Food spoilage & b. Sources of food spoilage micro-organisms

 Food spoilage:

b. Sources of food spoilage micro-organisms

Food spoilage:

According to the Oxford English Dictionary to spoil is to ‘deprive of good or effective qualities’.

When a food is spoiled its characteristics are changed so that it is no longer acceptable.

Such changes may not always be microbiological in origin; a product may become unacceptable as a result of insect damage, drying out, discolouration, staling or rancidity for instance, but by and large most food spoilage is a result of microbial activity.

Microbiological food spoilage can manifest itself in several different ways, some of which often occur in combination.

Visible microbial growth may be apparent in the form of surface slime or colonies, degradation of structural components of the food can cause a loss of texture, but the most common manifestation will be chemical products of microbial metabolism, gas, pigments, polysaccharides, offodours and flavours.

Food spoilage can be defined as “any sensory change (tactile, visual, olfactory or flavour)” which the consumer considers to be unacceptable.

Spoilage may occur at any stage along food chain.

Spoilage may arise from insect damage, physical damage, indigenous enzyme activity in the animal or plant tissue or by microbial infections.

Most natural foods have a limited life.

Food spoilage is a metabolic process that causes foods to be undesirable or unacceptable for human consumption due to changes in sensory characteristics.

Spoiled foods may be safe to eat, i.e. they may not cause illness because there are no pathogens or a toxin present, but changes in texture, smell, taste, or appearance cause them to be rejected.

B. Sources of food spoilage micro-organisms

Chemical reactions that cause offensive sensory changes in foods are mediated by a variety of microbes that use food as a carbon and energy source. 

These organisms include prokaryotes (bacteria), single-celled organisms lackingdefined nuclei and other organelles, and eukaryotes, single-celled (yeasts) and multicellular (molds) organisms with nuclei and other organelles.

Some microbes are commonly found in many types of spoiled foods while others are more selective in the foods they consume; multiple species are often identified in a single spoiled food item but there may be one species (a specific spoilage organism, SSO) primarily responsible for production of the compounds causing offodors and flavors. 

Within a spoiling food, there is often a succession of different populations that rise and fall as different nutrients become available or are exhausted. Some microbes, such as lactic acid bacteria and molds, secrete compounds that inhibit competitors .

Spoilage microbes are often common inhabitants of soil, water, or the intestinal tracts of animals and may be dispersed through the air and water and by the activities of small animals, particularly insects. It should be noted that with the development of new molecular typing methods, the scientific names of some spoilage organisms,particularly the bacteria, have changed in recent years and some older names are no longer in use. 

Many insects and small mammals also cause deterioration of food but these will not be considered here.

Yeasts

Yeasts are a subset of a large group of organisms called fungi that also includes molds and mushrooms. 

They are generally single-celled organisms that are adapted for life in specialized, usually liquid, environments and, unlike some molds and mushrooms, do not produce toxic secondary metabolites. 

Yeasts can grow with or without oxygen (facultative) and are well known for their beneficial fermentations that produce bread and alcoholic drinks. 

They often colonize foods with a high sugar or salt content and contribute to spoilage of maple syrup, pickles, and sauerkraut. 

Fruits and juices with a low pH are another target, and there are some yeasts that grow on the surfaces of meat and cheese.

There are four main groups of spoilage yeasts: Zygosaccharomyces and related genera tolerate high sugar and high salt concentrations and are the usual spoilage organisms in foods such as honey, dried fruit, jams and soy sauce.

They usually grow slowly, producing off-odors and flavors and carbon dioxide that may cause food containers to swell and burst. Debaryomyces hansenii can grow at salt concentrations as high as 24%, accounting for its frequent isolation from salt brines used for cured meats, cheeses, and olives. 

This group also includes the most important spoilage organisms in salad dressings . 

Saccharomyces spp. are best known for their role in production of breadand wine but some strains also spoil wines and other alcoholic beverages by producing gassiness, turbidity and offflavors associated with hydrogen sulfide and acetic acid. 

Some species grow on fruits, including yogurt containing fruit, and some are resistant to heat processing .

Candida and related genera are a heterogeneous group of yeasts, some of which also cause human infections. 

They are involved in spoilage of fruits, some vegetables and dairy products . Dekkera/Brettanomyces are principally involved in spoilage of fermented foods, including alcoholic beverages and some dairy products.

They can produce volatile phenolic compounds responsible for off-flavors .

Molds:

Molds are filamentous fungi that do not produce large fruiting bodies like mushrooms.

Molds are very important for recycling dead plant and animal remains in nature but also attack a wide variety of foods and other materials useful to humans. 

They are well adapted for growth on and through solid substrates, generally produce airborne spores, and require oxygen for their metabolic processes.

Most molds grow at a pH range of 3 to 8 and some can grow at very low water activity levels (0.7–0.8) on driedfoods. 

Spores can tolerate harsh environmental conditions but most are sensitive to heat treatment. 

An exception is Byssochlammys, whose spores have a D value of 1–12 minutes at 90ºC. Different mold species have different optimal growth temperatures, with some able to grow in refrigerators. 

They have a diverse secondary metabolism producing a number of toxic and carcinogenic mycotoxins. 

Some spoilage molds are toxigenic while others are not.

Spoilage molds can be categorized into four main groups: Zygomycetes are considered relatively primitive fungi but are widespread in nature, growing rapidly on simple carbon sources in soil and plant debris, and their spores are commonly present in indoor air.

Generally they require high water activities for growth and are notorious for causing rots in a variety of stored fruits and vegetables, including strawberries and sweet potatoes. 

Some common bread molds also are zygomycetes. Some zygomycetes are also utilized for production of fermented soy products,enzymes, and organic chemicals. The most common spoilage species are Mucor and Rhizopus. Zygomycetes are not known for producing mycotoxins but there are some reports of toxic compounds produced by a few species.

Penicillium and related genera are present in soils and plant debris from both tropical and Antarctic conditions but tend to dominate spoilage in temperate regions. 

They are distinguished by their reproductive structures that produce chains of conidia. 

Although they can be useful to humans in producing antibiotics and blue cheese, many species are important spoilage organisms, and some produce potent mycotoxins (patulin, ochratoxin, citreoviridin, penitrem).

Penicillium spp. cause visible rots on citrus, pear, and apple fruits and cause enormous losses in these crops.

They also spoil other fruits and vegetables, including cereals. 

Some species can attack refrigerated and processed foods such as jams and margarine. A related genus, Byssochlamys, is the most important organism causing spoilage of pasteurized juices because of the high heat resistance of its spores.

Aspergillus and related molds generally grow faster and are more resistant to high temperatures and low water activity than Penicillium spp. and tend to dominate spoilage in warmer climates. 

Many aspergilla produce mycotoxins: aflatoxins, ochratoxin, territrems, cyclopiazonic acid. Aspergilli spoil a wide variety of food and nonfood items (paper, leather, etc.) but are probably best known for spoilage of grains, dried beans, peanuts, tree nuts, and some spices.

Other molds, belonging to several genera, have been isolated from spoiled food. These generally are not major causes of spoilage but can be a problem for some foods. Fusarium spp. cause plant diseases and produce several important mycotoxins but are not important spoilage organisms. However, their mycotoxins may be present in harvested grains and pose a health risk. 

Bacteria

Spore-forming bacteria are usually associated with spoilage of heat-treated foods because their spores can survive high processing temperatures. 

These Gram-positive bacteria may be strict anaerobes or facultative (capable of growth with or without oxygen). Some spore-formers are thermophilic, preferring growth at high temperatures (as high as 55ºC). Some anaerobic thermophiles produce hydrogen sulphide (Desulfotomaculum) and others produce hydrogen and carbon dioxide (Thermoanaerobacterium) during growth on canned/ hermetically sealed foods kept at high temperatures, for example, soups sold in vending machines.

Other thermophiles (Bacillus and Geobacillus spp.) cause a flat sour spoilage of high or low pH canned foods with little or no gas production, and one species causes ropiness in bread held at high ambient temperatures.

Mesophilic anaerobes, growing at ambient temperatures, cause several types of spoilage of vegetables (Bacillus spp.); putrefaction of canned products, early blowing of cheeses, and butyric acid production in canned vegetables and fruits (Clostridium spp.); and "medicinal" flavors in canned low-acid foods (Alicyclobacillus) . 

Psychrotolerant sporeformers produce gas and sickly odors in chilled meats and brine-cured hams (Clostridium spp.) while others produce off-odors and gas in vacuum-packed, chilled foods and milk (Bacillus spp.).

Lactic acid bacteria (LAB) are a group of Gram-positive bacteria, including species of Lactobacillus, Pediococcus,

Leuconostoc and Oenococcus, some of which are useful in producing fermented foods such as yogurt and pickles.

However, under low oxygen, low temperature, and acidic conditions, these bacteria become the predominant spoilage organisms on a variety of foods. Undesirable changes caused by LAB include greening of meat and gas formation in cheeses (blowing), pickles (bloater damage), and canned or packaged meat and vegetables. Off-flavors described as mousy, cheesy, malty, acidic, buttery or liver-like may be detected in wine, meats, milk, or juices spoiled by these bacteria. 

LAB may also produce large amounts of an exopolysaccharide that causes slime on meats and ropy spoilage in some beverages.

Pseudomonas and related genera are aerobic, gram-negative soil bacteria, some of which can degrade a wide variety of unusual compounds. 

They generally require a high water activity for growth (0.95 or higher) and are inhibited by pH values less than 5.4. Some species grow at refrigeration temperatures (psychrophilic) while other are adapted for growth at warmer, ambient temperatures. Four species of Pseudomonas (P. fluorescens, P. fragi, P. lundensis, and P. viridiflava), Shewanella putrefaciens, and Xanthomonas campestris are the main food spoilage organisms in this group. 

Soft rots of plant-derived foods occur when pectins that hold adjacent plant cells together are degraded by pectic lyase enzymes secreted by X. campestris, P. fluorescens and P. viridiflava. 

These two species of Pseudomonas comprise up to 40% of the naturally occurring bacteria on the surface of fruits and vegetables and cause nearly half of post-harvest rot of fresh produce stored at cold temperatures. P. fluorescens, P. fragi, P. lundensis, and S. putrefaciens cause spoilage of animal-derived foods (meat, fish, milk) by secreting lipases and proteases that cause formation of sulfides and trimethylamine (off-odors) and by forming biofilms (slime) on surfaces . 

Some strains are adapted for growth at cold temperatures and spoil these foods in the refrigerator.

Enterobacteriaceae are gram-negative, facultatively anaerobic bacteria that include a number of human pathogens(Salmonella, E. coli, Shigella, Yersinia) and also a large number of spoilage organisms. 

These bacteria are widespread in nature in soil, on plant surfaces and in digestive tracts of animals and are therefore present in many foods. 

Erwinia carotovora is one of the most important bacteria causing soft rot of vegetables in the field or stored at ambient temperatures.

Biogenic amines are produced in meat and fish by several members of this group while others produce off-odors or colors in beer (Obesumbacterium), bacon and other cured meats (Proteus, Serratia), cheeses (several genera), coleslaw (Klebsiella), and shell eggs (Proteus, Enterobacter, Serratia). Temperature, salt concentration, and pH are the most important factors determining which, if any, of these microbes spoil foods.

Many Gram-negative bacteria, including pseudomonads and enterobacteriaceae, secrete acyl homoserine lactones (AHLs) to regulate the expression of certain genes, such as virulence factors, as a function of cell density. 

These AHL quorum-sensing signals may regulate proteolytic enzyme production and iron chelation during spoilage of some foods  although the role of these signals in other spoilage systems is not clear .

Other bacteria are associated with spoilage of chilled, high protein foods such as meat, fish, and dairy products.

They may not be the predominant spoilage organisms but contribute to the breakdown of food components and may produce off-odors.Most species are aerobic although some grow at lowoxygen levels and may survive vacuum packaging,and one (Brochothrix) is a facultative anaerobe. 

Some examples include: Acinetobacter and Psychrobacter, which are predominant bacteria on poultry carcasses on the processing line and have been isolated from a variety of spoiled meat and fish. 

Acinetobacter grows at a pH as low as 3.3 and has been detected in spoiled soft drinks. These two genera do not produce extracellular lipases, hydrogen sulfide, or trimethylamine (fishy odor) and so are considered to have a low spoilage potential.

Alcaligenes is a potential contaminant of dairy products and meat and has been isolated from rancid butter and milk with an off-odor. 

These bacteria occur naturally in the digestive tract of some animals and also in soil and water.

Flavobacterium is found widely in the environment and in chilled foods, particularly dairy products, fish, and meat.

It uses both lipases and proteases to produce disagreeable odors in butter, margarine, cheese, cream, and other products with dairy ingredients. Moraxella and Photobacterium are important constituents of the microflora on the surface of fish. 

Photobacterium can grow and produce trimethylamine in ice-stored, vacuum-packaged fish.

Brochothrix has been isolated from meat, fish, dairy products and frozen vegetables. 

During spoilage, it produces odors described as sour, musty, and sweaty.