Growth/no-growth models of propionic and sorbic acid for bread and cake moulds

Abstract

Propionic and sorbic acid are very effective weak-organic acid preservatives in bakery products. Their pH-dependent antifungal activity, the role of undissociated acid and the importance of the aqueous phase was studied. Besides looking for new natural preservation techniques the gain of further knowledge on their activity is still highly relevant to tackle the problem of food waste. In this study, the susceptibility of spoilage moulds for propionic (PA) and sorbic acid (SA) was tested in-vitro and in bread and cake. Aspergillus niger, Aspergillus montevidensis, Aspergillus ruber, Cladosporium ramotenellum, Penicillium brevicompactum and Penicillium roqueforti were subjected to PA and SA, both through macrodilution on malt extract agar (MEA) plates with adjusted aw (0.93, 0.95, 0.97), pH (5, 5.5, 6.0), acid (max. 2000 mg/L MEA), and through validation in wheat bread (max. 2901 mg PA/kg) and (lemon) cake (max. 1988 mg SA/kg). The undissociated acid concentrations (CHA) were estimated using either the Henderson-Hasselbalch (HH) equation or two modified HH equations. The influence of the food matrix (liquid oil, water, pH, aw), partitioning coefficient of the acids and spoilage fungi were evaluated and results were compared with in-vitro predictive models. In bread, the regular HH eq. proved to have good predictive power of CHA, whereas in cake a modified HH eq. (model 2) had better performance due to the inclusion of potential loss of CHA in the oil phase of complex food systems. The minimal inhibitory concentration (MIC) of PA in bread, after 30 days, was higher than 2901 mg/kg bread due to the strong resistance of P. brevicompactum, P. roqueforti and A. montevidensis. MIC of SA in lemon cake was 1429 mg SA/kg (pH 5.2), whereas in standard cake with pH 7.2, the max. level of 1988 mg SA/kg was insufficient for postponing mould growth to 30 days. The pH of cake had no influence on MIC of SA against A. niger and A. rubrum, whereas this effect was seen for C. ramotenellum, P. brevicompactum and P. roqueforti.