The use of freshness indicators in packaging M. Smolander, VTT Biotechnology, Finland 7.1 Introduction As we know from the definition, intelligent or smart packaging monitors and gives information about the quality of the packed food. According to Huis int Veld (1996)the changes taking place in the fresh food product can be categorised as (1) microbiological growth and metabolism resulting in pH changes, formation of toxic compounds, off-odours, gas and slime formation (ii) oxidation of lipids and pigments resulting in undesirable flavours, formation of compounds with adverse biological reactions or discoloration. The focus of this chapter is on intelligent concepts indicating the changes mainly belonging to the first category The intelligence of a package can be based on the package's ability to give information about the requirements of the product quality like package integrity (leak indicators) and time-temperature history of the product(time-temperature indicators). Intelligent packaging can also give information on product quality directly (see Fig. 7. 1). A freshness indicator indicates directly the quality of the product. The indication of microbiological quality is, for example, based on a reaction between the indicator and the metabolites produced during growth of microorganisms in the product. Of the indicators mentioned, time-temperature indicators and leak indicators are already commercially available and their use is increasing constantly. An indicator that would show specifically the spoilage or the lack of freshness of the product, in addition to temperature abuse or package leaks would be ideal for the quality control of packed products. The number of concepts of package indicators for contamination or freshness detection of food is still very low, however new concepts of freshness indicators are patented and new commercially available products are likely to become available in the near future
7.1 Introduction As we know from the definition, intelligent or smart packaging monitors and gives information about the quality of the packed food. According to Huis in’t Veld (1996) the changes taking place in the fresh food product can be categorised as (i) microbiological growth and metabolism resulting in pHchanges, formation of toxic compounds, off-odours, gas and slime formation, (ii) oxidation of lipids and pigments resulting in undesirable flavours, formation of compounds with adverse biological reactions or discoloration. The focus of this chapter is on intelligent concepts indicating the changes mainly belonging to the first category. The intelligence of a package can be based on the package’s ability to give information about the requirements of the product quality like package integrity (leak indicators) and time-temperature history of the product (time-temperature indicators). Intelligent packaging can also give information on product quality directly (see Fig. 7.1). A freshness indicator indicates directly the quality of the product. The indication of microbiological quality is, for example, based on a reaction between the indicator and the metabolites produced during growth of microorganisms in the product. Of the indicators mentioned, time-temperature indicators and leak indicators are already commercially available and their use is increasing constantly. An indicator that would show specifically the spoilage or the lack of freshness of the product, in addition to temperature abuse or package leaks, would be ideal for the quality control of packed products. The number of concepts of package indicators for contamination or freshness detection of food is still very low, however new concepts of freshness indicators are patented and new commercially available products are likely to become available in the near future. 7 The use of freshness indicators in packaging M. Smolander, VTT Biotechnology, Finland
128 Novel food packaging techniques Time-temperature Freshness ndicator Temperature abuse Microbial growth Microbiological of the package indicator Fig. 7.1 Quality indicators for packaged food products can be either on direct or indirect freshness evaluation In this chapter the potential microbial metabolites and other compound indicating the quality of packaged food are presented. Subsequently, freshness indicator concepts, which are commercially available or have been described literature are reviewed. Finally, the possibilities for the future are discussed 7.2 Compounds indicating the quality of packaged food products An essential prerequisite in the development of freshness indicators is knowledge about the quality indicating metabolites. These metabolites have also been studied because they offer a possibility to replace time-consuming sensory and microbiological analyses traditionally used in the quality evaluation of food products(Dainty, 1996). The formation of the different metabolites depends on the nature of the packaged food product, spoilage flora and the type of packaging. The chemical detection of spoilage has been extensively reviewed by Dainty(1996). Chemical changes in stored meat have been discussed by Nychas et al.(1998). They propose that some chemical compounds do indeed indicate the microbiological quality of food products but also that more information is needed about the correlation between the sensory quality and the concentration of the metabolites. In this chapter some of the quality-indicating metabolites and other compounds representing potential target molecules for the quality-indicating freshness indicators are discussed in detail
In this chapter the potential microbial metabolites and other compounds indicating the quality of packaged food are presented. Subsequently, freshness indicator concepts, which are commercially available or have been described in literature are reviewed. Finally, the possibilities for the future are discussed. 7.2 Compounds indicating the quality of packaged food products An essential prerequisite in the development of freshness indicators is knowledge about the quality indicating metabolites. These metabolites have also been studied because they offer a possibility to replace time-consuming sensory and microbiological analyses traditionally used in the quality evaluation of food products (Dainty, 1996). The formation of the different metabolites depends on the nature of the packaged food product, spoilage flora and the type of packaging. The chemical detection of spoilage has been extensively reviewed by Dainty (1996). Chemical changes in stored meat have been discussed by Nychas et al. (1998). They propose that some chemical compounds do indeed indicate the microbiological quality of food products but also that more information is needed about the correlation between the sensory quality and the concentration of the metabolites. In this chapter some of the quality-indicating metabolites and other compounds representing potential target molecules for the quality-indicating freshness indicators are discussed in detail. Fig. 7.1 Quality indicators for packaged food products can be either on direct or indirect freshness evaluation. 128 Novel food packaging techniques
The use of freshness indicators in packaging 129 7.2.1 Glucose Glucose is an initial substrate for many spoilage bacteria in air,vacuum packages and modified atmosphere packages (Dainty, 996) growth takes place glucose is depleted from meat surface and it has been proposed by Kress-Rogers(1993)that the measurement of the glucose gradient could be utilised to predict the remaining shelf-life. However glucose is not among the most promising quality-indicating compounds since the concentration decreases during storage and it would be more beneficial to have a quality-indicating compound with non-existent or low intitial concentration 7. 2.2 Organic acids Organic acids like lactic acid and acetic acid are the major compounds having a role in glucose fermentation by lactic acid bacteria. The amount of L-lactic acid has generally been reported to decrease during storage of fish and meat(Kakouri et al., 1997; Drosinos and Nychas, 1997; Nychas et al., 1998). On the contrary, the concentration of D-lactate has been reported to increase during storage of meat and therefore D-lactate seems to be a more promising freshness indicator ( Shu et al, 1993) Acetate concentrations have been reported to increase during storage of fresh fish(Kakouri et al, 1997). In our studies with modified atmosphere packaged poultry meat(Smolander et al. in preparation) we have also found that the concentration of acetic acid in the tissue fluid and homogenised meat increased as a function of storage time and temperature. We also studied the formation of formic acid. At the beginning of storage some decrease in the concentration in meat was seen but later the concentration increased however the increase was not as clearly dependent on storage temperature as in the case of acetic acid 7.2.3 Ethanol In addition to lactic and acetic acid, ethanol is another major end product of fermentative metabolism of lactic acid bacteria. It has been postulated that an increase in ethanol concentration in meat and fish indicates an increase of total viable count of the product. For instance, Rehbein(1993)studied the formation of ethanol in iced fish. He found that on an average the concentration of ethanol average of 1. 77 mg ethanol was found in 100 g fish. Rehbein(1993)also studied the formation of ethanol in smoked, vacuum-packed salmon. The concentrations in stored fish samples were considerably higher than in fresh, iced fish. At the end of shelf-life concentrations as high as 10 mg/100 g fish were observed Randell et al.(1995)studied the effect of storage time and package integrity on the formation of ethanol in modified atmosphere packaged, marinated rainbow trout slices. They found that the amount of ethanol in package headspace was increased together with storage time and size of the package leakage. In our unpublished studies analogous to Randell et al.(1995) we found also that
7.2.1 Glucose Glucose is an initial substrate for many spoilage bacteria in air, vacuum packages and modified atmosphere packages (Dainty, 1996). As bacterial growth takes place glucose is depleted from meat surface and it has been proposed by Kress-Rogers (1993) that the measurement of the glucose gradient could be utilised to predict the remaining shelf-life. However glucose is not among the most promising quality-indicating compounds since the concentration decreases during storage and it would be more beneficial to have a quality-indicating compound with non-existent or low intitial concentration. 7.2.2 Organic acids Organic acids like lactic acid and acetic acid are the major compounds having a role in glucose fermentation by lactic acid bacteria. The amount of L-lactic acid has generally been reported to decrease during storage of fish and meat (Kakouri et al., 1997; Drosinos and Nychas, 1997; Nychas et al., 1998). On the contrary, the concentration of D-lactate has been reported to increase during storage of meat and therefore D-lactate seems to be a more promising freshness indicator (Shu et al., 1993). Acetate concentrations have been reported to increase during storage of fresh fish (Kakouri et al., 1997). In our studies with modified atmosphere packaged poultry meat (Smolander et al. in preparation) we have also found that the concentration of acetic acid in the tissue fluid and homogenised meat increased as a function of storage time and temperature. We also studied the formation of formic acid. At the beginning of storage some decrease in the concentration in meat was seen but later the concentration increased, however the increase was not as clearly dependent on storage temperature as in the case of acetic acid. 7.2.3 Ethanol In addition to lactic and acetic acid, ethanol is another major end product of fermentative metabolism of lactic acid bacteria. It has been postulated that an increase in ethanol concentration in meat and fish indicates an increase of total viable count of the product. For instance, Rehbein (1993) studied the formation of ethanol in iced fish. He found that on an average the concentration of ethanol was increased as a function of storage time. At sensory rejection point an average of 1.77 mg ethanol was found in 100 g fish. Rehbein (1993) also studied the formation of ethanol in smoked, vacuum-packed salmon. The concentrations in stored fish samples were considerably higher than in fresh, iced fish. At the end of shelf-life concentrations as high as 10 mg/100 g fish were observed. Randell et al. (1995) studied the effect of storage time and package integrity on the formation of ethanol in modified atmosphere packaged, marinated rainbow trout slices. They found that the amount of ethanol in package headspace was increased together with storage time and size of the package leakage. In our unpublished studies analogous to Randell et al. (1995) we found also that The use of freshness indicators in packaging 129
130 Novel food packaging techniques storage temperature had a remarkable effect on the ethanol concentration. The higher the storage temperature was, the higher was the concentration of ethanol Randell et al(1995)also studied the volatile compounds in packages containing marinated chicken pieces in modified atmosphere (40%CO 60%N2). They found that ethanol concentration in the package headspace increased as a function of storage time In our own unpublished studies we also found some correspondence between sensory quality of modified atmosphere (40%CO2+ 60%N2) packaged, unmarinated poultry meat and ethanol concentration in the package headspace. However, in unmarinated poultry meat packaged in MAP with higher(80%)CO2 concentration we did not observe a clear trend in the formation of ethanol as a function of storage time and temperature 7. 2. 4 Volatile nitrogen c It is well known that high levels of basic volatile nitrogen compounds like ammonia,dimethylamine and trimethylamine give an indication about microbiological spoilage of fish(Ohlenschlager, 1997). The European Commission has even fixed TVB-N(total volatile basic nitrogen contributed by ammonia, dimethylamine and trimethy lamine) limits for some fish species (95/149/EEC). Trimethylamine, formed by microbial actions in fish muscle is generally considered as a major metabolite responsible for the spoilage odours of seafood. a drawback of using trimethylamine as a quality indicator for seafood is the variation in the concentration of its precursor trimethylamine N oxide according to the species and season(Dainty, 1996; Rodriguez et al. 1999) 7.2.5 Biogenic amines Biogenic amines (e.g. tyramine, cadaverine, putrescine, histamine)are specially widely considered as indicators of hygienic quality of meat products In addition to this indicative nature, they can have pharmacological physiological and toxic effects. Due to the health risks, a tolerance level of 100 mg/kg of fish has been established for histamine by FDA(Kaniou, et al 2001). Even if biogenic amines indicate the quality of food products they do not themself contribute to the sensory quality of the product Putrescine and cadaverine are formed from ornithine and lysine, respectively n enzymatic decarboxylation. It has been widely suggested that these diamines are indicators of the initial stage of decomposition of meat products. Okuma et al. (2000)describe an increase in the diamine concentration together with the increase of the total viable counts in aerobically stored chicken. Kaniou et al (2001)reported the formation of putrescine and cadaverine in unpacked beef. In addition to putrescine and cadaverine, histamine was also produced during the torage of vacuum-packed beef. Also in our studies we have found a clear correspondence between the microbiological quality of modified atmosphere
storage temperature had a remarkable effect on the ethanol concentration. The higher the storage temperature was, the higher was the concentration of ethanol. Randell et al. (1995) also studied the volatile compounds in packages containing marinated chicken pieces in modified atmosphere (40%CO2 + 60%N2). They found that ethanol concentration in the package headspace increased as a function of storage time. In our own unpublished studies we also found some correspondence between sensory quality of modified atmosphere (40%CO2 + 60%N2) packaged, unmarinated poultry meat and ethanol concentration in the package headspace. However, in unmarinated poultry meat packaged in MAP with higher (80%) CO2 concentration we did not observe a clear trend in the formation of ethanol as a function of storage time and temperature. 7.2.4 Volatile nitrogen compounds It is well known that high levels of basic volatile nitrogen compounds like ammonia, dimethylamine and trimethylamine give an indication about microbiological spoilage of fish (Ohlenschla¨ger, 1997). The European Commission has even fixed TVB-N (total volatile basic nitrogen contributed by ammonia, dimethylamine and trimethylamine) limits for some fish species (95/149/EEC). Trimethylamine, formed by microbial actions in fish muscle is generally considered as a major metabolite responsible for the spoilage odours of seafood. A drawback of using trimethylamine as a quality indicator for seafood is the variation in the concentration of its precursor trimethylamine Noxide according to the species and season (Dainty, 1996; Rodrı´guez et al., 1999). 7.2.5 Biogenic amines Biogenic amines (e.g. tyramine, cadaverine, putrescine, histamine) are especially widely considered as indicators of hygienic quality of meat products. In addition to this indicative nature, they can have pharmacological, physiological and toxic effects. Due to the health risks, a tolerance level of 100 mg/kg of fish has been established for histamine by FDA (Kaniou, et al. 2001). Even if biogenic amines indicate the quality of food products they do not themself contribute to the sensory quality of the product. Putrescine and cadaverine are formed from ornithine and lysine, respectively in enzymatic decarboxylation. It has been widely suggested that these diamines are indicators of the initial stage of decomposition of meat products. Okuma et al. (2000) describe an increase in the diamine concentration together with the increase of the total viable counts in aerobically stored chicken. Kaniou et al. (2001) reported the formation of putrescine and cadaverine in unpacked beef. In addition to putrescine and cadaverine, histamine was also produced during the storage of vacuum-packed beef. Also in our studies we have found a clear correspondence between the microbiological quality of modified atmosphere 130 Novel food packaging techniques
The use of freshness indicators in packaging 131 packaged poultry and the total amount of biogenic amines. Tyramine formation took place evenly during the storage period, the formation being however dependent on temperature. Storage temperature had a more striking effect on the formation of putrescine and cadaverine which were accumulated especially at the end of storage period(Rokka et al, submitted) Ruiz-Capillas and moral(2002)studied the effect of controlled and modified atmosphere on the production of biogenic amines during storage of hake. They found that controlled and modified atmosphere enera formation of biogenic amines and that cadaverine was a major biogenic amine formed in most of the studied atmospheres. Rodriguez et al.(1999) proposed that biogenic amines cadaverine and putrescine could indicate the freshness of freshwater rainbow trout either stored in air or in a vacuum package 2.6 Carbon dioxide Carbon dioxide(CO2) is generally known to be produced during microbial growth. COz is also typically added as a protecting gas to modified atmosphere packages, together with inert nitrogen, since it has bacteriostatic effects. A modified atmosphere package for non-respiring food typically has a COz concentration as high as 20-80 and, e.g., Fu et al. (1992)reported a further increase of CO2 during storage in modified atmosphere packages containing beef. Even if the indication of microbial growth by CO2 may be difficult in these modified atmosphere packages already containing a high concentration of CO2, it is possible to use the increase in CO2 concentration as a means of determining microbial contamination in other types of product. For instance Mattila et al. (1990)found a correlation between CO2 concentration and the growth of microbes in pea and tomato soup which were packaged aseptically either in air or in a mixture of O2(5%)and nitrogen 7.2.7 ATP degradation products K-value is defined as the ratio of the sum of hypoxanthine and inosine and the total concentration of ATP-related compounds(Henehan et al., 1997). This value, indicating the extent of ATP-degradation, correlates with the sensory quality of fish and also other types of meat and has been used as a freshness indicating parameter(Watanabe et al. 1989, Yano et al., 1995a). For fresh meat the value is low since the concentration of ATP-degradation products is low as compared to the concentration of all ATP-related compounds. The correlation between ATP-degradation products and fish quality has been extensively studied, e.g., by Hattula(1997 7.2.8 Sulphuric compoun Some sulphuric compounds have a remarkable effect on the sensory quality of meat products due to their typical odour and low odour threshold. Hydroger
packaged poultry and the total amount of biogenic amines. Tyramine formation took place evenly during the storage period, the formation being however dependent on temperature. Storage temperature had a more striking effect on the formation of putrescine and cadaverine which were accumulated especially at the end of storage period (Rokka et al., submitted). Ruiz-Capillas and Moral (2002) studied the effect of controlled and modified atmosphere on the production of biogenic amines during storage of hake. They found that controlled and modified atmosphere generally restricted the formation of biogenic amines and that cadaverine was a major biogenic amine formed in most of the studied atmospheres. Rodrı´guez et al. (1999) proposed that biogenic amines cadaverine and putrescine could indicate the freshness of freshwater rainbow trout either stored in air or in a vacuum package. 7.2.6 Carbon dioxide Carbon dioxide (CO2) is generally known to be produced during microbial growth. CO2 is also typically added as a protecting gas to modified atmosphere packages, together with inert nitrogen, since it has bacteriostatic effects. A modified atmosphere package for non-respiring food typically has a CO2 concentration as high as 20–80 % and, e.g., Fu et al. (1992) reported a further increase of CO2 during storage in modified atmosphere packages containing beef. Even if the indication of microbial growth by CO2 may be difficult in these modified atmosphere packages already containing a high concentration of CO2, it is possible to use the increase in CO2 concentration as a means of determining microbial contamination in other types of product. For instance Mattila et al. (1990) found a correlation between CO2 concentration and the growth of microbes in pea and tomato soup which were packaged aseptically either in air or in a mixture of O2 (5%) and nitrogen. 7.2.7 ATP degradation products K-value is defined as the ratio of the sum of hypoxanthine and inosine and the total concentration of ATP-related compounds (Henehan et al., 1997). This value, indicating the extent of ATP-degradation, correlates with the sensory quality of fish and also other types of meat and has been used as a freshness indicating parameter (Watanabe et al., 1989; Yano et al., 1995a). For fresh meat the value is low since the concentration of ATP-degradation products is low as compared to the concentration of all ATP-related compounds. The correlation between ATP-degradation products and fish quality has been extensively studied, e.g., by Hattula (1997). 7.2.8 Sulphuric compounds Some sulphuric compounds have a remarkable effect on the sensory quality of meat products due to their typical odour and low odour threshold. Hydrogen The use of freshness indicators in packaging 131