Food Protection Trends - August 2009 - 496

of dried egg white, which include heating the product in a closed container to at least 130°F (54.4°C) for 7 days or longer until Salmonella is no longer detected (as a practical matter, the egg industry routinely uses a more severe heat treatment in order to eliminate the avian influenza virus as well as Salmonella). both thermal and non-thermal control measures can be used for Salmonella inactivation to achieve the target log reduction. Various processing steps (e.g., cooking, frying, roasting, baking, heat extruding, fumigation) may be used to inactivate Salmonella in a low-moisture product. thermal processing is the most commonly used control measure to inactivate Salmonella. For example, the almond board of California’s technical expert review panel (abC terp) determined that oil roasting at or above 260°F (126.7°C) for 2 min will result in a 5-log reduction of Salmonella on the surface of whole almonds (1). the abC terp also provided minimum time and temperature combinations required for blanching processes to deliver a 4 or 5-log reduction of Salmonella on almonds (1). these parameters were determined on the basis of heat resistance data for Salmonella enteritidis pt 30 as the target organism. it is useful to review available scientific data for the processing method of interest, including high temperature short time or low temperature long time, when desirable for maintaining product quality. in order to assure appropriate validation, it is also necessary to evaluate scientific and processing equipment data and information specific to the processing technology under consideration. a process authority should be consulted where necessary. For example, the abC terp, which consists of experienced microbiologists and processing experts, evaluates the adequacy of various treatments to inactivate Salmonella in raw almonds and develops guidelines for validating individual processes, including propylene oxide (ppo) treatment for raw almond kernels, ppo treatment for in-shell almonds, blanching, oil roasting, dry roasting and other processes that may be proprietary (1). heat resistance of Salmonella is affected by factors during heating, as well as the Salmonella strains used (28). heat resistance observed in an aqueous system may not be applicable to a lowmoisture product. For example, a study by ng and colleagues (46) found that S. Senftenberg 775W was the most heat resistant among 300 strains evaluated in an aqueous solution, while this strain was found to be less heat resistant than S. typhimurium in chocolate (26). S. enteritidis pt 30, the target organism for raw almonds, was implicated in a foodborne illness outbreak and was found to be more resistant to dry heat than many of the strains evaluated on almonds (1, 58). a number of studies have been published on heat resistance of Salmonella in various low-moisture products. available d- and z-values for heat resistance of various Salmonella strains in low-moisture matrices are shown in table 1 for food matrices and in table 2 for model systems. these data indicate that heat resistance is much greater in a product with low aw than in a high-moisture product. For example, while reaching an internal product temperature of 160°F (71.1°C) without a hold time would eliminate Salmonella in raw poultry (23), the same temperature would result in little inactivation of Salmonella in milk chocolate, in which the d-value for S. typhimurium has been reported as 816 min at 71°C (26). table 1 shows d-values for Salmonella in wheat flour (7), milk chocolate (8, 26), almonds (28), corn flour (56), and dry animal feeds (36). in addition, recent research (18) found that, based on the non-linear Weibull model, 42 ± 8 min at 90°C achieved a 5-log reduction of a mixture of three outbreak-associated S. tennessee strains in peanut butter (49 ± 12 min were needed to inactivate a composite of other Salmonella isolates). liu et al. (36), who determined the heat resistance of S. Senftenberg 775W in meat and bone meal and chicken starter at moisture levels from 5% to 30%, found that the method used to prepare the inoculum (growing the cells in a laboratory medium vs. in meat and bone meal suspension) affected the heat resistance. akinleye (3) reported that d- and z-values were affected by water activity of a salt solution model system. d- and z-values relevant to low-moisture heat conditions from this study are shown in table 2, along with data from another study using sucrose as a model system (53). it should be noted that comparison of inactivation kinetics data from different studies can be difficult, and it is crucial to review the raw data and experimental procedures, as well as the d- and z-values reported, so as to apply the data appropriately. heat-inactivation of Salmonella in low water activity matrices was found be non-linear in many cases, such as in peanut butter (37), oil-roasted almonds (2), flour (7), and laboratory media (39). the Salmonella inactivation curve in low water activity foods can be complex, often showing a concave upwards curvature, and significant tailing has been observed (28, 38, 39). thus, the rate of inactivation may not be constant throughout the heating process, and caution must be used when interpreting and using heat resistance data to support the adequacy of the process parameters. in a study by archer et al. (7) of the heat resistance of Salmonella Weltevreden in wheat flour, the investigators observed that death kinetics were non-linear, with approximately a 1-log reduction in the first 5–10 minutes of heating, followed by a slower, linear decrease in survivors. to be conservative, the investigators calculated the d-value based on the second, slower phase of the inactivation curve. Sumner et al. (53) reported that the d-value of Salmonella typhimurium atCC 13311 increased by more than 100-fold as the aw was reduced from 0.98 to 0.83 in sucrose solutions; this trend was observed in the treatment temperature range of 65 to 77°C (149–170.6°F); the study did not investigate temperatures below 65°C for Salmonella inactivation. in laboratory media with aw adjusted with glucose and fructose, mattick et al. (39) reported that Salmonella typhimurium dt104 inactivation was non-linear in the range of 55 to 80°C (131–176°F). At temperatures ≥ 70°C (158°F), heat resistance increased as the aw decreased from 0.90 to 0.65; however, this trend was not observed for heat treatment at 65°C (149°F) or below, at which range decreasing aw from 0.90 to 0.65 either had little effect or slightly decreased the heat resistance of Salmonella. Some studies have also been published on the inactivation of Salmonella by non-thermal processing. For example, the efficacy of low-energy X-ray irradiation was examined for inactivating S. enteritidis pt 30 on almonds at different water activities (34). the organism was found to be more resistant at aw 0.65 (d10-value ~ 0.34 kgy) compared to aw 0.23 (d10value ~0.26 kgy). irradiation, for products where its use has been approved, can also be an effective control measure. irradiation with a dose up to 30 kgy (21 CFr 179.26) has been approved for use in inactivating microorganisms in dry aromatic vegetable substances such as herbs, spices and vegetable seasonings 496 FOOD PROTECTION TRENDS | AUGUST 2009

Food Protection Trends - August 2009

Table of Contents for the Digital Edition of Food Protection Trends - August 2009