Physiological alterations involved in inactivation of autochthonous spoilage bacteria in orange juice caused by Citrus essential oils and mild heat.

This study investigated physiological alterations involved in the inactivation of Levilactobacillus (L.) brevis and Leuconostoc (Lc.) mesenteroides in orange juice caused by Citrus lemon essential oil (CLEO) and C. reticulata essential oil (CREO) alone and combined with mild heat treatment (MHT). Damage in DNA, membrane integrity, membrane potential, metabolic and efflux activity of bacterial cells were measured after exposure (6 and 12 min) to CLEO or CREO (0.5 µL/mL) and/or MHT (54 °C) using flow cytometry. Limonene was the major constituent in CLEO (66.4%) and CREO (89.4%). The size of the damaged cell subpopulations increased (p < 0.05) after longer exposure time and varied with the tested essential oil and/or bacterial isolate. After exposure to CLEO and CREO alone, the cell subpopulations with damage in measured physiological functions were in a range of 19.6-66.8% and 23.8-75.9%, respectively. Exposure to CREO resulted in larger Lc. mesenteroides cell subpopulations (35.4-68.7%) with damaged DNA, permeabilized and depolarized membrane and compromised metabolic or efflux activity compared to L. brevis (23.8-58.0%). In contrast, exposure to CLEO led to higher damaged L. brevis cell subpopulations (35.1-77%) compared to Lc. mesenteroides (25.3-36.6%). Exposure to combined treatments (CLEO or CREO and MHT) affected the measured physiological functions in almost the entire L. brevis and Lc. mesenteroides cell population (up to 99%), although the damage extension on each isolate varied with tested essential oil. Results show that inactivation of L. brevis and Lc. mesenteroides cells caused by CLEO and CREO alone and combined with MHT in orange juice involves a multi-target action, which causes DNA damage, altered permeability and depolarization of membrane and compromised metabolic and efflux activities.

Effect of ultrasound on the physicochemical and microbiological characteristics of Italian salami.

Italian salami were sonicated in different times (0, 3, 6 and 9min) using ultrasound bath (US, 25kHz). The effect of sonication on microbial growth (lactic acid bacteria and Micrococcaceae), lipid and protein oxidation, total heme pigments (THP), non heme iron (NHI) and metmyoglobin (MMb) was investigated during processing (0, 2, 15, and 28days) and storage (1, 30, and 120days). US enhanced growth of microorganisms (P<0.05), mainly for the treatment 9min of sonication. The lipid (peroxide value and TBARS) and protein (thiol group) oxidative reactions were accelerated by US (P<0.05) and they should be considered to maintain Italian salami quality. Sonication contributed to maintenance of THP (P<0.05), especially during storage. MMb pigment was not affected by sonication (P>0.05). This study presented some features of US application that could be explored in the manufacture of Italian salami.

Stress response of some lactic acid bacteria isolated from Romanian artisan dairy products.

Understanding the mechanisms of stress response and adaptation to stress in the case of lactic acid bacteria (LAB), especially in the case of strains with functional properties, is very important when such strains are potential candidates for starter cultures or probiotics. In this context, our study shows the response of some LAB [four exopolysaccharide (EPS)-producing strains and one strain with potential probiotic effect] to the stresses induced by low and high incubation temperatures, acidity, NaCl, and bile salts, often encountered during the technological processes in food or during the passage through the human gastro-intestinal tract. The strains were able to grow at temperatures up to 40 °C (the mesophilic strains) and 47 °C (the thermophilic strain), in medium with an initial pH of at least 4.0 (Lactobacillus acidophilus IBB801), or in the presence of NaCl up to 10% (Weissella confusa/cibaria 38.2), or bile salts up to 0.2% (L. acidophilus IBB801). The protein and isoenzyme patterns of the strains subjected to various stress conditions presented several differences compared with the control patterns, among which the overexpression of some proteins of about 50-60 kDa, differences in the bands intensity in the case of the intracellular enzymes, or the complete loss of some of these bands. The best survival to low pH values and high temperatures was observed for strain L. acidophilus IBB801, the candidate probiotic strain. The EPS production of the four tested strains was, in general, directly related to the growth, the highest yields being obtained when strains were incubated at 24 °C.

A comparison of fluorescent stains for the assessment of viability and metabolic activity of lactic acid bacteria.

Lactic acid bacteria (LAB) are used as starter or probiotic cultures in the food and pharmaceutical industries and, therefore, rapid and accurate methods for the detection of their viability are of practical relevance. In this study 10 LAB strains, belonging to the genera Enterococcus, Lactococcus, Leuconostoc, Lactobacillus, Streptococcus and Weissella, were subjected to heat and oxidative stresses and cell injury or death was assessed comparing different fluorescent probes (Syto 9; Propidium Iodide, PI; 4,6-diamidino-2-phenylindole, DAPI; 5,(6)-carboxyfluorescein diacetate, cFDA) to identify the stain combination which most reliably allowed the detection of live/metabolically active and dead cells. Protocols for specimen preparation and staining were optimized and a simple procedure for automated cell counts was developed using NIH ImageJ macros. Cysteine and semi-solid agar solution were efficiently used as anti-fading agent and mounting medium, respectively. The double staining cFDA-PI apparently offered the best and most versatile indication of both cell metabolic activity and membrane integrity. An excellent correlation between manual and automated cell counts for the majority of strain/stain combinations was found. This work provides a simple protocol for specimen preparation and staining based on the use of safe, easy to prepare and inexpensive reagents as compared to other methods. Additionally, the automated cell count procedure developed can be applied to several bacterial species and allows an increase in the number of experimental trials and the reproducibility and sensitivity of the analysis.

Shelf life and sensory characteristics of baby spinach subjected to electron beam irradiation.

The use of ionizing radiation for the control of foodborne pathogens and extending the shelf life of fresh iceberg lettuce and fresh spinach has recently been approved by the U.S. Food and Drug Administration. The efficacy of electron beam irradiation for controlling foodborne pathogens has been reported. For this experiment, the effectiveness of electron beam irradiation on the microbiological and sensory characteristics of fresh spinach was studied. Total aerobic plate counts were reduced by 2.6 and 3.2 log CFU/g at 0.7 and 1.4 kGy, respectively. Lactic acid bacteria were reduced at both doses of e-beam but grew slowly over the 35 d of the experiment. Yeasts and molds were not reduced in samples exposed to 0.7 kGy whereas 1.4 kGy significantly reduced microbial counts. Gas compositions (O(2) and CO(2)) were significantly different than controls. Oxygen levels inside the spinach sample bags decreased over time; however, O(2) levels did not drop below 1% that can induce anaerobic fermentation. CO(2) levels for all treatments increased through day 4; yet 7 d after irradiation, CO(2) level differences were not significant in both control and irradiated samples. Irradiation dose did not affect the basic tastes, aromatics, or mouth feels of fresh spinach, however; hardness attributes decreased as irradiated dose increased and slimy attributes of fresh spinach were higher in control samples compared to irradiated samples.