1768 Journal o f Food Protection, Vol. 77, No. 10, 2014, Pages 1768-1772 doi: 10.4315/0362-028X. JFP-14-108 Copyright © , International Association for Food Protection
Research Note
Effect of Gamma Radiation on the Reduction of Salmonella strains, Listeria monocytogenes, and Shiga Toxin-Producing Escherichia coli and Sensory Evaluation of Minimally Processed Spinach (Tetragonia expansa) ANA CAROLINA B. REZENDE, MARIA CRYSTINA IGARASHI, MARIA TERESA DESTRO, BERNADETTE D. G. M. FRANCO, a n d MARIZA LANDGRAF Food and Experimental Nutrition Department, FCF, University o f Sao Paulo, Avenida Professor Lineu Prestes, 580, Bloco 13B, 05508-900, Sao Paulo, SP, Brazil MS 14-108: Received 28 February 2014/Accepted 3 June 2014
A BSTR A CT This study evaluated the effects of irradiation on the reduction of Shiga toxin-producing Escherichia coli (STEC), Salmonella strains, and Listeria monocytogenes, as well as on the sensory characteristics of minimally processed spinach. Spinach samples were inoculated with a cocktail of three strains each of STEC, Salmonella strains, and L. monocytogenes, separately, and were exposed to gamma radiation doses of 0, 0.2, 0.4, 0.6, 0.8, and 1.0 kGy. Samples that were exposed to 0.0, 1.0, and 1.5 kGy and kept under refrigeration (4°C) for 12 days were submitted to sensory analysis. D 10-values ranged from 0.19 to 0.20 kGy for Salmonella and from 0.20 to 0.21 for L. monocytogenes; for STEC, the value was 0.17 kGy. Spinach showed good acceptability, even after exposure to 1.5 kGy. Because gamma radiation reduced the selected pathogens without causing significant changes in the quality of spinach leaves, it may be a useful method to improve safety in the fresh produce industry.
The desire for a healthy diet, and the search for convenience in meal preparation, has led to increased consumption of fresh or partially processed vegetables; this has triggered the production of several products offered for immediate consumption. Despite the numerous advantages of minimally pro cessed products, they pose a potential risk to public health; many processing steps (e.g., cutting, washing, and packag ing) are done manually, increasing the likelihood of microbial contamination (9). Foodbome diseases associated with fresh produce increased by 5.3% from 1973 to 1997 in the United States (29). In many marketplaces, these products are offered to consumers at room temperature, which facilitates microbial growth. Data from the U.S. Centers for Disease Control and Prevention (5) indicate that vegetable products are involved in an increasing number of foodbome disease outbreaks. Some vegetables, such as lettuce and spinach, have been associated with outbreaks involving the enteric pathogen Escherichia coli 0157: H7 (6). In addition to Shiga toxinproducing E. coli (STEC), Salmonella strains and Listeria monocytogenes also deserve attention, being primarily responsible for foodbome diseases (7, 8, 33).
* Author for correspondence. [email protected].
Tel:
+55(11)99996-4734;
E-mail:
Because the use of sanitizers does not significantly reduce the population of microorganisms present in vegeta bles, the application of more efficient techniques, such as irradiation, is necessary. Gamma radiation has received special consideration because this process, depending on the applied dose, does not alter the natural characteristics of food; and, at the same time, it significantly reduces microbial flora, promoting food safety and extending shelf life. Among the minimally processed vegetables, spinach is noteworthy because its consumption is increasing in Brazil. Thus, the objective of this study was to evaluate the effect of irradiation on the reduction of Salmonella strains, STEC, and L. monocytogenes inoculated on minimally processed spinach, as well as on the sensory characteristics of minimally processed spinach. M A TER IA LS A ND M E TH O D S Substrate. Samples of spinach (Tetragonia expansa) were purchased directly from the producer in Itatiba city, State of Sao Paulo, Brazil, on the day of the experiments. The damaged leaves were discarded, and the remaining stems and leaves were washed in water at room temperature and were sanitized by immersion in a solution of sodium hypochlorite (200 ppm) for 15 min (25). Microorganisms. A cocktail of three strains, each, of Salmonella (Infantis, Enteritidis ATCC 13076, and Typhimurium ATCC 14028), L. monocytogenes (L. monocytogenes isolated from spinach, L. monocytogenes ATCC 19115, and L. monocytogenes
J. Food Prot., Vol. 77, No. 10
FIGURE 1. Population o f Salmonella spp. in spinach exposed to gamma radiation. ATCC 7644), and STEC (E. coli 0157:H7, E. coli 0157:H7 EDL933, and E. coli 0157:H7 3104 88) were used. The strains were maintained at -70 °C in tryptone soy broth (TSB; Oxoid, Basingstoke, UK) with 20% glycerol. Inoculum preparation. Each strain of Salmonella and STEC was inoculated into TSB (Oxoid), and each strain of L. monocytogenes was inoculated into TSB supplemented with 0.6% of yeast extract (TSB-YE) and was incubated at 37°C for 20 to 24 h. A loopful was transferred to 100 ml of TSB for Salmonella and E. coli and to 100 ml of TSB-YE for L. monocytogenes; the cultures were incubated at 37°C for 20 to 24 h. One 15-ml aliquot of each culture was added to each of two centrifuge tubes (45 ml of culture per tube) to make a stock culture. The cultures were centrifuged (Hettich, Tuttlingen, Germany) at 700 x g for 30 min. The supernatant was discarded, and the pellet was resuspended with 100 ml of 0.85% (wt/vol) NaCl solution (LabSynth, Diadema, Brazil). Inoculation of spinach with Salmonella strains, STEC, and L. monocytogenes. The stock culture of bacteria, previously washed with NaCl solution (100 ml), was mixed with 6 liters of cold (2 to 5°C) distilled water. This solution was homogenized, and aliquots of 1 ml were taken for further dilution and were pour plated using tryptic soy agar (TSA) (Salmonella and STEC) or TSA-YE (L. monocytogenes) with overlay of mannitol lysine crystal violet brilliant green agar and MacConkey sorbitol agar supplemented with potassium tellurite and cefixime (Oxoid) for Salmonella and E. coli, respectively, and Oxford agar (Oxoid) for L. monocytogenes. The final inoculum was 107 to 108 CFU/ml. The spinach was immersed into this suspension for 15 min. Then, the spinach was spun in a sanitized centrifuge (salad spinner type) to remove excess surface water (2J). The vegetables were divided into 40-g portions and were packed into polyethylene bags and vacuum sealed (AP 500, TecMaq, Sao Paulo, Brazil). Irradiation process. Packages of contaminated spinach, in triplicate, were transported in ice to the irradiation plant at the Institute of Energy and Nuclear Research (Sao Paulo, Brazil). All samples were irradiated using gamma radiation originating from 60Co. The samples were exposed to doses of 0.2, 0.4, 0.6, 0.8, and 1.0 kGy. Control samples (nonirradiated) were also used. The experiment was repeated three times. A red perspex dosimeter (Harwell Dosimeters, Didcot, Oxfordshire, UK) was used, with 75% accuracy and 72% precision. The slopes of the individual survivor curves were calculated with linear regression using Microsoft Excel 97 SR- 2 (Microsoft Corporation, Redmond, WA). The D 10-value was calculated by taking the negative reciprocal of the survivor curve slope.
SPINACH IRRADIATION
1769
FIGURE 2. Population o f L. monocytogenes in spinach exposed to gamma radiation. Enumeration of Salmonella strains, STEC, and L. monocytogenes. Irradiated and nonirradiated samples were maintained under refrigeration (4°C) until microbiological testing began. Portions of spinach (25 g) were weighed and homogenized with 225 ml of 0.85% NaCl solution (LabSynth) using a Stomacher 400 (Seward, London, UK) for 1 min and were serially diluted with 0.85% NaCl solution (LabSynth). From each dilution, 1 ml was pour plated using TSA-YE for L. monocytogenes and TSA for Salmonella strains and STEC (26). To determine the population of surviving organisms after solidification, an overlay of Oxford agar (Oxoid) was added for L. monocytogenes, an overlay of mannitol lysine crystal violet brilliant green agar for Salmonella strains, and an overlay of MacConkey sorbitol agar supplemented with potassium tellurite and cefixime for STEC. Two pour plates per dilution were incubated at 37°C for 24 h. Each analysis was performed three times. Sensory evaluation. Packages of minimally processed spinach (400 g), either uninoculated, exposed to 1 and 1.5 kGy, or nonirradiated, were submitted to sensory evaluation on days 0, 5, 9, and 12. Spinach samples were analyzed for acceptability, which was measured by an unstructured hedonic scale of nine points, ranging from extremely disliked (point 1) to extremely liked (point 9) (16). The panelists, consisting of 30 untrained members of the faculty, staff, and students of the University of Sao Paulo, evaluated the appearance, flavor, aroma, and texture of products. Spinach was maintained under refrigeration (4°C), and irradiated and nonirra diated samples (10 g) were served in white plastic dishes coded with three-digit random numbers. The samples were evaluated under white light in individual cabins, in the Sensory Food Analysis Laboratory, Faculty of Pharmaceutical Sciences, Univer sity of Sao Paulo. Data were submitted to a two-way analysis of variance (SAS Institute Inc., Cary, NC) followed by Tukey’s means comparison test (P £ 0.05).
RESULTS AND DISCUSSION Resistance of Salmonella strains, STEC, and Liste ria monocytogenes in irradiated spinach. Figures 1, 2, and 3 show, respectively, the inactivation curves of Salmonella, STEC, and L. monocytogenes inoculated in minimally processed spinach and exposed to doses of 0.2, 0.4, 0.6, 0.8, and 1.0 kGy. The Z)10-values determined in this study ranged from 0.19 to 0.20 kGy for Salmonella strains and from 0.20 to 0.21 kGy for L. monocytogenes; for STEC, this value was 0.17 kGy.
1770
REZENDE ET AL.
J. Food Prot., Vol. 77, No. 10
FIGURE 3. Population of Shiga toxin-producing E. coli in spinach exposed to gamma radiation.
The D io-values obtained in this study for Salmonella strains (Fig. 1) are similar to those obtained by Nunes et al. (26) on arugula (0.16 to 0.19 kGy) and those reported by Goularte et al. (13) in lettuce (0.16 to 0.23 kGy). Results presented here were lower than those that Dhokane et al. (10) found for Salmonella inoculated in carrot and cucumber, 0.31 and 0.35 kGy, respectively. Lee et al. (18) also obtained high results when the microorganism was inoculated in spinach and cucumber (0.37 and 0.40 kGy, respectively), as did Martins et al. (20) for Salmonella in watercress (0.33 to 0.42 kGy). ForL. monocytogenes, D,,)-values (Fig. 2) were similar to those reported by Bari et al. (3) on endive, cabbage, and tomato; Niemira (21) in lettuce; Niemira et al. (24) on endive; and Tsuhako (31) in lettuce; these studies obtained results ranging from 0.19 to 0.25 kGy. However, Lee et al. (18) reported higher values than those presented here for L. ivanovii inoculated into spinach and cucumber, 0.41 and 0.34 kGy, respectively; and Nunes et al. (26) reported values from 0.37 to 0.48 kGy for L. monocytogenes inoculated on arugula. Figure 3 shows D |0-values (0.17 kGy) for STEC that are similar to those obtained by Hsu et al. (15) in irradiated mint (0.17 kGy). However, Niemira et al. (25), working with several varieties of lettuce (iceberg, red lettuce, green
leaf, and Boston), found values that ranged from 0.12 to 0.14 kGy. Ionizing radiation has also been shown effective in reducing E. coli 0157:H7 internalized in lettuce and spinach (22, 23). This is important because, once internalized, bacteria are protected from antimicrobial surface treatment. The D iQ-values obtained by Niemira (22, 23) for E. coli 0157:H7 ranged from 0.30 to 0.45 kGy in several varieties of lettuce, whereas for spinach leaves the value was 0.27 kGy. £>io-values may vary depending on the type of vegetable, as well as different cultivars of the same vegetable, due to moisture content variation, the presence of chemical compounds that protect microorganisms from action from the radiolytic products, or other factors that so far have not been identified. This was confirmed by Niemira (21), whose analysis of four types of lettuce (Boston, iceberg, green leaf, and red lettuce) found D ev alu es of 0.31, 0.25, 0.23, and 0.24 kGy, respectively. Sensory evaluation. Whereas irradiation can improve the microbiological safety of minimally processed vegeta bles, it can also cause undesirable changes in the sensory characteristics of some vegetables, depending on the dose. In the case of spinach, in general, both the control and the irradiated (1 to 1.5 kGy) samples were accepted by the panelists. Table 1 shows the averages allocated to general appearance, aroma, flavor, and texture of the samples. Although a decay in grades was seen during the storage period, there was no statistically significant difference between irradiated and control samples. However, with respect to appearance, the use of 1.5 kGy significantly reduced (P < 0.05) the acceptability of this vegetable after the ninth day of storage. The attributes of aroma, flavor, and texture were not significantly different between the control and irradiated samples during the storage period, which shows that the product was well accepted by the panelists. Several authors also found no significant difference in the acceptability of other vegetables such as lettuce, watercress,
TABLE 1. Changes in sensory scores of irradiated minimally processed spinach during storage at 5 °Cfor 12 daysa Storage time (days) Sensory evaluation Appearance
Aroma
Flavor
Irradiation dose (kGy)
7.45 7.32 7.63
0
6 .6
1 1.5
7.0
a
a
6 .8
a
a
6.9 7.0 6.7 7.5 7.2 7.4
a
a
a a a a a
a
a
0
0
1 1.5 °
5
1 1.5
0
1 1.5 Texture
0
a
a a a
a a a
a
a a a
7.52 a a 7.05 a a 7.32 a ac 7.0 a a 7.2 a a 7.0 a a 7.0 a a 7.3 a a 7.1 a a 6.9 a a 7.6 a a 7.6 a a
12
9
6.33 6.03
a a
a a
6.42 6.47
a
a
a
a
a
bd
6 .3
a
bed
6 .0 2
6 .0
a
a
5.9 6.4 5.9 6.3 6.4
a
a
6 .0
a
a a
6 .6
a
a
6 .2
a
a
6 .1
a
a
6.5 6.3
a
a
a a
a a a a
a a a a
6 .8
a
a
6 .8
a
6.5 6.7
a
a a
6.7 6.4
a
a
a
a
a
Means followed by the same uppercase letter in the same column and by the same lowercase letter on the same line do not differ by Tukey’s test at 5% probability.
SPINACH IRRADIATION
J. Food Prot., Vol. 77, No. 10
parsley, coriander, mint, and cauliflower irradiated with 0.5, 1.0,1.5, and 2 kGy
Research Note
Effect of Gamma Radiation on the Reduction of Salmonella strains, Listeria monocytogenes, and Shiga Toxin-Producing Escherichia coli and Sensory Evaluation of Minimally Processed Spinach (Tetragonia expansa) ANA CAROLINA B. REZENDE, MARIA CRYSTINA IGARASHI, MARIA TERESA DESTRO, BERNADETTE D. G. M. FRANCO, a n d MARIZA LANDGRAF Food and Experimental Nutrition Department, FCF, University o f Sao Paulo, Avenida Professor Lineu Prestes, 580, Bloco 13B, 05508-900, Sao Paulo, SP, Brazil MS 14-108: Received 28 February 2014/Accepted 3 June 2014
A BSTR A CT This study evaluated the effects of irradiation on the reduction of Shiga toxin-producing Escherichia coli (STEC), Salmonella strains, and Listeria monocytogenes, as well as on the sensory characteristics of minimally processed spinach. Spinach samples were inoculated with a cocktail of three strains each of STEC, Salmonella strains, and L. monocytogenes, separately, and were exposed to gamma radiation doses of 0, 0.2, 0.4, 0.6, 0.8, and 1.0 kGy. Samples that were exposed to 0.0, 1.0, and 1.5 kGy and kept under refrigeration (4°C) for 12 days were submitted to sensory analysis. D 10-values ranged from 0.19 to 0.20 kGy for Salmonella and from 0.20 to 0.21 for L. monocytogenes; for STEC, the value was 0.17 kGy. Spinach showed good acceptability, even after exposure to 1.5 kGy. Because gamma radiation reduced the selected pathogens without causing significant changes in the quality of spinach leaves, it may be a useful method to improve safety in the fresh produce industry.
The desire for a healthy diet, and the search for convenience in meal preparation, has led to increased consumption of fresh or partially processed vegetables; this has triggered the production of several products offered for immediate consumption. Despite the numerous advantages of minimally pro cessed products, they pose a potential risk to public health; many processing steps (e.g., cutting, washing, and packag ing) are done manually, increasing the likelihood of microbial contamination (9). Foodbome diseases associated with fresh produce increased by 5.3% from 1973 to 1997 in the United States (29). In many marketplaces, these products are offered to consumers at room temperature, which facilitates microbial growth. Data from the U.S. Centers for Disease Control and Prevention (5) indicate that vegetable products are involved in an increasing number of foodbome disease outbreaks. Some vegetables, such as lettuce and spinach, have been associated with outbreaks involving the enteric pathogen Escherichia coli 0157: H7 (6). In addition to Shiga toxinproducing E. coli (STEC), Salmonella strains and Listeria monocytogenes also deserve attention, being primarily responsible for foodbome diseases (7, 8, 33).
* Author for correspondence. [email protected].
Tel:
+55(11)99996-4734;
E-mail:
Because the use of sanitizers does not significantly reduce the population of microorganisms present in vegeta bles, the application of more efficient techniques, such as irradiation, is necessary. Gamma radiation has received special consideration because this process, depending on the applied dose, does not alter the natural characteristics of food; and, at the same time, it significantly reduces microbial flora, promoting food safety and extending shelf life. Among the minimally processed vegetables, spinach is noteworthy because its consumption is increasing in Brazil. Thus, the objective of this study was to evaluate the effect of irradiation on the reduction of Salmonella strains, STEC, and L. monocytogenes inoculated on minimally processed spinach, as well as on the sensory characteristics of minimally processed spinach. M A TER IA LS A ND M E TH O D S Substrate. Samples of spinach (Tetragonia expansa) were purchased directly from the producer in Itatiba city, State of Sao Paulo, Brazil, on the day of the experiments. The damaged leaves were discarded, and the remaining stems and leaves were washed in water at room temperature and were sanitized by immersion in a solution of sodium hypochlorite (200 ppm) for 15 min (25). Microorganisms. A cocktail of three strains, each, of Salmonella (Infantis, Enteritidis ATCC 13076, and Typhimurium ATCC 14028), L. monocytogenes (L. monocytogenes isolated from spinach, L. monocytogenes ATCC 19115, and L. monocytogenes
J. Food Prot., Vol. 77, No. 10
FIGURE 1. Population o f Salmonella spp. in spinach exposed to gamma radiation. ATCC 7644), and STEC (E. coli 0157:H7, E. coli 0157:H7 EDL933, and E. coli 0157:H7 3104 88) were used. The strains were maintained at -70 °C in tryptone soy broth (TSB; Oxoid, Basingstoke, UK) with 20% glycerol. Inoculum preparation. Each strain of Salmonella and STEC was inoculated into TSB (Oxoid), and each strain of L. monocytogenes was inoculated into TSB supplemented with 0.6% of yeast extract (TSB-YE) and was incubated at 37°C for 20 to 24 h. A loopful was transferred to 100 ml of TSB for Salmonella and E. coli and to 100 ml of TSB-YE for L. monocytogenes; the cultures were incubated at 37°C for 20 to 24 h. One 15-ml aliquot of each culture was added to each of two centrifuge tubes (45 ml of culture per tube) to make a stock culture. The cultures were centrifuged (Hettich, Tuttlingen, Germany) at 700 x g for 30 min. The supernatant was discarded, and the pellet was resuspended with 100 ml of 0.85% (wt/vol) NaCl solution (LabSynth, Diadema, Brazil). Inoculation of spinach with Salmonella strains, STEC, and L. monocytogenes. The stock culture of bacteria, previously washed with NaCl solution (100 ml), was mixed with 6 liters of cold (2 to 5°C) distilled water. This solution was homogenized, and aliquots of 1 ml were taken for further dilution and were pour plated using tryptic soy agar (TSA) (Salmonella and STEC) or TSA-YE (L. monocytogenes) with overlay of mannitol lysine crystal violet brilliant green agar and MacConkey sorbitol agar supplemented with potassium tellurite and cefixime (Oxoid) for Salmonella and E. coli, respectively, and Oxford agar (Oxoid) for L. monocytogenes. The final inoculum was 107 to 108 CFU/ml. The spinach was immersed into this suspension for 15 min. Then, the spinach was spun in a sanitized centrifuge (salad spinner type) to remove excess surface water (2J). The vegetables were divided into 40-g portions and were packed into polyethylene bags and vacuum sealed (AP 500, TecMaq, Sao Paulo, Brazil). Irradiation process. Packages of contaminated spinach, in triplicate, were transported in ice to the irradiation plant at the Institute of Energy and Nuclear Research (Sao Paulo, Brazil). All samples were irradiated using gamma radiation originating from 60Co. The samples were exposed to doses of 0.2, 0.4, 0.6, 0.8, and 1.0 kGy. Control samples (nonirradiated) were also used. The experiment was repeated three times. A red perspex dosimeter (Harwell Dosimeters, Didcot, Oxfordshire, UK) was used, with 75% accuracy and 72% precision. The slopes of the individual survivor curves were calculated with linear regression using Microsoft Excel 97 SR- 2 (Microsoft Corporation, Redmond, WA). The D 10-value was calculated by taking the negative reciprocal of the survivor curve slope.
SPINACH IRRADIATION
1769
FIGURE 2. Population o f L. monocytogenes in spinach exposed to gamma radiation. Enumeration of Salmonella strains, STEC, and L. monocytogenes. Irradiated and nonirradiated samples were maintained under refrigeration (4°C) until microbiological testing began. Portions of spinach (25 g) were weighed and homogenized with 225 ml of 0.85% NaCl solution (LabSynth) using a Stomacher 400 (Seward, London, UK) for 1 min and were serially diluted with 0.85% NaCl solution (LabSynth). From each dilution, 1 ml was pour plated using TSA-YE for L. monocytogenes and TSA for Salmonella strains and STEC (26). To determine the population of surviving organisms after solidification, an overlay of Oxford agar (Oxoid) was added for L. monocytogenes, an overlay of mannitol lysine crystal violet brilliant green agar for Salmonella strains, and an overlay of MacConkey sorbitol agar supplemented with potassium tellurite and cefixime for STEC. Two pour plates per dilution were incubated at 37°C for 24 h. Each analysis was performed three times. Sensory evaluation. Packages of minimally processed spinach (400 g), either uninoculated, exposed to 1 and 1.5 kGy, or nonirradiated, were submitted to sensory evaluation on days 0, 5, 9, and 12. Spinach samples were analyzed for acceptability, which was measured by an unstructured hedonic scale of nine points, ranging from extremely disliked (point 1) to extremely liked (point 9) (16). The panelists, consisting of 30 untrained members of the faculty, staff, and students of the University of Sao Paulo, evaluated the appearance, flavor, aroma, and texture of products. Spinach was maintained under refrigeration (4°C), and irradiated and nonirra diated samples (10 g) were served in white plastic dishes coded with three-digit random numbers. The samples were evaluated under white light in individual cabins, in the Sensory Food Analysis Laboratory, Faculty of Pharmaceutical Sciences, Univer sity of Sao Paulo. Data were submitted to a two-way analysis of variance (SAS Institute Inc., Cary, NC) followed by Tukey’s means comparison test (P £ 0.05).
RESULTS AND DISCUSSION Resistance of Salmonella strains, STEC, and Liste ria monocytogenes in irradiated spinach. Figures 1, 2, and 3 show, respectively, the inactivation curves of Salmonella, STEC, and L. monocytogenes inoculated in minimally processed spinach and exposed to doses of 0.2, 0.4, 0.6, 0.8, and 1.0 kGy. The Z)10-values determined in this study ranged from 0.19 to 0.20 kGy for Salmonella strains and from 0.20 to 0.21 kGy for L. monocytogenes; for STEC, this value was 0.17 kGy.
1770
REZENDE ET AL.
J. Food Prot., Vol. 77, No. 10
FIGURE 3. Population of Shiga toxin-producing E. coli in spinach exposed to gamma radiation.
The D io-values obtained in this study for Salmonella strains (Fig. 1) are similar to those obtained by Nunes et al. (26) on arugula (0.16 to 0.19 kGy) and those reported by Goularte et al. (13) in lettuce (0.16 to 0.23 kGy). Results presented here were lower than those that Dhokane et al. (10) found for Salmonella inoculated in carrot and cucumber, 0.31 and 0.35 kGy, respectively. Lee et al. (18) also obtained high results when the microorganism was inoculated in spinach and cucumber (0.37 and 0.40 kGy, respectively), as did Martins et al. (20) for Salmonella in watercress (0.33 to 0.42 kGy). ForL. monocytogenes, D,,)-values (Fig. 2) were similar to those reported by Bari et al. (3) on endive, cabbage, and tomato; Niemira (21) in lettuce; Niemira et al. (24) on endive; and Tsuhako (31) in lettuce; these studies obtained results ranging from 0.19 to 0.25 kGy. However, Lee et al. (18) reported higher values than those presented here for L. ivanovii inoculated into spinach and cucumber, 0.41 and 0.34 kGy, respectively; and Nunes et al. (26) reported values from 0.37 to 0.48 kGy for L. monocytogenes inoculated on arugula. Figure 3 shows D |0-values (0.17 kGy) for STEC that are similar to those obtained by Hsu et al. (15) in irradiated mint (0.17 kGy). However, Niemira et al. (25), working with several varieties of lettuce (iceberg, red lettuce, green
leaf, and Boston), found values that ranged from 0.12 to 0.14 kGy. Ionizing radiation has also been shown effective in reducing E. coli 0157:H7 internalized in lettuce and spinach (22, 23). This is important because, once internalized, bacteria are protected from antimicrobial surface treatment. The D iQ-values obtained by Niemira (22, 23) for E. coli 0157:H7 ranged from 0.30 to 0.45 kGy in several varieties of lettuce, whereas for spinach leaves the value was 0.27 kGy. £>io-values may vary depending on the type of vegetable, as well as different cultivars of the same vegetable, due to moisture content variation, the presence of chemical compounds that protect microorganisms from action from the radiolytic products, or other factors that so far have not been identified. This was confirmed by Niemira (21), whose analysis of four types of lettuce (Boston, iceberg, green leaf, and red lettuce) found D ev alu es of 0.31, 0.25, 0.23, and 0.24 kGy, respectively. Sensory evaluation. Whereas irradiation can improve the microbiological safety of minimally processed vegeta bles, it can also cause undesirable changes in the sensory characteristics of some vegetables, depending on the dose. In the case of spinach, in general, both the control and the irradiated (1 to 1.5 kGy) samples were accepted by the panelists. Table 1 shows the averages allocated to general appearance, aroma, flavor, and texture of the samples. Although a decay in grades was seen during the storage period, there was no statistically significant difference between irradiated and control samples. However, with respect to appearance, the use of 1.5 kGy significantly reduced (P < 0.05) the acceptability of this vegetable after the ninth day of storage. The attributes of aroma, flavor, and texture were not significantly different between the control and irradiated samples during the storage period, which shows that the product was well accepted by the panelists. Several authors also found no significant difference in the acceptability of other vegetables such as lettuce, watercress,
TABLE 1. Changes in sensory scores of irradiated minimally processed spinach during storage at 5 °Cfor 12 daysa Storage time (days) Sensory evaluation Appearance
Aroma
Flavor
Irradiation dose (kGy)
7.45 7.32 7.63
0
6 .6
1 1.5
7.0
a
a
6 .8
a
a
6.9 7.0 6.7 7.5 7.2 7.4
a
a
a a a a a
a
a
0
0
1 1.5 °
5
1 1.5
0
1 1.5 Texture
0
a
a a a
a a a
a
a a a
7.52 a a 7.05 a a 7.32 a ac 7.0 a a 7.2 a a 7.0 a a 7.0 a a 7.3 a a 7.1 a a 6.9 a a 7.6 a a 7.6 a a
12
9
6.33 6.03
a a
a a
6.42 6.47
a
a
a
a
a
bd
6 .3
a
bed
6 .0 2
6 .0
a
a
5.9 6.4 5.9 6.3 6.4
a
a
6 .0
a
a a
6 .6
a
a
6 .2
a
a
6 .1
a
a
6.5 6.3
a
a
a a
a a a a
a a a a
6 .8
a
a
6 .8
a
6.5 6.7
a
a a
6.7 6.4
a
a
a
a
a
Means followed by the same uppercase letter in the same column and by the same lowercase letter on the same line do not differ by Tukey’s test at 5% probability.
SPINACH IRRADIATION
J. Food Prot., Vol. 77, No. 10
parsley, coriander, mint, and cauliflower irradiated with 0.5, 1.0,1.5, and 2 kGy
