MICROBIOLOGY OF FERMENTED FOODS 561 



some cases natural cultures are used. The pure cultures of selected strains give the 

 best results. These are grown at 50 to 52 in small thermostat regulated tanks. 

 At temperatures much below 50 butyric acid-forming organisms will develop; if 

 the temperature is much higher than this, the lactic organisms are weakened and 

 peptonization of the proteins is halted by destruction of proteases and coagulation 

 of the barley and rye proteins. Diastase works more rapidly at temperatures of 

 60 to 65 than at 50 but gives a much higher proportion of maltose to dextrins at 

 the lower temperature. 



Lactic acid activates the proteases and checks or prevents the growth of undesir- 

 able bacteria such as B. subtilis during yeast growth or storage of the finished yeast. 

 Success or failure depends as much upon the proper acidification of the mash as 

 upon any other single factor. 



Diastase rapidly converts the starch to maltose but peptonization of the pro- 

 teins is an extremely slow process. Where three to four hours would suffice for the 

 diastase eighteen to thirty should be allowed to permit the proteases to act. The 

 yield of yeast depends upon the degree of protein hydrolysis. This long period of 

 mashing can only be carried out by use of lactic acid cultures and accurate regulation 

 of the temperature to favor the growth of the lactic organisms and to check the 

 growth of other types. 



During mashing the plastic mass of crushed malt and water becomes thin and 

 watery because of conversion of starch to sugar. This change permits the liquid to 

 be drained from the grain husks and the husks to be washed with water. The 

 liquids so obtained are combined and filtered. The filtered liquid is of n to 14 

 degrees Balling and acid in flavor. In some factories the mash before straining or 

 the liquid after straining is heated to a temperature sufficient to destroy the lactic 

 organisms; in others this pasteurization is omitted. 



The liquid now known as "wort" is cooled to the pitching (yeast inoculation) 

 temperature of about 20 to 25. The cooled liquid is transferred to large open vats, 

 usually of 10,000 to 15,000 gallons capacity, either of wood or of enameled or otherwise 

 protected steel. These are equipped with large copper cooling coils through which 

 water may be circulated to prevent too great a rise in temperature of the wort from 

 the heat of fermentation. Too high temperatures favor alcoholic fermentation with 

 low yields of yeast. Compressed air is delivered to the liquid from perforated coils 

 placed in the bottom of the vat. Vigorous aeration is used to favor the production 

 of yeast and to limit alcohol formation. 



Pure cultures of selected strains of yeast are employed for "pitching" purposes. 

 This yeast starter is grown in sterilized wort in a small vat which can be kept free 

 from undesirable organisms. This pure culture is replaced frequently by new start- 

 ers propagated from pure cultures made by standard plating methods. This 

 practice is necessary to insure purity of the yeast turned out by the large vats. 

 Care must be taken to insure that the yeast used is true to type. 



Fermentation and yeast growth in the large vats proceed for about fourteen to 

 twenty-four hours. In some factories, the yeast is then allowed to settle. This 

 takes place rapidly and almost completely if an agglomerating (granule-forming) 

 type of yeast is used, very slowly if the yeast is of the fine-grained type. If settling 



