Effects of the indigenous microflora of minimally processed lettuce on the survival and growth of Listeria innocua

Interactions between the natural background microflora of shredded lettuce and Listeria innocua (in lieu of Listeria monocytogenes ) were studied. The effect of increasing the initial size of indigenous populations (from 10³ to 10⁶–10⁷ CFU g⁻¹) was tested for its ability to reduce L. innocua growth on shredded lettuce. Co-culture experiments were performed in model media, where bacterial isolates from the indigenous microflora were tested for possible inhibitory effects. Varying the size of the indigenous populations had no effect on L. innocua survival or growth. However, interactions with individual species and mixed populations from lettuce did affect the survival and growth of L. innocua in model media. In general, mixed populations diminished L. innocua growth. In the undiluted lettuce medium, the various species tested individually either reduced or did not affect the growth of L. innocua . However, when the medium was diluted, some species extended the survival of L. innocua . Competition between the indigenous microflora and L. innocua resided mostly with the Enterobacter spp. and not with the pseudomonads. Enterobacter cloacae was particularly effective in reducing L. innocua growth. Lactic acid bacteria also reduced L. innocua growth in undiluted media. It is concluded that interactions with the natural background microflora may play an important role in determining the dynamics of Listeria populations on shredded lettuce.     

Minimal water activity levels for growth and survival of Listeria monocytogenes and Listeria innocua.

Following initial range-finding experiments, total count determinations were used to determine minimal water activity (aW) levels for the growth and survival of Listeria monocytogenes (L.m.) and L. innocua (L.i.). Media containing three different humectants; NaCl, sucrose or glycerol were used to determine minimal aW levels for growth in the above media which were 0.92, 0.92 and 0.90 respectively. The growth minima for L.i. were similar, or slightly higher than for L.m. in these media. Survival rates were generally lower in NaCl-adjusted media than in systems adjusted with sucrose or glycerol. Survival of L.m. and L.i. in these experiments was similar.     

Effects of gas atmosphere, antimicrobial dip and temperature on the fate of Listeria innocua and Listeria monocytogenes on minimally processed lettuce

The growth and survival of inoculated strains of Listeria innocua and L. monocytogenes on minimally processed lettuce were studied. The effects of package atmospheres (lettuce sealed within packages after flushing with 100% N₂ or without flushing with N₂, lettuce sealed within perforated packages), antimicrobial dips (100 p.p.m. chlorine solution for 5 min, 1% citric acid solution for 5 min) and storage temperatures (3°C and 8°C) were investigated. Populations of L. innocua and L. monocytogenes on undipped lettuce stored at 3°C gradually decreased (by 1–1.5 log cycles) during a 14 day storage period. By contrast counts on lettuce stored at 8°C did not change significantly ( P > 0.05). Flushing packages of lettuce with 100% N₂ followed by storage at 8°C resulted in a significant increase ( P < 0.05, by 2–3 log cycles) in L. innocua and L. monocytogenes counts during storage. L. innocua , strain NCTC 11288, behaviour was similar to that of L. monocytogenes (strains ATCC, 19114 and NCTC 11994) under these storage temperatures and atmospheres. Using L. innocua as a model for L. monocytogenes , it was found that dipping lettuce in a chlorine or citric acid solution followed by storage at 8°C resulted in a significant increase ( P < 0.05, by 2 log cycles) in L. innocua populations compared with undipped samples. It is concluded that N₂ flushing or use of antimicrobial dips combined with storage at 8°C, both enhanced the survival and growth of Listeria populations on shredded lettuce.     

Characterization of Listeria monocytogenes and Listeria innocua from a vegetable processing plant by RAPD and REA

The incidence of Listeria monocytogenes in a vegetable processing plant was investigated over a 23-month period. Frozen ready-to-eat vegetable samples, well as the plant environment, were sampled. The molecular subtyping techniques, Random Amplified Polymorphic DNA (RAPD) and Restriction Endonuclease Analyses (REA), were performed to help investigate the origin and routes of Listeria dissemination.

The low and sporadic incidence of L. monocytogenes made it impossible to establish an epidemiological sequence in the processing plant, though a case of cross-contamination between tomato and ratatouille was detected. Listeria innocua subtyping, however, allowed us to determine the prevalence of several strains in vegetables, and their presence on machinery samples suggested the possibility of cross-contamination during processing.

The low incidence of L. monocytogenes indicated that the risk of listeriosis transmission by vegetable consumption is low. On the other hand, the isolation of the same strain of L. innocua in several surveys pointed out the risk of colonisation on surfaces and machinery. The persistence of Listeria spp. is a cause for concern as can lead to future contamination of vegetables processed in the plant and to a possible increased risk for health. Therefore, periodic controls for the presence of Listeria spp. and a further review of the cleaning and disinfection procedures used in frozen vegetable plants are recommended.     

Packaging Effects on Growth of Listeria innocua in Shredded Cabbage

Freshly shredded white cabbage was treated with citric acid and sodium erythorbate, inoculated with Listeria innocua (in lieu of Listeria monocytogenes), and packaged in 230g lots in four types of retail bags with oxygen transmission rates (OTRs) of 5.6, 1,500, 4,000, and 6,000 cc O₂/m²/24 hr, then stored 21 days at 11°C. After 14 days, L. Innocua decreased in cabbage stored in three films. After 21 days, Listeria population increased in all packages, but the increase was less (p<0.05), for cabbage packaged in film with the highest OTR (commercial film).     

Effects of steam and lactic acid treatments on inactivation of Listeria innocua surface-inoculated on chicken skins

Effectiveness of combined steam (10 and 60 s, 70 degrees C and 98 degrees C) and chemical treatments, using concentrated solutions of lactic acid (1 and 30 min, 5% and 10% lactic acid), on the inactivation of Listeria innocua inoculated on the surface of chicken skins have been studied. Surviving bacteria on the skin were enumerated immediately after treatment, and after 7 days of storage at 4 degrees C. The most effective treatment was the combination of steam of 98 degrees C and 10% lactic acid with its immediate efficacy being mainly attributed to the applied heat treatment. However, after 7-day storage, the treatment's effectiveness was mainly due to the applied acid treatment, which prevented growth of the bacteria that survived the heat treatment. Milder treatments (70 degrees C steam, 5% lactic acid) revealed a genuine synergy between the heat and acid treatments, paving the way for an effective means of reducing bacterial load on the surface of poultry without affecting the product's "raw" appearance.     

壳聚糖双胍盐酸盐对羊毛织物抗菌性能的影响

利用壳聚糖和双氰胺成功地合成了一种壳聚糖双胍盐酸盐(CGH),应用红外光谱确认了聚合物的结构.采用整理剂CGH,交联剂柠檬酸和催化剂次亚磷酸钠按照轧-烘-焙工艺整理羊毛织物,使其获得良好的抗菌性能.运用扫描电子显微镜SEM和傅里叶变换红外光谱图对抗菌剂CGH,交联剂柠檬酸与羊毛之间的交联机理进行了探讨.结果表明,整理后的羊毛织物具有良好的抗菌性和耐洗涤性能.     

Survival of Listeria monocytogenes, Listeria innocua, and Lactic Acid Bacteria in Chill Brines

ABSTRACT:  Listeria monocytogenes is the pathogen of concern in ready-to-eat (RTE) meat products. Salt brines are used to chill processed meats. L. monocytogenes and lactic acid bacteria (LAB) can grow under saline conditions, and may compete with each other for nutrients. The objective of this study was to determine the effect of lactic acid bacteria ( Enterococcus faecalis , Carnobacterium gallinarum , and Lactobacillus plantarum ) on the survival of L. monocytogenes and Listeria innocua in brines stored under low temperatures for 10 d. Sterile tap water (STW) and 2 brine solutions (7.9% and 13.2% NaCl) were inoculated with 1 of 5 cocktails ( L. monocytogenes , L. innocua , LAB, L. monocytogenes + LAB, or L. innocua + LAB) at initial concentrations of 7 log CFU/mL. Brines were stored for 10 d at 4 or 12 °C. Three replications of each brine concentration/cocktail/temperature combination were completed. No significant reductions of L. monocytogenes occurred in 7.9%[w/v] or 13.2%[w/v] brines when LAB were present; however, there were significant reductions after 10 d of L. monocytogenes in the STW solution when LAB were present (1.43 log CFU/mL at 4 °C and 3.02 log CFU/mL at 12 °C). L. innocua was significantly less resilient to environmental stresses of the brines than L. monocytogenes , both with and without LAB present ( P ≤ 0.05). These strains of lactic acid bacteria are not effective at reducing L. monocytogenes in brines at low temperatures. Furthermore, use of L. innocua as a model for L. monocytogenes is not appropriate under these environmental conditions.     

Effect of Combined Ozone and Organic Acid Treatment for Control of Escherichia coli O157:H7 and Listeria monocytogenes on Lettuce

ABSTRACT: This study was conducted to determine the effects of ozonated water (1, 3, and 5 ppm) alone with different exposure times (0.5,1,3, or5min), and combinations of 3 ppm ozone with 1% organic acids (acetic, citric, or lactic acids) during 1-min exposure for inactivating Escherichia coli O157:H7 and Listeria monocytogenes on lettuce and to observe the regrowth of these pathogenic bacteria on treated lettuce during storage for 10 d at 15°C. Results showed that 5 ppm ozone treatment for 5 min gave 1.09-log and 0.94-log reductions of E. coli O157:H7 and L. monocytogenes , respectively, indicating insignificant reductions compared with 3 ppm ozone treatment for 5 min. Treatment with 3 ppm ozone combined with 1 % citric acid for 1 min immersing resulted in 2.31 - and 1.84-log reductions ( P < 0.05), respectively. During storage at 15°C for 10 d after combined treatment and packaging, populations of E. coli O157:H7 and L. monocytogenes increased to approximately 9.0-log colony forming unit (CFU) /g, indicating that this treatment did not have a residual antimicrobial effect during storage. Although the storage study did not show control of these pathogens, the combined ozone-organic acid treatment was more effective in reducing population levels of these pathogens on lettuce than individual treatments.     

Application of the Weibull model to describe inactivation of Listeria monocytogenes and Escherichia coli by citric and lactic acid at different temperatures

Inactivation of Listeria monocytogenes and Escherichia coli by citric (10‐150 g L^?1) and lactic (1‐60 mL L^?1) acids at different temperatures (4, 20, 40 °C) has been investigated. Bactericidal effect of both acids was dependent on time and temperature of exposure and acid concentration. Survival curves of L. monocytogenes treated by lactic acid were concave downward and those treated by citric acid were linear. On the other hand, survival curves of E. coli treated by both organic acids were concave upward. Shape of survival curves depended on the type of acid but not on the treatment temperature. A mathematical model based on the Weibull distribution accurately described the kinetics of inactivation of both microorganisms by both acids. This model allowed quantification and comparison of the acid resistance of L. monocytogenes and E. coli. Lactic acid was more effective than citric acid and E. coli was more sensitive to both acids than L. monocytogenes. Copyright © 2006 Society of Chemical Industry     

Effects of packaging type, gas atmosphere and storage temperature on survival and growth of Listeria spp. in shredded dry coleslaw and its components

The survival and growth of Listeria populations inoculated on to dry coleslaw mix and its components were investigated, focusing on effects of storage temperatures and gas atmospheres within packaging films or storage chambers. There were few significant effects of packaging film at 3 °C, but at 8 °C the elevated CO₂/low O₂ atmospheres generated within orientated polypropylene (OPP) packages and used in controlled atmosphere chambers were inhibitory. Although two strains of Listeria monocytogenes had survival characteristics comparable with Listeria innocua, L. monocytogenes ATCC 19114 survived better at 3 °C and also in the elevated CO₂/low O₂ atmospheres within OPP at 8 °C. The effects of product components on the survival of L. innocua were linked to storage temperature. Shredded carrot reduced initial counts and at 8 °C inhibited survival of L. innocua in comparison with shredded cabbage.     

Effect of pH on the survival of Listeria innocua in calcium ascorbate solutions and on quality of fresh-cut apples

Fresh-cut apple slices were dipped in calcium ascorbate (CaA) solution at pH values ranging from 2.5 to 7.0 to inhibit browning. After treatment, the cut apples were stored at 4 and 10 degrees C for up to 21 days. Color and texture of the apples were determined on days 1, 14, and 21. In a separate experiement, the pH of CaA solution was adjusted with acetic acid to six different pH levels, and the solution was inoculated with Listeria innocua. The survival of the bacterium and the stability of CaA were determined at 0, 20, and 96 h. The cut apples maintained fresh quality when the pH of the CaA solution was above 4.5, but slight discoloration of apple slices dipped in pH 4.5 solution was observed after 14 days at 10 degrees C. At pH 5.0, the CaA dip maintained the quality of the apples at both temperatures for at least 21 days. The L. innocua population was reduced by 4 to 5 log CFU/ml at pH 4.5 after 96 h. At pH 5, the bacterial population in the CaA solution was reduced by approximately 2 log CFU/ml during the same period. The CaA solution was stable at pH 5 for at least 96 h. Reduction of the pH to between 4.5 and 5.0 might reduce the risk of foodborne illness due to consumption of fresh-cut apples treated with a CaA solution contaminated with Listeria.     

Antimicrobial activity of essential oils on Salmonella enteritidis, Escherichia coli, and Listeria innocua in fruit juices

The antimicrobial properties of essential oils (EOs) and their derivatives have been known for years. However, the information published about the minimal effective concentration of EOs against microorganisms in fruit juices is scarce. In this study, both MIC and MBC of six EOs (lemongrass, cinnamon, geraniol, palmarosa, or benzaldehyde) against Salmonella Enteritidis, Escherichia coli, and Listeria innocua were determined by the agar and broth dilution methods, respectively. All of the six EOs inhibited the microbial (Salmonella Enteritidis, E. coli, and L. innocua) growth at a concentration from 1 microl/ ml (MIC). These studies led to choosing the three most effective EOs. Lemongrass, cinnamon, and geraniol were found to be most effective in inhibiting the growth of the microorganisms and thus were used for the MBC analysis. On this last point, significant differences (P < 0.05) among EOs, their concentrations, and culture media (apple, pear, and melon juices, or tryptone soy broth medium) were found after comparing the results on MBC for each microorganism. A concentration of 2 microl/ml from lemongrass, cinnamon, or geraniol was enough to inactivate Salmonella Enteritidis, E. coli, and L. innocua in apple and pear juices. However, in melon juice and tryptone soy broth medium, concentrations of 8 and 10 microl/ml from cinnamon, respectively, or 6 microl/ml from geraniol were necessary to eliminate the three microorganisms, whereas lemongrass required only 5 micro/ml to inactivate them. These results suggest that EOs represent a good alternative to eliminate microorganisms that can be a hazard for the consumer in unpasteurized fruit juices. The present study contributes to the knowledge of MBC of EOs against pathogenic bacteria on fruit juices.     

柠檬酸和壳聚糖对棉织物抗菌整理的研究

针对柠檬酸和壳聚糖在棉织物抗菌整理中的应用,研究了壳聚糖的浓度对棉织物抗菌性的影响,多元羧酸的含量对壳聚糖固着率的影响及焙烘温度对壳聚糖固着率和白度的影响;并对整理后棉织物的性能进行了测试分析。结果表明,用柠檬酸和壳聚糖的混合液对棉织物进行抗菌整理的效果是明显的,整理后棉织物的耐久定型和折皱回复率都有提高,而且经反复洗涤后,其抗菌性仍达到80%以上。     

Effects of gallotannin treatment on attachment, growth, and survival of Escherichia coli O157:H7 and Listeria monocytogenes on spinach and lettuce

Food-borne illness outbreaks are increasingly associated with fresh produce. Their high prevalence may reflect the lack of methods to effectively remove pathogenic bacteria from the surface of fruits and vegetables. This study evaluated the effect of antimicrobial gallotannins on attachment, growth, and survival of food-borne pathogens on green leafy vegetables. Spinach leaves and interior leaves of lettuce harboring high and low cell counts of background microbiota, respectively, were washed with tap water with and without added gallotannins. To account for the variability among organisms, green leafy vegetables were inoculated with strain cocktails of Escherichia coli O157:H7 and Listeria monocytogenes. Cell counts of L. monocytogenes were significantly reduced by the gallotannin treatment. Lower cell counts after storage for 8?days at 4?°C demonstrated antimicrobial effects of gallotannins retained on the surface of green leafy vegetables. Gallotannin treatments with 1?g/L did not inhibit E. coli O157:H7 but hindered their attachment to filter paper by up to 94?%. The addition of gallotannin-containing extracts from mango (Mangifera indica L.) kernels to the washing water did neither alter color nor texture of bagged fresh-cut products. In conclusion, gallotannin treatment significantly reduced surface contamination of green leafy vegetables with L. monocytogenes and reduced the attachment of cells of E. coli O157:H7.     

Difference between Escherichia coli O157:H7 and non-pathogenic E. coli: survival and growth in seasonings

We examined the survival and growth of Escherichia coli O157:H7 cells incubated with several seasonings, in comparison with those of non-pathogenic E. coli. The cells were incubated at 25 degrees C for 24 h with several concentrations of NaCl, sucrose, soy sauce, worcester sauce and tomato ketchup, and their survival ratios were determined. The E. coli O157:H7 strains showed relatively higher survival ratios in 0.5-1.0 M sucrose, 25% soy sauce and 12.5-50% worcester sauce than the non-pathogenic strains, but slightly lower survival ratios in 0.5-2.0 M NaCl. A noteworthy difference between E. coli O157:H7 and the non-pathogenic strains was that incubation in the presence of 12.5% soy sauce allowed the growth of E. coli O157:H7 strains but reduced the viable cell numbers of non-pathogenic E. coli strains.     

Nisin和柠檬酸对鲜切皇冠梨储藏期间单增李斯特菌生长的影响

分别采用0.05mg/m L乳酸链球菌素(Nisin)、0.3%柠檬酸复配溶液和自来水清洗接种单增李斯特菌(LM)的鲜切皇冠梨,将清洗后的鲜切皇冠梨分别于4、12、24℃的条件下储藏,以未清洗的样品为对照,测定储藏过程中LM的生长情况。结果显示Nisin和柠檬酸清洗能显著降低(p<0.05)鲜切皇冠梨中LM的初始带菌量。在4℃储藏过程中,Nisin和柠檬酸清洗的鲜切皇冠梨中LM数量没有明显变化(p>0.05),自来水清洗和未清洗的鲜切样品中LM数量略有升高。在12℃和24℃储藏期间,三种处理方式的鲜切皇冠梨LM数量均显著上升(p<0.05)。Nisin和柠檬酸复配清洗处理结合低温储藏能有效控制鲜切皇冠梨中LM的生长。      

Inactivation of Listeria innocua on Frankfurters by Ultraviolet Light and Flash Pasteurization

ABSTRACT:  Listeria monocytogenes , a psychrotrophic foodborne pathogen, is a recurring postprocess contaminant on ready-to-eat meat (RTE) products including frankfurters. Flash (Steam) Pasteurization (FP) and ultraviolet light (254 nm-UVC) has been shown to reduce levels of L. monocytogenes and L. innocua on frankfurters. In this study, the use of UVC light followed by FP to inactivate L. innocua, a nonpathogenic surrogate for L.monocytogenes , on frankfurters that contained sodium diacetate and potassium lactate (SDA/PL) in a pilot-plant setting was investigated. Application of UVC (1.0 J/cm²), followed by FP (0.75 s steam/121 °C) resulted in inactivation of 3.19 log L. innocua , while application of UVC (4.0 J/cm²), followed by FP (3 s steam/121 °C) resulted in inactivation of 3.89 log of L. innocua . A refrigerated storage study (8 °C) of frankfurters that contained SDA/PL that were treated with UVC followed by FP revealed the growth of L. innocua was inhibited for approximately 8 wk following application of the interventions. The use of UVC in combination with FP had little effect on frankfurter color and texture. The combination of UVC, FP, and SDA/PL was found to be an effective hurdle process for decontamination of frankfurter surfaces.     

Effects of addition of malic or citric acids on fermentation quality and chemical characteristics of alfalfa silage

We studied the effects on alfalfa preservation and chemical composition of the addition of different levels of malic acid and citric acid at ensiling as well as the utilization efficiency of these 2 organic acids after fermentation. Alfalfa was harvested at early bloom stage. After wilting to a dry matter content of approximately 40%, the alfalfa was chopped into 1- to 2-cm pieces for ensiling. Four levels (0, 0.1, 0.5, and 1% of fresh weight) of malic acid or citric acid were applied to chopped alfalfa at ensiling with 4 replicates for each treatment, and the treated alfalfa forages were ensiled for 60 d in vacuum-sealed polyethylene bags (dimensions: 200 mm × 300 mm) packed with 200 to 230 g of fresh alfalfa per mini silo and an initial density of 0.534 g/cm³. The application of malic or citric acids at ensiling for 60 d led to lower silage pH than was observed in the control silage (0% of malic or citric acids). Application of the 2 organic acids led to higher lactic acid concentration in alfalfa silage than in the control silage except with the application rate of 1% of fresh weight. Silages treated with both organic acids had lower nonprotein nitrogen concentrations than the control silages, and the nonprotein nitrogen concentrations in ensiled forages decreased with the increase in malic or citric acid application rates. The application of the 2 organic acid additives led to lower saturated fatty acid proportions and higher polyunsaturated fatty acid proportions in ensiled alfalfa than in the control silage. The amount of malic and citric acids degraded during ensiling of alfalfa was 1.45 and 0.63 g, respectively. At the application rate of 0.5% of fresh weight, residues of malic acid and citric acid in alfalfa silage were 11.1 and 13.6 g/kg of dry matter. These results indicate that including malic or citric acids at the ensiling of alfalfa effectively improved silage fermentation quality, limited proteolysis, improved fatty acid composition of the ensiled forage, and could provide animals with additional feed additives proven to promote animal performance. However, when the application rate of both organic acids reached 1%, the concentration of lactic acid in silages decreased notably. Additionally, 0.5 and 1% application rates also increased the yeast count in ensiled alfalfa.