Estimation of an acceptable final level of pathogens in a food involves assessing the probability (Q) that, at no time in a given period, will a member of a given population be exposed to numbers of infective units of pathogens transmitted by foods equal to or greater than the minimal infectious dose (MID). This probability Q depends on (i) the average number of infective units of the particular pathogen in each portion of food consumed; (ii) the number of portions eaten by the whole population in a given period of time.
If the pathogens under review are randomly distributed in the entire bulk of the food and Nf infective units occur per portion of food at the time of consumption, the probability that one portion of food is not contaminated is e-Nf. If V = the size of the population at risk, and I = the number of portions eaten by one person in one year, Q = e-Nf vl, and hence Nf = -In Q (VI)-1.
Values of Nf at various values of Q and VI are presented in Table 1. The practical significance of the VI combinations chosen is illustrated in Table 2. However, only infrequently are the numbers of infective units whose absorption leads to overt disease as low as 1. Therefore calculations of Nf for Q = 0.99, a series of values of MID and the VI combinations of Table 2 are presented in Table 3.
MIDs depend on a variety of factors, in addition to the organism itself. Different subjects will react quite differently to a given challenge dose, depending on general health condition, nutritional status and gastric function. In addition, in a given individual the MID for an organism may vary considerably according to (i) the vehicle in which the organism is administered and ; (ii) whether or not the consumer has been fasting prior to infection (Mossel and Oei, 1975).