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BIOL 345 Set 3 Spoilage.pdf

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BIOL 345
Barbara Butler

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SP1 microbial food spoilage: needs… • microorganisms on the food item, from 1 or more sources • food environment favouring growth of 1 or more types • storage conditions (temperature) enabling multiplication of 1 or more types • storage time sufficient to attain high numbers necessary to effect detectable change(s) in food 103 /g or /cm2 or /mL microbial spoilage is 104 generally not observed at 105 these levels 106 107 some visible signs of spoilage; some odours 108 off-odours associated with meats, vegetables; slime 109 + definite structural changes in food at this stage influence of storage temperature, packaging on SP2 microbial population size: (MMK Fig (MMK Fig 2nd ed Fig 21.4; •aerobic bacteria in ground) • ground pork under aerobic & ed Fig 20.3) beef vacuum packaging at 0ºC influence of initial microbial load, storage SP2a temperature: 8 projected level for spoilage l 6 o 12°C ≤4°C g C F 12°C ≤4°C U 4 / Loading... m L 2 , g , cm 2 0 2 4 6 8 weeks in storage 4th ed.)ay & Bhunia, 2008. Fundamental Food Microbiology, SP3 spoilage of fresh fruits and vegetables: • alive for extended period after picking → no clear boundary between “plant pathogen” & “spoilage organism” derived damage for many plant products • spoilage may be active (plants pathogens), passive (opportunists) • gathering, transport, washing, sorting, processing affect degree of contamination (microbial load) • composition of fruit or vegetable influences likely spoilage pattern vegetables fruit ~88% water ~85% water ~8.6% carbohydrate ~13% carbohydrate ~2% protein ~0.9% protein ~0.3% fat ~0.5% fat ~0.8% ash ~0.5% ash not usually acidic pH generally more acid high Eh; lack poising capacity some poising capacity often contain accessory growth factors often deficient in B vitamins SP3a pH values of some fruits and vegetables: initial fungal spoilage initial bacterial spoilage fruits pH vegetables pH apples 2.9-3.3 asparagus 5.4-5.8 bananas 4.5-5.2 broccoli 5.2-6.5 grapes 3.4-4.5 carrots 4.9-6.3 Loading... grapefruit 3.0 lettuce 6.0-6.4 melons 6.2-6.7 runner beans 4.6 oranges 3.3-4.3 spinach 5.1-6.8 tomatoes 3.4-4.9 turnips 5.2-5.6 (data from Adams & Moss, 2008) vegetables: SP4 • microbes grow more rapidly in damaged or cut vegetables (ready-to-eat salads, sliced vegetables) bacterial soft rot: soft, mushy water-soaked appearance •pectinolytic gram negatives (Pectobacterium (or Erwinia), Pseudomonas, Xanthomonas); others • possess pectinesterases, polygalaturonidases •Pectobacterium will ferment rhamnose, cellobiose, arabinose, mannitol pectin • cellulolyticactivity – second major contributor to tissue destruction sliminess or souring: various saprophytic bacteria SP5 • there are many ~similar-looking spots, specks, wilts, blights, rots of vegetables, fruits; may be distinguishable on the basis of symptoms (by experts in the field anyway ) bacterial “spots”, “specks”, “cankers”: • Pseudomonas (e.g., P. syringae group – tomato speck), Xanthomonas (e.g., X. campestris – tomato, pepper spot, Clavibacter (e.g., C. michiganensis – tomato canker), Curtobacterium, Corynebacterium Agrobacterium tumefaciens : influence on plant hormone activity • increased synthesis of indoleacetic acid (IAA) → increased ethylene • carrots undergo senescence more rapidly examples of disease in tomato: SP5a bacterial spot bacterial spot bacterial speck these are both fungal bacterial canker early blight verticillium wilt (OMAFRAfactsheet; 069.htm ) SP6 fungal agents: • preharvest (e.g., flowers, lenticels during growth) or post-harvest invasion (e.g., stem, wounds or punctures) • defect often termed “rot”, described by spore colour, site of damage • colour change, off-odour, loss of texture examples: (M&M, 2nd ed Table 20.3, p. 290 lists more examples) gray mold rot: • Botrytis cinera (grey mycelium) • favoured by high humidity, warm temperatures sour (watery soft) rot: • Geotrichum candidum (and others) • widely distributed in soils, on decaying fruits, corn smut (Ustilago) vegetables (Raven et al., • spread by Drosophila melanogaster 1999) • fungus cannot enter through unbroken skin rhizopus soft rot: • Rhizopus stolonifer • small black dots of sporangia on surface of vegetable • tissue becomes soft, mushy Cladosporium spore masses (OMAF) SP7 fruits: • pH generally below that favouring bacterial growth → molds, yeasts are more usual dominant spoilage agents • Erwinia (Pectobacterium) rot of pears an exception (pH 3.8-4.6) • aciduric bacteria can be involved (lactics, Gluconobacter, Acetobacter) example yeasts: • Saccharomyces, Candida, Torulopsis, Hansenula: may affect apples, strawberries, citrus fruits, dates • fermentation of natural fruit sugars • often precede molds (grow faster) example molds: • Penicillium (blue mold rot), Aspergillus (black rot Rhizopus on or “smut”), Alternaria (alternaria rot), Botrytis, strawberries Rhizopus (Raven et al., • oxidize alcohols, decompose high molecular 1999) weight components (structural polysaccharides, rinds) spoilage of SP8 •rcereals dried post-harvest → low aw → xerophilic, -tolerant molds • e.g., wheat, rye, rice, maize, sorghum • various rots and blights • some contaminant fungi produce toxic metabolites (mycotoxins) “field fungi”: well-adapted to conditions on scenescing plant material in field • survive changing aw, temperature; optimum growth requires relatively high aw Loading... • may cause post-harvest spoilage if infected material stored at too high aw ergot (Claviceps purpurea) • e.g., Cladosporium, Alternaria, Fusarium (Raven et al., 1999) “storage fungi”: well-adapted to more constant conditions, lower aw • thrive in storage conditions • e.g., Penicillium, Aspergillus SP9 •processing tends to reduce microbial populations (e.g., milling →flour) • heat treatment (baking) inactivates most microorganisms in doughs (Frazier & Westhoff. 1988. Food Microbiology, 4th ed.) • ropy bread: stringy capsular material stretching between halves of loaf • usually mucoid variant of Bacillus subtilis or B. lichenformis • rare in commercial bread (preservatives present); may occur in home-baked loaves Spoilage of meats: SP10 • fresh meat is excellent culture medium → readily perishable • relatively low carbohydrate:high protein content → tends to favour non- fermenters “typical” adult mammalian muscle, post rigor mortis, predecomposition: water ~74-80% protein ~15-22% fat ~2.5-37% soluble nonprotein substances ~3.5% (incl. carbs (0-1.2%), vitamins, etc) without refrigeration: • spoilage likely due to bacteria of internal sources → anaerobic, proteolytic • clostridia, Enterobacteriaceae with refrigeration: • spoilage mainly a surface phenomenon (microbial biofilm develops) • reflective of external sources of microbes • molds (if dry), bacterial (if high humidity) microbial invasion of freshly-slaughtered carcass: SP11 • depends on slaughter-associated events • microbes from animal exterior, GI tract, knives, workers, plant air • spread via blood and lymph vessels, connective tissue interspaces inspection of sides of beef (OFAC Animal Agriculture Photo fresh meat spoilage under aerobic conditions: SP12 bacteria cause: • surface slime (Pseudomonas, Acinetobacter, Moraxella; Psychrobacter) • surface colouration (Serratia - red; Flavobacterium - yellow) • fat hydrolysis (Pseudomonas) • discolouration of tissue - red → green-brown or grey, due to H2O2 or H2S production (heterofermentative lactics (Leuconostoc, Lactobacillus), Shewanella) • “taints” - undesirable odour, taste (souring due to acids; musty flavour (actinomycetes)) yeasts: (e.g., Rhodotorula, Candida, Torulopsis) • may produce surface slime, coloured spots, lipolysis, taints molds cause: • stickiness • whiskers
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