BIO2041: Lecture 10

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  • Created on: 14-05-14 23:34
when are cereals usually harvested?
when Aw < 0.6- inhibits microbial growth
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what happens if stored cereals get moisture?
can get mould (mycelial fungi) growing – Aspergillus, Penicillium, Rhizopus
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what is the usual Aw of bread?
0.75-0.9- prevents bacterial growth usually
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when is bread usually contaminated?
post-baking as heat would kill them
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what characterises bread spoilage?
visual spoilage – white, green, yellow areas of growth, Ropiness due to germination and growth of B. subtilis spores
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which enzymes are possibly produced during spoilage that degrade starch and protein?
amylases and proteases- bread becomes soft, stringy, brown mass
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what is the Aw of meat?
high Aw of around 0.99
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after animal slaughter what happens to glucose?
metabolised to lactic acid in muscle to give low [glucose] and pH ~5.5
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what happens if the animal is stressed prior to slaughter?
little glucose available so less lactic acid so higher pH (DFD dry, firm, dark meat)
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does the surface of meat have high or low redox potential?
high redox potential
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where can meat be contaminated from?
surface or animal, from intestine and from air
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which organisms can infest meat if it is kept refrigerated?
psychotrophic aerobes and facultative anaerobes- pseudonomas sp particularly common
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what do these organisms metabolise first- glucose or amino acids?
glucose first then amino acids- Amino acid metabolism results in production of unpleasant smelling compounds – methyl sulphides, esters
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what does DFD stand for in relation to meat?
dry, firm, and dark
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which microorganisms grow on DFD meat?
Acinetobacter and Morexella- produce odour more rapidly as utilise amino acids from early stage- odour is followed by slime
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high heat processed meats are 'commercially sterile', what does this mean?
Only contaminated with spores of thermophilic organisms- remain dormant unless stored at high tempeatures
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low heat processed uncured meats are usually cooked for longer periods at what temperatures?
60-65 ºC
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spores of which organisms could survive this cooking?
Those of Bacillus, Clostridium, possibly a few heat resistant lactobacilli, enterococci, and micrococci
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which organisms might spoil meat in vacuum packed products?
1) Lactobacillus 2) Leuconostoc 3) Clostridium 4) Proteus 5) Hafnia
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how are low heat cured meats usually packaged?
MAP- Modified atmosphere packaging
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at ambient temperature which microorganisms are likely to affect low heat processed cured meats?
lactobacilli, micrococci, leuconostocs
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what do egg shells and membrane not prevent?
entry of organisms
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which compounds do eggs contain that inhibit microbial growth?
lysozyme, avidin
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what are the most common spoilage organisms of eggs?
G-ve motile rods- Pseudomonas, Proteus, Alcaligenes, Serratia
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what visual effect does spoilage have on eggs?
coloration of yolk- green, black, and red- called rot
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what is the major organism in fish spoilage?
Pseudomonas sp.
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what kind of organisms spoil fish?
G-ve aerobic and facultative anaerobic rods- (Flavobacterium, Alcaligenes, Vibrio)
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what do these organisms metabolise to produce foul smells?
metabolise non-protein nitrogen resulting in production of putrescine, cadaverine, H2S- foul smelling compounds
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what do they produce to release more amino acids?
Proteases- destroy fish texture- growth results in slime
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what pH do fresh fruit tend to have?
below 4.5
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what causes spoilage in fresh fruits?
yeasts, moulds and lactic acid bacteria
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which organisms cause rot?
Penicillium, Aspergillus, Botrytis and Rhizopus
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which organisms can ferment some fruits?
Saccharomyces, Torulopsis and Candida
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if there is high moisture during storage of fresh fruit, which organisms will grow?
moulds- Penicillium, Botrytis, Aspergillus
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which bacteria are involved in spoilage of fresh fruit?
Pseudomonas, Erwinia, Bacillus and Clostridium- Cause change in colour, loss of texture, off-odour. Called rots
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in canned products, what can under processing allow to grow?
spores of mesophilic anaerobes and facultative anaerobes- e.g. clostridium sporogenes
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what effects do these organisms cause (in canned products)?
putrid odour and production of CO2 and H2 gases – can bloats
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if the pH of the product is high which bacteria can grow?
thermophilic bacteria (Bacillus stearothermophilus) can survive and grow if temperature of storage above 37 ºC
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what is a major cause of spoilage in the canning process?
leakage after processing
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what can happen if there are flaws in the seam of the can?
Cans cooled in water and negative pressure inside can will **** cooling water in,
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what happens if stored cereals get moisture?


can get mould (mycelial fungi) growing – Aspergillus, Penicillium, Rhizopus

Card 3


what is the usual Aw of bread?


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Card 4


when is bread usually contaminated?


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Card 5


what characterises bread spoilage?


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