Reducing water loss
Cool room design
The main method of reducing the vapour pressure deficit (VPD), thereby also reducing moisture loss, is to increase RH in the cool room air. Humidifying storage rooms is relatively simple, as all that is required is to add water, usually as a fine mist. However, adding mist can also lead to soft, damp packaging, floor puddles and increased disease if products become wet. Moreover, adding free water at rooms close to zero increases frosting on the refrigeration coils, leading to longer and more frequent defrost cycles.
One of the best ways to increase RH in the storage environment is to design the cooling system so that the delivery air is only slightly colder than the setpoint for the room. This minimises the difference in temperature between the coils and the produce, and so avoids excess condensation and/or drying of the air. Using large coils with a high surface area for heat exchange can help achieve this.
Rooms that are poorly sealed, frequently opened or vary significantly in temperature will have lower and more variable humidity than those that are more accurately controlled. In the example shown below, both rooms have been set at 1.5°C and fluctuate by approximately ±0.5°C overall. However, the larger and more frequent temperature changes in room 2 result in lower and more variable RH inside the room, potentially causing condensation on and/or dehydration of the produce.
As well as the magnitude of the VPD, many other factors will affect how quickly a vegetable will lose moisture. These include:
- The surface area to volume ratio. All other factors being equal, a leafy vegetable such as spinach will lose water more quickly than a more solid product such as a sweetpotato or eggplant.
- The characteristics of the plant skin. A thick skin and/or a waxy surface, such as that of a pumpkin, can present a significant barrier to moisture loss.
- The presence of open stomata or lenticels.
- Stomata in leaves usually close at harvest or in darkness, reducing moisture loss.
- Stomata may remain open during rapid cooling or following chilling damage.
- Lenticels are open pores, but may be blocked with soil or waxes. For example, washing treatments can spread natural waxes in the skin more evenly over the surface, helping to block lenticels.
- The skins of solanaceous fruit vegetables (eggplant, chilli) don’t have stomata or lenticels; most gas diffusion is through the calyx area. Leaving tomato fruit attached to the stem (as normal practice for hydroponics) or waxing the calyx scar, can both reduce postharvest water loss.
- Mechanical injury, including bruising or abrasion. Moisture loss increases as the integrity of the skin is destroyed.
The surface of produce has a thin layer of stationary air around it — the ‘boundary layer’. Within this layer the vapour pressures of the air and the product will be in equilibrium. The boundary layer is thickest for products with hairy skins, such as the Asian gourd ‘chi qua’ or ‘hairy melon’. Uneven surfaces, such as found on bitter melon (fu qua) also increase the boundary layer.
Increased air movement disrupts and reduces the boundary layer. Rapid air movement can help cool produce and air movement during storage is needed to remove heat generated by respiration. However, reducing air movement around fully cooled vegetables will avoid disrupting the boundary layer and reduce moisture loss.
Packaging has a clear role in increasing the RH around a product, thereby reducing the VPD. Packaging holds products together, restricting air movement around individual items. This increases the boundary layer and so reduces water loss. Even a mesh bag may reduce dehydration, simply by interfering with air movement.
Plastic films are very effective barriers to water loss. However, it is important to avoid condensation inside the package that can increase rots and diseases. Films made of ultra-thin, plasticised materials or which have anti-fog coatings or microperforations have reduced risk of internal condensation while still protecting against moisture loss.
Waxes and other fruit coatings can have a somewhat similar effect to packaging materials, especially if the material blocks openings in the vegetable skin. In this case there is no issue with condensation.
Shrink wrap, which is commonly used to package telegraph cucumbers, can also provide RH control without significant condensation. In this case the plastic is in close contact with the produce skin. Condensation does not occur inside the wrapping because the film and the product are at the same temperature. Shrink wrapping has been shown to be effective for a number of different products, however, cost and consumer resistance are barriers to use.
Some types of packaging can also have the opposite effect on water loss in produce. Wooden bins or boxes, paper and cardboard can all absorb significant amounts of moisture even before they appear damp. For example:
- Wooden bins can gain 6-10% weight in a moist environment. This would represent up to 5kg water absorbed by one half-tonne bin.
- Cardboard can gain up to 17% additional weight. For example, 128g of moisture could be absorbed from 5kg of vegetables by a 750g cardboard carton, a direct weight loss of 2.5% due to the carton alone.
Storing in a high RH environment can reduce moisture absorption by bins, although this is not always feasible. Waxed, varnished or plastic-coated cardboard is less moisture absorbent than raw cardboard.
Even if the atmosphere around a product is close to saturation, some moisture loss will still occur due to metabolic activity and respiration.