Foods that have a water activity (aw) below 0.85 are defined as low moisture food products by the Codex Committee on Food Hygiene (CCFH) and US Food and Drug Administration (FDA) (CCFH, 2015; FDA, 2015, Sánchez-Maldonado, Lee and Farber, 2018a, Sánchez-Maldonado, Lee and Farber, 2018b). Lower water activity is considered to prevent the growth of pathogen microorganisms, including Salmonella (Podolak, Enache, Stone, Black, & Elliott, 2010) while significant number of outbreaks has been frequently linked to these products (Alshammari, Xu, Tang, Sablani, & Zhu, 2020; Podolak et al., 2010). The most frequently identified serotypes linked with these outbreaks are Salmonella Enteritidis and Salmonella Typhimurium (Beuchat et al., 2011; Syamaladevi et al., 2016). Sesame seed-based products are one of the cases repeatedly linked to Salmonella outbreaks within the last two decades, in November 2002, June 2003, September 2011 and January 2019 (Unicomb et al., 2005, Centers for Disease Control and Prevention (CDC), 2012, European Centre for Disease Prevention and Control, European Food Safety Authority, 2021). Tahini, sesame paste, is produced from sesame seeds by cleaning and soaking them in water, roasting, cooling, and grinding (Sanja et al., 2015; Torlak, Sert, & Serin, 2013; Zhang, Keller, & Grasso-Kelley, 2017). It is a common ingredient in halva and hummus and can also be consumed raw in homemade sauces (Lake, King, Cressey, & Gilbert, 2010; Zhang et al., 2017).

Torlak et al. (2013) investigated the inactivation rates of Salmonella during sesame seed roasting and storage of tahini. It was indicated that the roasting at 110 °C for 60 min, at 130 °C for 50 min or 150 °C for 30 min were adequate to achieve more than 5 log cycle reduction. Zhang et al. (2017) stated that the study carried out by Torlak et al. (2013) did not investigate the effect of the whole process which may eventually change the presence of Salmonella in tahini. They found that high initial aw values (≥0.90) at the beginning of roast can positively affect the inactivation of Salmonella, but when the aw levels are reduced to an aw of 0.45 prior to roasting, Salmonella might still survive through the roasting process. Formation of biofilms before roasting was also reported to have an impact on its thermal resistance. Brockmann, Piechotowski, and Kimmig (2004) also explained that that cross-contamination after heat treatments can be a reason for the presence of Salmonella in sesame seed products such as tahini. Hence, it would be important to introduce a following process prior to packaging and storing. For this purpose, various decontamination methods were presented. These included the use of natural anti-microbial agents (Al-Nabulsi et al., 2014; Olaimat et al., 2017) and gamma radiation (Al-Nabulsi et al., 2020; Osaili & Al-Nabulsi, 2016; Osaili, Al-Nabulsi, Abubakar, Alaboudi, & Al-Holy, 2016). Besides these, Osaili et al. (2021) presented the use microwave processing on (25 g) tahini samples and indicated the effect of high-power application for longer times for inactivation of Salmonella without negatively affecting the quality attributes. While these studies showed possible innovative approaches, an industrially applied process condition was not presented. In fact, an efficient novel approach is needed for a sustainable process that might be easily applied in industrial scale.

Microwave processing is considered to be an alternative to conventional heating with its possible volumetric heating feature. Various studies were reported for significance of microwave cavity geometric configuration, holding pipe orientation in continuous flow systems, and physical properties of the products (Basak & Meenakshi, 2006; Karatas, Topcam, Altin, & Erdogdu, 2022; Kumar et al., 2008; Nikdel, Chen, Parish, MacKellar, & Friedrich, 1993; Salvi, Boldor, Aita, & Sabliov, 2011; Topcam & Erdogdu, 2021; Topcam, Karatas, Erol, & Erdogdu, 2020; Tuta & Palazoğlu, 2017; Yang et al., 2022; Zhou, Puri, Anantheswaran, & Yeh, 1995). Besides, as also presented in a recent review, the shorter processing times with lower environmental footprint are additional features of microwave processing. A continuous flow microwave heating, hence, might considered to be possible to process tahini, but the effective temperature range for microbial inactivation should be known with the changes in final quality. The temperature distribution evolved within the sample during microwave processing is also an important feature to know.

Therefore, the objectives of this study were:

To develop a computational model to determine the temperature distribution of tahini during microwave processing and validate this model with experimental time-temperature data,

To experimentally identify the required temperature ranges for inactivating Salmonella sp. in tahini, and

To design an industrial scale continuous flow microwave process to attain the required temperature for tahini.