improper cooking is one of the main factors causing food borne illness [ 15 ] , and a large part , 4060% , of the cases of food borne illness are expected to originate from private households [ 69 ] .
most consumers do not use a meat thermometer [ 10 , 11 ] but determine the doneness of meat most often by cutting the meat to evaluate changes in color and texture , or by other subjective techniques . especially for chicken breast fillet
chicken breast fillet is , apart from minced meat , the most popular type of meat in the netherlands   and a pathogen associated with it is campylobacter jejuni , a microorganism responsible for 50% of confirmed cases of bacterial gastroenteritis in western europe ( austria , belgium , finland , france , germany , ireland , italy , the netherlands , norway , portugal , sweden , switzerland , and the uk ) and the usa [ 1 , 1418 ] .
furthermore , a predominant risk factor for c. jejuni infection is consumption of undercooked chicken meat [ 1922 ] . in a consumer study on food handling practices in the netherlands by our research group , focusing on the preparation of a chicken breast fillet salad artificially contaminated with nonmotile , nonpathogenic lactobacillus casei ,
it was shown that for consumers who apparently made no mistakes regarding cross - contamination , heating time of chicken meat ( boiled in chicken stock ) was negatively correlated with the bacterial contamination level of the prepared salad .
although this correlation may be expected , it is surprising that the bacteria present were not immediately killed upon contact with boiling water , as the bacteria were only present on the surface of the meat and not , as with ground meat , on the inside as well .
in addition , it was shown by bergsma et al .   that consumer - style cooking ( pan frying ) of chicken breast inoculated with c. jejuni resulted in high bacterial heat resistance levels of this pathogen .
ample research has been dedicated to heat inactivation of bacteria in meat , but most of these studies use thin - layered ground meat as a model system and normal cooking temperatures , that is , temperatures > 70c are hardly ever applied [ 2527 ] ( also see figure 3 ) . as these studies do not suffice to explain the observed heat resistance by bergsma et al . , the aim of this research was to further elaborate on this phenomenon .
therefore , in the current study , the chicken fillets were boiled instead of pan fried to obtain better reproducible temperatures in the experiments and decimal reduction times of bacteria present on each sample were determined .
heating profiles of chicken breast fillets during cooking were obtained both by measurement and by calculation . apart from c. jejuni
, salmonella and escherichia coli were also studied as these bacteria are as well associated with food borne illness caused by consumption of raw or undercooked meat .
in addition , l. casei was used , as this bacterium was used as indicator organism for c. jejuni to study real - life consumer practices by our research group .
furthermore , the effect of food matrix , matrix size , and refrigerated storage on the survival of bacteria during cooking was studied as well as the effect of refrigerated storage on cell attachment .
in addition , the obtained decimal reduction times were used to assess the relationship between the probability of illness ( campylobacteriosis ) due to consumption of chicken breast fillet and cooking time .
five c. jejuni strains ( nctc 11168 , nctc 11828 , b258 , lb99hu , and 82/69 ) , a lactobacillus casei strain ( isolated from a food ) , and escherichia coli wg5 ( atcc 700078 ) were used as described by de jong et al . .
c. jejuni , l. casei , and e. coli were cultured as described by de jong et al . .
s. typhimurium dt104 was grown in brain heart infusion broth ( 37c ; bhi , oxoid , basingstoke , uk ) .
the heat resistance of bacteria present on the surface of chicken breast fillets during cooking of the meat in boiling water was expressed as a decimal reduction time during boiling ( dboil ) : time needed to reduce the number of bacteria present on the outside of a large matrix exposed to boiling water by 10-fold ( min ) .
this decimal reduction time is comparable to the formal , frequently applied d - value .
however , to determine a dt - value of a bacterium on a matrix , the temperature of the matrix must be constant during the heating experiment .
therefore , d - values of bacteria on food items are determined using small sized food samples , mainly with a 1  mm thickness . as these small - sized samples are not representative for sizes of meats prepared by consumers during cooking , the heating experiments in this study
boil  as index as the temperature could not be given explicitly , since the product temperature was dynamically changing ( see figure 2 ) .
furthermore , the effect of food matrix and refrigerated storage on survival of c. jejuni during cooking was studied .
in addition , cell attachments studies were conducted . to study the matrix - effect on heat resistance of c. jejuni , cold - stored
( 4c , 24  h ) inoculated chicken breast fillets and carrots were used as matrix and survival of c. jejuni on these matrices after a 5 minute cooking time was compared .
the effect of refrigerated storage of chicken meat on heat resistance of c. jejuni present on the meat surface was studied by comparing survival of c. jejuni present after 5  min cooking using chicken breast fillets refrigerated ( 4c ) for 0 , 1 , or 24  h. the effect of refrigerated storage on cell attachment of c. jejuni to the matrix was studied for both chicken breast fillet and carrot .
the inoculated matrices were stored in the refrigerator ( chicken meat : 0 , 1 , and 24  h at 4c ; carrots : 0 and 1  h at 4c ) and subsequently manually rinsed for 10  s under cold running water and the number of c. jejuni cells attached was determined .
this was done by comparing the number of cells present on the food item after washing to the number of cells applied to the food item .
a multiple - species cocktail of c. jejuni / l . casei was obtained by mixing a multiple - strain cocktail ( 5 strains mixed in equal volumes ) of c. jejuni with a single strain of l. casei in order to compare the behavior of these different bacterial species under equal test conditions , similar to what was done by the authors in a study on cross - contamination during consumer cooking practices .
in addition , a multiple - species cocktail containing one strain of e. coli and one strain s. typhimurium was used .
the cocktails were prepared by combining equal volumes of each single - species culture . an inoculum level of 10  cfu  ml was used to inoculate the food items with .
cell counts of bacterial suspensions were determined ( n0 ) by spread plating appropriate dilutions on appropriate media .
whole chicken breast fillets ( 98218  g ; maximal thickness : 3.54  cm ) purchased in different batches at a local supermarket were used fresh or were stored frozen and defrosted before use . thawed or fresh fillets were dabbed with paper tissue to remove superficial moisture , after which each fillet was contaminated with 1  ml of inoculation culture ( each side 0.5  ml ) , which was evenly spread using a plastic , sterile rod .
each contaminated fillet was stored overnight in a separate plastic bag at 4c to mimic retail storage . the ( large ) carrot was longitudinally sliced to obtain slices of similar weight ( 77198  g ) and similar thickness as the chicken breast fillets .
both sides of these slices were allowed to dry in a laminar flow cabinet ( 30  min each side ) to remove moist , after which the carrot slices were similarly inoculated and stored as the chicken fillets . for the heating experiments ,
one single inoculated refrigerated food item was placed in a pan (  24  cm ) with boiling water ( 4 - 5  l ) at a time ( each data point is one single heating experiment ) , thus limiting the decrease of the water temperature due to the addition of the food item . the water was constantly heated and boiling . at time zero the food item was placed in the boiling water and then heated for times ranging from 0 ( not heated ) to 15  min . after heat treatment , the food item was immediately transferred into a sterile waring commercial blender , weighed , and cooled with 200  ml peptone ( 1  g  l ) physiological salt ( 9  g  l ) solution ( pps ) of 4c and blended for 1  min resulting in blended chicken slurry or blended carrot slurry .
the pan was washed up after cooking for every single food item and clean tap water was brought to the boil again .
experiments were conducted at least in duplicate , on different days . for the heat - inactivation test , a cooking pan (  24  cm ) with water ( 4 - 5  l ) was brought to the boil . the water was constantly heated and as the weight of the added matrices was small ( < 200  g ) compared to that of the water ( > 4000  g ) , the temperature profile of the water was considered to be constant at 100c . however , the temperature of refrigerated chicken meat has a come - up time and the temperature profile of the meat can be estimated using ( 1 ) : 
 ( 1)uslab = uchicken  fillet = tx , tt0tt0=1erf(x2at ) , 
 in which u is the fraction of nontransferred heat ( 0  u  1 ) , tx , t is the temperature of the product at place coordinate x at time t , t0 is the temperature of the product at t = 0 , t is the temperature of the surroundings , erf is the standard error function from excel , x [ m ] is the place coordinate , and a [ m  s ] is the product 's thermal diffusion coefficient , which is given by : 
 ( 2)a=cp , 
 where  [ w  m  k ] is the product 's thermal conductivity ,  [ kg  m ] is the product 's specific density , and cp [ ws  kg  k ] is the specific heat capacity of the product , and in which only conductive heat transfer effects via the determining dimension of the chicken fillet , the height , are taken into account ( the chicken fillet is thus considered to be an infinite slab , since the width and length of the fillet are more than 2.5 and 5 times as long as the height , resp . ) . in reality , the temperature of the chicken will increase more rapidly , so this is a fail safe assumption .
equation ( 1 ) is applicable when the external heat transfer resistance is negligible ( boundary condition 1 ) and when the temperature of the centre of the product has not changed yet ( boundary condition 2 ) . to determine whether boundary condition 1 is met
, the ratio between the transport resistance of the product and its surroundings , the biot number ( bi ) , can be calculated by : 
 ( 3)bi = al=transport  resistance  producttransport  resistance  surroundings , 
 where  [ w  m  k ] is the convective - heat - transfer coefficient of the surroundings , l [ m ] is half height of the product , and  [ w  m  k ] the product 's thermal conductivity . when the value of bi exceeds 10 , the transport resistance of the surroundings is considered to be negligible . with : 
 ( 4)fo = atl2 , 
 where fo ( fourier number )   is the dimensionless time , one can calculate until which heating time , the short times boundary , boundary condition 2 is met .
for an infinite slab , the short times boundary is set by fo = 0.20 .
the temperature profile of the chicken fillet ( at different distances from the surface of the meat ) in boiling water was estimated using ( 1 ) and the parameter values given in table 1 . a pt100 temperature probe attached to an applikon adi-1030 biocontroller ( applikon , schiedam , the netherlands )
the temperature probe was calibrated between 0c ( melting ice ) and 100c ( boiling water ) prior to use . according to the manufacturer ,
therefore , to measure the temperature at the surface of the meat , a 20  cm long stainless steel probe was bent in the shape of an ice - hockey stick , allowing the actual sensor ( last two cm ) to be pressed firmly to the surface of the meat .
the temperature profiles of the water and meat surface were generated using noninoculated chicken fillets ( weight : 142172  g ) .
the meat was placed on a plastic test tube rack so that only the last 7  cm of the temperature probe was under water .
after adding the meat in the pan with boiling water , the temperature probe was immediately pressed on the meat surface or held in the water and temperature recording was started .
culturability of the inoculated bacteria on the food items after heat - treatment was determined by use of the most probable number method ( mpn , see de man ) in combination with spread - plating suitable dilutions on agar plates .
chicken breast fillets were sampled and analyzed as described by de jong et al . ; the carrots were similarly treated . the lower level of detection used was 1.4 cfu per food item .
media used for the mpn method and plate counts , respectively , were as follows , as described by de jong et al . : preston broth and karmali agar for c. jejuni , mrs broth and agar for l. casei , and modified - tryptic soy broth and tryptic soy agar both supplemented with 1  g / l nalidixic acid for e. coli . for s.
typhimurium buffered peptone water ( nvi ) and brilliant green agar ( oxoid ) were used .
mpn samples were checked for growth of the respective organisms after incubation ( time / temperature details see ) by streak plating on abovementioned agar plates .
suspected colonies of c. jejuni and l. casei were confirmed by phase contrast microscopy . 
 c. jejuni media were microaerobically incubated ( broth : 48  h , agar : 72  h , at 37c ( to allow the recovery of any sublethally injured c. jejuni ) either in a three - gas incubator ( 5% co2 , 10% o2 , 85% n2 ) or in jars with bbl campypak ( becton dickinson , sparks , usa ) . for reasons of comparison , media for l. casei were aerobically incubated at 30c ( broth 48  h ; agar : 72  h ) , those for e. coli and s. typhimurium overnight at 37c .
the contamination levels of the food items after heat treatment were calculated , taking into account the exact weights used for enumeration , using an excel spreadsheet based on the mpn method described by de man .
data were represented as count data ( log cfu and mpn per fillet ) plotted versus heating time ( min ) , to which the linear and weibull inactivation models were fitted . levels below the detection limit were taken into account as censored data by using maximum likelihood estimation assuming poisson - distributed data .
the best fitting model was determined by applying an f - test using the rss of the weibull and linear model .
analyses were performed using mathematica 5.2 ( wolfram research inc , champaign , usa ) . according to the f - test , the linear model fitted best and therefore decimal reduction times during boiling ( dboil )
were calculated using the slope of the graphs , using : 
 ( 5)lognt = logn0tdboil 
 with log as the 10-based logarithm , nt as number of viable microorganisms at a given time , n0 as the initial number of microorganisms , t as time in min , and dboil as time needed to reduce the number of bacteria present on the meat surface by 10-fold when exposed to boiling water .
the effect of different heat treatment scenarios was tested for its significance with anova on the log transformed data in spss ( spss , chicago , usa ) .
for comparison of currently obtained data to literature data , d - values were collected from literature for different temperatures , strains , and products ( mainly meat ) or media following the approach of van asselt and zwietering .
microorganisms studied were campylobacter jejuni ( n = 176 ) , lactobacilli ( n = 6 ) , escherichia coli ( n = 79 ) , and salmonella ( n = 287 ) .
the relation between heating temperature and logd is linear : 
 ( 6)log  dref = intercept  ( log  d , t)trefz . 
logdref is the logarithm of the d - value ( log min ) at temperature tref , tref is the reference temperature ( c ) , and z is the temperature increase ( c ) needed to reduce the d - value with a factor of 10 .
the logd versus temperature plot was used to compare our decimal reduction times with data ( d - values ) published in literature .
decimal reduction times can be used to assess the human health risk of pathogen exposure consequential to undercooking . as an example , we studied campylobacter on chicken breast fillets using an empirical distribution of concentrations on fillets after cutting at the cutting plant , obtained from a dutch risk assessment model [ 21 , 33 ] .
it showed that most contaminated fillets carried between 0 between 4 log  cfu / fillet and less than 5% carried higher numbers , results which are similar to campylobacter contamination data reported for german retail products .
after incorporation of the survival after storage [ 21 , 33 ] , yielding a decrease in concentration described by a betapert distribution with minimum 0.1 , most likely value 0.9 and maximum 2.1 log cfu / fillet , these results can be used as a distribution for input ( n0 ) of ( 5 ) .
the exposure distribution after an inactivation time t is then given by the resulting distribution of nt .
the associated health risk , expressed as the probability of illness per fillet , can be assessed by implementing this distribution of doses into the 
dose response relationship for campylobacter : 
 ( 7)pill(nt)=0.33(1(+)(+nt)()(++nt ) ) 
 with dose response parameters  and .
the effect of cooking on bacterial survival on whole chicken breast fillets was studied for a cocktail of c. jejuni . during cooking
, cell numbers declined , following a straight line ( figure 1 ) . using ( 5 ) , we calculated that the decimal reduction time for c. jejuni when present on meat during cooking of the meat in boiling water was 1.90  min . in figure 2 ,
the estimated temperature profiles at the surface of a chicken fillet in boiling water and at 0.25  mm , 0.5  mm , and 0.75  mm depth are shown . to estimate the temperature profile of the chicken fillet in boiling water ,
the first condition states that the external heat transfer resistance is negligible , thus that bi > 10 ( 3 ) . to verify whether this boundary condition is met ,
for still water , bi ranges between 11.67 and 19.33 ; thus for boiling water bi will be higher and boundary condition 1 is met . to verify until which heating time boundary condition 2 was met , the short times boundary was calculated , using ( 4 ) .
this implies that for heating times > 10.8  min the temperature according to the heating profile estimated by ( 1 ) is overestimated .
after 30  sec , the average measured surface temperature of the chicken fillets approximated the estimated profile at 0.5  mm depth in the meat , and measured data ranged between the estimated profiles at 0.25  mm and 0.75  mm depth . within 1 
min , the surface temperature of chicken fillets ( 4c at time zero ) reached a value of at least 85c ( figure 2 ) .
the water temperature was only slightly affected by the addition of the refrigerated meat to the water and did not drop below 99.3c .
ample research has been dedicated to heat resistance levels of food borne pathogens , and in figure 3 an overview is given from a selection of heat inactivation data not only of campylobacter but also of lactobacilli , e. coli , and salmonella .
data are both from fluid and solid ( i.e. , meat ) matrices ( see also van asselt and zwietering .
this graph shows the extremity of our obtained bacterial heat resistance levels ( encircled data ) .
the heat resistance of bacteria depends on many factors . to investigate some specificities of the observed heat resistance , different heating experiments were conducted with c. jejuni . in addition ,
the heat resistance of c. jejuni was also determined when inoculated onto a slice of carrot of similar size as a chicken fillet to investigate whether the observed heat resistance was related to the matrix type .
no bacteria were recovered from cold stored inoculated carrots ( 24  h , 4c ) after 5  min of boiling ( data not shown ) .
the effect of refrigerated storage time on the heat resistance of c. jejuni on chicken meat was investigated .
we observed that after a 24  h storage period , a 5  min cooking time reduced cell numbers by 3.0 ( stdev 0.5 ) log units , for 1  h stored fillets this was 4.3 ( stdev 0.8 ) log units , and for noncold stored fillets a 4.7 ( stdev 0.5 ) log reduction was observed .
the heat resistances after 0  h and 1  h storage time were not significantly different ( p = 0.392 ) , but those observed after a 24  h storage time were significantly higher than the other two storage times ( 24  h versus 1  h storage time : p = 0.037 ; 24  h versus 0  h storage time : p = 0.002 ) .
refrigerated storage for various periods of time did have no effect on the level of attachment of cells to chicken meat , as rinsing for 10  s under cold running water resulted in a 1.41.9 log cfu removal of campylobacter from chicken meat , independent of the storage time ( 0 , 1 , and 24  h ) .
the heat resistance of l. casei , e. coli , and s. typhimurium was also tested under the same conditions as used for campylobacter . for all bacterial species tested cells
could still be recovered from meat after a 10 minute heating time in boiling water .
we calculated ( 5 ) decimal reduction times of 1.93 , 1.97 , and 2.20  min , respectively for l. casei , e. coli , and s. typhimurium .
we finally determined the effect of the observed heat resistance levels on food safety health risks .
for this purpose we applied a dutch risk assessment study on campylobacter in broiler meat , as explained in the methods section .
equation ( 1 ) was used to calculate the expected ingested doses nt after boiling the meat for t minutes , given the value dboil = 1.90  min , as found in our experiments .
after implementation of the dose response relationship , this resulted in figure 4 , showing the decreasing probability of illness per fillet bought at retail in the netherlands , as a function of cooking time .
consumption of chicken breast fillets cooked for 10 minutes resulted in an estimated probability of illness of 5.5  10 .
this research studied the effect of consumer cooking practices on survival of various bacterial species applied to chicken fillet meat .
frying is the most frequently used method for the preparation of chicken meat by dutch consumers .
the temperature at the surface of chicken meat during frying is on average 127c , but as the temperature is difficult to control ( stdev : 18c ) , we decided to study survival in boiling water .
all tested species died off following a straight line from which a d - value of approximately 2 minutes was calculated .
the advantage of the study design was that it immediately revealed the fate of bacteria present on meat during nonisothermal heating to high temperatures , mimicking a consumer 's style of cooking .
thus , no extrapolation of data obtained from isothermal d- and z - value experiments with small - sized matrices at temperatures < 70c was needed .
the disadvantage was the difficulty of determining the actual surface temperature of the meat and thus the exact temperature the bacteria were exposed to .
we measured the surface temperature and we used a mathematical approach to determine the surface temperature during cooking . with a temperature probe , we measured that the surface temperature of chicken fillets reached 85c within 1  min .
the mathematical approach can be applied when the external heat transfer resistance is negligible and when the temperature of the centre of the product has not changed .
we calculated that during the first 10 minutes of cooking , both conditions are met .
the calculated temperature at 0.5  mm depth in the meat approximated the average measured surface temperature of the chicken fillets .
the shape of the calculated heating profile was similar to that of the measured profile , and the results of our calculations are comparable to those of houben and eckenhausen , who calculated the temperature at certain surface depths during pasteurization of a model product in a water bath of 96c .
together this shows that both our measured and calculated temperature reliably reflect the temperature bacteria experience at the surface of a chicken breast fillet during cooking in boiling water .
heat resistance studies are generally conducted in solid ( thin patties of mainly ground meat ) or in liquid matrices ( broth , milk , etc . ) , with higher heat resistance levels obtained in solid matrices [ 25 , 4046 ] .
temperatures during such studies range from 55 to 72c , allowing accurate determination of d- and z - values . at higher temperatures , d - values
the surface temperature of chicken fillets reached 85c within 1 minute . according to the literature ( after extrapolation ) the d - value of bacterial cells at 85c is less than one second [ 32 , 41 , 47 ] , yet in our experiments it took 2 minutes to obtain a 1 log reduction of bacteria .
when inoculated on another solid matrix of approximately the same size ( slice of carrot ) bacteria showed different behaviour .
the study of van asselt and zwietering   revealed that heat resistance can be significantly increased for certain specific matrix ( high fat and low aw)/bacterium combinations .
but as both carrot and chicken meat are low fat / high aw products , our results indicate that chicken meat itself ( e.g. , the presence of some specific component like iron , or an amino acid ) affects the heat resistance of bacteria .   to determine the d - value of a bacterial species at a certain temperature ,
a small volume of a bacterial culture is transferred to a relatively large volume of a preheated menstruum . in this way ,
the temperature come - up time is as short as possible and bacteria experience a constant temperature . as the temperature of a large and solid matrix does not come up as fast as the temperature of a small or liquid matrix , the dimension of a product can affect the heating experiment . a weak size effect on heat resistance
is indeed demonstrated in a similar study conducted in our laboratory by bergsma et al .
, who pan fried chicken meat inoculated with c. jejuni using whole and diced fillets . in literature ,
purnell et al .   observed that elevated heat resistance levels of naturally c. jejuni contaminated whole chicken carcasses during water immersion treatments at temperatures 70c ( 2030  s ) .
the other campylobacter data points in figure 3 boxed by the dotted line were calculated using data from whyte et al . .
these authors conducted hot water immersion treatments ( 7580c ; 0 , 10 , and 20  s ) with naturally and artificially contaminated chicken thighs .
again high survival levels of campylobacter cells were obtained after heat treatments . in the study by purnell et al .   not only was the size of the test products larger than that used in most other studies , but also the challenge temperatures in this studies were higher than normally used in heat resistance testing .
interestingly , in small sized meat discs artificially inoculated with e. coli and salmonella ( see figure 3 in dotted box ) and pasteurized with hot steam ( 87c ) , also high survival levels of bacteria were measured after a 60  s heat treatment . although the calculated d - values or decimal reduction times for data from purnell , whyte , and mccann are based on very short heating times , 60  s , and may therefore be less accurate , these d - values or decimal reduction times and our current data show that bacteria on meat are not as easily killed by temperatures > 70c as predicted based on d - values obtained at temperatures < 70c .
both our data and those discussed previously indicate that the combination of products size and challenge temperature might affect the level of heat resistance of bacteria .
another difference of our study design compared to heating experiments described in literature is the overnight refrigerated storage of the contaminated fillets to mimic consumer / retail storage conditions .
although overnight cold storage did not affect the number of bacteria attached to the meat , we observed that storing the meat in a refrigerator increased the number of surviving cells . at a low temperature (
no growth , stress ) physiological properties may have changed which may have affected the heat resistance of the bacteria , a phenomenon known as cross - protection [ 49 , 50 ] .
although the pathogens used were all motile and thus can move from the surface to more inner parts of fillets , the observed high level of heat resistance can not be explained by such movement , as the heat survival of the non - motile l. casei was equal to that of the motile pathogens tested .
so chicken meat , challenge temperature , or heating rate and cold storage have their effect on the heat resistance of c. jejuni , s. typhimurium , e. coli , and l. casei .
because bacteria on the surface of poultry would be destroyed by limited cooking , the recurrent association of illness with undercooked poultry suggests either that the poultry is regularly re - contaminated after cooking or that campylobacter is somehow present deep in the tissues of a single poultry carcass , where it survives limited cooking  , which is also in concordance with fao / who conclusions .
however , data presented in our paper reveal that limited cooking does not necessarily eliminate all bacteria present on the surface of poultry meat .
furthermore , our data add to the deeper understanding of the frequent association of food borne illness with consumption of undercooked poultry . as a consequence ,
our data show that research conclusions based on the assumption that cooked chicken meat does not substantially contribute to the risk of food borne illness [ 5355 ] must be interpreted with more consideration .
consumption of chicken meat cooked for 10  min cooking still results in a probability of illness of 5.5  10 . the estimated probability of illness per meal containing chicken breast fillet and a salad cross - contaminated with c. jejuni from the chicken fillet to the salad , as assessed in the risk assessment of nauta et al .
this not only confirms that the risk of acquiring campylobacteriosis consequential to undercooking is much smaller than that consequential to cross - contamination   but also shows that the cooking time may be far more critical than previously assumed : with a cooking time of about 7.5  min the risk of undercooking is comparable to that calculated for cross - contamination ( see figure 4 ) .
taking consumer behavior into account , using data of the observational study of fischer et al .
, it becomes clear that undercooking of chicken meat by consumers is certainly not negligible , as 33% of the participants ( dutch consumers ) in their study applied heating times during cooking of chicken breast fillets of less than 7.5  min .
when inoculated on chicken breast fillets , the heat resistance of bacteria increased to unexpected high levels .
chicken meat , the challenge temperature , or heating rate and cold storage affected the level of resistance .
it can be concluded from our study that , until now , the effect of cooking on the survival of bacteria present on the outside of chicken meat has been overestimated .
it is therefore recommended to reconsider all statements made based on meat heating trials that do not use consumer - style meat types , sizes , and cooking techniques .
 the aim of this research was to determine the decimal reduction times of bacteria present on chicken fillet in boiling water . 
 the experiments were conducted with campylobacter jejuni , salmonella , and escherichia coli . 
 whole chicken breast fillets were inoculated with the pathogens , stored overnight ( 4c ) , and subsequently cooked . 
 
 the surface temperature reached 70c within 30  sec and 85c within one minute . 
 extremely high decimal reduction times of 1.90 , 1.97 , and 2.20  min were obtained for c. jejuni , e. coli , and s. typhimurium , respectively . chicken meat and refrigerated storage before cooking enlarged the heat resistance of the food borne pathogens . additionally , a high challenge temperature or fast heating rate contributed to the level of heat resistance . 
 the data were used to assess the probability of illness ( campylobacteriosis ) due to consumption of chicken fillet as a function of cooking time . 
 the data revealed that cooking time may be far more critical than previously assumed . 
1. Introduction
2. Materials and Methods
3. Results
4. Discussion