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Food Micro > Microorganisms and Spoilage > Flashcards

Microorganisms and Spoilage Flashcards

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1
Q

Spoiled Food

A

Damage or injuries that make food undesirable for human consumption.

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2
Q

Spoiled food can be the result of:

A
  1. insect damage
  2. physical injury
  3. enzymatic degradation
  4. microbial activity
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3
Q

Basic types of Food Spoilage

A

Appearance
Textural Changes
Changes in taste and Odor

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4
Q

Appearance of food spoilage :

A
  1. microbial growth
    mycelia or colonies visible on surface,
  2. Changes in food color due to heme or chlorophyll breakdown. colony pigments growth of mycelia, etc.
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5
Q

Textural changes in food spoilage:

A

1)Slime formation
due primarily to surface accumulation of microbial cells
also be a manifestation of tissue degradation
2)Tissue softening due to enzymatic degradation (e.g. soft rot in veggies)

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6
Q

Changes in taste and odor in food spoilage:

A

Development of :
nitrogenous compounds (ammonia, amines, etc)
sulfides
organic acids

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7
Q

The numbers and types of MO in a food are largely determined by:

A
  1. Environment from which the food was obtained.
  2. Microbiological quality of the food in its raw or unprocessed state (intrinsic factors).
  3. Handling and processing sanitation.
  4. Effectiveness of packaging, handling and storage conditions in restricting microbial growth (extrinsic factors).
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8
Q

Chemical composition of Fresh meats:

A

75% water
18% protein
3% fat
1% ash, traces of CHO, vitamins, etc.

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9
Q

The microflora of fresh (whole) meat is composed primarily of

A
  1. Gram negative aerobic rods such as Pseudomonas, Acinetobacter and Moraxella.
  2. Bacillus and clostridia (e.g. C. perfringens) are also common on all types of meat.
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10
Q

What is among the most perishable of foods?

A

Fresh meats

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11
Q

What is the single most important control factor for meat spoilage?

A

Storage temperature

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12
Q

Molds can’t grow on meat below

A

5 degrees C

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13
Q

Fresh cut meats in the fridge at high humidity usually undergo bacterial spoilage by:

A

Gram negative aerobes like Pseudomonas, Acinetobacter and Moraxella spp.
(extrinsic and intrinsic factors of ground beef favor these bacteria highly)

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14
Q

Bacterial count when meat spoilage is characterized by off odors and slime:

A

10^7 CFU/cm^2

10^8 is when organoleptic quality clearly reveals spoilage.

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15
Q

Meat spoilage occurs without any significant breakdown of the primary protein structure. Why?

A

spoilage bacteria utilize glucose, free amino acids or other simple nitrogenous compounds to attain population of about 10^8 CFU/cm^2, at which point the organoleptic quality of the meat will clearly reveal it is spoiled.

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16
Q

Why does ground meat have higher MO surface loads?

A
  • greater surface area which gives microbes better access to the food and also traps air to favor the growth of gram-negative, aerobic bacteria like Pseudomonas spp.
  • every handling or processing (storage utensils, cutting knives, grinders) step can contribute additional contamination to the final product.
  • one heavily contaminated piece (e.g. a lymph node) can contaminate an entire lot when they are ground together
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17
Q

Use of soy protein extenders and mechanically deboned meat:

A

does not change the microflora significantly but does raise the pH of meat which leads to more rapid spoilage

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18
Q

Vacuum Packed meats

A
  • 80% of beef leaves packing plant in vacuum package.
  • not all O2 is removed during packaging but residual is consumed by respiration of aerobic MO and the tissue itself
  • results in increased CO2 levels and thus get a longer shelf life.
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19
Q

Impermeable films used in vacuum packing:

A
  1. CO2 levels are higher

2. Eh lower

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20
Q

Microflora in Vacuum packing:

A

The microflora shifts from predominantly G- aerobes to G+ anaerobes and microaerophilic lactic acid bacteria (LAB) like Lactobacillus, Carnobacterium and Leuconostoc.

  • if nitrites have been added to the vacuum packaged meat (e.g. to inhibit C. botulinum in hams, bacon), LAB domination is even more pronounced

In general, vacuum packaged meats are considered very safe foods and free from most pathogenic species of bacteria.
-with the possible exception of S. aureus and Y. enterocolitica

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21
Q

Spoilage in vacuum packaged meats is manifest by:

A
  1. Slime development
  2. Greening caused by microbial production H2O2 or H2S.
    H2O2 production in meat has been associated with several types of lactic acid
    bacteria (primarily Lactobacillus) The oxidant (H2O2) reacts with nitrosohemochrome (cured meat color cmpd) to form a green porphyrin compound.
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22
Q

H2S greening occurs in

A

fresh meats that have been vacuum packaged and stored between 1-5oC.

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23
Q

H2S reacts with myoglobin to form

A

sulphmyoglobin in meats with a pH above 6.0.

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24
Q

H2S is produced by:

A
  1. Shewanella putrefaciens and Pseudomonas spp. (when O2- permeable films are used).
  2. Some lactobacilli (when O2- impermeable films are used).
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25
Q

Off odors (in vacuumed packaged food) which result from

A
  1. the release of short chain fatty acids
  2. the production of volatile compounds like acetoin, diacetyl and H2S (and many other compounds, depending on the dominant spoilage bacterium)
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26
Q

The type of spoilage bacteria that will dominate is influenced by several factors that include:

A
  1. Is the meat product raw or cooked?
    Cooked products have a higher pH (>6.0) which may allow growth of G- facultative anaerobic pathogens like Yersinia enterocolitica.
    Raw products have a pH of about 5.6 which favors lactic acid bacteria, esp. Lactobacillus, Carnobacterium, and Leuconostoc.
  2. Nitrite concentration in meat.
    High nitrite conc. favors lactic acid bacteria.
    Low nitrite levels may allow growth of Brochothrix thermosphacta (G+ rod, fac anaer, growth @ 0-30oC from pH 5.0-9.0 catalase+).
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27
Q

An important spoilage bacterium in anaerobically stored meats kept at low temperature, but is inhibited by nitrite.

A

B. thermosphacta

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28
Q

Most common cause of spoilage in processed meats:

A

yeasts and molds

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29
Q

Ways food spoilage in processed foods occur:

A

A) Slimy spoilage (buildup of yeasts, lactobacilli, enterococci or Brochothrix thermosphacta.)
B) Sour Spoilage from growth of lactic acid bacteria. Not harmful, but affects taste
C) greening due to H202 or H2S production.

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30
Q

Reasons Cured meats (bacon, hams) are resistant to spoilage:

A
  1. Use of nitrite/nitrate
    2. Smoking or brining of hams
    3. The high fat content (thus low aw) of bacon
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31
Q

Spoilage of cured meats often caused by:

A

molds from genera including: Aspergillus, Fusarium, Mucor, Penicillium, Rhizopus and Botrytis.

32
Q

When poultry is in the advanced stages of spoilage, the skin will often fluoresce under UV because so many ______ are present

A

fluorescent pseudomonads

33
Q

In poultry, off odors usually appear before ______ does

A

sliminess

34
Q

Visceral taint:

A

a condition manifest by off odors in the abdominal cavity of poultry

35
Q

Point to remember about poultry food spoilage:

A

During the initial stages of spoilage, the skin supports bacterial growth better than does the tissue (which remains essentially free of bacteria for some time). Thus, the skin can sometimes be removed to salvage the food

36
Q

Fish have high nitrogen content but no

A

carbohydrate

37
Q

The microbial quality of fish and shellfish is heavily influenced by the

A

quality of water from which they are harvested

38
Q

in general, ____ fish have lower MO loads than fresh fish

A

frozen

39
Q

Bacteria on fresh fish are concentrated on the

A

outer slime, gills and intestine.

40
Q

the most susceptible part of the fish to spoilage is

A

the gill region. Odors include ammonia, triethylamine and H2S to indicate spoilage

41
Q

Spoilage of crustaceans is similar to fish, but these products have some CHO and more free amino acids so spoilage occurs

A

more rapidly

42
Q

_______ have more CHO and less nitrogen than either fish or shelfish

A

mollusks

43
Q

Usual types of spoilage bacteria in Mollusks/ shellfish

A

Pseudomonas and Acinetobacter, Moraxella

44
Q

Chemical composition of vegetables:

A

88% water
8.6% CHO
1.9% protein
.3% fat
.84% minerals
also contain fat and water soluble vitamins and nucleic acids (<1%).
-pH of most veggies is around 6.0; within the growth range of many bacteria

45
Q

Types of MO that typically grown on vegetables:

A

molds, yeasts, bacteria

46
Q

what percent of all harvested fruits and vegetables is due to spoilage by MO

A

20%

47
Q

____ are of greater consequence in the spoilage of vegetables with intrinsic properties that support bacterial growth (favorable pH, Eh).

A

Bacteria

48
Q

Microflora of vegetables is primarily composed of:

A

G+ bacteria like lactic acid bacteria (e.g. leuconostocs, lactobacilli, streptococci.
Coryneforms and staphylococci (the latter coming from the hands of employees during processing.
3. Staphylococci are usually unable to proliferate but cross-contamination can introduce them into other foods where growth conditions are more favorable.

49
Q

One of the most common types of bacterial spoilage in veggies, soft rot, is caused by:

A

Erwinia carotovora and sometimes by Pseudomonas spp., which grow at 4oC

Softening can also be caused by endogenous enzymes

50
Q

FlavrSavr story:

A

a. polygalacturonase (PB); hydrolyzes a (1-4) glycosidic bond in pectin which leads to softening.
b. Calgene made antisense RNA to tomato pg, constructs soften slower and so can be harvested after they are ripe (better flavor).
c. First commercially avail. genetically engineered vegetable.

51
Q

Mold spoilage in Vegetables:

A

a. In vegetables where bacterial growth is not favored (e.g. low pH), molds are the principal spoilage agents.
b. Most molds must invade plant tissue through a surface wound such as a bruise or crack.
c. Spores are frequently deposited at these sites by insects like Drosophila melanogaster, the common fruit fly.
d. Other molds like Botrytis cinerea, which causes grey mole rot on a variety of vegetables, are able to penetrate fruit or vegetable skin on their own.

52
Q

The microflora of vegetables will reflect:

A

A. the sanitation of processing steps

B. the condition of the original raw product

53
Q

Sources of contamination in vegetables:

A
  1. Surface contamination – Soil, water, air, human pathogens from manure (night soil)
  2. Harvesting - hand picking vs. machines
    high damage if crop is ripe…harvest before ripe
    Geotrichium candidum – mold on harvestors
  3. Packaging: containers reused-sanitized
  4. Processing plant
  5. Markets – handling, cross-contamination
54
Q

Chemical composition of fruit:

A
  • 85% water
    - 13% CHO
    - 0.9% protein (a bit low on nitrogen sources)
    - 0.5% fat
    - 0.5% ash
    - trace amounts of vitamins, nucleotides, etc.
    - less water and more CHO than veggies
    - low pH (1.8-5.6)
55
Q

Types of MO that usually grow on fruit:

A

Molds and yeast

56
Q

Fruit spoilage:

A

Because these organisms grow faster than molds, yeast often initiate fruit spoilage. then molds finish the job by degrading complex polysaccharides in cell walls and rinds

57
Q

Specific spoilage organisms:

A
  1. Blue rot – Penicillium, fruits
  2. Downy mildews – Phytophora, large masses of mycellium (grapes)
    1. Black rot – Aspergillus, onions
    2. Sour rot – Geotrichum candidum
58
Q

Raw milk flora:

A

a. All MO found on the cow hide (which incl. soil and fecal bacteria), udder, and milking utensils
b. Can include G-, G+, yeasts and molds.
When properly handled and stored, the flora of pasteurized milk is primarily G+ bacteria.
Psychrotropic pseudomonads are common in bulk stored raw milk
-produce heat stable enzymes that can reduce milk quality and shelf life

59
Q

pastuerization kills

A

most G- (incl. Pseudo.), yeasts and molds

-some G- enzymes, thermotolerant G+ bacteria and spores survive

60
Q

pastuerized fluid milk spoiled by:

A

a variety of yeasts and molds

61
Q

Milk spoilage:

A

a. In the past, milk was usually soured by LAB such as enterococci, lactococci, or lactobacilli, which dropped the pH to 4.5 where milk proteins coagulate (curdling).
b. Today, milk is more frequently spoiled by aerobic sporeformers such as Bacillus, whose proteolytic enzymes cause curdling.
c. Molds may grow on the surface of spoiled milk, but the product is usually discarded before this occurs.

62
Q

Butter; high lipid content and low aw make it more susceptible to surface ___ growth than to bacterial spoilage.

A

mold

63
Q

“surface taint”

A

putrid smell, caused by the production of organic acids (esp. isovaleric) from P. putrefaciens

64
Q

rancidity of butter due to:

A

butterfat lypolysis caused by P. fragi

65
Q

most common cottage cheese spoilage:

A

slimy curd caused by: Alcaligenes spp (g- rod in soil)

Penicillum, Mucor and other fungi also grow well on cottage cheese and impart stale or yeasty flavors

66
Q

Ripened cheeses:

A

(1) low aw, (2) low pH and (3) high salt inhibit most spoilage microorganisms except surface mold growth.

67
Q

Spores of C. butyricum, C. sporogenes and others can germinate in cheeses (e.g. Swiss) with intrinsic properties that are less inhibitory (e.g. lower salt, higher pH). These organisms may:

A

metabolize citrate, lactose, pyruvate or lactic acid and produce butyrate or acetate plus CO2 or H2 gas which “blows” the cheese

68
Q

intrinsic factors that keep eggs from spoiling:

A

shell w/ associated membranes, lysozyme, conalbumin, high pH

69
Q

Cereal and baked goods generally spoiled by:

A

Molds like Rhizopus stolonifer

70
Q

refrigerated frozen bread dough can be spoiled by

A

lactic acid bacteria

71
Q

Beer and wine can be spoiled by:

A

yeasts and bacteria (LAB) The anaerobic bacterium Megasphaera cerevisiae can also spoil beer by producing isovaleric acid and H2S.

72
Q

Spoilage in packaged beer is often due to growth of the yeast

A

Saccharomyces diastaticus, which grows on dextrins that brewers yeast cannot utilize

73
Q

most important yeast in wine spoilage:

A

candida valida (shown as turbidity, off flavors and odors)

74
Q

Wines can also be spoiled by LAB which are able to convert malic acid to

A

lactic acid

75
Q

In mayonnaise or salad dressing, the first signs of spoilage are usually

A

off odors and emulsion separation

Food Micro (3 decks)

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