chemistry onions?

Question

what is the substance in onion that bring tears to the eyes

Answer 1

As onions are sliced, cells are broken, allowing enzymes called alliinases to break down sulfides and generate sulfenic acids (amino acid sulfoxides). Sulphenic acids are unstable and decompose into a volatile gas called syn-propanethial-S-oxide. The gas dissipates through the air and eventually reaches the eye, where it reacts with the water to form a dilute solution of sulfuric acid. This acid irritates the nerve endings in the eye, making them sting. Tear glands produce tears to dilute and flush out the irritant.

Supplying ample water to the reaction prevents the gas from reaching the eyes. Eye irritation can, therefore, be avoided by cutting onions under running water or submerged in a basin of water. Rinsing the onion and leaving it wet while chopping may also be effective. Chilling or freezing onions prevents the enzymes from activating, limiting the amount of gas generated. Using a sharp blade to chop onions will limit the cell damage and the release of enzymes that drive the irritation response.

The volume of sulphenic acids released, and the irritation effect, differs among Allium species.

Answer 2

Why onions make us cry

Onions come in many varieties and three main colors, yellow, white and red. They are rich in nutrients including vitamins B and C, protein, calcium and iron. Onions are low in sodium and are fat-free.

Onions add flavor to your meals and they have nutritional value. But why do these very useful plants make you cry when you slice them?

It is not the strong odor of the onion that makes us cry, but the substances that the onion releases when we cut this member of the lily family.

The onion contains some substances which contain sulfur. Sulfur is an irritant to both our noses and to our eyes. Cutting an onion releases these sulfur compounds. When these sulfur compounds come in contact with the water in our eyes, they produces sulfuric acid.

Sulfuric acid irritates the eyes. In response to this acid, our eyes automatically blink, and produce tears which wash the eye and flush out the acid.

An automatic reaction many people show is to rub their eyes with their hands, which often
makes the situation worse, because our hands are covered with the sulfur compounds from cutting the onion, which we then rub directly into our eyes.

The only remedy for this problem is to boil the onion, not to slice it or cut it up, which is not very practical. Some people suggest putting the onion in the fridge or the freezer for a few minutes because the cold decreases the speed of the chemical reaction. Another tip is to slice the area around the root of the onion last. Why? Because there are more sulfur compounds in the onion root.

The good news is that a new onion which doesn't make you cry when you are peeling it is now available.

The new kind of onion, called Supasweet onions, has been developed by British farmers. It is said to be so mild that it can be eaten like an apple. Supasweet onions have been on sale in Tesco supermarkets around England since August, 2003. The onion has a pale, thin skin, is easy to peel and quick to prepare.

Supasweet onions are not genetically modified. Their secret is that they have been specially grown in low-sulphur soils!


FROM : http://cyberend.com/science/821108_whyonionsmakeuscry.htm

Answer 3

The odour of onions comes from mercaptans (chiefly, methyl and ethyl mercaptan). These are sulfur-containing organic compounds. The tearing is also partially due to these chemicals, but there are other lachrimators present.

By the way, if you cut onions under running water, there is less crying. Also, cutting onions under lemon juice reduces the tearing.

Why is this question under "Travel/Thailand"?

Answer 4

Acidic onion juice.

Answer 5

chemistry question in a thai q&a? why and who cares?