350 The nutrition handbook for food processors al (1999) screened microorganisms associated with the spoilage and safety of minimally processed vegetables. In general, exposure to high oxygen alone(80 to 90%O2, balance N2) did not inhibit microbial growth strongly and was highly variable. A prolongation of the lag phase was more pronounced at higher O2 con- centrations. Amanatidou et al, (1999)as well as Kader and Ben-Yehoshua(2000) uggested that these high O2-levels could lead to intracellular generation of read tive oxygen species(ROS, O2, H2O2, OH*), damaging vital cell components and thereby reducing cell viability when oxidative stresses overwhelm cellular pro- tection systems. Combined with an increased CO2 concentration (10 to 20%),a more effective inhibitory effect on the growth of all microorganisms was noticed in comparison with the individual gases alone( Gonzalez Roncero and Day, 1998 Amanatidou et al, 1999; Amanatidou et al, 2000). Wszelaki and Mitcham(1999) found that 80-100%O, inhibited the in vivo growth of Botrytis cinerea on straw berries. Based on practical trials(best benefits on sensory quality and anti- microbial effects), the recommended gas levels immediately after packaging are 80-95% O2 and 5-20%o N2. Carbon dioxide level increases naturally due to product respiration(Day, 2001; Jacxsens et al, 2001a). Exposure to high O2 levels may stimulate, have no effect on or reduce rates of respiration of produce depend- ing on the commodity, maturity and ripeness stage, concentrations of O2, CO and C2 Ha and time and temperature of storage(Kader and Ben-Yehoshua, 2000) Respiration intensity is directly correlated to the shelf life of produce(Kader et al, 1989). Therefore, the quantification of the effect of high O2 levels on the res- piratory activity is necessary (Jacxsens et al, 2001a). To maximise the benefits of a high O2 atmosphere, it is desirable to maintain levels of >40%0O2 in the head space and to build up CO2 levels to 10-25%b, depending on the type of packaged produce. These conditions can be obtained by altering packaging parameters such Ls storage temperature, selected permeability for O2 and CO2 of the packaging film and reducing or increasing gas/product ratio(Day, 2001) High O2 MAP of vegetables is only commercialised in some specific cases, probably because of the lack of understanding of the basic biological mechanisms involved in inhibiting microbial growth, enzymatic browning and concerns about possible safety implications. Concentrations higher than 25% O2 are consid- ered to be explosive and special precautions have to be taken on the work floor BCGA, 1998). In order to keep the high oxygen inside the package, it is advised to apply barrier films or low permeable OPP films(Day, 2001). However, for high respiring products, such as strawberries or raspberries, it is better to combine high O2 atmospheres with a permeable film for O2 and COz, as applied in EMA pack aging, in order to prevent a too high accumulation of coz ( Jacxsens et al, 2001b) 16.5 The microbial safety of MAP Modified atmospheres containing CO2 are effective in extending the shelf life of many food products. However, one major concern is the inhibition of nor mal aerobic spoilage bacteria and the possible growth of psychrotrophic foodal (1999) screened microorganisms associated with the spoilage and safety of minimally processed vegetables. In general, exposure to high oxygen alone (80 to 90% O2, balance N2) did not inhibit microbial growth strongly and was highly variable. A prolongation of the lag phase was more pronounced at higher O2 con￾centrations. Amanatidou et al, (1999) as well as Kader and Ben-Yehoshua (2000) suggested that these high O2-levels could lead to intracellular generation of reac￾tive oxygen species (ROS, O2 - , H2O2, OH*), damaging vital cell components and thereby reducing cell viability when oxidative stresses overwhelm cellular pro￾tection systems. Combined with an increased CO2 concentration (10 to 20%), a more effective inhibitory effect on the growth of all microorganisms was noticed in comparison with the individual gases alone (Gonzalez Roncero and Day, 1998; Amanatidou et al, 1999; Amanatidou et al, 2000). Wszelaki and Mitcham (1999) found that 80–100% O2 inhibited the in vivo growth of Botrytis cinerea on straw￾berries. Based on practical trials (best benefits on sensory quality and anti￾microbial effects), the recommended gas levels immediately after packaging are 80–95% O2 and 5–20% N2. Carbon dioxide level increases naturally due to product respiration (Day, 2001; Jacxsens et al, 2001a). Exposure to high O2 levels may stimulate, have no effect on or reduce rates of respiration of produce depend￾ing on the commodity, maturity and ripeness stage, concentrations of O2, CO2 and C2 H4 and time and temperature of storage (Kader and Ben-Yehoshua, 2000). Respiration intensity is directly correlated to the shelf life of produce (Kader et al, 1989). Therefore, the quantification of the effect of high O2 levels on the res￾piratory activity is necessary (Jacxsens et al, 2001a). To maximise the benefits of a high O2 atmosphere, it is desirable to maintain levels of >40% O2 in the head￾space and to build up CO2 levels to 10–25%, depending on the type of packaged produce. These conditions can be obtained by altering packaging parameters such as storage temperature, selected permeability for O2 and CO2 of the packaging film and reducing or increasing gas/product ratio (Day, 2001). High O2 MAP of vegetables is only commercialised in some specific cases, probably because of the lack of understanding of the basic biological mechanisms involved in inhibiting microbial growth, enzymatic browning and concerns about possible safety implications. Concentrations higher than 25% O2 are consid￾ered to be explosive and special precautions have to be taken on the work floor (BCGA, 1998). In order to keep the high oxygen inside the package, it is advised to apply barrier films or low permeable OPP films (Day, 2001). However, for high respiring products, such as strawberries or raspberries, it is better to combine high O2 atmospheres with a permeable film for O2 and CO2, as applied in EMA pack￾aging, in order to prevent a too high accumulation of CO2 (Jacxsens et al, 2001b). 16.5 The microbial safety of MAP Modified atmospheres containing CO2 are effective in extending the shelf life of many food products. However, one major concern is the inhibition of nor￾mal aerobic spoilage bacteria and the possible growth of psychrotrophic food 350 The nutrition handbook for food processors