Control of microbes in foods

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Control of microbes in foods

• Keeping them out
• Physically removing them
• Preventing their growth
• Killing them

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General observations about microbial control

• Cells in log phase are more susceptible to injury
• More effective when fewer microbes are present
• Spores are more resistant than vegetative cells
• Gram-negative organisms are more susceptible to
  many methods
• Bacteria, molds, yeasts, and viruses have
  different susceptibilities

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What are some strategies for microbial
control/preservation?
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Control of access (cleaning and sanitation)

• What is sanitation?
• Reducing microbial count to acceptable level
• How do you achieve it?
• Consider food contact surface
• Ingredients
• Integrated into plant design
• Light, air, ventilation
• Water quality
• Workspace design

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Water, ice, brine, curing solutions

• Many uses for water
• Keep ice germ-free
• Chilled water can cause cross-contamination
• Warm water for washing thermoduric bacteria
• Brine and curing solutions should be made fresh
  and used frequently

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More essential plant features

• Dry air, free of dust
• Well-trained (and healthy) personnel
• Well-maintained equipment
• Detergent, high-pressure distribution
• Skin contact?
• Clean-in-place protocol?
• Cleaning schedule

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Sanitation

• Physical
• Hot water, steam, hot air, UV irradiation
• Chemical
• Desirable features effectiveness, non-toxicity,
  non-corrosiveness, ease of use, stability, cost
  effectiveness
• Detergent sanitizers and clean and sanitize

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Type of chemical sanitizer Advantages Disadvantages
Chlorine-based Wide range of effectiveness Fast acting cheap Unstable Can oxidize food Less active in hard water
iodophores Fast-acting, noncorrosive, easy to use, stable Expensive, less effective against spores and viruses, affect flavor, eract with starch
Quats (QACs) Very stable, noncorrosive, bacteriostatic Expensive, limited range, require rinsing
Hydrogen peroxide Can be sterilant can be used as liquid or vapor Organic materials reduce its effectiveness
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Decontamination and sanitation of raw fruits and
vegetables

• Washing
• Chlorine (50-200 ppm) ozone (0.1-2.5 ppm)
  peroxyacetic acid (lt80 ppm) plant essential oils
• Decontamination
• Chemical sanitizers in liquid or vapor form
• ClO2- chlorine dioxide gas
• Ozone
• Hydrogen peroxide, etc.
• Effective against Gram-negative pathogens

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Standards, specifications, guidelines

• Maximum acceptable levels standards enforced by
  regulatory agencies
• Examples 20,000 cfu/ml in milk, less than 10/ml
  coliforms (standard plate count)
• Most foods have specifications agreements
• Achievable if good cleaning, sanitation, and
  handling is observed

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Removing microbes

• Centrifugation
• Large contaminants in liquids
• Could be combined with heat treatment
• Filtration
• Heat-sensitive liquids
• air
• Trimming
• Washing
• Combined treatments can work better
• Heat, high pressure, chemicals
• Try to avoid biofilm (highly resistant to
  removal)

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Heat treatment

• One of the oldest antimicrobial methods!
• Mathematical precision (heat to what temperature
  and for how long?)
• Destroy microbes- some or all?
• Heat-stable enzymes, toxins
• Sometimes the first step in a fermentation
  process (kill off the competition)
• Overcome natural ability to react to heat

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Influencing factors

• Nature of food
• Small chunks vs large chunks (more heat
  susceptibility)
• High aw vs low aw
• Low pH vs high pH
• Nature of microbes
• Vegetative vs spores
• Low vs high numbers of microbes
• Exponential vs stationary phase of growth
• Nature of process
• Higher the temperature, shorter the holding time

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D values and thermal death time

• D value time in minutes to reduce number of
  cells by 90 by a specific treatment
• Thermal death time (TDT) complete killing
• Will be longer for spores than vegetative cells
• 12D process for canning high-pH foods

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Methods of heat treatment

• Low-heat (less than 100oC) Pasteurization
• Microwaves hot but problematic
• High heat (greater than 100oC)
• Low acid 12D to kill C. botulinum spores
• Other spores can survive, but wont germinate
  below 30oC
• High acid dangerous spore formers dont grow
• UHT will kill microbes, but may not destroy
  enzymes or toxins

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Control by low temperature

• Another ancient technique refrigeration really
  took off after WWII
• Frozen foods, new technologies increase shelf
  life
• Unintended consequences (new pathogens)
• How does cold control microbes?
• Slows metabolic activity
• Aw and pH are reduced
• Freezing and thawing disrupts cell structures

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Influencing factors

• Nature of process
• Most free water is frozen at -20oC
• Fluctuation in temperature can promote growth
• Slow cooling, slow thawing, can promote microbial
  growth
• Nature of food
• Neutral pH, high Aw, absence of inhibitors
• Nature of microbes
• Gram-positive cocci are more resistant to
  freezing damage
• Some spores can germinate at low temperatures do
  not lose viability

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Methods

• Ice chilling (0-1oC)
• Watch for temperature fluctuation,
  cross-contamination
• Refrigeration
• Combine with dryness, preservatives, low pH
• Freezing- will kill microbes

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Reduced water activity

• Removing free water
• Naturally
• Mechanically
• Freeze-drying
• Foaming
• Smoking
• Can kill some microbes, but some are resistant
• Usually combined with other methods to increase
  effectiveness

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Acidification denatures proteins, inhibits
nutrient transport, affects spore formation

• Natural
• Fermentation products
• Acetic acid (vinegar)
• Propionic acid-effective against molds and
  bacteria
• Lactic acid-bacteria
• Citric acid- chelates divalent cations
• Sorbic acid- more effective against yeasts and
  molds
• Benzoic acid-inhibits respiration
• Parabens-broad-spectrum antimicrobials

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Modified atmosphere (removing oxygen)

• CAP (controlled atmosphere packaging)
• Long-term, continuous monitoring
• Modified atmosphere packaging (MAP)
• Remove air and flush with gas
• like CO2 or nitrogen
• Vacuum packaging (VP)
• Prevents aerobic respiration
• CO2 slows growth rate
• Facultative anaerobes and
• anaerobes can benefit

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Antimicrobial preservatives

• Compounds that kill microbes (innate or added)
• Static or cidal
• Will not sterilize foods
• Criteria
• Must be safe to use (!)
• Should not affect quality of food
• Stable
• Effective in food environment

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Examples of antimicrobial preservatives (GRAS,
generally regarded as safe)

• Nitrates and nitrites-controls C. botulinum
• Sulfur dioxide and sulfites- broad spectrum, can
  be allergenic
• Epoxides- used as fumigants
• EDTA- helps destabilize Gram-negative cell walls
• Lysozyme-protects against Gram-positives
• Antibiotics- can be used as sprays
• Wood smoke- formaldehyde, phenols, cresols
• Spices- many antimicrobial compounds

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Gamma-(?)-Irradiation

• Microbicidal against a wide variety of microbes
• This type of radiation is focused and penetrating
• DNA is damaged and correlated to dose
• Can penetrate food packaging
• Cobalt-60 usually used (half-life of 5.3 years)
• Moldsgtyeastsgtbacteriagtviruses in sensitivity
• Toxins are not destroyed

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Use of irradiated foods is limited in
U.S. Spices, flour, potatoes before
1985 permitted irradiated foods Food prep
surfaces may be UV- irradiated
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Are there new and better technologies?

• Demand for minimal processing
• Long-wave electromagnetic waves
• Ohmic heating by electric currents passed through
  food
• Pulsed electric fields
• High-hydrostatic pressure processing (HPP)
• All high-energy, short time exposures
• Purpose to kill microbial cells

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Possible uses for HPP

• Adjust pressure to kill vegetative cells (low) or
  endospores (high)
• Dont disrupt internal environment, so foods are
  more natural
• Food processing tenderize meat, inactivate
  spoilage enzymes or other undesirable molecules,
  thaw food rapidly extend shelf life
• Potentially, could sterilize food

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Hurdle technology combine methods

• Treatment may be effective but affect food
  acceptability
• Methods that suppress one organism may enhance
  another
• Requires careful study

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Examples

• Combine low heat treatment with acidification and
  preservatives
• Some preservatives act synergistically (NaCl and
  BHA)
• Vacuum packaging and acidification to reduce
  anaerobe growth
• One treatment may stress a microbe to increase
  its susceptibility to killing
• Promising but preliminary

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Summary

• Food preservation technology has existed for
  millennia
• Many strategies required because of the diversity
  of food (and microbes)
• Microbes are extremely adaptable
• Combinations of techniques may be most effective
• Food cant just be safe, it has to be good!