James Grant, an English chemist and teacher of the bakery classes in the Manchester Bakery School, in England, gave the following illustrated lecture before the Bakery Students’ Society, on the Bacterial Contamination of Bread, which is of considerable interest to American bakers:

It is well known that wheats and other cereals, owing to the deep crease or furrow down the center of the ventral side, and to the hairs (especially in the case of wheat), known as the beard, at the top of the berry, are liable to cause bacterial diseases in our food supplies. It may be objected that washing during the preparation for milling will get rid of dust and its accompanying bacteria. Unfortunately, this is not the case, as may readily be shown by washing wheats that are ready for milling and incubating the washing water. Fruits, equally with cereals, are liable to this contamination. Wines, for example, for hundreds of years have been fermented by the yeasts which adhere to the grape in the “bloom” on the outside of the fruit. Hansen, the great expert on yeasts, has proved that during the period of the year when there are no grapes, the yeasts and other micro-organisms that exist in the soil in the form of spores, which are able to endure periods of stress that kill the adult micro-organisms. Similarly in the case of barley. We have found in our work, time after time, that germs of all kinds exist on the wrinkled surface of the grain. Not many years ago we were able to isolate pure cultures of the bacillus which induces tetanus or lockjaw. During the milling process it can be seen that germs left on the surface of the berry must necessarily pass into the finished flour. Flour, then, is not germ free.

It is claimed by certain millers, who bleach their flours, that one of the chief objects is to render it sterile or nearly so. Research has shown that this claim is justified only to a very limited extent. In the year 1904, Dr. F. M. Blumenthal studied the subject very thoroughly. Two of his results, as examples, will be quoted. In an unbleached rye meal there existed no less than 2,400 micro-organisms per gramme of the meal. After bleaching there still remained 1,600 micro-organisms per gramme. With flour unbleached he found 540 organisms per grain, and with bleached flour 170. In both cases the best figures are only given. It is pretty evident, then, that milled products are not germ free; and further, those spoken of as meals, or in other words, those that contain the husk, are much more contaminated than those from which the husk has been separated, e. g., the ordinary flour.

The chief object of this paper is to give students an idea as to the best methods of undertaking a research or investigation into the cause of contamination. Since taking up the study of bread-making, between five and six years ago, a number of very interesting cases of bacterial diseases of bread have come under my observation, but the one that impressed me more deeply than others was that of a case of bread baked in special tins at a very low temperature, and known in the trade as sandwich bread. For this purpose the bread must be cooked at the lowest possible temperature, so as to form little or no crust. In this particular case of sandwich bread, after a few days keeping, a peculiar formation, resulting in a hole, was developed in the center of the loaf and running in the direction of the length. Accompanying this development was a very unpleasant odor. All around the low flat hole the crumb had a dull, sodden appearance. The question to be settled was: What was the cause of this unpleasant formation? To one acquainted with the life history of very many of the lower forms of life, especially of vegetable life, there was little difficulty in ascribing it to filth bacteria. From the general appearance of a section of a loaf the only conclusion that could be arrived at was that the trouble was due to bacterial action, together with the products formed. Starting from these premises it became necessary to inquire into the sources of such contamination. These might be due to either (1) Dirty and unclean premises and plant, or (2) to the water used, or (3) to the yeast, or (4) to the flours, or (5) to bread improvers used (if any). It could not possibly be the salt, because salt is so strong an antiseptic that there could be no risk from this source. Numbers (1), (2) and (5) were easily eliminated. This narrowed down the work to a study of the flours and yeast. The details of the research will show the means taken to determine, if possible, the actual causes of the trouble. The work was still further narrowed down by the fact that if bacteria were at work it could only be a group capable of withstanding comparatively high temperatures. Again, a large number of expensive media were unnecessary, as bread was a suitable food for our purpose. The requisite appliances were those of an ordinary well-filled bacteriological laboratory.

Ordinary microscopic slides of the diseased bread were made with sterile water, and these examined by microscope. This revealed the presence of moulds and mucor spores, yeasts—both the ordinary cultivated and wild—and numerous bacteria. On further examination after incubation at suitable temperatures, most of the above-mentioned proved to be just the common micro-organisms existing in flours and bread. Some of the bread was then incubated at 80 degrees Fahrenheit for four days. The piece of bread was then found to be covered with a whitish-colored growth, which later developed into a dark yeasty color and possessed a very peculiar and strong odor. Samples of the flour and yeast used in the manufacture of the bread were treated in a similar manner. In four days the flour specimens showed the same peculiar growth which, in two days, changed to the dark fawn color possessing the same characteristic odor. The yeast, on the other hand, behaved quite normally and developed none of the strange symptoms.

The next step was to try to infect some sterile bread with this peculiar disease, if possible. To this end sterile bread was introduced into Petri dishes, moistened with sterile water, and some of the dish contents sprinkled with flour, and others with crumbs of the diseased bread. The incubation temperatures were 68 and 80 degrees Fahr., respectively. At the lower temperature, as well as at the higher, the cultures were all successful, but it took several days longer in the specimens—at the lower temperature. Various other cultures were now put on, with other media and different apparatus, with a view to isolating the special cause of the disease. All specimens, and also micro-slides from these, had to be examined regularly at fixed periods, entailing, of course, an enormous amount of detail work which cannot here be set forth. Suffice it to say that ultimately by varying the media and mode of cultivation swarms of very minute oval-shaped non-mobile bacteria, and also many rod-shaped mobile organisms, were isolated. By this time all yeasts, moulds, mucors, and other complex growing organisms had been eliminated. To ensure that all the apparatus and media were sterile, blank specimens were put on so as to be parallel with the special culture in each case.

By means of the plate (Petri dish) cultures and Bottcher moist cells, a group of minute bacteria belonging to the Thermo or film species was obtained by which this particular disease could be produced at will. Moreover, prepared in this way, the bacteria which cause the disease were, and still are, very virulent. It only remained now to identify the particular species of the Thermo-group, but this was not an easy matter, as the members of the Thermo group are exceedingly minute. The plate cultures yielded colonies which rapidly increased in size, the disease spreading over the media in all directions. It was finally identified as belonging to the Proteus division of the Thermo or septic bacteria. These exist in most fertile soils, hence the research showed that the flour was produced from near the outer skin of the wheat berry, or, in other words, a low grade of flour. Further, it proves that the miller, with all his modern machinery, has not yet perfected that portion which does the cleansing or washing of the wheat. It should be remembered that the complete washing of the wheat, so as to free it from dust and micro-organisms, especially in the deep crease, and the fine hairs or beard at the top of the berry, is not at all a simple matter; but much more could be done, even if only a very dilute antiseptic was used in the final or next to the last washing water, instead of finishing with the muddy fluid as at present.