Which acid is predominantly found in garlic and onions ?

sulfenic acid

A sulfenic acid is an organosulfur compound and oxoacid with the general formula RSOH, where R ≠ H. It is the first member of the family of organosulfur oxoacids, which also include sulfinic acids and sulfonic acids, RSO2H and RSO3H, respectively.

Properties
In contrast to sulfinic and sulfonic acids, simple sulfenic acids, such as methanesulfenic acid, CH3SOH, are highly reactive and cannot be isolated in solution. In the gas phase the lifetime of methanesulfenic acid is about one minute. The gas phase structure of methanesulfenic acid was found by microwave spectroscopy (rotational spectroscopy) to be CH3–S–O–H. Sulfenic acids can be stabilized through steric effects, which prevent the sulfenic acid from condensing with itself to form thiosulfinates, RS(O)SR, such as allicin from garlic. Through the use of X-ray crystallography, the structure of such stabilized sulfenic acids were shown to be R–S–O–H.The stable, sterically hindered sulfenic acid, 1-triptycenesulfenic acid, has been found to have a pKa of 12.5 and an O–H bond-dissociation energy (bde) of 71.9 ± 0.3 kcal/mol, which can be compared to a pKa of ≥14 and O–H BDE of ∼88 kcal/mol for the (valence) isoelectronic hydroperoxides, ROOH.