An Acid is an organic compound with acidic properties, mostly produced in the plants. Some organic acids are with carboxyl group -COOH, group -SO2OH. Other groups can also confer acidity, usually weakly: the thiol group -SH, the enol group, and the phenol group. Organice acids are commonly used for acidulant, antioxidant, flavoring agent, and some possess antibacterial activities.
An aldehyde is an organic compound containing a functional group with the structure -CHO, consisting of a carbonyl center (a carbon double-bonded to oxygen) with the carbon atom also bonded to hydrogen and to an R group, which is any generic alkyl or side chain. Many aldehydes are found in essential oils and often contribute to their favorable odors, e.g. cinnamaldehyde, cilantro, and vanillin. Possibly because of the high reactivity of the formyl group, aldehydes are not common in several of the natural building blocks: amino acids, nucleic acids, lipids. Most sugars, however, are derivatives of aldehydes. These aldoses exist as hemiacetals, a sort of masked form of the parent aldehyde. For example, in aqueous solution only a tiny fraction of glucose exists as the aldehyde.
