What is the "white frost" on fruits? Mold? Pesticide residue? Or...

When people buy fruits and vegetables, they are most worried about pesticide residues, especially when some fruits and vegetables have white frost on them. They wonder if it is pesticide residue and whether it is harmful to the body. What is the white frost and wax on the surface of fruits? Do you need to wash them? Can you eat them? Today, we will answer all these questions.

What is the "white frost" or "wax" on the surface of the fruit peel?

In our daily life, we often see a layer of "white frost" or "wax" on the surface of fruits such as apples and pears. What's going on? Is it the pesticides people use? In fact, many fruits will naturally form some "white frost" or "wax" during the growth process. This is their true appearance.

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Many fruits have a layer of "white frost" or "wax" on their surface. They are the main barrier to prevent water evaporation. They not only play a positive role in helping plants adapt to arid environments, but also help plants resist ultraviolet radiation . Since most of them are insoluble in water, they can prevent the formation of a moist environment on the surface, thereby preventing the infection of pathogens. Therefore, the "white frost" or "wax" on the surface of fruits is actually formed by the plants themselves, not pesticide residues .

After testing, scientists found that the most common of these "white frosts" or "waxes" are long-chain aliphatic compounds, including long-chain fatty acids, aldehydes, alcohols, etc., as well as cyclic compounds and sterol compounds, etc. They are all insoluble in water and are usually difficult to wash off .

Is the “white frost” on the surface of blueberries pesticide residue?

When buying blueberries, many people will find that there is a very conspicuous layer of "white frost" on the surface of blueberries, and many people worry that it is pesticide residue.

In fact, it is called fruit powder, which is secreted by blueberries themselves. It is not a pesticide and is harmless to the human body . Moreover, the white frost on the surface of blueberries can prevent the evaporation of water and resist the infection of bacteria, which can protect blueberries.

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In fact, the white frost on the surface of blueberries often indicates that the blueberries are relatively fresh, because the white frost on blueberries that have been stored for a long time will gradually disappear and will not be visible. Therefore, the white frost on the surface of blueberries is not pesticides at all, so don't worry too much, and blueberries with white frost are still good blueberries.

The white frost on the surface of blueberries is mainly composed of oleanolic acid and ursolic acid. In addition, the white frost also contains some alcohols and esters. These substances are insoluble in water and cannot be washed away by simply washing with water . If you really try hard to wash them off, it may damage the fruit.

In addition to blueberries, there are similar white frost on the surface of other fruits such as grapes, plums, and prunes. They are all the same and can be eaten with confidence. Just rinse them with clean water when eating. There is no need to scrub off the white frost.

Will eating waxed apples cause cancer?

There have been rumors online that "Apples are waxed and eating them can cause cancer!" "Five apples can produce half a pound of wax! Eating them can cause cancer!" What is the wax on the surface of apples? Is it artificial? Can we eat them without worry?

In fact, during the growth process of apples, its skin will secrete a layer of wax. This wax is an ester component. The wax is the first protective barrier against biological and non-biological stresses. It can prevent external microorganisms, pesticides, etc. from invading the flesh. It is a protective measure for apples against the external environment. This substance is harmless to the human body .

Moreover, waxing is actually a formal operation during the production and transportation of apples. Many countries in the world, including China, allow waxing on the surface of fruits to preserve them . China's "Sanitary Standards for the Use of Food Additives" stipulates that morpholine fatty acid salts (also known as fruit wax) and Brazilian palm wax can be used as coating agents on the surface of fresh fruits, which is commonly known as waxing.

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There are benefits to waxing apples properly. Waxing can prevent water loss, solve the problem of skin shrinkage, prevent pests from invading, and extend the shelf life of apples. Therefore, waxing apples is actually beneficial for long-term storage and long-distance transportation of apples, so that we can eat delicious apples anytime and anywhere. As long as it is waxed properly, it will not cause harm to human health and can be eaten with confidence .

What is the white frost on dried persimmons? Is the more the better?

Dried persimmons are also very popular. Traditionally, dried persimmons are dried by "drying": set up a rack in a ventilated and sunny place, select persimmons that have turned yellow but are still firm, peel them one by one (this will allow the moisture in the flesh to evaporate faster), and lay them out flat to dry. In the clear autumn, after about seven days of sun exposure and hand kneading, the dried persimmons can be formed and the tannins can be oxidized and removed. However, in some areas, dried persimmons are hung to dry.

People who like to eat dried persimmons know that there is a layer of white frost on the outside of dried persimmons, also known as persimmon frost. Some people say that the white frost is a preservative. In fact, the "white frost" of dried persimmons is a "microcrystalline polymer" condensed on the surface of the persimmons as the water evaporates during the drying process. The main component is actually sugar, including fructose, glucose, etc.

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Some people say that the more white frost on persimmons, the better. Many people also like to buy persimmons with more frost. In fact, the quality of persimmon cakes cannot be evaluated by the amount of white frost. The main component of white frost is sugar. From the current health perspective, we should eat less .

Moreover, this layer of white frost is easy to fake. Due to consumer preference, some unscrupulous merchants will use this to cheat, such as applying a layer of starch or talcum powder on the surface, on the one hand to prevent the frost from absorbing moisture, on the other hand to make the dried persimmons whiter and more beautiful . When buying dried persimmons, if there is a lot of white frost, and it falls off when you shake it, and there is a lot of white powder on your hands after you pick it up, it is basically cheated, so you should be more careful when buying.

References

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[2] Han Tong, Yu Shuli, Liu Yidong, et al. Research progress on epidermal wax and its relationship with fruit storage[J]. Journal of Food Safety and Quality, 2023, 14(10): 191-198. DOI: 10.19812/j.cnki.jfsq11-5956/ts.2023.10.029.

[3] Zhang Liping, Liu Ruiling, Han Yanchao, et al. Effects of blueberry epidermal wax components on postharvest disease resistance[J]. Journal of Chinese Institute of Food Science and Technology, 2021, 21(12): 205-213. DOI: 10.16429/j.1009-7848.2021.12.022.

[4] Zhang Liping, Liu Ruiling, Han Yanchao, et al. Effects of blueberry epidermal wax components on postharvest disease resistance[J]. Journal of Chinese Institute of Food Science and Technology, 2021, 21(12): 205-213. DOI: 10.16429/j.1009-7848.2021.12.022.

[5] Huang Shian, Lin Xin, Zhang Qin, et al. Comparison of structure and composition of wax in peel of three plum varieties[J]. Journal of Nuclear Agricultural Sciences, 2022, 36(06): 1155-1165.

[6] Ma Jie, Hu Yongchao, Yue Yitong, et al. Study on wax micromorphology and wax components of Lycium barbarum peel in Ningxia[J]. Journal of Northwest Botany, 2022, 42(04): 589-599.

[7] Huang Shian. Changes of wax content in plum fruit during storage and its relationship with storability[D]. Guizhou University, 2022.DOI:10.27047/d.cnki.ggudu.2022.000349.

[8] Cao Yue. Study on the difference of apple peel wax in dry and wet regions[D]. Shandong Agricultural University, 2020. DOI: 10.27277/d.cnki.gsdnu.2020.000104.

[9] Zhu Shouliang, Yuan Jianqi, Dong Xiaoqing. Research progress on apple peel wax[J]. Guizhou Agricultural Sciences, 2016, 44(02): 120-124.

[10] Yang Yanqing, Ren Xiaolin. Research progress on apple peel wax and its effect on postharvest storage and preservation of fruit[J]. Preservation and Processing, 2016, 16(06): 1-7+15.

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

Review丨Wang Kang, Director of the Science Museum of the National Botanical Garden