236 GROWTH OF PLANTS 



rather than the peroxidase. Recent work ^^ indicates that thiourea also 

 inactivates the oxidase system in fruits that destroys vitamin C. 



Fig. 89 shows the effectiveness of thiourea in preventing the browning of 

 shced apples and apple juice. If the thiourea is placed in the juice immedi- 

 ately upon pressing, no browning occurs; also the early browning of the juice 

 is completely or almost completely reversed if the thiourea is added within 

 an hour after pressing, but the later oxidation in the juice is irreversible. 

 The amount of thiourea required to prevent broAvning is very small. Sliced 

 fruit is dipped momentarily in a 0.1 per cent solution and then drained, 

 after which each pound of fruit retains about 0.027 gram of thiourea. If 

 used to treat the juices, the thiourea content is less than 0.05 gram per 

 pound. The same treatment is quite as effective in preventing sliced apri- 

 cots, bananas, nectarines, peaches, pears, and plums from browning. In 

 peaches a 0.05 per cent solution is effective. It has been suggested that 

 thiourea dip be substituted for sulphur dioxide treatment for drying, 

 canning, and quick-freezing of fruits. Thiourea is also excellent for treating 

 sliced fruits for salads and desserts. 



Thiourea ^'' has lower acute toxicity dosage for mammals than table salt, 

 and life- time feeding of mammals *^ with dosages many times that which 

 can be consumed in treated fruit has no effect on either the weight or life 

 span of the animals. For several years * it has been know^n that plants of 

 the cabbage family cause thyroid enlargement, i.e., are goitrogenic. It has 

 also been found ^^ that allyl thiourea, or some similar compound released 

 from mustard oils by glucoside-splitting enzymes, causes this goitrogenic 

 action and that various organic sulphur compounds (thiourea, thiouracil, 

 etc.) in high doses have goitrogenic action. Astwood ^ used thiourea and 

 thiouracil as therapeutic agents against hyperthyroidism. The initial doses 

 of 1 to 2 grams of thiourea a day were used until the basal metabolism 

 was reduced to the proper level and then 0.5 of a gram a day to maintain 

 it. The effect of the thiourea ceases soon after its use is discontinued. 

 When 1 to 2 grams per day were fed to normal individuals for 13 to 17 

 days, no change in metabolism was noted. Very recently ^^ it has been 

 observed that rats fed a diet containing 0.5 per cent thiourea for 12 or more 

 days are able to endure an atmospheric pressure of 200 mm of Hg, equiva- 

 lent to an altitude of 32,000 feet, for two hours, whereas the majority of 

 those not fed thiourea were killed by a like exposure to low pressure. 

 Leblond ^ found that rats given 1 per cent of thiourea in their drinking 

 w^ater for three months endured an atmospheric pressure of 100 mm of Hg 

 much better than did the controls. These are extremely high doses. It has 

 been suggested that aviators might be able to endure rarefied air better if 

 given thiourea in their diet. The therapeutic and goitrogenic doses are 

 enormous compared with the intake obtainable from eating treated fruits. 

 Even if all the largest annual apple and peach crops the United States has 

 ever produced were treated with thiourea to prevent browning and were 

 all eaten by the people of the United States, each person would get on the 



