Aspergilli with Neosartorya-type ascospores: heat resistance and effect of sugar concentration on growth and spoilage incidence in berry products

- Neosartorya ascospores (glucose solution): D87 = 3.7-13.5′; D90 = 1.5-3.5′; D95 = 0.3-0.4′
- Neosartorya ascospores (strawberry mix): D87 = 3.3-15.4′; D90 = 1.3-4.3′; D95 = 0.3-0.6′
- No growth was observed below aw 0.87 or 0.88 (blueberry- or strawberry medium)
- No growth was observed above 56.0 or 56.5 °Bx (blueberry- or strawberry medium)
- A relation between sugar content, HRMS contamination and spoiling incidence was found

This study focused on four different heat resistant aspergilli: two strains of Aspergillus hiratsukae (≡ Neosartorya hiratsukae), one strain of Aspergillus neoglaber (≡ Neosartorya glabra), and one strain of Aspergillus thermomutatus (≡ Neosartorya pseudofischeri), all isolated from spoiled pasteurized products. Their heat-resistance, the sugar concentration limiting their germination and growth in berry-based media, and a possible relation between the contamination levels of the raw materials used and the spoilage incidence in strawberry jams were assessed. Heat resistance data obtained from thermal death curves showed that the D values of the strains tested ranged between 3.7 and 13.5 min at 87 °C; 1.5 and 3.5 min at 90 °C; and 0.3 and 0.4 min at 95 °C in glucose solution. Similarly, D values ranged between 3.3 and 15.4 min at 87 °C; 1.3 and 4.3 min at 90 °C; and 0.3 and 0.6 min at 95 °C in strawberry-based formulation. For all strains, the corresponding z-values ranged between 5.7 and 8.3 °C in glucose solution and from 5.7 to 8.4 °C in strawberry formulation. With regard to the limitation of fungal germination and growth in fruit-based media, sucrose concentrations required to avoid growth varied between 45.0 and 55.0% for strawberry medium and between 42.5% and 50.0% for blueberry medium. Spore inactivation was observed below aw 0.88-0.91 for strawberries and aw 0.87-0.90 for blueberries; above 49.7-56.5 °Bx for strawberries and 49.6-56.0 °Bx for blueberries. The threshold optical refractometric residue proved strain-dependent, but substrate-independent, as for each strain the highest Brix degree value at which germination occurred was the same on both media, despite their different sucrose concentrations. With regard to the relation between contamination of raw materials by heat-resistant mould spores and spoilage incidence on final product, an equation was modelled to estimate the occurrence of fungal spoilage in strawberry jams for low contamination levels (26-46 CFU/kg). Although it could not be used as a definitive tool to predict final spoilage in such of products, it could give important practical information to jam producers in preventing spoilage of their products.