When an apple is cut, a coffee colored substance is produced on the flesh, so why is this a chemical

When a fruit is injured, its skin and the membrane that acts as a "protective wall" inside are damaged, and oxygen in the air enters the fruit and reacts with antioxidants such as phenols, such as cinnamic acid and flavonoids, as well as vitamin C, to oxidize them. In general, the compound is oxidized and brown, so when the apple is cut, the flesh will produce a brown color.

Browned fruit often gives people a dirty feeling, so how can you avoid fruit discoloration? We know that fruit changes color because of a chemical reaction with oxygen, and if there is a substance that can absorb it, then it will prevent this from happening. It has been found that lemon juice contains a citric acid, which is highly reducing and can quickly "rob" the oxygen away.

Therefore, if the peeled fruit is soaked in lemon juice, the fruit can not get enough oxygen to carry out a chemical reaction, so as to maintain its original color for a long time.

If the fruit is easy to change color after cutting, it means that the fruit contains more antioxidants, which is not only nutritious and beneficial to the body, but also can remove waste from the body and beautify the skin!

Phenolic compounds in apples. For example: polyphenols, catechols, etc.

Phenolic compounds are easily oxidized to quinones, that is, they change color and turn yellow, and the color gradually deepens with the increase of the amount of reaction, and finally turns dark brown. The oxidation reaction occurs due to the contact with oxygen in the air and the release of phenol oxidase in the cell.

Anyone who has studied chemistry knows it.

This is due to the fact that the substances in the apple are oxidized by the oxygen in the air to produce a coffee-colored substance, which produces a new substance, which is an oxidation reaction and is a chemical change.

The pulp reacts with oxygen in the air to form oxides. Some of the metal elements in it change color when they are oxidized.

A phenomenon after the oxidation of 2-valent ferric substances to 3-valent ferric substances in apples.

The phenomenon of oxidizing 2-valent iron to 3-valent iron in apples.

Oxidation reactions. I can't remember exactly what substance was oxidized.

(1) If Fe2+ in apples is oxidized to Fe3+ by oxygen in the air, the brownish-yellow Fe3+ meets the colorless KSCN solution and undergoes complexation reaction, Fe3++3SCN-?The complex Fe(scn)3 produced by Fe(scn)3 is red, so to test whether Fe2+ in apples is oxidized to Fe3+ by oxygen in the air, you can use KSCN solution to test So the answer is: KSCN; no red color appears;

2) One piece is placed in the air and the other is quickly immersed? The surface of the apple placed in the air in L-1NaCl solution gradually browns, and the other piece is immersed in the sodium chloride solution, isolated from the air, and there is no obvious change for a long time, indicating that the apple browning is related to oxygen Therefore, the answer is: oxygen;

Enzymes are proteins, and heating or strong acids denature enzymes, i.e., proteins, and polyphenols are easily oxidized by oxygen By putting the other two pieces into 90 95 boiling water and ? L-1 hydrochloric acid soaked for 2min, the protein was denatured, placed in the air, for a long time, there was no obvious change, indicating that the activity of polyphenols and oxidase in apple was destroyed, indicating that apple browning is related to the activity of phenol oxidase Therefore, the answer is: the activity of polyphenols and oxidase;

3）?The pH of L-1's NaHCO3 solution and Na2SO3 solution is similar, but Na2SO3 solution can prevent apple browning, and control experiments show that sodium sulfite is more reducible than phenol, and the reaction: 2SO3 occurs

2-+o2═2so4

2-, consumes oxygen, protects the phenol, so that the apple does not brown So the answer is: sodium sulfite is stronger than phenol, consumes oxygen, protects phenol; 2so3

2-+o2═2so4

2-；(4) Apples contain polyphenols and polyphenol oxidase, apple browning is caused by the joint action of oxygen, phenols, and phenol oxidase to produce quinones To prevent peeled apples from browning, you can take the method of isolating the air and destroying the activity of phenol oxidase Such as: put it in the refrigerator and refrigerate, wrap it in plastic wrap, soak it in salt water, etc., so the answer is: put it in the refrigerator, wrap it in plastic wrap, and soak it in salt water (other reasonable answers are acceptable).

Apples will oxidize and turn brown in the air because apples contain divalent iron ions (green, that is, the glass we see every day, are all green, because they contain divalent iron ions), oxidation into brown state fingers and divalent iron ions are oxidized by oxygen in the air to trivalent iron ions (brown Daily we see that the red bricks used in construction are caused by trivalent iron ions, and the green bricks are cooled by water immediately after the red bricks are burned. Water can reduce ferric iron to bivalent iron at high temperatures, so it turns green).

Neither salt water nor sugar water can react with divalent iron ions or trivalent iron, so it cannot be chemically saved.

But from a physical point of view, if the brine and sugar water are both thick (when it is thicker than the juice in the apple), the apple will lose water. As a result, the contact surface between the apple flesh and the air exposed to the air is reduced, which can be slowed down.

It's not like the second floor says, and those who sell fruit skewers certainly know that sugar is much more expensive than salt, and that sugar water is difficult to preserve.

a. Enzymes can work as long as there are suitable conditions, and they can only work outside the cell, so A is wrong;

b. If the phenolic substances can pass through freely, then before the biofilm is destroyed by the cell, the phenolic substances of small molecules will be contacted by phenolic oxidase to form brown substances, so B is wrong;

c. According to the title, "after the cell is destroyed, the phenol oxidase in it comes into contact with phenolic substances", indicating that before the cell is destroyed, the phenol oxidase and phenolic substrate in the cell are stored separately, which is the function of the biofilm system in the cell, and the biofilm in the cell separates the cell into small compartments, so that a variety of chemical reactions can be carried out in the cell at the same time without interfering with each other, ensuring that the life activities of the cell are carried out efficiently and orderly, so C is correct;

d. This content has nothing to do with the reason for the stem, so D is wrong

Therefore, C

(1) Fe3+ turns red when it encounters KSN solution, so to test whether Fe2+ in apples is oxidized to Fe3+ by oxygen in the air, you can use KSN solution to test, according to the conclusion: statement A is incorrect, and it can be seen that the phenomenon is not red;

So the answer is: kscn; no red color appears;

2) The apple placed in the air is browned, and the other piece is immersed in sodium chloride solution, isolated from the air, and there is no obvious change for a long time, which indicates that the apple browning is related to oxygen;

According to the title: the other two pieces are immediately put into 90 95 boiling water and ? L-1 hydrochloric acid was soaked in hydrochloric acid for 2 min and then taken out, and placed in air without obvious change, indicating that the activity of polyphenol oxidase in apple was destroyed, which proved that apple browning was related to the activity of phenol oxidase.

So the answer is: oxygen; the activity of phenol oxidase (heating or strong acids denature enzymes, i.e. proteins, etc., reasonable answers are acceptable);

3）?The pH of L-1 NaHCO3 solution and Na2SO3 solution is similar, but Na2SO3 solution can prevent apple browning, which may be due to the fact that sodium sulfite is more reducible than phenol, which consumes oxygen and protects phenol, so that apple does not brown; The ionic equation for the reaction is 2SO3

2-+o2═2so4

2-；So the answer is:

Sodium sulfite is more reducible than phenol, consumes oxygen, and protects phenol; 2so3

2-+o2═2so4

2-(4) To prevent the browning of peeled apples, you can take measures to isolate the air and destroy the activity of phenol oxidase, such as: put it in the refrigerator, wrap it in plastic wrap, soak it in salt water, etc.;

So the answer is: put it in the refrigerator and wrap it in plastic wrap (reasonable answer is acceptable).

(1)a

2) Esterification with carboxylic acid and the substance first, then oxidation, and then hydrolysis under acidic conditions.

1) If Fe2+ in apples

It is oxidized by oxygen in the air to Fe3+

Brownish-yellow Fe3+

Complexation reaction occurs when encountering a colorless KSCN solution, Fe3+

3scn-fe（scn）3

The resulting complex Fe(sCN)3

It is red, so it is necessary to check the Fe2+ in the apple

Whether it is oxidized to Fe3+ by oxygen in the air

It can be tested with KSN solution, so the answer is: KSCN; no red color appears;

2) One piece is placed in the air and the other is quickly immersed? l-1

The surface of the apple placed in the air in the NaCl solution gradually browns, and the other piece is immersed in the sodium chloride solution, isolated from the air, and there is no obvious change for a long time, indicating that the apple browning is related to oxygen Therefore, the answer is: oxygen;

Enzymes are proteins, and heating or strong acids denature enzymes that are proteins, and polyphenols are easily oxidized by oxygen By putting the other two pieces into 90 95 boiling water and ? l-1

Soak in hydrochloric acid for 2min to denature the protein, place it in the air for a long time, there is no obvious change, indicating that the activity of polyphenols and oxidase in apples is destroyed, indicating that apple browning is related to the activity of phenol oxidase, so the answer is: the activity of polyphenols and oxidase;

3）?l-1

NAHCo3

solution and Na2

The pH of the SO3 solution is similar, Na2

The SO3 solution can prevent the browning of apples, and the control experiment shows that the reduction of sodium sulfite is stronger than that of phenol, and the reaction occurs: 2SO3

o22so4

Oxygen is consumed, and phenol is protected, so that apples do not brown So the answer is: sodium sulfite is more reducible than phenol, which consumes oxygen and protects phenol; 2so3

o22so4

(4) Apples contain polyphenols and polyphenol oxidase, apple browning is caused by the joint action of oxygen, phenols, and phenol oxidase to produce quinones To prevent peeled apples from browning, you can take the method of isolating the air and destroying the activity of phenol oxidase Such as: put it in the refrigerator and refrigerate, wrap it in plastic wrap, soak it in salt water, etc., so the answer is: put it in the refrigerator, wrap it in plastic wrap, and soak it in salt water (other reasonable answers are acceptable).