440 CONTAMINATION AND DETERIORATION OF FOOD 



Substances containing free and readily fermentable sugars in small percentage are 

 commonly subject to lactic fermentation. Many species of bacteria may participate 

 in this activity, but the long rod forms (Lactobacilli) constitute the dominant agents 

 in the souring of vegetable products. This tendency in the foods themselves is utilized 

 in making the whole series of foods collectively known as "pickles," which are mere- 

 ly foods soured by the activity of the Lactobacilli under controlled conditions. 

 Ensilage and sauerkraut are simply pickled corn and cabbage. This fermentation is 

 essentially anaerobic. The active species, while able to grow more or less in the pres- 

 ence of oxygen, find their optimum, quickly dominate and suppress other forms un- 

 der anaerobic conditions and at temperatures near 30° C. 



The organisms when supplied sufficient sugar carry the acidifying process to con- 

 centrations varying from about i per cent to, perhaps, 2 per cent or a little higher, 

 calculated as lactic acid. Ordinarily lactic acid forms the major part of the acid 

 present, but it is usually supplemented with acetic, butyric, or other aromatic acids in 

 amount varying with the substratum and the complex of bacteria present. Lactic acid 

 at this concentration reduces other bacterial activity to negligible amount, and hence 

 is an effective preserving agent. The pickle fermentation has been widely and suc- 

 cessfully adapted to food preservation, and is effective, when supplemented by the 

 exclusion of air through close covering of the mass, or by hermetic seal, or by a pro- 

 gressive addition of salt. Unsupported by such measures, pickled masses are invaded 

 by the mycoderma yeasts which progressively destroy the acid and release destructive 

 bacteria. 



Foods containing larger percentages of fermentable sugars usually develop an 

 alcoholic fermentation if oxygen is present in necessary concentrations. The pre- 

 servative effect of the alcoholic fermentation is less stable. Unless carefully controlled 

 and protected, the acetic fermentation commonly intervenes before the yeasts have 

 completed their action. The alcoholic fermentation passes directly, therefore, toward 

 the vinegar fermentation unless it is controlled with great care and the resulting licjuor 

 concentrated or protected from contamination. The alcoholic fermentation indus- 

 tries are thus very closely specialized in the handling of selected organisms and the 

 attainment of particular combinations of flavor and alcohol concentration. They con- 

 tribute comparatively little, however, to the maintenance of the food supplies of the 

 world. They are destructive rather than constructive. 



Control of bacteria in our food supplies is the basis of a number of large indus- 

 tries. Such control may be attained by drying, by the use of preserving agents, or by 

 cold. Their exclusion as active agents is consistently sought by canning. 



DRYING AND PRESERVING 



Drying and preserving are very ancient practices in the ])reservalion of foods. 

 They rest upon a single principle, i.e., that spoilage of a food may be prevented or 

 delayed by raising the concentration of the osmotic constituents in the water present 

 high enough to render the solution unavailable to micro-organisms and to stop, or de- 

 lay greatly, enzymic activities. This may be accomplished by evaporating the water 

 present to a figure varying with the })roduct; for example, in dried apples to about 25 

 per cent, in dried milk to 1.5 per cent, in corn meal to 12 percent. Part of the water 



