Changes in microbial diversity of brined green asparagus upon treatment with high hydrostatic pressure.

The application of high hydrostatic pressure (HHP, 600MPa, 8 min) on brined green asparagus and the changes in bacterial diversity after treatments and during storage at 4 °C (30 days) or 22 °C (10 days) were studied. HHP treatments reduced viable cell counts by 3.6 log cycles. The residual surviving population did not increase during storage at 4 °C. However, bacterial counts significantly increased at 22 °C by day 3, leading to rapid spoilage. The microbiota of green asparagus was composed mainly by Proteobacteria (mainly Pantoea and Pseudomonas), followed by Firmicutes (mainly Lactococcus and Enterococcus) and to a less extent Bacteroidetes and Actinobacteria. During chill storage of untreated asparagus, the relative abundance of Proteobacteria as well as Enterococcus and Lactococcus decreased while Lactobacillus increased. During storage of untreated asparagus at 22 °C, the abundance of Bacteroidetes decreased while Proteobacteria increased during late storage. The HHP treatment determined a reduction of the Proteobacteria both early after treatment and during chill storage. In the HHP treated samples stored at 22 °C, the relative abundance of Pseudomonas rapidly decreased at day 1, with an increase of Bacteroidetes. This was followed by a marked increase in Enterobacteriaceae (Escherichia) simultaneously with increase in viable counts and spoilage. Results from the study indicate that the effect of HHP treatments on the viability ofmicrobial populations in foods also has an impact on the dynamics of microbial populations during the storage of the treated foods.

Microbial diversity in pitted sweet cherries (Prunus avium L.) as affected by High-Hydrostatic Pressure treatment.

Sweet cherries are a highly appreciated seasonal fruit rich in anthocyanins. The purpose of the present study was to determine the effect of High-Hydrostatic Pressure (HHP) processing on the microbiological quality and bacterial biodiversity of sweet cherries. Pitted cherries inoculated with their own epiphyte microbiota to simulate a worst-case scenario of contamination during preparation and processing were treated or not by HHP (600MPa, 8min) and stored at 4°C for 60days. HHP treatment reduced total viable counts by 4.65 log cycles. The surviving bacterial fraction did not increase significantly (p<0.05) for the first 15days of storage. Concentrations of yeasts and molds were reduced below detectable levels. Upon prolonged storage (60days), microbial growth was observed. Bacterial biodiversity studied by high-throughput sequencing of the 16S rRNA gene revealed that Proteobacteria had highest relative abundance (88.70%) in the spiked cherries followed by Firmicutes (11.04%). Gluconobacter and Enterobacteriaceae together with Leuconostoc were the most abundant Operational Taxonomic Units (OTUs). Upon application of HHP treatment, 97.62% of OTUs from the surviving fraction belonged to Proteobacteria. The relative abundance of Enterobacteriaceae also decreased markedly while Acetobacteraceae (represented mainly by Gluconobacter) increased to 89.18%. Gluconobacter dominated during storage. Results from the present study provide insights on the microbiota of sweet cherries and the dynamics of the bacterial populations surviving HHP treatments that may be useful to improve the non-thermal preservation of cherries.

Survival and High-Hydrostatic Pressure Inactivation of Foodborne Pathogens in Salmorejo, a Traditional Ready-to-Eat Food.

UNLABELLED: Salmorejo is a traditional tomato-based creamy product. Because salmorejo is not heat-processed, there is a risk of contamination with foodborne pathogens from raw materials. Even though bacterial growth in salmorejo is strongly inhibited because of its acidic pH (close to 3.9), the growth and survival of 3 foodborne pathogens in this food has not been studied before. In this study, 3 cocktails consisting of Escherichia coli O157, Salmonella enterica serovar Enteritidis, and Listeria monocytogenes strains were inoculated in freshly prepared salmorejo. The food was treated by high hydrostatic pressure (HHP) at 400, 500, or 600 MPa for 8 min, or left untreated, and stored at 4 °C for 30 d. Viable cell counts were determined on selective media and also by the triple-layer agar method in order to detect sublethally injured cells. In control samples, L. monocytogenes viable cells decreased by 2.4 log cycles at day 7 and were undetectable by day 15. S. enterica cells decreased by 0.5 or 2.4 log cycles at days 7 and 15 respectively, but still were detectable at day 30. E. coli O157 cells survived much better in salmorejo, decreasing only by 1.5 log cycles at day 30. Treatments at pressures of 400 MPa or higher reduced viable counts of L. monocytogenes and S. enterica to undetectable levels. HHP treatments significantly (P < 0.05) reduced E. coli counts by approximately 5.2 to 5.4 log cycles, but also yielded surviving cells that apparently were sublethally injured. Only samples treated at 600 MPA for 8 min were devoid of detectable E. coli cells during storage. PRACTICAL APPLICATION: Salmorejo is a traditional, vitamin-rich food, usually produced on a small scale. HHP treatment at 600 MPa for 8 min can be an efficient nonthermal method for industrial-scale preparation of preservative-free salmorejo with improved safety against transmission of foodborne pathogens L. monocytogenes serotyes 4a and 4b, S. enterica serovar Enteritidis, and E. coli O157.