Effect of packaging treatments and vacuum-cooling on quality of winter harvested iceberg lettuce

Heads of winter harvested iceberg lettuce (Lactuca sativa L. cv. Coolguard) individually packed or sealed in perforated (22 μm thickness) or unperforated (25, 30 or 40 μm thickness) polypropylene films were stored up to 2 weeks at 2°C (storage period) and then held for 2.5 days at 12°C (shelf-life period). Five packaging treatments together with passive or active modification of the atmosphere within the package were applied. Vacuum-cooling was superimposed on these five treatments. Weight loss, wilting, physiological disorders, bacterial decay and quality were monitored. In all treatments, both slight severity of pink rib and russet spotting occurred. Brown stain and heart leaf injury were only detected on heads in unperforated bags. Low oxygen was generally related to a high quality score. Passive MAP in 40 μm PP and active (initial 5% O₂ and 0% CO₂) MAP in 30 μm PP were the best treatments for overall visual quality. Vacuum-cooling reduced both pink rib and heart-leaf injury but the effect only showed up during shelf-life period.     

Evaluation of household sanitizers for reducing levels of Escherichia coli on iceberg lettuce

Diluted solutions of various household sanitizers (apple cider vinegar, white vinegar, bleach, and a reconstituted lemon juice product) were tested for their effectiveness in reducing counts of inoculated Escherichia coli and naturally present aerobic, mesophilic bacteria on lettuce. Sanitization treatments were carried out at 4 degrees C and at room temperature (ca. 21 degrees C) with and without agitation and at different exposure times (0, 1, 5, and 10 min). Of the sanitizers tested, 35% white vinegar (1.9% acetic acid) was the most effective in reducing E. coli levels (with a 5-log10 reduction after 5 min with agitation and after 10 min without agitation) and in reducing aerobic plate counts (with a >2-log10 reduction after 10 min with agitation). Lettuce samples treated with diluted household sanitizers were analyzed for consumer acceptability by sensory evaluation using a 9-point hedonic scale. The sanitized samples did not differ in acceptability (P > 0.05), except for samples treated with white vinegar. Samples treated with the white vinegar for 10 min were noticeably sour and slightly wilted in appearance. Consumer acceptability was maintained with all sanitization treatments, including those involving 35% white vinegar.     

Assessment of the microbiological quality and wash treatments of lettuce served in University restaurants

One hundred and forty-four samples of lettuce from 16 University restaurants were analyzed. The mesophilic aerobic counts of all samples ranged from 3.01 to 7.81 log10 CFU g(-1). Results of total coliforms ranged from < 0.47 to > 3.38 log10 most probable number (MPN) g(-1). Of the lettuce samples, 25.7% harbored Escherichia coli, 22.9% Staphylococcus aureus and 84% group D streptococci. Similarly, 10.4% of the samples harbored Aeromonas hydrophila, 2.8% Pseudomonas aeruginosa, and coliforms such as 14.6% Citrobacter freundii, 8.3% Klebsiella pneumoniae, 4.2% Enterobacter cloacae and 1.4% Providencia spp. Salmonella, Shigella and E. coli O157:H7 were not detected. When sodium hypochlorite or potassium permanganate solutions were used in washing procedures, the aerobic microorganisms were reduced by more than two log units, and total coliforms by at least one log.     

Growth of Listeria monocytogenes on iceberg lettuce and solid media

The growth of pathogenic bacterium Listeria monocytogenes on fresh-cut iceberg lettuce under constant temperatures was modelled in order to investigate microbial safety during distribution of this vegetable. We examined the effects of several incubation temperatures, ranging from 5 to 25 degrees C, on bacterial growth. These data were fitted to the Baranyi model and the curves showed a high correlation coefficient at all temperature (R2 > 0.95). In addition, the native bacterial flora of the lettuce did not affect the growth rate of L. monocytogenes regardless of incubation temperature. However, the lag time was reduced at a ratio of native bacteria to inoculated L. monocytogenes (100:1) at low incubation temperatures (5 and 10 degrees C). Furthermore, the maximum population density (MPD) was increased at a low ratio of native to inoculated L. monocytogenes (1:1) at all incubation temperatures. These results were compared with the previous work published by [Buchanan, R.L., Stahl, H.G., Whiting, R.C., 1989. Effects and interactions of temperature, pH, atmosphere, sodium chloride, and sodium nitrite on the growth of Listeria monocytogenes. J. Food Prot. 52, 844-851] that is being developed at the US Department of Agriculture (USDA) Agricultural Research Service's Pathogen Modelling Program (PMP). The broth-based Buchanan model for L. monocytogenes was found to markedly deviate from the observed data. In order to investigate this discrepancy, we examined the effects of medium environment and nutrient content on L. monocytogenes growth using tryptic soy agar plates (TSAP) and agar plates (AP) containing 1.7% sucrose. The inoculated bacteria on both TSAP and AP showed slower growth rates than that predicted by the PMP. The MPD of bacteria grown on TSAP was similar to the PMP model ( approximately 9 log10 CFU/ml or plate (circle of diameter of 90 mm)) regardless of the incubation temperature. By contrast, the MPD observed on AP was approximately 4 log10 CFU lower than that observed on TSAP or predicted by the PMP. Both the growth rate and the MPD of L. monocytogenes on AP were similar to those on lettuce. These results suggest that the solid medium and poor nutrient content inhibited the growth of L. monocytogenes on lettuce. The growth rates of the inoculated L. monocytogenes on all media were described using Ratkowsky's simple square root model.     

红曲菌发酵中添加紫苏叶提取物对其抗氧化活性的影响

通过将紫苏叶提取物添加到红曲菌GM075发酵培养基中进行发酵,考察了发酵产物中抗氧化活性物质Dimerumic acid (DMA)的含量变化以及发酵液离心后上清液的DPPH自由基和ABTS自由基清除活性.结果表明,添加超声法提取的紫苏叶滤液对红曲发酵产DMA的影响最为明显,使发酵液中DMA浓度达到了(3.104±0.086) g/L,与对照相比提高了46.94%;且红曲发酵液的自由基清除活性也有显著提高,对DPPH自由基清除活性的TEAC值为(1.800±0.306) mmol/L,比对照提高了157.96%,对ABTS自由基清除活性的TEAC值为(7.124±0.061) mmol/L,比对照提高了37.86%.     

结球生菜多酚氧化酶动力学特性研究

为解决生菜加工贮藏过程中出现的褐变问题,研究了结球生菜多酚氧化酶（polyphenol oxidase,PPO）动力学特性,分析了pH、温度及底物浓度等因素对PPO活性的影响,建立了结球生菜褐变的反应动力学方程。结果表明,结球生菜最适pH为8.0,最适温度为40℃。用Michaelis-Menten机理描述可得结球生菜PPO相应的动力学参数Km=0.0691mol/L,vmax=26.5957U/min。生菜叶片组织中的PPO活性较强,叶柄和茎部组织中的PPO活性差异不大,均低于叶片组织中的PPO活性,且结球生菜茎部切口处已经发生褐变的部分PPO活性为6.12U/min,远高于没有发生褐变的部分。     

Efficacy of disinfectants to reduce Listeria monocytogenes on precut iceberg lettuce

The efficacy of water, chlorinated water (100 ppm), peracetic acid solution (0.05%), and commercial citric acid-based produce wash (0.25%) to reduce the population of Listeria monocytogenes on precut lettuce was tested. Samples were inoculated with a mixture of equal amounts of five L. monocytogenes strains at a level of 4.7 log CFU/g, and analyzed on the day of washing and after 3 and 6 days of storage at 6 degrees C. Sanitizer reduced the number of L. monocytogenes at maximum 1.7 log CFU/g and number of L. monocytogenes reached the inoculation level during 6 days of storage. Thus, disinfectants do not eliminate L. monocytogenes on precut lettuce and cannot be solely relied on in producing precut lettuce safely. The inoculated L. monocytogenes strains were recovered at different rates after 6 days of storage; one of these strains was not recovered at all. Thus, strain-specific differences exist in the ability of L. monocytogenes to survive the washing treatments of the lettuce.     

Effect of ozonated water treatment on microbial control and on browning of iceberg lettuce (Lactuca sativa L.)

We examined the effect of ozonated water treatment on microbial control and quality of cut iceberg lettuce (Lactuca sativa L.). Fresh-cut lettuce was washed in ozonated water (3, 5, and 10 ppm) for 5 min at ambient temperature. The native bacterial population on the lettuce declined in response to a rise in ozone concentration. However, there was no further bacterial reduction (1.4 log CFU/g) above 5 ppm ozone. Although ozonated water treatment increased the phenylalanine ammonia lyase (PAL) activity of the lettuce stored at 10 degrees C compared with the water wash treatment after 1 day of storage, the concentration of ozone did not affect PAL activity. The a* value of the residue of the lettuce methanol extracts, which reflects the extent of browning, increased dramatically in lettuce treated with 10 ppm ozonated water compared with other treatments. Treatment with 3 or 5 ppm ozonated water resulted in more rapid changes in the a* value than after the water treatment. The combined treatment of hot water (50 degrees C, 2.5 min) followed by ozonated water (5 ppm, 2.5 min) had the same bactericidal effect as treatment with ozonated water (5 ppm, 5 min) or sodium hypochlorite (NaOCl, 200 ppm, 5 min), giving a reduction in bacteria numbers of 1.2 to 1.4 log CFU/g. The ascorbic acid content of the lettuce was not affected by these treatments. The combined treatment of hot water followed by ozonated water greatly inhibited PAL activity for up to 3 days of storage at 10 degrees C. Treatment with this combination greatly suppressed increases in the a* value, thus retarding the progress of browning compared with other treatments throughout the 6-day storage. NaOCl treatment also inhibited browning for up to 3 days of storage. Bacterial populations on the lettuce treated with sanitizers were initially reduced but then showed rapid growth compared with that of the water wash treatment, which did not reduce bacterial counts initially.     

鲜切莴苣护色处理的试验研究

研究贮藏条件及抗坏血酸、植酸和柠檬酸等护色剂对鲜切莴苣色泽变化的影响。结果表明,真空冷藏可较好地抑制酶促褐变,把鲜切莴苣片置入0.05%抗坏血酸、0.25%植酸、0.05%柠檬酸和0.5%氯化钙组成的复合护色液中,浸泡5min可有效护色,护色时间可达5d。     

The effects of washing treatment on antioxidant retention in ready-to-use iceberg lettuce

The influence of washing treatment on the retention of key antioxidants in minimally processed iceberg lettuce was examined. Shredded iceberg lettuce was subjected to one of three washing treatments: a domestic wash (tap-rinsed), immersion in distilled water (water-dipped) or immersion in chlorinated water (chlorine-dipped). Lettuce was subsequently packed in oriented polypropylene bags and flushed with 100% nitrogen and stored at 4 °C. Vitamin C (ascorbic acid and dehydroascorbic acid), total phenols, total antioxidants, individual phenols, individual carotenoids, colour measurements and pH were quantified on production day and throughout the storage period (8 days). Different groups of antioxidants were found to differ in their response to washing treatments. No significant ( P  > 0.05) effects of washing treatment were observed on total antioxidant activity, total phenolics and on the levels of individual phenolics (chicoric acid, chlorogenic acid and caffeic acid). Individual carotenoids were initially affected by washing treatment, with higher levels of lutein and beta-carotene retained with domestic washed lettuce. Ascorbic acid was the antioxidant most affected by washing treatments with domestic washing resulting in significantly ( P  ≤ 0.05) higher retention of ascorbic acid throughout the storage period compared with the chlorine-dipped lettuce.     

鲜切结球莴苣中单增李斯特菌生长预测模型的建立

为建立不同温度下鲜切结球莴苣中单增李斯特菌生长模型,将单增李斯特菌接种到鲜切结球莴苣表面,并于不同温度下贮藏,获得其在4、8、16、24和32℃下的生长数据,选用Gompertz模型进行拟合,建立初级生长模型。在此基础上建立二级模型研究温度对初级模型中单增李斯特菌生长动力学参数的影响,并进行数学检验。结果表明,对最大比生长速率和延滞时间建立平方根模型,结果呈良好的线性关系,相关系数R2分别为0.977 2和0.984 7,所建立的预测模型能很好地描述不同温度下单增李斯特菌的生长动态。     

茉莉酸甲酯处理对鲜切莴苣和甘蓝苯丙烷代谢的影响

本文以鲜切莴苣和鲜切甘蓝为实验材料,采用10μmol/L茉莉酸甲酯(以10％甲醇为溶剂)浸泡处理,研究了处理后鲜切莴苣和鲜切甘蓝苯丙烷代谢的变化.结果表明,MeJA处理可有效提高鲜切莴苣和鲜切甘蓝苯丙氨酸解氨酶(PAL)活性,同时,次生代谢产物如总酚、类黄酮以及木质素的含量也有所增加 由此可见,茉莉酸甲酯作为外界信号分子可诱导并激活苯丙烷代谢途径运行,合成多酚类化合物用以修复伤口,提高鲜切莴苣和甘蓝的自身修复能力与抗性.     

紫苏叶对萝卜乳酸发酵亚硝酸盐含量的影响

萝卜在4％的食盐浓度下进行乳酸发酵,加入紫苏叶的试验组,亚硝酸盐的最终含量低于对照组。     

不同热烫处理对马铃薯片中还原糖浓度的影响研究

以还原糖含量为评价指标,还原糖的含量通过DNS法测得,旨在研究不同热烫处理在降低马铃薯片中还原糖含量方面的作用,从而抑制切片型薯片油炸过程中致癌物质丙烯酰胺的生成。在单因素实验的基础上进行正交实验,考察热烫温度和时间两个因素在降低还原糖含量方面的作用。结果表明,热烫处理的最佳工艺条件为：热烫温度75℃、热烫时间135s。     

Sensorial and chemical quality of gamma-irradiated fresh-cut iceberg lettuce in modified atmosphere packages

A study was conducted to investigate the effects of various doses of irradiation on the quality of fresh-cut iceberg lettuce and to determine a suitable maximum dose. Fresh-cut iceberg lettuce packaged in film bags was exposed to 0, 1, 2, 3, and 4 kGy of gamma radiation and stored at 3 degrees C for 14 days. CO2 levels were higher and O2 levels were lower in packages containing irradiated lettuce than in those containing nonirradiated lettuce for most of the storage period. Comparison with nonirradiated lettuce indicated that total ascorbic acid (ascorbic acid plus dehydroascorbic acid) content and firmness were not significantly influenced by irradiation at 1 or 2 kGy. The overall visual appearance was best for lettuce irradiated at 1 or 2 kGy. This improved quality may be related to the high CO2 and low O2 levels observed for the irradiated samples. Electrolyte leakage for lettuce increased with higher radiation doses and was correlated (R2 = 0.99) with a soggy appearance. The leakage for lettuce irradiated at > or = 2 kGy was significantly more extensive than that for nonirradiated lettuce. The irradiation of fresh-cut lettuce in modified atmosphere packages at doses of 1 kGy and perhaps 2 kGy for safety enhancement and quality improvement is feasible.     

Differences in the binding of human norovirus to and from romaine lettuce and raspberries by water and electrolyzed waters

Food contamination by human norovirus (hNoV) is a major cause of gastrointestinal disease. We evaluated the effectiveness of removing inoculated hNoV from the surfaces of raspberries and romaine lettuce by a simple wash in tap water and in different forms of electrolyzed water (EW), including acidic EW (AEW), neutral EW (NEW), and basic EW (BEW). A simple rinsing or soaking in water was able to remove >95% of hNoV from surface-inoculated raspberries. In contrast, only 75% of hNoV was removed from surface-inoculated romaine lettuce by rinsing in tap water. An AEW wash enhanced the binding of hNoV to raspberries and lettuce. Only 7.5% (±10%) and 4% (±3.1%) of hNoV were removed by AEW wash from surface-inoculated raspberries and lettuce, respectively. When raspberries and lettuce were prewashed with NEW or BEW prior to surface inoculation, an AEW wash likewise resulted in significantly less removal of hNoV compared with untreated samples. A prewash with AEW significantly decreased the removal of hNoV from raspberries and lettuce when they were washed with NEW, from 90.6 to 51% and from 76 to 51.3% , respectively. There are minimal or no improvements gained by use of any of the EWs instead of a regular tap water wash in removal of hNoV from produce. However, use of AEW shows a significant decrease in the removal of hNoV from contaminated produce compared with other water rinses. The ability to remove hNoV from different types of produce varies, possibly due to differences among types of ligand-like molecules that bind hNoV. The distribution of hNoV on raspberries and lettuce was studied using recombinant Norwalk-like particles (rNVLP). By immunofluorescence microscopy, we were able to observe binding of rNVLP only to vein areas of romaine lettuce, suggesting that the virus was binding to specific molecules in these areas. Random binding of rNVLP occurred only with raspberries prewashed with AEW or washed with AEW.     

Effects of different sanitizing treatments on biofilms and attachment of Escherichia coli and Listeria monocytogenes on green leaf lettuce

The effects of ozone (2 mg/L), chlorine (100 mg/L) and organic acid (0.25 g/100 g citric acid plus 0.50 g/100 g ascorbic acid) treatments at 10 °C for 2 min on the removal of Escherichia coli and Listeria monocytogenes cells embedded inside biofilms on the surface of lettuce leaves were studied. None of the sanitizing treatments were found effective in removing the bacterial biofilms. Initiation of biofilms was observed after 24 h of incubation. Bacterial cells appeared as individual cells, rather than clusters after 6 h incubation, thus 99.9% reductions in both E. coli and L. monocytogenes counts were achieved with all the three treatments. However, after 48 h incubation, none of the treatments resulted in higher than 90% reduction in microbial counts. Biofilm formation was demonstrated for the 48 h incubated samples with SEM images.     

Alternative disinfection techniques to extend the shelf life of minimally processed iceberg lettuce

Chlorination continues to be widely used by the fresh-cut industry for washing and disinfecting minimally processed fruits and vegetables. However, as chlorination has been reported to produce unhealthy by-products such as, chloramines and trihalomethanes, many alternative methods have been reported in the literature. This study examined the use of silver and hydrogen peroxide as possible alternative to chlorination. The results revealed an obvious bactericidal effect of hydrogen peroxide, silver and their combination on spoilage organisms. Combination of electrochemically generated silver (5 ppm) and hydrogen peroxide (0.4 ppm) caused significant (P < 0.05) reduction in the total plate count (0.87 log), Pseudomonas (2.66 logs), Enterobactericeae (1.61 logs) and yeast and mould (1.60 logs) immediately after washing in comparison to water washed shredded lettuce. However, washing with chlorinated water (5 ppm) under same conditions revealed insignificant reduction in TPC (0.17 log), Pseudomonas (0.60 log), Enterobactericeae (0.15 log) or yeast and mould (0.81 log) counts. Results revealed also that both sources of silver (electrochemical silver and silver nitrate) have similar disinfecting effects, however, electrochemical silver maintained the quality of washed lettuce.     

Effect of the precutting process on sanitizing treatments for reducing pathogens in vegetables

Food Science and Biotechnology - The effectiveness of sanitizing treatments was investigated on reducing pathogens inoculated in whole or cut fresh vegetables, including Brussels sprouts, carrots,...     

Development of oscillation method for reducing foodborne pathogens on lettuce and spinach

In this study, the efficacy of an oscillator for reducing the numbers of foodborne pathogens on lettuce and spinach was tested. A cocktail of three strains each of Salmonella typhimurium, Escherichia coli O157:H7 and Listeria monocytogenes cells and of Bacillus cereus spores was inoculated onto lettuce and spinach leaves and followed by oscillation at 10 Hz and 20 Hz for up to 30 s. After treatment of inoculated lettuce leaf with an oscillator at 20 Hz for 30 s, 2.58, 2.82, 2.21 and 2.22 Log₁₀ CFU/g reductions were obtained with the cells of S. typhimurium, E. coli O157:H7 and L. monocytogenes and the spores of B. cereus, respectively. In the case of the oscillation treatment of spinach leaf, 2.89, 3.73, 2.46 and 2.25 Log₁₀ CFU/g reductions of those pathogens were achieved under the same condition. Statistically significant reductions were observed after oscillation treatment at 20 Hz for 5-10 s. The oscillation treatment at 10 Hz led to slightly less reductions of the pathogens tested as compared to the treatment at 20 Hz. In conclusion, the oscillation method developed shows to be highly efficacious in reducing foodborne pathogens on lettuce and spinach leaves.