a soft dough being left in a warm place to ferment naturally, the yeast cells probably being present in the flour. The yeast that spoils the canned fruit is present m the fruit, m the utensils, or can, and has not been killed as it should be in the canning process. The bacteria are also one-celled microorganisms, smaller than the yeast. Figure 31 shows the four types of bacterial cells. Their size is measured by the unit used in the A Buchanan's Household Bacteriology. FiQ. 31. — The four types of bacterial cells. A, cocci. B, bacilli. C, spirilla. 2>, branched filamentous organism. microscope, called the micron, which is about ^rwmr of one inch. Bacteria may measure from one to three or four of these microns in length. Some bacteria are reproduced by means of spores which form within the cell. Bacteria, as they develop in some material, produce substances from the material that may or may not be injurious to us. One im- portant truth about the bacteria is this : that many of them are harmless, and may even be made useful, as in the manufacture of fruit vinegar. The pleasant acid of butter- milk and of sour milk is due also to bacteria which are not harmful to us. However, there may be disease pro- ducmg bacteria present in milk that is not clean, and their presence must not be tolerated. Other bacteria, de- I 100 FOODS ASD HOUSEHOLD MANAGEMENT veloping in meat and fish, produce substances known as 'pUymaines, which are dangerous poisons ; or, more often, the kinds of bacteria which thrive in meats and fish may them- selves be directly injurious to man. It is evident, therefore, that the problem before us is the control of these lower organisms, that we may increase or destroy them as we wiU. The control of niicrodrganisms. — With warmth, water, and food all living things flourish and grow ; most organisnis require air, but some of the microorganisms do not. Where these conditions are best met, the organism is most active and multiplies most rapidly. To retard growth or to destroy life, the conditions must be the reverse of favorable. While warmth, say a temperature froin 70** to 90° F., promotes the life of most microorganisms, intense heat destroys it. The boiling temperature, 212° F., will kill these lower organisms, although this heat has to be continued for some length of time, particularly in the case of spores. The spores of certain bacteria are quite resistant. A temperature of 32° F. and lower retards growth, but it requires extreme cold to destroy bacteria. Since moisture is necessary to all the lower organisms, they do not develop in a dry material or dry place. We cannot destroy these lower forms of life by removing food from them, since they are ever present, but we can make the food unavailable to them through the introduction in the material of certain substances called preservatives which prevent their growth. The preservatives long familiar^ are salt, sugar, wood-smoke, spices, vinegar, and alcohol. While a small amount of sugar is necessary in the fermentar tion process, a large amount acts as a preservative, as in candied fruit. It is an interesting fact that alcohol and vinegar, products of fermentation processes, tend (when FRUIT AND ITS PRESERVATION 101 sufficiently concentrated) to stop the growth of the fermen- tation organisms.

AI-modernized reading of the original text

a soft dough being left in a warm place to ferment naturally, the yeast cells probably being present in the flour. The yeast that spoils the canned fruit is present m the fruit, m the utensils, or can, and has not been killed as it should be in the canning process. The bacteria are also one-celled microorganisms, smaller than the yeast. Figure 31 shows the four types of bacterial cells. Their size is measured by the unit used in the A Buchanan's Household Bacteriology. Fig. 31. — The four types of bacterial cells. A, cocci. B, bacilli. C, spirilla. 2>, branched filamentous organism. microscope, called the micron, which is about 1/25000 of one inch. Bacteria may measure from one to three or four of these microns in length. Some bacteria are reproduced by means of spores which form within the cell. Bacteria, as they develop in some material, produce substances from the material that may or may not be injurious to us. One important truth about the bacteria is this : that many of them are harmless, and may even be made useful, as in the manufacture of fruit vinegar. The pleasant acid of buttermilk and of sour milk is due also to bacteria which are not harmful to us. However, there may be disease producmg bacteria present in milk that is not clean, and their presence must not be tolerated. Other bacteria, de-developing in meat and fish, produce substances known as ptomaines, which are dangerous poisons ; or, more often, the kinds of bacteria which thrive in meats and fish may themselves be directly injurious to man. It is evident, therefore, that the problem before us is the control of these lower organisms, that we may increase or destroy them as we wiU. The control of niicrodrganisms. — With warmth, water, and food all living things flourish and grow ; most organisnis require air, but some of the microorganisms do not. Where these conditions are best met, the organism is most active and multiplies most rapidly. To retard growth or to destroy life, the conditions must be the reverse of favorable. While warmth, say a temperature froin 70** to 90° F., promotes the life of most microorganisms, intense heat destroys it. The boiling temperature, 212° F., will kill these lower organisms, although this heat has to be continued for some length of time, particularly in the case of spores. The spores of certain bacteria are quite resistant. A temperature of 32° F. and lower retards growth, but it requires extreme cold to destroy bacteria. Since moisture is necessary to all the lower organisms, they do not develop in a dry material or dry place. We cannot destroy these lower forms of life by removing food from them, since they are ever present, but we can make the food unavailable to them through the introduction in the material of certain substances called preservatives which prevent their growth. The preservatives long familiar are salt, sugar, wood-smoke, spices, vinegar, and alcohol. While a small amount of sugar is necessary in the fermentar tion process, a large amount acts as a preservative, as in candied fruit. It is an interesting fact that alcohol and vinegar, products of fermentation processes, tend (when sufficiently concentrated) to stop the growth of the fermentation organisms.

Foods and household management