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I recently completed a professional study trip to Europe. One of the goals was to learn more about the need to control the relative humidity in CA storages and the technology used to maintain desired humidity levels. This report includes some of the ideas and information I gathered while talking with industry personnel, working on a storage environment research project and reading research reports.

As I toured Germany, Switzerland and the South Tyrol (Italy), I visited with CA researchers, storage operators and equipment suppliers, and attended meetings with grower groups. Humidity monitoring and controlling humidity using humidifiers and coil operation are parts of daily storage management. Maintaining desired humidity levels in CA storage is considered essential to maintain fruit quality. It was interesting to note that several researchers recommend an optimal moisture loss of apples of 3% regardless of the length of storage (Josef Streif, Bavendorf, Germany; Carlo Nardin, Bolzano, South Tyrol; and Von R. Kirchhof, Jork, Germany). These researchers have determined that a 3% moisture loss prevents scald and internal browning of fruit, and the fruit can be run through the packingline with minimal bruising. Also, a 3% moisture loss does not shrivel fruit.

Most weight loss of stored fruit is caused by transpiration of moisture. Transpiration is the loss of moisture from living tissues. Some weight loss also occurs due to loss of carbon in respiration, but this is relatively small. The rate of transpiration is primarily affected by temperature and relative humidity, but other factors such as variety and maturity are also important. The major driving force in transpiration is the vapor pressure difference (VPD) between the essentially saturated internal atmosphere of the fruit and the less saturated atmosphere of the surrounding air. The vapor pressure (VP) inside the fruit is directly related to its temperature. The VP outside the fruit depends on the temperature and the relative humidity (RH) of the air. The fruit's skin and air touching the skin surface resist moisture transfer from the fruit to the air. For most long-term storage conditions for apples or pears, fruit's skin prevents most moisture loss. The velocity of air flowing over the surface of the fruit has relatively little effect on moisture loss, however it has a significant effect on heat transfer from the fruit, or cooling rate. Doubling the air velocity increases the heat flow by about 40%. According to a study by J. J. Gaffney et al. (1985), the cooling rate of apples increased almost eightfold when the air flow was increased from 2 ft/min to 200 ft/min, but the transpiration rate increased only 3%.

Moisture losses are highest during the filling period of the storage and until the fruit is cooled, which takes two to three weeks. For example, during this period the warm fruit may be at 70°F with an internal moisture vapor pressure of 0.74 psi, but the cooling air at 65°F and a relative humidity of 70% have a vapor pressure of only 0.31 psi. This results in a vapor pressure deficit of 0.43 psi.

These data were obtained in a CA storage in Washington in fall 1989. Shown are VPD between the cooling air flowing through the runner space in the center of the stacks and that of the fruit in the center of the bin immediately below for several 5-day periods starting at the time the room was sealed. Accumulated moisture loss was 1% during the first 12 days, and 1.5% after 1 month in storage. Similar losses were obtained in Fall 1988. This project demonstrates the correlation between moisture losses and the VPD between the fruit and surrounding air. It also shows that there are excessive moisture losses during the first few weeks in storage. These losses could possibly be reduced in two ways: (1) increasing the relative humidity in the storage with humidifiers and (2) cooling the fruit faster in storage or pre-cooling prior to bringing it into the storage. Both methods would reduce the VPD, thus reducing weight loss.

Regulating Transpiration Rate

Regulating transpiration rates involves decreasing or increasing the VPD. This means raising the relative humidity (RH) to lower transpiration, or lowering the RH to increase transpiration. Kirchhof (1990) provided guidelines for operation of the refrigeration system to obtain a 3% weight loss at the end of the storage season. These guidelines include estimating fruit shrinkage by periodic weighing of 40-lb. fruit samples and/or keeping track of condensed water from the evaporators in systems with hot gas or electric defrost. Kirchhof recommends the use of humidifiers for the first 2 weeks in storage when the relative humidity of the storage air is usually much below desirable levels to provide extra moisture for saturating the wooden bins. By plotting the daily accumulated condensate from the coils during the storage season, the transpiration loss can be estimated and compared to the desired moisture loss (transpiration rate). If the actual moisture loss is too high, the relative humidity in the room can be increased by operating the humidifiers or, if transpiration is too low, the relative humidity can be lowered by increasing the temperature difference (TD) across the coils. For most storages however, the problem is usually too much moisture loss.

Relative humidity of the cooling air is affected by various factors, the major ones being evaporator coil design and operation, storage design and the use of humidifiers. Sainsbury (1979) investigated the effect of evaporator design and operation on the relative humidity of the cooling air. He concluded that desirable storage conditions can be obtained by using large coil areas and operating the system at the lowest possible TD between the return air and coil temperature. Any factor that increases heat input into a storage such as high power fans, continuous fan operation and low insulation levels will increase moisture loss. Sainsbury also suggested mounting the fans on the return side of the coils (push-through system) to reduce condensation on the coils. Van Beck (1983) reported that reducing air velocities and reducing fan operation time after initial cool down reduce moisture loss. Schwarz (1983) recommended the use of 2-speed fans to reduce the air flow from 40 air changes per hour to 10-15 changes per hour for long-term storage. He also cautioned growers to not "close-stack" when using very low air flow rates, to make certain that adequate air flow and cooling throughout the whole room is provided.

Humidity management is important in modern fruit storage along with temperature and storage atmosphere control. For humidifiers to be effective, they must humidify the cooling air that flows through the stacks. To increase the relative humidity of the cooling air, evaporated water must be added to the air stream, that is, liquid water must change into vapor and disperse into the atmosphere between the time it is injected into the air stream in front of the evaporator and the time it enters the stacks. The process of vaporization requires the addition of heat to the liquid. This heat comes from the surrounding air. For water at 32°F, the heat of vaporization is 972 Btu per pound. (For comparison, it takes only 1 Btu per pound to raise the temperature of the water by 1°F.) It is extremely important to break up the moisture into minute particles so that it can be vaporized in a short time period. A small droplet has a large surface-to-volume ratio which greatly enhances heat transfer and evaporation. For this reason, only humidifiers that break the water up into very small droplets are suitable for CA storages.

It takes very little moisture to increase the relative humidity in a CA storage room at 32°F. For example, a typical 2,000-bin CA room, when filled, contains about 6,000 lbs. of air containing 22.6 pounds of water when saturated. The difference in moisture between 90% relative humidity and 95% is only about 1.1 pounds; however, with a fan capacity of 40 air changes per hour, 0.75 lbs/of water per minute, or 5.4 gallons per hour of water must be evaporated into the cooling air during the refrigeration cycle to raise the humidity level by 5%. Between refrigeration cycles the coil temperature is usually above the dewpoint of the storage air, so there is no condensation, and humidification is not needed unless the RH is too low.

Simply adding water to a storage room with low pressure nozzles or flooding the floor with water are relatively ineffective methods of humidification. Similarly, keeping room temperature above freezing to reduce or eliminate the need for defrost is ineffective because most of the condensed moisture runs down the drain. Large droplets may be carried in the air stream but they usually fall out in a relatively short distance or when the air comes in contact with walls or bins, thus low pressure water nozzles producing large droplets are undesirable, especially at low temperatures such as in fruit storages. This was demonstrated in a research project by Braun and Angruner (1990) in 1,500 bins of commercial Golden Delicious CA rooms with Freon refrigeration systems. With a low-pressure humidification system consisting of nozzles in front of the evaporators, it took 50 days to bring the relative humidity levels to 95%, and the monthly weight loss for the storage 1986/87 season was 0.72%, or 6.7%, for the entire 9-month storage season. In comparison, after installing high-pressure, low-volume nozzles, and making other improvements in the operation of the refrigeration system, the weight losses were reduced to 0.57% per month. With the improved system, the 95% relative humidity level was reached less than a month after filling the rooms.

Most new storages in Europe have humidifiers. There are many humidification systems, methods and equipment in use. Some operators simply run the condensate from the coils onto the floors and add extra water during the temperature pulldown period in an attempt to provide additional moisture to saturate wooden bins. Some have installed low-pressure nozzles in front of the evaporators, others use small diameter, high-pressure nozzles that run at pressures of up to 10 atmospheres. With most of these systems, the droplets are large enough to get the bins and the fruit near the ceiling wet, although this does not seem to cause any problems. More sophisticated systems use an air compressor to inject air into the nozzles to break up the moisture into smaller droplets. In this case it is important that air is recirculated out of the CA room to avoid bringing in any oxygen. Nozzles, especially those with small orifices, are subject to plugging and erosion. Malfunctioning can be a problem, especially with hard water, thus frequent maintenance is required. Some rotary-type humidifiers break up the water into a fine mist with shear action, thus they can operate on low pressure; also, they do not need small diameter orifices and are less likely to malfunction.

A general guideline used by many European growers provides 1.5 L/hr per metric tonne (0.35 gal/hr per ton) of fruit. Higher amounts are used during the first three to four weeks in storage to help saturate the dry wooden bins. Although many European fruit production areas are not nearly as dry as those in Washington, storage operators are concerned about the effect of wood bins on storage environment. Many storage operators in Italy use plastic bins to reduce the moisture deficits due to moisture absorption by wooden bins. These bins are made locally, and the cost is competitive with wood bins since the plastic can be recycled into new bins.

One storage operator near Bolzano, Italy, hung an empty bin on a scale (visible through the window) inside a CA room to determine moisture absorption. The weight increase during the first month was 4 kg (8.8 lb). Weight increases of 10% for 2-bushel wood containers were found by Schwarz (1991) at the Swiss Federal Research Station which receives less than 20 inches precipitation per year. Wood samples from bins held in a CA room in Washington increased in weight by 7% during the first month, 10% during the first 2 months, and a total of 12% during the entire storage season (Waelti et al., 1989).

Humidity sensors are widely used to monitor and control humidifier operation. They are periodically checked for accuracy by placing psychrometers inside the window in the door. Humidifiers should be run only when the RH must be increased. The sensors should be located in the cooling air stream, about 40 ft from the coils and a little to one side.

Several storages in Switzerland and Germany run fans only during the refrigeration cycles, plus an additional 10-minute period every hour to provide air movement at least once per hour. Others operate the fans an additional 10-15 minute period at the end of each refrigeration cycle. After temperature pulldown, the fan speeds are reduced to lower the air volume by half for the rest of the storage season. With these systems, about 10-12 refrigeration cycles per day are required to maintain desired storage temperatures.

Braun, H. and B. Angruner. 1990. Energiebilanz der Obstkuhllagerung, 3.Teil, Messungen 1988-1990. Bundesministerium fur Landund Forstwirtschaft, August 1990, Vienna, Austria.

Gaffney, J. J., C.D. Baird and K. V. Chau. 1985. Influence of Airflow Rate, Respiration, Evaporative Cooling, and Other Factors Affecting Weight Loss Calculations for Fruits and Vegetables. CH-85-15, No. 2. American Society of Heating, Refrigerating and Air Conditioning Engineers, Inc. Atlanta, GA.

Kirchhof, V.R. 1990. Hinweise fur die Lagerung unserer wichtigsten Apfel-und Birnensorten. 1990-1991. In Mitteilungen, OVR (Obstbauversuchsring) 45, pp. 326-344, Jork, Germany.

Sainsbury, G. F. 1979. Moisture Loss of Apples in CA Storage Rooms. Paper presented at the 15th International Congress of Refrigeration, Venezia, Italy. International Institute of Refrigeration, Paris.

Schwarz, A. 1983. Problemes techniques de l'entreposage frigorifique des fruits et legumes. Kaelteanwendung in Nahrungsmittelsektor. Schriftenreihe des Schweizerischen Vereins fuer Kaeltetechnik, Sonderheft der "Temperatur Technik," Nr. 5. Zurich, Switzerland.

Van Beck, G. 1983. Weight Loss of Hard Fruit During Storage as Influenced by Cold Room Climate and Transpiration Coefficient. 16th International Congress of Refrigeration, Paris.

Waelti, H., R. P. Cavalieri and K. R. Zaugg. 1989. Effect of Evaporator Fan Operation on CA Storage Environment. Proceedings, 5th Annual Warehouse Seminar and Trade Show, March 28, 29, 1989. Wenatchee. Washington State Horticultural Association.