Microbial spoilage of poultry products
Growth of spoilage bacteria lead to defects in the products and can be responsible for unwanted taste, colour, odour or texture. There are multiple spoilage mechanisms, and they can result from the production of various metabolites such as volatiles or exopolysaccharides. Once bacteria contaminate
poultry meat and constitute the initial microbiota, the storage conditions and the various treatments applied shape the fate of this microbiota. Causes of spoilage defects
Spoilage is caused by the accumulation of metabolic byproducts or the action of extracellular enzymes produced by psychrotrophic spoilagebacteria as they
multiply on poultry surfaces at refrigeration temperatures. Some of these by-products become detectable as off-odours and slime, as bacteria utilize nutrients on the surface of meat. Off-odours do not result from breakdown of the protein in skin and muscle, as previously thought, but from the direct microbial utilization of low molecular weight nitrogenous compounds such as amino acids, which are present in skin
and muscle. Concentrations of free amino acids increase as proteolysis occurs throughout the storage period. It has been demonstrated that measurement of these free amino
acids, due to the production of aminopeptidases and subsequent breakdown of protein, may be used to rapidly determine the bacteriological quality of beef Development of off-odours and slime Microorganisms appear first in damp pockets on the carcass, such as folds between the foreleg and breast of a carcass, and their dispersion is promoted by condensation which occurs when a cold carcass is exposed to warm, damp air.
An ester-like odour, which was described as a “dirty dishrag” odour, can develop on cut-up chickens. In most cases, off-odour precedes slime formation and is considered the initial sign of spoilage. Immediately after off-odours are detected, many small, translucent, moist colonies may appear on the cut surfaces and skin of the carcass. Eventually, meat surfaces become coated with tiny drop-like colonies, which increase in size and coalesce to form a slimy coating. In the final stages of spoilage, the meat may begin to exhibit a pungent ammoniacal odour in addition to the dirty dishrag odour, which may be attributed to the breakdown of protein and the formation of ammonia or ammonia-like compounds.
Microbial spoilage of milk and dairy products
Milk is an excellent medium for growth for a variety of bacteria. Spoilage bacteria may originate on the farm from the environment or milking equipment or in processing plants from equipment, employees, or the air. LAB are usually the predominant microbes in raw milk and proliferate if milk is not cooled adequately. When populations reach about 106 cfu/ml, off-flavours develop in milk due to production of lactic acid and other compounds. Some spore-formers grow in milk and milk products and can eventually produce rancid odours through the action of lipases and bitter peptides from protease action.
Microbial spoilage of baked foods and confectioneries Microbiological spoilage is often the major factors limitingthe shelf life of bakery products. Ropey bread is characterized by discoloration from brown to black, the release of a
rotten fruit odour and having an extremely moist, stringy bread crumb. Ropiness can develop very rapidly under warm and humid conditions. Ropey bread is caused mainly by Bacillus subtilis but other species of Bacillus are capable of causing rope and these include Bacillus licheniformis, Bacillus megaterium and Bacillus cereus. Ropy spoilage in bread is first detected by an odour similar to that of pineapple. Later, the crumb becomes discoloured, soft and sticky to the touch, which makes the bread inedible. The deterioration of bread texture is due to slime being formed as a result of the combined effect of the proteolytic and amylolytic enzymes produced by some Bacillus strains which results in slime formation.
Yeast problems occur in bakery products. Wild yeast include Trichosporon variable, Saccharomyces, Pichia and Zygosaccharomyces. Saccharomyces sp. produce white spots in bread, leading to the term chalk bread. Moulds cause mouldiness in baked products. The most common moulds found in bakery products are: Rhizopus sp., Aspergillus sp., Penicillium sp., Monilia sp., Mucor sp. and Eurotium sp.
Microbial spoilage of fruits and vegetables
In Fruits, due to ripening, cell wall weakens and the amounts of antifungal
chemicals in fruits decreases. Also, physical damage during harvesting causes
breaks in outer protective layers of fruits that spoilage organisms can exploit.
High levels of sugar and a low pH in fruits juices generally favours growth of
yeasts, moulds and acid-tolerant bacteria.
In Vegetables, bacterial spoilage start with softening of tissues as pectins are
degraded and the whole vegetable finally become slimy mass.
The higher moisture content of vegetables allows different fungi to proliferate.
Spoilage microorganisms cause sensory quality failure of fruits and vegetables.
i. Surface discoloration (e.g., pinking of cut lettuce, browning of cut potato, graying and browning with processed pineapple, and gray discoloration with cabbage), water-soaked appearance or translucency (e.g., cut watermelon, papaya, honeydew, and tomatoes), moisture loss (e.g., “baby” carrots and celery sticks).
ii. Off-aroma (e.g., broccoli florets and diced cabbage in low % O2 and high CO2 packages).
iii. Flavour changes (e.g., cut kiwifruit), and texture changes (e.g., processed
strawberry, grated celery, kiwifruit, and papaya).
iv. Slimy surface (e.g., “baby” carrots), wetness and soft rot (e.g., cut bell pepper).
v. Discoloration (e.g., apple wedges).
vi. Visual microbial growth/colonies (such as apple wedges, cantaloupe chunks, and cored pineapple).
Microbial spoilage of canned foods
Canning is intended to destroy harmful pathogens in the packed food materials, however, with improper handling, cans become breeding grounds for microbes. Certain canned foods receive low-heat treatments and as such are prone to contamination by large number of different microorganisms’ types. Canning destroys the microbial contaminants, however, products undergo microbial spoilage and could cause food borne illness as a result of under processing, inadequate cooling, contamination of the can resulting from leakage and pre-process spoilage.
Microbial spoilage is by far the most common cause of spoilage and may manifest itself as
i. Visible growth (slime, colonies)
ii. Textural changes (degradation of polymers)
iii. Off-odours and off-flavours
iv. Production of biofilm on the food matters and inner surface of the canned materials
v. Corrosion of the canned materials / enamel and the leached out metals’ accumulation in the canned foods
vi. Flat sour spoilage, this type of spoilage occurs mainly in low acid foods and is caused by species of Bacillus, such as Bacillus coagulase and Bacillus stearothermophilus.
vi. A spoilage caused by Clostridium thermosaccharolyticum, produces acid and gas in food. Due to gas production, can swells and finally burst if it is kept for long a time. The spoiled food usually has sour odour or taste.
vii. Decomposition of protein to produce H2S, indole, skatable and mercaptans by mesophilic Clostridium species give bad odour to the food.
viii. Obligate thermophile produces H2S that react with tin of can to form black spots of FeS in food and on inner wall of can. H2S produced give putrid odour which is widened when can is opened
Microbial spoilage of dried foods
Drying is one of the oldest and most common methods of processing and preserving food. Drying does not only inhibit the growth of spoilage microorganisms, but also halts the occurrence of colour change and other moisture-driven deterioration reactions and thus, preserving the structure, characteristics and nutritional value of the original material.
For example, the drying of fruits and vegetables, more importantly, minimises losses, enhances storage stability, reduces packaging and handling requirements,
and makes transportation easier and cheaper because of reduced weight and volume. However, traditional sun drying is often a slow process and this increases the chances of microbial contamination.
Microorganisms especially fungi can easily containate dried foods and cause spoilage such as
i. Mouldiness
ii. Slime formation
iii. Colour and texture changes
iv. Off-odour
Microbial spoilage of fish and fish products
Composition of the microflora on newly caught fish depends on the microbial contents of the water in which the fish live. Microbial growth and metabolism is a major cause of fish spoilage which produce amines, biogenic amines such as
putrescine, histamine and cadaverine, organic acids, sulphides, alcohols, aldehydes and ketones with unpleasant and unacceptable off-flavours.
For unpreserved fish, spoilage is a result of Gram-negative, fermentative bacteria (such as Vibrionaceae), whereas psychrotolerant Gram-negative bacteria (such as
Pseudomonas spp. and Shewanella spp.) tend to spoil chilled fish. Trimethylamine (TMA) levels are used universally to determine microbial deterioration leading to fish spoilage. Fish use Trimethylamine Oxide (TMAO) as an osmoregulant to avoid dehydration in marine environments and tissue waterlogging in fresh water. Bacteria such as Shewanella putrifaciens, Aeromonas spp., psychrotolerant Enterobacteriacceae and Vibrio spp. can obtain energy by reducing TMAO to TMA creating the ammonia-like off flavours. Pseudomonas putrifaciens, fluorescent pseudomonads and other spoilage bacteria increase rapidly during the initial stages of spoilage, producing many proteolytic and hydrolytic enzymes. The proteins are degraded by the proteases into peptides and amino acids, whereas lipids are degraded by lipases into fatty acids, glycerols and other compounds.
Microbial spoilage of cereals and cereal products Cereals are a rich source of vitamins, minerals, carbohydrates, fats, oils, and
protein. Some common cereals are: corn (maize), wheat, barley, rice, oats, rye, etc.
Examples of cereal products derived from cereal grains are oat flours, semolina, corn meal and corn grits.
Bacteria are found as most frequent surface contaminants of cereal. These bacteria need high moisture or water activity with high relative humidity to grow. Bacterial pathogens like Bacillus cereus, Clostridium botulinum, Clostridium perfringens, Escherichia coli, Salmonella and Staphylococcus aureus may contaminate cereal and cereal products and cause spoilage. Also, approximately more than 150 species of filamentous moulds and yeasts on cereal grains are present as surface contaminates. Yeasts are frequently less in
number as compare to moulds. Water activity and temperature are considered the most important environmental factors inducing the mould spoilage of cereals and cereal products with the Product production of mycotoxin. Some of these moulds are Penicillium
aurantiogriseum, Penicillium brevicompactum, Aspergillus flavus, Aspergillus candidus, Eurotium amstelodami and Willemia sebi. Methods for controlling the microorganisms involved in food spoilage
1. Prevent access of the microorganism to the food
i. Good agricultural practices
ii. Good manufacturing practices
iii. Appropriate hygienic and sanitary conditions
2. Microorganism removal
i. Washing of surfaces
ii. Centrifugation
iii. Filtration by membranes
3. Growth inhibition
i.. Cooling or freezing
ii. Reduce water activity (drying and addition of solutes)
iii. Food acidification
iv. Addition of inhibitors (weak organic acids, nitrites, nitrates, sulfites, and
bacteriocins)
v. Microbial competition
vi. Atmosphere modification (vacuum or modified atmosphere packaging)
4. Microbial inactivation
i. Thermal treatments (pasteurization, sterilization, blanching, and thermization)
ii. Radiation
iii. High hydrostatic pressures
iv. Pulsed electric field processing
v. Ultrasound
Storage conditions of meat and meat products
Traditional methods of meat preservation such as drying, smoking, brining and canning have been replaced by new preservation techniques such as chemical, bio-
preservative and non-thermal techniques. Current meat preservation methods are broadly categorized into three methods
i. Controlling temperature,
ii. Controlling water activity
iii. Use of chemical or bio-preservatives
Combination of these preservation techniques can be used to diminish the process
of spoilage.
Storage conditions of fish and fish products
Different types of preservation methods such as drying, smoking, freezing,
chilling, brining, fermentation and canning are reported to extend the shelf-life of
fish and fish products.
However, low temperature storage and chemical techniques for controlling water
activity, enzymatic, oxidative and microbial spoilage are the most common in the
industry today.
Storage conditions of baked foods and confectioneries
Several methods can be used to control mould growth on bakery products. These
methods include
i. Reformulation to reduce water activity
ii. Freezing
iii. The use of preservatives.
Storage conditions of milk and milk products
In order to make milk safe for human consumption, it should be protected from
external contamination at the time of its production, storage along with
transportation. The milk preservation strategies differ depending on available
facilities. The most commonly used methods are
i. High temperature treatment e.g. pasteurization
ii. Low temperature treatment e.g. refrigeration
ii. UV exposure
iii. Treatment by microwave
iv. Processing of membrane and microfiltration
Storage conditions of fruits and vegetables
Many thermal and non-thermal technologies have been developed to control
microorganisms in fruits and vegetables.
Types of thermal processing used include
i. Hot water
ii. Hot steam
iii. Hot sanitizing solution
There are a number of difficulties associated with the application of thermal
processes to fresh-cut fruits and vegetables. For example, thermal processing
cannot be used for fresh-cut commodities such as leafy vegetables and berries
because of deterioration of quality characteristics.
Non-thermal processing technologies can be classified as either physical or
chemical.
Physical technologies include
i. High pressure
ii. Irradiation
iii. Pulsed electric fields
iv. Pulsed white light
v. Ultrasound, and ultraviolet radiation.
Some of these methods are generally not applicable commercially because they are too expensive (high pressure and pulsed electric fields), do not have consumer
acceptance (irradiation), or require process validation of efficacy (UV and pulsed white light).
Chemical technologies can be divided into two based on the physical state of the chemical used
i. Gas-phase sanitation e.g. ozone and chlorine dioxide
ii. Liquid-phase sanitation
Microorganisms in food fermentation
Most commonly used microorganisms in food fermentation are bacteria, yeast and
moulds, or a combination of these.
Fermentation is a natural process in which microorganisms turn sugar into alcohol,
carbon dioxide, organic acids and esters.
Benefits of food fermentation
i. Develop and improve flavours, aromas and textures of food e.g. coffee beans to
coffee, grapes to wine.
ii. Extend the shelf life of food, e.g. milk to yoghurt and cheese
iii. Improve the nutritional value of the product, e.g.
a) improved digestibility e.g. wheat to bread
b) synthesis of probiotic compounds e.g. milk to yoghurt
Bacteria
Bacteria are used to make a wide range of products.
The most important bacteria in food fermentation is the Lactobacillus bacteria species, also known as lactic acid bacteria. Lactic acid bacteria is a group of bacteria that can decompose lactose (a form of sugar) in products to lactic acid. Examples are Lactobacillus, Lactococcus, Bifidobacterium and Leuconostoc.
Functions of bacteria in the production of fermented foods
Lactic acid bacteria are used as the starter cultures in the production of fermented foods such as
i. Cultured dairy products such as cheese and yoghurt from milk
ii. Fermented meat products such as salami and pepperoni iii. Pickled vegetables such as sauerkraut and kimchi from cabbage
iv. Fermented drinks such as fruit wine and alcoholic wine
Yeasts
Yeasts are used in the production of wide varieties of fermented products.
Saccharomyces cerevisiae is a very important type of yeasts in food industry as a
starter for producing fermented foods. Other yeasts include Candida spp and
Kluyveromyces spp.
Functions of yeasts in the production of fermented foods
Through fermentation, yeasts are used in the production of
i. Leavened bread and bakery products: Saccharomyces cerevisiae ferments sugars
to produce CO2, the gas that gives the porous structure of bakery products.
ii. Production of alcoholic drinks such as beer, wine, vinegar and pickles.
iii. Contribute to products flavour by formation of alcohols, aldehydes, esters, etc.
Moulds
Moulds are used to produce specific flavours and textures in several food products.
Examples of moulds used in food fermentation are Aspergillus oryzae, Rhizopus
oryzae and Penicillum roquefortii
Functions of moulds in the production of fermented foods
i. The moulds Aspergillus oryzae and Aspergillus sojae are used in the production
of soya sauce and miso
ii. The mould Penicillium is used in the production of cheese
iii. The production of dry salami by Penicillium and Scopulariopis moulds
iv. The production of tempeh from legumes by Rhizopus oligosporus.