Postharvest Management of Watercored Fuji Apples
Watercore in apples increases fruit susceptibility to internal breakdown (IB) caused by low oxygen (O2) and/or high carbon dioxide (CO2) concentrations. The potential for injury also increases when fruit oxygen demand and respiration increase with advanced maturity. Gas exchange in Fuji apples is slow compared to many other cultivars. The combination of Fuji's tendency to develop watercore, its sensitivity to CO2, late harvest to develop color and poor gas exchange combine to create a risk of developing IB during controlled atmosphere (CA) storage.
The potential for IB is low when watercored Fuji apples are stored in regular atmosphere (RA) storage, but the risk of injury increases with storage in low oxygen CA -- even when CO2 is controlled. The risk of IB also increases with storage duration.
The total potential for injury is the sum contributed by each risk factor. Development of a management strategy that minimizes total risk while maximizing storage potential requires information regarding the relative contribution of the various risk factors to the total injury potential. The objective of this ongoing project is to examine the contributions of harvest maturity, watercore, storage environment and duration to IB development during storage.
Materials and Methods
Fuji apples were harvested from a Wenatchee area orchard on October 20 and 30, 1995 and October 24 and November 4, 1996. Maturity analyses were conducted the day following harvest. Watercore was rated using a 1 (none) to 4 (severe) scale, starch using a 1 (full) to 6 (none) scale, and ground color using a 1 (green) to 5 (yellow) scale. Firmness was measured with a penetrometer on two pared surfaces. Soluble solids content (SSC) and titratable acidity (TA) of juice were measured with a refractometer and autotitrator, respectively. Ethanol concentration was measured by headspace sampling of apples cut into 4 pieces and equilibrated for 1 hour in a sealed quart jar.
Prior to storage in 1995, apples were held overnight at 34 °F and placed into CA chambers the day after harvest. Chamber atmospheres were established within 24 hours of loading. All CA treatments were maintained at 1% CO2. Concentrations of O2 in CA were 1%, 2%, 3%, 4%, or 5%. Due to similarity in responses, only results of 1%, 3%, and 5% O2 treatments are reported, except for ethanol accumulation. Apples were removed from CA storage after 3 or 6 months and held at 68 °F for 1 or 7 days prior to analysis. In 1996, fruit were stored in air at 30 °F for 30 days then transferred to 34 °F, or stored continuously at 34 °F.
After storage, fruit were visually rated for watercore and internal breakdown. Internal breakdown was observed as internal browning, cortex browning, cavitation or core flush. Internal browning was defined as dark brown areas related to watercore near the coreline. Cortex browning was lighter brown compared to internal browning and was located out into the cortex. Cavitation was holes or cavities usually near the coreline where watercore would have been at harvest. Core flush was a light brownish shadow within the coreline. The total amount of internal disorders was the percent of apples in each sample having one or more of these disorders.
Results and Discussion
All fruit examined at harvest in 1995 had scorable watercore (Table 1). Apples harvested October 30 were more mature compared to apples harvested October 20 based on watercore, starch, ground color, firmness and titratable acidity. Soluble solids content did not change between harvests. Watercore development was not as pronounced in 1996 (Table 2). However, starch conversion was more advanced at harvest compared to 1995.
The impacts of CA and RA treatments on fruit quality were evident (Tables 3 through 6). Watercore loss was slowest in apples from both harvests stored in low oxygen compared to RA. Scorable watercore was still present 6 months after harvest in CA stored apples. Fruit from both harvests stored at the low end of the 1% to 5% O2 range tended to have higher titratable acidity than fruit stored in air or 5% O2. This difference was most pronounced after 6 months storage plus 7 days ripening. There were no consistent differences in firmness, however, apples stored in low oxygen tended to have higher values. No consistent differences in soluble solids between treatments were observed.
Harvest maturity and storage oxygen concentration were two factors leading to ethanol accumulation during storage. Ethanol accumulated in apples harvested October 30 and stored in CA. The amount of ethanol detected after storage increased as oxygen concentration decreased. Ethanol in excess of 1000 ppm resulted in detectable off-flavors in this experiment, and only apples stored in 1% O2 accumulated ethanol excessively. Ethanol concentrations usually decreased during the 7-day ripening period, lessening the potential for off-flavor development.
After 3 months storage, internal browning was present in apples harvested October 20 stored in 1% to 3% O2 the day after removal from storage (Table 7). The incidence of internal browning increased slightly for several of the CA treatments as well as the RA fruit during the 7-day ripening period. The incidence of internal browning was higher for apples harvested October 30 (Table 8). All oxygen concentrations resulted in development of injury during 3 months storage, and some injury was present in RA fruit after the 7-day ripening period.
The incidence of disorders changed between 3 and 6 months storage (Tables 9 and 10). Less internal browning was present while cortex browning and cavitation appeared. Core flush also developed in fruit harvested October 30, particularly in RA fruit ripened for 7 days after storage. The total percentage of apples in each sample having one or more disorders increased for most CA treatments and RA-stored fruit between 3 and 6 months.
Apples harvested October 20 and stored in 5% O2 had the highest incidence of cortex browning. Cavitation in apples harvested October 20 was highest for apples stored in 1% O2, but it was only observed after the 7-day ripening period. Fruit harvested October 30 were more susceptible to cavitation, and injury was present at both 1 and 7 days ripening after storage for all CA treatments. Cavitation was also present in RA-stored fruit harvested October 30 after the 7-day ripening period.
The reduction in incidence of internal browning between 3 and 6 months storage appears to represent a progression of symptoms related to watercore at harvest. After 3 months storage, only internal browning was observed regardless of harvest date or storage conditions. After 6 months, the appearance of cortex browning and reduction of internal browning may be the result of migration of brown compounds initially formed in the watercored areas present at harvest. As the watercored tissues injured during the initial storage period break down and cellular contents are released, the brown compounds formed when the injury occurred could migrate into the cortex, diluting their concentration near the core. This would lighten the browning intensity of the watercored tissue near the core and decrease the amount of fruit rated with internal browning. Movement of these brown compounds would then impart a light brown appearance to more of the cortex. After 6 months storage, both internal browning and cortex browning were evident and visually distinct. Some apples had both types of browning but most fruit had one or the other. These disorders were rated separately because they looked different. The cavities may be a progression of cellular collapse in the watercored areas. The loss of cellular integrity that results in cavities would allow movement of the brown compounds out into the cortex.
The appearance of cortex browning and core flush after 6 months along with internal browning resulted in a high percentage of the samples having one or more disorder. Cortex browning and core flush were fairly lightly colored in many cases and may not be noticeable to consumers as long as other quality attributes were acceptable. This was very different from apples with internal browning, which was dark brown and ugly. Cavities are hard to miss but often are small and may be preferable to dark internal browning.
Fuji maturity at harvest and low oxygen CA both contributed to development of internal injury in storage. Nearly all fruit harvested October 30 had severe watercore and more injury developed during CA storage of this fruit compared to apples harvested October 20. Total injury due to low oxygen storage was variable for different harvests and storage durations. Based on these results, it is difficult to identify a safe oxygen concentration within the 1% to 5% range that can minimize storage disorders while maximizing other fruit quality attributes when late harvest apples with well developed watercore is present. It was apparent from these results, however, that late harvest Fuji apples with severe watercore should not be stored at 1% O2 due to the accumulation of ethanol in addition to the high percentage of fruit with internal disorders. Storage of late harvest fruit at higher oxygen concentrations appears to reduce ethanol accumulation, but many apples may still develop other internal disorders.
The percentage of apples with one or more storage disorders in this experiment was higher than expected even though harvest was timed to obtain fruit that would have high susceptibility to low oxygen injury. Apples were in storage at the target CA conditions within 48 hours of harvest, and this relatively rapid CA may have resulted in development of more injury compared to a longer initial cooling period prior to CA. Experiments with the 1996 crop compared rapid and slow establishment of CA conditions, however, the incidence of internal breakdown after 3 months storage was negligible.
Storage at 30 °F for 30 days after harvest in 1996 resulted in a high incidence of internal browning compared to no injury in apples stored at 34 °F (Table 11). These apples were examined 90 days after harvest, when browning was first visible is unknown. We have seen similar injury from low temperatures in previous experiments and caution is advised when air storage at temperatures below 32 °F is considered.
The excellent technical work of Dave Buchanan and Janie Gausman is gratefully acknowledged.
Tables 1 and 2
Tables 3, 4, 5, and 6
Tables 7, 8, 9, and 10
Jim Mattheis and John Fellman
(1)USDA, ARS Tree Fruit Research Laboratory
1104 N. Wenatchee Ave., Wenatchee, WA 98801
(2) Department of Horticulture and Landscape Architecture, Washington State University, Pullman, WA 99164-6414
13th Annual Postharvest Conference