workers failed to isolateC. botulinumor detect toxin in more than 500
samples analysed. When they deliberately inoculated these products with
C. botulinumspores and incubated at the abuse temperature of 10 1 C,
only in the case of vacuum packed whole trout was toxin produced
within the declared shelf-life of the product. Nevertheless, misuse of the
technique does have the potential for increasing risk and a Government
committee has recommended that all manufacturers of vacuum packing
machinery should include instructions alerting the user to the risks from
organisms such asC. botulinum.
In a variant of vacuum packing, known ascuisine sous-videprocessing,
food is vacuum packed before being given a pasteurization treatment
which gives it a longer shelf-life under chill storage. The technique was
developed in the 1970s in France and is said to give an improved flavour,
aroma and appearance. It is used for the manufacture of chilled ready
meals for various branches of the catering industry andsous-videmeals
are also available in the retail market in some European countries. They
have been slow to appear on the UK market due to the lack of appro-
priate UK regulations and concern over their microbiological safety with
respect to psychrotrophicC. botulinum. It has been recommended that
sous-videproducts with an intended shelf-life of longer than 10 days at
o 31 C should receive a minimum heat process equivalent to 90 1 C for 10
minutes; 70 1 C for 100 minutes should be sufficient for products with
shorter shelf-lives.
For other types of chilled, vacuum or modified atmosphere packed
foods, where there are no other controlling factors such as preliminary
heat treatment, low pH or high salt, an advisory 10 day rule is sometimes
applied. This states that such foods should have a designated shelf life
ofr10 days at storage temperaturesr 81 C. The same rule can also be
applied to foods packed in air as anaerobic niches suitable for
C. botulinumgrowth can exist in these products too.
Inmodified atmosphere packing, MAP, a bulk or retail pack is flushed
through with a gas mixture usually containing some combination of
carbon dioxide, oxygen and nitrogen. The composition of the gas
atmosphere changes during storage as a result of product and microbial
respiration, dissolution of CO 2 into the aqueous phase, and the different
rates of gas exchange across the packing membrane. These changes can
be reduced by increasing the ratio of pack volume to product mass
although this is not often practicable for other reasons.
The initial gas composition is chosen so that the changes which occur
do not have a profound effect on product stability. Some examples of
MAP gas mixtures used in different products are presented in Table 4.13.
Carbon dioxide is included for its inhibitory effect, nitrogen is non-
inhibitory but has low water solubility and can therefore prevent pack
collapse when high concentrations of CO 2 are used. By displacing oxygen
110 The Microbiology of Food Preservation