Physical Methods of Food Preservation Flashcards
What are the purposes of modern food preservation methods?
- increase shelf life
2. improve safety (inactivate/prevent pathogens)
What is “inactivation” of microbes in food?
damaging/reducing population to point where it will not recover
True/False; Physical methods of food preservation are rarely used today
false: still most common food preservation method
List the physical methods of food preservation:
- High temp
- Low temp
- Decrease Aw
- Ionizing irradiation
- High pressure
- Pulsed electric field
Describe the effect on bacteria as heat is increased:
enzyme activity and growth will increase -> reach optimum temp, peaks -> even higher temp will cause rapid decline/stop (cell death)
High heat will cause ____ in most bacteria. Why?
cell death damages membranes (collapse -> lysis), ribosomes, enzymes and proteins (denatured)
Describe the effects of high heat and very low temp on bacterial membranes:
low temp: membrane “gels” -> cannot transfer efficiently -> no growth
high temp: membrane lysis -> cell death
What is an issue with heat treatment in food?
overheating will damage quality of food
What factors affect heat transfer in food?
- product type
- container material (conductance)
- container shape (surface area)
- container size
- agitation
- heating medium temperature
What is the best container shape for fast heating of food?
small, tall, narrow
Liquids will heat (faster/slower) than solids
faster
What can be done to improve heating rate of viscous fluids?
agitation/mixing -> help better distribute heat
The (greater/less) the difference in temperature between product and heating medium, the greater the heat transfer
greater
The major parameters of heating processes for food:
time
temp
The microbial population will decline following a _____ pattern during heat treatment
logarithmic
What is a survivor plot?
depicts the inactivation of the bacteria population (logarithmic curve) over time
What is D value?
Decimal reduction time: time needed at given temperature to make a 1 log reduction (kill 90% of current population)
An organism with a high D value is (resistant/not resistant) to heat
resistant (take longer treatment)
Why might heat treatment times differ for different products?
- different bacteria -> different D values
2. different initial population (higher population need longer time)
As temperature increases, D value will (increase/decrease).
decrease
D value plotted vs. temperature gives a ____ plot. This follows a ___ pattern.
thermal resistance; linear
What is the z value?
thermal resistance constant
change in temp needed to decrease D value by 90%
negative reciprocal of slope on thermal resistant plot
The higher the Z value, the more _______ the organism
thermally resistant
________ measurements are taken to determine amount of time needed to sterilize food at a given temp
Lethality is represented by: ______
thermal lethality F value (minutes)
What is the difference between D value and F value?
D value: time for 1 log reduction
F value: time to reduce population to certain level (10^1 or 0)
F value is very important to consider in the ____ industry
canning
What standards are the canning heat treatments based on (especiallly low acid food)?
reducing C. botulinum population by 12 log
What is the difference between commercially sterile and sterile?
sterile: done by autoclaving; totally kills all microbes (extreme high heat, long time)
commercially sterile: free from microbes that can reproduce under common storage conditions; no viable cells/spores that could harm health
(less heat treatment to avoid totally destroying food)
What are the most heat resistant microbes in food?
Geobacillus stearothermophilus (spoilage) C. botulinum (A and B) - (toxin) C. sporogenes - (spoilage) B. coagulans - (spoilage) B. licheniformis - (spoilage)
What mechanisms allow for greater heat resistance in bacteria?
- Disulfide bonding (protein cross-linking, prevent total denaturation)
- DNA structural changes (more G-C: 3 bonds so stronger linking)
- form spores/stationary cells (more resistance when dormant, especially spores)
How can we lower the heat resistance ability of microbes?
add in another stressing factor: low pH, salt, reduce water, antimicrobials, etc
What is aseptic processing? What are its benefits, and what is it used for?
sterilize container & product separately, then package
avoids excess heating (better quality)
used for liquids (juice/dairy/sauce); NOT for low acid/chunky products
Describe the general aseptic process:
liquid pass through heat exchangers until reach target temp/time -> cooled (heat exchangers) -> packed in pre-sterilized packages & hermetically sealed
How do microwaves heat food, and why isn’t this used for heat treatment?
causes rotation of water molecules -> friction, heat generation (generates heat within food)
causes uneven heating - uneven lethality
Microwave heating is (faster/slower) than conventional heating. Why?
faster
generates heat within/throughout product, rather than penetrating from outside
How does freezing preserve food (microbiologically)?
Stops metabolic activity of microbes (except psychrophiles, psychrotrophs, cryophiles)
No water activity (frozen, not available)
freeze/thaw will damage cells of microbes
What are the impacts of freezing on food quality?
preserves:
stops chemical/biochemical deterioration (enzymes halted) - prevent lipid ox, Maillard rxn, enzymatic browning, lipolysis, proteolysis
but will rupture some cells (ice crystals) -> altered structured
True/False: freezing will destroy toxins
False; toxins not affected
microbes growing at subzero temperatures must be both cold-tolerant and ___tolerant. These microbes are mostly ____ and _____.
xerotolerant (lack of water)
yeast, fungi
Are Gram positive or Gram negative bacteria more susceptible to freezing? Why?
Gram negative (no cell wall for protection)
What process is done to eliminate parasites from pork?
Freeze briefly at -23C (parasites inactivated by freezing) - inactivates Trichenella spiralis (pork worm)
How does slow freezing compare to fast freezing in terms of quality and microbial death?
fast freezing (industrial): smaller ice crystals, shorter period of osmotic stress -> better quality, but less damage to microbial cells too
slow freezing (home): large ice crystals -> damaged food quality, but also more effective against microbes
Water in food is either ____ or _____ by ___, ____ groups, or ____ groups.
free
bound: salts, hydroxyl, carbonyl/amino (on proteins)
___ or ____ the ___ water in food will restrict microbial growth.
removing; restricting; free
How is the available water expressed?
Aw (water activity)
ratio of water vapor pressure (P) with water vapor pressure of pure water (Po)
How is water vapor pressure determined?
seal food in container -> water will escape from food into headspace until reach equilibrium; pressure at this point is P (water vap pressure)
Aw will have values between ___ and ____. The lower the value, the lower the _____ and the longer the ____
0 and 1
lower the water activity/microbial growth
longer shelf life
what is ERH?
equilibrium relative humidity %
Aw x 100%
How is moisture content and Aw different?
moisture content: total water in food (bound and free)
Aw: only free water (better indicator of biochem & chem activity)
Division of food based on Aw:
High moisture food: Aw>0.9, prone to spoilage (milk, meat, veg, etc)
Intermediate moisture food: Aw 0.65-0.9, relatively resistant (jam, raisins)
Low moisture food: Aw<0.65, long shelf life (dry milk, crackers)
Maple syrup has Aw of ______, making it a _____ moisture food.
0.88
intermediate
What is the maple syrup making process?
boil maple sap to 104C, Brix should be 66
Why is water necessary for microbial growth?
nutrient acquisition
waste removal
medium for metabolic rxn
How do water requirement differ among microbes?
Pathogens/spoilage bacteria: high water req (need 0.95 to grow)
Spore-forming bacteria: can grow at 0.91-0.95
Yeast: tolerate lower moisture than bacteria
Mold: most xerotolerant
What bacteria requires the least moisture to grow?
Staphylococcus aureus; can grow at as low as 0.86 (but cannot make toxins)
What are the usual spoilage microbes in intermediate moisture foods?
Yeast; bacteria inhibited by lower moisture but yeast are more tolerant
Low moisture may not be able to inhibit mold growth, but it will prevent: ____
yeast growth, bacteria growth, toxin production (prevent mycotoxins)
Agents used to lower Aw are called ____. Give examples:
How do their effects differ?
humectants
glycerol, salt, sugar
glycerol can permeate bacterial membrane
Are microbes killed by low Aw?
No; only dormant. Can be reactivated
How is water removal from food achieved?
drying: apply heat, mobilize water to surface and evaporate
osmotic dehydration: immerse in hypertonic solution; water drawn out by osmosis (less effective, usually used as pre-treatment)
freeze drying: frozen, then sublimated; high cost but preserves structure/flavor
what types of electromagnetic waves are effective in killing microbes? What is the mechanism?
short wavelengths: gamma and x-rays (ionizing radiation)
creates ROS that will attack DNA
What affects a microbe’s susceptibility to radiation?
size of genome; smaller genome will be more resistant
How are X-rays produced, and what foods is it used for What are the limitations?
produced when electron beam strikes metal (tungsten, stainless steel)
Used for fresh fruit/veg (<5MeV)
High power requirement
Lot of heat generation; need cooling system as well
How is gamma radiation produced? What are advantages and disadvantages?
emitted from radioisotope (excited) - Ce137 or Co60
high penetration power, constant energy during treatment, continuous source
safety concerns (need large facility, store source under water), cannot turn off
What are the dosage levels for irradiation in food?
Redappertization: high dose (10-75kGy) -> sterile
Radicidation: med dose (1-10kGy) -> similar to pasteurization (spores unaffected)
Radurization; low (0.05-1kGy) -> inhibit sprouting, kill insects
The main advantage of HPP and PEF processing is:
maintains food in “fresh” state, and microbiologically safe
What are the parameters and considerations for HPP?
High pressure processing:
treat with 100-600MPa -> inactivate most bacteria/yeast/mold/virus (spores survive)
need to consider package type, food composition (air pockets will cause deformity due to difference in compressibility)
PEF is _____. It is designed to treat _____ food.
pulsed electric field
pumpable (liquid)
The process for PEF involves: _________ to _______.
The success will depend on: _______
apply short (micro/milliseconds) burst of high voltage, to eliminate vegetative bacterial cells
food composition, conductivity, pH
Air pockets are problematic in PEF and HPP. Why?
non-uniformity:
PEF: air is poor conductor
HPP: air has different compressibility -> leads to deformation
