Salmonella is a leading agent of foodborne illness, accounting for thousands of foodborne illness outbreaks reported each year. The Centers for Disease Control and Prevention (CDC) estimates that Salmonella causes approximately 1.0 million illnesses per year in the United States (Scallan et al., 2011), amounting to a conservative annual average economic burden of approximately $4.4 billion (Scharff, 2012). Water activity (a_w), where a_w  =  P/P⁰, and P  =  partial pressure of water over the sample at equilibrium and P⁰ =  equilibrium pressure of pure water at the same temperature, is a critical concept in microbial safety, as the minimum a_w for growth of most bacteria is approximately 0.87. Foods with an a_w below 0.85 are commonly classified as low-water activity, and these include roasted peanuts (Beuchat et al., 2013). Although Salmonella contamination has traditionally been associated with high-water activity foods such as eggs and poultry, several recent outbreaks have involved lower-a_w foods such as almonds, peanut butter, powdered milk, cereal, chocolate, dry pet food, and various nuts (Beuchat et al., 2013). Salmonella growth generally occurs at a_w between 0.94 and 0.99 (Bell and Kyriakides, 2009); however, it can survive at lower a_w, and may be difficult to inactivate (Farakos et al., 2013). Survival of Salmonella is a concern in low-a_w foods since infection by Salmonella may occur with ingestion of even a single cell, depending on the health of the consumer (Food and Drug Administration, 2012).

Until recent years, nuts have been considered to be of low risk for Salmonella contamination due to low a_w. Although Salmonella does not generally grow on nuts, it can survive on nut surfaces for periods as long as a year, and develops increased resistance to thermal treatment over time (Danyluk et al., 2007). One of the largest food recalls in U.S. history was due to an outbreak linked to Salmonella in peanut products causing 714 documented illnesses and 9 deaths, leading to 3,900 products being recalled by 200 companies (Wittenberger and Dohlman, 2010). In addition to peanut butter, peanuts are used as an ingredient in a wide variety of products, including, snack foods and confections (Chang et al., 2013). Many small companies buy peanuts from larger manufacturers who produce bulk quantities of peanuts to distribute to numerous clients (Wittenberger and Dohlman, 2010).

Salmonella contamination can occur at any point in the production of foods and is generally traced back to human or animal sources. Eliminating the risk of Salmonella contamination at the manufacturing level involves proper control and hygiene during all stages of processing, including raw materials handling, roasting, post-roast processing and packaging (Bell and Kyriakides, 2009). Prevalence of Salmonella on raw, grade samples of peanuts from all production areas in 2008 to 2010 was estimated at 2.33% and the concentration levels of Salmonella in positive samples, as determined by a most-probable-number assay (MPN), were <0.03 to 2.4 MPN/g (Calhoun et al., 2013). In a separate study, Salmonella was found in 0.67% (68/10,162 samples) from raw, runner peanuts from 2009, 2010 and 2011sampled at labs located in both GA and TX (Miksch et al. 2013). No difference in prevalence data was observed for the two growing regions and concentration levels of Salmonella in positive samples averaged 1.05 MPN per 350 g.

The FDA recommends that peanut processors use a process that is validated to reduce Salmonella on peanuts, e.g., 5 logarithm of colony forming units/g (5-log CFU/g) (Food and Drug Administration, 2009). Commercial testing laboratories that perform roaster validations currently use a Salmonella surrogate, Enterococcus faecium (personal communication, Deibel Laboratories, Inc.). This organism was used in almond processing validations (Almond Board of California 2007a, 2007b) and in recent studies on peanuts was found to be slightly more heat tolerant than Salmonella (Sanders and Calhoun, 2014). Roasting is used as the critical control point in most peanut processing operations. The two most common types of roasting are oil roasting and dry roasting. Studies have been performed to investigate Salmonella reduction in the thermal processing of various nuts, and in the later processing operations for several peanut-based products, such as peanut butter (Shachar and Yaron, 2006; Ma et al., 2009 Sanders and Calhoun, 2014). Research on oil roasting of peanuts (Sanders and Calhoun, 2014) and almonds (Du et al, 2010) suggested that 5-log CFU/g or greater reductions were expected at normal oil roasting temperatures (150–160 C) because of the higher thermal conductivity of oil compared to air. Dry roasting is the most widely used roasting process in the peanut industry. Some information is available regarding reduction of Salmonella during dry roasting of almonds (Bari et al., 2009), sesame (Torlak et al, 2013), pecans (Beuchat and Mann, 2011), and peanuts (Sanders and Calhoun, 2014). Sanders and Calhoun (2014) reported an examination of E. faecium and a cocktail of 7 different Salmonella cultures inoculated onto peanuts which were then subjected to a series of both dry and oil roasting temperatures for different times. They utilized 25 g samples and bed depth and air flow were not considerations in the studies. The data reported demonstrated that at every time and temperature, E. faecium was slightly more heat tolerant than Salmonella. These data demonstrated that E. faecium was an excellent surrogate for Salmonella as it was previous shown to be for almonds.

Given the lack of information available on reduction of Salmonella on peanuts in industrial dry-roasting processes which include temperature, time, bed depth, air flow, and air flow direction, the objectives of this study were; (i) to estimate the effectiveness of various industry-typical dry roasting conditions in reduction of Salmonella; (ii) to determine if Salmonella reduction depends on location in the bed of peanuts in the dry roaster; (iii) to determine if Salmonella reduction correlates to any physical roasted peanut properties.

Most continuous, industrial dry roasters (Figure 2) consist of a perforated steel belt that conveys the peanuts through the nut roaster. Peanuts are typically stacked on the conveyor to depths between 50–125 mm (75 mm is a commonly used bed depth). As the conveyor moves the peanuts through the dry roaster, the bed of product is subjected to forced convection airflow at elevated temperatures, typically between 140 and 190 C. Airflow through the bed of product is between 0.7–1.3 m³/s per m² of bed (expressed as m/s in this study). The dry roaster is divided into independent roaster zones and the direction of the airflow through the bed of peanuts typically alternates between upflow and downflow from one zone to the next. The parameters used in this study (bed depth, process air temperature, airflow, airflow direction) were specifically chosen to simulate the industrial dry roaster conditions described above.

Moisture content of the peanuts prior to roasting was 6.1% wwb. After roasting at different time/temperature combinations (constant bed depth of 75 mm and air flow of 1.3 m/s) the highest moisture content observed was 1.7% wwb (Figure 3). At equivalent roast times, moisture content was reduced more at higher roast temperatures. For example, after 12 min roasting, samples prepared at 163 C and 177 C had greater (P<0.05) moisture contents than samples prepared at 191 C, whereas at 16 min roasting, samples prepared at 149 C had greater (P<0.05) moisture content than samples roasted at 163 C. Across these test conditions, linear (R² ≥ 0.66) trends were observed in moisture loss with increasing roast time at a given roast temperature. Many food processors control the final moisture of their product to ensure that the product is below a target water activity (a_w), where a_w  =  P/P⁰, and P  =  partial pressure of water over the food sample at equilibrium and P⁰ =  equilibrium pressure of pure water at the same temperature. Water activity is a critical concept in microbial safety, as the minimum a_w for growth of most bacteria is approximately 0.87. Foods with an a_w below 0.85 are commonly classified as low-water activity, and these include roasted peanuts (Beuchat et al., 2013). Importantly, many low-water activity foods have been implicated over recent years in microbial illness outbreaks, and it is now understood that some bacteria, including Salmonella, can survive for some limited time on low-water activity foods if contaminated (Beuchat et al., 2013). Water activity is also critical in governing various reactions important to final product quality, including lipid oxidation, hydrolytic reactions, non-enzymatic browning and enzyme activity (Fennema, 1996). Water activity for peanuts prior to roasting was approximately 0.73 and after roasting this value ranged from 0.33 to 0.13 (Figure 4). Water activity for roasted peanut samples decreased with increasing roast time at a given roast temperature (Figure 4), although linear trends were not as good as those observed for corresponding moisture content data (Figure 3). This is attributable to water activity data being inherently less accurate/precise for low moisture, high lipid products such as roasted peanuts. Regardless, at equivalent roast times, a_w was lower at higher roast temperatures (Figure 4). For all roasted peanut samples, a_w was less than 0.35, meaning no roasted peanut would be at risk for microbiological growth if properly packaged with a suitable moisture barrier; however, documenting survival after various roast conditions is critical.

During all roasting trials, the air flow direction was up through the peanut bed for the first half of the roasting time and down through the bed for the second half. This protocol simulates industrial roasting procedures in which air flow is alternated as the peanuts progress through the roaster zones. This alternating air flow prevents over roasting of either the bottom, or top, of the bed that would result from unidirectional airflow. Real time temperature data was collected for all roasted samples, and Figure 5 shows a typical temperature plot as measured via thermocouples placed at the top, middle and bottom of the peanut bed. As air flowed up for the first half of each roasting trial, the hot air first contacts the peanuts at the bottom of the bed and the air subsequently loses temperature as it flows upward through the bed, as evident in Figure 5. For this reason, peanuts at the top of the bed are initially subjected to lower temperatures than those at the bottom of the bed. However, half way through the prescribed roasting time, the airflow direction is reversed and the heated air contacts peanuts at the top of the bed first. As evident in Figure 5, the heated air loses energy as it passes down through the product bed and initially cools the peanuts at the bottom of the bed (Figure 5). After roasting, the nuts were cooled using up-flowing air (the industry standard), meaning nuts at the bottom of the bed cooled most rapidly.

The thermocouple measurements in Figure 5 show measurements of the local conditions in the interstices between peanuts and are most representative of the surface temperature of the peanuts. Since Salmonella will likely exist on the surface of peanuts, these measurements most closely represent the conditions to which Salmonella cells are exposed during the roasting process. Based on these temperature measurements, it was evident that thermal inactivation of Salmonella during dry roasting would vary throughout the depth of the product bed. A thermocouple was also embedded inside a peanut for each roast trial to better understand internal peanut temperature during roasting. Figure 6 shows an example of the relationship between the internal temperature of the peanut and the temperature in the interstices around the peanut. The internal temperature lags behind the surface temperature because of the conductive heat transfer through the peanut; however, the internal temperature of the peanut reaches the process air temperature.

Salmonella surrogate log reduction was measured at the top, middle and bottom of the peanut bed (75 mm depth) after 14 and 16 min of roasting at 163 C with a constant air velocity of 1.3 m/s (Figure 7). Log reductions were significantly (P<.05) lower at the middle of the bed, compared to the top or bottom, across these conditions (Figure 7). Furthermore, log reduction was greater (P<.05) after 16 min of roasting compared to 14 min, regardless of measurement location (Figure 7). When samples were roasted at 163 C with a bed depth of 75 mm for 16 min, increasing the air flow from 1.0 to 1.3 m/s significantly (P<.05) increased log reduction in the middle and top of the bed, with the greatest increase occurring at the middle of the bed (Figure 8). Data in Figs. 7 and 8 indicated that the middle of the bed always exhibited the lowest (P<.05) level of Salmonella surrogate reduction. The California Almond Board procedures for validation of almond dry roasters recommends “embedding” the inoculated sample in the bed of almonds on the roaster conveyor (Almond Board of California, 2007b), and these results support that approach. Data in Figs. 7 and 8 also show that any change in processing parameters that allow the hot air to more effectively heat the nuts at the middle of the bed will increase Salmonella reduction and decrease variation in Salmonella reduction when comparing reductions at the top, middle and bottom of the bed. Figure 7 shows that longer roasting time increases Salmonella surrogate reductions and ultimately results in more consistent inactivation across the bed depth. Figure 8 shows that higher air flow rates through the bed also increases Salmonella surrogate reduction and results in more consistent inactivation throughout the bed of peanuts. Higher air flow heats peanuts in the middle of the bed more rapidly since the air loses less energy as it travels through the peanut bed. Based on these results, all subsequent Salmonella surrogate inactivation data was collected at the middle of the bed, as this location represents the worst case in terms of inactivation.

Another processing parameter that impacts heating at the middle of the bed is the bed depth. Peanuts were roasted at 163 C for 16 min using 3 bed depths, 38, 75 and 113 mm and log reductions were measured at the middle of the bed (Figure 9). Log reduction was negatively linearly correlated (R²  =  0.67) with increasing bed depth. A deeper bed reduced the capacity of the process air to heat the peanuts in the middle of the bed. Over this limited data set, bed depth impact on log reduction was significant at the P = 0.16 level.

Figure 10 shows the impact of time and temperature of roasting on Salmonella surrogate log reduction at the middle of the bed (75 mm) with a constant air velocity of 1.3 m/s. Higher temperatures and longer roast times increased inactivation across these conditions, with log reductions generally increasing linearly with roast time at a given roast temperature. The 5-log reduction in CFU/g line is highlighted on this graph since this is a common target in the peanut roasting industry (Food and Drug Administration, 2009). A 5-log CFU/g reduction was achieved during dry roasting at the middle of the product bed under these conditions at 11, 15 and 21 min when roasting at 177, 163 and 149 C, respectively.

Standard industry practice is to roast peanuts to a target color, as color measurements are rapid, inexpensive, nondestructive and correlate well with final product quality, including flavor, texture, and nutritional composition (McDaniel et al, 2012). Specific L-values from the Hunter Lab scale are commonly targeted in the peanut industry as the L-value corresponds to the relative lightness or darkness of the sample, with 100 and 0 equaling perfectly white and dark, respectively. Figure 11 shows the same log reduction data from Figure 10 plotted versus Hunter-L values of the roasted, ground peanuts. L-values of 53 and 48.5 are industry commonly accepted L values for light and medium colored peanuts, respectively (McDaniel et al, 2012). This graph indicates most peanuts roasted to an L-value of 53 and darker had more than a 5-log CFU/g reduction. Further study is needed to understand and bolster this finding, but this data suggests a good correlation between E. faecium reduction (and thus Salmonella reduction) and Hunter-L color, regardless of roast conditions. As color darkens with both increasing time of roast and increasing temperature of roast, roast color is a good indicator of total heat exposure, which could explain the good correlation observed in Figure 11. However, since roast color is well established to be a function of seed maturity and production location, among other factors, further data is required to understand this correlation and processors should not use roast color as an indication of microbial reduction.

Because of the close correlation of E. faecium and Salmonella in relative heat stability, this study indicated that Salmonella reductions of 5-log CFU/g and higher may be obtained with typical industrial dry roasting parameters for peanuts. The peanuts located in the middle of the roasting bed were shown to have the lowest microbial reduction. Processing parameter changes that allowed the heated air used for roasting to more effectively heat the middle of the bed (longer time, increased airflow or shallower bed depth) improved reductions and reduced variation in reduction among the top, middle and bottom of the bed. Data from the study also suggested that Salmonella reductions correlate to the Hunter L-value of ground, roasted peanuts. This information provides processors practical guidance to better understand and utilize data from microbial validations performed on specific process parameters in their facilities and, as needed, in selection and modification of those parameters to achieve desired Salmonella reductions during industrial roasting of peanuts.