2.1. Design of the experiment
A Rotatable Central Composite Design of experiment (CCRD)
was employed to determine the combinations of the product (pH
and initial ascorbic acid, AA concentration) and process (temperature
and time) variables investigated in this study. The experimental
design consisted of 16 factorial combinations of high and
low settings, 8 axial combinations of very low/very high and intermediate
settings, and 12-time replicated center point combinations
where all factors were set to intermediate values. In the
CCRD, the ranges for the variable settings of pH and initial AA
concentration were based on the reported physicochemicalcharacteristics of fresh and processed citrus juice products in the
study of Gabriel et al. (2015). Furthermore, the temperature and
time ranges were based on the previously established decimal
reduction times (D-values) of E. coli O157:H7 in simulated fruit
juices at varying heating temperatures (Gabriel, 2012). Table 1
summarizes the CCRD combinations generated using the Stat-
Ease Design-Expert version 7.0.0, trial version (Stat-Ease Inc.,
Minneapolis, Minnesota, USA).