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-IOIO and this was only just attained a t 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 a t the thermocouple positions.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.Corresponding loaves were incubated a t 37".In btlffer solutions vegetative cells were killed in about z min. a t 75". ( I ) IntroductionThat certain bacteria can survive baking in bread is plain, for the well-known defect, ' rope ', is caused by their development from spores usually present in the original flour1 or in the yeast.= Most workers agree that some spores can resist baking. Thus, Aubertin et al.3 confirmed that sporing bacteGa (Bacillus mesentericus, Clostridium -+erfringens) survive, and although Edmonson, Thorn & Giltner4 failed to recover Clostridium botulinzlm that had been included in doughs and baked at 220" for 35 min., more recent workers have succeeded. For example, Ingram & Robinson5 record the isolation of C1. botulinum type A from experimental canned bread ; Soloski & Cryns,6 Kadavy & Dack7 and Bever & Halvorsons also showed that canned bread from doughs inoculated with Cl. botulinum sometimes developed toxin on storage, unless the bread had a pH below 5.0 and a moisture content below 3474, and viable spores but no toxin were found in bread even with these properties.I t seemed to us from a review of the literature that further study of the temperature distribution in a loaf was needed, as apparently there must be occasions when, in parts a t least, lethal temperatures are not attained.Thus we have found : (i) that when cloth contaminated with Staphylococcus aureus and Salmonella paratyphi was inserted in doughs, although the salmonellas were all killed the staphylococci sometimes were not, and other non-sporing bacteria survived too, though they were not identified ; (ii) that living baking-yeast can occasionally be recovered from the crumb, suggesting (cf. Lundg) that the localized temperature cannot have exceeded about 65" ; and...

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