In order to offer seasonal fruits and vegetables throughout the year, the fruit must be stored for several months. In commercial agriculture, gas-tight storage rooms with a controlled atmosphere (CA) are used for this purpose. In CA storage, the temperature, humidity and composition of the storage atmosphere are precisely monitored and controlled. This means that apples, pears and the like can be stored for 3 to 4 times longer than under normal conditions. One prerequisite for this is exact measurement of the temperature, humidity and CO₂ concentration in the CA warehouse using suitable sensors.

CA Storage – Constant Conditions Improve Storage Ability

To ensure that fruit retains its consistency, ingredients, colour and flavour for as long as possible, it is stored in cold rooms at constantly low temperatures and high humidity. In addition to this, the composition of the storage atmosphere also has a significant influence on storage ability. Normal air consists of about 78 % nitrogen, 21 % oxygen, and small amounts of carbon dioxide (0.04 %) and various inert gases. In CA storage, the oxygen content in the storage room is reduced to a constantly low oxygen level by adding nitrogen, while at the same time the CO₂ content is increased. This slows down the natural ripening process and maintains the quality of the fruit for a longer period of time.

DCA (Dynamic Controlled Atmosphere) Storage

DCA storage technology is an enhancement on conventional CA storage. It takes into account the fact that the stored fruit constantly releases heat, water, carbon dioxide and ethylene (C₂H₄) to the ambient air through cellular respiration, which changes the composition of the storage atmosphere. 

In DCA storage, the oxygen content and the ethylene and carbon dioxide concentrations are continuously monitored and dynamically controlled. The aim is to achieve the lowest possible oxygen level, just above the anaerobic compensation point (ACP*). 

In what are known as Ultra Low Oxygen (ULO) or Extreme Low Oxygen (XLO) storage facilities, the oxygen content is gradually reduced to around 0.7 % to 1 %. This puts the stored fruit into a kind of "coma" and reduces fruit metabolism to the absolute minimum. Variety-specific O₂ and CO₂ values must not be undershot or exceeded in this process, otherwise the fruit will be damaged by a lack of oxygen or an excessively high CO₂ concentration.

_{*The anaerobic compensation point (ACP) is the critical oxygen concentration at which fermentation occurs in the fruit metabolism process.}

Precise Sensors Are Prerequisite to Optimum Storage Conditions

To ensure ideal storage conditions, CA / DCA storage rooms are equipped with a wide variety of technical systems for cooling, air conditioning, humidification and gas management (nitrogen generator, carbon dioxide scrubber, ethylene converter). 

One prerequisite for precise control of the storage conditions is continuous monitoring of the pertinent climate parameters with the aid of suitable sensors. CA / DCA storage only offers the best possible result in terms of quality and storability if the measuring devices work reliably and deliver exact measured values. At the end of the day, the use of energy and resources must pay off for the operator of a CA / DCA storage facility.
 

Humidity, Temperature and CO₂ Measurement in CA / DCA Storage Rooms

Humidity, temperature and CO₂ are among the most important parameters which need to be monitored in CA / DCA storage. Due to the challenging conditions prevailing in the storage rooms, the following requirements arise for the sensors:
 

• High measurement accuracy (<2 % RH) and long-term stability under high humidity conditions
• Contamination-resistant measuring principle, ideally with auto-calibration
• Resistance to chemical contamination
• Condensation-resistant
• Robust enclosure with protection class IP65 or higher
• Easy maintenance and sensor replacement in case of service