Sulfur compounds are present in the plant in three forms: in the amino acids of proteins, i.e., cystine, methionine, and others; volatile compounds; and sulfates.

It is known that a portion of the protein sulfur is readily split off at boiling or higher temperatures. Peterson found volatile sulfur in clover, beet tops, blue grass, and milk. This was unexpected, and he suggests that it may come from two sources, volatile sulfur compounds in the materials used, or from the splitting off of sulfur from the protein and the formation of hydrogen sulfide. He thinks the latter the more probable explanation. He found a larger percentage of volatile sulfur compounds in plants grown in a sulfur-rich soil than in those grown in a sulfur-poor soil.

Volatile sulfur compounds. The volatile sulfur compounds are found in the plants as glucosides. When treated with acid or alkali and when acted upon by enzymes, these glucosides yield a sugar or some closely allied carbohydrate and one or more other substances, frequently phenols, allyl sulfide, or allyl isothiocyanate. Most of the investigations of sulfur-splitting enzymes have been made upon myrosin. The strong flavor of old or stored turnips, rutabagas, and cabbage is probably largely due to setting free of sulfur compounds by enzymes. When they are cooked, plant acids are liberated which cause hydrolysis of the volatile sulfur compounds. In addition, hydrogen sulfide is formed by decomposition of the sulfur compounds by heat.

The odor of many of these compounds is familiar, those of onions and garlic, cooking cabbage, etc., being typical examples.

Allyl isothiocyanate is found in mustard seed. It is prepared commercially by macerating the seed, the enzyme then splitting off the sulfur of the glucoside. The oil is then obtained by distillation with steam. Allyl sulfide is found in the onion family, and is prepared commercially from garlic. The strong flavor of onions, leeks, and garlic depends upon the concentration of allyl sulfide.

These volatile compounds escape from the food more rapidly with rise. in temperature and breakdown of the plant cells. Hence, the longer onions, leeks, and garlic are cooked the milder their flavor becomes. Also because finely minced onion has a greater surface area for volatilization to occur than when cut in large pieces, these substances volatilize more rapidly from finely cut pieces. This is why it is preferable to add onion juice, very finely minced onion, or cooked onion to some foods that are not cooked long or that do not reach a high temperature, such as hamburger or stuffing for fowl or meat. Large pieces of raw onion in a stuffing produce a very strong onion flavor, more concentrated in some areas than others, instead of a subtle, well-blended flavor, for the stuffing never reaches boiling temperature.

The amount of onion and/or garlic added, the fineness of division of the pieces, and the length of cooking all affect the flavor of the finished product to which they are added. The flavor of a food like catsup or chili sauce may be ruined by adding a large quantity of onion or garlic in large pieces near the end of the cooking process. The same amount of onion or garlic added in small pieces, or added in large pieces early in the cooking process, could blend with, instead of dominate, the flavor of the other ingredients.

Sulfur compounds and the cooking of vegetables. It is commonly known that long cooking of some vegetables such as cabbage, turnips, cauliflower, and Brussels sprouts develops strong, disagreeable flavors, and the eating of these vegetables may cause discomfort and digestive disturbances. The longer cooking and stronger flavor are accompanied by increased acidity of the vegetable. The length of time of cooking for the strong flavor to develop varies with the vegetable, whether the cooking is started in cold or boiling water, and the proportion of vegetable to water.

Simpson and Halliday have determined the total volatile sulfur and hydrogen sulfide evolved when cabbage and cauliflower are cooked for different lengths of time. They found that the most acceptable product is obtained when winter cabbage is boiled 7 to 8 minutes, and spring cabbage 5 minutes. Cauliflower required 8 minutes to become tender. They conclude that with prolonged cooking the decomposition products of sulfur compounds increase, and that these products produce the strong taste and odor associated with cabbage and cauliflower.

Simpson and Halliday report that the amount of hydrogen sulfide increases from the fifth to the twentieth minute of boiling cabbage, and the total volatile sulfur between the seventh and thirtieth minutes. Cauliflower gives off more volatile substances in the same period than cabbage. Masters and Garbutt have found that the amount of sulfide increases up to a certain point and then decreases, becoming fairly constant. Kohman states that in corn the amount of hydrogen sulfide evolved is greater during the first half hour and decreases with each succeeding half hour.

Shilling in determining the amount of hydrogen sulfide formed in cooking cabbage finds that it varies from 1 to 7 milligrams per 300 grams of fresh cabbage. The wide variation depends upon the amount of green compared with the white, the speed of cooking, the length of time of cooking, and the temperature involved in the method, i.e., boiling in water and the pressure cooker.

Bigelow has reported that the black spotting on the corn around the edges of the can in canned corn is due to the formation of hydrogen sulfide in processing. The amount of sulfide is very small and there is no objection to it except from appearance. Stevenson states that iron sulfide is formed to a greater extent in canned peas than in canned corn. It is not so notice-able in peas on account of their color. In peas it is deposited as scales that break up easily in shipping and handling.