Over the past two decades, high-pressure processing (HPP) has attracted considerable industrial attention as an alternative to conventional thermal processing in response to consumers' needs for safe, healthier, fresh-like food products (Oey, Lille, Van Loey, & Hendrickx, 2008; Olsen, Grunert, & Sonne, 2010). HPP technology has been successfully commercialized for pasteurizing various types of foods, including seafood, meat, vegetables, ready-to-eat (RTE) foods, and juices at ambient or chilled temperatures (Farkas, 2016). Although not commercialized yet, a processing method called pressure-assisted thermal processing (PATP) can inactivate both microbial cells and spores and it is feasible to use the technology to produce shelf-stable commercially sterilized food by combining high pressures and temperatures above 100 °C. In 2009 and 2015, FDA issued two letters of no objections for industrial petitions for PATP treated low-acid foods (Daryaei, Yousef, & Balasubramaniam, 2016; Stewart, Dunne, & Keener, 2016).

Although various pressure pasteurized products are commercially available, no PATP products are commercially available. With current industrial practice, both HPP and PATP technologies are essentially batch operations where the food products need to be individually pre-packaged, loaded into a pressure vessel, and processed at target process conditions (Hogan, Kelly, & Sun, 2005). The batch nature of HPP and PATP may limit their overall product throughputs (Lelieveld & Hoogland, 2016). Therefore, the food industry, especially the pumpable liquid beverage industry, is seeking continuous forms of high-pressure processing methods.

Ultra-shear technology (UST), or high-pressure homogenization (HPH), is a semi-continuous high-pressure processing method. During the process, liquid foods are first pressurized up to 400 MPa, and depressurized by discharging through a small opening known as a pressure-drop valve or shear valve. During the discharging process, the pressure energy is converted into kinetic energy, generating a shear effect and temperature elevation (Martínez-Monteagudo, Kamat, et al., 2017, Martínez-Monteagudo, Yan, & Balasubramaniam, 2017). UST can be used to alter the characteristics of liquid foods, producing value-added pasteurized or commercially sterilized liquid food products by generating mixing effects, dispersion, and nanoemulsion (Janahar, Marciniak, Balasubramaniam, Jimenez-Flores, & Ting, 2021; Janahar et al., 2022). From a microbial perspective, UST has the potential to inactivate microbial cells and spores with pressure, temperature, and shear as lethal factors.

Recent studies had evaluated the efficacy of the combined effect of high-pressure, thermal, shear against bacterial and fungal cells and spores (Georget et al., 2014; Janahar, Xu, Balasubramaniam, Yousef, & Ting, 2023). However, only a few studies were conducted using pressure- and thermal-resistant bacterial spores (such Clostridium sporogenes) used in industrial process validation studies. Moreover, there is a limited understanding of the individual or combined effect of various lethal factors (pressure, temperature, shear) on bacterial spore inactivation during UST processes. Therefore, the goal of this study was to evaluate the effects of pressure, shear, temperature, and the interaction of the three lethal factors on the inactivation of pressure- and thermal-resistant Clostridium sporogenes PA3679 spores during UST processing.