The influence of temperature and water activity on thermal resistance of Salmonella in milk chocolate
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Abstract
Salmonella contamination of chocolate-derived products has caused several outbreaks and recalls in recent years. Earlier research found that reducing moisture content or water activity of low-moisture foods sharply enhances the resistance of Salmonelladuring thermal treatments. However, there is a lack of data that correlates the relationship between temperature, water activity (aw), and thermal resistance of Salmonella in milk chocolate. In this study, milk chocolate was inoculated with SalmonellaEnteritidis PT 30 and conditioned to aw of 0.23, 0.33, and 0.43 at room temperature (21 ± 2 °C). The chocolate samples were heated at 70, 75, and 80 °C to obtain the D-values (time to inactivate 90% of the test microorganisms at a given temperature) of S.Enteritidis PT 30. The change of aw of milk chocolates at elevated temperatures (up to 80 °C) was also investigated. The results showed that the D-value of S. Enteritidis PT 30 decreased exponentially with the increase of aw or temperature. The maximum D-value was 47.4 ± 3.7 min obtained at 70 °C and aw of 0.23. The minimum D-value was 5.2 ± 1.0 min at 80 °C and aw of 0.43. The z-values were found as the followings: zaw,70°C = 0.42, zaw, 75°C = 0.36, and zaw, 80°C = 0.46 °C; zT, 0.23 = 18.8 ± 2.5, zT, 0.33 = 20.6 ± 4.1, zT,0.43 = 18.1 ± 0.5 °C. Overall, the results from this research may provide useful information to help the industry control the risk of Salmonella contamination and improve microbial safety in chocolate production.
Introduction
The risk of Salmonella contamination in chocolate has been a concern for the food industry over the past several decades. In 1970, Salmonella Durham caused 110 cases of infection through contaminated cocoa powder, an ingredient used in chocolate products (WHO, 1973). In 1982, a Salmonella-contaminated chocolate bar (Salmonella Napoli) resulted in 245 infections in the U.K. (Gill et al., 1983). The most severe Salmonellaoutbreak (Salmonella Oranienburg) can be traced back to 2001 in Europe, where over 400 people were reported sick from consuming contaminated German chocolate (Werber et al., 2005). The latest Salmonella outbreak in chocolate products was reported by the European Centre for Disease Prevention and Control (ECDC) starting from the beginning of 2022. This outbreak has caused over 450 illnesses so far (by August 8, 2022) due to contamination of Salmonella Typhimurium (ECDC, 2022). Overall, there have been two types of contaminations in milk chocolate that can cause Salmonella outbreaks: 1) from raw ingredients, like cocoa beans, that were contaminated from harvesting or transportation (Komitopoulou & Peñaloza, 2009); 2) from the processing plants of poor sanitary conditions. For example, it was reported by Hughes et al. (2008) that the Salmonella outbreak which occurred in the U.K. in 2006 was due to poor hygienic conditions.
Water activity (aw) of food is an important factor for the stability and growth of micro-organsms in a food product. Water activity can be defined as the ratio between the water vapor pressure of a food matrix and the water vapor pressure of pure water at the same temperature (FDA, 2014). In recent years, aw has been often reported as a key factor in the thermal resistance of bacteria in low-moisture foods. For example, He et al. (2013) showed that an increased aw (up to 0.8) of peanut butter reduced the thermal resistance of Salmonella in the sample at 90 and 126 °C. Earlier research documented the negative relationship between the moisture content of chocolate and the thermal resistance of Salmonella in the chocolate but did not correlate the result to aw (Barrile, Cone, & Keeney, 1970). Moreover, there is a lack of general information concerning how aw of milk chocolate changes with temperature in thermal treatments that are used for thermal control of Salmonella.
Chocolate (aw 0.3–0.5, moisture ＜2%) is a low moisture food with high sugar and fat content (Krapf & Gantenbein-Demarchi, 2010). Roasting is the first thermal processing for cocoa beans harvested from the field. After that, conching is the only thermal treatment in chocolate processing (Toker, Palabiyik, & Konar, 2019). During conching, all ingredients are mixed and cross-contamination may happen in this step. The purposes of conching are to evaporate moisture, reduce particle size, mix all the ingredients and accelerate aroma release but pathogen reduction was not taken into consideration (Bolenz, Thiessenhusen, & Schäpe, 2003; Counet, Callemien, Ouwerx, & Collin, 2002; Keogh, Murray, Kelly, & O’Kennedy, 2004; Owusu, Petersen, & Heimdal, 2012). In the chocolate industry, it has been acknowledged that a low-temperature treatment is preferred over a high-temperature treatment for the best quality of the product. Lin (2010) noted that 70–80 °C treatment temperatures can promote Maillard reactions and release volatiles which may improve the quality of chocolate. Yet, depending on the desired quality of the final products, the conching temperature varies from 50 °C to 75 °C and from a few hours to several days (Caligiani, Marseglia, & Palla, 2016; Glicerina & Romani, 2017). However, considering the low aw of chocolate and the low temperatures (like 50 °C) used for conching, it is not certain if such a process could serve as a kill step for a 5-log reduction of the target pathogens in chocolate (like Salmonella).
The objectives of this research were to 1) study the relationship between moisture content and aw at elevated temperature in milk chocolate, 2) determine thermal resistance (D-value) of Salmonella Enteritidis PT 30 in milk chocolate with certain aw(0.23,0.33, and 0.43) at three treatment temperatures (70, 75, and 80 °C).
