Enumeration Agar, Acid Exposure and Sampling Time Are Relevant Factors Accounting for the High-Pressure Inactivation of Vegetative Pathogens in Fruit Puree

High pressure processing (HPP) is a non-thermal technology with emerging application within the fruit and vegetable sector. The impact of the enumeration agar on the recorded HPP inactivation of <i>L. monocytogenes</i>, <i>Salmonella</i> spp. and <i>E. coli</i> in banana-apple and apple purees was evaluated. Additionally, the HPP inactivation and sublethal injury was quantified in apple puree, considering the impact of acid exposure (24 h before HPP) and sampling time. Inoculated purees were pressurized at 300 MPa for 2 min. Enumeration was performed immediately and 24 h after HPP. HPP inactivation was 0.9-to-4.5-fold higher in apple than banana-apple puree. Compared with nutrient-rich media, selective agar enumeration overestimated the inactivation. HPP inactivation and sublethal injury of <i>L. monocytogenes</i>, <i>Salmonella</i> and <i>E. coli</i> was variable, mainly dependent on the exposure to acid and the sampling time. The 24 h-delayed enumeration slightly increased the inactivation. In apple puree, the CECT5947 strain of <i>E. coli</i> O157:H7 was the most piezo-resistant strain (1.5 log reduction), while <i>L. monocytogenes</i> Scott A was the most piezo-sensitive (6-log reduction when exposed to acid and sampled 24 h after HPP). All the studied factors should be taken into account when designing HPP treatments, performing product-specific validation studies and setting verification procedures.