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Control of microbes in foods. Keeping them out Physically removing them Preventing their growth Killing them. General observations about microbial control. Cells in log phase are more susceptible to injury More effective when fewer microbes are present
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Control of microbes in foods Keeping them out Physically removing them Preventing their growth Killing them
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
What are some strategies for microbial control/preservation?
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
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
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
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
Decontamination and sanitation of raw fruits and vegetables • Washing • Chlorine (50-200 ppm); ozone (0.1-2.5 ppm); peroxyacetic acid (<80 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
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
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)
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
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
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
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 won’t germinate below 30oC • High acid: dangerous spore formers don’t grow • UHT will kill microbes, but may not destroy enzymes or toxins
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
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
Methods • Ice chilling (0-1oC) • Watch for temperature fluctuation, cross-contamination • Refrigeration • Combine with dryness, preservatives, low pH • Freezing- will kill microbes
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
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
“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
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
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
Gamma-(γ)-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) • Molds>yeasts>bacteria>viruses in sensitivity • Toxins are not destroyed
Use of irradiated foods is limited in U.S. Spices, flour, potatoes before 1985 permitted irradiated foods Food prep surfaces may be UV- irradiated
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
Possible uses for HPP • Adjust pressure to kill vegetative cells (low) or endospores (high) • Don’t 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
Hurdle technology: combine methods • Treatment may be effective but affect food acceptability • Methods that suppress one organism may enhance another • Requires careful study
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
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 can’t just be safe, it has to be good!