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Hanging out in the supermarket one day, although the editor is not good at cooking, he is still attracted by the dazzling variety of thick-cut steak and leg of lamb, the fresh red makes people and you look at the salmon next door, red, watermelon red, warm orange, light pink. A large area of gaudy red, the freshness of that kind of color transmission makes people really have an appetite. No, I suddenly thought of a question: why does fresh meat turn red?

Figure 1 Beef fillet photography [1] (enjoy the meat slices, no sugar is zero calories) when it comes to color, let's start with the visible light spectrum that the editor is good at. People can perceive color because visual cells are stimulated by electromagnetic waves in the range of visible light. The color of an object is determined by its reflection spectrum, which is related to the physical properties and surface structure of the material itself.

Figure 2 electromagnetic waves [2] We start with the most basic characteristic colors of elements. Taking the well-known flame color experiment as an example, the characteristic color of the flame appears when burning some metal elements or their compounds. The principle is that during combustion, electrons in the atom absorb energy and transition from lower-energy orbitals to higher-energy orbitals. Over time, electrons return to lower-energy orbitals, a process that releases photons.

Fig. 3 Flame color reaction of metal elements [3]

Fig. 4 the relationship between the photon emission wavelength (λ) and the atomic orbital energy difference (∆ E) from the excited state to the ground state radiated photon [4] is ∆ E = h v = hc/ λ, where h, v and c are Planck constant, frequency and speed of light, respectively. The energy level differences of electron orbitals in different elements are different, so the elements have a series of characteristic spectral lines. If the wavelength of the photon falls within the range of visible light, it can be perceived by the human eye. Take the metal Na as an example, its characteristic spectrum comes from the transition of electrons between 3p orbitals and 3s orbitals in the visible band. Due to its fine structure, there are double lines in the spectrum at the wavelengths of 589.0 nm and 589.6 nm, both in the yellow band, so we see a bright yellow flame in the flame color reaction of Na elements.

So when we go back to the meat we eat, why does vb behave differently in red? This mainly comes from myoglobin. First, let's take a look at myoglobin, which is a single-stranded protein with a molecular weight of 16700 Daltons. Myoglobin exists in muscle and can store and release oxygen, which plays an important role in muscle movement. The process of storing and releasing oxygen from myoglobin is accompanied by changes in iron coordination, as shown in figure 5.

Fig. 5 structure of myoglobin [5] when myoglobin does not bind to oxygen, iron is positive bivalent, and each ferrous ion coordinates with five nitrogen atoms to form (FeN5) groups with a pyramid structure. Four of the five nitrogen atoms come from the pyrrole ring and one from histidine in the polypeptide chain. When combined with oxygen, the valence of iron is controversial, which may have both bivalent and trivalent. In addition to coordination with nitrogen, iron ions also directly bond with oxygen molecules to form (FeN5O) groups, which are octahedral structures. Let's analyze the orbital arrangement of the electrons in detail:

When there is no oxygen binding, the electronic arrangement of Fe2 + ion in myoglobin is 1s22s22p63s23p63d6. Generally speaking, among these electrons, 3D electrons have the most obvious effect on the physical and chemical properties of Fe2+ ions. 3D orbit can be subdivided into five orbits: dxy, dxz, dyz, dx2-y2 and dz2. In isolated Fe2+ ions, these five orbitals have the same energy, but the shape or orientation of the electron cloud is different, as shown in figure 6.

Fig. 6 the electron cloud of 3D orbitals [6] when Fe2+ ions coordinate with five nitrogen atoms, the interaction between 3D orbital electrons and nitrogen atoms results in differences in the energies of the five orbitals, from low to high as dxz/dyz, dxy, dz2 and dx2-y2 (the energies of dxz and dyz are equal) [7], as shown in the figure on the left of figure 7. The horizontal lines in the picture represent electron orbitals, and their levels represent the energy of the orbitals. Each arrow represents an electron, and their orientation represents the spin direction of the electron. The electron arrangement of oxygen-free Fe2+ ions in the left of figure 7 only shows the case of high spin state, and there are many kinds of electron arrangements of Fe2+ ions in actual anaerobic myoglobin.

Fig. 7 Electron arrangement in the iron ion coordination group of myoglobin [8] another case is that oxygen molecules enter myoglobin. First of all, for oxygen molecules, σ / σ * bonds and π / π * bonds are formed between electrons from 2p orbitals of different oxygen atoms, and electrons are filled in the order of σ bond, π bond, π * bond and σ * bond. The result of filling is that the σ bond and π bond are filled, the σ * bond is not occupied by electrons, and the π * bond is only half filled. When myoglobin binds to oxygen, the iron ion forms a bond with the oxygen atom, and the two electrons on the π * bond in the Omure O bond form a bond with the 3D electron of the iron ion, namely, a σ / σ * bond with the dz2 orbital and a π / π * bond with the dyz orbital. After bonding, the 3D orbitals of iron ions and the Fe-O bond orbitals form a common Fe-O2 level system, and the energy of the electron orbitals in the system is as high and low as shown in figure 7. The electron arrangement in the figure only shows the case of low spin, and there are many ways of electron arrangement in the actual Fe-O2 system.

Now we discuss the electronic transitions related to myoglobin color. It is found that in both oxygen-free Fe2+ ion groups and Fe-O2 systems, electrons can transition between orbitals shown in figure 7, and the transition process can absorb visible light. However, the energy levels of the two are different, which leads to their different absorption spectra. Figure 8 shows the absorption spectra of oxygenated, anoxic, ferric and nitric oxide myoglobin. Oxygen-containing myoglobin can obviously absorb the light whose wavelength is less than 600nm, which leads to the red appearance of the reflected light (red band: 622770nm). In contrast, the light absorption of anaerobic myoglobin at wavelengths less than 550 nm was weakened, and the light absorption in the wavelength range of 600-700 nm was stronger than that of oxygenated myoglobin, resulting in a purplish red appearance. The muscles of living animals are bright red because myoglobin carries plenty of oxygen. The fresh meat transported to the supermarket is temporarily purplish red because it is separated from the mother's oxygen supply, and myoglobin releases oxygen. After being exposed to the air for a long time, the anaerobic myoglobin on the surface of the meat will recombine with the oxygen in the air to form oxygen-containing myoglobin and show bright red, but the interior of the meat is still in a state of hypoxia and purplish red. When overoxidized, such as bacon, myoglobin is converted into ferrimyoglobin, which appears brown. Some merchants add a certain amount of nitrite when pickling meat in order to keep the meat red. After self-redox reaction, part of nitrite will be converted into nitric oxide, and nitric oxide will combine with myoglobin to form nitric oxide myoglobin. Its absorption spectrum is similar to that of oxygen-containing myoglobin, so the meat is red. However, nitrite has a certain harm to the human body, and the state has a strict limit on its addition.

Fig. 8 the absorption spectrum of myoglobin [9] has been said for so long that it goes back to the gaudy salmon next door. When the proportion of myoglobin in meat changes, it will show different red macroscopically. Before different kinds of fish are made into sashimi, there are plenty of fitness "big fish" that jump up and down, such as crimson moon fish and tuna that look very much like beef, while others lie flat on the sea floor, such as pink-puffed yellow belt scads. Oh, and the good-looking orange salmon, which preys on small crustaceans to get astaxanthin and stores it in its own myored cells to dye the reddish halo of the meat orange. Different living environment, habits and other factors lead to different myoglobin content in animals, and then show different meat color. This is very much like the pigment blending in traditional Chinese painting. When white is mixed with different proportions of red, it can bring up the transition from milky white, light pink, dark pink, rose to positive red.

Fig. 9 Moon fish swimming fast [10]

Fig. 10 scad sashimi [11] (left) and salmon sashimi [12] (right)

Fig. 11 Color blending of traditional Chinese painting [13] finally, friends, exercise a lot, our meat will also change color.

reference

[1] from ZCOOL, https://img.zcool.cn/ community / 0780db6246a8b40002db57064bdee2.jpg

[2] from Wikipedia, https://wikipedia.org/ [J].

[3] extracted from instrument Network, https://www.yiqi.com/ retiao / detail_2596.html

[4] excerpt from Zhihu: why is the flame color reaction a physical change? Won't metal burn if you put it on the fire? Kevin Wayne's answer in, https://www.zhihu.com/ question / 436486528 / answer / 2375592574

[5] from Wikipedia, https://wikipedia.org/

[6] from Wikipedia, https://wikipedia.org/

[7] Churg A K, Makinen M W. The electronic structure and coordination geometry of the oxyheme complex in myoglobin. The Journal of Chemical Physics, 1978, 68 (4): 1913-1925.

[8] from Libretexts-Chemistry, https://chem.libretexts.org/

[9] Millar S J, Moss B W, Stevenson M H. Some observations on the absorption spectra of various myoglobin derivatives found in meat. Meat Science, 1996, 42 (3): 277288.

[10] Sina Weibo from fish in Salzburg

[11] Sina Weibo from fish in Salzburg

[12] from Wikipedia, https://wikipedia.org/

[13] from the official account of traditional Chinese painting, ID:quicksnowfall

This article comes from the official account of Wechat: Institute of Physics, Chinese Academy of Sciences (ID:cas-iop), author: Zhou Junyan, review: Jin Shifeng

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