Storage of Perishable Commodities – Importance Of Humidity

Refrigeration has been the principal known method of successful storage of fresh fruits and vegetables to retain their freshness and flavour. When fruits and vegetables are harvested, they are cutoff from their source of water and nutrition and soon start to deteriorate. They lose weight, texture, flavour, nutritive value and appeal. Both time and temperature are important factors in post-harvest product deterioration. Cooling the harvested product control the rate of quality loss by slowing the rate of respiration. The warmer the temperature, faster is the deterioration and the shorter is the storage life, conversely, the cooler the temperature, the slower the deterioration and the longer the storage life. The more quickly the product is cooled, the longer it will remain marketable. Storage is the art of keeping the quality of agricultural materials and preventing them from deterioration for specific period of time, beyond their normal shelf life. Cold storage Control ripening retards aging, softening, texture and colour change, retards moisture loss, wilting, microbial activity, spoilage, sprouting and undesirable growth.

However, when the temperature of air in the cold store drops below its dew point, the moisture of air condenses as water on to the cold surfaces, particularly the cooling coils in the refrigeration system. As this air circulates around the chill store it warms, reducing its relative humidity. The warmer air draws moisture from any surface it can, including the produce itself. This leads to product weight loss – as much as 20% – and cells become less turgid, affecting the appearance, quality and shelf-life of the produce, all of which reduce the value of the stock. The solution? Raise the relative humidity (RH) to a level that’s in equilibrium with the produce. Optimal conditions vary according to the specific type of fruit, vegetables, flowers or other food product being stored but, typically, a humidity of around 95% RH at a temperature just above freezing is needed but, even produce stored in warmer conditions benefit from a humidity level around 75% RH. The required moisture is introduced directly into the air, evenly throughout the store.

Humidification is a process by which moisture is provided inside a given space, where this is scarce or insufficient, by means of a humidifier. For storage of fresh products, the important factors are temperature, humidity, ethylene and odour, which need to be managed. Until recently, most of the stakeholders’ attention was focused only on temperature. Loss of water from produce is often associated with a loss of quality, as visual changes such as wilting or shriveling and textural changes can take place. Leafy crops tend to lose water the most quickly. If using mechanical refrigeration for cooling, the larger the area of the refrigerator coils, the higher the relative humidity in the cold room will remain. It pays however, to remember that water loss may not always be undesirable, for example if your produce is destined for dehydration or processing to puree or sauces. Water loss of 3 to 6% is generally enough to cause a noticeable loss of quality and value. Stone fruits (peaches, plums and apricots) look shriveled when they suffer water loss of 4 to 5%. Root crops (carrots, beets, turnips, radishes) will lose water much faster if their tops are left attached, since the leaves will continue to transpire, taking water from the roots. Reducing the rate of water loss slows the rate of shriveling and wilting, causes of serious post harvest losses. If you sell produce by weight, every bit of water lost is also lost profits. Produce handled and marketed at 27°C and 60% RH (very adverse conditions, but not uncommon in Punjab in April and May) can lose a lot of water in a short time. Accurate humidity control (Table 1.0) is the key to maintaining quality fresh produce in cold storage. Refrigerated air tends to be lower in relative humidity than is beneficial for storage of most horticultural crops. The simplest method of increasing relative humidity of the storage air is to wet the floor of the room or mist the storage containers with cold water and allow the water to evaporate. The dry air increases transpiration and water loss from the stored product, resulting in lost weight and lost value. Potatoes stored without added humidity can lose 10-12% of their weight over six months. The humidity of the air in storage rooms directly affects the keeping quality of the products held in them. If it is too low, wilting or shriveling is likely to occur in most fruits, vegetables, cut flower and related products. Maintaining humidity high enough in commercial storage is usually a greater problem than the occurrence of too high humidity. High humidity is beneficial for wound healing and periderm formation during curing of certain crops.

RH can be controlled by

• Adding moisture to air (water mist or spray)
• Regulating air movement and ventilation rates in relation to produce
• Maintaining refrigeration coils in a cold room within 1°C or 2°F of air temperature
• Providing moisture barriers (insulation, liners, plastic films)
• Wetting floors in storage rooms
• Adding crushed ice to packages and displays (for tolerant crops)
• Sprinkling produce during retail marketing (leafy vegetables, cool season root crops, immature fruit crops)

Benefits of humidification

• Raising the relative humidity inhibits moisture loss.
• The right humidity level ensures that product quality is maintained.
• Fresh, just picked appearance increases value.
• Improved profitability through maintained product weight.
• Longer storage and greater shelf life reduce waste.
• Direct room humidification ensures the right humidity level evenly around the store, maintaining produce quality throughout.
• Close humidity control means the right humidity for the right produce.

Relative humidity

Relative humidity is defined as the ratio of the water vapour pressure in the air to the saturation vapour pressure at the same temperature, and is normally expressed as a percent. The difference in vapour pressure can cause water vapour movement from or to objects in contact with the air. The water holding capacity of air increases as the temperature rises; hence, air at 90 percent relative humidity at 10°C contains more water by weight than air at 90 percent relative humidity at 0°C. Never-the-less, water would be lost from a product at about twice the rate in a room at 10°C than at 0°C if the relative humidity is 90 percent in both.

High relative humidities (85 to 100 percent) are recommended for most perishable horticultural products to retard softening and wilting from moisture loss. For most vegetables, the relative humidity should be about 90 to 100 percent. If it is necessary to increase the relative humidity in rooms used for common storage, or air-cooled (non-refrigerated) storage, water may be sprinkled on the floor occasionally or misters may be installed. The relative humidities recommended are those that will retard moisture loss and do not favour excessive growth of micro-organisms. Of major importance in maintaining adequate relative humidity in the storage air is providing good insulation, avoiding air leaks, and providing sufficient cooling surface so that the spread between the temperature of the refrigerating surface and desired commodity temperature is as small as possible.

Therefore, accurate control of refrigerant temperature is essential for maintain high humidities in a mechanically refrigerated storage. As the difference between the temperature of the refrigerating surface and the temperature of the air in contact with the refrigerating surface increases, the humidity decreases.

For example, if the air moving over the refrigerating surface is cooled to – 1.1°C and the surface temperature is – 3.9°, the relative humidity of this air will be about 78 percent, because any water vapour in excess of the amount that can be held at this relative humidity will condense on the coil. However, if air at -1.1°C moves over a refrigerating surface maintained at -2.8°, the relative humidity will be 89 percent and if, instead, the air moves over a refrigerating surface at -1.7° (a temperature difference of only 0.6°), then a relative humidity of 94 percent could be excepted. Actual humidities obtained under these conditions of refrigerating surface and air temperatures will usually be somewhat higher than indicated, because not all the air comes in contact with the refrigerating surface. Thus, air leaving the chamber will be a blend of that which has lost moisture and that which has retained its original moisture content.

As the difference between refrigerant temperature and air temperature becomes narrower, a greater refrigerating surface is necessary. If air is cooled from 0° to -1.1°C during passage through a dry-coil bunker, considerable more surfaces will be required to accomplish this temperature reduction. With an adequate refrigerating surface and with the temperature of the surface controlled by automatic devices, there should be no humidity problem. When the refrigerating surface is not adequate to maintain the desired atmosphere humidity, pressure atomized or heat-vaporized water can be added to the air.

A system capable of supplying 4L of water/hour/ton of refrigeration should be able to maintain 95 percent relative humidity under any reasonable conditions (figure 1.0). Up to half of the amount of water may still condense on the cooling surfaces, but the other half provides for absorption by dry containers, walls, ceiling, and floor, if needed. When the demand is less, water sprays or mist can be reduced.

Another method of maintaining high humidity is the jacket system. In this system, there is a jacket or envelop surrounding the room in which cool air circulates. Heat leaking into the room is absorbed by the flowing refrigerated air in the jacket. The main advantage of the jacketed system is that high relative humidity, approaching 100% can be maintained while condensation of water vapour in the insulation room is prevented. However, construction and operating costs of jacketed storages are somewhat higher than those for conventional storages.

Several different devices for measuring relative humidity (Figure 2.0) are available. A common instrument is a psychrometer with two thermometers. The bulb of one thermometer is left uncovered, and the bulb of the other is covered with a wick that has been wetted with distilled water. This wet and dry bulb psychrometer works on the principle that if the ambient air is not saturated, water will evaporate from the wet-muslin wick, thereby cooling the wet bulb. The wet bulb drops to a definite temperature when the heat received by convection and conduction from the air flowing over the bulb balances the heat required to evaporate the pure water on the wick. From the difference in temperatures of the wet and dry bulbs, the relative humidity can be determined from slide rules provided with the pshychrometer or from psychrometric tables or charts.

Direct readings of relative humidity may be taken with hair hygrometers or gold beater’s skin hygrometers. These measurements are based on the principle that organic filaments change length as a function of relative humidity. These devices are relatively inexpensive and not very accurate, especially at high humidity and they must be checked periodically for accuracy with a psychrometer. Electrical hygrometers are being used increasingly for humidity measurements and to control humidifying equipment. Operation of these is based on the ability of a hygroscopic film to change its electrical resistance instantly with small changes in relative humidity. They must be calibrated periodically.

Figure 1.0 Specially designed fogging head will put humidity only into the air, and not on the floor…

Spray humidifier

A commonly used humidifier in the multi commodity cold store (Namdhari Seed Pack-house at Ludhiana) is shown in figure 3.0. In this type of humidifier, the water is lifted upwards on the revolving PVC discs by a centrifuge, rotating on high speed (2850 rpm). Due to high centrifugal force, the water strikes the SS atomiser grid with great impact, where the water layer is broken into ultra fine mist. The flow of air carries the droplet free aerosol/mist into the store.

Steam humidifier

It is an electrode humidifier. It produces steam for humidification by passing electrode current through the water in the steam generator cylinder between metal electrodes. Steam output is directly proportional to the conductivity of the water (Figure 4.0), and the amount of electrode immersed in the water. On a call for humidity, the humidifier controller will open the water fill valve (1) and allow water to enter the system. A flow restrictor prevents the unit from filling too quickly or with too much pressure. As the water fills the cylinder, it will reach the electrodes and current will begin to flow. As the water continues to fill the cylinder, the current will increase, and this is monitored by an amperage transformer placed on one of the power wires. Periodically, based on the incoming water conductivity, the unit will drain some water to reduce the mineral concentration. If there is no water in the cylinder, there will be no current flow and no steam production. The electrodes do not burn out, but they will eventually become completely coated with mineral and the cylinder will then need to be replaced or cleaned.

It works on the principle of centrifugal atomization and produces droplets/mist with no wastage of water. Evaporation rate; 6-7 lts/hr Coverage ; 6000ft 3 , particle size 5 – 10 micron Air circulation ; 800 CMH

Figure 3 Humidifier used by Namdhari Seeds at its packhouse and cold store at Ludhiana

Figure 4.0   Steam humidifier…

Figure 5.0 Ultra sonic humidifier…

Ultra sonic humidifier

These use a piezo-electric transducer to create a high frequency mechanical oscillation in a body of water. The water tries to follow the high frequency oscillation but cannot because of its comparative weight and mass inertia. Thus, a momentary vacuum is created on the negative oscillation, causing the water to cavitate into vapour. The transducer follows this with a positive oscillation that fine mist, about one micron in diameter that is quickly absorbed into the air flow.

Since the mist is created by oscillation, not heat, the water Temperature need not to be raised. Ultrasonic humidifiers, therefore, can create instantaneous humidity, and don’t have to wait for a heating element to boil the water. This precise on/off humidity control is the hallmark of ultrasonic humidifiers. In addition, unlike wet pad humidifiers, ultrasonic units can be of comparatively small size, and still produce significant amounts of vapour. When the sensors, which pick up ultrasonic humidifier has decreased below a certain level of humidity of the air ultrasonic humidifier comes into operation. When, however, capture a high degree of moisture the cool mist humidifier is turned off. In this way, it is very simple to maintain a high degree of humidity within a room as the cool mist humidifier works automatically. Main feature of this humidifier is that no mechanical drive, no noise, the output fog grain diameter only be 1-10 micron.

Humidification requirements for multi commodity store:

Protocols set up by National Horticultural Board and adopted by other government agencies with regard to humidity requirements in cold stores are as follows: “Although higher humidity levels of 85-90% can be achieved by keeping low delta T in the cooling coil. But during loading periods and for RH>90%, humidification system is a must. Several techniques are available, but it should preferably be done using water mist with 2 – 10 micron and uniformly distributed all over the chamber ensuring that the product does not get wet.”

Conclusion

Ventilation and refrigeration in your walk-in cooler can hasten the dehydration process, robbing your vegetables of the moisture needed to maintain their freshness and appearance.

A walk-in cooler humidification system will help to prevent the ‘tired’ look that many vegetables can display each morning due to the effects of dehydration. Even non-misted items such as mushrooms, strawberries, and some citrus can benefit from proper RH levels, and now, new technologies have made humidification more affordable than ever.

With proper humidification, products generally last twice as long as they would otherwise, making it much easier to manage inventory and use the walk-in-cooler for the purpose it was intended.

If you want to share thoughts or feedback on this article, please leave a comment below.