By Benjamin Brits
When one thinks of cold or cooling, linking that function with humidity seems disjointed, however in many instances humidity control is critical in several aspects of the supply chain to support quality products and process consistency.
In the modern age, the thought of preservation of perishable goods has widely focused on items that should be in a fridge, however the heating and refrigeration engineering sector is in fact responsible for a far greater impact on the world’s supply chains that involve items classed as “perishable”.
The definition linked to this term is often stated as: any item with a limited shelf life that is likely to decay, spoil, go bad or die – outright or in an accelerated timeframe – if not kept under ideal conditions – such as refrigeration or in a controlled environment. Most often these items are classed into foodstuffs, beverages, and biologicals.
Taking a more detailed look into preservation of perishables will show that the role of engineering has far-reaching arms and the need for humidity control extends to many more functions to supply quality and consistency to the consumer – from everyday fresh fruit and vegetables to items that have a longer shelf life but are still classed as perishable, and even to the production of your favourite beverages, and more.
The history surrounding the understanding of humidity as well as the mechanical devices and techniques used in measurement – the hygrometer (humidity measure) and other devices influencing humidity such as the barometer (pressure) and anemometer (air flow/speed) date back many centuries and involve some well-known people of history such as Leonardo da Vinci, Francesco Folli, Sir John Leslie, Robert Hooke and Sir Isaac Newton – a subject matter clearly followed attentively around the world.
Natural humidity conditions vary quite broadly depending largely on geographic location and it can be an engineering challenge to implement the correct solutions while not breaking the bank in terms of energy consumption, although, some humidification solutions are amongst the cheapest and simplest forms of ‘environment conditioning’.
Around the world (and even within South Africa), environments can be drastically different from one another. This not only includes inland or coastal factors but differing weather cycles depending on regions. This could include the extremes of any of the earth’s elements – permanent high humidity levels because of continual rain, or low levels due to habitats where relative humidity (RH) can be as low as a couple of percent such as in the South Australian desert.
Relative humidity (RH) as a simple explanation is a measure of the water vapor content of air. More technically, it is the amount of water vapor present in air expressed as a percentage (%RH) of the amount needed to achieve saturation at a particular temperature and pressure.
Humidity control throughout the cold chain has always been regarded as “notoriously difficult”, owing to the fact that most ‘cold spaces’ are constructed and sealed to maximise the efficiency of the cooling system. With no/limited ventilation or mechanical systems, water (as vapour) has no way to ‘move freely’ which creates the difficult condition of regulation of the humidity levels. Therefore, depending on the product and process, humidification or dehumidification equipment would be required. Humidity conditions in the cold chain also have to take into account that fresh produce harvested may be coming out of the field where conditions can vary between 30 and 45 degrees Celsius.
Most commonly, humidity control (humidification and de-humidification) is applied where close control conditions are required to meet operational set points. An example of the need for simultaneous control is within indoor growing [organic] facilities where the transpiration rate varies during the growth stages, but relative humidity must be maintained to optimise plant growth. Alternatively, either humidification or de-humidification would be required if conditions outside of a desired range would have a negative impact – such as an item or product drying out or creating an environment for bacterial growth.
The ideal gas law describes the relationship between pressure, volume, and temperature in air. The reason why humidity control is required comes down to the nature of the expansion and contraction that occurs due to changing conditions. Expansion and contraction thus create differences in RH and the difference in RH creates various impacts for products that could be extremely sensitive to changes. Here examples could be pharmaceutical products that could experience altered characteristics while in storage, or water losses (shrivel) with fresh fruits, and even products that may experience a change in taste or aroma.
Optimal conditions vary according to the specific type of product, be this fruit, vegetables, flowers or other perishables – fresh or being stored. Incorrect humidity (most often) leads to product water loss which could be as high as 20% – cells then become less full or swollen, affecting the appearance, quality and shelf-life of the produce, which then in turn will result in a reduction in the value of the stock, or in worst case scenarios – a total loss. Fresh commodities as an example can be considered as ‘unique packages of liquid’. In fact, what the consumer perceives as ‘freshness’ is directly linked to that liquid/moisture content, and of course freshness is what is rewarded with the highest marketability and thus rates. A picture that anyone can associate with is a punnet of berries that is shrivelled up, a box of bananas that are all brown, or wilted spinach that looks like it has been in the sun all day – all of which could still be consumed but would not be appealing to the consumer.
It is critically important to be able to control the RH to a highly specific point as warm air can hold more RH, for example, when hot fruit comes into a room one needs that high RH to avoid quality problems with the product down the line. However, when the air gets colder the air holds less water. So, this process must be accurately controlled to avoid condensation forming, which will occur if too much RH is in the air that drops in temperature and then that water condensates.
It is imperative to understand that in cold storage and in cooling, ‘living materials’ are not the same as cooling a beverage – where RH would not feature in any case. Similarly, the cooling of open fruit versus boxed are also two totally different situations. For this reason, engineers need to take the natural processes occurring into account. Blower coils withdraw water out of the air. This process uses a natural force to control the RH as the temperature drops. So, cooling and retaining the correct RH per product must be understood correctly.
The effect of humidity control at storage facilities
As mentioned, the quality of fresh produce as well as that in storage depends to a great degree on the humidity of the immediate environment. Humidity is more difficult to control than temperature and often does not receive adequate consideration when storage facilities are designed.
If the air is too dry, there may be enough water loss to affect the texture of the product and cause visible effects such as shrivelling or wilting. It can even make the product unsaleable. There are certain varieties of stone fruits that experience deterioration from as little as 0.8% in weight loss while fruits such as apples and pears are most resistant to moisture loss, but still over several months of storage may lose 2-3% (or more) in weight because of water loss. A moisture loss of 4-5% results in a ‘spongy texture’ and visible shrivelling of these products. Table grapes would experience dried stems and brown berries.
Excessive humidity, on the other hand, is conducive to growth of mould and decay organisms, particularly when water droplets form on the surface of certain fruits. There is increasing evidence that extremely high humidity (or oversaturation of the air), particularly in the early part of the cold chain, can contribute to physiological disorders. With most commodities, however, the problem is one of maintaining enough moisture in the storage zone. (A few vegetables such as onions, garlic, squash, and pumpkin require low relative humidity.)
Vegetables are, in general, very susceptible to moisture loss in storage, with leafy vegetables losing moisture most readily. In an unfavourable environment they can suffer damage from water loss within a few hours. A moisture loss of 4% or more may necessitate trimming of the outside wilted leaves. Softening or wilting of root crops or heads is to be apparent when the total moisture loss exceeds 5-6%, whereas moisture loss more than 8% renders such product unsaleable.
Unlike pome and stone fruit, which is susceptible to increased decay and physiological disorders at high relative humidity, most vegetables requiring storage at high relative humidity are resistant to increased decay or physiological disorders.
For most vegetables that are susceptible to rapid water loss, the incidence of decay is usually not accelerated by the presence of condensation on the surface of the product if storage temperatures are maintained near those recommended for the product.
For a given relative humidity, moisture loss is greater with high produce-temperature. Thus, to minimise moisture loss it is essential to cool the produce promptly after harvesting. There are several techniques in order to achieve this mechanically.
In the case of processing and storage of meats such as beef, pork or lamb, although these products do not experience obvious ‘signs’ of drying out, when humidity control is not deployed or deployed incorrectly, reduced carcass weight below the optimal levels could result in millions of rands in lost revenues annually. In past issues of Cold Link Africa, particular details around this subject have been published by contributors.
In the case of grain storage, humidity control is also essential in order to avoid these types of products spoiling because of over-moisture and thus becoming contaminated with bacteria or overrun by pests. Grains when not kept at the correct moisture levels (too high) may also undergo the germination process – in which case limited use of the product (if not loss) would result.
Further to these obvious examples, humidity also plays a major role in the commercial production of products such as beers, wines and spirits – most importantly during the fermentation processes as well as reduction of what is known as ‘angel share’ (natural evaporation/loss) in barrel halls for these products. When not managed correctly, a very dry environment can cause barrel losses exceeding 20% as the internal liquid continually feeds the porous wood that is being dried out from the outside.
Products that ripen or mature over time such as cheeses also require particular humidity control during storage in order to produce a consistent product. This is also important for desired taste as deviation may result in the product experiencing undesirable sweetness or bitterness.
Flowers and herbs are further very susceptible to the correct humidity conditions once harvested and are some of the most sensitive products linked to the cold chain and shelf life.
For other stored products that experience changes in state dependant on particular temperature, even the occurrence of condensation forming on packaging could affect the product inside and spoil batches when RH levels are incorrect. These could include products like butter/margarine, dairy and even chocolate products.
Achieving the correct RH in storage
At a refrigerated storage facility, for the best way to maintain high humidity it is suggested to use an evaporator coil that is large enough to provide rapid cooling of the air without requiring operation at a low temperature and combined with an added humidification unit. Most products that will be kept in storage, require high RH levels of between 80 and 95% and therefore humidification rather than dehumidification would be the norm in South Africa.
“Flowers and herbs are further very susceptible to the correct humidity conditions once harvested”
An undersized cooling coil must be operated with a low surface temperature to cope with demands, especially during loading of the storage rooms, which cause moisture to condense and freeze on the coil and effectively remove water from the storage environment. This lowers the humidity and results in abnormal moisture loss from produce. Also, the accumulation of frost reduces the air flow over the coil and lowers its cooling efficiency further. There is evidence that humidity levels of the storage room atmosphere are related to the temperature of air leaving the coil.
Where adequate humidity can be obtained in no other way, water should be added to the storage room by humidifiers that introduce water as a fine spray. It is extremely important to note that specialists advise to never spray water directly on the produce as any water on the surface of the produce encourages microbial growth. Alternatively, produce stored in bulk bins or field boxes could be enclosed with a perforated polyethylene barrier film (or equivalent) to maintain an atmospheric humidity of 94-98%. However, caution should be applied with this technique as any barrier around produce that has not been precooled slows down field-heat removal and increases the potential of deterioration of the product.
Types of humidity control devices
For clarity, humidification and de-humidification should be identified as separate functions because the equipment for each process function is different from the other and prevailing conditions, application, and requirements per facility often determine what method is best suited for specification.
One type of humidifier typically boils water to create dry steam which could be supplied directly to the space. In some humidification techniques one must make sure that the water source is pure to avoid trace elements being left behind through the evaporative function that forms salt dust which is not good for a system in terms of corrosion or scaling, or for stored products. This may require various pre-treatments. However, some electrode-type units, depend on minerals in the supply water to function correctly.
Dehumidification equipment is also generally installed serving the space directly. Process air is re-circulated in multiple passes. These units can be free standing or mounted within the space.
Adiabatic or ultrasonic techniques of humidity control are also available. Simply, these use a process of either evaporative media, misting or fogging to push water into the air. Adiabatic techniques generally require very little energy too.
In a de-humidification process, techniques include the condensation method – which is only effective in warm climates, a heater that dries the air out, and then desiccant dehumidification which is required mostly in specialised applications of 20% RH or less at lower temperatures or for dew-points less than zero degrees C. Desiccant techniques are also ideal for cold and wet climates, as well as certain process cooling where a function creates condensation and needs to be removed.
Sources:
- Humidair
- Humiditas
- Specialised Climate Engineering
- Industry engagement
- RACA Journal