Control of microbes in foods

1. Control of microbes in foods Keeping them out Physically removing them Preventing their growth Killing them

2. 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

3. What are some strategies for microbial control/preservation?

4. 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

5. 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

6. 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

7. 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

10. 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

11. 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

12. 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)

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

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

20. 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

21. 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

22. “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

23. 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

24. 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

25. 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

26. Use of irradiated foods is limited in U.S. Spices, flour, potatoes before 1985 permitted irradiated foods Food prep surfaces may be UV- irradiated

27. 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

28. 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

29. Hurdle technology: combine methods • Treatment may be effective but affect food acceptability • Methods that suppress one organism may enhance another • Requires careful study

30. 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

31. 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!