Midterm 2 Flashcards
Extrinsic factors
properties of the environment “where you put the food”
- temperature
- presence and concentration of gases
- relative humidity
Intrinsic factors
properties of the food “what you put in the food”
- pH
- aw
- oxidation reduction potential
- content of nutrients or antimicrobial constituents
- physical structures
Psychrophiles
love cold
optimum
Psychrotrophs
tolerate cold
optimum >20, can go at
Mesophiles
love ambient optimum ~30-40 e.g. e coli and s aureus no growth at refrigeration temp most food borne pathogens are mesophiles - survive in intestines
Thermophiles
love warm optimum >45 eg. Geobacillus stearothermophilus relavent for spoilage in tropical climates have heat resistant spores
Hyperthermophiles
love very warm
optimum >80
not relavant in foods
Danger Zone
4.4-60 degrees - unsafe
optimum temperature for microbial growth of mesophiles
if you keep foods hotter than 60 and less than 4, growth of most pathogens is inhibited
Foods not to refrigerate
climacteric fruits (tomatoes, melons, bananas) oil, honey and bread
How to store a ham sandwich?
store the whole thing in the fridge because the safety issue of the ham is more important than the spoilage issue of the bread
Bacterial endospores
Heat resistant
cannot be destroyed by household heating
need to be controlled by pressure cooking (>121 degrees) or combined with low pH etc.
Fungal spores
most are as heat sensitive as vegetative cells
will usually be destroyed by cooking or pasteurization
exceptions** byssochlamys fulva, B nivea, Aspergillus fischeri which can withstand pasteurization (fruit juices)
Gaseous environment ambient conditions
20% o2
80% N2
Modified Atmosphere conditions
take away 02
- vacuum packaging
- N2 inhibits growth of aerobic organisms
add Co2
- inhibits growth of aerobic organisms but NOT LAB
Aerobic Spoilage
ex. pseudomonas sp.. can grow (in meat) metabolizes proteins - smell containing N or S odor and slime after 10^7 is reached
Anaerobic Spoilage
ex. LAB
metabolizes CHO - souring
some strains can convert amino acids - occasional S odor
3 magic numbers in food micro
10^6 - over this number will notice activity (dangerous because some infective doses are less than this)
10^9 bacteria will grow to this in food
10^12 max number of bacterial cells (12 d concept)
12 D concept
requires heating process to reduce the bacterial count by 12 log cycles.
ex. if you start at 10^6, and you heat for the time to reduce by 12 log cycles, you would have a population of 10^-6, which would provide a safe product.
- used in canning to ensure no C. botulinum
Can you store spinach in an anaerobic condition?
No - will wilt. Can add Co2 to decrease microbial growth, without suffocating the leaves
Examples of physical barriers (intrinsic factor)
banana peel, egg shells - prevents organism from getting to the nutritious part of the food
butter - makes an emulsion that results in small water droplets - must be small**
Antimicrobials - plant/animal defence systems - examples (intrinsic factor)
e. g. lysozome in egg white - hydrolyzes cell wall and protects if organism gets past the shell
e. g. lactoperoxidase in raw milk - not sufficient on its own
e. g. isohumulones (hops) in beer - also added for flavor - permeabilizes bacterial membranes
e. g. glucosinolates in garlic, onions, and mustard -
Most pathogens do not grow at a pH below…
4.5
exceptions - salmonella, e coli. and s. aureus, LAB
- if ph 4.5 pasteurize, refrigerate or sterilize to kill spores
min water activity examples
pseudamonas 0.97 e.coli and salmonella 0.96 c botulinum 0.94 most spoilage bacteria 0.9 s.aureus 0.86*
min aw for growth is dependent on the solutes** sugar solutions are less inhibitory than salt solutions at the same aw.
halophiles and halotolerant bacteria
loving salt and tolerating salt
aw >0.75
osmotolerant yeasts
tolerate high concentration of organic compounds
aw >0.61
xerophilic molds
tolerate dry conditions
aw. >0.61
How is aw decreased?
- drying
- dry salting (pulls water out - meat)
- brining (osmotic dehydration) (put in high salt or high sucrose concentration)
Growth Inhibition
bacterial growth is static but organisms remain alive
- cold, low aw, low pH
- enough for a pathogen with a high infective dose (i.e. clostridium perfringens)
- NOT enough for a pathogen with a low infective dose (i.e. ehec)
- bacteria resume growth when conditions change (i.e. temp abuse)
Microbial Death
Elimination of bacteria and spores
- heat, pressure, radiation, chemicals (low pH)
prevents regrowth during storage and ambient conditions
Pasteurization
- 60-80 degrees (less than 100)
- kills vegetative and fungal spores
- milk target organism coxiella burnetti
- alkaline phosphatase has a similar z value to pathogens - if you can detect its activity (like in raw milk), then you know that heating was not sufficient
- low temp holding 63 degrees for 30 minutes
- high temp short time - 72 degrees for 15 seconds (typical for milk
Commercial Sterilization
121 degrees
- typical for canning is 121 for 15 mins
- kills vegetative bacterial cells, fungal spores, and endospores of bacteria that grow at ambient temperature
- spores don’t have a metabolism so can still survive in a dry environment**
acid foods pH 4.5
- need heat inactivation of endospores
target organisms
- food safety - clostridium botulinum (most heat resistant pathogen and produces the most lethal toxin)
- food spoilage - C sporogenes, geobacillus stearothermophillus etc. more resistant than C botulinum so if you know you don’t have spoilage you also know the food is safe**
Process that are not predominantly aimed at food preservation
cooking
- 100 degrees, meal prep; kills bacterial cells and fungal spores
- creates optimum growth conditions for clostridium perfringens and s. aureus
blanching
- 70-80 degrees
- prior to canning, inactivates enzymes and removes o2
drying/concentration
- 40-100
- evaporates water (dry heat does NOT kill bacteria)
Benefits vs. Disadvantages of Thermal Preservation
Benefits
- quick, easy, cheap
- reference process, well understood and good safety record
- inactivates enzymes
- changes in flavour/texture are usually accepted by the consumer
- maintains most nutrients and vitamins
Disadvantages
- energy requirement
- changes flavour (still accepted by consumers)
- destroys some vitamins (
Shoulder effect
uneven heating of the food or clumping of organisms so that they are not evenly exposed to heat
Tailing effect
heat resistant mutants
Thermal preservation won’t work without __?
Water.
- as soon as you take away water, pasteurization and sterilization lose all of their effectivity
- dry vegetative cells have high heat resistance (i.e. salmonella on almonds, potato chips, peanut butter)
Sublethal Injury
When cells are not quite dead yet.
Heating may injure but not kill cells.
- unable to grow on selective media
- given time, they restore viability (extended lag phase)
- cell injury is represented by the growth difference between the non selective media line and the selective media line
- difference in lines reflects the level of selection media - important for industry to keep in mind (storage allows for repair)
Fo value
time to reduce population of target pathogen by given amount at 121 degrees
Alternatives to thermal processing
microwaves, ohmic heating, microfiltration, aseptic packaging, high pressure, pulsed electrical fields, ionizing radiation
Microwave pros and cons
pros - rapid, broad spectrum of activity (bacteria, yeast, viruses, told)
cons - thermodynamic gradients, water activity, survival of endospores
** needs h20
Micro filtration pros and cons
pros - no change in product, extended refrigeration shelf life
cons - less effective removal of bacteria than pasteurization, costly
pulsed electrical fields pros and cons
pros - retains product quality, short treatment time, heating enhances microbial effects
cons - low aw has a protective effect, ineffective against enzymes and spores
Ionizing radiation pros and cons
(x rays, gamma rays, beta particles) - destroys the DNA
pros - control achievable, death/destruction of microbes
cons - dose not effective for endospores, consumer acceptance, side effects (lipid oxidation)
Food Additive
affects the characteristics of the food, substance part of the food, label declaration
Processing Aid
no impact on food characteristics, not part of the food, negligable residues of substance in food, no label declaration
e.g. lactic acid bacteria
Functions of weak organic acids in foods
- flavour (taste and odor)
- pH modification
How are organic acids different from a low pH (protons)
Weak organic acids can cross the cytoplasmic membrane whereas protons cannot
- protonated form i.e. acetic acid vs deprotonated form acetate
Why do bacteria mind transport of protons into the cell?
1) metabolic energy - influx of protons decreases intracellular pH, which wastes energy
2) low intracellular pH - enzymes of most mesophyllic bacteria don’t function at a low pH
organic acids are good preservatives only if…???
the pH is equal to or less than the pkA
- most organic acids have a pKa of 4.2-4.8, which means they function well in acidic foods but not in neutral foods (except parabenes which don’t have a pkA - work well at high pH)
- the catch ** most organic acids have a characteristic smell at concentrations that are effective against bacteria and fungi
Pros vs. Cons of Organic Acids
pros
- wide applicability (bacteria, yeasts, moulds, endospores when combined with pH)
- generally accepted by consumers
- work in tandem with pH
cons
- pKa is a critical factor - may prevent use in food depending on pH
- may negatively affect flavour profile
- “clean” label and fewer additives may be preferred alternatives
Sulfites
SO2, NaSO2, etc
more effective at low pH
used in wines to control yeast or fermentation
Nitrites
- inhibit the outgrowth of C botulinum in meats
- works better in foods with a low pH (activity increases with decreasing pH)
- irreversibly inhibits the iron suffer cluster in enzymes found in clostridia that are essential for growth
- very selective ingredient** inhibits clostridia but other organisms may still grow
- risks - potential for carcinogenic nitrosamine formation, but the alternative is worse (can be controlled by adjusting usage levels and addition of ascorbic acid)
- even natural meats have nitrites - celery extract contains nitrates which when cultured in the right bacteria convert to nitrites
Allowable limits of nitrites in Canada
- 150 ppm
- 120 ppm in bacon - bacon can reach a higher temperature in its fat content, which favours nitrosamine formation
Bacteriophage processing aids
- viruses that kill bacteria
very specific targets ** - phage cocktails - 5-10 different phages to control all of target species
use level 10^8 pfu/g = 3 log kill
phage MUST come into contact with the bacteria**
approved for use to control L monocytogenes
considered “organic”
pros and cons of bacteriophage
pros - considered processing aids so don’t need to be labelled, have no impact on quality (small and have no metabolism)
cons - don’t move - an issue in solid foods because they might not come into contact with the bacterial cell
Bacteriocins
antimicrobial peptides or proteins produced by bacteria
- classified according to structure
- class I: lantibiotics (containing lanthionine, e.g., nisin)
- class II: small heat stable peptides
Nisin
- class I bacteriocin
- 5 ring structure
- produced by Lactococcus lactis spp. subs lactis
- used in milk, cheese, cans, alcohol, fermented sausage (partial replacement of nitrite), fish, eggs
control of viruses
- don’t multiply in food - want to keep them out of food supply*
- much more resistant to environmental stress and inactivation treatments than bacteria
Norovirus
12-72 h incubation, symptoms last 1-3 days
diarrhea, projectile vomiting, nausea, abdominal cramps
self limiting and relatively mild
more severe for elderly and immunocompromised
outbreaks in close contact settings** eg. cruise ships, residences, nursing homes
- infective dose is EXTREMELY LOW 18 particles - extremely contagious
- extremely stable at room temp/freezing
- extremely resistant to disinfection
- can shed for 3-4 weeks
- high rate of mutation - can get it again even if gotten it before
- high risk foods include berries, and produce (contaminated H2o; food handler contamination - typically eaten raw), seafood (oysters, clams, muscles - bivalves filter water and concentrate microorganisms), ready to eat foods
