Research in the 1990s: The Chemistry of Garlic
The volatile materials that can be distilled from plants were named essential oils by Paracelsus in the 16th century because they were thought to be the quintessence (literally, the "fifth essence," or vital principle) responsible for the odor and flavor of the plants from which they were isolated.
The Egyptians extracted essential oils from fragrant herbs more than 5000 years ago by pressing the herb or by extracting the fragrant material with olive or palm oil. Some essential oils are still obtained by pressing. Others are extracted into a nonpolar solvent, such as one of the fractions obtained when petroleum is distilled. The most common method for isolating essential oils, however, is steam distillation. The ground botanical is immersed in water which is heated to boiling, or boiling water is allowed to pass through a sample of the ground botanical. The oil and water vapor pass into a condenser, where the oil separates from the water vapor.
The function of the essential oils isn't fully understood. Some act as attractants for the insects involved in pollination. Most are either bacteriostats (which stop the growth of bacteria) or bactericides (which kill bacteria.). In some cases, they can be a source of metabolic energy. In other cases, they appear to be waste products of plant metabolism.
The essential oils are mixtures of up to 200 organic compounds, many of which are either terpenes (with 10 carbon atoms) or sesquiterpenes (with 15 carbon atoms). Although the three components shown in the figure below represent almost 60% of the mass of a sample of rose oil, 50 other components of this essential oil have been identified.
Garlic, onions, and mustard seed differ from most other sources of essential oils. In each case, the fragrance producing part of the plant must be crushed before the volatile components are released. For more than 100 years, chemists have known that the principal component of the oil that distills from garlic is diallyl disulfide [Semmler, F. W., Archiv der Pharmazie, 1892, 230, 434-448].
Only recently, however, have they explained how this compound is produced when a clove of garlic is crushed [Block, E., Angewandte Chemie, International Edition in English, 1992, 31, 1101-1264].
Before garlic is crushed, the intact cell contains S-2-propenyl-L-cysteine S-oxide or alliin which can be found in the cell cytoplasm.
Within the cell there are vacuoles that contain an enzyme known as alliinase. When the cell is crushed, the enzyme is released. The enzyme transforms the natural product alliin into an intermediate that reacts with itself to form a compound known as allicin.
Allicin has been described as an odoriferous, unstable antibacterial substance that polymerizes easily and must be stored at low temperatures. When heated, it breaks down to give a variety of compounds, including the diallyl disulfide obtained when oil of garlic is distilled from the raw material.
An alliinase enzyme can also be found in onions, where it converts an isomer of alliin known as S-(E)-1-propenyl-L-cysteine S-oxide into propanethial S-oxide.
The product of this reaction is known as the lachrymator factor of onion because it is the substance primarily responsible for the tears generated when onions are cut.
A great deal of progress has been made in recent years in identifying the various organosulfur compounds formed when garlic or onion are cut and in understanding the process by which these compounds are formed. The structures of some of the principal organosulfur compounds associated with garlic are shown below. In spite of the progress made so far, much still has to be learned about the compounds that can be isolated from the extracts of the genus Allium, which includes both garlic and onion.