Heat resistance of fungi isolated from frozen blueberries

Occurrence of Heat-Resistant Mold Ascospores in Pineapple and Sugarcane Field Soils in Thailand

Heat-resistant molds (HRMs) are important spoilage fungi of heat-processed fruit products worldwide. Ascospores of HRMs are widely distributed in the soil in which fruits are grown and are often found associated with raw fruit materials. To date, there is little available information on the distribution of HRMs in the soil and on their heat resistance. Thus, this study determined the presence and characterized the heat resistance of HRMs in soil samples from pineapple and sugarcane fields in Thailand. HRMs were detected in all soil samples, and the most dominant species was Aspergillus with 50-99.2% relative abundance. Other isolates, in descending order of frequency, were Penicillium, Talaromyces, Hamigera, and Paecilomyces. Then, 100 representative HRM isolates were identified based on a combination of morphological characteristics and ITS sequences. They were classified into 5 genera and 24 species. The heat resistance of ascospores aged 30 days produced by selected HRMs was qualitatively determined in a glucose-buffered solution. Based on their log reductions after heat shock at 75°C for 30 min, they were classified as less, moderately, or highly heat-resistant ascospores. HRMs belonging to A. chevalieri, A. denticulatus, A. siamensis, A. laciniosus, A. fennelliae, A. spinosus, Paec. niveus, H. pallida, and T. macrosporus produced high heat-resistant ascospores. In addition, soil physicochemical properties significantly influenced the prevalence of HRMs, depending on the fungal genus. The thermal resistance of ascospores was significantly and positively correlated to available phosphorus, whereas it was negatively correlated to soil pH. The results of this study confirmed the presence of HRMs in soils and potential HRM contamination, especially in fruits growing in acidic or high-nutrient soils, or both.

A Comparison of Three Methods for Determining Thermal Inactivation Kinetics: A Case Study on Salmonella enterica in Whole Milk Powder

ABSTRACT

Different methods for determining the thermal inactivation kinetics of microorganisms can result in discrepancies in thermal resistance values. In this study, thermal resistance of Salmonella in whole milk powder was determined with three methods: thermal death time (TDT) disk in water bath, pouches in water bath, and the TDT Sandwich system. Samples from three production lots of whole milk powder were inoculated with a five-strain Salmonella cocktail and equilibrated to a water activity of 0.20. The samples were then subjected to three isothermal treatments at 75, 80, or 85°C. Samples were removed at six time points and cultures were enumerated for survivors. The inactivation data were fitted to two consolidated models: two primary models (log linear and Weibull) and one secondary model (Bigelow). Normality testing indicated that all the model parameters were normally distributed. None of the model parameters for both consolidated models were significantly different (α = 0.05). The amount of inactivation during the come-up time was also not significantly different among the methods (α = 0.05). However, the TDT Sandwich resulted in less inactivation during the come-up time and overall less variation in model parameters. The survivor data from all three methods were combined and fitted to both consolidated models. The Weibull had a lower root mean square error and a better fit, according to the corrected Akaike's information criterion. The three thermal treatment methods produced results that were not significantly different; thus, the methods are interchangeable, at least for Salmonella in whole milk powder. Comparisons with more methods, other microorganisms, and larger varieties of food products using the same framework presented in this study could provide guidance for standardizing thermal inactivation kinetics studies for microorganisms in foods.

HIGHLIGHTS

TDT Sandwich: An open source dry heat system for characterizing the thermal resistance of microorganisms

The determination of the thermal death kinetics of microorganisms has traditionally been performed with liquid baths which have some disadvantages such as liquid spillage and liquid infiltration into samples. The TDT Sandwich was developed as a free, open source alternative that utilizes dry heat. The system is capable of heating samples up to 140 °C and maintaining it within 0.2 °C of the target temperature. Other features of the TDT Sandwich include adjustable heating rates up to approximately 100 °C/min, real-time display and recording of temperature readings at a nominal rate of 5 Hz, an optional thermocouple for acquiring temperature of samples, built-in heating timer, and customizable operating parameters. The modular nature of the TDT Sandwich allows multiple units to be connected to a computer or laptop. Operation of the TDT Sandwich is done through a computer program which, along with the build instructions and microcontroller program, are open source and are available for free to the public at https://doi.org/10.17605/OSF.IO/5Q3Y7.

Detection of Talaromyces macrosporus and Talaromyces trachyspermus by a PCR assay targeting the hydrophobin gene

Talaromyces species are typical fungi capable of producing the heat-resistant ascospores responsible for the spoilage of processed food products. Hydrophobins, which are unique to fungi, are small secreted proteins that form amphipathic layers on the outer surface of fungal cell walls. In this study, species-specific primer sets for detecting and identifying Talaromyces macrosporus and Talaromyces trachyspermus were designed based on hydrophobin gene sequences. A conventional polymerase chain reaction (PCR) assay using these primer sets produced species-specific amplicons for T. macrosporus and T. trachyspermus. The detection limit for each primer set was 100 pg template DNA. This assay also detected fungal DNA extracted from blueberries inoculated with T. macrosporus. Other heat-resistant fungi, including Byssochlamys, Neosartorya and Talaromyces species, which cause food spoilage, were not detected in PCR amplifications with these primer sets. Furthermore, a conventional PCR assay using a crude DNA extract as the template also yielded amplicons specific to T. macrosporus and T. trachyspermus. The simple and rapid PCR assay described herein is highly species-specific and can reliably detect T. macrosporus and T. trachyspermus, suggesting it may be relevant for the food and beverage industry. SIGNIFICANCE AND IMPACT OF THE STUDY: The heat-resistant ascospores of Talaromyces macrosporus and Talaromyces trachyspermus are responsible for food spoilage after pasteurization. Traditional methods for detecting fungal contamination based on morphological characteristics are time-consuming and exhibit low sensitivity and specificity. In this study, a conventional polymerase chain reaction (PCR) assay based on hydrophobin gene sequences was developed for the specific detection of T. macrosporus and T. trachyspermus. This detection method was simple, rapid and highly specific. These results suggest that the conventional PCR assay developed in this study may be useful for detecting T. macrosporus and T. trachyspermus in raw materials and processed food products.

Occurrence, distribution and contamination levels of heat-resistant moulds throughout the processing of pasteurized high-acid fruit products

Heat-resistant moulds (HRMs) are well known for their ability to survive pasteurization and spoil high-acid food products, which is of great concern for processors of fruit-based products worldwide. Whilst the majority of the studies on HRMs over the last decades have addressed their inactivation, few data are currently available regarding their contamination levels in fruit and fruit-based products. Thus, this study aimed to quantify and identify heat-resistant fungal ascospores from samples collected throughout the processing of pasteurized high-acid fruit products. In addition, an assessment on the effect of processing on the contamination levels of HRMs in these products was carried out. A total of 332 samples from 111 batches were analyzed from three processing plants (=three processing lines): strawberry puree (n = 88, Belgium), concentrated orange juice (n = 90, Brazil) and apple puree (n = 154, the Netherlands). HRMs were detected in 96.4% (107/111) of the batches and 59.3% (197/332) of the analyzed samples. HRMs were present in 90.9% of the samples from the strawberry puree processing line (1-215 ascospores/100 g), 46.7% of the samples from the orange juice processing line (1-200 ascospores/100 g) and 48.7% of samples from the apple puree processing line (1-84 ascospores/100 g). Despite the high occurrence, the majority (76.8%, 255/332) of the samples were either not contaminated or presented low levels of HRMs (<10 ascospores/100 g). For both strawberry puree and concentrated orange juice, processing had no statistically significant effect on the levels of HRMs (p > 0.05). On the contrary, a significant reduction (p < 0.05) in HRMs levels was observed during the processing of apple puree. Twelve species were identified belonging to four genera - Byssochlamys, Aspergillus with Neosartorya-type ascospores, Talaromyces and Rasamsonia. N. fumigata (23.6%), N. fischeri (19.1%) and B. nivea (5.5%) were the predominant species in pasteurized products. The quantitative data (contamination levels of HRMs) were fitted to exponential distributions and will ultimately be included as input to spoilage risk assessment models which would allow better control of the spoilage of heat treated fruit products caused by heat-resistant moulds.

Microbial Quality and Shelf Life of Blueberry Purée Developed Using Cavitation Technology

Blueberry purée was developed using hydrodynamic cavitation technology. The product was made from entire blueberries without adding any food additives. In this study, microbial reduction following each processing stage (at the industry setting) and after product pasteurization at 86, 88, 90, 92, 94, and 96 °C was investigated. Microbial quality including total plate counts, yeast and molds, and heat-resistant molds counts was determined. Shelf life of pasteurized products stored for up to 24 weeks at room temperature were assessed for microbial quality, soluble solids (°Brix), titratable acidity (citric acid %), pH, viscosity (cP) and flow rate (cm/30 s). Our results indicated that heat-resistant molds, initially present in frozen blueberries with counts at 2.03 log CFU/200g, were totally inactivated at 94 to 96 °C with 1 to 2 min holding time. Shelf life study showed that no product spoilage was caused by bacteria, yeasts and heat-resistant molds along with non-significant changes of textural characteristics. This study provided useful information for the food industry to develop variety of fruit purée products with no wastes of fruit materials.

PRACTICAL APPLICATION

This study provides useful information for the food industry to develop safe liquid food products using cavitation technology without wasting any raw materials.

Design of a species-specific PCR method for the detection of the heat-resistant fungi Talaromyces macrosporus and Talaromyces trachyspermus

Heat-resistant fungi occur sporadically and are a continuing problem for the food and beverage industry. The genus Talaromyces, as a typical fungus, is capable of producing the heat-resistant ascospores responsible for the spoilage of processed food products. Isocitrate lyase, a signature enzyme of the glyoxylate cycle, is required for the metabolism of non-fermentable carbon compounds, like acetate and ethanol. Here, species-specific primer sets for detection and identification of DNA derived from Talaromyces macrosporus and Talaromyces trachyspermus were designed based on the nucleotide sequences of their isocitrate lyase genes. Polymerase chain reaction (PCR) using a species-specific primer set amplified products specific to T. macrosporus and T. trachyspermus. Other fungal species, such as Byssochlamys fulva and Hamigera striata, which cause food spoilage, were not detected using the Talaromyces-specific primer sets. The detection limit for each species-specific primer set was determined as being 50 pg of template DNA, without using a nested PCR method. The specificity of each species-specific primer set was maintained in the presence of 1,000-fold amounts of genomic DNA from other fungi. The method also detected fungal DNA extracted from blueberry inoculated with T. macrosporus. This PCR method provides a quick, simple, powerful and reliable way to detect T. macrosporus and T. trachyspermus.

SIGNIFICANCE AND IMPACT OF THE STUDY

Polymerase chain reaction (PCR)-based detection is rapid, convenient and sensitive compared with traditional methods of detecting heat-resistant fungi. In this study, a PCR-based method was developed for the detection and identification of amplification products from Talaromyces macrosporus and Talaromyces trachyspermus using primer sets that target the isocitrate lyase gene. This method could be used for the on-site detection of T. macrosporus and T. trachyspermus in the near future, and will be helpful in the safety control of raw materials and in food and beverage production.

Diversity and Control of Spoilage Fungi in Dairy Products: An Update

Fungi are common contaminants of dairy products, which provide a favorable niche for their growth. They are responsible for visible or non-visible defects, such as off-odor and -flavor, and lead to significant food waste and losses as well as important economic losses. Control of fungal spoilage is a major concern for industrials and scientists that are looking for efficient solutions to prevent and/or limit fungal spoilage in dairy products. Several traditional methods also called traditional hurdle technologies are implemented and combined to prevent and control such contaminations. Prevention methods include good manufacturing and hygiene practices, air filtration, and decontamination systems, while control methods include inactivation treatments, temperature control, and modified atmosphere packaging. However, despite technology advances in existing preservation methods, fungal spoilage is still an issue for dairy manufacturers and in recent years, new (bio) preservation technologies are being developed such as the use of bioprotective cultures. This review summarizes our current knowledge on the diversity of spoilage fungi in dairy products and the traditional and (potentially) new hurdle technologies to control their occurrence in dairy foods.

Method for identifying heat-resistant fungi of the genus Neosartorya

Species of the genus Neosartorya are heat-resistant fungi that cause the spoilage of heat-processed acidic foods due to the formation of heat-resistant ascospores, and they produce mycotoxins, such as fumitremorgins and gliotoxin. Their anamorphs are phylogenetically and morphologically very close to Aspergillus fumigatus, which has never been reported as a spoilage agent in heat-processed food products. Therefore it is important to discriminate between the species of Neosartorya and A. fumigatus in the food industry. In the present study, we examined β-tubulin and calmodulin genes to identify Neosartorya and A. fumigatus at the species level and found a region for specifically detecting these species. We succeeded in developing the PCR method of differentiating and identifying Neosartorya and A. fumigatus using specific primer sets. Moreover, we developed specific primer sets to identify Neosartorya species, N. fischeri, N. glabra, N. hiratsukae, N. pseudofischeri, and N. spinosa-complex, which are important in food spoilage; these fungi vary in heat resistance and productivity of mycotoxins, depending on the species. PCR using these primer sets did not detect other fungi involved in food spoilage and environmental contamination. These identification methods are rapid and simple with extremely high specificity.

A rapid method for identifying Byssochlamys and Hamigera

Heat-resistant fungi, genera Byssochlamys, Talaromyces, Neosartorya, and Hamigera, contribute significantly to the spoilage of heat-processed acidic foods, due to the formation of heat-resistant ascospores. Here, we first evaluated the differences in the beta-tubulin gene between Byssochlamys and Hamigera and developed specific primers to identify the Byssochlamys species fulva, nivea, and spectabilis, and Hamigera. Using primers designed for B. fulva and B. nivea (B1F/1R), specific PCR products were detected for B. fulva and B. nivea, as well as B. langunculariae and B. zollerniae, two closely related species. Similarly, the Pae4F/4R-1 and H2F/2R primers produced specific PCR products for B. spectabilis and Hamigera, respectively. Using these three primer sets, strains involved in acidic food spoilage and environmental contamination were not detected. The detection limits of all primer sets were 1 ng of DNA by PCR and 10 pg of DNA by nested PCR. Each PCR assay was specific, even if the sample was contaminated 1,000-fold by other fungal DNA. Thus, this method has proved to possess an extremely high degree of specificity.