1. MICROBIAL FOOD SPOILAGE
Harish. R
Assistant Professor
SCHOOL OF BIOSCIENCES,
MAR ATHANASIOS COLLEGE FOR ADVANCED STUDIES
(MACFAST),
Tiruvalla-01. Kerala.
www.macfast.ac.in
2. Food spoilage
• A food is considered spoiled when it loses its acceptance
qualities.
• Detectable parameters associated with spoilage
– changes in color,
– odor,
– flavor (smell and taste),
– shape and texture;
– formation of slime;
– accumulation of gas (or foam); and
– accumulation of liquid (exudates).
3. • The acceptance qualities of a food can be lost due to-
– infestation with insects and rodents,
– undesirable physical and chemical actions, and
– growth of microorganisms.
• Physical spoilage (dehydration of fresh vegetables)
(wilting).
• Chemical spoilage includes oxidation of fat, browning of
fruits and vegetables, and autolytic degradation of some
vegetables (by pectinases) and fishes (by
proteinases).Heavy Metals. Pesticides.
• Microbial spoilage results either as a consequence of
microbial growth in a food or because of the action of
some microbial enzymes present in a food.
5. • Microbial food spoilage occurs as a consequence of
– Microbial growth in a food (increase in numbers)
– Enzymatic changes.
– Breakdown nutrients
– Producing Off-flavours.
– Synthesis of New compounds
– Changing texture etc.
• Spoilage by microbial growth occurs much faster than spoilage by
microbial extra- or intracellular enzymes in the absence of viable
microbial cells.
• Transient Vs Resident Mos.
• Many food items support the growth of Mos at least serve as a
carrier.
• Processing:
• Minimize the contact between Mos & foods.
• Eliminate Mos from foods
• Adjust the conditions of storage to prevent their growth (preservation)
6. •Stable/Non Perishable foods:
• Do not spoil unless they are handled carelessly
•Should be stored in cool dry place (ex: grain, dried meat,candy)
•No natural chemical breakdown (low moisture content)
•Semiperishable foods:
•Do not spoil for a fairly long time if stored properly
•Less likely to decay due to microbial contamination.
•Natural chemical breakdown is slower.
•Ex: flour,frozen foods,vegetables etc)
•Perishable foods:
• Very short shelf life
• High amount of protein & moisture,
• Spoil very easily by natural enzymatic changes.
• Ex: Milk & milk products,eggs, poultry, fish, beef etc.
13. Food poisoning vs spoilage
• Food poisoning
– food is eaten which looks normal, smells normal and tastes
normal
– you eat enough to make you ill from the ingested pathogens or
toxins
• Spoiled food
– does not normally cause food poisoning because it is rejected by
the consumer before ingestion
– Distorted shape, smell, taste, texture etc.
Putrefaction
Proteolysis and anaerobic breakdown of proteins,
yielding foul-smelling amine compounds
14. Sources of Contamination
• From Green plants & Fruits (Pseudomonas, Leucostonoc, Erwinia etc)
• Animals (Bacillus, Salmonella, Staphylococcus, Streptococcus etc)
• Soil (Bacillus, Acetobacter, Micrococcus, etc)
• Water & Sewage (E coli, Pseudomonas, Aeromonas, Enterobacter, Proteus
etc)
• Air and Dust (Staphylococcus, Streptococcus, fungal spores)
• Food Utensils & Other food contact surfaces.
• Gastrointestinal Tract (E coli, Enterobacter, etc)
• Food Handlers (Staphylococcus, Streptococcus)
• Animal Feeds (Salmonella, Shigella etc)
• Cross contamination.
Physical contaminations: unwanted edible materials (hair,stones, grit etc)
Insects & Rodents
Natural Enzymes (Autolysis)
15. Significance of microorganisms
MICROBIAL NUMBERS
• To spoil the food, microorganisms (mainly bacteria and yeasts)
must multiply and attain certain levels, often referred to as the
"spoilage detection level"
• As multiplication is an important component in spoilage, bacteria
(because of shorter generation time), followed by yeasts, are in
favorable positions over molds to cause rapid spoilage of foods.
• Depending on the specific nature of spoilage and microbial types,
the spoilage detection level can range from 106 – 108 cells/g, /ml
• Spoilage associated with H2S, some amines, and H2O2 formation
can be detected at a lower microbial load, whereas formation of
lactic acid may be detected at a higher microbial load.
16. • Thermophiles: (Flat sour spoilage of low acid canned foods by B.
stearothermophilus)
• Thermodurics: can survive pasteurization or get into the food following heating (as
post-heat contaminants). Bacillus Sp, S. thermophilus, etc.
• Psychrophiles & Psychrotrophs can grow in food stored at low temperatures-
chilling and refrigeration (-1°C and 7°C).
– Pseudomonas, Alcaligenes, Flavobacterium, Achromobacter, Micrococcus,
psychrophilic Clostridium spp
• Mesophilic Sp
– Yersinia enterocolitica and Listeria monocytogenes, Leuconostoc spp., several
Lactobacillus spp.,
(Spoilage of a heated food by microbial enzymes, in the absence of viable microbial cells,
can result from some heat-stable enzymes/Toxins produced by microorganisms in
the foods before heat treatment.)
• Proteolytic bacteria: Produces extracellular proteases, aerobic spore-formers
(Bacillus sp), acid proteolytic (Clostridium, Pseudomonas).
• Lypolytic bacteria: produces lipases which catalyze hydrolysis of fats to fatty acids
and glycerol (Pseudomonas, Alcaligenes, Staphylococcus, Serratia etc)
• Saccharolytic bacteria: hydrolyze disaccharides to simpler sugars, few are
amylolytic (Bacillus sp)
• Pectinolytic bacteria: Erwinia, Bacillus, Clostridium
17. •Halophilic bacteria: (Pseudomonas, Acinetobacter,Vibrio sp etc
•Pigmented bacteria: Flavobacterium, Micrococcus etc.
•Slime/Rope forming Bacteria: Alcaligenes viscolactis, E aerogenes,
Klebsiella oxytoca (ropiness of milk) Leuconostoc (surface slime).
•Gas-Forming bacteria: Produces CO2, H2 (Leuconostoc
,Propionibacterium,Enterobacter, E. coli etc.)
•Coliform & Fecal Coliform group: (E coli, Enterobacter, Klebsiella etc).
