Lesson 5 & 6 Flashcards
rationale for food preservation and preservation by thermal processing
Examples of food deterioration
- reaction with oxygen and light
- time
- physical stress or abuse
- gain or loss of moisture
- innappropriate temperatures
- microorganisms (e.g. bacteria, yeasts, moulds)
- food enzymes and other chemical reactions within food itself
- infestation by insects, parasites, and rodents
can be due to physical, biochemical (enzymatic), or biological (microbial) causes
3 types of shelf life
- perishable: not processed or only minimally processed with a shelf life of < 60 days
- semi-perishable: lasts between 2-6 mos due to some form of preservation method
- shelf stable: longer than 6 mos
all perishables and some semi-perishables need date labelling (required for foods with shelf life of 90 days or less)
3 types of microorganisms
- bacteria
- yeasts
- moulds
moulds are filamentous (i.e. have branches), making them well-suited for aerobic respiration
3 types of microorganisms according to function
- good (e.g. lactic acid bacteria from fermentation, probiotics) make us healthier or produce a different kind of food
- bad (i.e. spoilage-formers) don’t cause diseases but spoil our food
- ugly (i.e. disease-causing or pathogens)
4 kinds of microorganisms according to temperature requirement
- mesophiles can grow at room temperature (where most microorganisms belong)
- thermophiles can tolerate high temperature so require high thermal processing
- psychrophiles
- psychotrophs can grow at refridgerated temperature (majority are spoilage formers! like listeria monocytogenes)
psychrophiles and psychotrophes are cold-loving micrororganisms (i.e. grow and multiply at cold temperatures)
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3 kinds of microorganisms according to oxgen requirement
- aerobic require oxygen to grow
- anaerobic require the absence of oxygen to grow (e.g. Clostridium botulinum)
- facultative can tolerate both conditions but react to them differently (e.g. yeast)
yeast in aerobic condition multiplies (e.g. baker’s yeast) and undergoes fermentation, producing alcohol, in anaerobic condition
Viruses
- pseudo-organisms (not true organisms)
- require a living host to propagate
- do not ferment or spoil food, rather uses food as a vehicle to enter the body, causing foodborne illnesses
Growth and tolerance of bacteria
fast growth
* vegetative (active) are growing and metabolizing
* spores (dormant) produced by some bacteria when they encounter unfavorable conditions
* germinating cells transition from spores to vegetative
i.e. spores are waiting to germinate and return to active state
Growth and tolerance of yeast
slower growth than bacteria but more tolerant of low pH and water activity
Growth and tolerance of moulds
even more tolerant (than yeast) of low pH and water activity
Enzymatic browning
what causes deterioration!
- plant tissues have polyphenols
- once oxidized with enzyme polyphenol oxidase, forms brown pigments
browning would not proceed if you avoid oxygen exposure or inactivate the enzyme with acid and heat
Old preservation methods
- dehydration (oldest)
- smoking
- fermentation
- salting
- sweetening
turns food into a completely new product (e.g. cheese, bread, wine, smoked meats)
New preservation methods
- canning
- freezing
- ultra high-temp (UHT) preservation
- irradiation (not used often anymore)
canned foods will not undergo microbial or enzymatic spoilage as long as the physical integrity of the can is maintained, but may spoil due to chemical reactions (e.g. Maillard browning) which may proceed slowly even at ambient temperature
Difference between old and new preservation methods
- old methods cause noticeable changes in the food (e.g. fresh vs smoked salmon, grapes vs wine, milk vs cheese)
- new methods cause little change (e.g. freezing, pasteurization)
- no method can completely eliminate spoilage phenomena indefinitely!
- sometimes one method is not enough (i.e. hurdle technologies)
Dehydration
2 types
removal of free water to control chemical, enzymatic, and microbial activity
1. complete removal (drying)
2. partial removal (concentration)
e.g. condensed milk combine dehydration and canning; concentrated juices combine dehydration and freezing
Fermentation
use of the desired microorganism (e.g. production of acid, metabolites, antimicrobial compounds) to delay the growth of non-desireable ones (i.e. microbial antagonism)
requires other forms of preservation (e.g. vacuum or modified atmosphere packaging)
Low-temperature preservation
2 types
- refridgeration (milder) slows down the rate of chemical, enzymatic, and microbial activity
- freezing controls water activity and inhibits chemical, enzymatic, and microbial activity below a certain temperature
- food must be blanched prior to freezing and freezer burn (a defect) may occur
- enzymatic reactions can proceed throughout the freezing process and can resume when food is thawed
3 kinds of thermal processing preservation methods
- blanching (very mild) inactivtes enzymes
- pasteurization destroys disease-causing microorganisms (i.e. pathogens) but putrefactive microorganisms (can hydrolyze protein and produce foul odor) survive
- commercial sterilization (i.e. botulinum cook) destroys almost all disease-causing and spoilage-causing microorganisms, but also nutrients
pasteurization first used industrially in milk
Irradiation
uses ionization energy to inactivate microorganisms or biological systems
general public concern regarding its safety even though the required dose is regulated
Addition of chemicals as a preservation method
4 ways
- acids
- sugar and salt lower water activity
- antioxidants (e.g. vitamins C and E, Butylated hydroxyanisole or BHA) protect food from oxidation
- preservatives (e.g. Sodium proprionate, Sodium benzoate)
- BHA is currently under scrutiny
- Sodium proprionate is a mold-retarding or -inhibiting component in flour while Sodium benzoate is used in the beverage industry
Role of preservation
- eliminate any potential microbial harm to consumer
- maintain nutritional value within limits dictated by the production of a safe food product
- maintain quality of food
Principle of thermal preservation
regulated processes using heat that are performed comercially to control chemical, enzymatic, and microbial reactions
regulated = validated by authorities
e.g. blanching, pasteurization, commercial sterilization
Blanching
thermal preservation
a mild-intensity heat process that exposes fruit or vegetables to boiling water or steam for a short period of time
Preservation principle of blanching
- inactivate endogenous (i.e. natural) enzymes
- drive off oxygen and other gases entrapped in food matrix (minimizes pressure buildup)
Pasteurization
thermal preservation
a moderate-intensity heat process that requires a temperature below boiling point of water (60-80°C)
Example of pasteurization of milk
2 ways
- low temp, long time (LTLT) or batch pasteurization for 30-40 mins at 60°C
- high temp, short time (HTST) or flash pasteurization for 15 secs at 72°C
LTLT produces a cooked taste in milk
Principle of pasteurization in low-acid foods
e.g. milk, eggs
- to destroy pathogenic (i.e. disease-causing) bacteria and viruses
- to inactivate enzymes
many spoilage-causing MOs can survive (e.g. psychotrophic bacteria, putrefactive spoilage, lipolytic spoilage), requiring refridgeration with a durable life date at < 4°C
Preservation principle of pasteurization in acid foods
e.g. beer, wine, fruit juices
to extend product shelf-life by destroying spoilage-causing MOs and enzymes
acid foods are not a source of pathogens (i.e. disease-causing MOs) except for E.coli (0157:H7), an acid-tolerant MO that can remain in apple juice if not pasteurized
Why select pasteurization?
- for foods that will be consumed within a short period of time after processing
- partially extends storage life with a combination of pasteurization and refridgeration
e.g. milk, some cured meat products, smoked salmon
Commercial sterilization (CS)
aka canning (for canned products)
thermal preservation
a high-intensity heat process that requires a minimum of 121°C moist heat for 15 mins
CS products have a shelf life of 2 years or more (to ensure quality but are indefinitely safe) and a durable life date is voluntary, not required
Principle of commercial sterilization
- destroys spoilage-causing and disease-causing MOs
- make the product free from viable forms of MOs (including spores)
- ensures the spores of C. botulinum are destroyed
a small number of heat-resistant spores (not disease-causing) survive but aren’t able to multiply (or germinate) in the food product even if held at room temperature
Botulism
Latin botulus = sausage
health risk of C. botulinum
- bacteria present in soil, water, air that’s strictly anaerobic and grows well in low-acid foods
- produces a potent toxin, neurotoxin that affects the central nervous system
Symptoms of botulism
vertigo, blurred vision, slurred speech, difficulty breathing and swallowing
worst cases include death from respiratory collapse or cardiac arrest
Container specifications for commercial sterilization
- must withstand the high temp and pressure used
- must be hermetically sealed (i.e. impermeable to transmission of gases, liquids, and microorganisms)
CS is conducted once food is packaged in suitable containers!
Ultra high temperature (UHT) processing
injection of hot steam under pressure (140-150°C) for a short time (4-6 secs) followed by immediate cooling
Aseptic packaging
UHT food is aseptically placed into pre-sterilized containers and sealed in an aseptic environment
e.g. shelf-stable milk and juice like tetrapaks
Shelf life of UHT treated and aseptically packaged products
a form of commercial sterilization
≥ 6 mos without refridgeration
* not shelf-stable (i.e. can undergo post-processing recontamination at room temp)
* longer shelf life at refridgeration temperatures
some UHT treated foods aren’t aseptically packaged = not commercially sterile
Why select commercial sterilization?
longer storage times at room temperatures in sealed containers (e.g. canned goods, UHT/aseptic pack products)
Important factors to consider for commercial sterilization
- appropriate heat treatment
- thermal death curves (TDRC and TDTC)
- margin of safety
- heat transfer mechanism
- effect of food constituents
Selecting the appropriate heat treatment
commercial sterilization
- sufficient heat to destroy MOs and enzymes, minimizing the effect on other properties of the food (e.g. nutrients)
- mildest HT that guarantees freedom from pathogens and toxins while producing the desired storage life
Thermal death curves (TDC)
microorganisms are not killed instantaneously, rather they follow a logarithmic order of death
under constant thermal conditions, the same % of microbial population will be destroyed in a given time interval, regardless of the size of the surviving population
2 types of thermal death curves
and their 3 components
- thermal death rate curves (TDRC)
- thermal death time curves (TDTC)
D-value, Z-value, and F-value
Thermal death rate curve (TDRC)
i.e. survivor curve
if a given temp kills 90% microbial population in the 1st minute of heating, 90% of remaining population will be killed in the 2nd minute, so on and so forth…
described by a D-value
D-value (mins)
i.e. decimal reduction time
time (in minutes) at a particular temperature (°C) required to kill 90% of a microbial population
i.e. resistance at a specific temp
- remains constant regardless of changes in the initial microbial load
- decreases when temperature is increased
Thermal death time currves (TDTC)
time required for the destruction of a microbe under specific conditions at different temperatures
described by a Z-value (°C) and F-value (min)
Z-value (°C)
- resistance of MOs to temperature variation
- temp change causes a 1-log cycle change in the D-value
- if it takes an increase of 8°C for the D-value to change 1-log, then the Z-value is 8°C
- in reality, no instataneous heat to 121°C as product may already be cooked at 116°C
F-value (mins)
time required to kill all MOs at 121°C
i.e. lethality of heat treatment or the capacity of heat treatment to sterilize
Margin of safety (MS)
probability of viable survivors of C. botutlinum spores
intention is to have a wide margin of safety and minimize the probability of survivors
Margin of safety for low-acid foods
F-value
12D thermal process
* 12D = 12 log cycle reduction = can kill 10¹² (1 trillion) MOs
* large MS (usually # microbes < 10¹²)
Margin of safety for acid foods
F-value
5D thermal process
* C. botulinum will not grow in acid foods (pH below 4.6)
How can we determine if C. botulinum spores have been destroyed?
inoculated pack studies using Clostridium sporogenes (PA3679 spores), a putrefactive anaerobe, a non-toxic alternative to C. botulinum
more heat resistant than C-botulinum spores
Factors affecting the mechanism of heat transfer
in commercial sterilization
- location of the “cold point” (slowest heating part of the food with the greatest risk of microbial survival)
- consistency of the food (e.g. solid, viscous liquid, non-viscous liquid, a combination)
- chemical composition of the food (e.g. fat, protein)
2 types of heat transfer
- heating by conduction: molecule to molecule heat transfer in straight lines in solid foods (e.g. canned salmon)
- heating by convection: by fluid motion where fluid heats along the hot wall of the container and rises in non-viscous liquid foods (e.g. chicken broth, evaporated milk)
Effect of food constituents in thermal processing
- sugars, starches, fats, and proteins can protect spores and vegetative cells from the killing effects of heat and affect the heat transfer rate or mechanism
- other constituents (e.g. acids, spices, antimicrobial components) may decrease the resistance of MOs to thermal processes
e.g. D₉₀ for salmonella in chocolate milk = 78 mins but D₉₀ for salmonella in regular milk = 0.0008 min
Types of packaging for thermally processed foods
- cans (2 or 3 piece)
- retort(able) pouches
- tetrapak
- glass jars or bottles
- plastic
Steel body cans
packaging for thermally processed foods
- can withstand high temperatures and pressure differentials
- not readily breakable
- lids can indicate the presence of a vacuum and thus a hermetic seal
- steel, and often the tin plating, must be protected with lacquers to minimize the reaction of the metals with food constituents
tin cans (thin layer of tin) are most widely used!
Glass jars
packaging for thermally processed foods
- more resistant to corrosion and reaction with food constituents
- allows consumer to see the content inside the container
- glass is heavy and bulky, and must be packaged with extra protection to prevent physical breakage of the glass during transportation
- glass filled containers must also be processed in the retort with extra care to prevent breakage due to thermal shock
Laminates of sterile cartons (tetra paks)
packaging for thermally processed foods
made from laminated plastic, aluminum, and paper
* carton laminates are sterilized with hydrogen peroxide treatment
* forming and filling of cartons under sterile conditions
must use a chemical sterilizing agent because exposure of laminated materials to high temperature required for heat sterilization would destroy the packaging
Retortable pouch
relatively new!
packaging for thermally processed foods
- made of a laminate of plastic films and aluminum
- faster heat penetration due to thinner material
- superior nutrient retention
- pouches must be packaged in an outer protective carton to minimize physical damage (e.g. puncture) due to handling
Outcome of faster heat penetration in retortable pouches
12D thermal process at the cold point takes place in a much shorter period than the conventional metal can or glass bottle
Plastic bottles/cans
packaging for thermally processed foods
- new types of plastic packaging materials that can be produced in the shape of a can or bottle
- newer plastic bottles can be used with UHT-aspetic packaging technologies
- some containers are used for ready to eat foods and can be placed in the microwave (unlike metal cans and glass bottles)
- can be hermetically sealed and thermally processed in a steam retort to achieve the 12D process for low acid foods
3 major steps in milk processing
- clarification: centrifuge spins preheated milk to obtain a predetermined butter-fat content
- homogenization: milk forced through fine nozzles, breaking up fat globules to a size that keeps them suspended
- pasteurization: rapidly heated to 77°C for kept in holding tube for 16s
milk is placed in a heat exchanger (raw milk is preheated while pasteurized milk is cooled)
What should be considered when pasteurizing dairy products (other than milk)?
e.g. egg nog
ingredients added to milk after pasteurization can introduce spoilage
must re-pasteurize milk with a more severe process
