CA Regimes for Control of Superficial Scald of d'Anjou Pear
Superficial scald (scald), pithy brown core (PBC), and skin black speck (BS) are three major physiological disorders developed on/in d'Anjou pears during postharvest storage. Manipulation of storage conditions has been demonstrated to be an effective, commercially viable control measure of superficial scald on apple fruit (Little et al., 1982). Initial low-oxygen stress followed by CA storage has been used on a semi-commercial scale by the South African fruit industry to control superficial scald on Granny Smith apples (Truter et al., 1994).
In terms of d'Anjou pears, the use of low O2 CA storage combined with optimum CO2 levels could become a practical approach to non-chemical control of scald, since 60% d'Anjou crop harvested in the Pacific Northwest each year are routinely stored in CA facilities. In this report, we determined optimum CA regimes, which would simultaneously control scald, PBC, and BS on/in d'Anjou pears with minimal application of ethoxyquin after short-term, mid-term or long-term storage.
The Optimum Level of Oxygen
D'Anjou pears at commercial maturity with flesh firmness (FF) of 64.5 Newton (N) (Note: 1 Newton = 4.4 lb) were harvested September 1, 1994. Fruit were drenched with a fungicide solution (Mertect 34F), air-dried and transferred into 18-kg plastic bins with perforated polyethylene liners. Four packed bins were loaded into each of 24 gas-tight CA cabinets in a cold room at -1 °C. The core temperature of the fruit reached equilibrium at -1 °C after 96 hours (4 days).
Fruit were held at -1 °C in air for 10 days before flushing each cabinet with N2. Different O2 and CO2 concentrations were obtained by mixing cylinders of medical grade O2, pre-purified N2 and 25% CO2 balanced with N2 after each cabinet had been flushed with N2 generated from a Prism Alpha Nitrogen System to the desired O2 level.
The flow rate in each cabinet was maintained approximately at 50-mL/min-1. At each CA regime, fruit were stored for 4 months (short-term CA), 6 months (midterm CA) or 8 months (long-term CA). The desired CA regimes in CA cabinets were set as follows with 2 cabinets (replicates) per regime and storage interval:
- 0.5% O2/0.0336 CO2
- 1.0% O2/0.03% CO2
- 1.5% 02/0.03% C02
- 2.0% O2/0.0336 CO2
The concentration of O2 in each cabinet was maintained within ±0.10% accuracy by adjusting the inlet valve of O2 supply as needed and CO2 at < 0.05% by using hydrated lime (1 kg lime per 20 kg fruit) to absorb CO2 produced by the respiration of d'Anjou fruit. Exogenous CO2 was not supplied to each cabinet.
After 4, 6, or 8 months of storage, 2 cabinets from each set of CA regimes were returned to air and held at -1°C. After 8 weeks in air, fruit in 4 bins from each cabinet were transferred into a ripening room at 20 °C for 7 days. On day zero of ripening, 5 fruit from each bin were used for the determination of alpha-farnesene, and conjugated trienes by a previously described method (Chen et al., 1990b). Each fruit in each bin was also visually evaluated for the symptom of BS on day zero of ripening. On day 7 of ripening, fruit in each bin were individually evaluated for the symptom of scald and PBC.
For BS and scalds, fruit having approximately 0.5 cm2 or less, 0.6 to 1.0 cm2, 1.1 to 3.0 cm2, and greatly than 3.0 cm2 skin area affected were classified as very slight, moderate and severe symptom, respectively.
For PBC, each fruit was cut transversely through the central core area. Fruit having brown discoloration confined to the center of pith, spread to mesocarp and exocarp of the seed cavities, confined to inside the core line, or spread to outside the core line were classified as very slight, slight, moderate, and severe, respectively. Fruit with slight or worse symptoms were considered commercially unacceptable. The incidence of each disorder was expressed as the percentage of fruit affected with slight, moderate, and severe symptoms.
After short-term (4 months) low oxygen CA storage, fruit stored in 0.5% O2 and 1.0% O2 were free from scald and developed only negligible amounts of BS and PBC (Table 1). After mid or long-term (6 or 8 months) storage only fruit stored in 0.5% O2 were free from scald; however, fruit stored in less than 1.5% O2 developed unacceptable incidence of BS (Table 1). PBC was negligible regardless of oxygen concentration. The results confirmed that PBC in d'Anjou pear fruit was associated with elevated CO2 concentration in a prolonged CA storage (Hansen, 1957).
The optimum oxygen level is the lowest oxygen concentration in CA storage that can effectively control scald without inducing BS or PBC. In short-term CA this was found to be 0.5% O2. The results indicated that low oxygen storage regimes between 0.5% and 1.0% could be used to control scald effectively without causing BS and PBC when CO2 was held below 0.1% and the length of storage was no longer than 4 months. Oxygen concentrations between 0.5% and 1.0% with CO2 below 0.1% can be used commercially used for non-chemical control of superficial scald of d'Anjou pears stored for less than 4 months.
Following mid-term (6 months) or long-term (8 months) CA storage, fruit stored in 0.5%, O2 were free from scald, but developed a very high incidence of BS (Table 1). Fruit stored in 1.0% O2 developed high incidences of both scald and BS. Although fruit stored in 1.5% and 2.0% O2 developed only minor amounts of BS, almost all fruit developed scald (Table 1). Fruit stored in 0.5% O2 developed only minor amounts of PBC while those stored in 1.0% O2 or higher O2 regimes were free from PBC. It is evident that oxygen levels lower than 0.5% might suffocate the core tissue of d'Anjou pears resulting in PBC during mid-term or long-term CA storage. These results indicated that there was no optimum oxygen level for the control of all three physiological disorders without the addition of an antioxidant.
Alpha-farnesene and conjugated trienes in the peel of d'Anjou fruit were suppressed by low oxygen (Table 2). The lower the oxygen, the more effective it is in suppressing the accumulation of alpha-farnesene and conjugated trienes. When the level of conjugated trienes in the peel tissue accumulated to greater than 1.5 nmoles/cm-2, the development of superficial scald became inevitable. Conjugated trienes, the oxidative products of alpha-farnesene, have been reported to be the cause of scald of apple fruit (Huelin and Coggiola, 1970; Sal'Kova et al., 1975). In a previous study, the threshold level of conjugated trienes toxic to the peel tissue which induced scald was 2.0 nmoles/cm-2 (Chen et al., 1990b)
Control of Scald, PBC, and BS in Short-Term Low O2 Storage
In 1995, d'Anjou pears at commercial maturity with flesh firmness (FF) of 66.7 N were harvested from trees in the same block as in the previous experiment, on September 6 and 10. Fruit free from bruising and physiological disorders were transferred into 20-kg wooden bins with perforated polyethylene liners. Forty wooden bins of d'Anjou fruit were loaded into a gas-tight CA room at -1 °C to simulate 'field-run' untreated fruit transfer-ed directly from the orchard to the CA storage.
About 400 20-kg cardboard cartons packed with d'Anjou pears were also loaded into the CA room to simulate commercial CA storage. The core temperature of the fruit reached equilibrium at -1 °C after 96 hours. After 10 days of at -1 °C in air, 5 16-kg bags of fresh hydrated lime were put at the top of fruit boxes in the CA room to absorb CO2 produced by the fruit. The CA room was sealed and flushed with N2 until the O2 concentration reached 0.5% (approx. 72 hours). The concentrations of O2 and CO2 in the CA room were monitored; daily O2 concentration was maintained between 0.4 and 0.6%; CO2 was maintained at 0.08%.
After 108 days of storage, the CA room was opened and equilibrated to air within 5 hours. Fruit were held in air at -1 °C until use. At every biweekly interval, 4 boxes (24-kg wooden bin) of "field-run" fruit were removed from the cold storage and transferred into a ripening room at 20 °C for 7 days.
After short-term (108 days) low oxygen (0.5% 02 < 1% CO2) CA storage, d'Anjou fruit were completely free from scald during an additional 10 weeks of holding in air at 1 °C plus 7 days of ripening at 20 °C (Figure 1). However, fruit began developing minor amounts of scald (2% to 4%) 12 to 14 weeks in air following withdrawal from CA. These fruit developed more severe scald (28%) after 16 weeks (Figure 1) in air. Previous studies (Chen et al., 1990a; Chen et al., 1993) showed that 15 to 20% of optimally mature d'Anjou pears stored in air for 3 months (approximately 12 weeks) developed scald, which increased rapidly to 70% scalded fruit after 4 months in air. In this study, when fruit held at 0.5% O2 were returned to air for 3 to 4 months at -1 °C, they developed much lower incidence of scald than those fruit initially stored in air for a similar period. Thus, low oxygen reduced scald severity.
When fruit stored at 0.5% O2were returned to air at -1 °C, the concentration of alpha-farnesene in the skin increased linearly from 1.5 to 3.0 nmoles/cm-2 during 16 weeks of holding (Figure 2A), while the concentration of conjugated trienes also from 0.2 to 1.5 nmoles/cm-2 (Figure 2B). The "threshold level" of conjugated trienes that became toxic to the peel tissue of d'Anjou fruit to induce scald symptom was around 1.5 nmoles/cm-2, which was quite similar to the finding above. These results indicated that the biosynthetic pathways which lead to scald in the pear skin might be strongly suppressed by lowering O2 to 0.5% (Chen et al., 1993). When 0.5% O2, stored fruit were returned to air and held at -1 °C for further handling and marketing, they were much slower to regenerate those enzymes required in the pathways for synthesis of alpha-farnesene or conjugated trienes, as compared to regular air-stored fruit. In summary, 0.5% O2 CA-stored fruit took longer to develop scald during holding in air than fruit held in air the entire storage time. During 16 weeks of holding period, 0.5% O2 stored fruit were completely free from BS (Figure 1).
The minor setback of short-term low oxygen CA storage of d'Anjou pears was the development of PBC (8%) during 16 weeks of following removal from CA (Figure 1). PBC has been verified as a form of high CO2 injury (Blanpied, 1975; Hansen and Mellenthin, 1962). However, it is possible that less than the optimum level of O2 in CA might also be responsible for suffocation of the core tissue of d'Anjou pears resulting in PBC. In this experiment, the O2 chambers were set to maintain 0.4 to 0.6% with CO2 at 0.08% throughout the storage period.
An examination of the records of O2 and CO2 indicated that inadvertently the pears had been exposed to 0.4% O2 or lower frequently. Also CO2 levels had increased from 0.08% to 0.12% (Figures 3A and 3B). Maintaining 02 concentration at 0.7% (±0.1%) and CO2 concentration at <0.05% in short-term storage will be tested to verify its efficacy of minimizing PBC in d'Anjou pears.
Control of Scald, PBC, and BS in Mid- or Long-Term CA Storage
Since in the first experiment fruit stored at 1.5% to 2.0% O2 developed BS after mid- and long-term CA storage, the possibility of controlling scald by elevating CO2 from 1% to 3% in CA storage at 1.5% O2 was investigated.
In 1995, harvested fruit were drenched as above with a solution of Mertect 34F, except that 16 18-kg bins of fruit were drenched with Mertect 34F solution incorporated with 1,000 ppm ethoxyquin. The CA regimes were set as follows with 2 cabinets (replicates) per each CA regime at each storage interval:
- 1.5% O2/1.0% CO2
- 1.5% O2/2.0% CO2
- 1.5% O2/3.0% CO2
- 1.5% O2/< 0.05% CO2
(Fruit were drenched with 1,000 ppm ethoxyquin).
The concentrations of O2 and CO2 in each cabinet were maintained within ±0.10%. The concentrations of CO2 in 4 cabinets were maintained at < 0.05% by using hydrated lime (1 kg lime per 20 kg fruit) to absorb CO2. In this case [i.e., 4 cabinets], exogenous CO2 was not supplied to each cabinet.
After 6 months (mid-term CA), and 8 months (long-term CA) of storage, 2 cabinets from each CA regime were returned to air and held at 1 °C. After 1 and 2 months in air, fruit in 4 packed bins from each cabinet were transferred into a ripening room at 20 °C for 7 days.
After CA storage for 6 months, in 1.5% 02 at 1% CO2, plus 1 month in air, 9.6% of the fruit developed scald but were free from PBC. BS developed on 4.7% of these fruit. Scald was completely controlled if CO2 in the 1.5% 02 treatment was 2% or 3%. However, PBC increased from 6.6% to 19.5% at CO22 levels of 2% to 3% and BS increased from 3.7% to 15.6% (Table 3).
When fruit stored in 1.5% O2 at 1% CO2 were in air for 2 months, more than one-third (36.6%) of the fruit developed scald, while fruit stored in 2.0% and 3.0% CO2 developed much lower incidence of scald (11.2% and 4.6%, respectively). A 2-month holding period in air did not increase the incidences of PBC and BS, as compared to the fruit held in air for just 1 month, indicating that elevated CO2 in 1.5% O2 CA storage was the major cause of PBC and BS (Table 3).
After storage for 8 months, in 1.5% O2, 39.9% to 41.6% of fruit stored in 1.0% CO2 developed scald, regardless of the time in air. Elevating CO2 from 1% to 2% or 3% reduced scald to 6.2% and 4.1%, respectively. When the period was extended to 2 months, scald increased from 17.0% and 12.8%, respectively, for fruit stored in 2.0% or 3.0% CO2 (Table 3).
The development of PBC in d'Anjou pears stored in 1.5% 02 for 8 months was quite similar to that stored for 6 months. Fruit stored in 1% CO2 were free of PBC, while those stored in 2% and 3% CO2 developed proportionally greater amounts of PBC. These results confirmed that PBC is a CO2 related disorder as described previously (Blanpied, 1975; Hansen, 1957). After 8 months of 1.5% O2 CA storage, the incidence of BS increased dramatically to above 27%, regardless of CO2 concentration in the CA storage as well as the holding period. The results indicated that BS became inevitable if fruit were stored in long-term (8 months) 1.5% CA storage with CO2 at 1% or greater (Table 3).
Reducing Disorders Using Purge CA Storage
Using a purge CA storage system, Drake (1994) reported that d'Anjou pears stored in 1.5% 02 for 9 months developed less scald (22.3% to 12.3%). Internal breakdown (IB) was reduced (40.3% to 21.0%) when CO2 in CA storage was increased from 1% to 3%. The author suggested that the reduction of IB (presumably PBC) by elevating CO2 from 1% to 3% might be an action of a purge-type CA system that flushed out toxic volatile compounds. The incidence of scald or IB was assessed immediately after removal from CA storage and again after 8 days of ripening in air at 20 °C (Drake, 1994). Because many commercial CA rooms can hold approximately 40,000 20-kg packed cartons of fruit, it would require a minimal period of 3 weeks to be shipped. It would have been more realistic to assess disorders after they have been returned to air storage at -1 °C for at least 3 weeks.
Meeting USDA Standards
According to the United States Standards for winter pears (effective Sept. 10, 1955, USDA), commercially packed fruits classified as US extra #1, US #1, and US #2 should be free from any defects such as superficial scald, external markings, internal disorders, and decay, "In order to allow for variations incident to proper grading and handling, not more than a total of 10% of the pears in any lot may fail to meet the requirements of grade" (Section 51.1306 Tolerances).
Although elevated CO2 at 3% in 1.5% O2 CA storage suppressed the incidence of scald as compared to fruit stored in 1% or 2% CO2 the sum of scald, PBC, and BS of pears stored in 1.5% O2 plus CO2 at 1% or above was much greater than the commercially allowable tolerance level (i.e., <10%) as described above. Therefore, none of CO2 regimes in 1.5% O2 CA storage, including those reported in this study as well as those reported by Drake (1994), was feasible for the commercial application.
Ethoxyquin Treated Pears
A pre-storage drench of 1,000 ppm ethoxyquin controlled the development of scald completely after mid-term storage in 1.5% O2 with < 0.05% CO2 regardless, of the holding period in air. After long-term storage at 1.5% O2 with < 0.05% CO2, ethoxyquin treated fruit developed only minor incidence of scald (2.5% to 3.1%). These results indicated that a pre-storage drench of 1,000 ppm ethoxyquin was the most effective method for controlling scald of d'Anjou pears destined for mid-term or long-term storage at 1.5% O2.
Ethoxyquin-treated fruit were also completely free from PBC regardless of storage and holding period. Control of PBC in d'Anjou fruit was accomplished by holding the CO2 concentration in 1.5% O2 CA storage at less than 0.05% using lime.
Fruit developed only minor incidence of BS (5.7% to 7.0%) after long-term storage. Ethoxyquin might somewhat reduce the development of BS on d'Anjou pears, since fruit without pre-storage ethoxyquin treatment developed 10% and 20% incidence of BS respectively after 6 and 8 months of 1.5% O2 (with < 0.05% CO2) CA storage as shown above.
Pre-storage drench of 1,000 ppm ethoxyquin solution to d'Anjou pears was, therefore, the most effective measure for controlling scald, PBC, and BS simultaneously following mid-term or long-term CA (1.5% 2, < 0.05% CO2).
Low oxygen (0.5% O2, < 0.1% CO2 at -1 °C) controlled atmosphere (CA) storage of d'Anjou pears (Pyrus communis, L.) effectively reduced the superficial scald (scald) without inducing unacceptable levels of pithy brown core (PBC) or skin black speck (BS) disorders when stored for short-term (3 to 4 months) without treatment with ethoxyquin. Fruit remained free from scald after 8 to 10 weeks in air at -1 °C following CA. Thus these fruit could be safely packed and marketed within 2 months. The minor incidence of PBC (8%) which occurred in the fruit after this low oxygen CA storage was presumably caused by accidental and frequent drops of O2 concentration to 0.4% or lower in the CA room, rather than the desired low oxygen regime.
Fruit stored in low oxygen (0.5% O2) CA for mid-term (6 months) or long-term (8 months) were free from scald, but developed higher incidence of BS with a minor incidence of PBC.
Fruit stored in 1.0% O2 (< 0.1% CO2) for mid-term or long-term developed high incidences of scald and BS without developing PBC.
Increasing the oxygen from 1.0% to 1.5% or 2.0% caused d'Anjou fruit to suffer even higher incidence of scald, but reduced BS substantially.
CA regimes with 0.5% to 2.0% O2(< 0.1% CO2) could not control scald, PBC, and BS of d'Anjou fruit simultaneously over mid-term or long-term storage periods. In midterm or long-term CA storage with 1.5% 02, elevating CO2 from 1% to 2% or 3% suppressed the development of scald but aggravated high incidences of PBC and BS. For d'Anjou fruit destined for mid-term or long-term CA storage with O2 at 1.5%, a pre-storage drench of fruit with ethoxyquin (1,000 ppm) was necessary in order to control superficial scald effectively and CO2 in the CA storage must be kept to < 0.1% throughout the entire storage period to minimize the development of PBC and BS.
Summary of Results
- Short term storage - This study indicated that low
oxygen can effectively control the development of scald
without inducing unacceptable incidences of PBC and BS for
3 to 4 months storage. Fruit stored in short-term at 0.5%
O2 remained free from scald after 8 to 10 weeks
in air at
-1 °C following CA storage. Therefore, these fruit
safely packed and marketed within 2 months after being
returned to air storage. The minor incidence of PBC
occurred in fruit after short-term 0.5% O2
storage might be
minimized by increasing O2 level to 0.7% and
CO2 level to < 0.05% during the entire 3 to 4
- For fruit destined for mid-term or long-term
storage at 1.5% O2, a pre-storage drench with
ethoxyquin solution was required to control scald
effectively. The CO2 produced by fruit in the
be scrubbed by fresh hydrated lime (using the ratio of 1
lime per 20 kg fruit) to < 0.10% throughout the entire
storage period to minimize the development of PBC and
Storage Recommendations for Industry
On the basis of the results from these studies, in order to minimize the disorders of PBC, BS, and scald the authors have several suggestions for consideration by the industry.
According to marketing reports in 1995 1996, 42% of the d'Anjou pear crop was marketed prior to December 22, 1995 and 60% was stored in CA and marketed between late December early June. If we are to assume that this is a typical marketing schedule, I urge the industry to consider the following recommendations.
To minimize storage disorders, each packing house should consider placing 20% of the crop in air storage at -1 °C for marketing within the first 2.5 months. Ethoxyquin should not be needed if the fruit is of proper maturity.
The remainder of crop can be stored in CA. Twenty percent of the d'Anjou crop should be placed into CA storage immediately after harvest without any postharvest treatment and established in CA at 0.7% O2 at < 0.05% CO2 after the core temperature of the fruit reaches -1 °C. After up to 4 months of CA storage, this portion of fruit can be returned to air at -1 °C and packed and marketed within 2 months.
Sixty percent of the total d'Anjou crop should be drenched with 1,000 ppm ethoxyquin with a fungicide within 2 days of harvest and stored in CA at 1.6% O2 at < 0.1% CO2 at -1 °C. After 5 to 8 months in CA, this portion of fruit can be returned to air storage at -1 °C and packed and marketed within 2 months.
This will allow orderly marketing of the d'Anjou crop. However, these recommendations do not take into consideration issues involved with the control of shrivel, decay and scuffing from late season packing.
This study was supported by the Winter Pear Control Committee.
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Chen, P.M., Varga. R.J. and Xiao, Y.Q., 1993. Inhibition of a-farnesene biosynthesis and its oxidation in the peel tissue of d'Anjou pears by low O2/elevated C02 atmospheres. Postharvest Biol. and Tech., 3:215-223.
Chen, P.M., Varga, D.M., Mielke, E.A., Facteau,T.J. and Drake, S.R., 1990a. Control of superficial scald on d'Anjou pears by ethoxyquin: Effect of ethoxyquin concentration, time and method of application, and a combined effect with controlled atmosphere storage. J. Food Sci., 55:167-170.
Chen, P.M., Varga, D.M., Miellre. E.A., Facteau, T.J. and Drake, S.R., 1990b. Control of superficial scald on d'Anjou pears by ethoxyquin: Oxidation of a-farnesene and its inhibition J. Food Sci., 55:171-175 &180.
Drake, S.R., 1994. Elevated carbon dioxide storage of Anjou pears using purge-controlled atmosphere. HortSci., 29:299-301.
Hansen, E., 1957. Reactions of Anjou pears to carbon dioxide and oxygen content of the storage atmosphere. Proc. Amer. Soc. Hort. Sci., 69:110-115.
Hansen, E. and Mellenthin, W.M., 1962. Factors affecting susceptibility of pears to carbon dioxide injury Proc. Amer. Soc. Hort. Sci., 80:146-153.
Huelin, F.E., and Coggiola. L.M., 1970. Superficial scald, a functional disorder of stored apples. Oxidation of a-farnesene and its inhibition by diphenylamine. J. Sci. Food Agric., 21:44-xx
Little, C.R., Faragher, J.D. and Taylor, H.J., 1982. Effects of initial oxygen stress treatment in low oxygen modified atmosphere storage of Granny Smith apples. J. Amer. Soc. Hort. Sci., 107:320-323.
Sal'Kova, E.G., Morozova, N.P., Uturashvili, E.A. and Metlitskii, L.~,1975 The sesquiterpenic hydrocarbon farnesene and superficial scald of apples (in Russian). Prikladna Biokhimiya i Mikrobiologiya, 11:888-893.
Truter, A.B., Combrink, J.C. and Burger, S.A., 1994. Control of superficial scald in Granny Smith apples by ultra-low and stress levels of oxygen as an alternative to diphenylamine. J. Hort. Sci., 69:581-587.
Dr. Paul M. Chen and Diane M. Varga
Mid-Columbia Agricultural Research and Extension Center, Oregon State University
3005 Experiment Station Dr., Hood River, OR 97031
FAX: (541) 386-1905
13th Annual Postharvest Conference