14 - Physical Methods of Food Preservation Flashcards
what is the goal of modern preservation
extend shelf life; ensure safety by inactivating pathogens/preventing their growth
what is inactivation?
destruction of a bacteria judged by its inability to recover on microbiological media
_____ methods of food preservation are the most common
physical
what are the physical methods of food preservation?
- high temp treatment
- low temp preservation
- decreasing water availability
- ionizing radiation
- high pressure processing
- pulsed electric field
how does high temp inactivate microorgs?
going beyond optimal temp slows cell functions (multiplication) and eventually death.
damages membranes, proteins, and ribosomes
benefits of heat treatment? (2)
- eliminates almost any microbial target
- customizable to the product and organism of concern
limitations of heat treatment?
overheating can damage the product quality
what factors affect heat transfer? (6)
- product state
- container material
- container shape
- container size
- agitation
- temperature of heating medium
do liquids or solids transfer heat faster?
liquids
do glass or metal containers transfer heat faster?
metal
which shape container transfers heat the fastest?
tall and narrow
do large or small containers become thoroughly heated faster?
small
does agitation speed up heat transfer?
yes
what are the 2 most important parameters in high temp treatment?
temp and time
_____ _____ depict the logarithmic nature of population inactivation over time
survivor plots
what’s a D value?
decimal reduction time; the time it takes for a 10 fold reduction in the number of survivors
the greater the D value for a T, the more (resistant/sensitive) the organism is to heat
resistant
define a thermal resistance plot
when D value is plotted against temperature
what does a thermal resistance plot enable?
comparison of resistances of a microorganism at different temps
what is a z value?
change in temp required to change the D value of a microorganism by 10-fold
a larger Z value indicates a more heat (sensitive/resistant) organism
resistant
measurements taken to determine the amount of time required to commercially sterilize food at a given temp
thermal lethality measurements
what’s an F value
time (min) at a specified temp to require a targeted reduction in a homogenous population having a specific z value
differentiate between D and F values
D = time to reduce by 1 log
F - time to reduce to specified level (can be until sterile)
which value is the most important in the canning industry? (D, F, Z)
F
canning reduces C. botulinum by ___ log
12
define sterilization
renders a product free of any living organisms
define commercial sterilization
renders a product free of:
- miroorgs that can reproduce in food under normal non-refrigerated storage & distribution temps
- viable microbial cells or spores of public health significance
true or false: commercially sterilized items are completely free of microoganisms
false. they are heated just enough to produce a shelf-stable product
what mechanisms cause bacteria to be resistant to heat?
- protein cross-linking (disulfide bonding)
- DNA structural changes (high GC)
state of _____ is an important factor in heat resistance
dormancy
what phase of cells are more resistant to heat than exponential phase cells?
stationary phase
spore formation dramatically (increases/decreases) heat resistance
increases
can other hurdles (pH, aw) influence heat resistance?
yes
what is aseptic processing?
you have sterile can
you have sterile food
you put them together in aseptic conditions
what does aseptic processing achieve with regard to heat?
less excess heating
describe the heating process in aseptic processing
- passed through set of heat exchangers to specified time/temp
- food is passed through a set of cooling heat exchangers
- cooled product is placed in pre-sterilized packages & hermetically sealed
for what kind of commodities is aseptic processing applied?
fruit juices, dairy products, sauces
for what kinds of foods is aseptic processing unsuitable?
low acid, particulate containing fodos
why aren’t microwaves used in processing?
distribution of heat is very uneven
what does freezing accomplish?
stops metabolic activity of most food-borne microorganisms; slows biochemical deterioration (lipid ox, maillard, lysis) can also cause damage to living cells (for both food and microbiota)
are toxins affected by freezing?
no
what kind of microorganisms are resistant to low temperature preservation?
- cryophilic
- psychrophilic
- psychrotrophic
- xerotolerant
what does freezing do to aw?
decreases
are gram positive or negative better at surviving low temp preservation?
positive
how are parasites affected by freezing?
they’re killed
microbial cells are more susceptible to injury/death during (fast/slow) freezing
slow, like the freezer at home
what kind of water is essential for microbial growth and activity?
unbound
aw = ?
P/Po
ERH = ?
aw*100
true or false: there is a linear relationship between aw and moisture content of food
false
chemical and biological activities are better described using…
aw
give the aw ranges of high moisture, intermediate moisture, and dry foods
- high moisture foods: >0.9
- intermediate moisture: 0.65-0.90
- aw: <0.65
are intermediate moisture foods resistant to microbial spoilage?
moderately yes
what aw category does maple syrup belong in?
intermediate
spoilage need an aw in what range?
> 0.95
spore forming bacteria can grow in what aw range?
0.91-0.95
what aw can S. aureus grow?
> 0.86
which have a higher aw requirement: yeast or bacteria
bacteria
what are some spoilage yeasts associated with intermediate moisture foods?
- debaryomyces hansenii
- Candida pseudotropicalis
- Zygosaccharomyces bailii
- zygosaccharomyces rouxii
are moldes xerotolerant
yeah
can mycotoxins be produced under xerophilic conditions?
it’s restricted
what’s a humectant?
holds moisture in food
how can humectants microbial growth?
ability to permeate membranes and impart inhibitory effects
does reducing Aw kill microorganisms?
not always. they may go dormant until food is rehydrated
define drying
mobilizes water in food matrix, brining it to the food’s surface, then removing the water from the surface by evaporation. involves both heat and mass transfer
define freeze drying
- product is frozen
- frozen moisture is removed by sublimation
- retains original structure and flavor
define osmotic dehydration
removes water using hypertonic solutions (sucrose, salts, glycerols). relies on selective permeability of food cells to pass water out of food.
true or false: osmotic dehydration can remove all the free water
false - it will always have to reach equilibrium
what wavelengths are used for ionizing radiation
shorter (x ray and gamma)
what is the target of ionizing radiation?
cell’s DNA
a smaller genome will result in (greater/lower) resistance to ionizing radiation
greater
how is DNA damaged by ionizing radiation?
rxn of ROS
how are x rays produced?
- when electron beam strikes a metal
what are x rays good for treating?
fresh fruit and veggies for microbial lethality
what is the drawback of using x-rays?
high power requirement; high requirement ffor cooling treatment
how is gamma radiation produced?
from sources like Co-60 and Cs-137. it’s emitted by an excited nucleus of a radioisotope trying to go to its lowest energy state
advantages of gamma radiation?
- high penetration power
- constant energy
define redappertization
involves high dose (10-75 kGy) to produce commercial sterility
define radicidation
medium dose (1-10 kGy) to produce equivalent of thermal pasteurization. good against vegetative bacteria but does not inactivate spores.
define radurization
low dose (0.05-1 kGy) to control presence of insects in grains or stop the sprouting of potatoes or delay fruit ripening
expand HPP and PEF
HPP = high pressure processing PEF = pulsed electric field
define HPP
involves treating food w/pressures in the range of 100-600 MPa to inactivate most bacteria, yeasts, mold, and viruses (spores survive)
limitations of HPP
- food with air pockets may experience deformation
- appropriate packaging must be considered
define PEF
applies short burst of high voltage between 2 electrodes
what does PEF eliminate?
vegetative bacterial cells
what does the success of PEF treatment rely on??
- food composition
- pH
- electrical conductivity
true or false: foods with air bubbles are pumpable foods
false. air is not a good conductor.
