Acidothermophilic bacteria like Alicyclobacillus acidoterrestris and Bacillus coagulans can cause spoilage of heat-processed acidic foods because they form spores with very high heat resistance and can grow at low pH. The objective of this work was to study the germination and inactivation of A. acidoterrestris and B. coagulans spores by high hydrostatic pressure (HP) treatment at temperatures up to 60°C and both at low and neutral pH. In a first experiment, spores suspended in buffers at pH 4.0, 5.0 and 7.0 were processed for 10min at different pressures (100-800MPa) at 40°C. None of these treatments caused any significant inactivation, except perhaps at 800MPa in pH 4.0 buffer where close to 1 log inactivation of B. coagulans was observed. Spore germination up to about 2 log was observed for both bacteria but occurred mainly in a low pressure window (100-300MPa) for A. acidoterrestris and only in a high pressure window (600-800MPa) for B. coagulans. In addition, low pH suppressed germination in A. acidoterrestris, but stimulated it in B. coagulans. In a second series of experiments, spores were treated in tomato sauce of pH 4.2 and 5.0 at 100 - 800MPa at 25, 40 and 60°C for 10min. At 40°C, results for B. coagulans were similar as in buffer. For A. acidoterrestris, germination levels in tomato sauce were generally higher than in buffer, and showed little difference at low and high pressure. Remarkably, the pH dependence of A. acidoterrestris spore germination was reversed in tomato sauce, with more germination at the lowest pH. Furthermore, HP treatments in the pH 4.2 sauce caused between 1 and 1.5 log inactivation of A. acidoterrestris. Germination of spores in the high pressure window was strongly temperature dependent, whereas germination of A. acidoterrestris in the low pressure window showed little temperature dependence. When HP treatment was conducted at 60°C, most of the germinated spores were also inactivated. For the pH 4.2 tomato sauce, this resulted in up to 3.5 and 2.0 log inactivation for A. acidoterrestris and B. coagulans respectively. We conclude that HP treatment can induce germination and inactivation of spores from thermoacidophilic bacteria in acidic foods, and may thus be useful to reduce spoilage of such foods caused by these bacteria.

The Escherichia coli Rcs regulon is triggered by antibiotic-mediated peptidoglycan stress and encodes two lysozyme inhibitors, Ivy and MliC. We report activation of this pathway by lysozyme and increased lysozyme sensitivity when Rcs induction is genetically blocked. This lysozyme sensitivity could be alleviated by complementation with Ivy and MliC.

The inactivation of different spoilage organisms and surrogate pathogens in a cooked ham model product by high pressure (HP) treatment (100-700 MPa, 5-40°C, 10 min) was investigated. A 5 log reduction could be achieved at ≥ 600 MPa at ≥ 25°C. Subsequently, the shelf-life of packaged sliced product was studied during storage (7°C) after treatment at 600 MPa (10°C, 10 min) in combination with caprylic acid and Purasal ® . Without HP treatment, a plate count of 6 log CFU/g was reached after 40 days, both in presence and absence of antimicrobials. HP treatment delayed this initiation of spoilage to 59 days in absence of antimicrobials. However, microbial growth was completely suppressed during at least 84 days in the HP treated products containing caprylic acid or Purasal ® . HP treatment increased drip loss but had no or little effect on colour and sensorial evaluation by a taste panel. However, the antimicrobials had a negative influence on the flavour and aroma at the concentrations used. Industrial relevance: With a steadily increasing number of commercial applications being introduced on the market, HP pasteurization is growing out of its infancy. To further support this development, there is a need of integrated studies that translate fundamental scientific findings from simplified laboratory model systems to the complexity and scale of real food products. In this work, we determined HP processing conditions to control spoilage and pathogenic bacteria in a cooked ham model product, and subsequently conducted a large pilot scale experiment comprising a total of 432 individual packages of sliced cooked ham product, in which the microbiological, physicochemical and sensorial quality was evaluated during refrigerated storage after HP treatment. In addition, the usefulness of the natural preservatives caprylic acid and lactate-diacetate as an additional hurdle was also studied. This study is one of the most comprehensive available in the literature to document the shelf-life extension that can be achieved with HP treatment of cooked ham.

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