Detachment of Listeria innocua and Pantoea agglomerans from cylinders of agar and potato tissue under conditions of Couette flow

Cylinders of raw potato or agar were contacted with suspensions of Listeria innocua and Pantoea agglomerans and then used as replacement rotors in a rheometer in order to investigate detachment under the influence of known shear forces. These shear forces were functions solely of the rotational speed of the rotor and the fluid (glycerol) in which the cylinders were caused to rotate. With this system surface shear forces ranging from 1.3 to 125 Pa could be generated corresponding to rotational speeds of 12.5–775 rpm. Under these conditions detachment phenomena were quite rapid with in most cases complete detachment being achieved over timescales of the order of 30 s. In general, lower shear forces were required to detach L. innocua from both agar and potato. For agar cylinders an applied shear force of only 1.3 Pa was sufficient to achieve 98% detachment of L. innocua after 20 s. By contrast, relatively high shear forces were required to detach P. agglomerans particularly from potato; under an applied shear force of 2.8 Pa only 9.5% detachment was achieved after 30 s. The results obtained at the highest shear forces studied here (125 Pa) with potato cylinders were suggestive of mass transfer into glycerol of one or more constituents present in potatoes that caused detached cells to aggregate causing an apparent decrease in percentage detachment. The data obtained could be used as a basis for the rational design of washing processes for fresh ready to eat food products.