INCORPORATING COLD CHAIN
FEATURE 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 ,
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Grain processing and storage is another commodity reliant on the optimal humidity levels .
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 producetemperature . 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 overmoisture 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
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1-3 : Examples of installed humidification systems at various sited in South Africa .
COLD LINK AFRICA • May 2022 www . coldlinkafrica . co . za 29