japanese researchers believe it is linked to exposure to sun and excessive tree vigor, and report that highly colored sports of Fuji are more susceptible.

Symptoms can dissipate when fruit is held in a warm room, and there is less staining in fruit that is stored at warm temperatures. However, other aspects of fruit quality suffer when the apples are stored at 50°F, Mattheis pointed out.

In experiments, he has held fruit at high temperatures before placing them in storage. Since Fuji does not ripen rapidly, there is more potential for using such treatments than there would be with a variety like Gala, which deteriorates rapidly, he said.

He has also tried to find out if there is a critical period during the growing season when the fruit's exposure to light could be blocked in order to control the disorder. He found that fruit bagged for a two-week period in September and October developed considerably less stain after storage than fruit that was not bagged or was bagged earlier in the growing season.

Mattheis and his colleague Xuetong Fan also did experiments where they were able to induce staining by exposing fruit to certain levels of ultraviolet (UV) light. Apples evidently become more sensitive to UV light later in the season. Staining was most likely to develop on fruit that was afterwards stored in cold temperatures.

Risk of staining can be reduced by controlling the vigor of the trees and maximizing calcium levels in the fruit, which may entail applying calcium sprays. Bagging can reduce the disorder but is expensive.

Treatment with a dip of calcium chloride at the warehouse is another option. Mattheis urged packers to avoid storing packed boxes in the early season, because if the fruit develops staining, it will have to be repacked.

Fuji is one of several apple varieties that are prone to internal browning, particularly if the fruit is of advanced maturity or has watercore. High carbon dioxide levels or high storage temperatures can contribute to the problem.

Mattheis said internal breakdown can be controlled with a DPA drench of 2,000 parts per million. The drench water should be kept as clean as possible, and packers should not use the solution to treat more than the recommended number of bins.

Mattheis said it would be preferable not to have to use postharvest chemicals, and scientists are exploring other storage regimes that would protect the fruit from internal breakdown without DPA.

Packers should not be in too big a hurry to put the fruit into CA storage, because the fruit maintains its firmness well. The variety has poor gas exchange, and imposing rapid CA can increase the risk of internal breakdown.

Ideally, for long-term storage, a Fuji apple should have a starch reading of 3.5 to 4.5 on a scale of one to six, and there should be a break in the background color. Fruit harvested earlier will not develop the characteristic aroma and may develop strange flavors in storage. It should not have severe watercore.

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