The problem of balancing out the effect of order of presentation and the carryover effect of a preceding sample over a series of presentations of the same set of samples is addressed. A series of designs developed by Williams (1949) are used. The method of calculation is given. Tables containing about 50 consumers of each design for presenting from 4 through to I6 samples are given.

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A selective medium was used to enumerate Clostridium botulinum growing in the presence of natural spoilage organisms in a model cured pork slurry. The growth responses of a mixed spore inoculum of six strains of Cl. botulinum type A were studied at 15 degrees, 20 degrees and 27 degrees C with 1.5, 2.5, 3.5 or 4.5% (w/v) salt added (aw range 0.961-0.990). Gompertz and logistic curves, which have a sigmoid shape, were fitted to the data and lag times, growth rates, generation times and time to maximum growth rates were derived. Variation in germination rates of the spores occasionally gave a falsely extended lag time resulting in an exceptionally high estimate for growth rate. Products containing 4.5% (w/v) NaCl would be capable of supporting growth of proteolytic strains of Cl. botulinum, even at 15 degrees C, although the lag period would be extended. In products where absence of Cl. botulinum cannot be assured additional preservative measures are essential. The information obtained provides a framework to investigate the effects of a wider range of additives or variables on the growth responses of Cl. botulinum.

Received 23 M a y 1989 and accepted 31 M a y 1989Heat resistance data for Listeria monocytogenes are reviewed. The organism is appreciably more resistant than common Salmonella serotypes but less resistant than Salmonella senftenbmg 775W. Reports that the organism can survive heating at 80°C have not been substantiated and are incompatible with carefully determined D and z values in milk and a range of foods. Cooking food to an internal temperature of 70°C for 2 min is adequate to ensure destruction of L. monocytogenes. Normal pasteurization procedures will inactivate L. monocytogenes in milk but the margin of safety is greater for vat pasteurization than for high temperature short time treatment.Heat treatment is a critical point for controlling Listeria monocytogenes in many manufactured and prepared foods. Reliable data on heat resistance are therefore essential in defining safe heat treatments, especially as refrigeration cannot be relied upon to prevent growth in the event of underprocessing. An early report of exceptional heat resistance (Beams & Girard 1958) has not been confirmed (Donelly et al. 1987) but the organism does appear to be more heat-resistant than many other non-sporing food-borne pathogens. To assist those involved in the heat processing of food we have summarized published information on heat resistance of L. monocytogenes, though recognizing that data are absent or rudimentary for many products and are actively being sought in many laboratories.General collsideratiollsHeat resistance data for L. monocytogenes taken from papers listed in the References are summarized diagrammatically in Fig. 1 75Temperature ("C) Fig. 1. Heat resistance data for Listmia monocytogenes compiled from papers reviewed in the text and from Bradshaw et a/. (1985, 1987).

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