Effect of pH on eggshell penetration by Salmonellae

Experiments were conducted to study effects of pH on penetration of eggs by three species of Salmonella. Eggs having an average specific gravity of 1.078 were subjected to challenge by either S. typhimurium, S. st. paul, or S. derby. Challenge solutions ranged from pH 5.0 to 9.5 in .5 pH increments and contained an average of 7.5 x 10(3) Salmonella/ml. Egg temperature was 22 C and solution temperature 4.4 C when challenged. Tartaric acid (10%) or 1 N. NaOH were used to adjust solution prior to adding challenge organisms. Eggs were challenged for 3 min then allowed to dry and held at 22 C for 24 hr, after which they were opened aseptically. Salmonella penetration was determined by swabbing the inner shell membrane and incubating in selenite cystine and tetrathionate enrichment broths for 24 hr followed by plating on MacConkey and SS agars. Penetration rates for all three organisms were significantly less at pH 5.0 than at any higher pH tested. There was an increase in penetration from pH 5.5 to 7.0 for all species. Maximum penetration rates were 42% of eggs challenged at pH 7.5, 22% at pH 8.5, and 34% at pH 7.0 for S. typhimurium, S. derby, and S. st. paul, respectively. In no case was penetration of eggs at pH 9.0 significantly different from pH at maximum penetration of challenge eggs. Penetration by S. st paul, at pH 9.5 was significantly less (P less than .05) than at pH 7.0. Decalcification of the eggshell was less than .01%/min at pH 4.0. Shell losses at pH 3.5 and 3.0 were .03% and .33%/min, respectively.