Modeling the influence of electron beam irradiation on the heat resistance of Bacillus cereus spores.

The effect of electron beam irradiation (EBI) on Bacillus cereus spore heat resistance was investigated. Irradiation with accelerated electrons had an important heat-sensitizing effect on distilled-water spore suspensions. After irradiation doses of 1.3, 3.1, or 5.7 kGy followed by heating at 90 degrees C, calculated D(90)-values for strains Escuela Politécnica Superior de Orihuela (EPSO)-41WR and EPSO-50UR were reduced more than 1.3, 2.4, and 4.6 times, respectively. Plots of calculated log D(T)-values versus irradiation doses (1.3, 3.1, and 5.7 kGy) yielded straight parallel lines for the 85-100 degrees C heating temperature range, which made it possible to develop an equation to predict the changes in heat sensitivity of B. cereus spores that occurred with changing irradiation dose. Radiation-induced heat-sensitivity was characterized by a z(EBI)-value which was determined as the irradiation dose that should be required to reduce the decimal reduction time (D(T)) by one log(10) cycle when log(10)D(T) was plotted against irradiation treatment. A model is proposed to describe the influence of a pre-irradiation treatment with electron beams followed by heating on the heat resistance of B. cereus spores. This study also suggests the potential use of EBI followed by heating for food preservation.

Antibacterial activity of 11 essential oils against Bacillus cereus in tyndallized carrot broth.

The antibacterial activity of 11 essential oils from aromatic plants against the strain INRA L2104 of the foodborne pathogen Bacillus cereus grown in carrot broth at 16 degrees C was studied. The quantity needed by the essential oils of nutmeg, mint, clove, oregano, cinnamon, sassafras, sage, thyme or rosemary to produce 14-1110% relative extension of the lag phase was determined. Total growth inhibition of bacterial spores was observed for some of the antimicrobial agents assayed. The addition of 5 microl cinnamon essential oil per 100 ml of broth in combination with refrigeration temperatures of

Influence of pH and temperature on growth of Bacillus cereus in vegetable substrates.

Bacillus cereus is a food-borne pathogen which most often contaminates foods of plant origin. Spores of psychrotrophic strains have the ability to germinate and grow at refrigeration temperatures in different vegetable substrates, such as carrot broth, zucchini broth, and cooked carrot purée. In some circumstances, factors such as pH, heat treatment, and storage temperature play a fundamental role in controlling the growth of these psychrotrophic strains and in extending the shelf life of refrigerated, minimally processed vegetable-based products in relation to pathogenic spore-forming bacteria. The combination of mild acidification (pH 5.0) and refrigeration (

Predictive model to describe the combined effect of pH and NaCl on apparent heat resistance of Bacillus stearothermophilus.

The combined effect of pH and NaCl on the apparent thermal resistance of Bacillus stearothermophilus ATCC 12980 spores was studied. Spores were heated at different temperatures (115-125 degrees C) in mushroom substrate, acidified using glucono-delta-lactone to different pH levels (from 5.75 to 6.7), which contained concentrations of NaCl that ranged from 0.5 to 3% (w/v). The recovery medium was acidified to the same pH level and contained the same NaCl concentration as the heating menstruum. A factorial experimental design allowed a predictive model to be developed, which described the combined effect of heating temperature, pH and NaCl on the thermal resistance of B. stearothermophilus spores. Predictions from the model provided a valid description of the data used to generate the model, and agreed with observations from the literature and from an independent experiment performed using asparagus and bean substrates.

Effect of Intermittent Warming and Modified Atmosphere Packaging on Fungal Growth in Peaches.

Firm-breaker peaches (Prunus persica cv. Paraguayo), treated with 0.25 g liter-1 iprodione, were inoculated with 106 spores per ml of Alternaria tenuis and stored for up to 3 weeks at 0.5°C. Three intermittent warming (IW) cycles of 1 day at 20°C every 6 days at 0.5°C and modified atmosphere packaging (MAP) were applied. Fungicide treatment was ineffective in controlling growth of A. tenuis and Cladosporium spp. during storage. MAP prevented the growth of both fungal genera. IW alone or combined with fungicide seemed to increase total CFU counts and did not control decay (mainly due to exacerbated Cladosporium spp. development). CFU counts and losses through fungal attack were only occasionally significantly correlated.

Isolation of Acremonium Species Causing Postharvest Decay of Peaches in Spain.

Two Acremonium spp., A. tubakii and A. rutilum, were isolated from rotted peaches (Prunus persica (L.) Batsch 'Miraflores') after ripening at 15 and 20°C, respectively, for 10 days. Fruit harvested in September 1994 in Mula (Murcia) at a firm-ripe stage of maturity were either nontreated or washed with water at 18°C and sorted in a packing line. Sorted fruit were treated with an aqueous dilution of iprodione (Rovral 50% WP Rhône-Poulenc Agrochimie, Paris, France) at 2 g·liter-1, pH = 7, and 18°C for 5 min. Half of the iprodione-treated fruit were forced-air pre-cooled to reach 0°C at the endosperm in about 12 h. Isolations of fungi were made from subepidermic tissue of decay margins of fruit with potato dextrose agar (PDA). Fungal species were identified after axenic cultures were grown on PDA for 10 days at 30°C. Acremonium spp. were identified based on microscopic morphology of asexual reproduction structures, using taxonomic keys. No other fungi were isolated from fruit decayed by Acremonium spp. A. tubakii decayed 1.7% of the iprodionetreated and pre-cooled fruit ripened at 15 or 20°C and the fruit treated by iprodione without pre-cooling but ripened at 20°C. These fruit were completely decomposed and oozed liquid. Colonies of this species were light colored, with nonramified, nonseptate, and hyaline conidiophores. The one-cell, ovate, and hyaline conidia of the fungus were enclosed in slimy heads. A. tubakii did not cause any brown discoloration of the PDA media. A. rutilum affected 3.2% of the iprodione-treated fruit ripened at 20°C. In contrast to A. tubakii infection, A. rutilum affected only the surface of fruit, which remained dry. Cultures of A. rutilum were white to pink and the conidia were one-celled, ovate, and hyaline, forming a head on the tip of the nonramified conidiophores. Conidia were long, nonseptate, and distinct from the somatic hyphae. Acremonium spp. probably developed on injuries caused by handling and/or treatments, because these rots did not occur on untreated fruit. Rhizopus nigricans, and to a lesser extent other fungi such as Alternaria, Cladosporium, and Penicillium spp., also commonly caused decay of sampled fruit. A. tubakii is a ubiquitous soil fungus reported from various European countries. A. rutilum isolated from diseased apples were pathogenic upon reinoculation (1). Both fungi were not found in the Spanish type culture collection of 1990. Other Acremonium spp. have been identified in melon roots in Spain (2). To the best of our knowledge, this is the first report of Acremonium spp. causing decay of peach in Spain. References: (1) C. Brooks et al. Phytopathology 4:403, 1914. (2) J. García-Jiménez et al. Plant Dis. 78:416, 1994.