Spore rec-assay by dry sheet medium culture plate for bacterial counts

A simple and rapid method for spore rec-assay by utilizing dry sheet medium culture (Compactdry TC, CTC) for determining numbers of bacteria, instead of the spore agar plate, was developed. One mL of spore suspension (2 x 10(6)/mL) of Bacillus subtilis strain M45 Rec- or H17 Rec+ was inoculated in the center of the CTC plate. In the case of metabolic activation, 1 mL of mixed solution (spore suspension of M45 or H17: 9,000 x g supernatant of rat-liver homogenate treated with Aroclor 1254 = 19:1) was used. The spore suspension spreads over the whole sheet in seconds and gels. A paper disk impregnated with 20-40 microL of the sample solution and 20 microL of the cofactor solution was placed on the surface of CTC plate. For the assay of samples that do not require metabolic activation, use of the cofactor solution can be omitted. After 48 hr incubation at 37 degrees C, 0.01% MTT [3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide] aqueous solution (0.5 mL) was dropped uniformly on the plate. The plate was left for 5 min, and the diameter of the inhibition circle was measured with slide calipers. The samples for which the difference in inhibition zone between M45 and H17 was more than 2 mm were judged positive. Under these conditions, the DNA damaging activities of sodium sulfite, sodium benzoate and citric acid, used as food additives, were investigated by the proposed method. Sodium sulfite and sodium benzoate gave positive results and citric acid gave a negative result with or without metabolic activation, in agreement with the results obtained by the conventional method.     

A new medium for determining the total plate count in food

SimPlate for Total Plate Count-Color Indicator (TPC-CI, IDEXX Laboratories, Inc., Westbrook, Me.) is a new medium that incorporates the redox dye resazurin to detect and quantify bacteria in food. Enumeration is achieved by the most probable number method using a SimPlate device. Viable bacteria are detected in each well of the SimPlate device by the biochemical reduction of resazurin, which is blue, to the pink resorufin or the clear dihydroresorufin indicators. Results after 24 h of incubation for TPC-CI are highly correlated with standard plate count agar after 48 h of incubation. Correlation coefficients from studies conducted at five laboratories ranged from 0.94 to 0.98 in side-by-side comparisons against standard plate count agar. Four additional test sites, using alternative methods for determining the aerobic plate count in food, reported similar results in comparison studies (r = 0.91 to 0.97). The slopes from linear regression analysis at all sites ranged from 0.91 to 0.98, with y intercepts ranging from 0.11 to 0.84. Samples used for the validation of TPC-CI included raw food products (i.e., liver and grains), which may contain natural enzymes that interfere with enzyme-based detection methods. No interference was seen from the foods tested. These results suggest that TPC-CI is a suitable alternative to existing plate count methods and has reduced incubation time.     

Comparison of the compact dry TC and 3M petrifilm ACP dry sheet media methods with the spiral plate method for the examination of randomly selected foods for obtaining aerobic colony counts

Two hundred thirty-six randomly selected food and milk samples were examined to obtain aerobic colony counts by two dry sheet media methods and a standard Public Health Laboratory Service spiral plate method. Results for 40 samples were outside the limits of detection for one or more of the tested methods and were not considered. (The limits of detection for the spiral plate method were 200 to 1 x 10(8) CFU/ml for the spiral plate method and 100 to 3 x 10(6) CFU/ml for the dry sheet media methods.) The remaining 196 sets of results were analyzed further. When the results from the three methods were compared, correlation coefficients were all >0.80 and slopes and intercepts were close to 1.0 and 0.0, respectively. Mean log values and standard deviations were very similar for all three methods. The results were evaluated according to published UK guidelines for ready-to-eat foods sampled at the point of sale, which include a quality acceptability assessment that is based on aerobic colony counts. Eighty-six percent of the comparable results gave the same verdict with regard to acceptability according to the aerobic colony count guidelines. Both dry sheet media methods were comparable to the spiral plate method and can be recommended for the examination of food.     

Comparison of dry culture medium and conventional plating techniques for enumeration of bacteria in pasteurized fluid milk

Standard plate counts, psychrotrophic bacterial counts, and coliforms were determined by conventional plating techniques and by Petrifilm TM plates, a dry culture medium, for 48 commercially processed milk samples (24 whole milk and 24 skim milk). The Petrifilm SM plate counts were compared with counts on standard methods agar for the standard plate count, psychrotrophic bacterial count, and rapid psychrotrophic bacterial count. The Petrifilm violet red bile plate counts were compared with counts on violet red bile agar for coliform test with a solid medium and the preliminary incubation method for detection of coliforms. Standard plate counts were determined within 24 h of packaging and after 7, 10, and 14 d of storage at 6.1 degrees C. Psychrotrophic bacterial counts and coliform counts were determined with 24 h of packaging and after 7 d storage. There was a strong linear relationship between Petrifilm SM and standard methods agar plates (excluding counts on samples plated within 24 h of packaging) and for the psychrotrophic bacterial count method. Petrifilm SM had a weak linear relationship with Standard Methods Agar plates for the rapid psychrotrophic bacterial count. Coliform counts determined on Petrifilm violet red bile plates were generally within the same range as counts on violet red bile agar plates. The positive predictive values for the Petrifilm violet red bile plates and violet red bile agar plates were essentially the same for samples plated within 24 h of packaging.     

提高培养液中乳酸菌数的方法探讨

乳酸菌在发酵食品中应用非常广泛,发酵乳制品主要依靠添加乳酸菌类发酵剂进行发酵.要得到较高含菌量的发酵剂,首先应该提高培养液中乳酸菌数,以保证后续浓缩干燥等工艺的实施.概述了国内外制备直投式发酵荆工艺中提高培养液中乳酸菌菌数的方法,以及行业发展前景及趋势.     

Bioluminescence-based identification of nisin producers - A rapid and simple screening method for nisinogenic bacteria in food samples

We present a simple and rapid method for screening nisin producers that directly identifies nisinogenic bacteria by induction of bioluminescence within the Lactococcus lactis NZ9800lux biosensor strain (Immonen and Karp, 2007, Biosensors and Bioelectronics 22, 1982-7). An overlay of putative nisinogenic colonies with the biosensor strain gives identification results within 1h. Functionality and specificity of the method were verified by screening nisin producers among 144 raw milk colonies and a panel of 91 lactococcal strains. Studies performed on strains and colonies that did not induce bioluminescence but inhibited growth of the biosensor demonstrated that only nisinogenic bacteria can cause induction. Bacteria known to produce bacteriocins other than nisin failed to induce bioluminescence, further verifying the specificity of the assay. We discovered a non-inducing but inhibitory lactococcal strain harboring a modified nisin Z gene, and demonstrated that the source of the inhibitory action is not a non-inducing variant of nisin, but a bacteriocin of lower molecular weight. The concentration of nisin producers in a raw milk sample was 1.3×10(2)CFU/ml. We identified from raw milk a total of seven nisin Z producing L. lactis subsp. lactis colonies, which were shown by genetic fingerprinting to belong to three different groups. Among the panel of 91 lactococci, four strains were nisin A producers, and one strain harbored the modified nisin Z gene. The method presented here is robust, cost-effective and simple to perform, and avoids the pitfalls of traditional screening methods by directly specifying the identity of the inhibitory substance.     

Comparison of dry sheet media and conventional agar media methods for enumerating yeasts and molds in food

A study was done to compare Nissui Compact Dry Yeast and Mold plates (CDYM), 3M Petrifilm Yeast and Mold count plates (PYM), dichloran-rose bengal chloramphenicol (DRBC) agar, and dichloran 18% glycerol (DG18) agar for enumerating yeasts and molds naturally occurring in 97 foods (grains, legumes, raw fruits and vegetables, nuts, dairy products, meats, and miscellaneous processed foods and dry mixes). Correlation coefficients for plates incubated for 5 days were DG18 versus DRBC (0.93), PYM versus DRBC (0.81), CDYM versus DG18 (0.81), PYM versus DG18 (0.80), CDYM versus DRBC (0.79), and CDYM versus PYM (0.75). The number of yeasts and molds recovered from a group of foods (n = 32) analyzed on a weight basis (CFU per gram) was not significantly different (alpha = 0.05) when samples were plated on DRBC, DG18, PYM, or CDYM. However, the order of recovery from foods (n = 65) in a group analyzed on a unit or piece basis, or a composite of both groups (n = 97), was DRBC > DG18 = CDYM > PYM. Compared with PYM, CDYM recovered equivalent, significantly higher (alpha = 0.05) or significantly lower (alpha = 0.05) numbers of yeasts and molds in 51.5, 27.8, and 20.6%, respectively, of the 97 foods tested; respective values were 68.8, 15.6, and 15.6% in the small group (n = 32) and 43.1, 33.8, and 23.1% in the large group (n = 65) of foods. The two groups contained different types of foods, the latter consisting largely (73.8%) of raw fruits (n = 16) and vegetables (n = 32). Differences in efficacy of the four methods in recovering yeasts and molds from foods in the two groups are attributed in part to differences in genera and predominant mycoflora. While DG18 agar, CDYM, and PYM appear to be acceptable for enumerating yeasts and molds in the foods analyzed in this study, overall, DRBC agar recovered higher numbers from the 97 test foods, thereby supporting its recommended use as a general purpose medium for mycological analysis.     

Evaluation of the petrifilm plate method for the enumeration of aerobic microorganisms and coliforms in retailed meat samples.

This study was designed to compare the effectiveness and applicability of the Petrifilm plate method with the Association of Official Analytical Chemists' (AOAC) standard aerobic count method and violet red bile agar method for meat products. The comparison was carried out using 303 meat samples collected from various retailers: 110 pork samples, 87 chicken samples, and 107 beef samples. In the comparison of the correlation coefficient (R) between the conventional method and the Petrifilm plate method by a linear regression analysis, the correlation coefficient in total microorganisms was 0.99, 0.95, and 0.94 in pork, beef, and chicken samples, respectively. The correlation coefficient in coliform count was 0.83, 0.96, and 0.81 in pork, beef, and chicken samples, respectively. Based on the high correlation in the total microorganism count, it might be possible to replace the conventional methods with the Petrifilm plate method. For coliform counts, the Petrifilm plate method also showed a generally high correlation coefficient, except for pork samples, which are more subject to contamination. The Petrifilm plate method was simpler and less time-consuming in sample preparation and, in procedures, faster than the conventional method. These results suggested that the 3M Petrifilm plate method could replace the conventional methods in the analysis of microorganism contamination measurement in meat products.     

食品感官货架期确定的一般原则与方法

感官品质是描述和判断食品质量最直观的指标,将此品质作为货架寿命的评价指标而确定的货架期为感官货架期,则对应的感官评价方法是确定感官货架寿命的关键技术之一。由此,结合感官评价方法与货架期预测原理,提出了食品感官货架期确定的一般原则与方法,旨在指导构建我国不同类型食品感官品质的货架期确定指南,完善食品货架期检测标准,为我国食品质量与控制的保障提供标准化支撑。     

Kinetic-spectrophotometry method for determination of ultra trace amounts of aluminum in food samples

A simple and sensitive kinetic-spectrophotometry method is developed for the determination of trace amounts of aluminum in food samples based on its catalytic effect on the oxidation of Nile Blue A by potassium bromate in sulfuric acid medium. The absorbance is measured at 595.5 nm with the fixed-time method. The optimization of the operating conditions regarding concentrations of the reagents, temperature and interferences are also investigated. The calibration curve is linear over the concentration range 0.07–0.9 μg ml⁻¹ of aluminum with good precision and accuracy and the detection limit was down to 0.034 μg ml⁻¹. The relative standard deviation for a standard solution of 0.4 μg ml⁻¹ of aluminum is 1.73% (n = 10). The proposed method proved highly sensitive, selective and relatively rapid for the assay of aluminum at ultra trace level without any pre-concentration and separation step. The method was applied to the determination of aluminum in food samples (rice, tea and potato). The analytical results of the real samples were in good agreement with the standard method.     

Analysis of radio frequency (RF) power distribution in dry food materials

The objectives of this research were to investigate the influence of various factors on radio frequency (RF) power distribution in dry food materials, placed in a 12 kW, 27.12 MHz parallel plate RF system, using a validated finite element computer model. The factors investigated were sample size, shape, relative position between the RF electrodes, and dielectric properties (DPs) of the sample and the surrounding medium. Effects of electrode gap and top electrode configuration on the RF power distribution behavior of the sample were also studied. The RF power uniformity in the samples was compared using RF power density uniformity index (PUI). Simulated results showed that the RF power uniformity in cuboid shaped samples, placed on the bottom electrode, first decreased and then increased with the increase in sample size. The sample shape and its vertical position between the fixed gap parallel plate electrodes also affected the RF power distribution and uniformity. A cuboid sample had higher RF power densities at the edges, while an ellipsoid had higher power densities in the center parts. Simulated results showed that the smaller values of DPs resulted in better RF power uniformities in the samples. Reducing the electrode gap improved the RF power uniformity of the sample. While studying the influence of the top electrode configuration on the RF power distribution and uniformity, the results showed that optimum RF power uniformity in a particular sample size could be achieved with a particular top electrode bending position and angle. The results are useful in understanding complex RF heating, designing and scaling up of efficient RF systems.     

2,4-二硝基苯肼比色法测定食品中甲醛的改进

通过改进2,4-二硝基苯肼比色法测定食品中甲醛的不足,对显色条件进行了优化,并评价了方法的性能指标,对市售腐竹、米粉、毛肚三种食品中的甲醛含量进行了测定分析。建立一种将甲醛衍生成甲醛-2,4-二硝基苯腙,甲醛-2,4-二硝基苯腙与强碱作用生成酒红色的醌类物质,进行吸光度值的测定的方法。试验方法:甲醛溶液与2.8 m L 2,4-二硝基苯肼乙醇饱和溶液在酸性条件下于50℃水浴衍生30 min,冷却室温,先后加入无水乙醇、KOH-乙醇溶液,稳定10 min在432 nm波长下测得吸光度值。结果表明:甲醛标准曲线的线性方程为Y=0.6788X+0.0102,相关糸数R 2=0.999,线性范围为1~12μg/m L;平均回收率为102.8%(RSD=3.99%);最低检出限为0.0025μg/m L。结论:改进的方法灵敏、准确、精密度高,能够满足食品中甲醛分析准确度的要求。     

A simple method for determining the shear modulus of food dispersions and gels

The application of a simple commercially available apparatus to the measurement of the dynamic shear modulus for viscoelastic food dispersions and gels is described. For vibrant gels the shear modulus can be determined to a precision of ±5% in the range 50 to 5 × 10⁵ Nm^?2 and the value is independent of the frequency as the gel behaves like a simple elastic solid. The measurement causes minimal mechanical disturbance to a gel or dispersion and is thus ideally suited for monitoring weak network structures. With weak or “dead” gels, or pastes, the dynamic modulus at just one frequency, 200 Hz, is found.     

萃取比色法测定铬鞣剂中的六价铬

以异戊醇和 4 -甲基 - 2 -戊酮作为萃取剂 ,对 Cr( )与显色剂二苯碳酰二肼生成的紫色络合物 ,进行了萃取试验的研究 ,并确定了最佳萃取条件。为铬鞣剂中 Cr( )含量测定 ,排除 Cr( )的干扰提出可靠依据。结果表明 :4 -甲基 - 2 -戊酮萃取效率优于异戊醇 ,其水相 p H值控制在 4 .6左右 ,利用分光光度法对铬鞣剂进行测定 ,该方法简便快速 ,准确可靠 ,灵敏度高     

苹果汁中耐热菌培养基的优化及生长曲线

通过对苹果汁中分离耐热菌和德国标准耐热菌菌株基础培养基的正交优化,确定最佳的培养基。得出最佳培养基配比是(质量分数):德国标准菌:蛋白胨0.5%,果糖0.2%,酵母粉0.2%,MgSO4·2H2O0.1%,CaCl20.5%,KH2PO40.12%,MnSO4·4H2O0.05%;分离菌:蛋白胨0.5%,葡萄糖0.2%,酵母粉0.2%,MgSO4·2H2O0.1%,CaCl20.5%,KH2PO40.12%,MnSO4·4H2O0.05%。并计算得出吸光度与细菌浓度之间的关系公式及用优化培养基培养绘制耐热菌生长曲线。     

食品中双乙酸钠含量测定方法的研究

双乙酸钠(SDA)是一种新型的多功能绿色食品添加剂.主要用作粮食、食品、饲料的防腐剂、防霉剂、保鲜剂、调味剂、pH调节剂、品质改良剂、肉制品的保存剂,也是复合型防霉剂的主要配料.论述了食品中双乙酸钠含量的定性鉴别以及运用高效液相色谱进行定量分析的方法.     

Evaluation of dry medium (Sanita-Kun) for enumeration of coliforms and Escherichia coli in milk, ice cream, ham, and codfish fillet

This study was performed to compare the efficiency and accuracy of Sanita-Kun dry medium with those of conventional culture and Petrifilm dry medium methods for enumerating coliforms and Escherichia coli in milk, ice cream, ham, and codfish fillet. All 3 methods showed high correlations each other and no significant differences in enumerating coliforms and E. coli. In addition, the Sanita-Kun method was easier to count microbial load than the Petrifilm method. Therefore, we concluded that the Sanita-Kun method can be used as an alternative to conventional culture and Petrifilm methods for the analysis of coliforms and E. coli in milk, ice cream, ham, and codfish fillet products.     

Prediction of the shelf-life of pasteurized milk using dry medium culture plates (3M Petrifilm, Hygicult and Millipore Sampler)

Petrifilm SM Plate. Hygicult and Millipore Total Count Sampler were compared with normal plate count procedures for the enumeration of bacteria after preincubation of pasteurized milk. There were good correlations between results obtained using normal plate counts and those obtained with each of the other methods. Coupling these counting methods with preincubation procedures of 21°C for 25 hours in the presence of crystal violet-penicillin-nisin allowed prediction of the pass rate of the recently introduced 6°C/5 day storage test for pasteurized milk. The predictive accuracy was slightly improved by the use of preincubation conditions of 15°C for 25 hours. The availability of these devices enables quality control procedures to be set up on even the smallest processing site.