Epidemiology teams Microbial Ecology Team 1 Team 2 Tyann Blessington Kristin Ahrens of Foods Rebecca Garabed Sharon Hunt Gerardo Kullanart Tongkhao Bradley Olson Team 3 Team 4 Dean O. Cliver Thuyvan Lam Shirin Jamshidi Anika Singla Thomas Kohler Christopher Theofel A food is an ecosystem A food is an ecosystem for microbes for microbes � Viruses & parasites can only � They don't “know” they persist or be inactivated (die, are in food! lose infectivity). � Bacteria & molds may � Most attention devoted to fates multiply, survive, or die. of bacterial pathogens. Pathogenic bacteria in food: Pathogenic bacteria in food: potential “outcomes” potential “outcomes” � Death: another rate parameter � Persistence: viable, numbers (cf. viable-nonculturable) unchanged (lag or stationary � Sporulation: another defense phase or sporulation) (species) � Growth (multiplication): rate � Toxigenesis: growth is parameter (variable) based on necessary, but possibly not doubling time sufficient
Growth curve biology Growth curve biology � Spores & lag phase cells � Multiplying (doubling) cells are quiescent; adaptation to metabolically active, often environmental conditions = adapting; not all metabolically selecting needed enzymes active cells are multiplying. (activating appropriate genes) � Stress causes adaptation or from broad bacterial repertoire. injury. Bacteria in broth vs food Growth curve biology � Broth: “planktonic cells” � Stationary phase may represent � Bacteria tend to aggregate, quiescence or (more often) attach to surfaces, form growth rate = death rate. colonies or biofilms � Some injured cells appear dead � Foods = solid matrix, (“viable nonculturable”). microenvironments � Some dead cells autolyze. � Pathogens outnumbered Research vs real food Research vs real food � At high levels, bacteria signal � Food contaminants (water, each other chemically air, soil, raw material, feces) (“consensus”) have mixed microflora. � Different species interact � Food ecosystem may select competitively, but sometimes one organism beneficially
Major factors (interact) Research vs real food � Temperature � Nutrients � “Programmed” successions available � Genetic exchanges among � E h � Physical strains or species � a w structure � Toxigenic agents (including � pH (specific � Microflora molds) grow under conditions cations & that do not permit toxigenesis. � Antimicrobial anions) agents Temperatures for Temperatures for Thermophiles Mesophiles � Minimum: 5–15°C � Minimum: 40–45°C � Optimum: 30–45°C � Optimum: 55–75°C � Maximum: 35–47°C � Maximum: 60–90°C Temperatures for Temperatures for Psychrophiles Psychrotrophs � Minimum: -5–+5°C � Minimum: -5–+5°C � Optimum: 12–15°C � Optimum: 25–30°C � Maximum: 15–20°C � Maximum: 30–35?°C (cf. handout)
Warm = near optimum? Cold: liquid or solid water? � Food spoilage promoted; test of � Freezing kills some cells, sanitation frozen storage preserves � “Danger Zone”: 4–60 ° C (40– � Psychrotrophs grow slowly in 140 ° F) or 5–57 ° C (41–135 ° F) refrigerated food � Rapid transition from hot to cold or cold to hot DANGER ZONE FOR NEUTRAL FOODS “Danger zone” depicted Averages of Aeromonas hydrophila, Bacillus cereus, E. coli O157:H7 , Danger Zone Listeria monocytogenes, Salmonella spp. , Shigella spp. , Staphylococcus aureus, & Yersinia enterocolitica °F 32 48 64 80 96 112 128 140 40 2 (T 1/2 ) -1 [min] (T 2 ) -1 [h] 1.5 1 0.5 0 0 10 20 30 40 50 60 °C -0.5 -1 -1.5 Hot—temps > max for D value example growth cause death 8 8 8 � D value: time for decimal D t°C = 5 min 7 reduction at t ° C; organisms NUMBER 6 6 are in log death phase 5 LOG � z value: temperature change ( ° C) to reduce the D value 10- 0 5 10 15 20 fold HEATED (MIN) AT t o C
z value example Heat � Cooking, blanching, 100 pasteurization not for z (°C) = 15 10 “commercial sterility” D 1 � Cells in log phase are 0.1 0.01 more heat-sensitive 70 80 90 100 TEMPERATURE ( o C) Heat Tyndallization: boiling � “Heat-shock” proteins aid on 3 days � Day 1: vegetative cells killed, adaptation; some produced in spores heat-shocked response to other stresses. � Day 2: veg cells from spores � Mesophiles or psychrotrophs— killed, last spores heat-shocked infectious agents must be able � Day 3: vegetative cells from final to multiply at body spores killed; endpoint: sterility temperature. Eh Eh � Facultative organisms often use � Aerobic (>0 mV), available energy more efficiently microaerophiles, facultative, under aerobic conditions anaerobic (<0 mV) � C. perfringens may not start � “Strict” aerobes E h > 0 mV, growing under aerobic “obligate” anaerobes E h < -300 conditions, but is not inhibited mV by oxygen once growth begins.
Eh Water activity—" a w " � E h hard to measure in foods Water available for microbial � Live foods metabolize or bind growth, based on water present oxygen and on binding by solutes such � Packaging, modified atmosphere as salt or sugar; equilibrium relative humidity ÷ 100; range � Molds generally strict aerobes is 0 to 1.00 Approximate a w of some foods Minimum a w for some foodborne pathogens � Fresh fruit or vegetables >0.97 � Salmonella 0.93 � Fresh poultry or fish >0.98 � Fresh meats >0.95 � C. botulinum 0.93 � Juices, fruit & vegetable 0.97 � Staphylococcus aureus 0.85 � Cheese, most types >0.91 � (Most yeasts) 0.88 � Honey 0.54–0.75 � Cereals 0.10–0.20 � Most molds 0.75 pH: hydrogen-ion potential pH values of some foods � Egg white 7.6–9.5 � Foods range from pH 7 � Milk 6.3–6.8 downward. � Chicken 5.5–6.4 � Acidification inhibits � Beef 5.3–6.2 spoilage & growth of many � Cheeses, most 5.0–6.1 pathogens. � Tomatoes 3.7–4.9 � Apples 2.9–3.5 � “Low acid” (bot) pH > 4.6
Important minimum pH values pH � “Organic” acids (e.g., lactic, for growth of microbes in foods acetic, etc.) more effective antimicrobials than mineral � Clostridium botulinum 4.8 – 5.0 acids � Salmonella (most types) 4.5 – 5.0 � Most effective undissociated; at � Staphylococcus aureus 4.0 – 4.7 a given pH, molar quantity of � Yeasts & molds 1.5 – 3.5 organic acid >> than that of a mineral acid. Nutrients available Physical structure � Bacteria grow on surfaces � C & N sources required, when they can. sometimes “growth factors” � Some surfaces (melon rind, � Foods generally good C & N eggshell) limit access to sources nutrients. � Other factors, then nutrients � Food matrix: molds often decide which organism penetrate better than bacteria. predominates Physical structure Microflora � If water & solutes cannot diffuse � Bacteria in foods: variety & freely, local variations in E h , a w , competition and pH are highly possible. � Microbial growth may � High viscosity or strongly cellular structure can greatly lower E h & pH; molds use limit heat transfer (both heating organic acids as carbon and cooling) in foods. sources & raise pH.
Microflora Competing organisms � Bacteria may produce acetic, lactic, and other acids as � Staphylococcus aureus fermentation products. � Clostridium botulinum � Some produce bacteriocins— proteins that have a highly- specific lethal effect on closely related organisms. “Programmed succession” Antimicrobials: preservatives � Milk: rapid lactic acid producers � Materials added specifically to (lactococci), then inhibit microbial growth � Slower acid producers � Nitrite for “curing” meats, vs (lactobacilli) that tolerate lower C. botulinum . pH's, then � Acid-stable putrefactive � Sorbates, benzoates, & other (proteolytic) bacteria and finally, salts of organic acids � Molds (metabolite tolerance). bacteriostatic, not bactericidal Antimicrobials: preservatives Antimicrobials: radiation � UV widely applicable to � CO 2 & SO 2 long used in foods; decontamination of food SO 2 is highly toxic to a small surfaces, food contact segment of the population. surfaces, & water used in � Spices — especially those with food processing; limited strong flavors — often viewed as penetration. preservatives or disinfectants. Probably bacteriostatic, at best.
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