Modelling of the antifungal activity of propionic and sorbic acid against spoilage moulds and validation in bread and cake

Propionic and sorbic acids are weak acids widely used as antifungals in baked products. Their use increases shelf life and reduces food waste and mycotoxins production. This study, conducted at Ghent University (Belgium), investigates the relationship between weak acids, pH and six molds in bakery products. More specifically, the susceptibility of Aspergillus niger, Cladosporium ramotenellum, Aspergillus montevidensis, Aspergillus ruber, Penicillium brevicompactum, and Penicillium roqueforti to the antifungal action of propionic acid and sorbic acid was studied, both in-vitro, on MEA (Malt Extract Agar) plates with established conditions of water activity (0.93, 0.95, and 0.97), pH (5.0, 5.5, 6.0), and preservative concentration (0, 500, 1000, 1500, and 2000 mg acid/L MEA), and by validation carried out on bread (0, 497, 990, 1967, and 2901 mg propionic acid/kg), cake pH 7.2 (0, 493, 1004, 1488, and 1988 mg sorbic acid/kg), and lemon cake pH 5.2 (0, 475, 951, 1429, and 1897 mg sorbic acid/kg). The undissociated component (CHA) was estimated through three different Henderson-Hasselbalch equations: the standard one, model 1 (proposed in 2000 by Wilson et al.), and model 2 (Wemmenhove et al., 2018). The chemical and physical characteristics of the matrix (W/O, pH, aw, moisture) were studied to evaluate their relationship with the anti-mold effect of acids. In-vitro growth/non-growth patterns were correlated with shelf-life studies on bread and cakes. On bread, due to the strong propionic acid resistance of Penicillium brevicompactum and Penicillium roqueforti, the minimum inhibitory concentration (MIC) was above 2901 mg propionic acid/kg. In the cake at pH 7.2, the antifungal action of sorbic acid was unsatisfactory, and the maximum amount of sorbic acid (1988 mg sorbic acid/kg) was not enough to achieve 30 days of shelf-life. In lemon cake at pH 5.2, the minimum inhibitory concentration (MIC) was 1429 mg sorbic acid/kg; the shelf life reached thirty days. The standard Henderson-Hasselbalch equation was adequate to calculate CHA in low-fat matrices (MEA and bread), and model 2 effectively calculated CHA in high-fat matrices (cakes).