1. MICROBIAL FOOD SPOILAGE
Dr. Ir. Eni Harmayani, M.Sc.

2. Important Factors in Microbial Food Spoilage

3. Significance of Microbial Types
Raw and most processed foods normally contain many types of
bacteria (the first important component in spoilage due to a shorter generation time),
yeasts (favorable positions over molds to cause rapid spoilage of foods),
molds,
capable of multiplying and causing spoilage.
However, in food where bacteria and yeasts do not grow favorably and the foods are stored for a relatively longer period of time (breads, hard cheese, fermented dry sausages, and acidic fruits and vegetables) spoilage due to mold growth is more prevalent.

4. Significance of Microbial Numbers
The spoilage detection level can range from 106 – 108 cells/g, mL or square centimeter.
Spoilage associated with H2S, some amines, and H2O2 formation can be detected at a lower microbial load, while formation of lactic acid may be detected at a higher microbial load.

5. Significance of Predominant Microorganisms
The microbial profile of a food is quite different from that of a pure culture growing in a laboratory medium.
An unspoiled, nonsterile food generally contains many types of microorganisms consisting of bacteria, yeasts, and molds (also viruses) from different genera and may be more than one species from the same genus.
When the same food is spoiled, it is found to contain predominantly one or two types, and they may not even be present initially in the highest numbers in the unspoiled product.

6. Some Important Food Spoilage Bacteria
Psychrotrophic Bacteria
Aerobic : Pseudomonas fluorescens, P. fragi, other Pseudomonas species, Acinobacter, Moraxella, and Flavobacterium. (some molds and yeasts)
Facultative anaerobic : L. viidescens, L. sake, L. curvatus, Leuconostoc mesenteroides etc.
Thermoduric psychrotrops :
Facultative anaerobs : spores of Bacillus coagulans and Bacillus megaterium, some strains of L. viridescens.
Anerobes : spores of Clostridium laramie, Clo. estertheticum, Clo. algidicarnis, Clo.putrefaciens, and unidentified Clostridium spp.
When a food is temperature abused above 50C, some true mesophiles (growth temperature range 15 – 45 0C, optimum 25 – 40 0C) can also grow.
However, at 10 – 15 0C storage temperature psychrotrophs will generally grow much faster than these mesophiles.

7. Thermophilic Bacteria
The bacteria in this group grow between 40 – 90 0C, with optimum growth at 55 – 65 0C.
Spores of some thermophilic Bacillus and Clostridium spp. can be present in these heat treated foods, which at warm temperatures germinate and multiply to cause spoilage.
Some thermoduric vegetative bacteria surviving low heat processing (such as pasteurization) or thermophiles getting in food as post-heat contamination can also multiply in this warm foods, especially if the temperature is close to 50 0C.

8. Aciduric Bacteria
Bacteria capable of growing relatively rapidly in food at pH 4.6 or below are generally regarded as aciduric (or acidophilic).
Spoilage of acidic food products :
Fruit juices, pickles, salsa, salad dressings, mayonnaise, and fermented sausages.
Heterofermentative LAB and homofermentative LAB have been associated with such spoilage.
Yeast and molds are aciduric and thus are associated with spoilage of such foods.

9. Food Types Intrinsic factors that can cause food spoilage :
Aw & pH : food with a lower Aw (0.90) or a lower pH (5.3) is less susceptible to bacterial spoilage than one with Aw = 0.98 or pH = 6.4. Yeast and molds will probably grow equally well under both conditions.
O – R potential, nutrient content, antimicrobial substances, protective structures.
Extrinsic factors : storage conditions

11. Metabolism of Food Nutrients
Carbohydrates
Food nutrients
Lipids
Proteinaceous and NPN compounds
CO2, H2, H2O2, lactate, acetate, formate, butyrate, isobutyrate, isovalerate, ethanol, propanol, butanol, isobutanol, diacetyl, acetoin, butanediol, dextran, levans.
CO2, H2, NH3, H2S, amines, keto acids, mercaptans, organic disufides, putrescine, cadaverine, skatole
Fatty acids, glycerol, hydroperoxides, carbonyl compounds (aldehydes, and ketones), nitrogenous bases.

12. Preference for Utilization of Food Nutrients
In general, microorganisms prefer to use carbohydrate first, followed by NPN and proteinaceous compounds, and then lipids.
It again depends on whether or not a particular species has the ability to use specific carbohydrate.
In mixed microbial population, as normally present in food, availability and amount of metabolizable carbohydrates greatly affect the spoilage pattern.

13. Microbial Growth in Succesion
The intrinsic factors or environments of a food dictate which, among the mixed microbial species normally present, will multiply rapidly and become predominant to cause spoilage.
If sufficient time is given, the predominant microbial types and the nature spoilage of a food can change.

14. Spoilage of Specific Food Groups

16. Prevention of spoilage
Knowing types of microorganisms associated with spoilage of specific foods is essential for developing preventive measures
Intrinsic and extrinsic factors can be manipulated to prevent microbial growth

17. Fresh and Ready to Eat Meat Products
Raw Meat
From animals and birds contain a large group of spoilage bacteria
The predominant spoilage flora in a meat is determined by :
the nutrient and oxygen availability,
storage temperature,
pH,
the storage time of the product
generation time of the spoilage microorganisms under a given environment.

18. Meat are more perishable than other food commodities
Abundance all nutrients required for growth of bacteria, yeasts and molds
All of these nutrients readily available
Carcasses and cuts may be spoiled by bacteria or by molds
Whiskers (Mucor, Rhizopus), black or green spots (Cladosporium, Penicillium)
Ground meats and steaks are almost exclusively spoiled by bacteria
Storage temperature selects for psychrotropic spoilage organisms
Pseudomonas, Moraxella, Acinetobacter, Psychrobacter
Steaks tend to undergo surface spoilage

19. Ready to Eat Meat Products
High heat processed uncured and cured meat products (heat treatment to make them commercially sterile)
Low heat processed uncured and cured meat products
Sources of microorganisms :
Raw meat
Spices and other ingredients
Natural casings

20. Spoilage in ready to eat meat products :
Slimy spoilage
Occurs on the outside of casing
Yeast, LAB, B. thermosphacta
Souring
Occurs underneath the casing
Due to growth of LAB
Greening
Due to H2O2 production (Lactobacillus)
Due to H2S production
Reacts with myoglobin to form sulphmyoglobin
Pseudomonas mephitica

21. Eggs and Egg Products Rotting
Green rots (Pseudomonas fluorescens)
Colorless rots (Pseudomonas, Acinetobacter)
Black rots (Proteus)
Pink rots (Pseudomonas)
Red rots (Serratia)
Custard rots (Proteus vulgaris, P. intermedium)
Pinspots (molds, Penicillium, Cladosporium, Mustiness, P. graveolans, Proteus)

22. Hen’s egg structure includes barriers against microbial entry and growth :
External
Outer waxy shell membrane
Shell
Inner shell membrane
Internal (white)
Lysozyme
Avidin (chelates biotin)
High pH (~ 9.3)
Conalbumen (chelates iron)
Yolk is an excellent medium for bacterial growth

23. Milk and Milk Products Raw Milk Microbiota of milk from healty cows
Micrococcus, Staphylococcus, Lactic acid cocci (Lactococcus and Streptococcus)
Microbiota of milk from cows with mastitis
Staphylococcus aureus, Streptococcus agalactiae, Coliforms, Pseudomonas, and other streptococci
Count > 103 in the bulk tank (if not separated)
Spoilage of refrigerated milk consists usually of bitter, rancid, fruity flavors due to putrefaction and caused by psychrotrophs
Spoilage of milk at room consists usually of souring due to fermentation and LAB

24. Pasturized Milk Usually associated with Gram-negative psychrotrophs
Bitter, rancid, fruity, or unclean flavor
Gram-positive, psychrotrophic sporeformers (Bacillus spp.), can grow and cause spoilage (sweet curdling)

25. Vegetables and Fruits
Widely varied products (raw, frozen, canned, dehydrated, fermented)
Sources of m.o in fresh vegetables and fruits
Soil (Bacillus, Clostridium, fungi)
Wide distribution in nature (Lactobacillus, Leuconostoc, Streptococcus)
Fertilizers (non-fermented manure)
Water (irigation, solvent, washing)
Dust (air)
Animals, insects, humans
Harvesting equipment and utensils
Packing equipment
Ice, transporting vehicles, inadequate storages, cross contamination, handling previous to consumption

26. Types of spoilage on vegetables and fruits
Bacterial soft rot
Soft, mushy product, sometimes with off-odors
Souring by LAB
Gray mold rot (Botrytis sp.), Rhizopus soft rot (Rhizopus sp.) and many other rots caused by species of molds
Bacteria are of less impotance in the spoilage of fruits because of the lower pH

27. Fish, Crustaceans, and Mollusks
Microbial spoilage is determined by the microbial types, their level, fish environment, fish types, methods used for harvest, and subsequent handling
Crustaceans
Microbial spoilage in shrimp is more prevalent than that of crabs and lobsters because crabs and lobsters remain alive until they are pocessed
Mollusks
As compared to fish and crustaceans, oyster, clam, and scallop meats are lower in NPN compounds but higher in carbohydrates
The mollusks are kept alive until processed; thus, microbial food spoilage occurs only after processing

28. Soft Drink, Fruit Juices and Preserves, and Vegetable Juices
Among the microorganisms that can be present in these products, only aciduric molds, yeasts, and bacteria (Lactobacillus, Leuconostoc, and Acetobacter) are able to cause spoilage if appropriate preservation methods are not used
To prevent of these potential spoilage microorganisms, several additional preservation methods are used, include :
heat treatment, to kill vegetative microorganisms in tomato juices,
freezing,
refrigeration,
addition of specific chemical preservatives.

29. (Harmayani & Sumedi, 2006)

30. Cereals and Their Products
Cereal grains
If the Aw increase above 0.6, some species of fungi (Aspergillus, Penicillium, and Rhizopus) can grow and cause spoilage
Yeats are common on all cereals, although they represent only a minority of the microbial flora
Refrigerated Dough (biscuits, roles, and pizza)
Susceptible to spoilage (gas formation) from the growth of psychrotrophic heterolactic species of Lactobacillus and Leuconostoc then the gas can blow the containers, especially when the storage temperature increases to 100C or above
Breads
A specific type of bread spoilage, designated as ropiness and characterized by soft, stringy, brown mass with fruity odor, caused by the growth of some mucoid variants of Bacillus subtilis

31. Pastas
Anaerobic packing and refrigeration storage can prevent mold growth and slow down the growth of yeasts, anaerobic and facultative anaerobic psychrotrophic bacteria
Pastries
They can spoilage by microorganisms coming with the ingredients that are added after baking such as icing, nuts, toppings, and cream
Due to low Aw, most products will allow only molds to grow

32. Liquid Sweeteners and Confectioneries
Most of these products have an Aw of 0.8 or below and are normally not susceptible to bacterial spoilage.
Under aerobic conditions, some xerophilic molds can produce visible spoilage
Osmophilic yeasts Zygosaccharomyces rouxii, Saccharomyces cerevisiae, Torulopsis holmii, and Candida valida can ferment these products

33. Mayonnaise, Salad Dressing, and Condiments
Zygosaccharomyces bailii
Lactobacillus fructivorans
Bacillus vulgatus (Thousand Island dressing)

34. Canned Foods Thermophilic sporeformers
Can cause some types of spoilage of low acid (high pH) foods (such as corn, beans, peas) when the cans are temperature abused at 430C and above, even for short duration
Spoilage due to insufficients heating
Clostridium and some Bacillus spp.
Spoilage due to container leakage
Damage and leakly containers will allow different types of microorganisms to get inside from the environment after heating

35. Fermented Foods Fermented meat products
If the acid production of homofermentative LAB is slow, undesirable bacteria can grow (Clostridium, Bacillus, and other mesophilic bacteria have been reported to cause spoilage in such conditions.
Products with pH < 5.0 but Aw 0.92 or above and vacuum packaged can be spoiled by heterofermentative Leuconostoc and Lactobacillus spp. with accumulation of gas and liquid inside the package and creamy white growth of bacterial cells
If they are not vacuum packaged and have low Aw ( ), yeast and molds can grow on the surface, resulting in slime formation, discoloration, and undesirable flavor of the products.

36. Yeast In Specific Types of Foods
Yeast will actually grow to the extent that spoilage will result depends on intrinsic and extrinsic factors
Many yeast species are associated with plant and animal products and can also be found in food-processing environments
Yeast are most likely to cause spoilage in products such as fruits and soft drinks, which contain fermentable sugars, and in those types of food, e.g., alcoholic beverages and high-sugar and/or high-acidity products, which restrict the growth of competing bacteria

37. Food fermented with mixed cultures of bacteria and fungi
Products
Molds
Yeasts
Bacteria
Substrate
Use
Ragi
Amylomyces rouxii
Endomyces spp.
Pichia burtonii
Pediococcus pentosaceus
Steptococus faecalis
Uncoked rice
Inoculum
Peuyem
A.rouxii
P.Burtonii
Endomycopsis fibulinger
P.Pentosaceus
Enterococcus faecalis
cassava
Snack
Miso
Aspergillus oryzae
A.sojae
Zygosaccharomyces rouxii
T.versatilis
P.Halophilus
E.faecalis
Soybean
+ rice/barley
Flavoring
Soy sauce
A.Oryzae
A.Sojae group
Zy.rouxii, Zy.sojae, Zy.major, Ha. Spp., Torulopsis spp., C.etchellsii, C.versatilis
Lactobacillus delbrueckii
P.halophilus
P.damnosus
+wheat
+salt
Sake
A.oryzae
Ha.anomala
S.Cerevisiae (sake)
Lactobacillus mesenteroides var.sake
Lactobacillus sake
Rice
Liquor
Tempe
Rhizopus oligosporus
R.chinensis
R.Oryzae
Mucor indicus
Trp.beigelii
Cla.lusitaniae
C.maltosa
C.intermedia
Ya.lipolytica
Klebsiella pneumoniae
Enterobacter cloacae
Lactobacillus spp.
Mostly soybeans
Protein food snack
(Deak dan Beuchat, 1996)

38. Pustaka :
Deak, T. dan Beuchat, L.R Handbook of Food Spoilage Yeast. CRC Press, New York.
Ray, B Fundamental Food Microbiology. CRC Press, New York.
Nickerson, J.T. dan Sinskey, A.J Microbiology of Food and Food Processing. American Elsevier Publishing Co., Inc.