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Potentially Hazardous Food: The Evolving Definition of
Temperature Control for SafetyByDon Zink, PhDLead Scientist Food
ProcessingOffice of Plant and Dairy FoodsCenter for Food Safety and
Applied NutritionU.S. Food and Drug Administration
andShirley B. Bohm, RS, MPHConsumer Safety OfficerOffice of
Compliance, DCP, Retail Food ProtectionCenter for Food Safety and
Applied NutritionU.S. Food and Drug AdministrationNEHA Annual
Educational Conference, June 26, 2005
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Food Preservation and Potentially Hazardous FoodFood safety is
constantly evolving:
1825 U.S. patent granted for preserving food in tin cans1890
Commercial pasteurization of milk1908 Sodium benzoate approved as a
food preservative1917 Frozen foods available at retail1962 PHF is
defined as perishable food that supports the rapid and progressive
growth of infectious or toxigenic microorganisms1976 Added
crustacea and synthetic ingredients to definition, Exempted foods
with pH 4.6; aw 0.85; clean, uncracked shell eggs; food in
unopened, hermetically sealed containers1993 Added toxin production
of C. botulinum; growth of SE in eggs; non-acidified garlic-in-oil;
shell eggs; cut melons; raw seed sprouts2005 Added TCS Food as an
equivalent/transition term for PHF, pH and aw Interaction Tables,
and consideration for pathogenic microorganisms instead of only
Clostridium botulinum and Salmonella Enteritidis (1-201.10 in FDA
Food Code)
TCS Food stands for Time/temperature control for safety
food.Both the terms will be retained in the Food Code for several
editions to assist in the transition from PHF to TCS food.
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Potentially Hazardous FoodWhy is the definition of potentially
hazardous food changing in the Food Code?
The rapid and progressive growth of infectious and toxigenic
microorganisms" in the old definition was not clearly definedThe
slow growth of low infectious dose pathogens such as Listeria
monocytogenes was not consideredWater activity of 0.85 as a level
of safety against Staphylococcus aureus was conservatively lowpH of
4.6 may also be inappropriate as Listeria monocytogenes grows at
4.39, Salmonella spp. at 4.2 and Yersinia enterocolitica at 4.2 BUT
pH 4.6 is appropriate when only spore forming pathogens are
present
FDA submitted an issue to the 2004 Conference for Food
Protection recommending this change in the definition of
Potentially Hazardous Food. The issue was based on an IFT report,
Evaluation and Definition of Potentially Hazardous Foods, found at
http://www.cfsan.fda.gov/~comm/ift4-toc.html, which recommended
certain changes. FDA contracted with IFT to do the evaluation.
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Potentially Hazardous FoodWhy is the definition of potentially
hazardous food changing in the Food Code?
Sufficient research has been done on food products to construct
tables that predict the growth of pathogenic microorganisms when
the interaction of both pH and aw are considered (See Tables A
& B)Many factors control or prevent the growth and toxin
production of foodborne pathogens besides pH and aw aloneThere is
concern about the use of the word hazard which is used in a
different manner in HACCP
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Potentially Hazardous FoodWhy are some foods considered PHF?
PHF/TCS food means a food that requires T/T control for safety
to limit pathogenic microorganism growth or toxin formation
No or inadequate hurdles to pathogenic bacterial growth in the
foodIntrinsic factors of the food support the growth of bacterial
pathogens
Nutrients Energy source (sugars, alcohols, amino acids)Nitrogen
source (amino acids)Vitamins and growth factorsMinerals Available
water (aw ), acidity (pH), redox potential (Eh).
etc.Epidemiological evidence associates food with foodborne
outbreaks
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Sliced/Diced TomatoesNot heat-treated to destroy spore
formersNot treated with any other anti-microbial processpH is <
4.6aw is > 0.99Considered PHF unless a product assessment proves
otherwise
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Cut MelonNot heat-treated to destroy spore formersNot treated
with any other antimicrobial processpH of melons;Honeydew pH = 6.3
6.7Watermelon pH = 5.2 5.6Cantaloupe pH = 6.2 7.1aw is >
0.99Considered a PHF unless a product assessment proves
otherwise
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Dye Infiltration in Cantaloupe
Photo courtesy of Dr. Michelle Smith, FDA/CFSAN
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Dye Infiltration in CantaloupePhoto courtesy of Dr. Michelle
Smith, FDA/CFSAN
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Dye buildup on canker(rind blemish)Photos courtesy of Dr.
Michelle Smith, FDA/CFSAN
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Raw Seed SproutsNot heat-treated to destroy spore formersNot
treated with any other antimicrobial processpH is > 6.5aw is
> 0.99Considered a PHF, unless a product assessment proves
otherwise
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Potentially Hazardous FoodWhy are some foods considered
non-PHF?
If the foods pH 4.6, it is below the pH at which proteolytic
Clostridium botulinum can grow and produce toxin:
Because of inherent acidity fruitsBecause of acid from bacterial
activity fermented sausages, fermented milks, sauerkrauts,
picklesBecause of acidification added vinegar
If the foods aw 0.85, it is below the water activity at which
Staphylococcus aureus grows and produces toxin:
Not enough water is available for metabolic activities of
pathogenic bacteriaLow aw increases the length of the bacterial lag
phase and decreases the growth rate
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Potentially Hazardous FoodWhich foods are considered
non-PHF?
Unopened containers of hermetically sealed containers are
commercially sterileFoods with laboratory evidence showing that T/T
control is not required AND the food contains:
A preservative to inhibit pathogens see 21 CFR 172 Subpart B,
Food PreservativesOther barriers/hurdles to pathogenic growthA
combination of barriers/hurdles to inhibit pathogenic growth
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Examples of Non-PHF/Non-TCS FoodAir-cooled, hard boiled egg
shell intactShell eggs treated to destroy all SE pasteurized shell
eggsA food that does not support the growth of pathogenic
microorganisms even though they may be present (i.e., E.coli
O157:H7 in apple cider or norovirus on crackers)Some foods that are
refrigerated for quality, not safety
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Interaction TablesThe hurdle effect is used several inhibitory
factors used together to control or eliminate pathogens that would
otherwise be ineffective when used alone The effect of a heat
treatment which destroys vegetative cells is consideredThe effect
of packaging which prevents re-contamination is consideredWhen
tables indicate Product Assessment Required (PA), the food must be
treated as PHF/TCS Food until laboratory evidence shows
otherwiseTables A & B consider the interaction of pH and aw
under certain conditions of heat-treatment and packaging
Refer to Factors Affecting the Growth of Some Foodborne
Pathogens in FDAs Foodborne Pathogenic Microorganisms and Natural
Toxins Handbook (Bad Bug Book) at
http://www.cfsan.fda.gov/~mow/factors.html
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Water Activity in FoodsWater activity (aw ) is the water in
foods that is available for metabolic purposesaw = p / po (pure
water is 1.00)Effect of aw on microorganismsMost spoilage organisms
do not grow below 0.91Spoilage molds can grow as low as
0.80Staphylococcus aureus can grow as low as 0.86Clostridium
botulinum can grow & produce toxin as low as 0.94Some parasites
(Trichinella spiralis) survive at low aw Water activity ranges for
growth are affected by temperature and nutrient levelsWater
activity in a food can be changed by adding salt or sugar or by
drying
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Acidity in FoodspH is a measure of acidity in food using a scale
of 0 to 14, with 7.0 being neutralMicroorganisms grow best in
neutral or slightly acidic conditionsYeasts and molds can grow at
pH 3.5Clostridium botulinum can grow and produce toxin as low as pH
4.7Staphylococcus aureus can grow at pH 4.2Listeria monocytogenes
and Yersinia enterocolitica can grow down to pH 4.4The minimum pH
for growth of microorganisms is dependent on many factors inherent
acidity, type of acid, salt concentrationThe further out (above or
below) the optimum pH for growth, the longer the lag phase will
be
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Other Factors AffectMicrobial GrowthOther factors affect the
growth of pathogenic microorganisms besides pH and aw Redox
potential (ease of transferring electrons in food during energy
metabolism)Atmosphere within packaging (i.e., ROP)Antimicrobials
and bacteriosins (i.e., nisin)If other factors besides pH and aw
are used to show that the food is non-PHF, a pathogen modeling
program* or laboratory evidence must be provided
*USDAs Pathogen Modeling Program can be downloaded at
http://www.ars.usda.gov/Services/docs.htm?docid=6784
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Interaction Table A
Table A. Interaction of pH and aw for control of spores in food
heat-treated to destroy vegetative cells and subsequently
packaged.aw ValuespH Values4.6 or less> 4.6 5.6> 5.60.92 or
lessNon-PHF*/non-TCS**Non-PHF/non-TCSNon-PHF/non-TCS> 0.92
0.95Non-PHF/non-TCSNon-PHF/non-TCSPA***>
0.95Non-PHF/non-TCSPAPA* PHF means Potentially Hazardous Food** TCS
means Time/Temperature Control for Safety Food*** PA means Product
Assessment Required
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When to Use Interaction Table ATable A can be used to determine
if a food which is heat-treated and packaged is PHF, Non-PHF or
Requires Product AssessmentFood must meet cooking requirements of
Food Code section 3-401.11 (no partial cooks) to eliminate
vegetative pathogensSpore forming pathogens are the only remaining
biological hazards of concernFood is packaged to prevent
re-contaminationTherefore, higher pH & aw can be safely
tolerated
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Interaction Table B
Table B. Interaction of pH and aw for control of vegetative
cells and spores in food not heat-treated or heat-treated but not
packaged.aw ValuespH Values< 4.24.2 4.6> 4.6 5.0> 5.0<
0.88Non-PHF*/non-TCS**Non-PHF/non-TCSNon-PHD/non-TCSNon-PHF/non-TCS0.88
0.90Non-PHF/non-TCSNon-PHF/non-TCSNon-PHF/non-TCSPA***> 0.90
0.92Non-PHF/non-TCSNon-PHF/non-TCSPAPA>
0.92Non-PHF/non-TCSPAPAPA* PHF means Potentially Hazardous Food**
TCS means Time/Temperature Control for Safety Food*** PA means
Product Assessment Required
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When to Use Interaction Table BTable B can be used to determine
if a food which is not heat-treated or heat-treated but not
packaged is PHF, Non-PHF or Requires Product AssessmentFood not
heat-treated may contain vegetative cells and pathogenic sporesFood
that was heat-treated but not packaged may become re-contaminatedpH
values considered in Table B must include 4.2 because
Staphylococcus aureus can grow at that level
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Use of aw/pH Interaction Tables Decision Tree
Is the food heat-treated?NoYesIs the food treated using some
other method?Is it packaged to
preventRecontamination?YesNoYesNoFurther product assessment(PA) or
vendor documentation requiredUsing the foods known pH &/oraw
values, position the food inthe correct table
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Use of aw/pH Interaction Tables Decision Tree
Using the foods known pH and/or aw, position the food in the
appropriate tableUse Table A(heat-treated andpackaged)Use Table
B(not heat-treated or heat-treated but not
packaged)Non-PHF/non-TCSFood may be heldout of temperature control
and is consideredshelf stableProduct AssessmentFurther product
assessment or vendor documentation requiredNon-PHF/non-TCSFood may
be heldout of temperaturecontrol and isconsidered safe from
bacterial pathogens Product AssessmentFurther product Assessment or
vendorDocumentationrequired
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Application of Interaction TablesPreliminary questions:Is the
food held refrigerated for quality, not safety NOT enforceable What
is the scientific basis?Consider the foods safety history if not
associated with foodborne outbreaks, scientific rationale should be
able to explainAny pH and aw values must be accurate and replicable
from a competent laboratory
pH value chemistry grade pH paper accurate to 0.05 or calibrated
equipmentaw value homogenous sample with calibrated
equipmentHeat-treatment must destroy vegetative cellsPackaging must
be sufficient to prevent recontaminationProduct assessment may
result in a finding of non-PHF, limited shelf life or Time as a
Public Health Control, required temperature control or
reformulation of product
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Application of Interaction Tables - Parmesan CheeseParmesan
Cheese: aw = 0.68 0.76 pH = 6.5 curd heated to ~ 130F & cured
2-3 years, then packagedAmbient storage desired & no history of
related illnessThe food is heat-treated/cured & packagedUsing
this information, Table A is chosenLocate the cheeses aw (0.68
0.76) in the correct line and pH (6.5) in the correct columnThey
intersect at Non-PHF/Non-TCSNo temperature control is required
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Application of Interaction TablesAmerican Process Cheese
Slices
American Process Cheese Slicesaw = 0.94 0.95pH = 5.5 5.8Heat
processing and packaged to retailAmbient storage desired for 24
hrs.Cheese is heat-treated and unpackagedTable B is chosen because
it may become recontaminatedLocate the cheeses aw (0.94 0.95) in
the correct line and pH (5.5 5.8) in the correct columnThey
intersect at PA Product Assessment RequiredChallenge testing with 4
pathogens at 86F showed no growth for 24 hrs. and no growth for 210
days when refrigerated
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Application of Interaction TablesSushi Roll with Raw FishSushi
roll with raw fish:Cooked rice:
aw = 0.98 0.99pH = 6.0 6.7 (acidified rice pH = 4.2)Raw
fish:
aw = > 0.99pH = 5.2 6.1 (tuna), 6.1 -6.3 (salmon), 6.8 7.0
(shrimp)Ambient temperature display desired for buffet lineOnly
rice, not fish is heat-treated & not packagedLocate the foods
aw and pH in the correct line and columnThey intersect at PA
product assessment required
The food is PHF unless reformulated in some wayIf room
temperature display (for 4 hrs.) is desired, TPHC can be used if a
marking system is used and any left after 4 hrs. is discardedIf the
sushi roll with raw fish was packaged for retail sale, Table B is
still used because of the raw fish
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Evaluation of Laboratory EvidenceWhen is laboratory evidence
likely to be used?Variance applicationPerformance
standardPreservatives addedNew technologies usedpH and aw
Interaction Tables say PA Product Assessment
RequiredMulti-ingredient or combination foods with two or more
distinct food components - the interface may have different
properties than either of the individual ingredients
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Evaluation of Laboratory EvidenceMicrobiological challenge
testingDesign, implementation and assessment must be done by an
EXPERT MICROBIOLOGISTFailure to account for a specific product or
environmental factors in the design could result in a flawed
conclusionA competent laboratory should be used to conduct the
challenge testing
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Evaluation of Laboratory EvidenceWhat factors should be
considered in designing a challenge study?*
Selection of challenge organism(s)Level of challenge
inoculumsInoculums preparation and methodologyDuration of the
studyFormulation factors and storage conditionsSample analysis
*For more information, refer to Ch. 6 in Evaluation and
Definition of Potentially Hazardous Food at
http://www.cfsan.fda.gov/~comm/ift4-toc.html
TCS Food stands for Time/temperature control for safety
food.Both the terms will be retained in the Food Code for several
editions to assist in the transition from PHF to TCS food.
FDA submitted an issue to the 2004 Conference for Food
Protection recommending this change in the definition of
Potentially Hazardous Food. The issue was based on an IFT report,
Evaluation and Definition of Potentially Hazardous Foods, found at
http://www.cfsan.fda.gov/~comm/ift4-toc.html, which recommended
certain changes. FDA contracted with IFT to do the evaluation.