Research Digest April 2008

Freezing to reduce campylobacteriosis

Campylobacter spp. is one of the major causes of foodborne illness in the developed world and chicken has been shown to be a major carrier of these bacteria. Exposure to Campylobacter spp. has been linked to chicken occurring through undercooking, internal meat contamination and cross-contamination with ready-to-eat foods due to mishandling. This study has taken a similar approach as the previous study in applying a risk management model to determine which factors related to the handling of raw chickens have the greatest impact on humans getting sick from ingesting Campylobacter spp. Factors were categorized into the steps leading to chicken consumption including processing storage and preparation. The dose-response was another factor and included specific factors such as the probability of infection from one bacterium and probability of illness given an infection. Survey data on chicken consumption rate, home cross-contamination, research on inactivation during freezing or refrigeration, and Campylobacter spp. populations during processing steps were used in the model. The model calculated that there was a one-to-one relationship between the number of illnesses and a reduction in positive carcasses at the processing plant. This is in contrast to the effect of reducing the number of bacteria on a carcass. Reducing the average carcass population by 100 times only reduced the risk by 12 and 30 times for fresh and frozen carcasses, respectively. Thus preventing carcasses from becoming campylobacter-positive would have a greater impact on reducing illness than reducing overall campylobacter population on carcasses. Furthermore, the risk of illness was 5.4 times greater for refrigerated vs. frozen chicken. Among feasible strategies that would have the greatest impact on the number of campylobacteriosis illnesses, diverting all campy-positive flocks to freezing would have the greatest impact by reducing the number of illness by 57 percent or to 43 percent of the current level. Another proposed strategy was to divert all chickens from high-risk producers to freezing, since timing of determining positive and negative flocks might be problematic. According to the model, this approach reduced campy-related illness by 46 percent or 54 percent of the current level. Lack of good baseline data for true frequency of mishandling and the dose-response relationship for Campylobacter spp. were highlighted by the authors, however, the input data for flock prevalence and processing factors are based on longer-term sampling and research making these results more substantive. This study supports previous research (Rosenquist, et al. 2006. The effect of slaughter operation on the contamination of chicken carcasses with thermotolerant campylobacter. International Journal of Food Microbiology, 108: 226-232) reported here in 2006 that found reductions in campylobacter due to freezing were so substantive that Denmark, Norway and Iceland instituted freezing as a campylobacter intervention step to be used for positive flocks.

R. Lindqvist and M. Lindblad; 2008. Quantitative risk assessment of thermophilic Campylobacter spp. and cross-contamination during handling of raw broiler chickens evaluating strategies at the producer level to reduce human campylobacteriosis in Sweden. International Journal of Food Microbiology, 121: 41-52.

Salmonella can migrate during marination

Marination is used to improve meat yield and sensory properties of value-added products such as prepared meals. Shelf life of these products has been shown to be limited by bacteria found on surface of meat and skin. Limited research has reported on the safety of marination when pathogens might be present on the meat prior to marination. The interior of meat pieces is generally considered to be bacteria-free; however, since one objective of marination is to infuse the meat with spices and flavorings, the question as to whether bacteria can be carried into the meat interior during marination has been raised.

Previous research has determined that needle, blade or mechanical tenderization can introduce bacteria into the interior of whole muscle. These researchers theorized that since vacuum marination can facilitate the penetration of liquid marinades into the meat interior that this process might also result in greater penetration of bacteria.

The current study determined that salmonella on the surface of turkey breast could be carried into the meat interior during marination. This was determined by inoculating the surface of turkey breast meat with salmonellae after which the meat was exposed to a marinade solution and tumbled with and without vacuum. The depth of bacteria penetration was measured by enumerating bacteria from meat core sections taken after marination. Salmonellae migrated more than 2 cm into the intact meat during marination, while vacuum tumbling caused even deeper and faster penetration. The marinated meat was also sampled for bacteria while being held for 60 days at either 37 C or 4 C. The meat held at 4 C maintained high levels of salmonella throughout the 60 days while meat held at 37 C decreased in population to below detection within 9 days.

Thus, the traditional belief that interior tissue of intact meat is sterile is not supported by these and other data, especially for marinated products. Based on these results, marinated meat products may require a post-marinade HACCP heating step to ensure product safety. The authors suggest additional research is needed to determine the route(s) taken by bacteria to penetrate the muscle tissue during marination and whether muscle tears and certain processing conditions promote bacterial penetration into the meat.

C.R. Warsow, A. Orta-Ramirez, B.P. Marks, E.T. Ryser and A.M. Booren; 2008. Single directional migration of salmonella into marinated whole muscle turkey breast. Journal of Food Protection, 71(1): 153-156.

Listeria transfers to turkey during slicing

Slicing can transfer bacteria from knifes to deli meats resulting in possible foodborne illness. Listeria strains differ in their ability to attach to food preparation surfaces including cutting blades. The attachment of bacteria to surfaces is followed by the development of biofilms, which can affect the survival and transfer of bacteria on and from these surfaces.

In the current study, two types of listeria were tested for transfer from cutting knife blades to cooked turkey during slicing. The two listeria types tested were groups of strong and weak biofilm forming strains. The knife blades were held six or 24 hours after being inoculated to simulate possible food service or home preparation conditions.

The study found that listeria was transferred from knife blades to roast turkey through 16 slicings and that a greater number of bacteria were transferred for strong versus weak bio-filming forming strains. Also, a greater transfer of cells was observed after six hours (4.6 logs) as opposed to 24 hours (3.3 logs) of drying on the knife blade before slicing.

In a previous but similar study, (Vorst, et al., 2006. Transfer of Listeria monocytogenes during slicing of turkey breast, bologna, and salami using simulated kitchen knives. Journal of Food Protection, 69: 2939-2946) listeria was transferred out to 30 slices when the knives were used 1 hour after inoculation at a level of 8 logs but transfer was found only out to 16 slices when used six or 24 hours after inoculation at a 6 log level. Furthermore, the 2006 Vorst study found that listeria transfer was greater when cutting blades made from AISI grade 304 stainless was used compared to using blades made from grade 316 stainless steel. The difference in listeria transfer due to stainless steel types was believed to be related to better cleanability of the 316 grade. This finding relates to the current study that found strong biofilm forming Listeria survived drying as a 51.4 percent rate compared to a 38.7 percent rate for weak biofilm formers.

These studies show that the slicing step in meat processing and preparation cannot be overlooked in a HACCP plan to prevent food contamination and that cleaning, blade material and bacteria type affect microbial transfer from the knife to the meat.

L. Keskinen, E. Todd and E. Ryser; 2008. Transfer of surface-dried Listeria monocytogenes from stainless steel knife blades to roast turkey breast. Journal of Food Protection, 71(1): 176-181.