Effect of storage temperature, water activity, oxygen headspace concentration and pasteurization intensity on the time to growth of Aspergillus fischerianus (teleomorph Neosartorya fischeri)

Characteristics of fungal communities and the sources of mold contamination in mildewed tobacco leaves stored under different climatic conditions

Tobacco mildew is a common postharvest problem caused by fungal growth. It can directly decrease product quality and cause serious economic loss in the tobacco industry. However, the fungal community characteristics of mildewed tobacco leaves and the related influencing factors remain unknown. Here, next-generation sequencing was used to characterize the fungal communities present in mildewed and healthy tobacco leaves stored under three different climatic conditions. Mildewed leaves showed a higher pH and total nitrogen content as well as a lower carbon nitrogen ratio than healthy leaves. Fungal diversity and richness were significantly lower in the mildewed tobacco leaves than in healthy tobacco leaves, with saprophytic fungi such as Xeromyces, Aspergillus, and Wallemia being the dominant molds. Network analysis showed that the complexity, connectivity, and stability of the fungal network were significantly poorer in heavy mildew tobacco leaves than in healthy leaves. NMDS and PERMANOVA analysis showed that the distribution of fungal communities in warehoused tobacco leaves differed significantly across different regions, and temperature and humidity were the key factors affecting these differences. Mildew-causing fungi were significantly enriched in tobacco leaf samples collected in the period between the completion of flue-curing and the start of pre-re-curing. This study demonstrated that mildew is an irreversible process that destroys the balance of the tobacco ecosystem, and that environmental factors play important roles in shaping fungal communities in tobacco leaves.Key points• The diversity and composition of the fungal communities in mildewed tobacco leaves were significantly different from those in healthy tobacco leaves.• Climatic factors may play an important role in shaping fungal communities in tobacco leaves.• Tobacco leaves were most vulnerable to mold contamination between the post-flue-curing and pre-re-curing period.

Combined effect of water activity and pH on the growth of food-related ascospore-forming molds

Purpose

The contamination of raw materials, packaging, or processing environments by fungal ascospores is a real concern for food industries, where variable rates of spoilage can be reached in pasteurized acidic products such as fruit juices, fruit jams, or soft drinks. The aim of this work was to assess the combined effect of aw and pH on the growth of six isolates from three genera of ascospore-forming molds that may occur in raw materials and in food industrial environments, in order to determine the environmental conditions that prevent the spoilage of pasteurized foods and beverages.

Methods

Growth tests were carried out on 60-day-old ascospores from Aspergillus hiratsukae (≡Neosartorya hiratsukae), Aspergillus thermomutatus (≡Neosartorya pseudofischeri), Chaetomium flavoviride, Chaetomium globosum, Talaromyces bacillisporus, and Talaromyces trachyspermus. The tests were performed up to 90 days at 25 °C, using sucrose solutions at different aw (0.85, 0.88, 0.92, 0.95) and pH (3.20, 3.50, 3.80, 4.20, 4.60) values. Growth was characterized by fitting an ordinary logistic regression model to the collected growth data.

Results

The explained percentage of the growth/no growth models ranged between 81.0 and 99.3%: aw exerted the largest influence on the growth of all tested species, while pH was significant only for Chaetomium isolates. The minimum conditions for germination and growth were aw 0.92 and pH 3.50 or 3.80, respectively, for C. flavoviride (46 days) and C. globosum (39 days), aw 0.92 and pH 3.20 for T. trachyspermus (13 days), aw 0.88 and pH 3.20 for T. bacillisporus (39 days), and aw 0.88 and pH 3.20 for the two aspergilli (33 and 27 days, respectively, for A. hiratsukae and A. thermomutatus).

Conclusions

Most of the spoiling mycetes tested were well-adapted to the formulations considered; therefore, foods strategies aiming to inhibit their growth should explore also the hurdle effect exerted by other factors (e.g., antioxidants, organic acids, oxygen levels).