Food Spoilage

Definition
n Food spoilage can be defined
as a disagreeable change in a
food's normal state.
n NOT acceptable for human
consumption and for food
industrial usage
n Such changes can be detected
by smell, taste, touch, or sight
(disgusting).
n Food safety ↔ food quality

Classifications of food spoilage
n Stable or non perishable foods (sugar, flour,
dry beans)
n Semi perishable foods Foods handled and
stored properly → remain unspoiled for long
period (potatoes, apples)
n Perishable foods Spoil readily without special
preservative methods (fish, meat, eggs, milk)

Main causes of food spoilage
q Physical changes
n aw, temperature, mechanical effects
n Caused by the inappropriate transport, handling and storage
q Biological factors
n Microbiological
q bacteria, yeasts, moulds (most common)
n Macrobiological
q rodents, insects, birds, parasites
q Chemical, biochemical factors
n non-microbial or enzymatic changes usually involving
oxygen → oxidation processes (e.g., rancidity of fats and oils)
n activity of endogenous tissue enzymes (food of vegetable or
animal origin)

Consequences/effects
of food spoilage
q changes in nutritional value
n Decomposition of proteins, carbohydrates,
vitamins
q changes in organoleptic features
n colour, flavour, taste, consistency →
mucilaginous surface, unpleasant odour,
q unwholesome effects
n Biogenic amines, toxins,
n Metabolites of microorganisms
n Pathogen microbes

Sources of microbial spoilage
n Ubiquiter microorganisms
q Can originate from: natural
habitat of microorganisms;
e.g. soil, water, air,
n Special sources
contamination
q Spoiled raw materials
q Food waste
q Biofilm on the surface of
equipments
q Human → personal hygiene

Ubiquiter microorganisms
• natural habitat of microorganisms is the soil
(except for pathogen bacteria)
• Way of contamination of animals and plants can
be direct from soil or indirect by water or air
• Related microorganisms:
– Gram (-): Pseudomonas, Acinetobacter, Alcaligenes,
Enterobacter spp.;
– Gram (+): Micrococcus, Arthrobacter, Streptomyces
spp.;
– Aerobic and anaerobic sporoforms: Bacillus,
Clostridium

Ubiquiter microorganisms
• Microorganisms can contaminate the surface of
plant or fur and skin of animals
• These are not harmful for plants or animals, but after
harvesting or slaughtering can cause spoilage during
storage of products (Micrococcus, Lactobacillus
spp.).
• Meat products also can be contaminated by
microorganisms from mouth (Streptococcus spp.) or
from the gastrointestinal tract (Bacteroides,
Eubacterium, Clostridium spp)

Special sources contamination
• Surfaces of equipment, instruments, utensils, the
wall and floor surfaces,
– Inappropriate hygiene of plant → Food waste → resulting a
specified, adapted micro-flora typical to plant and products
• Spoiled raw material
– Plant: mechanical lesions during the harvesting, transport
can enhance the penetration of microbes into the deeper
tissues
– Animal: contaminated mainly during slaughter
• Rodents, insects (fly, fruit fly, bee, wasp) can
contaminate the products during storage and
processing

Characteristic of
microorganisms
• Reproductive potential
– Among optimal circumstances the fastest
– The fastest growing microbe becomes dominant
(mainly bacteria, but …)
• Metabolism, nutrient requirements
– Substrate → metabolism in cells → metabolites
(useful or harmful)
– The process is influenced by the environment
(presence/absence of oxygen, available nutrients)

Factors affecting microbial spoilage
n Intrinsic factors of foodstuffs
q Physical-chemical properties: aw,
redox circumstances, pH,
q Chemical composition: Nutrient
materials, vitamins, inhibitors
q Biological structure
n Extrinsic factors of foodstuffs
q Temperature, humidity, atmosphere
composition
q Processing effects
q Hygiene, cleaning, disinfections
n Implicit parameters
q Interactions of microorganisms
q Reproductive potential

Intrinsic factors – water
activity• Water content of foods is quite high:
– Meat, egg: > 70%
– Milk, fruits, vegetables: >80%
• For microbes only the free water is available (free
water content is characterized by the aw)
• The minimal aw is the limit to microorganisms for
growth/reproduction
– Bacteria: highest water requirement (>0.91aw)
– Yeasts: water requirement is lower than the bacteria
(≥0.88aw)
– Moulds: low water requirement (>0.78aw)

Minimum water activity requirement
of microorganisms
Group of microorganisms Minimum aw of growth
_______________________________________________________
Most Gram-negative bacteria 0.97
Most Gram-positive bacteria 0.90
Halophilic bacteria 0.75
Most yeasts 0.88
Osmophilic yeasts 0.62
Most filamentous fungi 0.80
Xerotolerant fungi 0.71
Xerophilic fungi 0.61
Xeromyces bisporus 0.60
_______________________________________________________

Water activity of some foods
Food aw
Fresh vegetables, meat, milk, fish 0.98<
Cooked meat, bread 0.95 – 0.98
Cured meats, ham, cheese 0.91 – 0.95
Dry cheese, salami 0.87 – 0.91
Flour, rice, beans, cereals 0.80 – 0.87
Jams 0.75 – 0.80
Dried fruits, caramels 0.60 – 0.75
Spices, milk powder 0.20 – 0.60

Intrinsic factors
n water activity „tolerance”: aw=0,7 → food is
sufficiently protected from spoilage
n Microorganisms growing in the food change the
level of available moisture by release of metabolic
water
q moulds can destruct the tissues → water available to
yeasts and bacteria

Intrinsic factors – water activity
• If we want to ensure the safe storage life the aw has to
be reduced under 0.7
• Water content of dried foods belonging to aw of 0.7:
7-10Cocoa
7-10Milk powder (whole)
10Skim milk powder
18-25Dehydrated fruits
13-21Dehydrated stock
12-22Dried/dehydrated vegetables
10Dried whole egg
10Dried meat, fish
Maximal water content %Food

Intrinsic factors of foodstuffs
Physical-chemical properties: aw,
redox circumstances, pH,
Chemical composition: Nutrient
materials, vitamins, inhibitors
Biological structure
Extrinsic factors of foodstuffs
Temperature, humidity, atmosphere
composition
Processing effects
Hygiene, cleaning, disinfections
Implicit parameters
Interactions of microorganisms
Reproductive potential

Factors affecting the moisture/water
requirements of organisms
n Nutritive properties of the substrate
n pH
n Content of inhibitory substances
n Availability of free oxygen
n Temperature

Influence of aw on the growth and toxin-
production of moulds
Mycotoxin Microbes
Min. aw
Growth
Toxin-
production
Aphlatoxin
Aspergillus flavus 0,78-0,84 0,83-0,87
Ochratoxin Aspergillus ocraceus
Penicillium cyclopium
0,77
0,82-0,85
0,85
0,87-0,90
Patulin Penicillium expansum, 0,81 0,95

n Intrinsic factors of foodstuffs
q Physical-chemical properties: aw, redox
circumstances, pH,
q Chemical composition: Nutrient materials, vitamins,
inhibitors
q Biological structure
n Extrinsic factors of foodstuffs
q Temperature, humidity, atmosphere composition
q Processing effects
q Hygiene, cleaning, disinfections
n Implicit parameters
q Interactions of microorganisms
q Reproductive potential

Intrinsic factors: pH and buffering capacity
n pH of foods: mostly acidic (pH=4-7)
n Most of the bacteria are inhibited under pH 4
n Yeast and moulds can tolerate the lower pH
Min. pH Opt. pH Max. pH
Most of the bacteria
4-4.5 6.5-7 8-9
Acidophil bacteria (Lactobacillus spp,
Acetobacter, spp., Clostridium butyricum,
3-4 5 6-9
Alkalinetolerant bacteria (Vibrio spp.) 5 7-8 10-11
Yeast 2.5-3 5 8-9
Mould 2 5 9-10

pH and buffering capacity
n The pH of food can be effective also against the activity of
pathogens
n pathogens:
q Staph. aureus → no toxin production < pH=4.5
q Salmonella no growth ≤ pH 3.8 ( pH 4.5)
q Clostridium botulinum: no growth ≤ 4.5
n acids (non-dissociated form)→ antimicrobial effect, preservative
function (e.g. benzoic acid, sorbic acid, )
q ↔ spoilage (pickled cabbage, cucumber)
n meat ageing: mammals → pH=5,5
q → DFD pH>6,0
n fish → pH 6,2-6,5
n moulds: aphlatoxin ≤ 3,3 also!
n high pH is also not tolerable for microbes, but: Pseudomonas, Vibrio
spp.
n eggs → CO2 lost → pH≈9

Redox circumstances – redox potential
q Redox circumstances can be characterized by the redox
potential (Eh) depending on the presence/and
concentration of oxidizing and reducing agents
q The Eh is also influenced by the redox-capacity of the
product → resistance against the change of redox potential
q Also remarkable factor, that the product in what rate can be
impregnated by air (minced meat)
q Redox potential requirements for microorganisms:
q Aerobic microbes: ≥300mV
q Anaerobic microbes: ≤-300mV
q Redox potential of foods:
q Plant origin: 300-400mV
q Meat, meat products, cheese: -20 to -200mV (e.g. minced
meat: -200 → +200mV

Composition of foods
q nutrients necessary for microbial growth
q foods rich in protein → putrefaction
q foods rich in carboydrates → fermentative
spoilage
q Limiting factor:
q vitamins → fruits low vitamin B content → mostly
moulds and yeasts, as spoilage organisms (they
can synthetize them)

Composition of foods - Inhibitory substances
n Originally presented in the food, added purposely or accidentally,
or developed by growth of microorganisms or by processing
methods
q Natural:
n Lysozyme, lactoferrin, lactoperoxidase - fresh milk,
n Lysozyme - egg white
n Benzoic acid - cranberries
n Egg-white/albumen: avidin → neutralizes biotin →
no longer available for microorganisms → inhibition
of bacterial growth
q antimicrobial substances (capsaicin, fitoncid)

Biological and physical structure of foods
q protection barriers
q natural biological structures: shell, shell membrane
q cuticle of intact plant organs
q fresh meat: fascia → aw ↓ → against Gram-
psychrotropic bacteria can be effective protection
(minced meat: lack of fascia → ↑ susceptibility)

Extrinsic factors: Storage temperature
n Most important extrinsic factor
n spoilage microorganisms: -10°C → 80°C
n Under the minimum temperature growth stop
n Above the maximum temperature microbes are
killed
n Mesophiles: 5-45 °C
n Psychrotrophic: -5 - +35 °C
n Psycrophiles: -10 - +20 °C
n Thermotrophic: 20 - 50 °C
n Thermophiles: 40 – 80 °C

n most of the pathogens → mesophiles →↓
temperature markedly decrease their growth (except:
Listeria monocytogenes, Yer. enterocolitica, Vib.
parahaemolyticus, Cl. botulinum E, F, Francisella
tularensis); (C. jejuni 30-46°C)
n at chilling temperature spoilage occurs earlier than
any health hazard due to pathogens
q Cl. botulinum: min. temperature for growth 10°C
q Staph. aureus: min. temperature for growth 7°C
q Salmonella: min. temperature for growth 5-6°C

n chill storage (5-7°C): pathogens will not destroyed
necessarily, but their growth will stop
n psychrophilic and psychrotrophs still growing
n stronger chilling (-1 → +2°C): also these organisms
almost entirely inhibited
n growth retarding/inhibiting effect of chilling supported by:
aw, pH, pO2
n freezing: -18°C → growth of all microorganisms entirely
inhibited
- most sensitive → Gram- bacteria, but survival of some pathogens
may be important (e.g., Salmonella in frozen poultry)
- activity of microbial enzymes → -30°C
n dominant spoilage flora of foods in relation to storage
temperature

Extrinsic factors: Relative humidity
n equilibrium between the moisture content of
the food and the air (if the packaging is not
completely moisture proof)
n Fresh food can lost partly its water content
during storage (e.g. fresh cheese → drier
atmosphere → dry surface → flora shift
towards moulds)
n Products with low aw in humid environment
→ adsorb moisture → go mouldy
n food materials inhibit water diffusion →
equilibration nearly complete at the surface
→ interior unaffected
n → difference between the spoilage
association → moulds on the surface,
xerotrophic bacteria inside

Extrinsic factors: Relative humidity
n surface spoilage may also be complex → fatty tissues
→ water diffusion slow→
n chill-stored meat dry → moulds and some yeasts may grow
n lean meat → bacteria → slimy
n chilled foods exposed to warm damp air → moisture
condenses onto foods „sweating” → rapid bacterial
growth → spoilage

Extrinsic factors: atmosphere
composition
n O2 ↓ CO2↑→ inhibit the aerobe
microbes and moulds
n (can be used in case of fruits/vegetables
storage – altogether with chilling)
n Modified atmosphere packaging
n Can be modified by passive or
active way
n Active: making vacuum in the package
or using oxygen adsorbent
n Passive: caused by the respiration of
the product’s micro-flora
n (meat- lactobacillus)

Extrinsic factors: processing
n Contaminated raw materials (always)
n Washing, sorting, peeling, → decrease microbe
number or contamination
n Comminuting, mincing, +additives → increase
microbes number
n Chilled, minced meat
q no fascia (low aw), high O2, high Eh → potential spoilage
caused by Pseudomonas-Acitenobacter-Alcaligenes is quite
high
q Technological hygiene, cleaning, disinfection

Implicit parameters
n Growth/reproduction rate
n Generation time/reproduction circle: less than 1 hour
n spoilage associations: dominant species → ↑ specific
growth rate
n under given conditions (temperature, aw, pH, Eh)
n aw >0,98 + pH > 4.5 → necessarily bacteria
n marked differences among bacteria (e.g., Pseudomonas
and bacilli) grow much faster than Lactobacillaceae,
these latter become dominant → the former inhibited
(e.g., anaerobic conditions)

Implicit parameters
n interactions → synergistic, antagonistic = competition
(indifferent)
n interactions may be between spoilage and pathogen
organisms and between spoilers
n mechanisms:
q Utilization of nutrients
q Alteration of pH,
q Alteration of redox potential,
q Alteration of aw
q production metabolites (inhibitors, antibacterial
substances, Vitamin B, etc.)

Implicit parameters
• public health importance: inhibition of
pathogens based on the antagonistic effects
Bacillus, Enterococcus, Lactobacillus,
Micrococcus, Pseudomonas,
Streptococcus
Staph. aureus
Enterococcus, Lactobacillus,
Lactococcus
L. monocytogenes
Pseudomonas
E. coli
Micrococcus
Cl. botulinum, Cl. perfringens
Enterococcus, LactobacillusBacillus cereus
Microbe caused spoilage
(inhibitor)
Pathogen (inhibited)

Special spoilage associations (micro-
flora) of certain foods
n Non-heat treated products
q chilled fresh meat, fish, milk (rich in proteins, ↑aw,
pH>4.5):
q psychrotrophic, rod-shaped Gram-negative bacteria,
(Pseudomonas-Acinetobacter-Alkaligenes) → slime
formation
q vegetables: Gram-negative bacteria (Pseudomonas,
Erwinia) → which can break down cellulose and
protein → rotting
q cured meat products without heat treatment (↑pH, ↓↓
aw): moulds
q fermented foods (pH<4.5): yeasts, moulds, lactobacilli

Special spoilage associations (micro-
flora) of certain foods
n Heat-treated foods
q pasteurized products (milk, ham, sausages):
thermotolerant streptococci (enterococci) and
lactobacilli (aerobic and anaerobic spore-forming
species → Bacillus, Clostridium)
q baked products: yeasts, moulds; bacteria: Bacillus
spores → ropiness
n Dried foods
q Cereal flours, dried vegetables, fruits (↓↓aw): moulds
q Chocolate, candy (high carbohydrate content, ↓↓aw):
yeasts

Chemical, biochemical spoilage
n Oxidation (effect of oxygen, generally
enhanced by light, temperature, metals)
q Result: Taste, odour, colour changes
n Pepper-fade
n Meat-grey
n Rancidity of fats and oils

Rancidity of fat
n Sign of rancidity: unpleasant, disgusting taste,
odour;
n Lipid peroxidation – or autooxidation
q Need 3 factors:
n Oxygen
n Substance that can be oxidated: fat contains
unsaturated fatty acid
n Catalisator: light, high temperature, metal ions
n Fotooxidation → activate oxygen
n Lard-fat heat treated at high temperature: 140 °C →
activate oxygen
n Lipid peroxidation: alcohol, ketone, aldehide are
produced →unpleasant, strange odour, taste

Rancidity of fat
n Inhibit the peroxidation:
q decreased exposure to O2
n Proper instruments
n Cutting
n Short time
q Antioxidants
n Vitamin E, C
n Artificial antioxidants
q Lower temperature
n Lypolysis of triglycerides ↓

Rancidity of fat
n Ketone rancidity
q Moulds in presence of oxygen and water can release
fatty acids from lipids → methylketone → unpleasant
taste
q Hydrolytic rancidity
q Enzymatic reactions
q Original enzymes, microbial enzymes

Enzymatic spoilage
n Mainly not heat treated raw materials
n Plants
q Fruits, vegetables after harvesting →
polysaccharides decomposition → sweet
taste decrease
q Protopectin decomposition → to soluble
pectin → softening (over matured fruit)
q Mechanical effects: membrane damaged →
polyphenols, oxidize enzymes released →
enzymatic reactions → browning

Enzymatic spoilage
q Meat
n After slaughtering similar
biochemical reaction than in the live
animal
n Glycogen decomposition →
n final pH value
q Milk
n Lipase
n Cream, butter, sour cream
n Can be deactivated by heat treatment
q Egg
n Water amount decrease, weight loss,
n Viscosity of white ↓, ovomucine
decomposition

Food Spoilage

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