Keeping fruits alive- An insight into Controlled Atmosphere Storage Method

Hi everyone. It’s good to have you here again

Every living thing continues to respire for as long as it is alive. Some crops such as fruits, vegetables keep on “living” even after harvest until a time they can no longer do so because they have used up all the supplies needed. The termination of this ability to respire leads to death but in crops, we call it spoilage. The length of life of a crop during post-harvest season is dependent on many factors; respiration rate is one of them. While some crops can last for months due to slow respiration rate and low water content, perishable crops are known to have very short shelf lives largely because of their higher respiration rates and water content.

I don’t like stating the obvious but this is necessary to drive out a point here. The point is that maintaining the respiration of crops can extend their shelf life. This fact forms the basis of the storage methods that I will be discussing today. Let your imagination take a ride with me.

Understanding Controlled Atmosphere (CA) Storage

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Storage is not a new thing and one important fact to recognize is that no storage system can actually increase the quality of agricultural produce; all that is ever done is maintenance of quality for a certain time period except if you are storing a fermented product. There are many storage methods but the choice of selection sometimes depends on the processing method employed and the nature of food products or crops to be stored. Controlled atmosphere can be used for many food products, grains, legumes etc. but for better understanding, I will narrow down this discussion to its use for perishable fruits and vegetables such as watermelon, tomato, apple, mango etc.

_{Inside a SCS Controlled Atmosphere room for apples from wikimedia under
the CC-BY-SA 4.0 International license by Bdcook}

So what is Controlled Atmosphere CA?

I am sure you have heard of greenhouse farming; a situation where the farming environment and climatic conditions are artificially regulated so that the plants can survive where they would normally not. Yeah. You guessed right. The situation here is quite similar. As the name implies, controlled atmosphere method involve regulating the temperature, humidity as well as the concentrations of gases (nitrogen, oxygen and carbon dioxide) of a storage room. The storage room is usually air tight such that all the gases present within it are from artificial sources.
In simple words simply put, controlled atmosphere = refrigeration + gas control.

The focus of controlled atmosphere is to lower the respiration rate of crops for as long as possible without killing them.
At this point, it will interest you to know that every fruit has an optimal range of temperature value for which it is to be stored. For example, uncut apple and watermelon fruit are best stored at -1.1⁰C-4.5⁰C and 10⁰C - 15⁰C respectively.
Also, there are specific ranges of CO2, O2 and Nitrogen that are best suited for different produce.

This is sometimes why the fruits and vegetables we store in our refrigerators at home still experience decrease in flesh firmness and other qualities just few days after their normal shelf lives at room temperature. It is also important to know the maximum shelf life of a fruit under different conditions before storage.

Controlled atmosphere storage capitalizes on this fact by employing sensors and control systems that keep the temperature and gas concentrations within the optimal range and this is how it’s able to prolong the shelf lives of the fruit.

Let’s do some chemistry

Respiration is the process of releasing energy from the breakdown of glucose

So, it would not be incorrect to say that respiration is just a chemical reaction. For instance in animals, glucose are broken with air to form carbon dioxide, water and energy. This is called aerobic respiration and this reaction can be represented as follow:

C₆H₁₂O₆ + 6O₂ 6CO₂ + 6H₂0 + energy (say 3780 kJ/mol)
Similarly photosynthesis in plant can be represented as
Carbon dioxide + water Glucose + Oxygen.

From these, it is obvious that respiration reaction can be manipulated in various ways. For example, the photosynthesis process will be slower if the carbon dioxide in the atmosphere is reduced.

Now let us talk in details about the methods of control

• Temperature and humidity control: Let’s me remind you of a simple experiment you may have done knowingly or unknowingly. If you put an unripe fruit say banana inside a polyethylene bag, paper bag or any other enclosed container, you would realize that the banana will get ripened within a day or two. This is due to the increased temperature within that enclosure which facilitates the production of a natural ripening agent called ethylene present in the fruit. Though, some fruits called non-climacteric fruits do not ripe any longer after harvest. In the same vein, vegetables exposed to higher temperature will wither faster than the ones in a humid environment.

In other words, this experiment establishes the fact that high temperature hastens ripening. Similarly, respiration reaction happens faster when the temperature of the surrounding is a little beyond normal.

Now, this is where refrigeration comes into play. Recall that I mentioned earlier that CA storage rooms are usually air-tight; refrigeration is employed here to lower the temperature within the range best suited for the fruit stored. Once the temperature of the storage room is exceeding the maximum/minimum value, refrigeration is stopped until the temperature begins to fall/rise again. This process is repeated for as long as the fruit is kept within the storage room.

• Gas Control

As stated earlier, oxygen and carbon dioxide gases are very essential in the respiration reactions.

By volume, the composition of dry air is 78.09% nitrogen, 20.9% oxygen, 0.93% argon, 0.04% carbon dioxide and small amount of other gases. source

So in the natural environment, oxygen is chief among the causes of food spoilage because it promotes the growth of microorganisms and oxidizing enzymes that speed up spoilage. In order to combat this, CA systems make use of reduced concentration of oxygen. The concentration of carbon dioxide on the other hand is usually increased within the specified limit best suitable for the fruits to be stored. This is because carbon dioxide can inhibit the production of ethylene in fruits.

Similarly, nitrogen gas is used with/in place of carbon dioxide in CA systems. Nitrogen gas is known in food processing and packaging for its ability to preserve taste and freshness. It is used in this case for the same purpose and to reduce the amount of humidity and oxygen present in the storage. Some other gases with similar properties may also be used.

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Pros and con

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Controlled atmosphere is a big leap over many similar storage methods e.g. cold storage. It is an innovative storage method involving the delay of ethylene action (ripening) and other microbial activities. The interactive effect of the controlled gases also brings about delay in respiration which ultimately leads to an extension in storage life. Most of the essential qualities of fruits such as firmness, taste, etc. are kept intact in CA systems.

However, it is worth nothing that controlled atmosphere does not extend the shelf life of the fruit indefinitely; just about 2-4 times the normal shelf life depending on the nature of the crop in question. I want to believe you understand the reason for that by now.

Furthermore, the huge cost of gases and different composition for each product type are setbacks associated with CA systems. A particular CA system cannot be used for more than one type of fruit. Reason being that, the composition required for one fruit is different from the other. Apart from that, some fruits will ripen others when they are stored together.

Conclusion

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Fruits and vegetables are best consumed fresh but can we consume them all? The losses associated with farm produce and other food products especially the perishable ones is a major setback affecting the profitability of many stakeholders and individuals in the farming business. Like every other storage method, Controlled atmosphere helps in minimizing these losses while assuring quality. In spite of the aforementioned setbacks, controlled atmosphere is widely used to extend the shelf lives of many products and it is most suitable for produce that respire even after harvesting.

When CA is used for different products other than fruits and vegetables, the principle is the same. Remember that CA is just a method and so it can be employed in different storage systems e.g. silos. Don't also forget that farm produce are best stored at certain temperatures. So the next time you are storing, ensure you put that in consideration
I hope you enjoyed the ride. Be sure to drop your comments below. See you next time.

References

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• Controlled atmosphere
• Respiration and Ethylene and their Relationship to Postharvest Handling
• Fruit processing
• Respiration of fruits and vegetables
• THE 3000-MILE CAESAR SALAD— A quick guide to controlled atmosphere storage
• Thompson, A.K. 1998. Controlled Atmosphere Storage of Fruit and Vegetables, CAB International, UK.

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