560 MICROBIOLOGY OF FOODS 



The manufacture of compressed yeast begins with the preparation of the malt. 

 Two cereals, barley and rye, are commonly used in most factories some use barley 

 only. The barley is first "steeped" in cold water as for brewing (page 623). It 

 is then placed in large slowly revolving drums maintained by a current of moist cool 

 air at 10 to 25. The air is cooled by being drawn through water sprays and wet 

 coke. The usual temperature is about 15 to 18. The barley is allowed to 

 germinate until the acrospire (embryo blade) has reached the length of the kernel. 

 Rootlets several times the length of the kernel form and cause the sprouted barley 

 to mat together. 



The object of malting is to cause the barley through the process of germination 

 to form large amounts of diastase and of proteases. The formation of the latter 

 enzymes is as important as that of diastase because the yield and vigor of the yeast 

 depend upon the amount of assimilable nitrogen compounds present in the nutrient 

 liquid. These are derived largely from the complex insoluble proteins of the grain 

 by the hydrolyzing action of protein-splitting enzymes. Too short a period of too 

 high a temperature of malting results in low yields of the enzymes; too long a 

 period results in loss of enzymes formed during germination and loss of starch. 

 Under normal conditions of temperature and humidity about five to seven days' 

 malting is used for barley. A typical record of temperature and rate of rotation of 

 a drum during malting is given by Wahl-Henius as follows: 



Day Temperature Period of Rotation 



ist 13 Once in 2 hours. 



2d 1 6 Once in 2 hours. 



3d 1 8 Once in 2 hours. 



4th 21 Once in 1^2 hours. 



5th 24 Once in 40 minutes. 



Rye is said to be used to furnish protein and mineral salts which increase the yield 

 and vigor of the yeast. It is malted on cement floors in layers about eight inches 

 thick. Much less of the rye is used than of the barley. The sprouted barley and 

 rye grains are crushed between rough steel rolls which revolve at different speeds, 

 the difference in speed resulting in a grinding action upon the grains. The malt is 

 not dried before grinding; in this respect the practice is different from that fol- 

 lowed in brewing. Drying results in loss of enzyme-content and increases the cost 

 of operation without in any way improving the quality of the yeast. 



The crushed malt is mixed with water in a large wooden tank known as the 

 'Mash tun" and which is equipped with revolving stirring arms and steam coils. 

 The coils are usually controlled by a thermostat in order that an exact temperature 

 may be maintained in the mash tun during acidification of the mash. 



In the mash tun the starch is converted into maltose; a high concentration of 

 lactic acid is formed and the insoluble proteins of the grains are converted to a con- 

 siderable degree into peptones and to a certain extent into amino acids. The forma- 

 tion of acid and hydrolysis of the proteins are obtained largely by maintaining a 

 temperature of 50 to 52. A vigorous culture of a lactic acid forming organism is 

 added to the mash as a starter. B. delbruckii is often used for the purpose in 



