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BIOL 345 Set 1 Ecology Of Microbes.pdf

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BIOL 345
Barbara Butler

ME1 Microbial ecology of foods: crop, animal which/how many microorganisms? transpor t processing, handling do numbers increase/decrease? storage, does community makeup transport change? spoilage implications? retail, storage economic loss? transpor t do numbers increase/decrease? home, storage, other processing community change? (cooking) spoilage implications? infection/poisoning implications? consumption ME2 microbial ecology: how a microorganism interacts with physico-chemical and biological characteristics of its environment (foodstuff) or the study of the interactions between the chemical, physical and structural aspects of a niche and the composition of its specific microbial population (International Commission on Microbiological Specifications for Foods (ICMSF), 1980) organisms of interest: bacteria: gram positive, gram negative; aerobes and anaerobes fungi: filamentous (molds), single-celled (yeasts); almost always aerobes viruses: e.g., hepatitis A, noroviruses parasites: worms, protozoa Microbial ecology of foods: ME3 • the food environment – what is it like? ~30,000 items in typical grocery store • why do certain microorganisms grow (or not grow) in particular food environments? • when growth occurs, what do the microorganisms do? (activity) Food quality safety Loading... spoilage microorganisms microbial pathogens shelf life of food health risk of food NOW LATER in course ME3a examples of food spoilage by fungi recall from Biol 241 ME3b Bacterial growth in batch culture lag, exponential phases are critical in spoilage Loading... • the food is analogous to the “flask of growth medium” Factors affecting microbial growth in food : ME4 intrinsic parameters: those that are an inherent part of the tissues • nutrient content • water activity (aw) • pH and buffering capacity • oxidation-reduction potential (Eh) • antimicrobial constituents • biological structures extrinsic parameters: those of the storage environment that affect both the foods and their microorganisms • temperature of storage • relative humidity of environment • presence and concentration of gases implicit parameters: properties and interactions of the microorganisms present • specific growth rates • mutualism, antagonism, commensalism • cell-cell communication (quorum sensing, signal transduction) processing factors: slicing, washing, pasteurization, etc. ME5 (i) nutrient content • major needs for growth: • water • source of C and source of energy → vast majority of microbes of • source of N concern in foods are • vitamins and related growth factors chemoorganotrophic, • minerals heterotrophic general requirements: molds < yeasts < gram negative bacteria < gram positive bacteria least fastidious most fastidious foods differ in relative proportions of what they provide: meats: rich in protein, lipids, minerals, vitamins, poor in carbohydrates plant-derived foods: rich in carbohydrates, poor in proteins, minerals, some vitamins milk, many prepared foods: good source of all 5 groups • simpler compounds (sugars, amino acids) generally attacked before more complex compounds (polysaccharides, proteins, fats) carbon, ME6 major carbohydrates (natural or added as ingredients) monosaccharides: hexoses: glucose, fructose, mannose, galactose pentoses: xylose, arabinose, ribose, ribulose, xylulose disaccharides: lactose (galactose+ glucose) sucrose (fructose + glucose) maltose (glucose + glucose) oligosaccharides: raffinose (glucose + fructose + galactose) stachyose (glucose + fructose + galactose + galactose) polysaccharides: glycogen, cellulose, starch, dextrans (glucose units) inulin (fructose units) hemicellulose (xylose, galactose, mannose) pectins gums, mucilages (after Ray & Bhunia, 2008. Fundamental Food Microbiology, 4th ed) ME7 lipids (fats): attacked by relatively small number of microbes in foods than carbohydrates • extracellular lipases, lipid oxidases • generally higher in animal-derived foods than plant-derived proteins: e.g., albumins (egg), globulins (milk, myoglobin, hemoglobin), muscle collagens • simple, complex (e.g., glycoproteins, metalloproteins) • proteases, peptidases • resultant amino acids metabolizable by wider range of microbes amino acids = important N source in foods other N-containing compounds (e.g., nucleotides): used by some microorganisms B vitamins: gram positives may require one or more if unable to synthesize their own • meats supply more than fruits minerals: P, Ca, Mg, Fe, S, Mn, K • most foods supply adequate amounts for microbial growth (ii) water activity (aw) ME8 • a measure of free or available water • aw x 100 = equilibrium relative humidity (ERH) aw = ρ/ρ0, where ρ is vapour pressure of solution {food}, ρ0 is vapour pressure of the solvent {water; by definition ρ0 =1 for pure H2O} unavailable water: • water of crystallization • ice • water dispensed in a colloid gel • water “tied up” by solutes or ions in solution bell RH of airspace = 90% jar aw = 0.90 initiall at y equilibrium ME8a Loading... Aqualab series 3 water activity meter (Decagon Devices) • works by measuring dew point; relating ERH to aw (Fig 2.4, Montville et al, 2012; Fig 2.6 2nd ed) ME9 (Fig 25.2 MMK 3rd ed; Fig 2.6 2nd ed; also see Tables 2.6, 2.7 MMK, 3rd ed) relationship between water activity and concentration of salt solution: ME10 water activity NaCl (%, weight/vol) 0.995 0.9 0.99 1.7 0.98 3.5 0.96 7 0.94 10 0.92 13 0.90 16 0.88 19 0.86 22 (modified from Table 3-4, p. 41, Jay, 2000; see also MMK, 3rd ed, Table 2.5) halotolerant microorganisms: can survive in salt concentrations greater than 10% • e.g., Bacillus, Staphylococcus, Vibrio, Corynebacterium ME11 water content versus water activity • water (moisture) content does not give indication of how available that water may be for example: food water content aw grains 4–9 % 0.7 rice, legume seeds 12–15% 0.7 dried fruits 18–25% 0.7 M&M 2nd ed)ble 25.2 MMK 3rd ed; Table 2.6, ME12 (iii) effect of food pH • most microorganisms of importance in foods are neutrophiles; optimal pH for growth ~pH 6.6-7.5 mold s approx pH range yeast s bacteri a 0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 p H • foods tend to be neutral or acidic; rarely basic • acidic foods are typically spoiled by fungi • neutral foods often susceptible to bacterial spoilage (faster growth rate); also fungal ME13 food-borne bacterium approx pH range for growth Alicyclobacillus spp. 2 – 6 Salmonella spp. 3.5 – 9.5 Acetobacter spp. 4.0 – 9.2 Listeria monocytogenes 4.2 – 9.6 Yersinia enterocolitica 4.2 – 9.0 Escherichia coli 4.5 – 9.0 Clostridium botulinum 4.5 – 8.5 Bacillus cereus 5.0 – 9.5 Campylobacter spp. 5.0 – 9.0 Shigella spp. 5 0 – 9.2 Vibrio parahaemolyticus 5.0 – 11.0 Clostridium perfringens 5.0 – 8.3 (modified from Jay, 2005) aciduric microorganisms: can survive at low pH (~below pH 4) e.g., Lactobacillus, Lactococcus, Enterococcus, Streptococcus (all examples of lactic acid bacteria) foods classed by pH: ME14 low acid pH greater than 5.3 vegetables, meats, milk, cereals medium acid pH 3.7-5.3 some vegetables, many cheeses high acid pH less than 3.7 lemons, cranberries, sauerkraut • medium/high acid foods are more microbiologically stable • buffering capacity: due to presence of compounds able to resist pH change • vegetables - low buffering capacity • milk, meats - higher buffering capacity • microorganisms have some ability to modify environmental pH • e.g., ami
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