The Student Room Group

Please help this is due tomorrow

Explain in terms of molecular structure and chemical interactions the changes in colour and texture which occur during the boiling of vegetables

My Answer:
Most vegetables attain their green colour through chlorophyll. This is a pigment which has a cyclic structure with four nitrogen atoms arranged around a Mg2+ ion. Chlorophyll is mainly the reason for changes in vegetable colour when being boiled. When put into an acidic solution, the Mg2+ is removed irreversibly to form a brown-green product known as pheophytin. The low pH (e.g. pH 3) gives rise to a change in colour in contrast to what the vegetable should look like. To prevent this from happening, green vegetables are placed in higher pH solutions (e.g. pH 8) by adding substances such as sodium hydrogen carbonate (NaHCO3) when being boiled. Metal ions can also be placed into the chlorophyll structure to help fill the empty space left by the Magnesium ion. An example is Cu2+, which leads to a blue-green colour with pheophytin when vegetables are boiling. Here we can see that two different interactions between chlorophyll in vegetables determines the colour while being boiled.

Another molecule which is responsible for the changes in texture that occur during the boiling of vegetables is pectin. This is a polysaccharide in plants (and therefore vegetables) with the function of holding cells together. Pectin is usually insoluble in water however, boiling in alkaline solutions causes the molecule to become soluble. This change causes pectinic acid salt to be formed, which softens the texture of vegetables on boiling. This is because pectin breaks down with the heat, releasing bonds between cells.

Hydrogen bonds also play an important role in the structure of vegetables while being boiled. These occur in between polymer chains where water molecules are trapped, creating a structure known as gel. This type of structure is 99.5% by mass of water.

Is this enough?
Any suggestions on how to improve?

Quick Reply