Bacteria also have another method of reproducing themselves which they resort to when conditions become unfavorable for their growth and they can no longer multiply themselves by the simple division method. Such an unfavorable condition would be the drying up of the moist surface on which they are growing. In this case the rod-shaped bacterium forms one or more spores within itself. These bacteria spores are somewhat similar to mold spores, but are much smaller. However, they are only produced by bacteria under unfavorable conditions, while the mold plant produces them under all conditions. The bacteria spores, like mold spores, have a tough surface which gives them high resistance, and they contain the germ of bacteria life which can reproduce the species when favorable conditions for growth are again encountered. The bacteria spores, therefore, float through the air as do the mold spores, remaining in the dry state without nourishment for months at a time, and as soon as they light upon a moist surface containing the elements for growth, even to a very small degree, they reproduce the rod-shaped bacteria form, which begins to multiply itself again by the simple division method.
Although most bacteria spores are killed by boiling for 10 or 15 minutes, there are a few species of extremely high resistance which have been known to remain alive after several hours’ boiling. Spores of this nature are infrequent, however, and the packer might as well forget that they exist. Not one case of spoilage out of a hundred is caused by the presence of such spores, and if all food products were cooked a length of time sufficient to kill all bacteria spores which might possibly be present, the foods would be cooked to death, and unfit to eat.
Yeasts
Yeasts are very tiny individual cells, and multiply by budding, instead of by simple division, as bacteria do. Instead of the yeast cell dividing in half, thus producing two cells, it remains intact, but a bud forms on its outer surface. This bud grows in size until it becomes almost as large as the cell which produced it, and then it separates from the mother cell, and we have two yeast cells. Each of these two cells then produces buds, which again separate, and so on. Like bacteria, the multiplication is very rapid under favorable conditions. Favorable conditions for yeasts and bacteria are the same as they are for mold, that is, a warm temperature, preferably near blood heat, and a moist substance which is not too strongly acid, and which contains the elements necessary for growth, preferably the natural sugar of fruits and vegetables.
Fig. 6.—Yeasts and spherical bacteria from decaying tomatoes. (× 500.) [The oval bodies are the yeasts, some in budding stage; the bacteria appear as small spheres, or pairs of spheres.]
Yeasts are the principal gas-forming agents in the fermentation of pulp. In the production of this gas they consume the natural sugar of the tomato. They will multiply by the budding method described above as long as they are in a medium which is favorable for their growth. As soon as this medium drys up, or for some other reason becomes an unfavorable medium for the multiplication of the yeast cells, the yeasts resort to spore forming, just as bacteria do. These spores are formed within the yeast cell, and when the cell wall breaks the spores pass off into the air. Like all other spores, they have a high resistance, and will float around in the dry state and remain alive for a very long time. When they light upon a medium in which they can grow they reproduce the yeast species, in the same manner in which the bacteria spore reproduces the bacteria species from which it was derived.
Spores
The spores, then, which are found in tomato products are of three different kinds, namely, mold spores, yeast spores, and bacteria spores. The greater number are almost always mold spores, since the mold plant produces spores under all conditions, while yeasts and bacteria only produce them under unfavorable conditions. When microscopical analyses of tomato products are reported, the mold spores and yeast spores are included with the yeasts in the term “Yeasts and Spores.” This is because a large number of the spores are very similar to yeast cells in appearance, and as it would be a laborious process for the analyst to separate them, the yeasts and spores are included in one figure, which greatly simplifies the analysis. The bacteria spores are not included in the count of “Yeasts and Spores.”