Growth of Fungi in Low Oxygen and Intermediate Water Activity Model Systems

Introduction: Hot-filled products are expected to have long shelf lives, however there are heat resistant, osmophilic fungi that can grow under extremely low oxygen conditions. Very few studies have been published regarding vacuum strength in low water activity (a_w), hot-filled products and the resulting oxygen concentration in the headspace nor the ability of known heat resistant spoilage fungi to grow at these levels.

Purpose: The objective of this study was to evaluate the effect of a_w, vacuum strength and oxygen concentration of hot-filled bottles and the survival of spoilage fungi Byssochlamys, Talaromyces, Cladosporium, Penicillium and Aspergillus under low oxygen conditions.

Methods: Glass bottles were hot-filled at 180, 190 and 200°F with sucrose, sodium chloride and water mixtures resulting in 0.70, 0.80, 0.90 and 1.00 a_w. Headspace oxygen and vacuum strength were measured using an AquaLab water activity meter and PreSens Fiber Optic oxygen meter, after equilibrium.

Spoilage fungi, isolated from foods, were inoculated into 70°Brix apple juice concentrate and Malt Extract Broth adjusted to 0.85, 0.90 and 0.93 a_w. Plates were incubated in anaerobic chambers adjusted to 400 ppb (1.0%) and 200 ppb (0.5%) oxygen for 60 days at 86°F.

Results: A linear trend in the hot-fill study was observed. Vacuum strength decreased and headspace oxygen increased as a_w decreased. At 0.70 a_w, the average vacuum was 8.3±0.3, 10.0±0.5, and 10.5±0.0 inHg and headspace oxygen concentration was 6.04±0.2, 5.5±0.1, 5.35±0.17 ppm at 180, 190 and 200°F respectively. At 1.00 a_w the average vacuum was 11.33±0.6, 12.7±0.3, and 13.7±0.6 inHg and headspace oxygen concentration was 5.42±0.2, 4.26±0.19, 3.82±0.37 ppm at 180, 190 and 200°F respectively. Spoilage fungi were capable of growth at reduced oxygen levels in low a_wmodels.

Significance: Our results will provide guidelines to control growth of heat resistant and osmophilic spoilage fungi in a_w controlled hot-filled foods.