LEARNING OBJECTIVES
At
the conclusion of this unit the students will be able to
Discuss different methods of food preservation including:
1. An ability to differentiate between various methods of
heat transfer;
2. Discuss difference between processing conditions for
pasteurization & blanching vs. canning;
3. Discuss appropriate terminology related to canning;
4. Discuss low temperature preservation and distribution of
foods and their attendant problems related to producing
quality food products for consumers
5. Define and describe the critical aspects of dehydration,
ohmic heating, and preservation of foods by the use of
additives: chemicals - acid, sugar and salt, smoke - and
physical-electromagnetic radiation & high pressure
INTRODUCTION
Food processing is the term applied to the series of activities used to
preserve a food by inhibiting its bio-processes, killing deteriorative
or pathogenic microbes, and sealing the food in a package to stabilize
it and prevent or slow the foods from perishing. Common processes
used to accomplish these ends are heat treatments, low temperature,
water removal, or chemical or physical additives to insure stability.
THERMAL
PROCESSING
Preservation of foods through the use of heat is accomplished
through the transfer of the thermal energy
Heat transfer is accomplished by
1. Conduction - molecule to molecule or particle to particle
through a solid, through direct contact, SLOW
2. Convection - waves or currents of energy passing
through the mass of food or the liquid or gas it is
processed in, MORE RAPID
3. Radiation - the electromagnetic energy emanates from
an object of surface walls of an oven (like the rays
of the sun) or an electrical coil; any surface
between the source of energy and food absorbs
energy and reduces the amount striking the food
Methods of heat treatment
Severity of heat treatment - goal or end result driven
Mild ---Aim kill pathogenic organisms in the food
Reduce population of spoilage organisms
NOT STERILE
Minimizes flavor, texture, and nutritional quality
Short shelf-life
Process used in conjunction with other
preservation techniques
Refrigeration, Freezing, Chemical
preservatives
Blanching - applied mainly for fruits and vegetables
in preparation for further processing,
ie: freezing
Purpose - inactivate enzymes that cause
deterioration during frozen storage
Conditions: 1-10 min at temps. 70 -
100 oC
(Any microbial destruction is
coincidental)
Pasteurization - applied to milk, liquid and whole
eggs, fruit juices, and beer
Purpose
Destroy Pathogens
Reduce spoilage microbe population
Inactivate enzymes - xanthine
oxidase limiting one and phosphatase
test for adequacy
Extend shelf-life
NOT A STERILE PRODUCT - HARMLESS
BACTERIA ARE STILL PRESENT
Conditions
MILK
65 oC
30 min
72 oC
15 sec
Eggs (liq) 60 - 62 oC
3.5 4.0 min
Eliminate Salmonella hazard
Fruit juices - Temperatues sufficient
to inactivate yeast and molds
More severe --- a stand alone process
KILL ALL BACTERIA
CREATE AN ENVIRONMENT TO INHIBIT
SPORE FORMERS!
Called commercially sterile
Lengthens shelf-life
No other preservatives ofr preservation
needed
Texture less desireable (overcooked)
Substantial alteration of color, texture
flavor and unavlidable losses of
nutrients
Canning (tinning, flexible pouch)
Historical credit for discovery given to a
Frenchman, Nicholas Appert (1809) --won a
prize for developing a process for
preserving a food to further the war effort
under Napoleon
Conditions: TIME: Dictated by the food
characteristics, pH (low acid > 4.5),
viscosity re: method of heat transfer
conduction vs. convection
TEMPERATURE: 116 - 121 oC
(or 241 - 250 oF)
Effects on microorganisms kills bacteria, yeast
and molds
Creates an environment inhibitory to
spore forming bacteria (vacuum)
Critical terms
From the thermal death-rate curve:
D-value - time to produce a one log
cycle reduction in the
microbe population
From the thermal death-time
curve:
Z-value - temperature change to
to produce a 1 log
cycle reduction in the
time to kill 1 log cycle
of microbes
Commercial sterilization achieved through sufficient
heating time at 121o C to prevent growth of Cl.
botulinum in low acid canned foods "A 'bot. cook'"
LOW TEMPERATURE PRESERVATION
Refrigeration - maintenance of sub-ambient temperatures
(< 4oC)
Problems
Microbial - cold loving organisms can grow and do
products must be protected from the
environment
Cross contamination
Chemical - as a minimum odor adsorption,
condensation
Microbial - organisms growing in the refrigerated
storage can grow and contaminate
unprotected foods or atmosphere
of open and not sealed containers
Freezing - Preservation technique practiced by traditional
cultures for centuries (Labrador, Greenlanders,
Icelanders, Eskimos, Scandinavians, Finns,
Laplanders, etc.)
Methods
Plate freezing - relatively slow, not the most
efficient, double plate better
Blast freezing - forced chilled air passing over
the surface of the food
particles or packages
Scrapped surface - liquid systems (i.e. ice
cream mix)
Cryogenic freezing - using liquid nitrogen or
carbon dioxide to
quickly freeze individual
pieces
Quality and rates of freezing - slow rates of freezing
produces large ice crystals and
damage food quality;
less damage with rapid
freezing
Interrelationship between quality and storage conditions
Recrystallization - freeze thaw cycles allow melting
and ice crystal growth
Freezer burn - water migration due to crystal
growth toughens and damages food's
quality
Oxidation - off-flavor development due to lipid
oxidation, some vitamins and amino
acids oxidized on air exposure in frozen
storage
Changes in physical states - colloidal systems [starch
gels, liquid yolk preparations
gel, & emulsions destabilized
on freezing
Chemical changes - failure to inactivate enzymes by
blanching allows flavor and texture changes
and nutrient losses, flavors accumulate
from oxidation too
Moisture control - freezer burn is dehydration if
packages are not sealed
Cryoprotectants - chemical additives which stabilize
{SUGAR OR SALT} colloidal dispersions preventing
failure {providing protection while
frozen} - lower frzg. pt.;
by binding water, inhibits
microbial growth when thawed
DEHYDRATION - Preservation by some degree of water removal,
inhibiting microbial growth
MOST COMMON METHODS
Partial removal
Concentration - water removal under vacuum, commonly applied
to milk [evaporated and condensed milk] & fruit
juices
APPLIED IN CONJUNCTION WITH CANNING OR FREEZING
Nearly complete removal
Sun Drying ('Natural') - direct sunlight and hot air currents
Mechanical heated air - tunnel, tray, cabinet air picks up
water
Drum drying - dries the product on single or two heated
stainless steel drums, for purees of
vegetables --(i.e. instant potatoes)
Freeze drying - products frozen and dried under a vacuum;
water sublimed -- solid to gas which is
collected separately from the product in
the equipment condenser -coffee, meats,
fruits and vegetables
Puff drying - heat/vacuum --> produces expanded
product volume -puffed cereals
Smoking - dehydrates and supplies flavor and microbial
inhibition
Spray drying - liquid sprayed in fine mist into a cone of hot
air flowing in opposite direction --> instant
water removal
Outcome
SHELF-STABLE PRODUCTS with reduced weight which
lowers freight costs in distribution,
susceptible to browning and flavor
changes even at low moisture
PRESERVATION BY ADDITIVES - applying additives to preserve
quality and extend the shelf-life of foods
Chemicals commonly used {approved by the government as safe
and useful} often with another
processing method
Acid - inhibits microbial growth by lowering the pH acid
fermented foods along with soft drinks and some fruit
drinks
Sugar and Salt - inhibits microbial growth by binding water,
raising osmotic conditions, dehydrating
microbes
Smoke - application of flavors and other chemicals in wood
smoke which inhibit microbial growth along with
dehydration by the heat
Miscellaneous chemicals - Previously approved safe chemicals
effective in inhibiting microbial growth are
added to food systems to prolong their
shelf-life benzoates and proprionates
added to bread, beverages, and a host of
other formulated products
Physical
Electromagnetic radiation - irradiation - G-rays, electron
beams like the ISU and Sioux City
facilities, or X-rays, electro-magnetic
energy kills bacteria, done frozen or
chilled to preserve quality
MUST STILL HANDLED CAREFULLY TO PREVENT
RECONTAMINATION!
PRESERVATION WITH ELECTROMAGNETIC RADIATION -
only limited
preservation effects heating mostly
Radio frequency -- microwave heating HEATS BY MOLECULAR
FRICTION FLIPPING OF WATER MOLECULES
uneven heating process hot and cold spots
in foods
DIFFERENTIALLY HEATS; TOUGHENS
PROTEINS AND GELS
Infra-red - radiant heating only little to no preservation effect
Ohmic heating - DIRECT APPLICATION OF ELECTRICAL CURRENT
TO RAPIDLY HEAT FOOD
PRESERVATION WITH HIGH PRESSURE - IN
EXPERIMENTAL STAGE; APPLIES HIGH
PRESSURE TO PRESERVE FOOD IN
CONJUNCTION WITH THERMAL
PROCESSES TO INSURE SAFETY
CONCLUSIONS
Food preservation
is the series of processes employed to extend the shelf-life and
availability of food stuffs through the application of
thermal energy, electromagnetic
energy, water removal, and the addition of additives
{chemical and physical} to raw food
commodities and systems to prevent their deterioration
and insure their safety.
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