Quantitative microbial spoilage risk assessment (QMSRA) of pasteurized strawberry purees by Aspergillus fischeri (teleomorph Neosartorya fischeri)

Abstract

Aspergillus fischeri ascospores are known as potential spoilage microorganisms of pasteurized fruit products due to their high incidence in fruits, the ability to survive pasteurization and to grow in acidic conditions. This study aimed to develop a quantitative microbial spoilage risk assessment (QMSRA) model approach to estimate the spoilage risk of packaged strawberry purees due to A. fischeri under various scenarios regarding product formulation, processing and storage conditions. The development of the risk assessment comprised three steps: (1) initial contamination level of raw material by ascospores (N0), (2) inactivation of ascospores during thermal processing (Np) and (3) determination of the number of ascospores which are able to survive thermal processing and develop visible mycelia (D = 2 mm) during storage (Nf). Data of visible growth (tv, days) comprised distributions previously obtained as function of water activity (aw) (0.860–0.985), oxygen (0–21%), temperature (8–30 °C) and pasteurization (95–105 °C/15 s). The simulations were performed in triplicate with 100,000 iterations using the software R. The outcome “spoilage risk” was defined as the probability of having at least one ascospore (Nf) capable of forming visible colonies in 100 g-pack strawberry puree within the typical use-by dates. Overall, high probabilities of spoilage were estimated for purees pasteurized at milder treatments at 85 °C/15–60 s (67%) and 90 °C/15–60 s (≥40%) stored at ambient temperature (22 °C). The spoilage risk was only effectively reduced (0.02%) by increasing pasteurization conditions to 95 °C for at least 45 s. Moreover, the microbial stability of such purees, i.e., spoilage risk <0.001% (=less than 1 spoilage pack out of 105 produced units) was predicted to occur for purees treated at 100 °C/15 s or stored at chilled conditions (≤8 °C) or at strict anaerobic conditions or produced as concentrates (aw ≤ 0.860). Based on the outcomes obtained, a set of specifications for Heat-Resistant Moulds (HRMs) in raw material and pasteurized purees aimed to be used as an ingredient was suggested. Furthermore, the results can be used to support risk management decisions in identifying and quantifying the impact of possible interventions during formulation, processing and storage conditions of fruit purees to effectively reduce this risk.