GROWTH 237 



demonstrated that other streptococci cease to grow in 

 milk for lack of available nitrogenous food. It has 

 further been found that Ps. fluorescens in broth ceases to 

 grow on account of accumulation of a thermolabile cell 

 secretion, and the same is true for streptococci in broth. 



(6) ENDPOINT AND CHEMICAL COMPOSITION OF FOOD 



The final crop or the endpoint of growth will depend 

 largely upon the kinds and amounts of construction 

 material if a reasonably good and continuous source of 

 energy is taken for granted. The type of construction 

 material seems quite essential. Most streptococci grow 

 to much larger numbers in milk if peptone, meat extract, 

 or yeast extract is added (see p. 149). 



Very extensive quantitative studies on the influence 

 of the kind of building stones upon the crop have been 

 made by the yeast industry. The yeast factory tries to 

 obtain the largest possible amount of cells from a given 

 amount of sugar. Since molasses is the cheapest source 

 of sugar available, the manufacture of yeast from 

 molasses is a common process. The building material 

 consists of different organic or inorganic nitrogenous 

 compounds, including the nitrogenous material in molas- 

 ses. While most of the results of these studies are 

 considered trade secrets, the following two tables show a 

 number of unpublished experiments which I owe to 

 the kindness of Dr. H. Claassen, of the ^'Rheini- 

 scher Aktienverein ftir Zuckerfabrikation/' Dormagen, 

 Germany. 



These experiments were carried out with 5 to 7 liters of wort under 

 plant conditions, being well aerated from perforated copper tubes 

 at the bottom of the vessel. The normal aeration was 15 to 20 

 liters of air per minute during the first hour, 50 to 60 liters during the 

 next seven hours, and 15 to 20 Uters during the last hour. The 



