Pear Cool-Down and Mass Loss: Comparing Plastic and Wooden Bins
The mass loss of fruit from the time it is harvested until it is consumed significantly affects not only the quality, but also the economic return to the grower, storage operator, wholesaler, and retailer. Growers and storage operators in the Pacific Northwest are continuing to extend storage periods of d'Anjou pears to lengthen the traditional marking period. Two primary factors that influence fruit mass loss are the respiration rate and the difference in the partial pressure of the water vapor between the interior of the fruit and its surroundings. Respiration rate is exponentially proportional to fruit temperature, and the partial pressure gradient is proportional to humidity level of the atmosphere in the storeroom. The products of respiration are carbon dioxide, water, and heat. The mechanism by which the water and organic carbon are lost from the fruit is termed transpiration. The goal of successful storage operators is to minimize both respiration and transpiration. How storage rooms are operated and controlled both during fruit cool-down and during long-term storage are important in this minimization effort. Therefore, this study was conducted to determine if differences in fruit cool-down rates and mass loss could be determined between two similar commercial storerooms, one filled with fruit placed in wooden bins and the other filled with fruit placed in plastic bins.
Two previously instrumented rooms located at Duckwall-Pooley Fruit Company's Odell site were used for the 1999-2000 storage season study. Each room had one empty bin located in the geographical center of the top layer to hold mass loss and dew point temperature measurement instrumentation. Controlled atmosphere (CA) Room 16 (28 ft wide by 61 ft long by 28 ft high) was filled as a tight-stacked room with 1017 wooden bins of field-run d'Anjou pears. Bins were stacked 11 high, and pallets with hydrated lime were placed on the top bins to control carbon dioxide evolution. Based on the formula: 24 boxes of pears per bin times 44 lb per box, Room CA 16 held approximately 1.07 million lb of fruit.
Room CA 13 (30 ft wide by 61 ft long by 28 ft high) was filled as a tight-stacked room with 957 plastic bins of field-run d'Anjou pears. Bins were stacked 10 high and pallets of hydrated lime were placed on the top bins. Duckwall-Pooley used MacroPlastic, Incorporated Model-28 MacroBins, which have a rated capacity of 56,300 cubic inches. Careful measurement of the wooden bins used in Room CA 16 indicated that internal capacity was 50,749.5 cubic inches per bin, which indicated the plastic bins had 10.937% greater capacity than the wooden bins. Because the sides of the wooden bins bulge outward when filled with fruit, it was estimated that each plastic bin contained 10.0% more fruit than each wooden bin. Based on this assumption, Room CA 13 held approximately 1.11 million lb of fruit.
Room CA 16 began filling September 17, 1999, and was sealed September 22, 1999. Room CA 13 began filling September 19, 1999, and was sealed September 24, 1999. Thermistors to measure fruit temperature were placed among the fruit at floor level, six bins above floor level, and in the top bin. Fruit temperature measurements were taken at 27 locations throughout each room. Additional data recorded included dew point temperature at one location, refrigerant temperature into and out of the evaporator coil, and air temperatures at three locations: as air entered the rear of the evaporator coil, approximately 1 ft in front of the coil, and at mid-room at the same elevation as the evaporator coil. Temperature data were recorded at 10-minute intervals throughout the storage period.
Fruit mass loss was measured two ways in each room. Prior to room closure, 40 fruit were randomly selected from bins being placed in each room. Each fruit was numbered and weighed. The 40 fruit were then placed on the load-cell mass loss instrument and continuous measurement of mass was initiated. Data were recorded at 5-minute intervals throughout the study. At the conclusion of the storage period, each fruit was reweighed and mass loss was compared to the load-cell data. A complete description of fruit temperature measurement locations was presented by Hellickson and Baskins, 1996.
Results and Discussion
Fruit Cool-Down Comparisons. Initial temperatures of fruit placed in Room CA 16 ranged from 43.5 °F to 83 °F. The time required to cool from its placement temperature to 32 °F ranged from 40 hours at the top of the stack just in front of the evaporator coil to 236 hours at mid-room, near the right side on the floor. Fruit entering Room CA 13 ranged from 30 °F (cooled overnight in an adjacent room and placed in Room CA 13 the morning of September 19, 1999) to 76 °F. Because some fruit entered Room CA 13 already cooled to the room set-point temperature, comparison to warm fruit placed in Room CA 16 could not be justified. Of the 27 locations that fruit temperatures were monitored, 14 appeared to be comparable between rooms. Based on those data, the fruit in the wooden bin room cooled slightly more efficiently than in the plastic bin room. However, two locations directly in front of the evaporator coil in Room CA 16 were highly efficient. Locations at the farthest distance from the evaporator coil could not be compared and may have evened out any differences in overall room cooling performance.
Although a complete comparison of fruit cooling could not be made, important differences in bin stacking patterns were observed. The wooden bins had one-way pallets which required cross-stacking three rows near the door, leaving sizable spaces between stacks near the door. The plastic bins have two-way pallets and allowed precise placement in the room near the door without blocking airflow channels created by the runners or requiring open spaces in which to maneuver the forklift. Consequently, Room CA 13 had a much tighter, uniform bin stacking pattern and contained more fruit mass than Room CA 16. (See Fig 1.)
Figure 1. Comparison of bin stacking patterns and storage capacities.
Fruit Mass Loss Comparisons. Total fruit mass loss during the 69 days of storage in Room CA 16 was 1.73%. The slope of the mass loss line in Room CA 16 from 24 hours after room closure for the next 7 days was -0.00844. The slope of the mass loss line in Room CA 16 from 9 days to 16 days after room closure was -0.00540. The slope of the mass loss line in Room CA 16 from October 22 to 29, 1999 was -0.004016.
Total mass loss during the 64 days fruit were stored in Room CA 13 was 1.27%. The slope of the mass loss line in Room CA 13 from 24 hours after room closure for the next 7 days was -0.00538. The slope of the mass loss line in Room CA 13 from 7 days to 14 days after room closure was -0.00393. The slope of the mass loss line in Room CA 13 from October 22 to 29, 1999 -0.003337.
Comparison of the total mass loss and each of the linear regressions performed to determine the slope of the mass loss lines indicated that Room CA 13 was performing superior to Room CA 16 in reducing mass loss. The mass loss values documented during the 1999 storage period were compared to values recorded during the 1996-97 storage season and are presented in Table 1. These data also show that fruit stored in plastic bins (1999) in Room CA 13 appeared to lose less mass than when the same room was filled with wooden bins (1996).
Table 1. Mass loss comparisons.
|Year||Room||No. of Bins||Bin Type||Storage Time||Mass Loss|
|1999-2000||CA 13||957||Plastic||64 days|
|CA 16||1017||Wooden||69 days||1.73 %|
|1996-97||CA 13||1048||Wooden||118 days||2.26 %|
|CA 16||825||Wooden||107 days||2.25 %|
|*Values in parentheses are projected values based on slope of mass loss curve.|
Humidity Development Comparisons: Room CA 16 required approximately 10 days from closure to reach 90% relative humidity while Room CA 13 required only 1.5 days. Figure 2 illustrates the relative humidity development in each room during the first days of storage. Room CA 13 consistently remained 3% to 5% higher in relative humidity than Room CA 16 throughout the storage period.
Figure 2. Comparison of relative humidity development.
Although Room CA 13 was filled with fewer bins of fruit than Room CA 16, it contained approximately 38,000 lb more pears. This additional mass and the use of plastic bins allowed Room CA 13 to develop more rapidly and maintain room relative humidity higher than in the wooden bin room. Ammonia temperatures recorded in each room show the evaporator coils were operating at the same temperature once cool-down had been achieved. No specific conclusions could be drawn about cool-down rates of plastic versus wood; however, variations in cooling efficiency at specific locations within each room reinforced the need for improvements in air flow uniformity. Room CA 13 did experience some coil frosting problems when defrost cycles were reduced to every other day. That problem was eliminated by defrosting on a daily basis.
Hellickson, M. L. and R. Baskins. 1996. Cool-down comparisons of d'Anjou pears tight-stack versus conventional stacking. Or. AES Tech. Paper No. 10,956. Proceedings of the 12th Annual Washington Tree Fruit Postharvest Conference. Wash. State Horticultural Association. P.O. Box 136, Wenatchee, WA. 98807. March.
Dr. Martin Hellickson(1) and Robert A. Baskins(2)
(1)Oregon State University
Department of Bioresource Engineering
Gilmore Hall, Corvallis, OR 97331-3906
(2)Duckwall-Pooley Fruit Company
3430 Davis Dr., Hood River, OR 97031
16th Annual Postharvest Conference, Yakima, WA
March 14-15, 2000