Microbial evaluation of minimally processed vegetables and sprouts produced in Seoul, Korea

The aim of this study was to evaluate the microbial quality of some minimally processed vegetables. A total of 345 samples of minimally processed vegetables were acquired at a department store, a local supermarket, and a restaurant in Seoul, Korea. Samples were tested for microorganism distributions and for the presence of pathogenic bacteria. The aerobic mesophilic counts ranged between 2.0 and 9.7 log CFU/g, with the highest count recorded from the sprouts. Counts of psychrotrophic microorganisms were as high as those of the mesophilic microorganisms. Total coliform populations between 1.0 and 8.8 log CFU/g were found in 98.3% of the samples. Microbiological counts for fresh-cut fruits were very low. Sprouts were highly contaminated with microorganisms and showed a high incidence of Escherichia coli and Clostridium perfringence. Salmonella species and Listeria monocytogenes were detected in 1.5 and 0.3% of samples, respectively. E. Coli O157:H7 and Yersinia enterocolitica were not detected in any of the samples.     

The microbiology of minimally processed fresh fruits and vegetables

Minimally processed fresh (MPF) fruits and vegetables are good media for growth of microorganisms. They have been involved in outbreaks because of the consumption of products contaminated by pathogens. They are also sensitive to various spoilage microorganisms such as pectinolytic bacteria, saprophytic Gram‐negative bacteria, lactic acid bacteria, and yeasts. Contamination of MPF fruits and vegetables occurs at every stage of the food chain, from cultivation to processing. Polluted environments during cultivation or poor hygienic conditions in processing increase the risk of contamination with foodborne pathogens. Although MPF fruits and vegetables may harbor psychrotrophic microorganisms such as fluorescent pseudomonads or Listeria monocytogenes, good control of refrigeration temperature limits growth of spoilage and pathogenic microorganisms. Modified atmospheres are often efficient to maintain or improve visual and organoleptic quality of MPF fruits and vegetables, but their effects on microorganisms are inconsistent. Chemical disinfection can partially reduce the initial bacterial contamination; irradiation seems to be more efficient. The applications of legislations and quality assurance systems to control contamination, survival, and growth of foodborne pathogens in MPF fruits and vegetables are discussed.     

Microbial antagonists of foodborne pathogens on fresh,minimally processed vegetables

On many types of raw or minimally processed foods, the bacterial microbiota is often composed of mixed species. The activities of one bacterial species may influence the growth and activities of others that are present. The objective of this project was to evaluate the microbial composition of fresh and minimally processed vegetables to determine if naturally occurring bacteria on produce are competitive with or antagonistic to potentially encountered pathogens. Naturally occurring bacteria were obtained from ready-to-eat salad vegetables on four occasions to allow for seasonal variation. Minimally processed vegetables were sampled at various stages in their processing from raw vegetables to packaged products. Some portions were analyzed microbiologically within 24 h, while other portions were stored refrigerated and analyzed after 72 h. Microbiological analysis was conducted for bacterial enumeration and to obtain isolates. An agar spot method was used to screen isolates for antimicrobial activity against Staphylococcus aureus ATCC 27664, Escherichia coli O157:H7 E009, Listeria monocytogenes LCDC 81-861, and Salmonella Montevideo. Of the 1,180 isolates screened for inhibitory activity, 37 (3.22%) were found to have various degrees of inhibitory activity against at least one test pathogen. Many isolates showed inhibitory activity against all four pathogens. The isolates with the most extensive inhibition were removed from finished lettuce piece shreds. Of the 37 inhibitory isolates, 34 (91.9%) were gram negative. All isolates with inhibitory activity are able to multiply at both 4 and 10 degrees C.     

Occurrence and characterization of enterotoxigenic Staphylococcus aureus isolated from minimally processed vegetables and sprouts in Korea

Staphylococcus aureus has long been recognized as an important pathogen in food-borne disease in the world. Minimally processed vegetables and sprouts are often contaminated with enterotoxigenic strains of this bacterium. This paper reports the results of a 3-year survey (2006–2008) on the occurrence of S. aureus in minimally processed vegetables and sprouts. Of 345 examined samples, 40 samples (11.6%) were contaminated with S. aureus. A total of 25 enterotoxigenic S. aureus strains were biotyped and their resistance to antibiotics was examined. Most isolated strains produced Staphylococcal enterotoxin A (SEA) (n=23) followed by Staphylococcal enterotoxin I (SEI) and Staphylococcal enterotoxin G (SEG) and mainly belonged to the human biotype (88%). At least 96.1% of the analyzed strains showed antibiotic resistance properties, while 56% of the analyzed strains exhibited multiple antibiotic resistance to the antibiotics tested. Two of the analyzed strains were resistant to methicillin. Moreover, a strain which had multi-resistance to 6 antibiotics was found. The results indicate that enterotoxigenic, antibioticresistant strains of S. aureus are widely proliferated in minimally processed vegetables and sprouts.     

净菜保鲜剂选用的研究

主要研究上海青、香菜和油麦菜的保鲜技术,通过正交实验确定了在0～8℃时保鲜剂的最佳配方,结果如下:对于上海青,保鲜剂最佳配方为柠檬酸0.08%,氯化钙0.05%,苯甲酸钠0.10%;对于香菜,保鲜剂最佳配方为柠檬酸0.08%,氯化钙0.10%,苯甲酸钠0.05%;对于油麦菜,保鲜剂最佳配方为柠檬酸0.08%,氯化钙0.10%,苯甲酸钠0.05%。实验表明,通过保鲜剂处理可以明显延长净菜贮藏期。      

Antimicrobial edible films and coatings for fresh and minimally processed fruits and vegetables: a review

The use of edible films and coatings is an environmentally friendly technology that offers substantial advantages for shelf-life increase of many food products including fruits and vegetables. The development of new natural edible films and coatings with the addition of antimicrobial compounds to preserve fresh and minimally processed fruits and vegetables is a technological challenge for the industry and a very active research field worldwide. Antimicrobial agents have been successfully added to edible composite films and coatings based on polysaccharides or proteins such as starch, cellulose derivatives, chitosan, alginate, fruit puree, whey protein isolated, soy protein, egg albumen, wheat gluten, or sodium caseinate. This paper reviews the development of edible films and coatings with antimicrobial activity, typically through the incorporation of antimicrobial food additives as ingredients, the effect of these edible films on the control of target microorganisms, the influence of antimicrobial agents on mechanical and barrier properties of stand-alone edible films, and the effect of the application of antimicrobial edible coatings on the quality of fresh and fresh-cut fruits and vegetables.     

Ultrasound decontamination of minimally processed fruits and vegetables

The effectiveness of power ultrasound for the microbial decontamination of minimally processed fruits and vegetables was studied. Reductions in Salmonella typhimurium attached to iceberg lettuce obtained by cleaning with water, chlorinated water, ultrasound with water and ultrasound with chlorinated water were 0.7, 1.7, 1.5 and 2.7 logs, respectively, for small-scale (2 L) trials. The cleaning action of cavitation appears to remove cells attached to the surface of fresh produce, rendering the pathogens more susceptible to the sanitizer. For large-scale (40 L) trials, the addition of chlorine to water in the tank gave a systematic difference in Escherichia coli decontamination efficiency. However, the frequency of ultrasound treatment (25, 32–40, 62–70 kHz) had no significant effect on decontamination efficiency ( P  > 0.69). With the potentially high capital expenditure together with the expensive process of optimization and water treatment, it is unlikely that the fresh produce industry would be willing to take up this technology. Furthermore, the additional one log reduction achieved by applying ultrasound to a chlorinated water wash does not completely eliminate the risk of pathogens on fresh produce.     

The microbiological safety of minimally processed vegetables

Summary Demand for fresh, convenient, minimally processed vegetables has led to an increase in the quantity and variety of products available to the consumer. Modified atmosphere packaging, in combination with refrigeration, is increasingly being employed as a mild preservation technique to ensure quality and storage-life. The fresh nature of these products, together with the mild processing techniques and subsequent storage conditions, have presented indigenous and pathogenic microorganisms with new ecosystems and potential infection vehicles; a number of outbreaks of foodborne disease being attributed to their consumption. Psychrotrophic pathogens and pathogens which are capable of maintaining an infectious potential under mild preservation regimes are of particular concern; Listeria monocytogenes , Aeromonas hydrophila and Clostridium botulinum being amongst the most notable. This review describes the processing, packaging and storage procedures involved in the production of minimally processed vegetables, and details their impact upon the survival and growth of associated pathogens. Gaps in our current understanding of the consequences of this novel technology for microbiological safety are highlighted.     

Microflora of minimally processed frozen vegetables sold in Gaborone, Botswana

Two hundred samples of minimally processed, frozen, and prepacked potato chips, peas, corn, and a variety of combined vegetables from supermarkets in Gaborone, Botswana, were examined microbiologically. Determination of aerobic mesophilic plate count, aerobic psychrotrophic plate count, lactic acid bacteria, yeasts and molds, coliforms, Listeria spp., and Staphylococcus aureus were done. Chips had the lowest mean log values for all of the microorganisms enumerated except yeasts and molds. The mean log values for single vegetables ranged from 3.6 to 9.1, 3.4 to 8.9, 2.9 to 5.6, and 2.1 to 6.5 log CFU/ g aerobic mesophilic plate count, aerobic psychrotrophic plate count, lactic acid bacteria, and yeasts and molds, respectively. The microbial profiles of peas and corn were almost similar (P < 0.001). The mean values for combined vegetables were clustered within 4.6 and 5.4 and 4.2 and 5.2 log CFU/g aerobic mesophilic plate count and aerobic psychrotrophic plate count, respectively. All of the vegetables had a coliform population distribution ranging from 0 to < 10(4) most probable number per g. The predominant gram-negative bacteria isolated included members of Enterobacteriaceae and Pseudomonaceae (86.2%). Escherichia coli was not detected in all of the samples. The organisms isolated included those responsible for spoilage in frozen vegetables, namely Pseudomonas, Klebsiella, Corynebacterium, lactic acid bacteria, and Flavobacterium. The predominant lactic acid bacteria were Lactobacillus spp. (55.9%). Other spoilage organisms were yeasts, and Cryptococcus spp. (55.4%) was predominant. Pathogens, namely Listeria monocytogenes, were also isolated at a rate of 2 to 10%, of which 4% was from corn, 2% each from peas and country crop, and 10% from stir-fry. Bacillus cereus was also isolated and accounted for 7.7% of the microorganisms from corn. S. aureus was isolated from all of the vegetables. Enterotoxigenic strains were from corn, peas, mixed vegetables, and stir-fry, and all of them produced enterotoxin A. In addition, the isolates from stir-fry vegetables also produced enterotoxins B and C. The study reveals the presence of pathogens and emerging opportunistic pathogens in the ready-to-use or ready-to-eat vegetables. If E. coli is the only indicator for safety and acceptability, consumers may be exposed to foodborne diseases. Inclusion of other groups as indicator organisms is suggested. Retailers are urged to invest in standby generators to maintain the cold chain.     

Soluble protein content in minimally processed vegetables during storage

Soluble protein content (SPC) and electrophoretic protein profile in minimally processed broccoli, Dutch carrot and Cos lettuce were determined after 0, 3, 5, 7, 10 and 12 days of storage at 12 °C and 95±2% relative humidity. An increase in SPC in broccoli tissues (florets, stems and whole) was observed on day 3, followed by a slight fluctuation thereafter. Similar observations were recorded in shredded carrot, which showed a significant (P<0.05) increase in SPC after 3 days at 12 °C. However, changes in SPC in Cos lettuce were different from broccoli and carrot, and showed a significant (P<0.05) decrease after 3 days. The SDS-PAGE profile revealed a continuous decrease in the band intensity of soluble proteins from broccoli, Dutch carrot and Cos lettuce throughout the storage period. Complete disappearance of some bands was observed in Cos lettuce leaves and shredded carrot after 12 days of storage at 12 °C.     

Microbiological quality of fresh, minimally-processed fruit and vegetables, and sprouts from retail establishments

A survey of fresh and minimally-processed fruit and vegetables, and sprouts was conducted in several retail establishments in the Lleida area (Catalonia, Spain) during 2005-2006 to determine whether microbial contamination, and in particular potentially pathogenic bacteria, was present under these commodities. A total of 300 samples--including 21 ready-to-eat fruits, 28 whole fresh vegetables, 15 sprout samples and 237 ready-to-eat salads containing from one to six vegetables--were purchased from 4 supermarkets. They were tested for mesophilic and psychrotrophic aerobic counts, yeasts and moulds, lactic acid bacteria, Enterobacteriaceae, presumptive E. coli and Listeria monocytogenes counts as well as for the presence of Salmonella, E. coli O157:H7, Yersinia enterocolitica and thermotolerant Campylobacter. Results for the fresh-cut vegetables that we analyzed showed that, in general, the highest microorganism counts were associated with grated carrot, arugula and spinach (7.8, 7.5 and 7.4 log cfu g(-1) of aerobic mesophilic microorganisms; 6.1, 5.8 and 5.2 log cfu g(-1) of yeast and moulds; 5.9, 4.0 and 5.1 log cfu g(-1) lactic acid bacteria and 6.2, 5.3 and 6.0 log cfu g(-1) of Enterobacteriaceae). The lowest counts were generally associated with fresh-cut endive and lettuce (6.2 and 6.3 log cfu g(-1) of aerobic mesophilic microorganisms; 4.4 and 4.6 log cfu g(-1) of yeast and moulds; 2.7 and 3.8 log cfu g(-1) lactic acid bacteria and 4.8 and 4.4 log cfu g(-1) of Enterobacteriaceae). Counts of psychrotrophic microorganisms were as high as those of mesophilic microorganisms. Microbiological counts for fresh-cut fruit were very low. Sprouts were highly contaminated with mesophilic (7.9 log cfu g(-1)), psychrotrophic microorganisms (7.3 log cfu g(-1)) and Enterobacteriaceae (7.2 log cfu g(-1)) and showed a high incidence of E. coli (40% of samples). Of the samples analyzed, four (1.3%) were Salmonella positive and two (0.7%) harboured L. monocytogenes. None of the samples was positive for E. coli O157:H7, pathogenic Y. enterocolitica or thermotolerant Campylobacter.     

Characterization of Bacillus cereus isolates from fresh vegetables and refrigerated minimally processed foods by biochemical and physiological tests

Bacillus cereus contamination has been evaluated in a total of 56 samples of fresh peppers, cucumbers, tomatoes, carrots, zucchini, garlic and onions, used commonly as ingredients to manufacture refrigerated minimally processed foods. It has been also evaluated in some of these products, such as gazpacho (Mediterranean vegetable cold soup made from fresh vegetables), salmorejo (vegetable cold purée containing bread and egg), ajoblanco (cold cream containing bread, almond, and olive oil as main ingredients) and zanaranja (carrot and orange juice mixture). The average counts did not exceed 10⁴ cfu g⁻¹ in any case and all the vegetables and processed products except garlic and zanaranja presented contamination of this foodborne pathogen. Cucumbers presented the highest count among all the vegetable products analysed, followed by carrots. According to the ISO 7932:1993(F) confirmation procedure, a total of 36 presumptive B. cereus strains were isolated from the 85 samples of various vegetable products analysed. Isolates were further characterized and identified by API 50CH/20E phenotypic system using APILAB Plus software, combined with additional tests of motility, oxidase activity and anaerobic growth. Of the 36 presumptive B. cereus isolates, 32 (88·9%) were confirmed to be B. cereus, two strains (5·5%) were identified as B. licheniformis, and other two strains (5·5%) were identified as B. firmus. About 81·3% (26 strains) of 32 B. cereus tested isolates hydrolysed starch and 71·9% (23 strains) produced diarrheal enterotoxin in TSB culture as detected by the BCET-RPLA test. The heat resistance at 90°C for spores of the 32 B. cereus strains ranged from 1·4 to 21·2 min. Strains unable to hydrolyse starch were the most heat-resistant, with D₉₀ values higher than 10·8 min. All the enterotoxin-positive strains were able to hydrolyse starch.     

Keeping quality and safety of minimally fresh processed melon

Fresh-cut Amarillo melon was stored under passive modified atmosphere packaging (MAP) for 14 days at 5 °C. Three commercial films were tested: microperforated polypropylene (MPP), bioriented PP (BPP), and oriented PP (OPP). As a control, a macroperforated PP film was used. The effect of a citric acid dip treatment (0.52 mM) on the quality of the melon pieces packaged in OPP film was also evaluated. A similar final gas composition (4 kPa O2 plus 12–13 kPa CO2) within packages was achieved by using the three tested films. This atmosphere was effective for maintaining sensorial quality and microbial safety and for avoiding weight loss and translucency. When citric acid dip was applied, a reduction of microbial counts, a low discoloration, an increase of lightness, and a general improvement of visual appearance were found. However, after 14 days of storage, neither MAP treatments nor citric acid dip were enough to avoid softening of at least 23%, although the highest value (32%) was found in control pieces. Taking into account safety, quality attributes and consumer acceptance, shelf life of fresh processed melon stored under the studied MAP conditions could be stabilised for 10 days.     

Decontamination methods to prolong the shelf-life of minimally processed vegetables, state-of-the-art

Minimally processed vegetables (MPV) are any fresh vegetables that have been physically altered from their original form, but remains in a fresh state. Microorganisms present in MPV can cause foodborne illnesses or spoilage; hence, decontamination of MPV can produce more stable products. The present review examines the difficulties to decontaminate and prolong the shelf-life of MPV, evaluating the current way of data analysis and interpretation. It addresses the different aspects of the problem of the accessibility of sanitizers to microorganisms (irregularities of produce surface, injuries, internalization, attachment, and biofilms). It also includes a critical exposition of the methodological problems to estimate the prolongation of shelf-life due to a decontamination method, namely: the variability among samples, the reproducibility of the results, and the interpolation when lacking some crucial data. Furthermore, it revises the difficulties to control the microbial loads of decontaminated MPV during storage (the enhanced growth rate of microorganisms in decontaminated MPV, the patterns of microbial growth in non decontaminated and decontaminated MPV, and the role of temperature in keeping the decontamination effect).     

Recent advances in edible coatings for fresh and minimally processed fruits

The development of new edible coatings with improved functionality and performance for fresh and minimally processed fruits is one of the challenges of the post harvest industry. In the past few years, research efforts have focused on the design of new eco-friendly coatings based on biodegradable polymers, which not only reduce the requirements of packaging but also lead to the conversion of by-products of the food industry into value added film-forming components. This work reviews the different coating formulations and applications available at present, as well as the main results of the most recent investigations carried out on the topic. Traditionally, edible coatings have been used as a barrier to minimize water loss and delay the natural senescence of coated fruits through selective permeability to gases. However, the new generation of edible coatings is being especially designed to allow the incorporation and/or controlled release of antioxidants, vitamins, nutraceuticals, and natural antimicrobial agents by means of the application of promising technologies such as nanoencapsulation and the layer-by-layer assembly.     

HPLC assay of ascorbic acid in fresh and processed fruit and vegetables

A method is reported to estimate ascorbic acid (AA) content in fresh and preserved fruit and vegetables. Following its extraction by HPO₃, AA is separated by HPLC on a strong anionic pellicular (Partisil 10 SAX) column, isocratically eluting with 0·1 m sodium acetate buffer solution at pH 4·25, and detecting at 250 nm by a variable wavelength UV detector. The amount of AA is estimated by adding benzoic acid as an internal standard and processing data by an attached electronic printer plotter integrator.The method has shown a high selectivity and speed together with a very good reliability in the range 0·04% to 1% AA.     

Nitrate and nitrite contents of some fresh and processed Egyptian vegetables

The nitrate and nitrite contents of sixteen fresh vegetables, widely consumed in Egypt, were determined. The highest values of nitrate were observed in leafy vegetables, followed by root vegetables and then pulses. Of the leafy vegetables, spinach, roquette and chard contained the highest concentration of nitrate. Nitrite concentrations were detected at low levels in a few samples, while the others were free of nitrite.Cooking had the effect of lowering the levels of nitrate in all types of fresh vegetables studied; no nitrite was formed during cooking.Storage of frozen vegetables for six months decreased the levels of nitrate, while nitrite was formed at low levels after storage for three or four months.     

Moulds, yeasts and aerobic plate counts in ginseng supplements

Forty six ginseng supplement samples including Siberian ginseng root, Chinese ginseng herb and root, and American ginseng root and extract were purchased from retail in the Washington, DC area and from Penn Herb Co. (Philadelphia, PA) and tested for mould and yeast (MY) contamination and the presence of aerobic mesophilic bacteria (APC). Results indicated that 100% of the Siberian ginseng samples were contaminated with fungi and bacteria. MY counts ranged from 8.0 x 10(2) to 1.4 x 10(3) cfu/g whereas the APCs were between 2.3 x 10(4) and 1.0 x 10(6) cfu/g. Most common fungi encountered in this commodity were Penicillium spp., Eurotium rubrum, E. chevalieri and Rhizopus spp. Seventy-eight percent of the Chinese ginseng herb samples were contaminated with fungi and 89% with bacteria at levels ranging between <100 and 6.0 x 10(4) and <100 and 1.2 x 10(6) cfu/g, respectively. Moulds commonly isolated were Alternaria alternata, Aspergillus niger, Aspergillus spp., Cladosporium spp., E. chevalieri, Penicillium spp. and Rhizopus spp. Fifty six percent of the Chinese ginseng root samples tested contained fungi (A. niger, Rhizopus spp. and yeasts), and 100% contained bacteria. Fungal counts ranged between <100 and 1.4 x 10(3) cfu/g and APCs were between 3.0 x 10(2) and 6.8 x 10(5) cfu/g. Forty-eight percent of the American ginseng root samples contained moulds and 30% showed bacterial contamination. MY counts were between <100 and 4.3 x 10(5) cfu/g whereas APCs were between <100 and 4.5 x 10(4) cfu/g. A. flavus was isolated from 9% and Penicillium spp. were recovered from 39% of the tested samples. This is the first report of A. flavus contamination in ginseng supplements. No moulds or yeasts were found in ginseng extract, but 50% of these samples contained bacteria at levels ranging between <100 and 1.0 x 10(3) cfu/g.