284 ANISrUAL REPORT SMITHSONIAN INSTITUTION, 1951 



satisfactory bread. The rising of dough is due to the action of en- 

 zymes in the flour and added yeast ; these cause break-down of starch 

 into sugars which the yeast ferments to produce carbon dioxide, the 

 gas which makes the dough rise, as well as a little alcohol, some or- 

 ganic acids and materials with pleasing flavors. It is enzymes that 

 make apples and potatoes go brown after cutting, a change due to 

 the enzymatic oxidation in air of certain colorless compounds pres- 

 ent to form pigmented substances. Many industrial processes are 

 based on enzym.atic action; usually living organisms such as yeast, 

 molds or fungi, are used, as, for example, in the manufacture of 

 malt vinegar, penicillin, citric acid, and so on ; a discussion of these 

 is rather outside the scope of the present article. 



Two examples can, however, be mentioned of industrial processes 

 that use special enzyme preparations, as opposed to living organisms. 

 When fruit juices are being manufactured, pectins, which form a 

 part of the structure of plant cells, may separate out from solution 

 as gelatinous precipitates and make filtration difficult or spoil the 

 appearance of the product. (If pectins are present in sufficient 

 amount, a fairly rigid jelly may result, as in successful jam making.) 

 Clarification of fruit- juice drinks is now achieved usually by adding 

 pectinases, enzymes that degrade pectins to soluble products. The 

 enzyme preparations used are made from certain molds grown for 

 the purpose. At one stage in leather manufacture, it is necessary to 

 remove degraded products of hair, glands, and certain tissue proteins 

 from the hides. This used to be a secret process that involved soak- 

 ing the hides in a warm suspension of dog dung. The same result 

 is now achieved more pleasantly by using enzymes prepared from 

 large-scale cultures of suitable strains of bacteria. 



Human life begins with an enzymatic reaction. Spermatozoa lib- 

 erate hyaluronidase, an enzyme that attacks the envelope around the 

 egg and allows a single spermatozoon to enter and fertilize it. En- 

 zymes keep the flame of life burning until death, when enzymatic 

 decomposition returns the building material to the great store from 

 which fresh life can draw its raw materials to be assimilated and re- 

 built into new life forms. 



The living cell may be pictured as a remarkable factory, which not 

 only makes a vast range of different products simultaneously, but 

 also builds its own extensions, does its own repairs and makes its own 

 machine tools. Changes in the nature of the raw materials available 

 or sudden demands for material for new tissue building do not 

 change the smooth-running efficiency of this factory. An elaborate 

 control system must be operating that regulates the speed at which 

 the different machine tools are working and thus which departments 



