Fatty acid is actually a chain of carbons.
- if they are linked only by single bonds, it is a saturated fatty acid (because each carbon atom is saturated with two hydrogen atoms)
- If there is exactly one double bond in the chain, it is a monounsaturated fatty acid. At the point of double bond, each of the adjacent carbons has “discarded” one of their hydrogens (and connects to the other using the freed bond) . They’ve now got only one hydrogen left each, hence remain unsaturated.
- if there is more than one double bond in the chain, it is a polyunsaturated fatty acid (because at the point of double bonds the corresponding carbons have remained unsaturated with hydrogen)
At the point where there is a double bond in the chain (hence, one of them is redundant), the unsaturated fat can react chemically (“utilize” the situation, release the excess bond, and bind another atom instead – saturate itself):
- the place can be saturated with hydrogen again and saturated fat is created (hydrogenation)
- two oxygen atoms from the air can be attached to the site – oxidation (this is exactly the way the poorly stored fat becomes rancid)
- it is easier to connect here with another substance that can change it, transport it, decompose it…
The double bond connects the adjacent carbons more firmly (fixes them in position), and the individual molecules “adhere” less. The fewer double bonds, the more it leads to the following effects:
- the whole fat tissue is “more motionless”, ie it has a higher viscosity (worse flow), is stronger and needs a higher temperature for melting
- the fat is more chemically stable, ie less prone to chemical reactions (which occur at an unsaturated “weak spot”), especially to oxidation
Saturated fats are therefore stronger and more stable than unsaturated fats (the same applies to MUFA vs PUFAs). This can be an advantage (better storage and durability, better handling, unlikely to be industrially processed, resistant to thermal processing), but also a disadvantage (resistance to some body processes) .