FEATURE
INCORPORATING COLD CHAIN 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 .
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 dehumidification 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 ,
Cold Link Africa | Benjamin Brits
A tell-tale sign of the incorrect humidity at harvest as I have learned is dry stems on grapes . This photo also illustrates condensation on the lower packaging which is not ideal for most fresh produce as it encourages bacterial growth .
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 . CLA
Sources :
• Humidair
• Humiditas
• Specialised Climate Engineering
• Industry engagement
• RACA Journal
Flowers and herbs are further very susceptible to the correct humidity conditions once harvested
30 www . coldlinkafrica . co . za COLD LINK AFRICA • May 2022