Raw almonds are common food commodities that can be directly consumed without cooking or used as an ingredient in confectionery, snacks, and other purposes. As an important source of protein, lipids, monounsaturated fat, and others, the consumption of almond products has vastly increased over the past 50 years in the United States (USDA, 2019). However, raw almonds have been implicated in Salmonella outbreaks in the past, resulting in international recalls of up to 13 million pounds of raw almonds (CDC, 2004; Isaacs et al., 2005). This pathogen is still a safety concern for almonds, as approximately 350,000 pounds of California-produced raw almonds were recently recalled due to the possible presence of Salmonella (FSN, 2022). In addition, Listeria monocytogenes is another notorious pathogen that leads to frequent recalls of food products containing almonds (FDA, 2018b, FDA, 2020, FDA, 2023). The contamination of almonds may happen during the in-field harvest, where mature almonds are shaken off the tree and left on the ground to dry until reaching the desired moisture content. This practice allows the direct contact of almonds with soil and dust that could be contaminated (Danyluk et al., 2008). Almond kernels could also be potentially contaminated with foodborne pathogens during the hulling and shelling process, especially if the equipment, containers, and utensils in the processing facilities are not properly sanitized. Once contaminated, Salmonella and L. monocytogenes can persist in almond kernels (Kimber et al., 2012) and almond meals (Zhu et al., 2020; Zhu et al., 2021) for up to a year at various storage temperatures. To improve the microbial safety of almond products, federal regulations require that all processed whole almonds be tested negative for Salmonella and Listeria spp. in every 375 g sample (USDA, 2022), making it crucial to control their contamination in raw almonds.

To reduce the risk of contamination with pathogenic microbes, the Almond Board of California mandates that whole almonds undergo a validated treatment, such as propylene oxide fumigation, steam, heat treatments, radiofrequency pasteurization, or other methods, to achieve a 4 log reduction of Salmonella, and those treated whole almonds must be packaged within 45 days (USDA, 2022). Propylene oxide fumigation is a traditional method to pasteurize conventional almonds. However, propylene oxide raises concerns about potential chemical residues (Danyluk et al., 2005), and it has been classified as a probable human carcinogen (EPA, 2016). Fumigation of almond kernels using gaseous chlorine dioxide is another method to inactivate Salmonella. However, its antimicrobial efficacy is limited, achieving only ~1 log10 CFU/g reduction of Salmonella in almonds after a 4-h exposure to gaseous chlorine dioxide at 1.34 mg/L (Rane et al., 2021). Dry heat can be used for microbial reduction in almond kernels (ABC, 2007), whereas this method is time-consuming and less effective; the 60 min heating at 90 °C only resulted in 0.86–2.49 log CFU/g Salmonella reductions on almonds depending on strains (Song and Kang, 2021). Radiofrequency heating has been approved for pasteurizing raw almonds (ABC, 2016), and it exhibited a higher efficacy against Salmonella on almonds than convection heating (Jeong et al., 2017). However, the heating uniformity of radiofrequency treatment depends on many variables (Ozturk et al., 2017), and the equipment and operation of radiofrequency heating are costly. Thus, alternative treatment methods are proposed for controlling microbial contamination in almonds.

Steam treatments have been used to improve the microbial safety of raw almonds (ABC, 2020; USDA, 2022). A previous study showed that short steam treatments have no impact on the total polyphenols, flavonoids, and phenolic acids, and the antioxidant activity of almond skins (Bolling et al., 2010). The existing limited studies indicated that saturated steam and superheated steam were effective methods to decontaminate Salmonella and L. monocytogenes in almonds and pistachios (Ban and Kang, 2016), and in-shell pecans (Ban et al., 2018). For example, a 10 s exposure to saturated steam at 100 °C reduced L. monocytogenes and S. Enteritidis PT 30 on almonds by ~1.5 and ~ 2 log10 CFU/g, respectively (Ban and Kang, 2016). Increasing the steam temperature from 100 °C to 200 °C for a 10 s contact time effectively caused ~4 and ~ 5 log10 CFU/g reductions of L. monocytogenes and S. Enteritidis PT 30 on almonds, respectively (Ban and Kang, 2016). However, steam temperature at 200 °C might negatively impact the quality of almonds and increase carbon footprint. Consequently, it is necessary to fully evaluate the effectiveness of steam treatments at practical temperature ranges.

Water activity (aw) of low-moisture foods is a critical factor impacting the resistance of L. monocytogenes and Salmonella to heat treatments (Dhowlaghar et al., 2021; Tsai et al., 2019a; Tsai et al., 2019b; Villa-Rojas et al., 2013; Zhu et al., 2020; Zhu et al., 2021). For example, S. Enteritidis PT 30 became more resistant to heat treatment at ~70 °C as the aw of almond flour was reduced from 0.95 to 0.60 (Villa-Rojas et al., 2013). Salmonella in almond kernels with aw 0.27 was more difficult to decontaminate by dry heating at 75 °C in comparison to that in almonds with aw 0.52 (Xu and Chen, 2023). Those studies demonstrated the importance of controlling the aw of almonds during thermal treatments. However, the impacts of aw on the effectiveness of steam treatments against L. monocytogenes and Salmonella on almonds remains unknown and thus were evaluated in this study at two aw levels of 0.25 and 0.45 within the common almond aw storage conditions.

In addition, to comply with the Food Safety Modernization Act (FSMA) (FDA, 2018a), the food industry is required to validate the treatment process and document microbial reductions. A non-pathogenic surrogate is commonly used instead of the target pathogen in validation studies to avoid the risk of introducing target pathogens to food facilities. Enterococcus faecium NRRL B-2354 has been recognized as a suitable Salmonella surrogate in validating the thermal treatments of almonds (ABC, 2014), and L. innocua is a well-known L. monocytogenes surrogate. Therefore, this study aimed to (1) evaluate the effectiveness of steam treatments in inactivating E. faecium and L. innocua on almond kernels at practical steam treatment temperatures, and (2) assess the impact of initial almond aw on the efficacies of steam treatments against the selected surrogates.