Following a 5 min incubation at room
temperature, three 25 g portions were cut from each group and
placed into sterile bags to which 225 mL of sterile peptone water
was added. The bags were hand mushed briefly and stomached
(Stomacher 400, Seward) for a approximately 30 s. The number of
viable L. monocytogenes in the samples was determined by plating
aliquots (0.1 mL and 0.5 mL) of the stomached lettuce/peptone
water mixture onto separate Oxford plates, containing Oxford Listeria
Selective Supplement, and incubated at 35 ± 2 C for 24 ± 2 h.
The resulting colonies were counted and the CFU/g was calculated
taking sample size and dilution into account.