Dry rehydratable film method for rapid enumeration of coliforms in foods (3M Petrifilm Rapid Coliform Count plate): collaborative study

A rehydratable dry-film plating method for coliforms in foods, the 3M Petrifilm Rapid Coliform Count plate method, was compared with the U.S. Food and Drug Administration's Bacteriological Analytical Manual method for nondairy foods and the American Public Health Association's Standard Methods for the Examination of Dairy Products (SMEDP) method for dairy foods. Six food types, vanilla ice cream, cheddar cheese, fresh refrigerated uncooked pasta, wheat flour, prepared frozen macaroni and cheese, and frozen hash browns, were analyzed for coliforms by 11 collaborating laboratories. For each food product tested, the collaborators received 8 blind samples consisting of a control sample and 3 levels of inoculated sample, each in duplicate. The mean log counts for the methods were comparable. The repeatability and reproducibility variances of the Petrifilm Rapid Coliform Count method at 14 and 24 h were not significantly different from those of the standard methods.     

Evaluation of a dry, rehydratable film method for rapid enumeration of Staphylococcus aureus

Results with the new 3M Petrifilm Rapid S. aureus Count (RSA) Plate method were compared with those of the classical Baird-Parker agar (BPA) method for detection and enumeration of Staphylococcus aureus. Studies on 219 bacterial strains demonstrated that the Petrifilm RSA plate is more sensitive than and as specific as the classical BPA method for confirmed identification of S. aureus. Counts of colonies from 71 pure cultures, 61 naturally contaminated food samples, and more than 750 artificially inoculated food samples showed that the Petrifilm RSA method was as effective as the classical BPA method for identification and enumeration of S. aureus. The Petrifilm RSA method gave results in one-third the time required for the classical method.     

Dry rehydratable film method for enumerating confirmed Escherichia coli in poultry, meats, and seafood: collaborative study

A rehydratable dry-film plating method for Escherichia coli, the Petrifilm E. coli/Coliform (EC) Count Plate in foods, has been compared with the AOAC INTERNATIONAL most probable number (MPN) method. Eleven laboratories participated in the collaborative study. Three E. coli levels in 8 samples each of frozen raw ground turkey, frozen raw ground beef, and frozen cooked fish were tested in duplicate. Mean log counts for the Petrifilm plate procedure were not significantly different from those for the MPN procedure for cooked fish samples inoculated with low or high inocula levels, for samples of raw turkey inoculated at medium level, and for beef inoculated at low, medium, and high levels. Repeatability and reproducibility variances of the Petrifilm EC Plate method recorded at 24 h were as good as or better than those of the MPN method. The dry rehydratable film method for enumerating confirmed E. coli in poultry, meats, and seafood has been adopted first action by AOAC INTERNATIONAL.     

Comparison of Petrifilm Staph Express Count System with the bacteriological analytical manual direct-plating method for enumeration of Staphylococcus aureus in artificially contaminated hard cheese

The 3M Petrifilm Staph Express Count System was compared with the U.S. Food and Drug Administration's Bacteriological Analytical Manual (BAM) direct-plate count method for the enumeration of Staphylococcus aureus in 6 types of artificially contaminated hard cheese (Asiago, Cheddar, Gruyère, Parmesan, Romano, and Swiss). Five different samples of each cheese type were inoculated with S. aureus (ATCC 25923) to achieve low, medium, and high inoculum levels. S. aureus was enumerated by the Petrifilm and BAM methods, and the results were compared. Multivariate analysis of variance revealed no significant differences (P<0.05) between the 2 methods. The Petrifilm method compared favorably with the BAM procedure. The rapid method was more convenient to use, considerably faster, and less expensive to perform than the BAM method.     

3M Petrifilm Staph Express Count plate method for the enumeration of Staphylococcus aureus in selected types of processed and prepared foods: collaborative study

The 3M Petrifilm Staph Express Count plate method was compared with AOAC Official Method 975.55 for the enumeration of Staphylococcus aureus in selected foods. Five foods--frozen lasagna, custard, frozen mixed vegetables, frozen hashbrowns, and frozen batter-coated mushrooms--were analyzed for S. aureus by 13 collaborating laboratories. For each food tested, the collaborators received 8 blind test samples consisting of a control sample, a low inoculation level, a medium inoculation level, and a medium inoculation level with background flora, each in duplicate. The mean log10 counts for the methods were comparable for all 5 foods. The repeatability and reproducibility variances of the 24 h Petrifilm Staph Express Count plate method were similar to those of the 72 h standard method.     

3M Petrifilm Staph Express Count plate method for the enumeration of Staphylococcus aureus in selected types of meat, seafood, and poultry: collaborative study

The 3M Petrifilm Staph Express Count plate method was compared with AOAC Official Method 975.55 for the enumeration of Staphylococcus aureus in selected foods. Four foods--cooked, diced chicken; cured ham; smoked salmon; and pepperoni--were analyzed for S. aureus by 12 collaborating laboratories. For each food tested, the collaborators received 8 blind test samples consisting of a control sample, a low inoculation level, a medium inoculation level, and a medium inoculation level with background flora, each in duplicate. The mean log10 counts for the methods were comparable for all 4 foods. The repeatability and reproducibility variances of the 24 h Petrifilm Staph Express Count plate method were similar to those of the 72 h standard method.     

3M Petrifilm Staph Express Count plate method for the enumeration of Staphylococcus aureus in selected dairy foods: collaborative study

The 3M Petrifilm Staph Express Count plate method was compared with AOAC Official Method 975.55 for the enumeration of Staphylococcus aureus in selected foods. Five foods--ice cream, raw milk, yogurt, whey powder, and cheese--were analyzed for S. aureus by 12 collaborating laboratories. For each food tested, the collaborators received 8 blind test samples consisting of a control sample, a low inoculation level, a medium inoculation level, and a medium inoculation level with background flora, each in duplicate. The mean log10 counts for the methods were comparable for all 5 foods. The repeatability and reproducibility variances of the 24 h Petrifilm Staph Express Count plate method were similar to those of the 72 h standard method.     

3M Petrifilm enterobacteriaceae count plate method for enumeration of enterobacteriaceae in selected foods: collaborative study

The practice of detecting and enumerating all oxidase-negative, glucose-fermenting-Gram-negative rods (i.e., the family Enterobacteriaceae) is used to indicate unsanitary or inadequate food processing conditions. The objective of this interlaboratory collaborative study was to evaluate and compare the methods described in Standard Methods for the Examination of Dairy Products (SMEDP) and the Compendium of Methods for the Microbiological Examination of Foods (Compendium) with a commercial product, the 3M Petrifilm Enterobacteriaceae Count Plate, for the recovery of Enterobacteriaceae in foods. Six foods--cheddar cheese, milk, flour, frozen prepared meals, frozen broccoli, and nut pieces--were analyzed for Enterobacteriaceae by 12 collaborating laboratories. For each food tested, the collaborators received 8 blind test portions consisting of a control test portion and 3 levels of inoculated test portion, each in duplicate. Each test portion was tested by the Petrifilm Enterobacteriaceae Count Plate method as well as the SMEDP or Compendium methods. The precision estimates (repeatability or within-laboratory variation, and reproducibility or between-laboratory variation) were calculated with standard statistical techniques.     

Validation of RAPID E. coli 2 for enumeration and differentiation of Escherichia coli and other coliform bacteria in selected foods--Performance-Tested Method 050601

RAPID'E. coli 2 agar (Bio-Rad Laboratories, Hercules, CA) is a chromogenic medium for differentiation and enumeration of E. coli and non-E. coli coliform bacteria in food. The principle of RAPID'E. coli 2 medium relies on simultaneous detection of 2 enzymatic activities, P-D-glucuronidase (GLUC) and beta-D-galactosidase (GAL). Coliforms, other than E. coli (GAL+/GLUC-), form blue to green colonies, whereas, specifically, E. coli (GAL+/GLUC+) form violet colonies. Eleven foods (raw ground beef, raw boneless pork, fermented sausage, processed ham, processed turkey, frozen turkey breast, raw ground chicken, cottage cheese, ricotta cheese, raw milk, and dry infant formula) were validated, comparing the performance of RAPID'E. coli 2 agar to the reference method AOAC 966.24. Two sample incubation temperatures were evaluated, 37 and 44 degrees C, testing a mixture of naturally and artificially contaminated foods. If naturally contaminated food was not available, the matrix was artificially inoculated with one E. coli strain and one non-E. coli coliform strain. Method comparison studies demonstrated some statistical differences between the 2 methods, which are expected when a plating method is compared to a most probable number method. Inclusivity and exclusivity rates of the medium were 99 and 94%, respectively. The RAPID'E. coli 2 method was shown to be stable when minor variations were introduced.     

Singlepath Salmonella. Performance-Tested Method 060401

Singlepath Salmonella is an immunochromatographic (lateral flow) assay for the presumptive qualitative detection of Salmonella spp. in food. The AOAC Performance-Tested Method study evaluated Singlepath Salmonella as an effective method for the detection of Salmonella spp. in the following selected foods: dried skimmed milk, black pepper, dried pet food, desiccated coconut, cooked peeled frozen prawns, raw ground beef, and raw ground turkey. When the foods were inoculated with Salmonella spp. at levels ranging from low [0.23-1.08 colony forming units (CFU)/25 g] to high (2.3-6.0 CFU/25 g), a Chi-square value of 0.9 indicated that there was no significant difference between Singlepath Salmonella and the ISO 6579:2002 reference method. Singlepath Salmonella gave a false-positive rate of 7.3% and a false-negative rate of 2.5%. For the inclusivity study, all 105 Salmonella serovars reacted with Singlepath Salmonella. For the exclusivity study, 58 non-Salmonella spp. were tested. There were no cross-reactions with Singlepath Salmonella from these strains.     

Evaluation of VIDAS Salmonella (SLM) immunoassay method with Rappaport-Vassiliadis (RV) medium for detection of Salmonella in foods: collaborative study

A collaborative study was conducted to compare the VIDAS Salmonella (SLM) with Rappaport-Vassiliadis (RV) method for detection of Salmonella in foods to the current standard method presented in the U.S. Food and Drug Administration's Bacteriological Analytical Manual (BAM) and the culture method presented in AOAC's Official Methods of Analysis. The VIDAS SLM with RV method uses tetrathionate broth in combination with RV medium in place of selenite cystine broth for selective enrichment, thereby eliminating the hazardous waste issue for laboratories. Twenty five laboratories participated in the evaluation, each testing one or more of 8 test products: nonfat dry milk, dried egg, soy flour, lactic casein, milk chocolate, raw ground pork, raw ground turkey, and raw peeled shrimp. Results of the study showed no significant differences in the numbers of confirmed positive samples with the VIDAS SLM with RV procedure and the BAM/AOAC culture procedure. The VIDAS SLM with RV method was effective for rapid detection of Salmonella in foods. It is recommended that AOAC INTERNATIONAL modify the VIDAS Salmonella SLM procedure to include the RV method.     

Simultaneous determination of sulfoxaflor in 14 daily foods using LC-MS/MS

Sulfoxa?or residues in 14 daily foods, including rice, sorghum, chilli, cucumber, white pear, apple, egg, beef brisket, chicken breast, fish, pork liver, milk, pine nut and honey, were simultaneously determined using a modified QuEChERS and LC–MS/MS method. These foods were classified into three categories to be purified. A combination of 25 mg of octadecylsilane (C18) + 25 mg of primary and secondary amine (PSA) + 50 mg of graphitised carbon black (GCB) + 150 mg of MgSO₄ was used to purify the rice, sorghum, honey, apple and white pear. A combination of 25 mg of C18 + 50 mg of PSA + 50 mg of GCB + 150 mg of MgSO₄ was used to purify the chilli and cucumber. A combination of 50 mg of C18 + 25 mg of PSA + 50 mg of GCB + 150 mg of MgSO₄ was used to purify the pine nuts, egg, beef brisket, chicken breast, fish, pork liver and milk. The linearity coefficient values were greater than 0.9975. The limit of detection and limit of quantification were in the ranges of 0.7?1.8 and 2.0?5.0 μg kg^?1, respectively. Average recoveries of the sulfoxa?or at the 14 food matrices at spiking levels of 5.0, 10 and 50 μg kg^?1 ranged from 74.0% to 100.8%, and the relative standard deviation ranged from 2.2% to 11.2%. This is a simple and rapid method for the determination of sulfoxa?or residues in various kinds of daily foods.     

Validation of RAPID'Staph for enumeration of Staphylococcus aureus in foods. Performance-Tested Method

RAPID'Staph (Bio-Rad Laboratories, Hercules, CA) is a medium for differentiation and enumeration of coagulase-positive Staphylococcus aureus in food. RAPID'Staph medium is based on a Baird Parker formula optimized for the detection and enumeration of S. aureus in 24 h. The principle of the medium relies on the capacity of S. aureus to reduce tellurite (production of black colonies) and to provoke proteolysis of egg yolk (production of clear halo around the colony). Four foods (pasteurized whole milk, custard pie, processed ham, and smoked salmon) were selected to compare the performance of RAPID'Staph agar to AOAC Official Method 975.55. Method comparison studies demonstrated excellent agreement between the methods. Inclusivity and exclusivity rates of the medium were 100%. RAPID'Staph agar performed as expected when minor procedural variations were introduced, validating the ruggedness of the method. There was no difference in performance between the dehydrated and ready-to-use formulations of the media.     

Detection of Salmonella in fresh cheese,poultry products,and dried egg products by the ISO 6579 Salmonella culture procedure and the AOAC official method: collaborative study

Three food types were analyzed for the presence of Salmonella by the AOAC culture method and by the International Organization for Standardization (ISO 6579:2002) culture method. Paired test portions of each food type were simultaneously analyzed by both methods. A total of 21 laboratories representing federal government agencies and private industry, in the United States and Europe, participated in this interlaboratory study. Foods were artificially contaminated with Salmonella and competing microflora if naturally contaminated sources were not available. No statistical differences (p < 0.05) were observed between the AOAC and ISO culture methods for fresh cheese and dried egg products. A statistically significant difference was observed for one of the 2 lots of poultry from the first trial. The poultry meat used in this run was radiation sterilized, artificially contaminated with Salmonella and competitive flora, and then lyophilized. A second trial was conducted with 2 separate lots of raw ground chicken that were naturally contaminated. The results from the second trial showed no statistical difference between the 2 culture methods. A third trial involving 4 laboratories was conducted on 2 separate lots of naturally contaminated raw poultry. Again, no statistically significant differences occurred. It is recommended that ISO 6579:2002 culture method for Salmonella be adopted Official First Action for the analysis of fresh cheese, fresh chilled and frozen poultry, and dried egg products.     

TECRA Unique test for rapid detection of Salmonella in food: collaborative study

The TECRA Unique Salmonella test uses the principle of immunoenrichment to allow rapid detection of Salmonellae in food. A collaborative study was conducted to compare the TECRA Salmonella Unique test with the reference culture method given in the U.S. Food and Drug Administration's Bacteriological Analytical Manual. Three food types (milk powder, pepper, and soy flour) were analyzed in Australia and 2 food types (milk chocolate and dried egg) were analyzed in the United States. Forty-one collaborators participated in the study. For each of the 5 foods at each of the 3 levels, a comparison showed no significant differences (p > or = 0.05) in the proportion of positive test samples for Unique and that for the reference method using the Chi-square test for independence with continuity correction.     

高效液相色谱-串联质谱法测定动物源性食品中矮壮素残留

建立了高效液相色谱-串联质谱测定动物源性食品中矮壮素残留的分析方法。样品经含1%（v/v）乙酸的乙腈溶液提取、正己烷脱脂、阳离子固相萃取柱（PCX）净化,采用Venusil MP C18（2）色谱柱（150 mm×2.1 mm,3 μm）分离,以乙腈和0.1%（v/v）甲酸水溶液为流动相进行梯度洗脱,采用电喷雾电离、正离子模式扫描,多反应监测模式（MRM）检测,基质匹配标准曲线内标法定量。结果表明：矮壮素在0.200~500 μg/L范围内呈良好线性,相关系数（r²）均不低于0.9993,方法的定量限为0.500 μg/kg;以猪肉、牛肉、羊肉、鸡肉、鸡蛋、猪肾、牛肝、羊肾、鸡肝、牛奶为基质,矮壮素的平均加标回收率为93.4%~101%,相对标准偏差为2.3%~8.0%。该方法基质干扰小,灵敏度高,准确可靠,适用于动物源性食品中矮壮素残留的定量检测。     

QuEChERS-超高效液相色谱-串联质谱法测定动物源食品中痕量五氯酚及其钠盐

采用改良的QuEChERS-超高效液相色谱-串联质谱（UPLC-MS/MS）技术,建立了动物源食品中痕量五氯酚及其钠盐的测定方法。样品中五氯酚钠在酸性条件下转化为五氯酚,采用1%（v/v）乙酸乙腈超声提取2次,提取液浓缩后分散固相萃取净化,以回收率及基质效应为考察指标,对吸附剂进行了优化。采用Waters Acquity UPLC HSS T3色谱柱梯度洗脱分离,在电喷雾电离（ESI）源、负离子模式和多反应监测模式下检测,基质匹配外标法定量。在1.0、2.0、10.0 μg/kg添加水平下6种基质（猪肉、猪肝、鸡肉、鱼肉、牛奶、鸡蛋）中五氯酚的加标回收率为73.2%~108.4%,相对标准偏差为4.0%~14.8%;定量限（S/N>10）为1.0 μg/kg。该方法简便、灵敏、准确、环保,适用于动物源食品中痕量五氯酚及其钠盐残留的定性定量分析。     

QuEChERS-同位素内标-高效液相色谱-串联质谱法测定动物源性食品中植物生长调节剂类农药残留

建立了高效液相色谱-串联质谱法测定动物源性食品中植物生长调节剂类农药残留量的方法。选取猪肉、牛肉、鸡肉、猪肝、鸡蛋和牛奶作为样品,样品经乙腈提取,4 g无水硫酸镁(MgSO₄)和1 g氯化钠(NaCl)盐析脱水后,取上清液经50 mg N-丙基乙二胺(PSA)+50 mg十八烷基硅烷(C18)粉末净化(含150 mg MgSO₄)。采用Agilent ZORBAX Eclipse Plus C18柱分离待测物,电喷雾电离,正负离子切换多反应检测模式检测,以乙腈和5 mmol/L乙酸铵水溶液作为流动相进行梯度洗脱,基质匹配内标法定量。在猪肝、鸡蛋基质中,矮壮素、噻苯隆和多效唑在0.1~100 μg/L范围内线性关系良好;在猪肉、牛肉和鸡肉中3种植物生长调节剂在0.1~50 μg/L范围内线性关系良好;在牛奶基质中,噻苯隆和多效唑的线性范围为0.05~10 μg/L,矮壮素的线性范围为0.05~5 μg/L,相关系数(r²)均大于0.990。以信噪比(S/N)≥3对应的添加水平作为检出限(LOD), S/N≥10对应的添加水平作为定量限(LOQ),矮壮素、噻苯隆和多效唑在不同基质下的LOD为0.01~0.1 μg/kg, LOQ为0.5~5 μg/kg。分别添加LOQ、2倍LOQ和10倍LOQ 3个水平的目标化合物,平均回收率为70.0%~117.4%, RSD为0.8%~16.1%。该方法操作简单、灵敏度高,采用基质匹配内标法定量,能最大限度地消除基质干扰,使检测结果更加精确,可满足动物源性食品中矮壮素、噻苯隆和多效唑残留的定量检测工作。     

固相萃取-超高效液相色谱-串联质谱法测定动物源食品中氯苯胺灵残留北大核心CSCD

建立了固相萃取-超高效液相色谱-串联质谱法(SPE-UHPLC-MS/MS)测定动物源食品中氯苯胺灵残留的分析方法。动物源食品经乙腈溶液提取、正己烷脱脂、ProElut PLS固相萃取柱富集净化,采用C18色谱柱进行分离,以0.2%(v/v)甲酸水溶液和乙腈为流动相进行梯度洗脱,采用电喷雾电离、正离子模式扫描,多反应监测模式(MRM)检测,基质匹配标准曲线外标法定量。研究通过对不同填料的固相萃取柱的考察,最终选择了ProElut PLS固相萃取柱,其能对动物源食品复杂基质中的氯苯胺灵进行有效富集和净化;通过考察3种规格的C18柱对氯苯胺灵的保留能力和出峰效果,选择了Agilent ZORBAX SB-C18(150 mm×2.1 mm,5μm)作为氯苯胺灵的分离色谱柱;通过考察氯苯胺灵在正负两种质谱电离模式下的响应情况,选择了正离子模式,确定氯苯胺灵一级质谱准分子离子和相应的碎片离子。结果表明:氯苯胺灵在0.5~100.0μg/L范围内呈良好的线性关系,相关系数(r^(2))均不低于0.9929;方法定量限(LOQ)为3μg/kg(S/N≥10);以猪肉、牛奶、牛肉、鸡肉、鸭肉、鸡蛋、鸡胗、鸭蛋、猪腰、猪肝、牛肝、羊肉、鸭胗共13种动物源食品为基质,在0.003、0.006、0.060 mg/kg添加水平下,氯苯胺灵的平均回收率为74.9%~97.6%,相对标准偏差(RSD)为2.9%~9.5%(n=6)。本方法前处理简便易行、基质干扰小、灵敏度高、准确性高,适用于多种动物源食品中氯苯胺灵的定性和定量检测。