Again, the only good table vinegar is the result of the activity of a species of acid-producing bacteria, and even the lactic bacterium, which incurs the enmity of the unthinking by "souring" the milk, is a very good friend—in this particular case the flavor of the milk is spoiled for many people, but the lactic acid formed makes it an especially health-giving drink and prevents for a time other noxious bacteria from rendering it dangerous by decomposition. Indeed, milk that has been "preserved" from souring by checking the formation of lactic acid may prove distinctly dangerous for consumption even though the fresh flavor is retained.
These instances give some idea of the good services rendered under certain con ditions by many kinds of bacteria—and they are also indispensable to agriculture and other industries—but in the retailer's establishment and the household they are best regarded as enemies to be fought at every turn, for their uncontrolled access to fresh food is certain to result in loss and sometimes in danger to health. They are far more generally destructive than either wild yeast or molds. All real putrefac tion is due to the action of bacteria—the breaking down of the structure of the food as they feed on certain elements in it and other changes caused by their growth and multiplication—and, as already stated, they are present everywhere, being especially plentiful in and around human habitations. Thoroughly dry, salted, smoked and
(under certain conditions) spiced and pickled foods are safe from their depreda tions, but any fresh foods that contain from 25% to 30% moisture, except those that are very acrid or very heavily sugared, offer suitable soil for their growth and multiplication—if undisturbed, they rapidly take them through the various stages of putrefaction to the culminating point of decay.
Daylight, sunshine and cleanliness are opposed to bacteria, so stores and homes, and especially kitchens, should be blessed with all three as a preliminary safe guard. Next, fresh meats, canned goods (after opening) and similar foods should be eaten as fresh as possible. When immediate consumption is impossible, a good refrigerator offers a considerable measure of temporary protection, but it is only temporary, for the growth of some kinds of bacteria is checked by nothing short of freezing.
As already stated, boiling continued for an hour or so after the full heat has permeated every part of the food will kill all kinds of bacteria—will sterilize it—but this must be followed by immediate and hermetical sealing while still boiling hot, or new bacteria may get into it and start propagation afresh.