Heat inactivation of Escherichia coli 0157 : H7 in apple juice exposed to chlorine

Exposure of Escherichia call O157:H7 to chlorine before heat treatment resu lts in increased production of heat shock proteins. Current heating regimen s for pasteurizing apple cider do not account for chlorine exposure in the wash water. This research determined the effect of sublethal chlorine treat ment on thermal inactivation of E. coli O157:H7. D-58-values were calculate d for stationary-phase cells exposed to 0.6 mg/liter of total available chl orine and unchlorinated cells in commercial shelf-stable apple juice (pH 3. 6). D-58-values for unchlorinated and chlorine-exposed cells in buffer were 5.45 and 1.65 min, respectively (P < 0.01). Death curves of chlorine-expos ed and unchlorinated cells in apple juice were not completely linear. Unchl orinated cells heated in apple juice exhibit a 3-min delay before onset of linear inactivation. Chlorine treatment eliminated this shoulder, indicatin g an overall loss of thermotolerance. The linear portion of each curve repr esented a small fraction of the total population. Dss-values calculated fro m these populations are 0.77 min for unexposed cells and 1.19 min for chlor ine-exposed cells (P = 0.05). This indicates that a subpopulation of chorin e-treated cells is possibly more resistant to heat because of chlorine trea tment. The effect of chlorine treatment, however, is insignificant when com pared with the effect of losing the shoulder. This is illustrated by the ti me required to kill the initial 90% of the cell population. This is observe d to be 3.14 min for unchlorinated versus 0.3 min for chlorine-exposed cell s (P < 0.001). These observations indicate that current heat treatments nee d not be adjusted for the effect of chlorine treatment.