168 Chilled foods not apparent until the late 1970s. Reported cases in the UK increased dramatically during the 1980s, and decreased during subsequent years. The symptoms of disease are protean and range from a mild flu-like illness to meningitis, septicaemia, stillbirths and abortions(Ralovich 1987). In general the major symptoms of disease are restricted to the pregnant mother, foetus level can be high(McLauchlin 1987). The epidemiology or s, the mortality elderly and immunocompromised. With the latter three groi monocytogenes as been discussed by Schuchat et al.(1991) A very wide range of foods including meat, poultry, dairy products, seafoods and vegetables have been reported to be contaminated with L. monocytogenes and have been reviewed by Bell and Kyriakides(1998b). Whilst the total absence of L. monocytogenes from raw meats, poultry and vegetables is difficult to ensure, the bacterium has been isolated from products which have undergone a listericidal thermal process ( Lund 1990). Such isolations are of concern as many of these chilled foods may be consumed without further heating. The presence of L. monocytogenes on cooked foods suggests that post-process contamination may have occurred. Several studies have shown this bacterium has been isolated from a wide range of sites in several types of factory( Cox et al. 1989)and may be spread by some cleaning procedures(Holah et al. 1993) Sites of particular concern include those where water is present. Environmental control of Listeria, particularly in key areas of production(e.g. after cooking)is Or clal to the prevention of product contamination. Whilst the number of cases eported listeriosis in England and Wales peaked dramatically between 1986 and 1988 which was associated with contaminated imported pate. Following public warnings about this, the number of cases declined to the annual rate prior to this(100-150 cases per year)(Border and Norton 1997) The major concern with L. monocytogenes is its ability to grow at low temperatures, and a minimum growth temperature of -0 4C has been reported Walker et al. 1990). Temperature control will however, retard the rate of growth(Fig. 7. 2). Conversely, temperature abuse during storage of a food ca exacerbate problems. Listeria monocytogenes is more resistant than many other vegetative bacteria to some, but not all, of the preservation mechanisms used in food manufacture (e.g. chilling, reduced water activity)and these have been reviewed by Walker (1990). Whilst resistance may be noted to these preservation systems when examined individually, foods are complex and interactions may occur which effectively prevent growth. The use of predictive models(see Section 7.9) for microbiology is an efficient method to identify such Interactions L. monocytogenes is not considered to be a classically heat-resistant bacterium. It is generally accepted that conventional HTST milk pasteurisation (717C/15 seconds) will eliminate this microorganism when freely suspended in milk(Bradshaw et al. 1991). In other foods, decimal reduction times of 8-16 seconds have been reported at 70oC(Gaze et al. 1989). It has been recommended (Anon. 1989)that foods subject to a cook-chill process be heated to a minimum of 70.C for 2 minutes (or the thermal equivalent) to ensure the effectivenot apparent until the late 1970s. Reported cases in the UK increased dramatically during the 1980s, and decreased during subsequent years. The symptoms of disease are protean and range from a mild flu-like illness to meningitis, septicaemia, stillbirths and abortions (Ralovich 1987). In general, the major symptoms of disease are restricted to the pregnant mother, foetus, elderly and immunocompromised. With the latter three groups, the mortality level can be high (McLauchlin 1987). The epidemiology of L. monocytogenes has been discussed by Schuchat et al. (1991). A very wide range of foods including meat, poultry, dairy products, seafoods and vegetables have been reported to be contaminated with L. monocytogenes and have been reviewed by Bell and Kyriakides (1998b). Whilst the total absence of L. monocytogenes from raw meats, poultry and vegetables is difficult to ensure, the bacterium has been isolated from products which have undergone a listericidal thermal process (Lund 1990). Such isolations are of concern as many of these chilled foods may be consumed without further heating. The presence of L. monocytogenes on cooked foods suggests that post-process contamination may have occurred. Several studies have shown this bacterium has been isolated from a wide range of sites in several types of factory (Cox et al. 1989) and may be spread by some cleaning procedures (Holah et al. 1993). Sites of particular concern include those where water is present. Environmental control of Listeria, particularly in key areas of production (e.g. after cooking) is crucial to the prevention of product contamination. Whilst the number of cases of reported listeriosis in England and Wales peaked dramatically between 1986 and 1988 which was associated with contaminated imported paˆte´. Following public warnings about this, the number of cases declined to the annual rate prior to this (100–150 cases per year) (Border and Norton 1997). The major concern with L. monocytogenes is its ability to grow at low temperatures, and a minimum growth temperature of
0.4ºC has been reported (Walker et al. 1990). Temperature control will however, retard the rate of growth (Fig. 7.2). Conversely, temperature abuse during storage of a food can exacerbate problems. Listeria monocytogenes is more resistant than many other vegetative bacteria to some, but not all, of the preservation mechanisms used in food manufacture (e.g. chilling, reduced water activity) and these have been reviewed by Walker (1990). Whilst resistance may be noted to these preservation systems when examined individually, foods are complex and interactions may occur which effectively prevent growth. The use of predictive models (see Section 7.9) for microbiology is an efficient method to identify such interactions. L. monocytogenes is not considered to be a classically heat-resistant bacterium. It is generally accepted that conventional HTST milk pasteurisation (71.7ºC/15 seconds) will eliminate this microorganism when freely suspended in milk (Bradshaw et al. 1991). In other foods, decimal reduction times of 8–16 seconds have been reported at 70ºC (Gaze et al. 1989). It has been recommended (Anon. 1989) that foods subject to a cook-chill process be heated to a minimum of 70ºC for 2 minutes (or the thermal equivalent) to ensure the effective 168 Chilled foods