132 Novel food packaging techniques sulphide(H2s)is produced from cysteine and triggered by glucose limitation (Borch et al, 1996). H2S forms a green pigment, sulphmyoglobin, when it is bound to myoglobin(Paine and Paine, 1992, Egan et al, 1989). However, ulphmyoglobin is not formed in anaerobic conditions H2s and other sulphuric compounds have been found to be produced during the spoilage of poultry by pseudomonas, Alteromonas sp. and psycrotrophic anaerobic clostridia( Freeman et al, 1976; Lea et al, 1969 Russell et al, 1997 Vieshweg et al, 1989; Arnaut-Rollier et al, 1999; Kalinowski and Tompkin, 1999). According to Dainty(1996), production of H2s can be used as an indication of Enterobacteriacae and hence also of hygienic problems in aerobically stored meat. H2s production by Alteromonas putrefaciens Enterobacter liquefaciens and pseudomonas was discovered in high ultimate pH beef from stressed animals(Gill and Newton, 1979, Nicol et al., 1970). It has also been found that in vacuum packed meat His indicates the growth of particular strains of lactic acid bacteria(Egan et al., 1989). Also in fish the volatile sulphuric compounds have been suggested as the main cause of putrid spoilage aromas(Olafsdottir and Fleurence, 1997) 7.3 Freshness indicators a variety of different concepts for freshness indicators have been presented the scientific literature. Most of these concepts are based on a colour change of the indicator tag due to the presence of microbial metabolites produced during spoilage(e.g Smolander et al, 2002; Wallach and Novikov, 1998; Kahn, 1996 Namiki, 1996), but also concepts for indicators relying on more advanced technology have been presented. For instance, a miniaturised gas detector based on conducting polymers has been patented by Aromascan, a manufacturer of electronic nose equipment(Payne and Persaud, 1995). Fibre optics can also be used to construct indicators for the volatile compounds produced in microbial spoilage(Honeybourne, 1993, Wolfbeis and List, 1995). Freshness indicator or detector concepts have been proposed, for example, for CO2, diacetyl, amines, ammonia and hydrogen sulphide(see Table 7. 1). These concepts are discussed in detail in following chapters 7.3.1 Indicators sensitive to pH change he majority of concepts described in the literature are based on the use of dyes, which change colour in the presence of volatile compounds produced during spoilage. As early as the 1940s Clark (1949)filed a patent application describing ' an indicator which exhibits an irreversible change in visual appearance upon an appreciable multiplication of bacteria in the indicator The idea was that if microbiological growth inducing a pH change has been possible in the indicator, the conditions might have been such that the food product itself may also have been subject to deterioration. A direct
sulphide (H2S) is produced from cysteine and triggered by glucose limitation (Borch et al., 1996). H2S forms a green pigment, sulphmyoglobin, when it is bound to myoglobin (Paine and Paine, 1992, Egan et al., 1989). However, sulphmyoglobin is not formed in anaerobic conditions. H2S and other sulphuric compounds have been found to be produced during the spoilage of poultry by pseudomonas, Alteromonas sp. and psycrotrophic anaerobic clostridia (Freeman et al., 1976; Lea et al., 1969; Russell et al., 1997, Vieshweg et al., 1989; Arnaut-Rollier et al., 1999; Kalinowski and Tompkin, 1999). According to Dainty (1996), production of H2S can be used as an indication of Enterobacteriacae and hence also of hygienic problems in aerobically stored meat. H2S production by Alteromonas putrefaciens, Enterobacter liquefaciens and pseudomonas was discovered in high ultimate pH beef from stressed animals (Gill and Newton, 1979; Nicol et al., 1970). It has also been found that in vacuum packed meat H2S indicates the growth of particular strains of lactic acid bacteria (Egan et al., 1989). Also in fish the volatile sulphuric compounds have been suggested as the main cause of putrid spoilage aromas (Olafsdottir and Fleurence, 1997). 7.3 Freshness indicators A variety of different concepts for freshness indicators have been presented in the scientific literature. Most of these concepts are based on a colour change of the indicator tag due to the presence of microbial metabolites produced during spoilage (e.g. Smolander et al., 2002; Wallach and Novikov, 1998; Kahn, 1996; Namiki, 1996), but also concepts for indicators relying on more advanced technology have been presented. For instance, a miniaturised gas detector based on conducting polymers has been patented by Aromascan, a manufacturer of electronic nose equipment (Payne and Persaud, 1995). Fibre optics can also be used to construct indicators for the volatile compounds produced in microbial spoilage (Honeybourne, 1993; Wolfbeis and List, 1995). Freshness indicator or detector concepts have been proposed, for example, for CO2, diacetyl, amines, ammonia and hydrogen sulphide (see Table 7.1). These concepts are discussed in detail in following chapters. 7.3.1 Indicators sensitive to pH change The majority of concepts described in the literature are based on the use of pHdyes, which change colour in the presence of volatile compounds produced during spoilage. As early as the 1940s Clark (1949) filed a patent application describing ‘an indicator which exhibits an irreversible change in visual appearance upon an appreciable multiplication of bacteria in the indicator’. The idea was that if microbiological growth inducing a pH change has been possible in the indicator, the conditions might have been such that the food product itself may also have been subject to deterioration. A direct 132 Novel food packaging techniques
The use of freshness indicators in packaging 133 Table 7.1 Examples of freshness and contamination indicator systems for food Author/ Patent applicant or Metabolite Principle of the indicator holder/Trade name Freshness indicators Visual Spoilage Indicator bromet g hymol blue pany(Eaton et al. 1977) omothymol blue Horan(1998,2000) E. g. CO,. SO nge of the indicator(e.g. xylenol blue, bromocresol purple halein. bromothymol blue, neutral red) incorporated in the packaging material Neary(1981) CO, H2, NH4 Colour change of liquid ndicator AVL Medical Instru CO2, NH4 Colour change of co olfbeis and List S NHi_, and amine-sensitive dyes, formation of colour of heavy-metal sulfides(H,s Biodetect Corporation g. acetic acid, Visually detectable colour (Wallach and Novikov, 1998) lactic acid hange of a pH-dye(e.g. acetaldehyde, phenol red. cresol red omonIa cresol purple) mines Mattila and Auvinen Not specified Colour change of methylene blue or 2, 6-dichlorophenol ndophen Miller et al.(1999) Volatile amines Colour change of a food dye calix[4]arene-based dye VTT Biotechno Colour change of myoglobin nathen e Cameron and Talasila(1995) Alcohol oxidase-peroxidase- 93) De Cicco and Keeven(1995) Microbial Colour change of chromogenic substrates of the microbial enzymes
Table 7.1 Examples of freshness and contamination indicator systems for food packages Author/ Patent applicant or holder/Trade name Metabolite detected Principle of the indicator Freshness indicators Holte (1993) CO2 Colour change of e.g. bromothymol blue Visual Spoilage Indicator Company (Eaton et al. 1977) CO2 Colour change Mattila et al. (1990) CO2 Colour change of bromothymol blue Horan (1998, 2000) E.g. CO2, SO2, NH4 Colour change of the indicator (e.g. xylenol blue, bromocresol purple, bromocresol green, cresol red, phenolphalein, bromothymol blue, neutral red) incorporated in the packaging material Neary (1981) CO2, H2, NH4 Colour change of liquid crystal/liquid crystal + indicator AVL Medical Instruments (Wolfbeis and List, 1995) CO2, NH4, amines, H2S Colour change of CO2ÿ, NH4ÿ, and amine-sensitive dyes, formation of colour of heavy-metal sulfides (H2S) Biodetect Corporation (Wallach and Novikov, 1998) E.g. acetic acid, lactic acid, acetaldehyde, ammonia, amines Visually detectable colour change of a pH-dye (e.g. phenol red, cresol red, mcresol purple) Mattila and Auvinen (1990a, b) Not specified Colour change of methylene blue or 2,6-dichlorophenol indophenol Miller et al. (1999) Volatile amines Colour change of a food dye Loughran and Diamond (2000) Volatile amines Colour change of calix[4]arene-based dye VTT Biotechnology (Ahvenainen et al., 1997) H2S Colour change of myoglobin Cameron and Talasila (1995) Ethanol Alcohol oxidase-peroxidasechromogenic substrate system Honeybourne (1993) Diacetyl Detection of optical changes in aromatic orthodiamine DeCicco and Keeven (1995) Microbial enzymes Colour change of chromogenic substrates of the microbial enzymes The use of freshness indicators in packaging 133