196 Chilled foods conventional direct conductance methods, but detection is made easy by the use of indirect conductance monitoring(Betts 1993). The increased use of indirect methods in the future could considerably enhance the ability of electrical systems to detect microorganisms that produce little electrical change in direct ystems, thus increasing the number of applications of the technique within the food 8.4.2 Adenosine triphosphate(ATP) bioluminescence The non-biological synthesis of ATP in the extracellular environment has been demonstrated(Ponnamperuma et al, 1963), but it is universally accepted that such sources of ATP are very rare(Huernnekens and Whiteley 1960). ATP is a high-energy compound found in all living cells(Huernnekens and Whiteley 1960), and it is an essential component in the initial biochemical steps of substrate utilisation and in the synthesis of cell material McElroy (1947)first demonstrated that the emission of light in the bioluminescent reaction of the firefly, Photinus pyralis, was stimulated by ATP. The procedure for the determination of ATP concentrations utilising crude firefly extracts was described by McElroy and Streffier(1949) and has since been used in many fields as a sensitive and accurate measure of ATP. The light yielding reaction is catalysed by the enzyme luciferase, this being the enzyme found in fireflies causing luminescence. Luciferase takes part in the following reaction: 1. Luciferase Luciferin+ ATP-Mg- Luciferase- Luciferin- AMP+ PP The complex is then oxidised 2. Luciferase-Luciferin- AMP O2- Luciferase-Luciferin- AMP= The oxidised complex is in an excited stage, and as it returns to its ground stage a photon of light is released 3. Luciferase-Luciferin- AMP=0-(Luciferase- Luciferin- AMP=0) light The light-yielding reaction is efficient, producing a single photon of light for every luciferin molecule oxidised and thus every ATP molecule used (Seliger and McElroy 1960) Levin et al.(1964)first described the use of the firefly bioluminescence assay of ATP for detecting the presence of viable microorganisms. Since this initial report considerable work has been done on the detection of viable organisms in environmental samples using a bioluminescence technique(Stalker 1984). As all viable organisms contain ATP, it could be considered simple to use a bioluminescence method to rapidly enumerate microorganisms. Research, however. has shown that the amount of atP in different microbial cells varies depending on species, nutrient level, stress level and stage of growth(Stannardconventional direct conductance methods, but detection is made easy by the use of indirect conductance monitoring (Betts 1993). The increased use of indirect methods in the future could considerably enhance the ability of electrical systems to detect microorganisms that produce little electrical change in direct systems, thus increasing the number of applications of the technique within the food industry. 8.4.2 Adenosine triphosphate (ATP) bioluminescence The non-biological synthesis of ATP in the extracellular environment has been demonstrated (Ponnamperuma et al., 1963), but it is universally accepted that such sources of ATP are very rare (Huernnekens and Whiteley 1960). ATP is a high-energy compound found in all living cells (Huernnekens and Whiteley 1960), and it is an essential component in the initial biochemical steps of substrate utilisation and in the synthesis of cell material. McElroy (1947) first demonstrated that the emission of light in the bioluminescent reaction of the firefly, Photinus pyralis, was stimulated by ATP. The procedure for the determination of ATP concentrations utilising crude firefly extracts was described by McElroy and Streffier (1949) and has since been used in many fields as a sensitive and accurate measure of ATP. The light￾yielding reaction is catalysed by the enzyme luciferase, this being the enzyme found in fireflies causing luminescence. Luciferase takes part in the following reaction: 1. Luciferase + Luciferin + ATP
Mg2+ Luciferase Luciferin AMP + PP The complex is then oxidised: 2. Luciferase Luciferin AMP + O2
(Luciferase Luciferin AMP = O) + H2O The oxidised complex is in an excited stage, and as it returns to its ground stage a photon of light is released: 3. Luciferase Luciferin AMP = 0
(Luciferase Luciferin AMP = 0) + Light The light-yielding reaction is efficient, producing a single photon of light for every luciferin molecule oxidised and thus every ATP molecule used (Seliger and McElroy 1960). Levin et al. (1964) first described the use of the firefly bioluminescence assay of ATP for detecting the presence of viable microorganisms. Since this initial report considerable work has been done on the detection of viable organisms in environmental samples using a bioluminescence technique (Stalker 1984). As all viable organisms contain ATP, it could be considered simple to use a bioluminescence method to rapidly enumerate microorganisms. Research, however, has shown that the amount of ATP in different microbial cells varies depending on species, nutrient level, stress level and stage of growth (Stannard 196 Chilled foods