Transporting perishable commodities in Controlled Atmosphere Reefer Containers
Daily, perishable goods such as fruits and vegetables are transported around the globe to meet consumer demand for fresh produce. To extent shelf life and ensure freshness, such commodities are transported in refrigerated containers in which the gaseous composition of the storage atmosphere is controlled and closely monitored throughout the transportation chain.
Commodities such as fruits, vegetables, meats, and other perishable foodstuffs are sensitive to changes in their surrounding atmosphere. In fact, 14 % of all food is lost between harvest and the wholesale market, estimated at a total value of 400 billion USD per year, states the Food and Agriculture Organization of the United Nations.
Maintaining product freshness and salability from farm to fork highly depends on the control of gases, temperature, and humidity during transportation. To control these factors, the cargo loads are transported in refrigerated containers, where the atmosphere is fully controlled in so-called Controlled Atmosphere Reefer Containers.
What is Controlled Atmosphere?
Controlled Atmosphere (CA) refers to control of the gaseous composition of the storage atmosphere.
By default, the atmosphere in the container will be equivalent to that of our atmosphere – 78 % Nitrogen, 21 % Oxygen, and 1 % Argon. But as the products transported under CA-conditions are fresh produce, they both consume O2 and produce Co2 on equal terms as humans. To maintain an ideal atmosphere in equilibrium with the products’ deterioration rates, the atmospheric composition is changed to ideal levels of O2, CO2, and other gases that match the specific cargo load.
Why CA? Adjusting ripening to meet consumer demands
The trade patterns for perishable products are determined by consumer demand. Generally, reefer cargo is transported over long distances for up to as much as 45 days. To make sure that the time-sensitive commodities remain fresh and salable when reaching the destinations, the gaseous composition is adjusted to slow ripening by reducing oxygen levels and increasing the concentration of CO2, and thereby extending shelf life.
Sensors ensure stable set points from farm to fork
When the commodity is loaded in the CA Reefer Container, a Pre-trip Inspection is carried out. This is where the ideal values for the adjustable factors, including temperature, O2, CO2, and humidity levels, are set. The atmosphere will then be kept matching the entered set points throughout the transportation chain.
The ideal set points vary depending on the cargo commodity.
Reliable sensor data as proof of commodity quality
Data on set point values are constantly logged across the cold chain. On the Reefer Container, the data is available directly on the control panel as well as in remote monitoring modules.
This means that customers can track and make sure that all factors have been kept at the desired set point during the cargo’s entire journey. For container owners and shipping companies, this data can act as proof of quality at final delivery of the cargo load, adding to the demand on reliable sensor data.
CO2 sensors based on thermal conductivity measurement principle
CO2 sensors from Senmatic are based on the thermal conductivity measurement principle rather than the traditional nondispersive infrared (NDIR) type of measurement. Thermal conductivity refers to a material’s ability to conduct heat: the higher the thermal conductivity, the more heat is conducted by a material. As for CO2 sensors, we are looking at the thermal conductivity of gases.
All gases have individual conductivity capabilities. To detect the concentration of the target gas – CO2 – in the background gases, knowledge on each gas’ conductivity is vital. If, for example, the atmosphere in the container was equivalent to that of our atmosphere – 78 % Nitrogen, 21 % Oxygen, and 1 % Argon – these values would be factored in to determine the concentration of the target gas. As described, the background gases depend on the commodity being transported.
In short, the thermal conductivity measurement in the CO2 sensor works by diffusing gas into the sensor, where a heating element heats the gas and measures the amount of heat that is transferred to determine the thermal conductivity. These values are then translated into a concentration of the target gas – CO2 – in the background gas, explains Sensirion with whom Senmatic’s CO2 sensors are developed.
Senmatic’s CO2 sensor also factor in temperature, pressure, and humidity; factors which all affect the accuracy of the translated values.
Combined, these features result in more robust and resistant sensors.
Why temperature is essential in Reefer Container
It probably comes as no surprise that temperature is an essential element in Reefer Containers; hence the name Reefer originating from Refrigeration. Being an essential element, it is crucial to monitor temperature levels during transportation to be able to react on any fluctuations.
Temperature essentially impacts two key procedures: the operating temperature in the cooling circuit and the temperature of the cargo itself.
Monitoring the operating temperature in the cooling circuit is necessary to ensure that the wanted set-points are kept at a constant, while the ambient temperature is also continuously factored in. To monitor the cooling circuit, 5-7 Type PTE sensors are placed in the cooling unit. Type PTE is a resistance temperature sensor (Pt-100 or Pt-1000) and has been triple sealed for moisture ingress protection. The high IP class makes it ideal for this type of temperature monitoring. Type PTE is marine classified and designed for harsh environments to withstand vibrations, shock, and large temperature fluctuations – from the shaking Arctic cold to suffocating heat in the Sahara Desert.
Temperature log based on sensor data
Measuring temperature in the cargo itself, meaning the actual produce’s temperature, is twofold. During the entire cold chain from farm to fork, a complete temperature log is created based on sensor data. For businesses in the cold chain, such logs provide critical information about any fluctuations which might cause damage to the cargo and, should such damage occur, determine at which stage in the chain the damage has been caused. For consumers, the close control of temperature is a vital element to guarantee a high level of food safety. To control cargo temperature, three Type PTI or three USDA sensors – or cargo probes as they are also known – are strategically placed directly in the produce, typically one in the front, one in the middle, and one in the back.
Monitoring relative humidity keeps produce fresh
Maintaining high water content levels in the fresh produce transported is necessary to keep the produce fresh and crisp all the way to your supermarket’s shelves. By monitoring relative humidity, shipping lines ensure optimal cargo conditions for perishables; that is why humidity sensors are installed in the majority of Reefer Containers.
Our MHT humidity sensor is typically placed in the evaporator section. It uses a capacitive element for measuring. A micro-controller compensates for unlinearity and fluctuation of temperature. In this way, accurate measurements are guaranteed.
MHT humidity sensors operate with an accuracy of +/- 3 % RH.
Type PTI
Insertion thermometer - for measurement of temperatures in e.g. meat and fruit
Type USDA
To monitor and control temperature during transportation of food
CO2 sensors - O2 sensors
CO2 and CO2 sensors used as stand-alone units or integrated units
MHT humidity sensor
For measuring in e.g. refrigeration plants, ventilating plants and reefer units.
If you have any questions regarding the above or if are interested in knowing more about our solutions for the Reefer industry, you are welcome to contact me.
Max Jensen, Global Sales Director - Sensor Phone: +45 6389 2223, e-mail: maj@senmatic.com