BIO2041: Lecture 5

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  • Created by: LMoney
  • Created on: 14-05-14 17:14
which organisms does chill storage prevent the growth of?
mesophiles and thermophiles- doesn't always kill
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which organisms does it reduce the growth rate of but not stop growing?
Reduces but does not prevent growth of psychrotrophs and psychrophiles
2 of 50
what decrease in temperature cuts enzyme activity in half?
10C- Growth rate similarly affected but may be greater than 2X effect at temperatures close to freezing
3 of 50
what is the other factor influences growth at low T?
composition of membrane phospholipids- Membranes change from a fluid matrix to a solid gel at low temperatures affecting ability to transport nutrients
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effectiveness of chill storage depends on what factors?
1) Infective load of organisms at start of process 2) How rapidly food is chilled 3) How low the storage temperature is 4) Fluctuations in temperature
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when spoilage eventually occurs, what kind of microbes will dominate?
G-ve rods
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when is storage classified at freezing?
below -18C
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what happens when food is frozen?
1) As food is cooled slowly below –2 ºC free water in the food starts to freeze 2) Concentrates solutes in unfrozen water 3) Decreases Aw Water molecules move out of microbial cells decreasing Aw within the cells Eventually get plasmolysis and death
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what happens if freezing is very rapid?
avoid these changes developing and organisms little damaged but no growth- Will grow again when food is thawed
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what happens with an Intermediate rate of freezing?
injures cells due to formation of ice crystals from free water inside cell, Ice damages membranes and DNA structure, Increases H+ concentration inside and outside cells, Damage to intracellular compounds that are essential for growth and survival
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what can happen as a result of many cells dying during freezing?
Food can be spoiled due to release of enzymes from killed/damaged cells
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can injured cells survive thawing?
yes, if sufficient nutrient available rapidly to allow repair- Frozen food is not sterile
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Microorganisms cannot grow at a Aw less than about what?
0.60 and this for very specific types
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which are least tolerant to low Aw, yeast, moulds or bacteria?
Bacteria are less tolerant (Aw>0.9) than yeasts (Aw >0.85) than moulds (Aw >0.8)
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what processes reduce Aw?
1) Dehydration 2) Drying 3) Freeze-drying (lyophilisation) 4) Smoking 5) Intermediate Moisture Foods (IMF) a) Salting b) Addition of sugar or alcohol 6) Freezing
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describe dehydration?
1) Water removed by heat of the sun 2) Slow process, most used in hot climates 3) Fruits, vegetables, meat, fish preserved this way 4) Risk of microbial growth during dehydration process
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describe drying?
1) Mechanical process which can be very rapid 2) Several processes including spray-drying 3) Used for preservation of many types of products – meat, vegetables, fruits, milk, coffee 4) Spores usually survive 5) heat can kill injure some cells
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describe freeze drying?
1) Food is frozen then exposed to a strong vacuum 2) Water removed by sublimation 3) Food quality least affected than for other drying processes
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describe smoking
1) Meat or fish exposed to low heat and smoke simultaneously 2) Low heat reduces Aw and may have a direct temperature effect on cells 3) Smoke often contains antimicrobial substances
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what is the Aw of Intermediate Moisture Foods?
Aw 0.85-0.61
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what temperature are IMFs stored at?
Can be stored at ambient temperature- Limited types of organisms can still grow
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what are some examples of IMFs?
Some traditional foods – salami, semidry and dry sausages, dried fruits, jams, honey
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addition of what contributes to making IMFs?
salt or sugar
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few bacteria will grow below what level?
below pH 4.5-5.0
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is the effect of pH or temperature more predictable?
Effect of pH not as predictable as effect of temperature
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the effect depends not only on pH but what other factor?
the dissociation of the acid at that pH
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which gives a greater effect- more dissociated or less dissociated acid?
The less dissociated gives the greatest effect
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what happens to undissociated acid?
Undissociated acid transported into cell where it dissociates
28 of 50
what are 4 organic acids and which are most potent?
29 of 50
how can acids be involved with foodstuff?
1) may be present naturally in food 2) may be component of food production 3) may be added as preservative
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what is acetic acid used in?
used as vinegar in pickles and sauces, most effective against bacteria
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what is propionic acid used?
bread, bakery products. Little effect against yeasts
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what is lactic acid used in?
carbonated drinks, low heat-processed meat, pickled vegetables. Effective against bacteria, little effect against yeasts and moulds
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what is citric acid used in?
jams, jellies, baking products, canned vegetables. Antibacterial only but less effective than lactic acid. Mechanism may be by chelating divalent cations rather than/as well as pH
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modified atmosphere prevents growth of what?
35 of 50
what is ROP?
reduced oxygen packaging- food is surrounded by an atmosphere containing little or no oxygen
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what can still grow in ROP packaged goods?
Cl. botulinum
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what is controlled atmosphere packaging (CAP)?
atmosphere in a storage facility continually monitored and regulated – long term store of fruit or vegetables
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what o2 and Co2 conditions are there in CAP conditions?
O2 2-5%; CO2 8-10%
39 of 50
what is modified atmosphere packaging (MAP)?
packaging of a product in an atmosphere that has had a one-time modification of gaseous composition so that it is different from air- food enclosed inside a high gas-barrier packaging material
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what % is CO2 at in MAP conditions?
CO2 most often used at 20-60%
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what is vacuum packing?
removal of almost all air from the packaging to leave a near-perfect vacuum then sealing
42 of 50
when irradiating food what kind of rays from what metal is used?
gamma (y) rays from 60Co
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what effect does radiation have on bacteria?
Damages bacterial DNA, can delay ripening of fruit but does not increase background radioactivity of food
44 of 50
what is the accepted dosage of radiation?
10 gGy – no toxicological hazard, no special nutrient or microbiological problems
45 of 50
what are the traditional curing agents for meat?
Nitrate, nitrite- mechanism not fully understood but inhibits growth and toxin production by Cl botulinum
46 of 50
what is the drawback of using nitrates and nitrites as preservatives?
Can result in formation of nitrosamines - carcinogenic
47 of 50
which substances control microorganisms in soft fruits, fruit juices, wines, sausages?
Sulphite, sulphur dioxide- More effective against yeasts and moulds than bacteria
48 of 50
what are some other examples of preservatives?
H2O2, ethylene oxide, EDTA
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Card 2


which organisms does it reduce the growth rate of but not stop growing?


Reduces but does not prevent growth of psychrotrophs and psychrophiles

Card 3


what decrease in temperature cuts enzyme activity in half?


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Card 4


what is the other factor influences growth at low T?


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Card 5


effectiveness of chill storage depends on what factors?


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