Elevated Carbon Dioxide Storage of Loose Packed 'Bosc' Pears
'Bosc' pears were placed in controlled-atmosphere (CA) storage immediately after harvest and held for 180 days at 1 ºC (34 ºF). Oxygen was 1.5% and CO2 was 1%, 3% or 5%. Atmospheres in a purge-type system were computer-controlled at ±0.1%. After removal from CA storage, pears were evaluated immediately, and after ripening at 21 ºC for 7 days. Pears stored in 3% CO2, were firmer, had a superior finish, with significantly reduced decay, than pears stored in 1% CO2. In addition no internal discoloration of 'Bosc' pears was evident when held in 3% CO2. 'Bosc' pears, held in 3% CO2, retained their ability to ripen after long-term storage. 'Bosc' pears stored at 5% CO2 were similar in quality to pears stored in 3%. A 10-day delay in atmosphere establishment had little or no influence on the long-term keeping quality or ripening ability of 'Bosc' pears. Firmness, soluble solids content and starch either alone or together were good indices of maturity for 'Bosc' pears.
Most previous research in CA pear storage has been conducted with static systems where fruit is held in the same atmosphere over the entire storage period and only O2 and CO2 levels are regulated. Many new CA facilities employ a flow-through system where fruit is exposed to a flowing atmosphere of desired gas concentrations. This research was conducted to determine if 'Bosc' pears could be stored under higher than normal CO2 levels and maintain quality over an extended storage period with possible storage cost reduction.
The author would like to express appreciation to the Washington State Tree Fruit Research Commission for partially funding this study.
Materials and Methods
This study was conducted over 4 crop seasons using 'Bosc' pears (Pyrus communis) grown in the Wenatchee, WA area. Starting 3 weeks prior to estimated commercial harvest, 10 pears from each of 4 trees in 2 locations were randomly harvested and maturity assessed. The same locations were sampled each year, but different trees were used. One to two days prior to commercial harvest, 120 pears were selected from each of the 4 trees at the 2 locations. Pears from each tree were kept separate and trees were used as replications. Immediately after harvest the pears were divided into 3 groups. Within 12 hours of harvest all pears were placed in CA chambers (Ca. 0.14M3) and CA conditions of 1% CO2, 3% CO2 or 5% CO2 at 1.5% O2 and 1 ºC were established with 24 hours. In one year of the study the atmosphere conditions were established on one-half of the pears and the remaining pears were held in RA at 1 ºC and CA established after 10 days. The CA condition of 5% CO2 was used only during one season. Pears were stored for 6 months before evaluation. All atmospheres were established and maintained throughout the storage period at ± 0.1% using a computer control system. Nitrogen for this purge type CA system was supplied by a membrane separation system and bottled CO2 was used as the CO2 source.
Pears from the orchard were evaluated on a weekly basis (maturity) for firmness, soluble solids content (SSC), titratable acidity (TA), external and internal color and starch. Storage evaluation consisted of 20 pears from all combinations of atmosphere, time delay and replication. Ten pears were evaluated immediately after removal from storage and the remaining 10 were allowed to ripen for 7 days at ambient temperature (21 ºC) before evaluation. Quality factors evaluated were: firmness, SSC, TA, external and internal color, visible disorders and carbohydrates.
Results and Discussion
'Bosc' pears sampled over a 3-week period differed due to growing location and weeks prior to harvest (Table 1). Regardless of the maturity indices, differences between locations were constant. Rapid change in maturity indices, as related to time before harvest, was apparent particularly for firmness, SSC and starch. Firmness, SSC and starch changed weekly with uniform separation between weeks. TA and weight measurements changed only between the first and second weeks. Color change was not evident until the final week of sampling and 1 day prior to commercial harvest. Firmness, SSC or starch alone or in combination, were the best indicators of maturity, due to their relatively uniform change over the 3 week sampling period. TA, weight and color were of limited value to determine changes in maturity prior to harvest for 'Bosc' pears.
'Bosc' pears stored in 3% CO2 were 4% firmer (2.2 N) than those pears stored in 1% CO2 after 180 days of storage (Table 2). When allowed to ripen for 7 days at ambient temperature, pears from either 1 or 3% CO2 storage ripened to similar firmness levels (17 N). When pears were held in cold storage (l ºC) for 10 days before atmosphere establishment a similar firmness pattern was present and no advantage for rapid CA establishment was evident (data not shown).
There was a distinct difference in SSC of the 'Bosc' pears due to CO2 concentration. Pears stored in 3% CO2maintained a higher SSC than pears stored in 1% CO2. This difference in SSC was present immediately after removal from storage and after a 7-day ripening period. Differences for individual carbohydrates concentrations (sucrose, fructose, glucose and sorbitol) between pears from the two CO2 concentrations were minimal. Differences in individual carbohydrates were evident between years and growing locations, but CO2 concentration in the atmosphere had no influence. TA values were different for pears stored in 1 or 3% CO2 regardless of storage or ripening. Pears stored in 1% CO2 maintained a higher level (6%) of TA than pears stored in 3% CO2. Pears from both CO2 storage environments had different SSC:TA ratios after storage and ripening. The SSC:TA for pears stored in 3% CO2 was much higher than the ratio for pears stored in 1% CO2. The SSC/TA ratio is a better flavor indicator than either the SSC or TA alone and higher ratios can be perceived to have superior flavor in apples (Boylston et al., 1994). The SSC/TA ratio determined for 'Bosc' pears in this study were in a range determined acceptable (Kappel et. al., 1993).
'Bosc' pears stored in 3% CO2 were less yellow (lower b values) than pears stored in 1% CO2 both after storage and ripening (Table 3). This difference in external color for pears from the two atmospheres was not consistent when L values or hue were considered. Pears from both atmospheres had similar L values both after storage and ripening. Exterior hue values were higher for pears in 3% CO2 immediately after storage but after 7 days of ripening hue value were similar for pears from both atmospheres. This difference in external hue values (arctan b/a) indicates that pears stored in 3% CO2 were less brown immediately after storage, but during ripening changed color more rapidly than pears stored in 1% CO2. Due to the normal color (brown) of 'Bosc' pears, differences in color between pears from the two atmospheres would be difficult to determine.
Hansen and Mellethin (1962) and Claypool (1963) reported that high CO2 in the storage results in a darker internal color of 'Anjou' pears. Drake (1994) found no internal color differences in 'Anjou' pears stored at 1% or 3% CO2 type atmosphere. The internal color of 'Bosc' pears was dependent upon the level of CO2 in the storage atmosphere immediately after storage. After 7 days of ripening internal color of the pears were similar regardless of CO2 level in the storage atmosphere (Table 3). This color difference (b and hue values) immediately after storage would indicate a less yellow (more immature) internal color for pears stored in 3% CO2 than in 1% CO2, but after ripening no internal color difference was apparent. These differences in external and internal color were slight and would be considered of no commercial consequence even though a change of 1.0 unit is visibly detectable (Hunter and Harold 1987). In one storage season using 5% CO2 similar external and internal color patterns were noted (data not shown).
Pear finish was enhanced and decay was reduced in 'Bosc' pears that were stored in 3% CO2 when compared to pears in 1% CO2 (Table 4). Pears stored in 3% CO2 were graded as having a better finish (1.0) than pears stored in 1% CO2 (1.2). After 7 days of ripening, scores were comparable between pears from the two storage atmospheres. The difference in decay as related to atmosphere was not evident immediately after removal from storage. After a 7-day ripening period, pears that had been stored in 3% CO2 had 60% less visible decay than pears stored in 1% CO2.
Internal breakdown (core browning) was of particular concern, but in this study no difference between atmospheres was noted. The pear industry in Washington State has been reluctant to store pears in a CO2 atmosphere above 1%, based on past experience of a higher incidence of breakdown using static-type CA. Drake (1994) found reduced internal breakdown in 'Anjou' pears stored in 3% CO2 versus 1% CO2. Dilley (1993), working with apples (Malus domestica Borkh.), suggested that purge CA may flush volatile compounds that contribute to physiological disorders. Gast (1993) noted a reduction in volatile production when apples were stored in a purge-type CA. In this study no internal problems or physiological disorders could be related to either storage environment of 1% or 3% CO2 of 180 days during the entire four years of the study.
Maintaining a higher CO2 in the storage atmosphere reduces operational cost (Wealti and Cavalieri, 1990). Three percent CO2 in the storage atmosphere is well above the recommended CO2 atmosphere of 1% to 1.5% for 'Bosc' pears. In this study there were definite quality advantages for 'Bosc' pears stored in 3% CO2 for 180 days relative to 1% CO2 and no apparent disadvantages. The advantages included reduced firmness loss, higher SSC:TA ratio and much less decay with no enhanced physiological disorders. A 3% CO2 atmosphere for the storage of 'Bosc' pears enhances keeping quality while allowing for normal ripening and possible reduced storage cost. 'Bosc' pears harvested at a firmness of 68 N with a starch content of 2.0 are good candidates for CA storage of 180 days in an atmosphere of 1.5% O2 and 3% CO2.
Dr. Steve Drake
USDA, ARS Tree Fruit Research Laboratory
1104 N. Western Ave., Wenatchee, WA 98801
14th Annual Postharvest Conference,
March 10-11, 1998