Allyl propyl disulfide is an

organosulfur compound Organosulfur compounds are organic compounds that contain sulfur. They are often associated with foul odors, but many of the sweetest compounds known are organosulfur derivatives, e.g., saccharin. Nature abounds with organosulfur compounds—sulfu ...

with the

chemical formula In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, ...

C₃H₅S₂C₃H₇. It is a volatile pale-yellow liquid with a strong odor. It is a major component of onion oil and is used in food additives and flavors. This substance is present in garlic and onion. When onion or garlic is sliced, the substance evaporates and causes eyes to irritate.CDC - NIOSH Pocket Guide to Chemical Hazards
/ref> When garlic or onion is cooked, it also evaporates, ridding them of the spicy taste, and leaving a sweet taste in them .

References

Organic disulfides Allyl compounds {{organic-compound-stub