Grab an apple – or how intelligent fruit storage made fresh, tasty fruit a non-seasonal experience

Controlled atmosphere (CA) applications – the main theme of the Angst+Pfister Sensors and Power booth at this year’s ACHEMA show – are used to control and maximize the performance of thousands of organic and inorganic processes in the industry, leading to products with a higher quality and value for the customers.

For instance, by controlling the oxygen content in various packaged food products, the shelf-lifetime can be extended, thereby reducing food waste. Fresh fruit, all year round, has long been a fact of everyday life, but what are tricks that are used to make fresh, tasty fruit a non-seasonal experience?

One trick of many is the application of longlife, stable gas sensors to control the atmosphere in a large storage box filled with premature harvested fruit and to be able to time the ripening process of the fruit. This article describes gas sensor solutions for fruit storage and ripening – a worldwide business that grows, and where our expertise is the key to success.

«Enter with me the fascinating world of controlled atmosphere applications – two applications out of thousands. Every single application requiring expertise in design-in and the ability to level with the customers.»

Dr. Thomas Clausen, Product Manager Gas Sensors, Angst+Pfister Sensors and Power


Gases that make fresh fruit out of the storage facilities possible

In my home country, Denmark, bananas were long considered a luxury good and therefore taxed accordingly high. Likewise for cars, chocolate and other things that make every day another good day. Stories about green snakes and poisonous spiders hiding in the banana baskets during the long overseas journey were invented to keep us kids from stealing the bananas. Bananas and fruit in general are now no longer considered a luxury good, but a way to a more healthy life/lifestyle by eating more fruits. My one apple a day, certainly have kept the doctor(s) away. This is, in effect, a story about how I got rid of my last childhood traumas (snakes, spiders, worms, beetles and bugs) by being able to eat fresh fruit every day - fresh out of a storage facility.

The storage facilities for early harvested fruit and vegetables have normally multiple storage boxes in sizes up to container size and with volumes of up to 30 m3 for each box. Some boxes are virtually air tight to be able to lower the oxygen level in the box to a very low level, while other boxes are easily accessible from a port opening and thus not or only partially gas tight. The gas management system is normally installed in the service area and gas from the storage box is pumped through the gas sensors in order to be able to control and monitor the storage process and to be able to react upon unwanted changes in the environment in the storage box during storage.

Gases of main interest for the manufacturers of fruit and vegetable controlled ripening storage facilities are oxygen, humidity, carbon dioxide and ethylene. The strategies of storage are very different from fruit to fruit type, but also within the different fruit types (apples, pears,...) the strategies can be very different. I will limit myself to two cases, which I will describe in some detail and I will show what Angst+Pfister Sensors and Power can offer with respect to products and expertise. First a short description of each gas and how they impact fruit and vegetable ripening:

Oxygen - O2 – we live and breathe oxygen. Take away the oxygen and virtually any living organism will have a problem. So by reducing oxygen in a storage facility, the problem of parasites is effectively eliminated ! Reduce oxygen and you will slow down the metabolism – this is the trick to store fruit for more than 9-12 months and to make just-intime delivery of fresh fruit possible.

Carbon dioxide - CO2 – normally a gas with a very bad reputation, but for the food industry, CO2 is a central gas for food conservation. In a storage facility, CO2 is either used to slow down the ripening speed or it is used to reduce the oxygen concentration in air in a storage box, where the oxygen concentration is otherwise not controlled.

Ethylene - C2H4 – is the exhaust product of ripening. Ethylene is also used to speed up ripening or it is monitored in order to be able to prevent unwanted ripening from happening. One rotten apple exhausting ethylene, can make the complete batch unsellable. This makes ethylene scrubbers popular.

Moisture – or water content in air is also a gas – fruit contain a lot water and by logic, fruit would dry out if stored under too low humidity conditions. Therefore, most fruits are stored in a high humidity environment in order for the water in the fruit not to evaporate.


Figure 1. FCX-U finished sensor (left), FCX-U sensor opened; note the white,
ceramic element on top of the white ceramic wool (middle), sensor principle (right)

Oxygen sensors from Angst+Pfister Sensors and Power

Juicy, sweet tasting or sometimes sour apples are a delicacy in the early autumn months and until start/middle of the winter season. Modern storage techniques have extended the delicacy period beyond even 12 months. This is how.

Most apple types are being harvested in a premature state in the first few weeks of autumn. They are then stored in so-called ULO (ultra-low oxygen) gas tight storage boxes, adding humidity of up to 95% and CO2 gas up to 2.5%, while lowering the temperature below 4°C. The apples are put to sleep. When the apples are needed, they are slowly being cultured, primed and made ready for sales. The ULO principle for storage of fruit is not limited to apples alone – pears, kiwis, blueberries, mangos, grapes and cherry berries can also be stored under ULO conditions and be delivered as fresh fruit all year long. As also onions, garlic, cabbage and asparagus.

In order to measure stable, low concentration values of oxygen, a highly reliable oxygen sensor is needed. We have such a sensor in our program – the FCX-U (see photo) amperometric oxygen sensor from Fujikura Ltd. Fujikura Ltd is our most important partner – not only for oxygen sensors, but also for pressure sensors. Fujikura makes ~ 600,000 pcs oxygen sensors per year – all handmade and highly competitive in price. ~ 95% of all of these oxygen sensors are sold to the medical industry for mobile respiration equipment. The rest we take - almost.

The FCX-U is a ceramic type of sensor with a very long lifetime – ideal for many applications, and in particular when it comes to controlled atmosphere applications. The sensor needs a predefined, very precise heater voltage, so that the temperature of the sensing element can be kept constant at 450°C (Figure 1). At 450°C, and when a potential is put between the sensor element anode and the cathode, a current is flowing through the element (Figure 1). The current is proportional to the oxygen concentration. This is the ideal world - low oxygen concentration equals low current and so forth. In reality, the sensor has certain cross influences from both CO2 and humidity. The cross influence behavior is very much dependent on the potential across the sensor element. When the potential is too high, the water and the carbon dioxide molecules break and more oxygen is generated (2H2O → 2H2 + O2) – leading to a systematic failure. When the potential is too low, the sensor stops working after only a couple of years in service, although a minimum of 4 years is guaranteed.

We manufacture customized oxygen sensor modules based on the FCX-U sensor. The most popular module, shown in Figure 2 below, is the FCX-MC25-FLOW-A-CH module, which is specifically developed for controlled atmosphere applications, taking into account varying conditions, such as CO2 (up 50%) and humidity (up to 100%). The potential is chosen to minimize impact of molecules breaking up and still maintaining a long operational lifetime. The modules are produced in Switzerland and we have a capacity of more than 10,000 modules per year. The module is highly popular among manufacturers of fruit and storage facilities, but is also sold for many other controlled atmosphere applications such as anaerobic bioreactors, additive manufacturing systems and nitrogen/ oxygen generators.



Figure 2. FCX-MC25-FLOW-A-CH OEM oxygen
sensor module developed for in-line controlled
atmosphere applications


Ethylene gas sensors from Angst+Pfister Sensors and Power

Banana ripening rooms are almost a scientific discipline of its own. Terms like “chilling the banana” or “cooking the banana” have nothing to do with gas or gas sensing, but is a consequence of temperature variations below and above an optimal storage temperature – therefore focus is maintaining the temperature constant and optimal during the ripening process. Humidity is a must – otherwise the bananas dry out. What make bananas special from a point of view of gas sensing, is the controlled speeding up of the ripening process in special ripening storage facilities..

The most advanced fast ripening storage rooms for bananas are pressurized and the air is circulated/recirculated to maintain the best conditions during the fast ripening process. Some studies indicate an advance of lowering the oxygen in the storage room, but for the acceleration of the process another gas comes into play – namely Ethylene. Ethylene is actively used for the ripening process and the higher the ethylene concentration the faster the ripening. Most manufacturers recommend constant levels of ethylene of 100-300 ppm, but it probably comes as no surprise, when I mention that most owners of fast banana ripening facilities run at levels of 400-500 ppm. Ethylene gas is not easy to detect and here is why.

Ethylene has an optical footprint, which means that it absorbs infrared (IR) energy at a certain wavelength. The absorption amplitude is proportional to the ethylene concentration. It is relatively simple in theory to build a sensor based on the IR absorption principle, but in reality, since it is a relatively weak absorption, the sensor output is subject to a lot of potential variations. Nevertheless, infrared absorption based ethylene sensors are being used for measuring and controlling the banana ripening process, because they offer a good compromise between price and performance.

Anything that interacts with radiation (and thus also IR energy) is excited into an intermediate, unstable state and relaxation into the ground state follows pretty quickly. The same goes for ethylene being irradiated with IR light at a certain wavelength. During the relaxation process, energy is desorbed from the ethylene atoms and this energy has an audial footprint. The amplitude of the audial signal is proportional to the ethylene concentration. A microphone is used to pick up the amplitude of the audial signal and this is the fundamental principle in a photoacoustic (PA) ethylene gas sensor, which has indeed a better performance in comparison to an absorption based ethylene, but is also more expensive. In some cases, also for banana ripening, the extra money is well spend, because the output of a PA ethylene sensor is more stable and reliable in comparison with an IR ethylene sensor.

In the presence of enough potential energy and oxygen, it is possible to break the ethylene atom. If the process is done on electrodes in contact with an electrolyte, a current can be generated in an external circuit, and the generated current is proportional to the ethylene concentration. It is a very simple and well-known technique and the so-called electrochemical (EC) sensors make up most of the gas sensor market. Ideal, one would think as a basis for a good, cheap sensor for banana ripening. The downside of something so cheap is cross influence from a lot of other gases such as ethanol, carbon monoxide, nitrogen dioxide and other gases that are probably also present during a ripening process. One truck loading bananas, while still emitting exhaust gas (motor running) and the ethylene sensors go bananas. The use of electrochemical ethylene sensors are limited, but it is possible to make it less cross sensitive to other gases. But, the design is complicated and the yield is low.

Our main business is on the IR type of sensors, where we have a corporation with a renowned supplier for these sensors. In Figure 3 below is shown a 0-2,000 ppm ethylene IR sensor, together with a rough sketch of the detection principle, which is perfect for controlling the banana ripening process. The sensor has a long absorption hollow tube made in aluminum and polished to a state in which the reflection from the polished surfaces are close to 100%. The IR light travels from end of the tube, where it is emitted from an IR source, to the other end of the tube, where the remaining light (i.e. the light that has not been absorbed) is detected. Gas input and output is via flow adapters mounted on the outside of the sensor. The detector is not just one detector, but in fact two detectors are used to control the IR light. One detector measures the light that has not been absorbed by ethylene in the long tube and the other sensor measures the intensity of the IR source, independent of the absorption from ethylene gas.


Closing remarks and business outlook

The climate debate is part of our every day life and most of us agree that something is maybe moving in the wrong direction. Over the last two years, we have received more oxygen sensors back from our customers that have suffered from a premature breakdown and failure. We try to offer our customers the best service by making an analysis of what could have caused the unexpected breakdown. Mostly, it has been the FCX-UC sensors that have suddenly stopped working and this is suspicious. Suspicious, because the sensor lifetime is normally very long and the signal output very stable over the lifetime of the sensor. After having received > 10 sensor modules back over a period of 6 months, we decided to open the sensor and look for possible root causes on the sensor elements. It turned out that a combination of dry weather (from climatic changes), combined with a fixed procedure of how to get the apples ready for harvesting caused the problems.

Just before harvesting the apples, the apples are sprayed with a copper sulfate solution in order to eliminate problems with mildew and apple scab. Because of the dry weather, the solution was not naturally washed away from the surface of the apples and the apples were taken to the storage rooms, where a significant amount of the solution was still on the surface of the apples. In a storage facility, the humidity is high, which caused the solution to be dissolved in the air with the humidity. The air, now having high concentrations of humidity and copper sulfates, is being flown through the oxygen sensors and if there is one thing a FCX-UC sensor does not like, it is the combination of humidity, copper and sulfur.

When we opened the sensor and looked with very large magnifying glasses in an electron microscope facility, we found traces of copper in the oxygen sensing grain boundaries on the surface of the sensor element (see figure 4). Our customer was simply unlucky with the weather and was responsible for his own "mishap", so to speak.

The business outlook for gas sensing solutions for long-time storage of not only fruit and vegetables, but also other types of food, chemical and medical products, is good and the business is growing. Most of the business worldwide is supported and dominated by specialized domestic companies working with domestic customers. Whatever works in Korea, is not necessarily working well in Denmark. Our job is to select the right gas sensing solution for a certain product based on experience, expertise and the ability to discuss requirements with the customer – be it the specified, hard and written down requirements or the non-specified, subtle and soft requirements that can win us the deal.

Figure 4. SEM pictures of the surfaces of a)
perfectly working sensor and 2) a failed sensor.
Note the difference in magnification. Essential
for a perfect working sensor is that the grain
boundaries are perfect and clean. In the picture
to the right (large magnification) it is clearly
demonstrated that the grain boundaries are filled
with pollutants. A X-ray analysis revealed that
the pollutants was mainly copper.


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published: 10 Mar 2022 15:19:00  by: Angst+Pfister Magazin2022