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8.8 X 102

8.4 X 102

Note: Samples were massaged in a Stomacher for 1 min in sterile diluent. Viable cells counts were made according to standard method. Incubation time was 48 h at 32°C. All samples were done in duplicate. Correlation coefficient between the two methods is r = 0.99. Source: Ref. 22.

Note: Samples were massaged in a Stomacher for 1 min in sterile diluent. Viable cells counts were made according to standard method. Incubation time was 48 h at 32°C. All samples were done in duplicate. Correlation coefficient between the two methods is r = 0.99. Source: Ref. 22.

injected into the instrument automatically, and readout is reported as relative light units (RLUs). By knowing the number of microorganisms responsible for generating known RLUs, one can estimate the number of microorganisms in the food sample. In some food systems, such as wine, the occurrence of any living matter is undesirable; thus, monitoring of ATP can be a useful tool for quality assurance in the winery. Recently, much interest has been expressed in using ATP estimation not for total viable numbers but as a sanitation check by companies such as Lumac, BioTrace (Plainsboro, N.J.), Lightning (IDEXX, Westbrook, Maine), Hy-Lite (Glengarry, Biotech, Cornwall, Canada), Charm 4000 Luminometer (Charm Sciences, Maiden, Mass.), and others.

As microorganisms grow and metabolize nutrients, large molecules change to smaller molecules in a liquid system and cause a change in electrical conductivity and resistance in the liquid as well as at the interphase of electrodes. By measuring the changes in electrical impedance, capacitance, and conductance, scientists can estimate the number of microorganisms in the liquid because the larger the number of microorganisms in the fluid, the faster the change in these parameters that can be measured by sensitive instruments. A detailed analysis on the subject of impedance, capacitance, and conductance in relation to food microbiology has been made by Eden and Eden (27).

The Bactometer (bioMerieur Vitek, Inc., Hazelwood, Mo.) is an instrument designed to measure impedance changes in foods. Samples are placed in the wells of a 16-well module. After the module is completely or partially filled, it is plugged into the incubator unit to start the monitoring sequence. At first, there is a stabilization period for the instrument to adjust to the module, then a baseline is established. As the microorganisms metabolize the substrates and reach a critical number (105-106 cells/ mL), change in impedance increases sharply, and the monitor screen shows a slope similar to the log phase of a growth curve. The point at which the change in impedance begins is the detection time, and this is measured in hours from the start of the experiment. The detection time is inversely proportional to the number of microorganisms in the sample. By knowing the number of microorganisms per milliliter in a series of liquid samples and the detection time of each sample, one can establish a standard curve. From the curve one can decide the cutoff points to monitor certain specifications of the food products.

Impedance methods have been used to estimate bacteria in milk, dairy products, meats, and other foods. This method has been used for determining the shelf life potential of pasteurized whole milk by Bishop et al. (28).

The Malthus system (Crawley, UK) works by measuring the conductance of the fluid as the organisms grow in the system. It also generates a conductance curve similar to the impedance curve of the Bactometer, and it also uses detection time in monitoring the density of the microorganisms in the food.

The Malthus system has been used for microbial monitoring of brewing liquids and hygiene monitoring. Gibson and colleagues (29-31) have done considerable work using the Malthus system to study seafood microbiology.

Besides estimating viable cells in foods, both the Bactometer and the Malthus systems can detect specific organisms by the use of selective and differential liquid media. An automatic instrument for measuring direct and indirect impedance has been developed in Europe and named RABIT (Rapid Automated Bacterial Impedance Technique).

An instrument called the Omnispec bioactivity monitor system (Wescor, Inc., Logan, Utah) is a tri-stimulus reflectance colorimeter that monitors dye pigmentation changes mediated by microbial activity. Dyes can be used that produce color changes as a result of pH changes, changes in the redox potential of the medium, or the presence of compounds with free amino groups. Samples are placed in mi-crotiter wells or other types of containers and are scanned by an automated light source with computer interface during the growth stages (0-24 hours). The change of color or hue (a*, b*, L) can be monitored similar to impedance curve and conductance curve. Manninen and Fung (32) evaluated this system in a study of pure cultures of Listeria monocytogenes and food samples and found high correla tion coefficients (r) of 0.90 to 0.99 for pure bacterial cultures and 0.82 for minced beef between the colony counts predicted by the colorimetric technique and the results of the traditional plate count method. They also showed that detection times for bacterial cultures such as Enterobacter aerogenes, E. coli, Hafnia alvei, and several strains of L. monocytogenes were substantially (2-24 h) shorter using the instrument than using the traditional method and concluded that the colorimetric detection technique employed by the Omnispec system simplifies the analyses, saves labor and materials, and provides a high sampling capacity. Tuitemwong (33) completed an extensive study using Omnispec 4000 to monitor growth responses of food pathogens in the presence or absence of membrane-bound enzymes. This instrument is highly efficient in large-scale studies of microbial interaction with different compounds in liquid and food.

The catalase test is another rapid method for estimation of microbial populations in certain foods. Catalase is a very reactive enzyme. Microorganisms can be divided into catalase positive and catalase negative. Both groups are important in food microbiology; however, under certain food-storage conditions, a certain group predominates. Most perishable foods (commercial as well as domestic) are cold stored under aerobic conditions. The organisms causing spoilage of these foods are psychrotrophs. The predominant psychrotrophic bacteria are Pseudomonas spp., which are strongly catalase positive. Other important psychrotrophs such as Micrococcus, Staphylococcus, and a variety of enterics are also catalase positive. Thus, one can make use of the presence of catalase to estimate the bacterial population. Catalase activity can be detected by a simple capillary tube method (34). Recently Binjasass and Fung (35) completed an extensive study using the capillary catalase tube method for monitoring microbial load and endpoint cooking temperature offish and found the method to be reliable and simple to use.

Ang et al. (36) also showed that heating poultry meat to 71°C (a legal requirement for these products) will destroy both bacterial and animal catalase. The test is 99% accurate and is simple and inexpensive to perform.

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