| Scanning electron and light microscopy demonstrated no disruption of asparagus stem or tip cell wall structure after boiling water blanching or the combination of boiling water blanching and HHP treatment. Boiling water blanched asparagus with a 1 to 0.75 ratio of spears to brine was firmest compared to steam or microwave blanched asparagus. Blanched asparagus were lighter, less green and more yellow compared to raw green asparagus. The equilibrium pH of acidified asparagus was approximately 4.0. Single log reduction times from HHP of 310 MPa at 21°C survivor curves for Escherichia coli, Listeria innocua and Enterococcus faecium in nutrient broth (pH 4.0) were 0.86, 0.15, and 4.58 min, respectively, whereas single log reduction times from HHP of 379 MPa at 21°C survivor curves in asparagus broth (pH 6.8) were 4.47, 0.17, and 21.11 min, respectively. The viable cfu/ml on nonselective media were greater than viable cfu/ml on selective media, relating to the recovery of HHP injured cells on nonselective media. The addition of 0.7% salt enhanced HHP resistance of E. faecium in nutrient and asparagus broth systems at pH 4.0 or pH 6.8. Whereas, acid pH was markedly synergistic to HHP susceptibility of E. faecium. As HHP increased, the number of surviving E. faecium decreased. D-values and z-values for E. faecium in nutrient or asparagus broths treated with HHP were determined from linear regression analyses of survivor or decimal resistance curves. D-values of E. faecium in nutrient and asparagus broths with pH 4.0 were less than D-values with pH 6.8. z-values of E. faecium in asparagus broth with both pH 4.0 and 6.8 were greater than in nutrient broth. D-value (0.54 min) of E. faecium with pH 4.0 treated with HHP at 448 MPa at 21°C was selected for acidified asparagus processing. The selection of D-values and z-values for a documented HHP process will result in a food that is microbiologically safe.; The combination of acidification and boiling water blanching provided effective inactivation (107) of E. faecium inoculated on asparagus spears. Therefore, HHP treatment is not necessary for safe processing of asparagus spears acidified in acid brine. |
