190 PLANT BIOCHEMISTRY 



was impossible. Recently the geneticists have also succeeded in produc- 

 ing a high-yielding corn with a very high amylose content. Amylose 

 can be used to prepare transparent fdms similar to cellophane but 

 with one important difference: amylose films are digestible. Such films 

 will provide an edible coating for meats and various frozen foods. 



Crops that are valuable sources of commercial starch include the 

 cereals (corn, rice, and wheat) and the tubers (Irish and sweet pota- 

 toes). Of these corn is the most important because it is produced in 

 greater quantities than any other American cereal. Although ap- 

 proximately 3 billion bushels of corn are produced annually in the 

 United States, only about 3 per cent of this amount finds its way into 

 non-food industrial channels. The remaining 97 per cent serves as 

 food for human beings and domestic livestock. Much of the corn 

 utilized industrially is used for starch manufacture, although appreci- 

 able quantities are purchased by the fermentation industries. 



Sucrose. Sucrose (page 64) is not only a major photosynthetic 

 product and sugar of translocation; it is also one of the principal forms 

 of carbohydrate storage in a number of plants, particularly certain 

 members of the cane family, and in some root crops, for example, sugar 

 beet. Sucrose is found in small quantities in all metabolically active 

 plant tissue. Fruits and juices of many plants contain appreciable 

 quantities of sucrose, as does the nectar of flowers. 



The principal uses for sucrose are as foods and candy products. 

 Large quantities are also consumed by the fermentation industry for 

 the production of alcohol, citric acid, lactic acid, and levulinic acid. 



Other reserve carbohydrates. Although starch and sucrose are the 

 storage forms of reserve carbohydrates in the major economically valu- 

 able plants, a number of other carbohydrates are found in a variety of 

 plants. Members of the Compositae and Graminaceae families ac- 

 cumulate D-fructofuranose polymers in stems, rhizomes or tubers. The 

 fructosans (page 69) are of low molecular weight and readily water 

 soluble. 



Legume entlosperms accumulate a branched-chain polymer com- 

 posed of D-galactosc and D-mannose as food reserves. The polymer 

 consists of linear chains of mannosc linked P-\ -^ 4, with o-galac- 

 topyranose units linked a-] -> 6. Guar is grown as a commercial source 

 of this polymer. The galactomannans are excellent solution thicken- 

 ers. 



Lipides 



The term lipides includes a variety of organic compounds found in 

 the plant (Chapter 4). In many cases the term is synonymous with 



