Alicyclobacillus acidoterrestris spores as a target for Cupuacu (Theobroma grandiflorum) nectar thermal processing: kinetic parameters and experimental methods

Abstract

The kinetic parameters of thermal inactivation of a spore former, Alicyclobacillus acidoterrestris, in a tropical fruit nectar [25% of Cupuaçu (Theobroma grandiflorum) pulp and 15% sugar] were determined by the isothermal method (IM), under batch heating, and by the paired equivalent isothermal exposures (PEIE) method, under non-isothermal continuous conditions. The isothermal experiments were repeated three times, every 4 months, with the same spore suspension kept frozen between experiments. The aging of spores, under frozen storage, seemed to produce a notorious increase in the z-value from experiment to experiment: Experiment 1 (z=7.8±2.6 °C, D95 °C=5.29±0.96 min), Experiment 2 (z=22±5 °C, D95 °C=5.99±0.63 min), and Experiment 3 (z=29±10 °C, D95 °C=3.82±0.48 min). The evaluation of the kinetic parameters by the PEIE method was carried out in parallel with Experiment 3, with the same aged spores, and the results (z=31±6 °C, D95 °C=5.5±1.2 min) were close to the ones obtained in this experiment. From this work, it seems that the PEIE method can also be applied to evaluate the reduction parameters of a spore-forming microorganism, and in a more realistic way, since the continuous system eliminates the errors caused by come-up and cool-down times (CUT and CDT) that are unavoidable in isothermal experiments. Therefore, when designing a thermal process for a continuous system, the PEIE method should be used, or the chances are that the process would be underdesigned, risking that the desired level of spore inactivation would not be achieved. An optimization of the thermal processing conditions was next performed for Cupuaçu nectar, considering a 5D reduction in A. acidoterrestris spores. If a pasteurization process is considered, the conditions that ensure safety (9 min at 98 °C) only allow a 55% retention of ascorbic acid (AA). If sterilization is considered, 8 s at 115 °C will ensure a safe product and retain 98.5% of the original ascorbic acid. Therefore, if A. acidoterrestris is considered as the target microorganism, the nectar should undergo an aseptic high temperature short time principle (HTST) process to achieve a 5D reduction in this acidophilus spore former. However, if the hot-fill-and-hold pasteurization process is preferred, the product should be fortified with ascorbic acid.