How to make fruit sweeter as it ages? The key is...

Friends who often eat fruits will have this feeling: some fruits become sweeter as they are stored after they are bought, while some fruits do not become sweeter as they are stored, and may even taste worse and become less and less sweet.

Why is this? This article will reveal the secret!

Fruits that become sweeter as they age **——climacteric fruits**

Some fruits may not be very ripe when they are in the fruit store, but after they are left at home for a while, you will find that they become more ripe and taste sweeter. This is because they are respiratory climacteric fruits, such as apples, bananas, mangoes, kiwis, pears, peaches, plums, papayas, durian, etc.

Climax fruits refer to the period between when the fruit stops growing and begins to age, when its respiration rate suddenly increases. At the same time, it produces a substance that can ripen itself - ethylene. Ethylene not only allows the fruit to gradually mature, but also improves the taste of the fruit, making it more beautiful and delicious!

The role of ethylene:

1. Change the color of the peel

Ethylene can accelerate the degradation of chlorophyll through the production and accumulation of carotenoids in fruits, changing the color of the fruit skin from green to orange-yellow, thus enhancing the appearance of the fruit and making it more beautiful. This effect is effective for both climacteric and non-climacteric fruits. [1]

2. Soften the fruit

The cell walls of unripe fruits contain water-insoluble protopectin and cellulose, which provide support for the fruit’s firmness. Ethylene not only activates and expresses genes involved in cell wall metabolism, but also accelerates the metabolism of intracellular substances through respiration, thereby promoting the degradation of pectin and cellulose in the fruit’s cell walls, reducing the firmness of the fruit and allowing the fruit to mature and soften. [1]

3. Convert starch into sugar

Unripe fruits contain more starch granules. As the fruit matures, the activity of α-amylase increases, and the starch is hydrolyzed into soluble sugars, increasing the sweetness of the fruit.

4. Reduce the acidity of fruits

While ethylene promotes the ripening of fruits, it also gradually reduces the titratable acid content in the fruits, lowers the acidity, and gradually increases the sugar-acid ratio, making the taste sweeter and more palatable. [2]

Image source: pixabay

Under the combined influence of these factors, respiratory climacteric fruits have the ability to become sweeter as they age. Don't underestimate the ripening ability of ethylene. If one of the fruits you bought is ripe, it will ripen the other fruits around it, and they may all ripen together. At this time, you have to eat them quickly.

This is why many fruits sold in fruit stores are always raw. Climax fruits are usually picked before they are fully ripe, which is more convenient for transportation and storage. After you buy them home, they will ripen themselves and other fruits around them, and they will become sweeter as time goes by.

Fruits that won’t become sweet or may not be sweet after being stored for a long time - non-climacteric fruits

Not all fruits will become sweeter as they age. Some fruits will become less sweet if left for too long. We call this type of fruit non-climacteric fruit.

Non-climacteric fruits do not have obvious respiratory peaks and ethylene release peaks. The respiratory rate does not change significantly from the time the fruit stops growing to the time it begins to age.

For example, grapes, strawberries, oranges, cherries, blueberries, pineapples, lychees, etc. They are all slowly ripening fruits, and can only complete the process of fruit ripening on the tree. If they are picked when they are not ripe, they cannot ripen themselves like respiratory climacteric fruits.

Although the peel will change color when placed together with respiratory climacteric fruits, this is not due to premature ripening. Instead, the chlorophyll in the peel has faded under the action of ethylene, which has little effect on the flesh.

Since these fruits do not have a ripening effect, they will not become sweeter as they are stored. Instead, they will become less sweet if they are stored for too long. This is because these fruits are already ripe when they are picked. If they are stored for too long or in improper conditions, they will gradually deteriorate and ferment, and then the acidity will increase, accompanied by a wine taste, and they will become less sweet and taste worse.

Because non-climacteric fruits are picked only after they are ripe, they must be eaten as soon as possible after they are brought home.

Image source: pixabay

How to store different types of fruit?

There are differences in the storage methods of respiratory climacteric fruits and non-respiratory climacteric fruits. Many people are accustomed to storing all the fruits they buy in the refrigerator. In this regard, it is not recommended to treat them equally.

How should these two types of fruits be preserved?

1. Climax Fruit

Do not put it in the refrigerator when it is not ripe. Leave it at room temperature to ripen itself. It will taste sweeter and should be eaten quickly after it is ripe.

If you want to speed up the ripening, you can seal it with a bag or cardboard box, so that the ethylene concentration is higher and the ripening is faster; if you don’t want the fruits to ripen quickly in a short period of time, put them in a breathable paper bag and isolate the fruits from each other, and then store them in the refrigerator after they are ripe.

Storage conditions for fruits with respiratory climacteric rate Storage conditions for fruits when they are raw Storage conditions and time after ripening [3] Apples Green, hard, sour, astringent, red or green, sweet, not astringent 3 weeks at room temperature, 4-6 weeks in the refrigerator Pears Green, hard, yellow, soft 3-5 days in the refrigerator Apricots Green, hard, sour, astringent, yellow, soft, sweet 2-5 days in the refrigerator Plums Hard, sour, astringent, red, sweet 3-5 days in the refrigerator Peach Green, hard, red, soft 3-5 days in the refrigerator Kiwis Hard, sour, soft, sweet 3-6 days in the refrigerator Papaya Green, yellow or orange-red 3-5 days at room temperature, 1 week in the refrigerator Mangos Hard, not sweet, soft, sweet 3-5 days in the room temperature, 1 week in the refrigerator Bananas Green, hard, soft, with black spots, sweet 3 days in the refrigerator Avocados Green, hard, dark green or black, soft 3-4 days in the refrigerator Tropical fruits such as mangoes, bananas and avocados are sensitive to temperature and can easily get frostbite if the refrigerator temperature is too low. If the temperature at home is low, or there is a cooler balcony, you can also not put it in the refrigerator. Buy less each time and eat it as soon as possible after it is ripe.

2 Non-climacteric fruits

Generally, it is picked when it is ripe, so there is no need to ripen it at room temperature. You can put it in the refrigerator when you buy it home. This can reduce its cell metabolism rate and delay the progress of rotting and deterioration.

Storage conditions for non-climacteric fruits Fruit selection Storage method and time [3] Grapes are plump and have white frost Refrigerate for 1 week Strawberries, except for white strawberries, choose reddish ones Refrigerate for 2-3 days Blueberries are dark purple and have white frost Refrigerate for 1-2 weeks Pineapples are heavy and have fresh and upright leaves Refrigerate for 5-7 days Citrus fruits have orange-yellow skin and are elastic when pressed Refrigerate for 10-21 days Cherries have fresh stems and the fruits are reddish and plump Refrigerate for 2-3 days Pomegranates have reddish, plump, and wrinkle-free skin 2-5 days at room temperature, 1-3 months Refrigerate Summary

The reason why some fruits become sweeter as they are stored is because they are respiratory climacteric fruits; however, non-respiratory climacteric fruits will not only not become sweeter if stored for a long time, but may become less sweet and their taste will deteriorate. It is best to store these fruits in the refrigerator as soon as possible after purchase!

References

[1] Wang Jianjun, Zhou Yahan, Zeng Kaifang. Effects of ethylene ripening on storage quality of postharvest fruits[J]. Science and Technology of Food Industry, 2012, 33(21): 361-364. DOI: 10.13386/j.issn1002-0306.2012.21.061.

[2] Zhou Mengqi, Chen Changlin, Meng Jiajun, et al. Effect of ethylene slow-release agent on ripening quality of Xuxiang kiwifruit[J]. Food Industry, 2022, 43(01): 176-180.

[3] U.S. Food Safety Network https://www.foodsafety.gov/keep-food-safe/foodkeeper-app

Author: Xue Qingxin, member of Chinese Nutrition Society, registered nutritionist, health manager, public nutritionist

Review丨Liu Guangyu Senior Engineer, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences

Ruan Guangfeng, Deputy Director of Kexin Food and Health Information Exchange Center