Microbial Ecology They don't know they of Foods are in food! - PDF document

A food is an ecosystem for microbes Microbial Ecology � They don't “know” they of Foods are in food! � Bacteria & molds may Dean O. Cliver multiply, survive, or die. A food is an ecosystem Pathogenic bacteria in food: for microbes potential “outcomes” � Viruses & parasites can only � Persistence: viable, numbers unchanged (lag or stationary persist or be inactivated (die, phase or sporulation) lose infectivity). � Growth (multiplication): rate � Most attention devoted to fates parameter (variable) based on of bacterial pathogens. doubling time Pathogenic bacteria in food: Growth curve biology potential “outcomes” � Spores & lag phase cells � Death: another rate parameter quiescent; adaptation to (cf. viable-nonculturable) environmental conditions = � Sporulation: another defense selecting needed enzymes (species) (activating appropriate genes) � Toxigenesis: growth is from broad bacterial repertoire. necessary, but possibly not sufficient

Growth curve biology Growth curve biology � Multiplying (doubling) cells are � Stationary phase may represent metabolically active, often quiescence or (more often) adapting; not all metabolically growth rate = death rate. active cells are multiplying. � Some injured cells appear dead � Stress causes adaptation or (“viable nonculturable”). injury. � Some dead cells autolyze. Bacteria in broth vs food Research vs real food � Broth: “planktonic cells” � Food contaminants (water, � Bacteria tend to aggregate, air, soil, raw material, feces) attach to surfaces, form have mixed microflora. colonies or biofilms � Food ecosystem may select � Foods = solid matrix, one organism microenvironments � Pathogens outnumbered Research vs real food Research vs real food � At high levels, bacteria signal � “Programmed” successions each other chemically � Genetic exchanges among (“consensus”) strains or species � Different species interact � Toxigenic agents (including competitively, but sometimes molds) grow under conditions beneficially that do not permit toxigenesis.

Major factors (interact) Temperatures for � Temperature � Nutrients Thermophiles available � E h � Minimum: 40–45°C � Physical � a w � Optimum: 55–75°C structure � pH (specific � Microflora � Maximum: 60–90°C cations & � Antimicrobial anions) agents Temperatures for Temperatures for Psychrophiles Mesophiles � Minimum: 5–15°C � Minimum: -5–+5°C � Optimum: 30–45°C � Optimum: 12–15°C � Maximum: 35–47°C � Maximum: 15–20°C Temperatures for Cold: liquid or solid water? Psychrotrophs � Freezing kills some cells, � Minimum: -5–+5°C frozen storage preserves � Optimum: 25–30°C � Psychrotrophs grow slowly in � Maximum: 30–35?°C refrigerated food (cf. handout)

Warm = near optimum? “Danger zone” depicted � Food spoilage promoted; test of sanitation � “Danger Zone”: 4–60 ° C (40– 140 ° F) or 5–57 ° C (41–135 ° F) � Rapid transition from hot to cold or cold to hot DANGER ZONE FOR NEUTRAL FOODS Averages of Aeromonas hydrophila, Bacillus cereus, E. coli O157:H7 , Hot—temps > max for Danger Zone Listeria monocytogenes, Salmonella spp. , Shigella spp. , Staphylococcus aureus, & Yersinia enterocolitica growth cause death � D value: time for decimal °F 32 48 64 80 96 112 128 140 40 2 reduction at t ° C; organisms (T 1/2 ) -1 [min] (T 2 ) -1 [h] 1.5 1 are in log death phase 0.5 � z value: temperature change 0 0 10 20 30 40 50 60 °C -0.5 ( ° C) to reduce the D value 10- -1 -1.5 fold z value example D value example 8 8 8 100 z (°C) = 15 D t°C = 5 min 7 10 NUMBER 6 6 D 1 0.1 5 LOG 0.01 70 80 90 100 0 5 10 15 20 TEMPERATURE ( o C) HEATED (MIN) AT t o C

Heat Heat � “Heat-shock” proteins aid � Cooking, blanching, adaptation; some produced in pasteurization not for response to other stresses. “commercial sterility” � Mesophiles or psychrotrophs— � Cells in log phase are infectious agents must be able more heat-sensitive to multiply at body temperature. Tyndallization: boiling Eh on 3 days � Aerobic (>0 mV), � Day 1: vegetative cells killed, microaerophiles, facultative, spores heat-shocked anaerobic (<0 mV) � Day 2: veg cells from spores � “Strict” aerobes E h > 0 mV, killed, last spores heat-shocked “obligate” anaerobes E h < -300 � Day 3: vegetative cells from final mV spores killed; endpoint: sterility Eh Eh � Facultative organisms often use � E h hard to measure in foods available energy more efficiently � Live foods metabolize or bind under aerobic conditions oxygen � C. perfringens may not start � Packaging, modified atmosphere growing under aerobic � Molds generally strict aerobes conditions, but is not inhibited by oxygen once growth begins.

Approximate a w of some foods Water activity—" a w " � Fresh fruit or vegetables >0.97 Water available for microbial � Fresh poultry or fish >0.98 growth, based on water present � Fresh meats >0.95 and on binding by solutes such � Juices, fruit & vegetable 0.97 as salt or sugar; equilibrium � Cheese, most types >0.91 relative humidity ÷ 100; range � Honey 0.54–0.75 is 0 to 1.00 � Cereals 0.10–0.20 Minimum a w for some pH: hydrogen-ion potential foodborne pathogens � Foods range from pH 7 � Salmonella 0.93 downward. � C. botulinum 0.93 � Acidification inhibits � Staphylococcus aureus 0.85 spoilage & growth of many pathogens. � (Most yeasts) 0.88 � “Low acid” (bot) pH > 4.6 � Most molds 0.75 Important minimum pH values pH values of some foods for growth of microbes in foods � Egg white 7.6–9.5 � Milk 6.3–6.8 � Clostridium botulinum 4.8 – 5.0 � Chicken 5.5–6.4 � Salmonella (most types) 4.5 – 5.0 � Beef 5.3–6.2 � Cheeses, most 5.0–6.1 � Staphylococcus aureus 4.0 – 4.7 � Tomatoes 3.7–4.9 � Yeasts & molds 1.5 – 3.5 � Apples 2.9–3.5

pH Nutrients available � “Organic” acids (e.g., lactic, � C & N sources required, acetic, etc.) more effective sometimes “growth factors” antimicrobials than mineral acids � Foods generally good C & N sources � Most effective undissociated; at a given pH, molar quantity of � Other factors, then nutrients organic acid >> than that of a decide which organism mineral acid. predominates Physical structure Physical structure � Bacteria grow on surfaces � If water & solutes cannot diffuse when they can. freely, local variations in E h , a w , and pH are highly possible. � Some surfaces (melon rind, � High viscosity or strongly eggshell) limit access to cellular structure can greatly nutrients. limit heat transfer (both heating � Food matrix: molds often and cooling) in foods. penetrate better than bacteria. Microflora Microflora � Bacteria may produce acetic, � Bacteria in foods: variety & lactic, and other acids as competition fermentation products. � Microbial growth may � Some produce bacteriocins— lower E h & pH; molds use proteins that have a highly- organic acids as carbon specific lethal effect on closely related organisms. sources & raise pH.

“Programmed succession” Competing organisms � Milk: rapid lactic acid producers (lactococci), then � Staphylococcus aureus � Slower acid producers � Clostridium botulinum (lactobacilli) that tolerate lower pH's, then � Acid-stable putrefactive (proteolytic) bacteria and finally, � Molds (metabolite tolerance). Antimicrobials: preservatives Antimicrobials: preservatives � Materials added specifically to � CO 2 & SO 2 long used in foods; inhibit microbial growth SO 2 is highly toxic to a small segment of the population. � Nitrite for “curing” meats, vs C. botulinum . � Spices — especially those with strong flavors — often viewed as � Sorbates, benzoates, & other preservatives or disinfectants. salts of organic acids Probably bacteriostatic, at best. bacteriostatic, not bactericidal Antimicrobials: radiation Antimicrobials: radiation � Surface efficiency enhanced � UV widely applicable to by pulsed laser application decontamination of food (some pulsed laser surfaces, food contact applications use visible light). surfaces, & water used in food processing; limited � Ionizing radiation discussed penetration. earlier in course.