A Canadian team has sought to explain why sourdough bread has an extended mould-free storage-life compared to conventionally leavened products. It has long been known that the presence of metabolites from specific strains of lactobacilli contributes to the prolonged storage-life of sourdough bread.
Several compounds have been identified as antifungal metabolites of sourdough lactobacilli but these are either not produced in effective quantities in sourdough fermentations, or adversely affect the quality of the product when produced in active concentrations. Hence, some other explanation is needed, and the aim of this study published in Applied and Environmental Microbiology was to determine whether lactobacilli can convert linoleic acid into metabolites with antifungal activity, and whether this conversion delays fungal spoilage of bread. Various preparations of sour dough were prepared containing different amounts of lactobacilli (including L. hammesii) and challenged with three fungal organisms, and a range of chemical analyses. The basic conclusion was that L. hammesii converts linoleic acid to a mono-hydroxy octadecenoic acid with antifungal activity. This conversion was observed in sourdough fermentations supplemented with linoleic acid but generation of hydroxy fatty acids in sourdough also occurred through enzymatic or chemical oxidation.
Mono-hydroxy octadecenoic acid in combination with substrate derived coriolic acid (mono-hydroxy octadecadienoic acid) inhibited mould growth on sourdough bread. The authors also note that the use of coriolic acid and antifungal metabolites from linoleic acid as natural antifungals is not limited to food preservation. Antifungal metabolites from lactobacilli may complement or substitute these fungicides for use in seed treatment and crop protection.