Alicyclobacillus Acidoterrestris: The Organism, the Challenge, Potential Interventions

Impact of Heating Rates on Alicyclobacillus acidoterrestris Heat Resistance under Non-Isothermal Treatments and Use of Mathematical Modelling to Optimize Orange Juice Processing

Alicyclobacillus acidoterrestris is a spoilage microorganism responsible for relevant product and economic losses in the beverage and juice industry. Spores of this microorganism can survive industrial heat treatments and cause spoilage during posterior storage. Therefore, an effective design of processing treatments requires an accurate understanding of the heat resistance of this microorganism. Considering that industrial treatments are dynamic; this understanding must include how the heat resistance of the microorganism is affected by the heating rate during the heating and cooling phases. The main objective of this study was to establish the effect of heating rates and complex thermal treatments on the inactivation kinetics of A. acidoterrestris. Isothermal experiments between 90 and 105 °C were carried out in a Mastia thermoresistometer, as well as four different dynamic treatments. Although most of the inactivation takes place during the holding phase, our results indicate the relevance of the heating phase for the effectiveness of the treatment. The thermal resistance of A. acidoterrestris is affected by the heating rate during the heating phase. Specifically, higher heating rates resulted in an increased microbial inactivation with respect to the one predicted based on isothermal experiments. These results provide novel information regarding the heat response of this microorganism, which can be valuable for the design of effective heat treatments to improve product safety and stability. Moreover, it highlights the need to incorporate experimental data based on dynamic treatments in process design, as heating rates can have a very significant effect on the thermal resistance of microorganisms.

Bacterial Spore Inactivation in Orange Juice and Orange Peel by Ultraviolet-C Light

Spore-forming bacteria are a great concern for fruit juice processors as they can resist the thermal pasteurization and the high hydrostatic pressure treatments that fruit juices receive during their processing, thus reducing their microbiological quality and safety. In this context, our objective was to evaluate the efficacy of Ultraviolet-C (UV-C) light at 254 nm on reducing bacterial spores of Alicyclobacillus acidoterrestris, Bacillus coagulans and Bacillus cereus at two stages of orange juice production. To simulate fruit disinfection before processing, the orange peel was artificially inoculated with each of the bacterial spores and submitted to UV-C light (97.8-100.1 W/m²) with treatment times between 3 s and 10 min. The obtained product, the orange juice, was also tested by exposing the artificially inoculated juice to UV-C light (100.9-107.9 W/m²) between 5 and 60 min. A three-minute treatment (18.0 kJ/m²) reduced spore numbers on orange peel around 2 log units, while more than 45 min (278.8 kJ/m²) were needed to achieve the same reduction in orange juice for all evaluated bacterial spores. As raw fruits are the main source of bacterial spores in fruit juices, reducing bacterial spores on fruit peels could help fruit juice processors to enhance the microbiological quality and safety of fruit juices.

Assessment of Spoilage Potential Posed by Alicyclobacillus spp. in Plant-Based Dairy Beverages Mixed with Fruit Juices during Storage

ABSTRACT

The scope of the present study was to assess the spoilage potential of different Alicyclobacillus spp. in commercial pasteurized (ambient-stable) plant-based dairy beverages mixed with fruit juices at different inoculation levels and storage temperatures. Different products (coconut and berry [CB]; almond, mango, and passionfruit [AMP]; and oat, strawberry, and banana [OSB]) were inoculated with 10 or 2 × 103 spores per mL of either Alicyclobacillus acidoterrestris, Alicyclobacillus fastidiosus, or Alicyclobacillus acidocaldarius strain composites, whereas noninoculated samples served as controls. Samples inoculated with A. acidoterrestris and A. fastidiosus were stored at 30 and 45°C, whereas A. acidocaldarius storage took place at 50°C for 240 days. Gas composition, Alicyclobacillus spp. populations, total viable counts, pH, water activity, color, and guaiacol off-taste were monitored. CB and AMP supported growth of A. acidoterrestris and A. fastidiosus, reaching populations of 4.0 to 5.0 log CFU/mL. In OSB, populations of A. fastidiosus remained close to the initial inoculation levels during storage at 30°C, whereas at 45°C, the populations declined <1 CFU/mL. A. acidocaldarius growth was supported in CB samples, but not in AMP and OSB samples, reaching ca. 3.0 log CFU/mL at 50°C, regardless of initial inoculum size. Total color change was increased during storage; however, the instrumentally recorded color changes were not macroscopically visible. Spoilage in terms of guaiacol off-taste was identified only in CB and AMP samples inoculated with A. acidoterrestris after 60 days at 30 and 45°C. The increased popularity of these products along with the scarcity of existing literature related to their spoilage by Alicyclobacillus spp., render the contribution of the findings and data of present study critical for assessing the significance of Alicyclobacillus spp. as a potential spoilage hazard in these products and for assisting in the design and implementation of effective mitigation strategies by the beverage industry.

HIGHLIGHTS

Inactivation of spores by nonthermal plasmas

Bacterial and fungal spore contamination in different industries has a greater economic impact. Because of the remarkable resistance of spores to most physical and chemical microbicidal agents, their inactivation need special attention during sterilization processes. Heat and chemical sporicides are not always well suited for different sterilization/decontamination applications and carries inherent risks. In recent years, novel nonthermal agents including nonthermal plasmas are emerging as effective sporicides against a broad spectrum of bacterial and fungal spores. The present review discusses various aspects related to the inactivation of spores using nonthermal plasmas. Different types of both low pressure plasmas (e.g., capacitively coupled plasma and microwave plasma) and atmospheric pressure plasmas (e.g., dielectric barrier discharges, corona discharges, arc discharges, radio-frequency-driven plasma jet) have been successfully applied to destroy spores of economic significance. Plasma agents contributing to sporicidal activity and their mode of action in inactivation are discussed. In addition, information on factors that affect the sporicidal action of nonthermal plasmas is included.

Development of a Polyclonal Antibody-Based Sandwich Enzyme-Linked Immunosorbent Assay for the Detection of Spores of Alicyclobacillus acidoterrestris in Various Fruit Juices

A polyclonal rabbit antibody-based sandwich ELISA for the rapid and specific detection of spores of Alicyclobacillus acidoterrestris was established. The reactivity of the antisera with spores was confirmed by immunofluorescence. For a thorough evaluation of the ELISA, 61 strains and isolates of Alicyclobacillus spp. were characterized regarding their guaiacol production ability and genetic variability. The ELISA was highly sensitive, the detection limits were isolate-dependent and ranged from 2.1 × 10(3) - 3.8 × 10(4) spores/mL, except for one isolate, for which a slightly lower sensitivity (5 × 10(5) spores/mL) was observed. Inclusivity tests revealed that the ELISA reacts with all tested A. acidoterrestris, while no cross-reactions with spores of 30 strains of Bacillus spp. and Clostridium spp. were observed. Further on, the assay applicability was tested with orange, apple (clear and unfiltered), tomato, pink grapefruit, pear, and white grape juices. Juices were inoculated with 1 or 10 spores/mL of A. acidoterrestris. After enrichment for 48 h, the established ELISA enabled the reliable and reproducible detection of contaminated samples. The enriched samples could be applied directly to the assay, underlining the robustness of the developed ELISA method.

Effect of Nisin and Thermal Treatments on the Heat Resistance of Clostridium sporogenes Spores

The aim of this research was to evaluate the effect of thermal treatments (isothermal or nonisothermal) combined with nisin, a natural antimicrobial, on the survival and recovery of Clostridium sporogenes spores. The addition of nisin to the heating medium at concentrations up to 0.1 mg liter(-1) did not reduce the heat resistance of C. sporogenes. Without a thermal treatment, nisin added at concentrations up to 0.1 mg liter(-1) did not reduce the viable counts of C. sporogenes when added to the recovery medium, but inactivation of more than 4 log cycles was achieved after only 3 s at 100°C. At 100°C, the time needed to reduce viable counts by more than 3 log cycles was nine times shorter when 0.01 mg liter(-1) nisin was added to the recovery medium than without it. The heat resistance values calculated under isothermal conditions were used to predict the survival in the nonisothermal experiments, and the predicted values accurately fit the experimental data. The combination of nisin with a thermal treatment can help control C. sporogenes.

Food Targeting: A Real-Time PCR Assay Targeting 16S rDNA for Direct Quantification of Alicyclobacillus spp. Spores after Aptamer-Based Enrichment

Spore-forming Alicyclobacillus spp. are able to form metabolites that induce even in small amounts an antiseptical or medicinal off-flavor in fruit juices. Microbial contaminations could occur by endospores, which overcame the pasteurization process. The current detection method for Alicyclobacillus spp. can take up to 1 week because of microbiological enrichment. In a previous study, DNA aptamers were selected and characterized for an aptamer-driven rapid enrichment of Alicyclobacillus spp. spores from orange juice by magnetic separation. In the present work, a direct quantification assay for Alicyclobacillus spp. spores was developed to complete the two-step approach of enrichment and detection. After mechanical treatment of the spores, the isolated DNA was quantified in a real-time PCR-assay targeting 16S rDNA. The assay was evaluated by the performance requirements of the European Network of Genetically Modified Organisms Laboratories (ENGL). Hence, the presented method is applicable for direct spore detection from orange juice in connection with an enrichment step.