The survival of Bacillus subtilis spores in the baking of bread

Abstract

The survival of Bacillus subtilis spores at temperatures near 100° was determined in buffered suspensions. Known numbers (about 10⁵/g.) of these spores were mixed into dough which was baked normally into bread. Temperatures taken near the centre of the loaf, near the crust, and midway between the two showed that the temperature within the loaf was 100‐101° and this was only just attained at the centre. These temperatures were comparable with similar measurements in various baked products. Viable counts were made on the dough just before entering the oven and on baked samples taken at the thermocouple positions. Corresponding loaves were incubated at 37°.In buffer solutions vegetative cells were killed in about 2 min. at 75°. The thermal death/time curves for spores were not exponential, a small proportion of the spores surviving for relatively long periods. In dough, about half the spores germinated between mixing the dough and putting it into the oven. Comparison of the survival of spores in the bread with that expected from the temperature treatment and from the experiments in buffer solutions suggested that the heat resistance in bread is roughly the same as, certainly no less than, that in phosphate buffer of pH 6.5. Only a small proportion of the surviving spores generated ropy patches within a week, owing perhaps to peculiarities in water distribution within the loaf.