After harvest, fruits and vegetables continue to ‘live’. The respiration of fruits and vegetables keeps on going until all suplies have been depleted. For keeping fruits and vegetables fresh it is essential that you try to keep respiration going on for long as possible. That why, in this post we’ll be focussing on the respiration of fruits and vegetables. We’ll discuss how it can be controlled and influenced to store your fruits and vegetables for as long as possible.
Respiration sets free energy which can be used by the fruits and vegetables for their internal processes. Controlling respiration is essential for properly keeping your produce. As long as the respiration process can continu, the fruits and vegetables can be kept well. Since the produce isn’t connected to roots or other parts of the plant anymore, it will not get any new nutrients, it will have to live of what it’s got. So once it’s all finished, the fruits and vegetables will deteriorate.
Respiration of fruits and vegetables = chemistry
The respiration process is nothing more than a very common chemical reaction. This chemical reaction is not unique for fruits and vegetables, it takes place in all plants and animals. In respiration glucose (C6H12O2) is transformed into carbon dioxide (CO2) and water (H2O) using oxygen (O2). This reaction releases energy as well which can be used for all sorts of processes in the plant. The reaction can be represented like this:
C6H12O2 + 6 O2 –> 6 CO2 + 6 H2O + energy
Glucose is a carbohydrate and is often stored in a plant as starch. Once all glucose has been depleted no energy is being formed anymore to sustain the fruit or vegetable and decay sets in.
Respiration rates differ per product
The respiration rate of a product determine how fast the chemical process occurs. The higher the rate the faster glucose will be transformed and the faster all supplies will be depleted. Therefore, when it comes to storage of fruits and vegetables we want to keep the respiration rates as low as possible without killing the produce.
We will zoom into various ways to do that in a little while. However, we first have to look at all the different types of fruits and vegetables and their influence on the respiration rate. Each variety has a different respiration rate. The respiration rate of a tomato will be very different of that of a carrot which again will be different of that of a strawberry.
It is a lot harder to keep products with a very high respiration rates. They simply run out of energy more quickly. Below you can find some examples of very slow up to very fast respiring products (USDA):
- Very slow: nuts & dates
- Slow: apple, citrus, grape, onion, potato
- Moderate: banana, cherry, pear, fig, lettuce, tomato
- Fast: strawberry, cauliflower, avocado
- Very fast: artichoke, Brussel sprouts
- Extremely fast: broccoli, mushroom, spinach, sweet corn
The definitions used above use the respiration rates (mg CO2 kg-1 hour-1) at 5°C. The categories are as follows: very slow (<5), slow (5-10), moderate (10-20), fast (20-40), very fast (40-60), extremely fast (>60).
The unit of respiration rate is: mg CO2 kg-1 hour-1. The rate describes the amount of carbon dioxide produced per kg of produce in 1 hour. As you can see in the equation above, carbon dioxide is formed during respiration. If you know this amount you can also calculate how much glucose has been converted.
Influencing respiration rate of fruits and vegetables
Since respiration is just a chemical reaction, you can influence the rate of its reaction in various ways:
- Temperature: the chemical reaction goes faster at a higher temperature, though, when the temperature is too hot, enzymes will break down and respiration will stop.
- Oxygen concentration: The reaction needs oxygen, so if there is no oxygen, no respiration occurs. In general, less oxygen leads to a slower reaction rate.
- Carbon dioxide concentration: The influence of carbon dioxide concentration depends strongly on the fruit or vegetable. Some might increase in respiration rate, whereas with others more carbon dioxide might lead to slower reactions.
- Stress in a vegetable: Yes, vegetables can be stressed, for instance if they are cut or damaged. This will initiate all sorts of reactions, including those that accelerate respition.
- Ripening: Some fruits (and vegetables) continue to ripe after they have been harvested (climacteric fruits). During ripening the respiration rate might increase or decrease, depending on the product. This can also be linked with ethylene concentrations.
Let’s dive a little deeper into some of these influences to see how we can handle those at home or in the food production chain. This will provide you with a general understanding. If you want to learn more about how this can be applied to packaging, read my post on modified atmosphere packaging after this one.
Storage temperature of fruits and vegetables
There is no such thing as the ideal storage temperature for fruits and vegetables, unfortunately. From a respiratory point of view you should just about always store fruits and vegetables in the fridge, where it’s nice and cool. However, since fruits and vegetables are living plants there are a lot more reactions going on as well. Not all of these reactions might appreciate a low temperature.
Products that had better not be kept in a fridge for instance are bananas and pineapples (browning) as well as tomatoes (they lose flavour rapidly). However, once these have been cut it is better to store them in the fridge. If you wouldn’t do this they would spoil in no time due to all the readily available sugars!
Oxygen concentration and respiration rate
As I mentioned before, respiration requires oxygen. If there is no oxygen anymore respiration will stop and decay of the fruit or vegetable will set in. What’s more, absence of oxygen can cause fermentation to occur which will produce strong off flavours.
In the air the oxygen concentration is around 20%. This is pretty high and most fruits and vegetables can easily be stored at lower oxygen concentrations, sometimes as low as 1 or 2%.
This article shows various nice graphs on the optimal oxygen (and carbon dioxide) concentration for several fruits and vegetables.
Too much carbon dioxide will kill most fruits and vegetables. Again it’s a balancing act. The normal concentration of carbon dioxide is well below 0,1%. By adding even as much as 1% of carbon dioxide to the environment of a plant the respiration will behave totally different.
Stress & respiration rate
The three factors influecing the respiration rate of fruits and vegetables mentioned above are pretty straight forward. The influence of temperature on a rate will always be more or less the same, the same goes up for oxygen and carbon dioxide. However, for these last two that is not the case. These are examples of processes that will change over time, will just about never have the exact same effect and depend on a lot of variables. These are what makes storing fruits and vegetables so hard.
The first is the effect of stress. Cutting vegetables causes them to dry out more quickly, spoil more quickly because of moulds but it also causes them to decay faster because of a high respiration rate.
When fruits and vegetables are cut a lot of things happen. One of them is that all of a sudden there is a lot more surface area with easy access to oxygen. Also, more sugar has come free, both resulting in higher respiration rates. At the same time, most plants react with some sort of a defense mechanism when they are wounded. This will often cause respiration rates to increase further because more energy is needed.
Generally, the respiration rate will slow down again some time after the product has been cut. It will stabilize again at a rate well below its highest rate not too long after cutting.
Ripening and respiration rate of fruits and vegetables
In this post we will not dive into detail on the effect of ripening on the respiration rate. Reason being that ripening is a complex process again and it is well wordth it to write a separate, more extensive post on the topic. For now all we will say is that ripening often influences respiration rates strongly. You can probably imagine that during some stages of a ripening process more energy is required, thus more respiration.
The bigger picture
The respiration rate is just one of the many (chemical) processes playing a role in storage of harvested fruits and vegetables. Read one of the posts mentioned below to gain more insights into other, related topics:
- There are various ways to translate the knowledge on respiration rate into packaging design.
- Storing fruits and vegetables at home for a longer period of time by using proper temperature control.
I have used various very useful sources for this article. Some I would advise you to go through if you’d like more details are: Modelling respiration rate (scientific article), Postharvest physiology and pathology of vegetables (book), Respiration rates of various products (scientific article)