Optimization of acidified warm water treatment to improve the microbiological status and sensory quality of iceberg lettuce

The optimal conditions of washing iceberg lettuce with acidified warm water to improve the hygienic and sensory product quality were determined with the aid of a central composite design. To determine the criteria of product quality, in the course of storage for 13 days at 4 °C lettuce was subjected every 2 days to sensory evaluation. The sensory values were found to correlate with the physical measurements of color (a* value) (R²=0.83) and texture (specific energy of deformation) (R²=0.77), respectively. Color, texture and overall visual quality correlated with product acceptability (R²=0.96, 0.97, 0.96, respectively). In the experiments of series I, the effect of temperature at washing (45–50 °C), pH (4–5) and treatment time (5–10 min) were investigated and the results were used for the construction of a model to describe the effects on color and texture and to define an area of sensory product acceptance. In series II the parameters were changed to ranges of temperature (45–50 °C), pH (4.6–6) and treatment time (2–5 min), and the effect on the reduction of bacterial counts within the area of product acceptance was defined. In that area reduction rates of total counts by 0.9–2.9 log cfu/g and Enterobacteriaceae by 0.8–3.0 log cfu/g were achieved. The initial germ- reducing effect was sustained until the end of storage for 7 days at 4 °C. At 50 °C, washing for 5 min at pH 4.93, the total bacteria and Enterobacteriaceae were reduced by 2.9 and 3.7 log cfu/g, respectively. The removal of the two groups of bacteria exhibited characteristic differences. Two independent experiments confirmed the reliability of the models.     

Efficacy of slightly acidic electrolyzed water in killing or reducing Escherichia coli O157:H7 on iceberg lettuce and tomatoes under simulated food service operation conditions

Abstract: The objective of this study was to evaluate the efficacy of slightly acidic electrolyzed (SAEO) water in killing or removing Escherichia coli O157:H7 on iceberg lettuce and tomatoes by washing and chilling treatment simulating protocols used in food service kitchens. Whole lettuce leaves and tomatoes were spot‐inoculated with 100 μL of a mixture of 5 strains of E. coli O157:H7. Washing lettuce with SAEO water for 15 s reduced the pathogen by 1.4 to 1.6 log CFU/leaf, but the treatments did not completely inactivate the pathogen in the wash solution. Increasing the washing time to 30 s increased the reductions to 1.7 to 2.3 log CFU/leaf. Sequential washing in SAEO water for 15 s and then chilling in SAEO water for 15 min also increased the reductions to 2.0 to 2.4 log CFU/leaf, and no cell survived in chilling solution after treatment. Washing tomatoes with SAEO water for 8 s reduced E. coli O157:H7 by 5.4 to 6.3 log CFU/tomato. The reductions were increased to 6.6 to 7.6 log CFU/tomato by increasing the washing time to 15 s. Results suggested that application of SAEO water to wash and chill lettuce and tomatoes in food service kitchens could minimize cross‐contamination and reduce the risk of E. coli O157:H7 present on the produce. Practical Application: SAEO water is equally or slightly better than acidic electrolyzed (AEO) water for inactivation of bacteria on lettuce and tomato surfaces. In addition, SAEO water may have the advantages over AEO water on its stability, no chlorine smell, and low corrosiveness. Therefore, SAEO water may have potential for produce wash to enhance food safety.     

Chlorine stabilizer T-128 enhances efficacy of chlorine against cross-contamination by E. coli O157:H7 and Salmonella in fresh-cut lettuce processing

During fresh-cut produce processing, organic materials released from cut tissues can rapidly react with free chlorine in the wash solution, leading to the potential survival of foodborne bacterial pathogens, and cross-contamination when the free chlorine is depleted. A reported chlorine stabilizer, T-128, has been developed to address this problem. In this study, we evaluated the ability of T-128 to stabilize free chlorine in wash solutions in the presence of high organic loads generated by the addition of lettuce extract or soil. Under conditions used in this study, T-128 significantly (P<0.001) decreased the rate of free chlorine depletion at the presence of soil. T-128 also slightly decreased the rate of free chlorine depletion caused by the addition of lettuce extract in wash solution. Application of T-128 significantly reduced the survival of bacterial pathogens in wash solutions with high organic loads and significantly reduced the potential of cross-contamination, when contaminated and uncontaminated produce were washed together. However, T-128 did not enhance the efficacy of chlorinated wash solutions for microbial reduction on contaminated iceberg lettuce. Evaluation of several produce quality parameters, including overall visual appearance, package headspace O2 and CO2 composition, and lettuce electrolyte leakage, during 15 d of storage indicated that iceberg lettuce quality and shelf life were not negatively impacted by washing fresh-cut lettuce in chlorine solutions containing 0.1% T-128. PRACTICAL APPLICATION: Reported chlorine stabilizer is shown to enhance chlorine efficacy against potential bacterial cross-contamination in the presence of high organic loads without compromising product quality and shelf life.     

Switchgrass extractives to mitigate Escherichia coli O157:H7 and Salmonella enterica serovar Typhimurium contamination of romaine lettuce at pre- and postharvest

The antimicrobial potential of switchgrass extractives (SE) was evaluated on cut lettuce leaves and romaine lettuce in planta, using rifampicin-resistant Escherichia coli O157:H7 and Salmonella Typhimurium strain LT2 as model pathogens. Cut lettuce leaves were swabbed with E. coli O157:H7 or S. Typhimurium followed by surface treatment with 0.8% SE, 0.6% sodium hypochlorite, or water for 1 to 45 min. For in planta studies, SE was swabbed on demarcated leaf surfaces either prior to or after inoculation of greenhouse-grown lettuce with E. coli O157:H7 or S. Typhimurium; the leaf samples were collected after 0, 24, and 48 h of treatment. Bacteria from inoculated leaves were enumerated on tryptic soy agar plates (and also on MacConkey's and XLT4 agar plates), and the recovered counts were statistically analyzed. Cut lettuce leaves showed E. coli O157:H7 reduction between 3.25 and 6.17 log CFU/leaf, whereas S. Typhimurium reductions were between 2.94 log CFU/leaf and 5.47 log CFU/leaf depending on the SE treatment durations, from initial levels of ∼7 log CFU/leaf. SE treatment of lettuce in planta, before bacterial inoculation, reduced E. coli O157:H7 and S. Typhimurium populations by 1.88 and 2.49 log CFU after 24 h and 3 h, respectively. However, SE treatment after bacterial inoculation of lettuce plants decreased E. coli O157:H7 populations by 3.04 log CFU (after 0 h) with negligible reduction of S. Typhimurium populations. Our findings demonstrate the potential of SE as a plant-based method for decontaminating E. coli O157:H7 on lettuce during pre- and postharvest stages in hurdle approaches.     

贮藏温度对鲜切莴笋品质的影响

以莴笋为原料,研究贮藏温度(2,8,25℃)对鲜切莴笋贮藏期间失重率、呼吸强度、PPO活性、ΔE值、VC含量、菌落总数和感官指标等理化指标的影响。结果表明,鲜切莴笋在2℃和8℃的低温下贮藏能有效降低失重率,抑制呼吸作用和VC损失,降低PPO活性,减轻酶促褐变,控制微生物的生长和繁殖,贮藏12d后仍具有良好的感官指标,其中2℃下贮藏保鲜效果最好。     

Antimicrobial activity of oregano oil on iceberg lettuce with different attachment conditions

In this study, the antimicrobial activity of oregano oil was investigated under different attachment conditions of Salmonella spp. to iceberg lettuce. Inoculated lettuce was either not dried or dried for 30 min, 60 min, or 120 min, under either static air or moving air. Washing iceberg lettuce with 500 ppm oregano oil for 1, 5, and 10 min reduced the population of Salmonella spp. by (respectively) 1.3, 1.65, and 2.28 log cfu/g following the most challenging inoculation conditions, an inoculum drying period of 2 h under moving air. Across all inoculation conditions, increasing the treatment time significantly increased the reductions in the populations of Salmonella spp. (P < 0.05). Browning and softening of the lettuce leaf surface was observed after 10 min of treatment with oregano oil. For each treatment time, attachment times and drying under static compared with moving air did not significantly affect the antimicrobial efficacy of the various oregano oil treatments (P > 0.05). The results obtained in this study suggest that oregano oil can effectively reduce populations of Salmonella attached to lettuce leaf surfaces. PRACTICAL APPLICATION: The use of essential oils as an antimicrobial treatment can help to ensure the safety of leafy green products. As used in this study, oregano oil effectively reduced Salmonella spp., even after the pathogen had dried onto the lettuce leaves. Treatments that incorporate oregano oil therefore hold promise as a biocide treatment for process and packaged lettuce.     

Acidified sodium chlorite optimisation assessment to improve quality of fresh-cut tatsoi baby leaves

BACKGROUND: The fresh‐cut industry commonly uses sodium hypochlorite (NaClO) for disinfection. However, there are certain problems related to its use, and acidified sodium chlorite (ASC) could be an alternative sanitiser to replace it. There is limited research evaluating the effect of ASC on the overall quality of fresh‐cut produce, especially sensory quality. In this study the decontamination efficacy and quality attribute effects of ASC on fresh‐cut tatsoi after application and during storage were investigated. RESULTS: Tatsoi baby leaves were minimally processed at 8 °C and stored under passive modified atmosphere packaging for up to 11 days at 5 and 10 °C. Low to moderate doses of ASC (100–500 mg L^?1) showed an initial antimicrobial efficacy on natural microflora and Escherichia coli as effective as that of NaClO. Regarding contact time, ASC was effective in reducting the E. coli population during the first 30 s of washing, and an increase in contact time did not improve the antimicrobial effect. Sensory quality attributes were well kept for up to 11 days at 5 °C but for only 5 days at the abusive temperature of 10 °C. CONCLUSION: ASC provides an alternative sanitising technique to NaClO for maintaining the quality and safety of fresh‐cut tatsoi baby leaves for up to 11 days at 5 °C. Copyright © 2011 Society of Chemical Industry     

Indigenous microorganisms from iceberg lettuce with adherence and antagonistic potential for use as protective culture

The antagonistic effect of the indigenous microflora on pathogens was investigated by using Salmonella Typhimurium, Staphylococcus aureus and Listeria innocua as challenge microorganisms. 18 strains strongly adherent to the leaves of iceberg lettuce were selected and their taxonomic status was determined with the aid of API NE20, BBL CRYSTAL™ E/NF systems and 16 S rDNA sequencing. In the majority of cases (15/19), the identification by biochemical testing did not agree with that of 16 S rDNA sequencing. Six strains exerted antagonistic effects on S. Typhimurium and L. innocua, when overnight cultures or cell free culture supernatants were investigated in the agar diffusion test. In vivo experiments showed that the inoculation of lettuce with P. putida LTH 5878 by dipping and pre-incubation decreased the numbers of S. Typhimurium, L. innocua and S. aureus below the level of detection (< 100 cfu/g) after storage for 7–8 d at 4 °C. The dependency of the antagonistic efficacy on the numbers of P. putida LTH 5878 was determined using point inoculation. The study of the effect of the pseudomonades/pathogen ratio on the reduction of pathogens showed that the antagonistic activity of Pseudomonas is stronger against S. Typhimurium than against L. innocua. A major reduction was achieved for the counts of L. innocua at ratios of greater than 100:1 whereas for S. Typhimurium a ratio of 0.1:1 was already effective. The sensory properties of the inoculated and stored lettuce were in general better than those of the untreated control.

Industrial relevance

The increasing importance of minimally processed ready-to-eat (RTE) vegetables has initiated many studies using surface treatments for microbial decontamination. However, most treatments such as the use of chlorine or ozone have not proven highly effective or desirable. This study is of high relevance because it evaluates the potential of endogenous microbial populations with high surface adherence properties as a competitive tool against pathogenic microorganisms. This is an intriguing concept which deserves further development because of its potential as an new biological food preservation process.     

Ozone Microbubble Treatment at Various Water Temperatures for the Removal of Residual Pesticides with Negligible Effects on the Physical Properties of Lettuce and Cherry Tomatoes

In this study, the effects of ozone microbubbles (OMCB) treatment at various water temperatures on the removal of residual fenitrothion (FT) pesticides and on the physical properties of lettuce and cherry tomatoes was examined. The residual FT percentage in lettuce and cherry tomatoes was shown decreased by using higher water temperatures, particularly at 30 °C, resulting in a decrease within the range of 32% to 52%, after the 20‐min treatment. In addition, bubbling OMCB treatment did not alter the color and pulling strength of lettuce and cherry tomatoes. These results indicate that bubbling OMCB treatment at 30 °C is a highly effective method for the removal of FT residues in lettuce and cherry tomatoes, with relatively little effect on crop quality. Practical Application : The bubbling OMCB has a potential of reducing the FT reduces on vegetables with no adverse effect on the sensory quality of vegetables.     

莴笋渗透脱水有效扩散系数研究

以葡萄糖溶液浓度(10%～40%)和温度(35～65℃)为影响因素,研究了莴笋渗透脱水的动力学过程。分别使用Azuara模型和Fick第二扩散定律计算出了平衡时刻的失水率、固形物增加率以及相应的水分和固形物有效扩散系数。设计了均匀实验,通过曲面拟合的方法得到了水分、固形物有效扩散系数与因素的回归方程。结果表明:失水率随着葡萄糖溶液浓度增加而增大,但随着温度的升高而降低;固形物增加率随着溶液浓度和温度的增加而增加。Azuara模型可用来预测失水率和固形物增加率,通过曲面拟合得到的有效扩散系数回归方程拟合性较高。有效扩散系数反映了失水率和固形物增加率达到平衡时刻的快慢程度。