I bought a bunch of fruits in a group purchase. Can they become sweeter if I let them sit?

Written by | Wulianhuakai

Many people have this experience: bananas and kiwis taste better after they are soft after two days. In fact, not only bananas and kiwis, but also apples, peaches, mangoes, papayas, pears, apricots, plums, persimmons, avocados, cantaloupes, etc., have the opportunity to become sweeter (if not spoiled) after they are brought home.

In addition, although the taste of tomatoes will not become noticeably sweeter, the flesh will become softer and the acidity will decrease. It is also a model organism for studying climacteric plants.

There is another obvious phenomenon in life. Some other fruits, such as watermelon, pineapple, grapes, strawberry, blueberry, cherry and lychee, will not change their taste significantly no matter how they are stored. The same is true for cucumber, potato, cabbage and carrot. Moreover, these fruits and vegetables should be eaten immediately after they are ripe to avoid them from going bad.

The reason for all this is that their ripening patterns are different. The examples given above are all respiratory climacteric fruits and vegetables, while the examples given below are all non-respiratory climacteric fruits and vegetables.

Figure 1. Examples of climacteric and non-climacteric fruits (Source: umd.edu)

Fruits and vegetables do not stop growing after harvesting, and both climacteric and non-climacteric plants can continue to respire. However, the two modes are very different.

The main difference between these two types of fruits and vegetables is their different sensitivity to the plant hormone ethylene. After being harvested, the respiratory climacteric fruits can still produce a peak in respiration rate under the catalysis of ethylene and convert large molecules in the body into small molecules - for example, starch is hydrolyzed into sugar. In this process, the flesh will become soft, the acidity will decrease, and the sweetness will increase. Moreover, the ethylene released by each fruit will also catalyze other individuals, so that the fruits gathered together will become more mature.

Non-climacteric fruits will not continue their original development process after being picked, and their respiration rate will decay. Immature fruits will not continue to ripen, and mature fruits will decay. Therefore, cold chain express delivery mainly delivers non-climacteric fruits, such as cherries and lychees, which cannot be artificially ripened and can only be picked when they are close to maturity. Once picked, they must be eaten/refrigerated in a short time, otherwise they will go bad. Climacteric fruits can be picked in advance and then ripened, thereby reducing logistics costs.

Figure 2. Cherries in a courier box (Photo provided by the author)

This mode difference is presented in pictures like this:

Figure 3. Changes in ethylene release and respiration rate after harvesting fruits and vegetables | Source: UC Davis

We can see that the properties of non-climacteric fruits are basically determined the moment they leave the branches, so the cucumbers we buy will wilt quickly, and the pineapples we buy (especially for those of us who live in the north like me) will basically prick your mouth.

Why is this so? Let’s take Apple[1] as an example.

Figure 4. Ethylene synthesis pathway in apple. Arrows represent promotion; flat arrows represent inhibition; solid lines represent confirmed processes; dashed lines represent unknown processes [1]

In the respiratory climacteric fruit, the ethylene synthesis pathway includes two systems: system 1 (S1) and system 2 (S2). The main function of ethylene is to reduce the growth rate of plants and promote early fruit maturity. The S1 stage occurs during the young fruit period, which is a self-inhibiting reaction. At this stage, methionine/methionine synthesizes S-adenosylmethionine (SAM) under the catalysis of S-adenosylmethionine synthetase, SAM synthesizes 1-aminocyclopropanecarboxylic acid (ACC) under the catalysis of ACC synthetase, and ACC is converted into malonyl ACC (MACC) under the catalysis of malonyl transferase. This can avoid the production of ethylene and prevent the early maturity of fruits.

The S2 process is an autocatalytic reaction. ACC synthesizes ethylene under the action of ACC oxidase, and then the autocatalytic reaction of ethylene is carried out through [2] ethylene → ethylene receptor ETR family → CTR family → EIN2 → EIN3/EIL → ERF → ethylene reaction-related gene expression.

Pathways for ethylene production in plants[2]

Therefore, during the ripening process of respiratory transition fruits and vegetables, only exogenous/endogenous ethylene release is needed to continuously produce ethylene to catalyze the ripening of fruits and vegetables.

However, the ripening mechanism of fruit is a highly complex process with multiple levels of regulation (DNA, RNA and protein), which requires the coordinated action of multiple plant hormones. There are also some new research progress. In addition to ethylene, another plant hormone abscisic acid (ABA) is also believed to play an important role in this process. In kiwifruit [3], freezing treatment and then returning to room temperature showed better results than applying ethylene.

Figure 5. Ethylene synthesis, mediation and other multi-level regulation during fruit ripening [3]

New research shows that not only does the classic ethylene model play an important role, but the cytochrome c (CYTc) pathway and the alternative oxidase (AOX) pathway also contribute to increasing mitochondrial activity levels. The highlighted area on the right side of the figure above suggests that low temperature can stimulate the production of NADPH oxidase, thereby stimulating the accumulation of reactive oxygen species (ROS), which enter the cytoplasm to mediate gene expression, thereby activating the AOX pathway.

Of course, this is a relatively microscopic molecular biology study. One time I bought raw kiwis, but they never softened, so I checked it out.

From a macroscopic perspective, in fruits with respiratory cessation, the respiratory rate will form a peak as the amount of ethylene released increases, and stopping the ethylene supply will interrupt this process.

In production and life, people are also using some chemicals to regulate the ripening progress of fruits and vegetables with respiratory climacteric reactions, such as 1-methylcyclopropene (1-MCP), which has a similar structure to ethylene and can compete with ethylene receptors, thereby delaying the ripening of fruits. Aminoethoxyvinylglycine (AVG) is also used, which can convert ACC into MACC and prevent the production of ethylene in fruits.

Figure 6. AVG working principle | Source: Fan Huifen et al. 2008

This is similar to the S1 stage of ethylene production in artificially simulated fruits.

Of course, the most commonly used one is ethephon. Ripening the fruit is what end customers like me need to do most.

In short, fruits and vegetables that can become sweet are usually respiratory climacteric fruits and vegetables. They can continue to respire after harvesting, thus producing better flavor. However, we do not know the whole picture of this process at present, but ethylene is considered to be a key role. Fruits that cannot become sweet or even become unsweet are usually non-respiratory climacteric fruits and vegetables. They are basically fixed after harvesting. The best way is to harvest them after they are completely ripe.

References

[1] Ji Y, Wang A. Recent Advances in Phytohormone Regulation of Apple-Fruit Ripening. Plants. 2021; 10(10):2061. https://doi.org/10.3390/plants10102061 https://www.mdpi.com/2223-7747/10/10/2061#cite

[2] Liu Changyu, Chen Xun, Long Yuqing, Chen Ya, Liu Xiangdan, Zhou Ribao. Research progress on genes related to flower senescence in ethylene biosynthesis and signal transduction pathways[J]. Biotechnology Bulletin, 2019, 35(3): 171-182 http://html.rhhz.net/SWJSTB/html/2019-3-171.htm

[3] Hewitt S and Dhingra A (2020) Beyond Ethylene: New Insights Regarding the Role of Alternative Oxidase in the Respiratory Climacteric. Front. Plant Sci. 11:543958. doi: 10.3389/fpls.2020.543958 https://www.frontiersin.org/articles/10.3389/fpls.2020.543958/full

This article is authorized to be reproduced from the author Wulianhuakai's Zhihu. It has been modified by "Fanpu". Click "Read Original Text" to see the original text.

Special Tips

1. Go to the "Featured Column" at the bottom of the menu of the "Fanpu" WeChat public account to read a series of popular science articles on different topics.

2. Fanpu provides a function to search articles by month. Follow the official account and reply with the four-digit year + month, such as "1903", to get the article index for March 2019, and so on.