Microorganisms
and Food Spoilage
Spoiled
Food
Basic Types of Food Spoilage
mycelia or colonies
visible on surface
development of cloudiness
in liquids
colony pigments, growth of mycelia, etc.
due primarily to surface accumulation of microbial
cells
also be a manifestation of tissue degradation
1.
nitrogenous compounds (ammonia, amines, etc.)
2.
sulfides
3.
organic acids
The
numbers and types of MO in a food are largely determined by:
Specific
Food Groups
Chemical composition:
-75%
water
-18%
protein
-3%
fat, 1% ash, traces of CHO, vitamins, etc.
1. Whole Meats:
The microflora of fresh
meat is composed primarily of:
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.
Although subsurface portions of meat are generally
sterile, some parts such as lymph nodes may be heavily contaminated.
Mechanical disruption of the tissue during
processing can distribute microorganisms from the meat surface throughout the product.
Fresh meats are among the most perishable foods.
Storage
temperature
is the single most important control factor for meat spoilage.
Handout - Sources of Contamination
Several genera of molds grow on the surface of meat and can cause spoilage, but
cannot grow on meat stored below 5oC.
Usually, fresh cut meats in the refrigerator at
high humidity undergo bacterial spoilage by:
Gram negative aerobes like Pseudomonas, Acinetobacter and Moraxella spp.
The intrinsic and extrinsic parameters of ground
beef favor these bacteria so strongly that they are almost exclusive
spoilage agents.
Meat spoilage is characterized by the appearance of
off odors and slime, which are manifest when surface loads exceed 107 CFU/cm2.
Handout - Figure 4.1 pg 81
The slime is due to the accumulation of bacterial
cells.
Interestingly, meat spoilage (including poultry and
fish) occurs without any significant breakdown of the primary protein
structure.
Instead, spoilage bacteria utilize glucose, free
amino acids or other simple nitrogenous compounds to attain population of about
108 CFU/cm2,
at which point the organoleptic quality of the meat will clearly reveal it is
spoiled.
Handout - Figure 4.2 pg 82 Total count vs spoilage
2. Ground Meats:
Same MO as whole meats,
but always have higher microbial loads.
Why?
- 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.
Use of: (a) soy protein
extenders (b) mechanically deboned meat (MDM)
-does not change the microflora significantly but
does raise the pH of meat which leads to more rapid spoilage
-ground beef pH=5.1-6.2,
add extenders raise it to 6.0-7.0)
3. Vacuum packaged meats
- 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.
Impermeable films used:
1.
CO2 levels are higher
2.
Eh lower
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
Spoilage in
vacuum packaged meats is manifest by:
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)
Handout - Meat Pigments
The oxidant (H2O2) reacts with nitrosohemochrome
(cured meat color cmpd) to form a green porphyrin compound.
H2S
greening occurs
in fresh meats that have been vacuum packaged and stored between 1-5oC.
H2S
reacts with myoglobin to
form sulphmyoglobin in meats with a pH above 6.0.
H2S
is produced by:
1.
Shewanella putrefaciens and Pseudomonas spp. (when O2-
permeable films are used).
2.
Some lactobacilli (when O2- impermeable films are used).
Off odors which result from:
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)
The type of spoilage bacteria that will
dominate is influenced by several factors that include:
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+).
B.
thermosphacta is an important spoilage bacterium in anaerobically stored meats kept
at low temperature, but the bacterium is inhibited by nitrite.
4. Processed meats (hot dogs, sausage and
luncheon meats)
These products are composed of a variety of blended
ingredients, any of which can contribute microorganisms to the food.
Yeasts and bacteria are the most common causes of
spoilage, which is usually manifest in 3
ways:
A. Slimy spoilage
Like other meat products, this occurs on the
surface and is caused by the buildup of cells of yeasts, lactobacilli,
enterococci or Brochothrix thermosphacta.
Washing the slime off with hot water can restore
the product quality.
B. Sour
spoilage.
Results from growth of lactic acid bacteria (which
originate from contaminated ingredients like milk solids) under the casing.
These organisms ferment lactose and other CHOs in
the product and produce organic acids.
Taste
is adversely affected but the product is not harmful if eaten.
C. Greening
due to H2O2 or H2S production.
Because greening indicates more extensive product
breakdown, I would not recommend eating green wieners.
Reasons Cured
meats (bacon, hams) are resistant to spoilage:
1. Use of nitrite/nitrate
2. Smoking or brining of hams
3. The high fat content (thus low aw) of bacon
Instead, spoilage of these products is often caused
by molds from several genera including Aspergillus,
Fusarium, Mucor, Penicillium, Rhizopus and Botrytis.
5. Poultry:
a. general trends are the same as
other fresh meats
b. similar microflora on fresh birds
c. whole birds have lower counts
than cut-up parts
d. additional processing steps add
to the microbial load
When poultry is in the advanced stages of spoilage,
the skin will often fluoresce under UV because so many fluorescent pseudomonads
are present.
Off odors generally appear before sliminess develops.
The same bacteria can produce visceral taint, a
condition manifest by off odors in the abdominal cavity or poultry.
Point to
remember:
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.
6. Fish:
a. Fish have high nitrogen content but
no carbohydrate.
b. The microbial quality of fish and especially shellfish
is heavily influenced by the quality of
the water from which they were harvested.
Unsanitized processing steps are principal
culprits in fish products with high microbial loads.
In general, frozen fish products have lower counts
than fresh products.
Bacteria on fresh fish are concentrated on the outer
slime, gills and intestine.
Spoilage of salt- and freshwater fish occurs in
similar ways; the most susceptible part of the fish to spoilage is the gill
region, and the best way to detect spoilage in fresh fish is to sniff this
area for off odors produced by Pseudomonas
and Acinetobacter-Moraxella bacteria.
The odors include ammonia, triethylamine, H2S
and other compounds.
If fish are not eviscerated quickly, bacteria will
move through the intestinal walls and invade the meat that lies next to the
abdominal cavity.
Spoilage of crustaceans
(shrimp, lobsters, crabs and crayfish) is similar, but these products have some
CHO (0.5%) and more free amino acids so spoilage can occur more rapidly.
Mollusks
(oysters, clams, mussels, squid and scallops) have more CHO (3-5%) and less
nitrogen than either fish or shellfish.
Microflora of mollusks can vary a great deal
depending on the quality of the water from which they were harvested.
Shellfish are filter feeders and can be expected to
contain almost any microorganism or virus that occurs in the water where they
were obtained.
If these products were taken from clean waters,
then the usual Pseudomonas and Acinetobacter-Moraxella types of
spoilage bacteria dominate.
B. Vegetables
Typical composition:
-88% water
-8.6 % CHO.
Includes readily available mono- and disaccharides like glucose and
maltose, as well as more complex oligosaccharides, which are available to fewer
types of microorganisms.
-1.9% protein
-0.3 % fat
-0.84 % 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
Vegetables are a good substrate for yeasts, molds
or bacteria
It is estimated that 20% of all harvested fruits
and vegetables for humans are lost to spoilage by these microorganisms.
Because bacteria grow more rapidly, they usually
out-compete fungi for readily available substrates in vegetables. As a result, bacteria are of greater
consequence in the spoilage of vegetables with intrinsic properties that
support bacterial growth (favorable pH, Eh).
Microflora of vegetables is primarily composed of:
3. Staphylococci are usually unable to proliferate but
cross-contamination can introduce them into other foods where growth conditions
are more favorable.
Soft rot
a. One of the most common types of bacterial spoilage.
b. caused by Erwinia
carotovora and sometimes by Pseudomonas
spp., which grow at 4oC
Softening can also be
caused by endogenous enzymes.
FlavrSavr story:
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.
Handout - Table 8.5 pp 155 –
(note the number of pseudomonads)
Mold spoilage
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.
The microflora of vegetables will reflect:
a. the sanitation of processing steps
b. the condition of the original raw
product
Soil-borne MO such as clostridia are common on raw
vegetables, and some species, like C.
botulinum, are of such great concern that they are the focus of processing
steps designed to destroy MO.
Sources
of Contamination
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
C. Fruits
Average composition
-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)
Handout - Fig. 7.1 Type pH of
vegetables and fruits
Like vegetables, fruits are nutrient rich
substrates but the pH of fruits does not favor bacterial growth. As a result, yeasts and molds are more important than bacteria in the spoilage
of fruits.
a. Several genera of yeasts can be
found on fruit.
b. Because these organisms grow faster than molds, yeast
often initiate fruit spoilage.
c. then molds finish the job by degrading complex
polysaccharides in cell walls and rinds.
Specific
Spoilage Organisms:
1. Blue rot
– Penicillium, fruits
2. Downy mildews – Phytophora, large masses of mycellium (grapes)
3. Black rot
– Aspergillus, onions
4. Sour rot
– Geotrichum candidum
D. Other Foods
1. Dairy Products - Milk is a very rich
medium
Raw milk flora may include:
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
Pasteurization
kills most G- (incl. Pseudo.), yeasts and molds
-some
G- enzymes, thermotolerant G+ bacteria and spores survive
-Psychrotropic
Bacillus spp. are also common in raw
milk
Pasteurized fluid milk – spoiled
by a variety of bacteria, yeasts and molds.
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.
Table 20.1 pp 281 – Defects of fluid milk
Butter; high lipid content and
low aw make it more susceptible to surface mold growth than to
bacterial spoilage.
Some pseudomonads can be a problem; Òsurface taintÓ
-putrid smell, caused by the production of organic acids (esp. isovaleric) from
P. putrefaciens
Rancidity due to butterfat lypolysis caused by P. fragi are common.
Cottage
cheese
can be spoiled by yeasts, molds and bacteria.
The most common bacterial spoilage is Òslimy curdÓ caused by Alcaligenes spp. (G- aerobic
rod bound in soil, water, and intestinal tract of vertebrates).
Like Campylobacter,
these species do not oxidize CHOs but instead use amino acids and TCA
intermediates.
Penicillum,
Mucor and
other fungi also grow well on cottage cheese and impart stale or yeasty
flavors.
Ripened
Cheeses –
(1) low aw, (2) low pH
and (3) high salt inhibit most spoilage microorganisms except surface mold
growth.
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 metabolize citrate, lactose,
pyruvate or lactic acid and produce butyrate or acetate plus CO2 or H2 gas which ÒblowsÓ the cheese.
Defects
Table 20.2 pp 282
Eggs
Eggs have several intrinsic parameters
which help to protect the nutrient-rich yolk from microbial attack. These include the shell and associated
membranes, as well as lysozyme, conalbumin, and a high pH (>9.0) in the
white. Freshly laid eggs are
generally sterile, but soon become contaminated with numerous genera of
bacteria.
Eventually, these MO will penetrate
the eggshell and spoilage will occur.
Pseudomonads are common spoilage agents, but molds
like Penicillium and Cladosporium sometimes grow in the air
sac and spoil the egg.
Cereal
and Bakery Goods
These products are characterized by a
low aw which, when stored properly under low humidity, restricts all
MO except molds. Rhizopus stolonifer is the common bread
mold, and other species from this genus spoil cereals and other baked goods.
-Refrigerated frozen dough products
have more water and can be spoiled by lactic acid bacteria.
Fermented
Foods and Beverages
The low pH or ethanol content of these
products does not allow growth of pathogens, but spoilage can occur.
Beer and wine (pH 4-5) can be spoiled
by yeasts and bacteria. Bacteria
involved are primarily lactic acid bacteria like lactobacilli and Pediococcus spp., and (under aerobic
conditions) acetic acid bacteria like Acetobacter
and Gluconobacter spp. Acetic acid bacteria convert ethanol to acetic acid in the
presence of oxygen.
The anaerobic bacterium Megasphaera cerevisiae can also spoil beer by producing isovaleric
acid and H2S.
Spoilage in packaged beer is often due to growth of the
yeast Saccharomyces diastaticus,
which grows on dextrins that brewers
yeast cannot utilize. Candida valida is the most important
spoilage yeast in wine. In either
case, spoilage by yeasts results in the development of turbidity, off flavors
and odors.
Wines
can also be spoiled by lactic acid bacteria which are able to convert malic
acid to lactic acid (malo-lactic
fermentation). This reduces
the acidity of the wine and adversely affect wine flavor. In some areas (e.g. Northwest), wine
grapes have too much malic acid so this fermentation is deliberately used to
reduce the acidity of grape juice that will be used for wine.
Yeasts, molds and lactic acid bacteria
can also spoil fermented vegetables such as sauerkraut and pickles, as well as
other acid foods like salad dressings and mayonnaise. Spoilage in fermented vegetables is often manifest by off
odors or changes in the color (chromogenic colony growth) or texture
(softening) of the product. In
mayonnaise or salad dressing, the first signs of spoilage are usually off odors
and emulsion separation.