Development of thermal and non-thermal preservation methods for production of microbiologically safe apple cider

Lise Lotte Dock, Purdue University

Abstract

The survival of a pathogenic acid resistant strain of Escherichia coli (E. coli O157:H7) in unpasteurized apple cider caused serious illness and death. The overall objective of this work was to develop thermal and non-thermal processing methods that result in a 5-log reduction of E. coli O157:H7 in apple cider. The effect of temperature, potassium sorbate, sodium benzoate and malic acid treatments on the inactivation of E. coli O157:H7 was investigated. Inactivation by ozone treatments was also studied. Based on D and z values, empirical models were developed to predict 5-log reduction of E. coli O157:H7, and the organoleptic quality of such 5-log-treated cider was optimized. Addition of malic acid and benzoate accelerated inactivation of E. coli O157:H7, while addition of sorbate had a lesser but significant effect. In cider without additives, D50 was 65 min, whereas with various amounts of additives, D50 ranged from 0.3–13.9 min. Furthermore, z-values were higher in cider containing additives than in cider without additives, indicating that a longer processing time may be required to pasteurize cider with additives at high temperature. It was possible to produce safe (5-log-treated) cider at 20°C by using appropriate amounts of additives. For example, cider with a pH of 3.6 containing 0.1% benzoate and 0.2% sorbate required a holding time of only 3 hr at 20°C for a 5-log reduction of E. coli O157:H7. Organoleptic and other quality measurements in 5-log-treated and untreated cider indicated that: (a) turbidity and sedimentation were not affected by any treatment; (b) cloudiness decreased with increasing processing temperature; (c) sorbate addition significantly increased off-flavors in cider; (d) cider with more than 0.05% benzoate and a pH below 3.6 was organoleptically different than untreated cider; and finally (e) in order to produce safe (5-log-treated) and organoleptically acceptable cider at 20°C, pH should be between 3.7–3.9, benzoate below 0.04% and sorbate between 0.08–0.16%. Inactivation of E. coli O157:H7 in cider by gaseous ozone was faster at 20°C than at lower temperatures. Safe cider was obtained by a treatment of 860 ppm ozone for 6.0 min at 20°C and 12.4 min at 5°C.

Degree

Ph.D.

Advisors

Floros, Purdue University.

Subject Area

Microbiology

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