- Email: [email protected]
Capacitance method to determine the microbiological quality of raw shrimp (Penaeus setiferus)
ARTICLE IN PRESS FOOD MICROBIOLOGY Food Microbiology 21 (2004) 361–364
www.elsevier.nl/locate/jnlabr/yfmic
Short Communication
Capacitance method to determine the microbiological quality of raw shrimp (Penaeus setiferus) Allie M. Metcalfe, Douglas L. Marshall* Department of Food Science and Technology, Mississippi Agricultural and Forestry Experiment Station, Mississippi State University, Box 9805, Mississippi State, MS 39762-9805, USA Received 8 July 2003; accepted 4 September 2003
Abstract The Bactometer monitoring system was used to determine the correlation between capacitance detection times and mesophilic and psychrotrophic plate counts of raw shrimp. Current methodologies to determine shrimp quality have problems with real-time analysis, which necessitates the need for development of more rapid, accurate, and ‘‘user-friendly’’ methods. Fresh raw shrimp were subjected to simulated retail display conditions at 5 C. The shrimp were sampled daily for one week for aerobic (APC) and psychrotrophic (PPC) plate counts and for Bactometer capacitance detection times. Capacitance detection times were highly correlated with APC (linear regression R ¼ 0:91; polynomial regression R ¼ 0:95) and PPC (linear regression R ¼ 0:89; polynomial regression R ¼ 0:95). This study suggests that capacitance detection times can be used as a rapid alternative method to measure shrimp microbial counts. r 2003 Elsevier Ltd. All rights reserved. Keywords: Shrimp quality; Impedance technology; Rapid methods; Capacitance
1. Introduction Increasing demand for seafood products and consumer awareness of food quality has called for better quality seafood products. However, the extremely perishable nature of seafood products makes this difficult (Miget, 1991). Many factors influence consumer acceptance of seafood products, including obvious factors such as taste, odor, and appearance (Marshall and Lehigh, 1993). Seafood quality is dependent upon the degree of spoilage or product decomposition. Several methods of evaluating seafood product freshness have been employed including sensory, microbiological, chemical, or physical properties (Botta, 1994). Currently, the most common methods used to gauge shrimp quality are microbiological and sensory analyses. These methods can be problematic in that they require trained technicians, require extended incubation times to obtain results, and have potential problems with analyst subjectivity (Marshall and Lehigh, 1997). Thus, there
*Corresponding author. Tel.: +1-601-325-8722; fax: +1-601-325-8728. E-mail address: [email protected] (D.L. Marshall). 0740-0020/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.fm.2003.09.002
is a need for development of rapid, less subjective methods for the seafood industry. Due to long time periods required to obtain results, microbiological testing is generally not a useful ‘‘realtime’’ method to deem product acceptability. However, microbiological standards still are necessary to permit monitoring and verification that processes are under control (Buchanan, 1991). Swartzentruber et al. (1980) using information by the International Commission of Microbiological Specifications for Foods, has recommended minimum standards for aerobic plate count (APC) values for shrimp and lobster tail. The marginally acceptable quality limit was 106/g for any five samples tested from a lot and an unacceptable level of 107/g. These standards are applicable to frozen cooked and raw peeled or un-peeled shrimp as well as frozen raw lobster tail. Although microbial enumeration is a relatively accurate measurement of shrimp quality, this methodology requires trained laboratory personnel and long time periods (24–48 h) to obtain results. Hence, microbial analysis is generally employed only in research environments and as quality assessment tools or for verification of microbiological control measures (Huss, 1988). Impedance-based techniques use electrical methods to measure microbial activity. Impedance is defined as the
ARTICLE IN PRESS 362
A.M. Metcalfe, D.L. Marshall / Food Microbiology 21 (2004) 361–364
resistance to flow of an alternating current as it passes through a conducting medium (Silley and Forsythe, 1996) and is the vector sum of conductance and capacitance of the medium (Marshall and Lehigh, 1993). Bacterial proliferation leads to metabolism of large weakly charged molecules within a growth medium (i.e., polysaccharides, proteins, fats) resulting in smaller, more highly charged molecules (i.e., organic acids, amino acids, fatty acids). These smaller metabolic byproducts are more highly conductive due to their charge. This increase in conductance can be monitored by instruments that measure impedance, conductance, and capacitance and can be related back to the number of micro-organisms initially present in the sample. Such methods have been developed for milk, meats, fish, and vegetables (Hardy et al., 1977; Martins and Selby, 1980; Firstenburg-Eden and Tricarico, 1983; FirstenburgEden and Klein, 1983; Russell, 1998). A novel impedance measurement (Bactometer Ivalue) has been used previously as a very rapid (o30 min) methodology to predict crustacean freshness (Wiese-Lehigh and Marshall, 1993; Marshall and Lehigh, 1993, 1997; Cotton and Marshall, 1998). This work demonstrated high correlation between impedance I-value and standard sensory odor or microbiological tests. The more traditional use of the Bactometer to measure capacitance detection times of shrimp has not been reported. Therefore, the proof of concept objective of the present study was to determine if capacitance detection times of shrimp correlated with traditional plate counts to enumerate micro-organisms.
2. Materials and methods 2.1. Preparation of shrimp Shrimp used for this study were 20–30 count headsoff, shell-on white Gulf Coast shrimp (Penaeus setiferus) obtained from wholesale dealers in Gulf Shores, Alabama during the summer months. The average individual weight of the shrimp was 18 g. Shrimp were placed in plastic bags and packed in ice chests and transported (approximately 6 h) to the laboratory. To simulate chill storage conditions, shrimp were layered to a depth of approximately 8–12 cm in an area of approximately 60 30 cm on a bed of crushed ice in a refrigerated display case. This entire setup was placed in a low temperature incubator at 5 C for 7 days. Ice was replenished 3–5 times daily as needed.
were commingled in a sterile sampling bag. Shells were not removed from shrimp for analysis. Shrimp samples were then diluted 1:10 (w/v) in sterile distilled water and homogenized on high speed for 2 min in a Waring Blender (Winsted, MA). Aliquots from these homogenate samples were then subjected to microbiological and capacitance analyses. Distilled water was used as the initial diluent because previous impedance-based studies of shrimp used this medium (Wiese-Lehigh and Marshall, 1993; Marshall and Lehigh, 1993, 1997). Subsequent dilutions were made with sterile 0.1% peptone water. 2.3. Microbial enumeration Aerobic plate counts (APC) and psychrotrophic plate counts (PPC) of appropriate dilutions were obtained using aerobic count plates (3M Petrifilmt, St. Paul, MN). Homogenized samples, prepared as stated previously, were serially diluted in sterile 0.1% peptone water. One-milliliter aliquots of appropriate dilutions were plated in duplicate and then incubated at 32 C for 24 h (APC) or at 21 C for 48 h (PPC). Following incubation, colony-forming units (CFU) were counted. Counts were then converted to CFU/g following guidelines by Vanderzant and Splittstoesser (1992). 2.4. Capacitance methodology A Bactometer (bioMerieux, Hazelwood, MO) monitoring system was used to monitor capacitance detection times. One-milliliter aliquots of each homogenized shrimp sample (1:10), as prepared above, were placed in quadruplicate Bactometer module wells. An additional 0.5 ml aliquot of sterile Tryptic Soy Broth (TSB; Difco, Ann Arbor, MI) supplemented with 1% glucose (w/v) was aseptically added to each well as an additional nutrient source. Modules were inserted into a Bactometer incubator and analysed at 30 C using test code 3 (standard capacitance algorithm) for a 24-h run cycle. Capacitance detection times of each well are automatically recorded by the Bactometer software when enough microbial growth has occurred to cause a change in capacitance over initial values recorded at the beginning of an analytical run. Detection times occur early when large numbers of target microbes are present in the sample and occur later when fewer numbers are present. Detection will not occur if the sample is free of target microbes. 2.5. Statistical analyses
2.2. Experimental protocol On each sampling day, 5 shrimp were removed with sterile forceps using a rotating sampling protocol to insure even distribution of sampling sites. The 5 shrimp
Microbiological and impedance analyses were done using three complete replications. All microbiological data were converted to log values before statistical analysis. Regression analysis using the Excel program
ARTICLE IN PRESS A.M. Metcalfe, D.L. Marshall / Food Microbiology 21 (2004) 361–364
(Microsoft Co., Seattle, WA) was performed to determine correlation coefficients between APC and capacitance detection time and PPC and capacitance detection time.
363
10
9
8
3. Results and discussion Use of capacitance microbiology to rapidly estimate the numbers of bacteria present in a given sample via detection times was investigated. Detection times were shown to be highly correlated with APC and PPC. Fig. 1 depicts correlation of capacitance detection times with APC. Regression analysis trend lines were done with both a linear fit, resulting in R ¼ 0:91; and a polynomial fit with R ¼ 0:95: Fig. 2 depicts correlation of capacitance detection times with PPC. Again regression analysis trend lines were fit with a linear model resulting in R ¼ 0:89 and a polynomial model with R ¼ 0:95: As log counts increased, detection times decreased. Russell (1998) used capacitance microbiology as a means of determining the quantity of bacteria on fish fillets. His results from this study were obtained in less than 9 h versus 24–48 h to obtain results using microbiological plating methods. In the present study, high correlations of capacitance detection times with both
Detection Times (Hours)
7
6
5
Linear Regression y = -1.94x + 17.2 R = -0.89 4
Polynomial Regression y = -0.85x2 + 8.31x - 12.6 R = -0.95
3
2
1
0 0
1
2
3
4
5
6
7
8
9
log 10 cfu/g
Fig. 2. Correlation of capacitance detection times with psychrotrophic plate counts.
9
8
7
Detection Time (Hours)
6
Linear Regression y = -1.81x + 15.6 R = -0.91
5
Polynomial Regression y = -0.56x 2 + 4.54x - 1.65 R = -0.95
4
3
2
1
0 0
1
2
3
4 log
5 10
6
7
8
9
cfu/g
Fig. 1. Correlation of capacitance detection times with aerobic plate counts.
APC and PPC suggests that detection times can be used as a rapid alternative to traditional microbiological plating to estimate microbial loads of shrimp. Capacitance detection times were attained within 9 h. Further refinements with media and incubation temperatures may improve performance of the detection time technique. The use of capacitance detection times to rapidly determine whether shrimp are microbiologically acceptable using specifications of Swartzentruber et al. (1980) revealed that unacceptable product with APC above 107/g could be detected using the Bactometer within 1– 3 h (Fig. 1). Marginal shrimp (APC between 106/g and 107/g) were detected within 3–5 h. Acceptable shrimp with APC less than 106/g were detectable with the Bactometer after 5 h. If PPC rather than APC is the acceptability indicator of choice, capacitance detection times were slightly longer to determine unacceptable (o4 h), marginal (4–5.5 h), and acceptable (> 5:5 h) quality. The present study utilized Bactometer capacitance detection times to rapidly determine shrimp microbial counts. The major advantages of the Bactometer versus other microbial count methods is the speed in which results can be obtained (at least 15 h earlier than
ARTICLE IN PRESS 364
A.M. Metcalfe, D.L. Marshall / Food Microbiology 21 (2004) 361–364
standard plate count methods) and the fact that time consuming and tedious dilutions are not needed. An additional advantage is that all results can be archived in the Bactometer computer. Additional work is needed to define the best medium, incubation time, and incubation temperature for routine microbial count determinations of shrimp.
Acknowledgements Approved for publication as Journal Article No. J10373 of the Mississippi Agricultural and Forestry Experiment Station. This work was supported in part by an USDA-CSREES Special Grant No. 98-342316002 and by the Mississippi Agricultural and Forestry Experiment Station under project MIS-081040.
References Botta, J.R., 1994. Freshness quality of seafoods: a review. In: Shahidi, F., Botta, J.R. (Eds.), Seafoods Chemistry, Processing, Technology and Quality. Blackie Academic & Professional, London, pp. 140–167. Buchanan, R.L., 1991. Microbiological criteria for cooked, ready-toeat shrimp and crabmeat. Food Technol. 45 (4), 157–160. Cotton, L.N., Marshall, D.L., 1998. Rapid impediometric method to determine crustacean food freshness. In: Tunick, M.H., Palumbo, S.A., Fratamico, P.M. (Eds.), New Techniques in the Analysis of Foods. Plenum Publishing Corp, New York, pp. 147–160 (Chapter 13). Firstenburg-Eden, R., Klein, C.S., 1983. Evaluation of a rapid impedimetric procedure for the quantitative estimation of coliforms. J. Food Sci. 48, 1307–1311.
Firstenburg-Eden, R., Tricarico, M.K., 1983. Impedimetric determination of total, mesophilic and psychrotrophic counts in raw milk. J. Food Sci. 48, 1750–1754. Hardy, D., Kraeger, S.J., Dufour, S.W., Cady, P., 1977. Rapid detection of microbial contamination in frozen vegetables by automated impedance measurements. Appl. Environ. Microbiol. 34, 14–17. Huss, H.H., 1988. Fresh Fish-Quality and Quality Changes. FAO Fisheries Series No. 29, FAO, Rome, pp. 1–118. Marshall, D.L., Lehigh, P.L.W., 1993. Nobody’s nose knows. CHEMTECH 23, 38–42. Marshall, D.L., Lehigh, P.L.W., 1997. Comparison of impedance, microbial, sensory, and pH methods to determine shrimp quality. J. Aquatic Food Product Tech. 6, 17–31. Martins, S.B., Selby, M.J., 1980. Evaluation of a rapid method for the quantitative estimation of coliforms in meat by impedimetric analysis. Appl. Environ. Microbiol. 39, 518–524. Miget, R.J., 1991. Microbiology of crustacean processing: shrimp, crawfish, and prawns. In: Ward, D.R., Hackney, C.R. (Eds.), Microbiology of Marine Food Products. Van Nostrand Reinhold, New York, pp. 65–87 (Chapter 4). Russell, S.M., 1998. Capacitance microbiology as a means of determining the quantity of spoilage bacteria on fish fillets. J. Food Prot. 61, 844–848. Silley, P., Forsythe, S., 1996. Impedance microbiology—a rapid change for microbiologists. J. Appl. Bacteriol. 80, 233–243. Swartzentruber, A., Schwab, A.H., Duran, A.P., Wentz, B.A., Read Jr., R.B., 1980. Microbiological quality of frozen shrimp and lobster tail in the retail market. Appl. Environ. Microbiol 40, 765–769. Vanderzant, C., Splittstoesser, D.F., 1992. Compendium of Methods for the Microbiological Examination of Foods, 3rd Edition. American Public Health Association, Washington, DC, p. 1219. Wiese-Lehigh, P.L., Marshall, D.L., 1993. Determination of seafood freshness using impedance technology. In: Spanier, A.M., Okai, H., Tamura, M. (Eds.), Food Flavor & Safety: Molecular Analysis and Design. ACS Symposium Series No. 528. American Chemical Society, Washington, DC, pp. 248–261 (Chapter 20).
www.elsevier.nl/locate/jnlabr/yfmic
Short Communication
Capacitance method to determine the microbiological quality of raw shrimp (Penaeus setiferus) Allie M. Metcalfe, Douglas L. Marshall* Department of Food Science and Technology, Mississippi Agricultural and Forestry Experiment Station, Mississippi State University, Box 9805, Mississippi State, MS 39762-9805, USA Received 8 July 2003; accepted 4 September 2003
Abstract The Bactometer monitoring system was used to determine the correlation between capacitance detection times and mesophilic and psychrotrophic plate counts of raw shrimp. Current methodologies to determine shrimp quality have problems with real-time analysis, which necessitates the need for development of more rapid, accurate, and ‘‘user-friendly’’ methods. Fresh raw shrimp were subjected to simulated retail display conditions at 5 C. The shrimp were sampled daily for one week for aerobic (APC) and psychrotrophic (PPC) plate counts and for Bactometer capacitance detection times. Capacitance detection times were highly correlated with APC (linear regression R ¼ 0:91; polynomial regression R ¼ 0:95) and PPC (linear regression R ¼ 0:89; polynomial regression R ¼ 0:95). This study suggests that capacitance detection times can be used as a rapid alternative method to measure shrimp microbial counts. r 2003 Elsevier Ltd. All rights reserved. Keywords: Shrimp quality; Impedance technology; Rapid methods; Capacitance
1. Introduction Increasing demand for seafood products and consumer awareness of food quality has called for better quality seafood products. However, the extremely perishable nature of seafood products makes this difficult (Miget, 1991). Many factors influence consumer acceptance of seafood products, including obvious factors such as taste, odor, and appearance (Marshall and Lehigh, 1993). Seafood quality is dependent upon the degree of spoilage or product decomposition. Several methods of evaluating seafood product freshness have been employed including sensory, microbiological, chemical, or physical properties (Botta, 1994). Currently, the most common methods used to gauge shrimp quality are microbiological and sensory analyses. These methods can be problematic in that they require trained technicians, require extended incubation times to obtain results, and have potential problems with analyst subjectivity (Marshall and Lehigh, 1997). Thus, there
*Corresponding author. Tel.: +1-601-325-8722; fax: +1-601-325-8728. E-mail address: [email protected] (D.L. Marshall). 0740-0020/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.fm.2003.09.002
is a need for development of rapid, less subjective methods for the seafood industry. Due to long time periods required to obtain results, microbiological testing is generally not a useful ‘‘realtime’’ method to deem product acceptability. However, microbiological standards still are necessary to permit monitoring and verification that processes are under control (Buchanan, 1991). Swartzentruber et al. (1980) using information by the International Commission of Microbiological Specifications for Foods, has recommended minimum standards for aerobic plate count (APC) values for shrimp and lobster tail. The marginally acceptable quality limit was 106/g for any five samples tested from a lot and an unacceptable level of 107/g. These standards are applicable to frozen cooked and raw peeled or un-peeled shrimp as well as frozen raw lobster tail. Although microbial enumeration is a relatively accurate measurement of shrimp quality, this methodology requires trained laboratory personnel and long time periods (24–48 h) to obtain results. Hence, microbial analysis is generally employed only in research environments and as quality assessment tools or for verification of microbiological control measures (Huss, 1988). Impedance-based techniques use electrical methods to measure microbial activity. Impedance is defined as the
ARTICLE IN PRESS 362
A.M. Metcalfe, D.L. Marshall / Food Microbiology 21 (2004) 361–364
resistance to flow of an alternating current as it passes through a conducting medium (Silley and Forsythe, 1996) and is the vector sum of conductance and capacitance of the medium (Marshall and Lehigh, 1993). Bacterial proliferation leads to metabolism of large weakly charged molecules within a growth medium (i.e., polysaccharides, proteins, fats) resulting in smaller, more highly charged molecules (i.e., organic acids, amino acids, fatty acids). These smaller metabolic byproducts are more highly conductive due to their charge. This increase in conductance can be monitored by instruments that measure impedance, conductance, and capacitance and can be related back to the number of micro-organisms initially present in the sample. Such methods have been developed for milk, meats, fish, and vegetables (Hardy et al., 1977; Martins and Selby, 1980; Firstenburg-Eden and Tricarico, 1983; FirstenburgEden and Klein, 1983; Russell, 1998). A novel impedance measurement (Bactometer Ivalue) has been used previously as a very rapid (o30 min) methodology to predict crustacean freshness (Wiese-Lehigh and Marshall, 1993; Marshall and Lehigh, 1993, 1997; Cotton and Marshall, 1998). This work demonstrated high correlation between impedance I-value and standard sensory odor or microbiological tests. The more traditional use of the Bactometer to measure capacitance detection times of shrimp has not been reported. Therefore, the proof of concept objective of the present study was to determine if capacitance detection times of shrimp correlated with traditional plate counts to enumerate micro-organisms.
2. Materials and methods 2.1. Preparation of shrimp Shrimp used for this study were 20–30 count headsoff, shell-on white Gulf Coast shrimp (Penaeus setiferus) obtained from wholesale dealers in Gulf Shores, Alabama during the summer months. The average individual weight of the shrimp was 18 g. Shrimp were placed in plastic bags and packed in ice chests and transported (approximately 6 h) to the laboratory. To simulate chill storage conditions, shrimp were layered to a depth of approximately 8–12 cm in an area of approximately 60 30 cm on a bed of crushed ice in a refrigerated display case. This entire setup was placed in a low temperature incubator at 5 C for 7 days. Ice was replenished 3–5 times daily as needed.
were commingled in a sterile sampling bag. Shells were not removed from shrimp for analysis. Shrimp samples were then diluted 1:10 (w/v) in sterile distilled water and homogenized on high speed for 2 min in a Waring Blender (Winsted, MA). Aliquots from these homogenate samples were then subjected to microbiological and capacitance analyses. Distilled water was used as the initial diluent because previous impedance-based studies of shrimp used this medium (Wiese-Lehigh and Marshall, 1993; Marshall and Lehigh, 1993, 1997). Subsequent dilutions were made with sterile 0.1% peptone water. 2.3. Microbial enumeration Aerobic plate counts (APC) and psychrotrophic plate counts (PPC) of appropriate dilutions were obtained using aerobic count plates (3M Petrifilmt, St. Paul, MN). Homogenized samples, prepared as stated previously, were serially diluted in sterile 0.1% peptone water. One-milliliter aliquots of appropriate dilutions were plated in duplicate and then incubated at 32 C for 24 h (APC) or at 21 C for 48 h (PPC). Following incubation, colony-forming units (CFU) were counted. Counts were then converted to CFU/g following guidelines by Vanderzant and Splittstoesser (1992). 2.4. Capacitance methodology A Bactometer (bioMerieux, Hazelwood, MO) monitoring system was used to monitor capacitance detection times. One-milliliter aliquots of each homogenized shrimp sample (1:10), as prepared above, were placed in quadruplicate Bactometer module wells. An additional 0.5 ml aliquot of sterile Tryptic Soy Broth (TSB; Difco, Ann Arbor, MI) supplemented with 1% glucose (w/v) was aseptically added to each well as an additional nutrient source. Modules were inserted into a Bactometer incubator and analysed at 30 C using test code 3 (standard capacitance algorithm) for a 24-h run cycle. Capacitance detection times of each well are automatically recorded by the Bactometer software when enough microbial growth has occurred to cause a change in capacitance over initial values recorded at the beginning of an analytical run. Detection times occur early when large numbers of target microbes are present in the sample and occur later when fewer numbers are present. Detection will not occur if the sample is free of target microbes. 2.5. Statistical analyses
2.2. Experimental protocol On each sampling day, 5 shrimp were removed with sterile forceps using a rotating sampling protocol to insure even distribution of sampling sites. The 5 shrimp
Microbiological and impedance analyses were done using three complete replications. All microbiological data were converted to log values before statistical analysis. Regression analysis using the Excel program
ARTICLE IN PRESS A.M. Metcalfe, D.L. Marshall / Food Microbiology 21 (2004) 361–364
(Microsoft Co., Seattle, WA) was performed to determine correlation coefficients between APC and capacitance detection time and PPC and capacitance detection time.
363
10
9
8
3. Results and discussion Use of capacitance microbiology to rapidly estimate the numbers of bacteria present in a given sample via detection times was investigated. Detection times were shown to be highly correlated with APC and PPC. Fig. 1 depicts correlation of capacitance detection times with APC. Regression analysis trend lines were done with both a linear fit, resulting in R ¼ 0:91; and a polynomial fit with R ¼ 0:95: Fig. 2 depicts correlation of capacitance detection times with PPC. Again regression analysis trend lines were fit with a linear model resulting in R ¼ 0:89 and a polynomial model with R ¼ 0:95: As log counts increased, detection times decreased. Russell (1998) used capacitance microbiology as a means of determining the quantity of bacteria on fish fillets. His results from this study were obtained in less than 9 h versus 24–48 h to obtain results using microbiological plating methods. In the present study, high correlations of capacitance detection times with both
Detection Times (Hours)
7
6
5
Linear Regression y = -1.94x + 17.2 R = -0.89 4
Polynomial Regression y = -0.85x2 + 8.31x - 12.6 R = -0.95
3
2
1
0 0
1
2
3
4
5
6
7
8
9
log 10 cfu/g
Fig. 2. Correlation of capacitance detection times with psychrotrophic plate counts.
9
8
7
Detection Time (Hours)
6
Linear Regression y = -1.81x + 15.6 R = -0.91
5
Polynomial Regression y = -0.56x 2 + 4.54x - 1.65 R = -0.95
4
3
2
1
0 0
1
2
3
4 log
5 10
6
7
8
9
cfu/g
Fig. 1. Correlation of capacitance detection times with aerobic plate counts.
APC and PPC suggests that detection times can be used as a rapid alternative to traditional microbiological plating to estimate microbial loads of shrimp. Capacitance detection times were attained within 9 h. Further refinements with media and incubation temperatures may improve performance of the detection time technique. The use of capacitance detection times to rapidly determine whether shrimp are microbiologically acceptable using specifications of Swartzentruber et al. (1980) revealed that unacceptable product with APC above 107/g could be detected using the Bactometer within 1– 3 h (Fig. 1). Marginal shrimp (APC between 106/g and 107/g) were detected within 3–5 h. Acceptable shrimp with APC less than 106/g were detectable with the Bactometer after 5 h. If PPC rather than APC is the acceptability indicator of choice, capacitance detection times were slightly longer to determine unacceptable (o4 h), marginal (4–5.5 h), and acceptable (> 5:5 h) quality. The present study utilized Bactometer capacitance detection times to rapidly determine shrimp microbial counts. The major advantages of the Bactometer versus other microbial count methods is the speed in which results can be obtained (at least 15 h earlier than
ARTICLE IN PRESS 364
A.M. Metcalfe, D.L. Marshall / Food Microbiology 21 (2004) 361–364
standard plate count methods) and the fact that time consuming and tedious dilutions are not needed. An additional advantage is that all results can be archived in the Bactometer computer. Additional work is needed to define the best medium, incubation time, and incubation temperature for routine microbial count determinations of shrimp.
Acknowledgements Approved for publication as Journal Article No. J10373 of the Mississippi Agricultural and Forestry Experiment Station. This work was supported in part by an USDA-CSREES Special Grant No. 98-342316002 and by the Mississippi Agricultural and Forestry Experiment Station under project MIS-081040.
References Botta, J.R., 1994. Freshness quality of seafoods: a review. In: Shahidi, F., Botta, J.R. (Eds.), Seafoods Chemistry, Processing, Technology and Quality. Blackie Academic & Professional, London, pp. 140–167. Buchanan, R.L., 1991. Microbiological criteria for cooked, ready-toeat shrimp and crabmeat. Food Technol. 45 (4), 157–160. Cotton, L.N., Marshall, D.L., 1998. Rapid impediometric method to determine crustacean food freshness. In: Tunick, M.H., Palumbo, S.A., Fratamico, P.M. (Eds.), New Techniques in the Analysis of Foods. Plenum Publishing Corp, New York, pp. 147–160 (Chapter 13). Firstenburg-Eden, R., Klein, C.S., 1983. Evaluation of a rapid impedimetric procedure for the quantitative estimation of coliforms. J. Food Sci. 48, 1307–1311.
Firstenburg-Eden, R., Tricarico, M.K., 1983. Impedimetric determination of total, mesophilic and psychrotrophic counts in raw milk. J. Food Sci. 48, 1750–1754. Hardy, D., Kraeger, S.J., Dufour, S.W., Cady, P., 1977. Rapid detection of microbial contamination in frozen vegetables by automated impedance measurements. Appl. Environ. Microbiol. 34, 14–17. Huss, H.H., 1988. Fresh Fish-Quality and Quality Changes. FAO Fisheries Series No. 29, FAO, Rome, pp. 1–118. Marshall, D.L., Lehigh, P.L.W., 1993. Nobody’s nose knows. CHEMTECH 23, 38–42. Marshall, D.L., Lehigh, P.L.W., 1997. Comparison of impedance, microbial, sensory, and pH methods to determine shrimp quality. J. Aquatic Food Product Tech. 6, 17–31. Martins, S.B., Selby, M.J., 1980. Evaluation of a rapid method for the quantitative estimation of coliforms in meat by impedimetric analysis. Appl. Environ. Microbiol. 39, 518–524. Miget, R.J., 1991. Microbiology of crustacean processing: shrimp, crawfish, and prawns. In: Ward, D.R., Hackney, C.R. (Eds.), Microbiology of Marine Food Products. Van Nostrand Reinhold, New York, pp. 65–87 (Chapter 4). Russell, S.M., 1998. Capacitance microbiology as a means of determining the quantity of spoilage bacteria on fish fillets. J. Food Prot. 61, 844–848. Silley, P., Forsythe, S., 1996. Impedance microbiology—a rapid change for microbiologists. J. Appl. Bacteriol. 80, 233–243. Swartzentruber, A., Schwab, A.H., Duran, A.P., Wentz, B.A., Read Jr., R.B., 1980. Microbiological quality of frozen shrimp and lobster tail in the retail market. Appl. Environ. Microbiol 40, 765–769. Vanderzant, C., Splittstoesser, D.F., 1992. Compendium of Methods for the Microbiological Examination of Foods, 3rd Edition. American Public Health Association, Washington, DC, p. 1219. Wiese-Lehigh, P.L., Marshall, D.L., 1993. Determination of seafood freshness using impedance technology. In: Spanier, A.M., Okai, H., Tamura, M. (Eds.), Food Flavor & Safety: Molecular Analysis and Design. ACS Symposium Series No. 528. American Chemical Society, Washington, DC, pp. 248–261 (Chapter 20).