Not only flesh and other animal substances, but vegetables can undergo putrefaction and fermentation (butyric) which make it dangerous to eat them. Many accidents have occurred in man as the result of deteriorated preserved fruit. Vegetables, preserved in silos to feed cattle, sometimes go wrong. “If, for instance, rainy days come after sunny days, so that the uncovered fodder is wetted again, the resulting ensilage is poor and has an extremely unpleasant butyric odour, so that the animals turn from it.” Sometimes the fodder grows black in the silo, and acquires a special smell. “The animals will take it only in the absence of other food; their excreta become black, and if they are kept on such a diet for a time they waste in a marked manner.”[131] In popular practice, the value of acids for preserving animal and vegetable food and for preventing putrefaction has long been recognised. Meats of all kinds, fish and vegetables have been “marinated” with vinegar, as the acetic acid in that substance, the product of bacteria, wards off putrefaction. If the materials which it is desired to preserve give off acids themselves, the addition of vinegar may be unnecessary. For this reason some animal products such as milk, or vegetables rich in sugar become acid spontaneously and so can be preserved. Soured milk can be made into many kinds of cheese, and these last for longer or shorter times. Many vegetables can undergo a natural process of souring, when they “keep” without difficulty. Thus cabbage becomes “sauerkraut” and beetroot and cucumbers pass into an acid state. In many countries, as for instance in Russia, the use of acidified vegetables is of great importance in the food-supply of the populace. Fresh fruit and vegetables cannot be obtained in the long winters, during which the people consume large quantities of cucumbers, melons, apples, and other fruits which have undergone an acid fermentation in which lactic acid is the chief product. During summer, milk, which acidifies readily, is the chief source of acid materials for consumption. The chief beverage is “kwass,” of which black bread is the main ingredient, and this passes through not only an alcoholic fermentation, but an acidifying change in which lactic acid is the most important product. Rye bread, the chief food of the populace, is also a product of fermentations amongst which the lactic acid fermentation is most important, but in other kinds of bread also there is a fermentation in which some of the sugar is transformed to lactic acid.