The loyal "guardian" of warmth and quality: cork oak

Thermos bottles are essential daily necessities for every household, but what many people don’t know is that the core component of thermos bottles is actually the humble wooden cork. Without it, the thermos bottles will lose their heat preservation function. In addition, in order to ensure that the wine is mellow and does not change its taste, wine bottles also use wooden corks. But not all wood can be used to make corks that can keep heat and preserve quality. What are the advantages of these “selected” woods?

What is cork?

The wood used to make these bottle stoppers must be cork. Cork comes from the surface protective tissue derived from the thickening growth of the stems and roots of trees, mainly the cork layer, which contains multiple layers of tightly arranged cork cells/cork cells. These cork cells die after maturity, the cell walls become corky, and the cell cavity is filled with air. Some also contain tannins, resins and other substances. The cork layer can isolate water and gas from passing through, play a protective role for trees, control the loss of water inside the tree, and prevent diseases, insect pests and external factors from mechanical damage to the internal tissues of the plant body. It should be noted that the cork layer is not the bark, it is just one of the components of the bark.

The cork layer is the raw material of cork. Its special structure makes it light and soft, elastic and stretchable, sealed and waterproof, wear-resistant and shock-proof, acid-corroded, and chemically stable. It is also a poor conductor of heat, electricity, and sound. In addition, it is non-toxic and odorless, has a small specific gravity, is soft to the touch, and is not easy to catch fire. These advantages make it play an important role in many fields, such as making corks and sound insulation and thermal insulation materials. It can also be used in daily lightweight insulating materials and life-saving equipment. So far, there is still no other industrial substitute that can match it.

The cork layer of the stems of some trees can accumulate all year round without falling off, so the cork layer is particularly developed (the external manifestation is that the bark is very thick), such as Quercus variabilis and Phellodendron amurense, so they become commercial cork, that is, the main source of cork manufacturing.

The name "cork oak" comes from the well-developed cork layer under the bark surface. The cork produced from the cork layer of cork oak has a large output, fine quality and light weight. It is widely used in the production of insulators, cold storage, life jackets for navigation, buoys and other items, and has become an important raw material for light industry and even defense industry. The bottle stoppers of the world-renowned Portuguese Porto wine and French champagne are made of high-quality cork.

The bark of the cork oak tree cracks longitudinally, revealing a thick layer of cork (Photo by Bai Ruixing)

The bark of the cork oak tree cracks longitudinally, revealing a thick layer of cork (Photo by Bai Ruixing)

Individual characteristics of cork oak

The cork oak is a tall deciduous tree of the Fagaceae family. Its bark is deeply cracked, revealing a thick layer of cork. When you press it, it feels soft and very elastic. It also has a notable appearance feature, that is, there is a circle of sharp serrations on the edge of the leaves, which are very slender, like the thorns on a caterpillar.

The slender serrations on the edge of the cork oak leaf (Photo by Bai Ruixing)

The cork oak is an important tree species in China, grown on sunny slopes below 800 meters above sea level in Liaoning, Hebei, Shanxi, Shaanxi and other regions of China. It has deep roots, a well-developed root system, strong germination ability and adaptability, and is wind-resistant, low-temperature-resistant, drought-resistant and barren. It can grow in acidic, neutral and calcareous soils, and can live for more than 200 years. It is very suitable for afforestation in the ecologically weak mountainous areas in the north. In addition, its leaf color changes significantly with the seasons, so it is also a good tree species for viewing yellow leaves in autumn.

The bark of the cork oak planted as an economic forest will be peeled off and used to make cork (the bark in a narrow sense consists of three parts: the cork layer, the cork cambium and the cork inner layer). This can easily damage the phloem inside the bark. Once the plant loses the phloem that transports nutrients, it will easily die. This is what the saying "people are afraid of hurting their bones, and trees are afraid of hurting their bark" means.

However, the cork oak is not afraid of being peeled. The bark of a cork oak can be collected about 10 times in its lifetime and it can still grow robustly. After the bark of cork oaks that are over 15 to 20 years old or have a breast diameter of over 15 cm is peeled, the cells will be reactivated and accelerated to divide, which is not only more conducive to secondary growth, but also the peeled cork oaks need more CO2 for bark growth. The CO2 absorption is five times that of unpeeled cork oaks, which has an important contribution to ecological and social benefits. On the contrary, if the bark is not peeled in time, the ecological benefits and comprehensive utilization level of the cork oak forest will be reduced, resulting in a waste of resources.

The Cork Oak is a treasure

The fruits of many Fagaceae trees are collectively called acorns. Because they are rich in starch, they were once used as one of the important food sources for our ancestors in prehistoric times. During the long years of social development, they have helped humans resist famine many times.

Mei Yaochen, a famous poet in the Northern Song Dynasty, described in detail in his "Miscellaneous Poems of Xuanzhou" how poor people in Anhui who had no land to farm collected wild oak nuts as food: "Wild grains are collected as acorns, and pine torches are lit in mountain houses. There are only trees to plant, but no land to cultivate."

As a type of oak, the fruit of the cork oak also contains a high proportion of starch. The extracted starch can be used for sizing yarn, brewing wine, and as feed in modern times.

In addition, the fruit of the cork oak can be used as medicine. According to "Guizhou Herbal Medicine", its fruit shell has the effects of relieving cough, treating tinea capitis, and diarrhea, and can also be used to extract black dye.

It can be said that the whole body of the cork oak is a treasure. It is one of the country's Class A precious tree species, and it combines many advantages such as "native, shade tree, longevity, resistance to adversity, edible source, colorful leaves, and new and excellent".

Fruits of the cork oak (Photo by Bai Ruixing)

Author: Wang Jue, Associate Professor, Beijing Forestry University

Scientific review: Hu Dongmei, Senior Laboratory Technician, College of Science and Technology, Beijing Forestry University

Editor: Xie Yun