16) Physical Methods of Food Preservation Flashcards
why were food preservation methods originally developed?
to extend shelf life of food pdts by protecting them from microbio spoilage
define inactivation of bacteria
destruction of the organism as judged by its inability o recover on microbio media
what form of food preservation is most common?
physical
what are some physical methods of food preservation?
- high temp treatment
- low temp preservation
- decreasing water availability
- ionizing irradiation
- high pressure processing (HPP)
- pulsed electric field
describe preservation by high temp
includes:
increasing temp above optimal temp of an organism, which slows and stops cell multiplication, resulting in cell death
- damages to cell membranes, proteins and ribosomes
why is heat treatment a good choice for preservation?
what is a drawback to heat treatment?
good:
- can eliminate almost any microbial target
- customizable to the pdt and organism of concern
bad:
- may damage pdt quality
what are factors that affect heat transfer?
- pdt type
- container material
- container shape
- container size
- agitation
- temp of heating medium
what should be considered about pdt type when using heat treatments for preservation?
liquid absorbs heat faster than solid foods
what should be considered about container material when using heat treatments for preservation?
glass containers heat slower than metal contains
what should be considered about container shape when using heat treatments for preservation?
tall and narrow containers heat quicker than other shapes
what should be considered about container size when using heat treatments for preservation?
small containers heat quicker
what should be considered about agitation when using heat treatments for preservation?
increases heat transfer, especially in viscous pdts
what should be considered about temp of heating medium when using heat treatments for preservation?
greater difference in temp between heat transfer medium and pdt = faster rate of heat transfer to pdt
what are major parameters in thermal treatments?
temp and time
kinetics of microbial inactivation at a temperature is determined by what?
the decline in population survivors during heating time
what are survivor plots?
shows the log nature of population inactivation over time
what is the D value?
- decimal reduction time
- the time it takes of a 10 fold reduction in number of survivors at a given temp
greater D value = ___ (higher/lower) resistance of the organism to heat
higher resistance
D value ___ (decr/incr) as the processing temp increases
decreases
what is the thermal resistance plot?
what shape does it have?
what is it used for?
graph of D value vs temp
linear
used to compare difference resistances of a MO at diff temps by calculating the z value
what is the z value?
what does it represent?
how can it be calculated?
thermal resistance constant
represents the change in temp needed to change the D value of a MO by 10 fold
can be found by calculating the negative reciprocal of the slope of the thermal resistance plot
larger Z value = ___ (higher/lower) heat resistant organism
higher resistance
how can you determine the time required to commercially sterilized food?
by taking thermal lethality measurements
what is the F value?
time (mins) at a specific temp to achieve a targeted reduction in a homogeneous population with a specific z value
compare F and D values
D: time to reduce population by 1 log
F: time to get the population to a specific level (ie. sterile or 10^1)
what is the key pathogen of concern in canned low acid foods?
C. botulinum
the canning industry often calculates which value to achieve commercial sterility?
F value
how does sterility affect C.bot population?
should reduce C. bot by 12 log
define sterilization
process of rendering a pdt free of any living organism
what is the problem with sterilization of food pdts?
heating at very high temp can result in unacceptable quality
a food product that undergoes commercial sterilization will be free of… (2)
- MOs capable of reproducing in the food under normal non-refrigerated storage and distribution temp
- viable microbial cells of public health significance
what are mechanisms which cause bacteria to be more naturally resistant than others? (2)
- protein cross-linking (disulfide bonding)
2. DNA structural changes (higher GC)
which is more resistant to heat?
a) stationary phase cells
b) exponential phase cells
stationary
how does spore formation affect heat resistance
more spores = increased heat resistance
define aseptic processing
what is a benefit of this?
industrial processed where:
- foods and cans are sterilized separately
- food is packaged under aseptic conditions
benefit: avoids excess heating of the food which improves food quality and nutrition
describe how aseptic processing is done
- pdt is heated by passing through a set of heat exchangers until the holding time and temp is reached
- food is passed through cooling heat exchangers
- cooled pdt is placed in pre-sterilized packages and is hermetically sealed
what is aseptic processing used for and not used for?
used for: fruit juices, dairy pdts, sauces
not used for: low acid or particulate containing foods
describe how microwaving works
what are advantages of this?
microwaves cause rotation of H2O molecules in food which generates friction and heat, which generates heat throughout the material
this leads to faster heating and shorter processing times
why aren’t microwaves used in processing? why are they not good for ensuring safety of a food?
they have uneven heat distribution, which leads to uneven lethality of MOs
how does freezing affect MOs?
what does it not affect?
it stops the metabolic activity of most food-borne MOs
what does freezing not affect?
does not affect:
- cryophilic, psychrophilic and psychrotropic MOs
- toxins
does affect:
- chemical and biochemical deterioration (eg. lipid oxidation, maillard rxns, enzymatic browning, proteolysis, lipolysis)
- water activity
which survives freezing better?
a) gram-pos
b) gram-neg
gram-pos
how does freezing affect parasites?
they easily destroy them
compare rapid freezing go slow freezing
rapid freezing:
- produces smaller ice crystals
- exposes cells to osmotic stress for shorter time period
slow freezing
- more susceptible to injury and death
what are 2 forms in which water can occur in?
bound
unbound
what can water be bound by?
- hydroxyl groups
- carbonyl and amino acid groups of proteins
- salts
what two aspects of water influences microbial activity of a food?
amount and form
what form of water does MOs need for growth and survival?
unbound water
what form of water can be removed or restricted to decrease microbial activity and extend pdt shelf life?
unbound water
describe the water activity of a moist food within a package with headspace
what term describes this?
water leaves food into the surrounding atmosphere until equilibrium is reached between air and food
water vapor pressure (P)
greater P = ____ (greater/lower) moisture content in package air
greater
more water binding = ___ (higher/lower) P
lower
what is the equation for Aw?
Aw = P / Po
Po = vapor pressure of pure water P = vapor pressure at the same temp
what is ERH? what is the equation?
equilibrium relative humidity: air surrounding the food
ERH (%) = Aw x 100
describe the relation between Aw and moisture content of food
not linear. They are different
Aw: describes chemical and biological activities
moisture content: describes water present in the food
what is the Aw for pure water?
what is the Aw for all other foods?
pure water: Aw = 1
other foods: Aw = 0-1
what are 3 categories of food based on Aw?
for each, describe:
- Aw
- example
- ability of MOs to grow
- high moisture foods
- Aw > 0.9
- eg. milk, meat, fresh fruits
- foodborne microbes can readily grow - intermediate moisture foods
- Aw = 0.65-0.9
- eg. raisins or jam
- relatively resistant to microbial spoilage - dry foods:
- Aw < 0.65
- eg. milk powder or crackers
- don’t support growth of foodborne MOs; has long shelf life
what do living cells rely on water for?
- nutrient acquisition
- waste removal
- medium for metabolic rxns
what is the range of Aw for:
a) spoilage and pathogenic bacteria
b) spore forming bacteria
c) S. aureus
d) yeasts and molds
a) Aw > 0.95 (very high)
b) Aw = 0.91-0.95
c) Aw >0.86
d) lowest Aw reqs than bacteria
describe the growth of molds in terms of Aw
- xerotolerant (grows in “dry” conditions)
- grows at lower moisture contents than other spoilage MOs
- can produce mycotoxins under xerophilic conditions
what can be used to lower the Aw of a food?
humectants
Does lowering Aw kill MOs in food?
what happens when Aw is increased again?
what is an example of this?
- lowering Aw may inhibit microbial growth, but may not kill MOs in food
- MOs may be dormant until food is rehydrated, then resume metabolic activity and multiply
- eg. salmonella in chocolate bars
describe drying and how it decreases water availability
- the process of mobilizing h2o present in a food matrix to its surface, then removing it from the surface by evaporation
- involves heat and mass transfer
describe freeze-drying and how it decreases water availability
- pdt is frozen first, then moisture is removed via sublimination
- minimal impact on structure and flavor
- high cost (used as a value added technique)
describe osmotic dehydration and how it decreases water availability
- involves removing h2o from fresh food w/ a hypertonic solution (sucrose, salts, glycerol)
- food cells permit water out of food w/o surrounding solutes entering cells
- final result: food is mostly dehydrated
- can be used as a pre-treatment to other methods of dehydration
what are 3 methods of preservation by decreasing water availability?
- drying
- freeze drying
- osmotic dehydation
describe preservation by ionizing radiation
- uses electromagnetic radiation with shorter wavelengths (such as gamma radiation), which have lethal effects on MOs
- primary target is the cell’s DNA (smaller genome = greater resistance to ionizing radiation) by using reactive oxygen species (ROS) generated during radiation
what is an example of short and long wavelengths?
which is more effective at killing MOs?
short: gamma radiation
long: microwaves
longer wavelengths just generate heat = less effective
smaller genome in cell = ____ (greater/lower) resistance to ionizing radiation
greater
what is ROS?
how do they function in ionizing radiation?
reactive oxygen species
they are generated during radiation and causes DNA damage to MOs
how are x rays produced?
when an electron beam strikes a metal
what are X rays effective at treating? why?
- fresh fruit and veg for microbial lethality
- b/c X rays for food are restricted to 5MeV or less
what is a drawback of X rays?
- requires high power
- requires cooling treatment
how is gamma radiation emitted?
emitted by an excited nucleus of a radioisotope, causing the nucleus to go to its ground state (lowest energy)
what are radiation sources for gamma radiation?
how are they stored?
cobalt 60
cesium 137
stored: under water
during treatment: in concrete and steal
what is an advantage of gamma radiation?(3)
- high penetration power
- constant energy throughout treatment
- continuous source that doesn’t need to be turned off
what is:
a) redappertization
b) radicidation
c) radurization
a) redappertization: high dose; produced commercial sterility
b) radicidation: medium dose; produces thermal pasteruization; effective against vegetative bacteria but not spores
c) radurization: low dose; controls insects in grains or sprouting of potatoes
what is HPP? describe it.
high pressure processing
- treats food with pressure
- inactivates most bacteria, yeasts, molds, viruses
- does not affect spores
- may cause deformation in food materials with air pockets due to compression
what is PEF processing? describe it
pulsed electric field
- treats pumpable food
- involves short burst of high voltage to a pdt between two electrodes
- eliminates vegetative bacterial cells
- need to know food comp, pH and electrical conductivity for successful treatment
- should not have air bubbles in food
