Weird, why are the fruits and vegetables in my refrigerator "sweating"?

In daily life, we are always used to putting fresh fruits and vegetables in the refrigerator to keep them fresh and taste good. However, have you ever found that the surface of these fruits and vegetables lying quietly in the refrigerator is covered with tiny water droplets, as if they are "sweating"? What is going on?

Physical reason: liquefaction

The temperature inside the refrigerator is lower, and it is usually used to keep food fresh and extend its shelf life. However, the surface and internal temperatures of fruits and vegetables that have just been put in from outside are relatively high, and they carry a certain amount of heat.

When these hot fruits and vegetables are placed in the refrigerator, their surfaces will cool down quickly. At this time, if there is a certain amount of water vapor inside the refrigerator (although the humidity inside the refrigerator is low, there will still be a small amount of water vapor), these water vapors will liquefy when they encounter the surface of the fruits and vegetables with lower temperature, that is, the gaseous water vapor will turn into liquid water droplets and adhere to the surface of the fruits and vegetables, forming a " sweating " visual effect.

Biological factors: Fruits and vegetables also breathe

Fruits and vegetables will still perform a weak respiration in the refrigerator, releasing carbon dioxide and water vapor. The released water vapor may also encounter a cold surface and liquefy. When the condensed water droplets on the surface of fruits and vegetables react with carbon dioxide, a slightly acidic environment will be formed.

This environment is conducive to the growth and reproduction of fungi, so care should be taken to keep fruits and vegetables dry and ventilated to avoid rotting.

What is respiration of fruits and vegetables?

After fruits and vegetables are harvested, assimilation basically stops, but they are still alive and their main metabolic process is respiration.

Respiration is the process in which respiratory substrates are decomposed into simple substances and chemical bond energy is released through many intermediate links under the biological oxidation of a series of enzymes.

There are two types of respiration in fruits and vegetables, aerobic respiration and anaerobic respiration. Respiration is closely related to the physiological and biochemical processes of various fruits and vegetables. The more vigorous the respiration, the faster the various physiological and biochemical processes proceed, and the shorter the life span after harvest.

Therefore, during the storage and transportation of fruits and vegetables after harvest, we must try to inhibit the respiration of fruits and vegetables while maintaining the normal life process of fruits and vegetables.

How to inhibit the respiration of fruits and vegetables?

Lower the ambient temperature: By lowering the temperature of the storage environment, the activity of respiratory enzymes in fruits and vegetables can be inhibited, thereby reducing the respiratory intensity. Most fruits are best stored at 4-5°C, while thermophilic fruits and vegetables are best stored at around 12°C.

Adjust the oxygen and carbon dioxide concentrations in the environment: Properly increasing the carbon dioxide concentration and reducing the oxygen concentration can inhibit the respiration of fruits and vegetables. This method is called controlled atmosphere storage, and commonly used methods include vacuum packaging, nitrogen filling packaging, etc.

Reduce water content: The water content of fruits and vegetables will also affect their respiration. By properly drying the fruits and vegetables to reduce their water content, their respiration intensity can be reduced, but care should be taken to avoid excessive drying that may cause cell inactivation.

Choose the appropriate storage method: According to the type and maturity of fruits and vegetables, choose a suitable storage method, such as refrigeration, controlled atmosphere storage, vacuum packaging, etc., to maximize the inhibition of their respiration and extend the storage life.

Why can refrigerators extend the shelf life of fruits and vegetables?

Under low temperature, the activity of enzymes related to respiration in fruit and vegetable cells is inhibited, such as hexokinase, fructokinase and pyruvate kinase in the tricarboxylic acid cycle (TCA), glycolysis (EMP) pathway and pentose phosphate (PPP) pathway. The reduction in the activity of these enzymes slows down the process of respiration, thereby reducing the consumption of organic matter.

Low temperatures can also inhibit the growth of microorganisms that can exacerbate the spoilage process of fruits and vegetables. Low temperatures slow down the growth of active microorganisms and delay the onset of disease, helping to extend the shelf life of fruits and vegetables.

In a low temperature environment, the respiratory metabolism of fruits is weakened, and the production and release of ethylene is also reduced. Ethylene is a gas that promotes the ripening and aging of fruits. Reducing its production of ethylene helps maintain the quality of fruits and vegetables.

How to prolong the shelf life of fruits and vegetables? The method is very simple!

Remove moisture

Adding moisture absorbent or absorbent paper to the bags of vegetables stored in the refrigerator can absorb the moisture inside the bags and effectively reduce the formation of water droplets. Check the inside of the fruit and vegetable bags frequently to see if there is condensation to ensure that the bags are always dry.

Adjust refrigerator temperature and humidity

Adjust the temperature in the refrigerator to between 2-8°C according to the type of fruits and vegetables. At the same time, pay attention to maintaining appropriate humidity to prevent fruits and vegetables from losing water and wilting. You can use fresh-keeping bags or plastic wrap to wrap fruits and vegetables to maintain their humidity.

Open cooling

For fruits and vegetables packed in fresh-keeping bags, you can put them in the refrigerator with the bags open first, and let the temperature of the fruits and vegetables gradually drop to the temperature inside the refrigerator. When the temperature of the fruits and vegetables is close to the temperature inside the refrigerator, tie the bag tightly.

Close the refrigerator door promptly

Close the refrigerator door in time after each time you take out or put in food. Avoid opening the door frequently and try to reduce the number and duration of opening the door to reduce the risk of cold air loss and food spoilage.

In addition, avoid putting hot or uncooled food directly into the refrigerator to avoid increasing the temperature and humidity inside the refrigerator. Clean the refrigerator regularly to ensure that the internal environment is clean and hygienic to avoid cross contamination. These measures can effectively inhibit the respiration of fruits and vegetables and extend their shelf life.

END

References

Lu Heng. Experimental study on the effect of storage temperature on the quality of different fruits and vegetables[D]. Tianjin: Tianjin University of Commerce, 2023.

Wu Xiulan, Ren Shixin, He Junjie, et al. Current status and prospects of fruit and vegetable preservation technology[J]. Shandong Chemical Industry, 2022, 51(12): 111-114.

Wang Yadong. Study on postharvest respiration and storage and preservation of fruit and vegetable products[J]. Horticulture and Seedlings, 2022, 42(02): 58-59.

Wang Yanhong. Study on the mechanism of water loss during low temperature storage of fresh fruits and vegetables[D]. Tianjin: Tianjin University of Commerce, 2010.

Author: Ren Yanan

Member of Beijing Refrigeration Society, Master's student of Institute of Agricultural Product Processing, Chinese Academy of Agricultural Sciences

Audit expert: Duan Yuquan

Director of Beijing Refrigeration Society, Associate Researcher of the Institute of Agricultural Product Processing, Chinese Academy of Agricultural Sciences

Editor: Dong Xiaoxian