Growth of food-borne pathogenic bacteria in oil-in-water emulsions: II—Effect of emulsion structure on growth parameters and form of growth

The growth rates and yields of Listeria monocytogenes and Yersinia enterocolitica were determined in liquid culture media, and in model oil-in-water emulsions that contained 30, 70 or 83% (v/v) hexadecane. In emulsions with a mean droplet size of 2 μm containing 83% (v/v) hexadecane, the growth of both organisms resulted in decreased yields. Additionally, in these emulsions adjusted to pH 5·0 or 4·4 the growth rate of L. monocytogenes was significantly less than in other model systems which had an aqueous phase of equivalent chemical composition. Microscopic examination of the 83% (v/v) emulsion showed that its microstructure immobilized the bacteria, which were constrained to grow as colonies. Bacteria behaved similarly in model emulsions of either hexadecane or sunflower oil. Manipulation of the droplet size distribution of the emulsions changed the form and rate of growth of bacteria within them.     

Antimicrobial activity of extracts of the lichen Parmelia sulcata and its salazinic acid constituent

The antimicrobial activity of the acetone, chloroform, diethyl ether, methanol, and petroleum ether extracts of the lichen Parmelia sulcata and its salazinic acid constituent have been screened against twenty eight food-borne bacteria and fungi. All of the extracts with the exception of the petroleum ether extract showed antimicrobial activity against Aeromonas hydrophila, Bacillus cereus, Bacillus subtilis, Listeria monocytogenes, Proteus vulgaris, Yersinia enterocolitica, Staphylococcus aureus, Streptococcus faecalis, Candida albicans, Candida glabrata, Aspergillus niger, Aspergillus fumigatus, and Penicillium notatum. Salazinic acid did not show antimicrobial activity against L. monocytogenes, P. vulgaris, Y. enterocolitica, and S. faecalis but showed activity against Pseudomonas aeruginosa and Salmonella typhimurium as well. The MIC values of the extracts and the acid for the bacteria and fungi have also been determined.     

Emulsifying behaviour of gum arabic. Part 1: Effect of the nature of the oil phase on the emulsion droplet-size distribution

The influence of the nature of the oil phase on the emulsifying behaviour of gum arabic has been investigated at neutral pH. Time-dependent droplet-size distributions are reported for oil-in-water emulsions (1% wt gum, 10% vol. oil) made with n-hexadecane, -limonene and orange oil. Three different gum samples of known analytical composition have been compared, and it is found that the gum giving the most rapid lowering of the tension at the n-hexadecane-water interface also gives the most stable n-hexadecane-in-water emulsions as well as the smallest droplets with all three oils. On the other hand, the same gum gives the poorest stability of the -limonene-in-water and orange oil-in-water emulsions.     

Antimicrobial activity of extracts of the lichen Xanthoparmelia pokornyi and its gyrophoric and stenosporic acid constituents

The antimicrobial activity of the diethyl ether, acetone, chloroform, petroleum ether, and ethanol extracts of the lichen Xanthoparmelia pokornyi and its gyrophoric acid and stenosporic acid constituents has been screened against some foodborne bacteria and fungi. Both the extracts and the acids showed antimicrobial activity against Aeromonas hydrophila, Bacillus cereus, Bacillus subtilis, Listeria monocytogenes, Proteus vulgaris, Staphylococcus aureus, Streptococcus faecalis, Yersinia enterocolitica, Candida albicans and Candida glabrata. The extracts were inactive against the tested filamentous fungi. The MIC values of the extracts and the acids for the bacteria have also been determined.     

Bacteria-induced de-emulsification of oil-in-water petroleum emulsions

Summary Nocardia amarae has been shown to be a potent biological de-emulsifier of model oil-in-water emulsions and petroleum water-in-oil field emulsions. N. amarae is reported here to de-emulsify petroleum oil-in-water field emulsions. Parameters affecting the growth and the de-emulsifying ability of this bacterium have been examined. Bacteria-induced de-emulsifying activity could be useful in clarifying tailings ponds and sludges.     

YdfH Identified as a Repressor of rspA by the Use of Reduced Genome Escherichia coli MGF-01

The effects of carnauba wax addition on the physical state of palm kernel oil-in-water emulsions were investigated. The oil-in-water emulsion (40 wt% oil + 60 wt% aqueous phase) kept the liquid state at 25°C irrespective of the presence or absence of carnauba wax in the oil phase. The emulsion containing the wax transformed from the liquid state to the solid state by shearing after storage for 20 h at 4°C, although the liquid-solid transition was not observed for the emulsion not containing the wax upon the same treatment. The viscoelasticity of the solid emulsions was demonstrated by small-deformation mechanical testing. Analysis of flow behavior of the emulsions showed that the change in physical properties of the emulsion containing the wax at 4°C was caused by the shearing at a low shear rate, around 50 s^?1–100 s^?1. According to the transition from the liquid state to the solid state of the emulsion containing the wax, the aggregation of oil droplets was found to occur to a large extent. The results of differential scanning calorimetry and surface pressure–surface area isotherms suggested that triglyceride molecules of palm kernel oil were more oriented at the oil–water interfaces in the emulsions after the wax addition. Based on these results, it is thought that carnauba wax is important in destabilization of palm kernel oil-in-water emulsions by modifying the physical state of the oil triglyceride molecules at the interfaces.     

Formation of Fine Oil-in-Water Emulsions by the Gel Emulsification Method with Saccharide and Protein

An emulsification method using a gel-like phase of a saccharide and protein mixture has been developed. In the method, which is called a gel emulsification method, an oil is added to the highly concentrated saccharide solution containing protein to form a clear gel-like phase, which followed by dilution with water to form a fine oil-in-water emulsion. This emulsion was investigated as to its emulsifying activity and emulsion stability as compared with that obtained by high-shear equipment, which was called a homomixer method. The emulsifying activity of the emulsions prepared by the gel emulsification method was much higher than that of the emulsions prepared by the homomixer method.

The emulsions prepared by both methods were highly stable in terms of the stability against coalescence. On the other hand, the stability against creaming of the emulsions prepared by the gel emulsification method was much higher than that of the emulsions prepared by the homomixer method.

The surface hydrophobicity of the protein and the unfreezable water content in the highly concentrated saccharide solution containing protein were not correlated to the emulsifying properties of the emulsions prepared by the gel emulsification method, which appeared to be dependent on the viscosity of the highly concentrated saccharide solution containing protein.     

A novel procedure for preparation of submicron liposomes-lyophilization of oil-in-water emulsions

A novel liposome preparation method is described involving the freeze-drying of oil-in-water emulsions containing sucrose in the aqueous phase as a lyoprotectant and phospholipids in the oil as emulsifiers. The oil-in-water emulsions can be prepared by emulsification and then lyophilized to obtain dry products that will, upon rehydration, form unilamellar liposomes with a mean size of less than 200 nm, as proved by dynamic light scattering, atomic force micrographs, freeze-fracture electron micrographs, and small-angle X-ray scattering (SAXS). With or without combination with active loading, this method can be used successfully to encapsulate amphiphilic or lipophilic drugs into liposomes, although it cannot be applied to hydrophilic drugs. The lyophilized products are stable and can be rehydrated to form liposomes, even after a storage period of 10 months. Based on the encapsulation efficiency for hydrophilic drugs, as well as the scanning electron micrograph and SAXS of the freeze-dried products, a unique liposome formation mechanism is proposed.     

Screening antimicrobial activity of various extracts of Artemisia dracunculus L

The antimicrobial activities of chloroform, acetone and two different concentrations of methanol extracts of Artemisia dracunculus L. were studied. These extracts were tested against nine bacteria and four yeasts strains by the disc diffusion method. The results indicated that the methanol extract of A. dracunculus is more effective against tested microorganisms than chloroform or acetone extracts. The chloroform and acetone extracts were inhibitory only towards Pseudomonas aeruginosa (ATCC 27853). While the methanol extract that was diluted with 10 ml distilled water showed inhibition zones against Shigella (RSHI), Listeria monocytogenes ATCC 7644, P. aeruginosa (ATCC 27853), the methanol extract that was diluted with 5 ml distilled water showed inhibition zones against two different strains of Escherichia coli (RSHI, ATCC 25922), Shigella (RSHI), L. monocytogenes (ATCC 7644), and P. aeruginosa ATCC 27853. The cells of microorganisms treated with plant extracts and normal microorganism cells were observed by scanning electron microscope. It was apparent that cells are damaged after treatment with A. dracunculus.     

Growth rate and growth probability of Listeria monocytogenes in dairy, meat and seafood products in suboptimal conditions

AIMS: To evaluate the performances of models predicting the growth rate or the growth probability of Listeria monocytogenes in food. METHODS AND RESULTS: Cardinal and square root type models including or not interactions between environmental factors and probability models were evaluated for their ability to describe the behaviour of L. monocytogenes in liquid dairy products, cheese, meat and seafood products. Models excluding interactions seemed sufficient to predict the growth rate of L. monocytogenes. However, the accurate prediction of growth/no-growth limits needed to take interactions into account. A complete and a simplified form (preservatives deducted) of a new cardinal model including interactions and parameter values were suggested to predict confidence limits for the growth rate of L. monocytogenes in food. This model could also be used for the growth probability prediction. CONCLUSIONS: The new cardinal model including interactions was efficient to predict confidence limits for the growth rate of L. monocytogenes and its growth probability in liquid dairy products, meat and seafood products. In cheese, the model was efficient to predict the absence of growth of the pathogen. SIGNIFICANCE AND IMPACT OF THE STUDY: The suggested model can be used for risk assessment and risk management concerning L. monocytogenes in dairy, meat and seafood products.     

A novel fiber-optic photometer for in situ stability assessment of concentrated oil-in-water emulsions

The purpose of this research was to evaluate a novel fiberoptic photometer for its ability to monitor physical instabilities occurring in concentrated emulsions during storage. For this, the fiber-optic photometer was used to measure transmission of oil-in-water emulsions stabilized with hypromellose (HPMC) as a function of oil volume fraction and droplet size distribution (DSD). To detect physical instabilities like creaming and coalescence, the transmissivity of the samples was studied at 2 different hight levels over a certain period of time. The corresponding droplet size distributions were determined by laser diffraction with PIDS. Transmissivity was found to depend on the number of dispersed droplets and thus is sensitive to both the variation of phase volume fraction as well as the emulsions droplet size distribution. At constant DSD, light transmission decreased linearly with increasing oil content within a large interval of phase volume fractions from 0.01 to 0.3. At constant phase volume fraction, an increase in droplet size increased light transmission. Investigation of creaming on emulsions with different droplet size distributions showed changes in the initial delay times and creaming velocities. In contrast to creaming phenomenon coalescence can be identified by height independent changes of the transmissivity. In conclusion, transmissivity of oil-in-water emulsions observed by the novel fiber-optic photometer is sensitive to phase volume fraction, droplet size distribution, and thus can be used as a tool for stability studies on concentrated emulsions. Published: August 31, 2007     

Improvement of the detection of Listeria monocytogenes by the application of ALOA, a diagnostic, chromogenic isolation medium

A new selective agar medium, ALOA, for the selective and differential isolation of Listeria monocytogenes has been evaluated. All stressed cultures of L. monocytogenes serovars tested grew on the medium as bluish colonies surrounded by a distinctive opaque halo and gave a productivity ratio of at least 0.95. Non-pathogenic Listeria sp. produced bluish colonies without a halo as was also the case for some enterococci and bacilli. Special attention must be paid to some Bacillus cereus strains and L. ivanovii since their colony appearance can be misleading. Only some unidentified listeria-like bacteria gave false-positive results. ALOA detected 4. 3% more positives from naturally contaminated dairy and meat samples compared with the ISO procedure when used with GenprobeTM or VidasTM for confirmation of presumptive colonies; 13.9% false negatives were found compared with 38.9% using PALCAM/Oxford. ALOA was also clearly superior to Oxford and PALCAM when samples containing both L. monocytogenes and L. innocua were examined. The introduction of ALOA in standard isolation procedures as an additional medium would enhance the detection ratio and reduce the time and cost of analysis for L. monocytogenes.     

Growth of Listeria monocytogenes and Yersinia enterocolitica colonies under modified atmospheres at 4 and 8 degrees C using a model food system

The growth of Listeria monocytogenes and Yersinia enterocolitica colonies was studied on solid media at 4 and 8 degrees C under modified atmospheres (MAs) of 5% O2: 10% CO2: 85% N2 (MA1), 30% CO2: 70% N2 (MA2) and air (control). Colony radius, determined using computer image analysis, allowed specific growth rates (mu) and the time taken to detect bacterial colonies to be estimated, after colonies became visible. At 4 degrees C both MAs decreased the growth rates of L. monocytogenes by 1.5- and 3.0-fold under MA1 (mu = 0.02 h(-1)) and MA2 (mu = 0.01 h(-1)), respectively, as compared with the control (mu = 0.03 h(-1)). The time to detection of bacterial colonies was increased from 15 d (control) to 24 (MA1) and 29 d (MA2). At 8 degrees C MA2 decreased the growth rate by 1.5-fold (mu = 0.04 h(-1)) as compared with the control (mu = 0.06 h(-1)) and detection of colonies increased from 7 (control) to 9 d (MA2). At 4 degrees C both MAs decreased the growth rates of Y. enterocolitica by 1.5- and 2.5-fold under MA1 (mu = 0.03 h(-1)) and MA2 (mu = 0.02 h(-1)), respectively, as compared with the control (mu = 0.05 h(-1)). At 8 degrees C identical growth rates were obtained under MA1 and the control (mu = 0.07 h(-1)) whilst a decrease in the growth rate was obtained under MA2 (mu = 0.04 h(-1)). The detection of colonies varied from 6 (8 degrees C, aerobic) to 19 d (4 degrees C, MA2). Refrigerated modified atmosphere packaged foods should be maintained at 4 degrees C and below to ensure product safety.     

Heat resistance of Listeria monocytogenes in dairy products as affected by the growth medium

Listeria monocytogenes strains 1151 and Scott A were grown in broth at 30 °C and transferred to half cream, double cream and butter stored at 5 °C to determine the influence of dairy product composition on heat resistance at 52, 56, 60, 64 and 68 °C. Strain 1151 showed a higher heat resistance than strain Scott A. The heat resistance of both strains was higher in the dairy products than in broth, particularly at lower temperatures. A significant difference was observed between log 10 of the D -values in the different dairy products. The D -values obtained for both strains resuspended in all the dairy products would result in efficient elimination of the pathogen at 72·7 °C for 15 s. The highest D -value was 11·30 s at 68 °C and by using a z -value of 6·71 °C it can be determined that at 72·7 °C the D -value would be 1·5 s. The 15 s process would therefore achieve 10 log reductions. The effect of growth conditions on the heat resistance at 60 °C of L. monocytogenes Scott A was also investigated. When the cells were grown in the dairy products themselves, and particularly butter, the heat resistance of Scott A was enhanced; for example, the D -values were 7·15 times higher than in broth. Further studies are required to investigate if this protection against heating exists at higher temperatures, in which case the efficiency of pasteurization treatments or other heat treatments would be considerably lowered.     

De-emulsification of oil-in-water emulsions by Bacillus subtilis

Bacillus subtilis grown on glucose produced a de-emulsifier that broke down model oil-in-water emulsions. The deemulsification increased with contact time and the recovery of oil was 98% with 0.6 mg de-emulsifier/ml emulsion. GLC and i.r. analysis of the molecule extracted from cell-free broth demonstrated that the de-emulsifier was acetoin. It may be better to use acetoin, a metabolite of glucose metabolism, to recover oil from oily sludges instead of microbial cells, because of its stability and de-emulsification activity.The authors are with the Director's Research Division, National Environmental Engineering Research Institute, Nehru Marg, Nagpur 440 020, India     

A new chromogenic agar medium for detection of potentially virulent Yersinia enterocolitica

Several outbreaks of foodborne yersiniosis have been documented and this disease continues to be source of infections transmitted through foods. The selective agars most commonly used to isolate Yersinia enterocolitica in clinical, food and environmental samples, cefsulodin-irgasan-novobiocin (CIN) and MacConkey (MAC) agars, lack the ability to differentiate potentially virulent Y. enterocolitica from other Yersinia that may be present as well as some other bacterial spp. This study proposes the use of an agar medium, Y. enterocolitica chromogenic medium (YeCM), for isolation of potentially virulent Y. enterocolitica. This agar contains cellobiose as the fermentable sugar, a chromogenic substrate and selective inhibitors for suppression of colony formation by many competing bacteria. All strains of potentially virulent Yersinia of biotypes 1B, and biotypes 2-5 formed convex, red bulls-eye colonies on YeCM that were very similar to those described for CIN agar. However, Y. enterocolitica biotype 1A and other related Yersinia formed colonies that were purple/blue on YeCM while they formed typical red bulls-eye colonies on CIN agar. When a mixture of potentially virulent Y. enterocolitica biotype 1B, Y. enterocolitica biotype 1A and 5 other bacterial species was used to artificially contaminate tofu and then spread-plated on three selective agars, Y. enterocolitica biotype 1B colonies were easily distinguished from other strains on YeCM. However, Y. enterocolitica biotype 1B colonies were indistinguishable from many other colonies on CIN and only distinguishable from those of C. freundii on MAC. When colonies were picked and identified from these agars, typical colonies from YeCM were confirmed only as Y. enterocolitica biotype 1B. Typical colonies on CIN and MAC were found to belong to several competing species and biotypes.     

Rheology and Stability of Beverage Emulsions in the Presence and Absence of Weighting Agents: A Review

Cloudiness or opacity (cloudy appearance) is an important property in citrus beverages, since it enhances their juice-like appearance and gives it a natural fruit juice appeal. This property is achievable through the addition of oil-in-water emulsions known as clouding agents. These emulsions are thermodynamically unstable and tend to break down during storage. Moreover, product and legal constraints put severe limits on materials that can be used to insure emulsion stability, particularly the introduction of weighting agents into the oil phase. Weighing agents (density-adjusting agents) are lipophilic compounds with specific gravity higher than that of water and have a restricted use because of the perceived health risk disadvantage, undesirable taste, and oxidative instability. The stability of beverage emulsions is a problem of serious concern faced by the flavor and beverage industry. This paper provides an overview of research carried out by the authors on basic factors affecting the physical stability of beverage cloud emulsions having a bearing on droplet size/distribution, rheological properties of emulsion and phases components, and the stability of emulsion in concentrated and diluted forms with or without addition of weighting agents. Delivery of Functionality in Complex Food Systems: Physically-Inspired Approaches From Nanoscale To Microscale University of Massachusetts, Amherst, October 8–10, 2007     

Liposome-stabilized oil-in-water emulsions as adjuvants: increased emulsion stability promotes induction of cytotoxic T lymphocytes against an HIV envelope antigen

Protective or therapeutic immunity against HIV infection is currently believed to require both antibody and CTL responses against the envelope protein. In the present study, the adjuvant activity of a unique oil-in-water emulsion, in which liposomes containing lipid A (LA) and encapsulated antigen served as the emulsifying agent, was examined in mice using oligomeric gp140 (ogp140) derived from the HIV-1 envelope as the antigen. Emulsions rendered either highly stable or unstable by altering the ratio of liposomes to oil were used to examine the effect of stability of the emulsion on adjuvant activity. Stable and unstable emulsions had similar potencies for inducing both IgG antibodies to ogp140 and antigen-specific T-lymphocyte proliferation. Stable emulsions, but not unstable emulsions, induced antigen-specific CTL responses, possibly because of the depot effect of the stable emulsions. Furthermore, stable emulsions induced lower IgG2a/IgG1 ratios than the unstable emulsions. We conclude that stable liposomal oil-in-water emulsions provide an effective means of obtaining both antibody and CTL responses against an HIV envelope antigen.     

An improved selective isolation medium for the recovery of Listeria monocytogenes from smoked fish

AIMS: The aim of this study was to improve the selective isolation of Listeria monocytogenes from smoked haddock fillets. METHODS AND RESULTS: Listeria selective agar (LSA)--Oxford formulation was supplemented with 25 microg x ml(-1) of colistin sulphate and 30 microg x ml(-1) of nalidixic acid. Inocula from four smoked haddock fillets produced colonies (approx. 2-13 bacteria x g(-1)), identified as L. monocytogenes, on LSA supplemented with antimicrobial compounds (MLSA). Moreover, there was only negligible evidence of bacteria which were not L. monocytogenes on MLSA. In contrast, LSA supported dense bacterial growth, which was not equated with L. monocytogenes. SIGNIFICANCE AND IMPACT OF THE STUDY: The modified medium permitted the recovery of L. monocytogenes from smoked haddock fillets and reduced the growth of contaminating bacteria.