Contamination and Spoilage of Foods: Causes and Effects in Various Food Types

Cereals are important foods which provide bulk of our dietary requirements. They are also source of carbohydrates which are metabolized by body for energy generation. Besides, cereals also provide minerals, proteins and vitamins. Nigeria produces a large variety of cereals such wheat, maize, barley, millets, etc. Various types of products are prepared from cereals. Cereal products can be broadly classified into the following groups:

Whole cereals where only the husk of the grain is removed such as rice, wheat, gram, lentils, etc. Milled grain products are made by removing the bran and usually the germ of the seed and then crushing the kernel into various sized pieces to produce wheat flour, maida, semolina, etc.

Processed cereals like weaning food, breakfast cereals, etc. Ready mixes like cake mix, idli mix, vada mix etc. India is self-sufficient in grain production and is the second largest rice producer in the world with a 20% share. But due to constantly increasing population there is still a shortfall in cereals. A large amount of these cereals is spoilt every year due to various factors.

Spoilage Factors in Cereals and Cereal Products

The grains are low moisture commodities and are less susceptible to spoilage, hence, have greater shelf life. The spoilage mainly occurs due to moisture absorption during storage leading to fungal growth at high temperature and humidity.

Before bulk packaging and storage, the whole grains are usually fumigated to reduce microbial load and increase storage period. The three factors influencing the quality of cereals are: Physical factors Physical losses are caused by spillages occurring due to use of faulty packaging materials.

Physiological losses occur as a result of respiration and heating in grains, temperature, humidity and oxygen. Biological factors Biological losses occur due to microorganisms, insects, rodents, and fungi. The sources of contamination in cereals are: Soil Air Insects Natural microflora of harvested grains

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Microbial Contamination in Cereal Grains and Flours

At initial stages, the grains are contaminated by Pseudomonas, Micrococci, Lactobacillus and Bacillus. The initial bacterial and mold population varies from one grain to another.

Due to low moisture content, grains and flours usually have long shelf life if properly harvested or stored under proper conditions. Spoilage of stored grains by molds is attributed to the following factors: Type and number of microorganisms

Moisture content of more than 12-13%

Storage temperature

Physical damage

Most common species of molds are Aspergillus, Rhizopus, Mucor and Fusarium. A significant aspect of spoilage of molds is production of mycotoxins which may pose danger to health.

The process of flour making such as washing and milling reduces the microbial load of product. Moisture content of less than 15% does not allow growth of molds. Most molds and bacteria in flours can grow only above 17% moisture, thus moistening of flours is essential for spoilage by microorganisms.

Spoilage Mechanisms in Bread Products

Spoilage of Bread Bread is prepared from flours which undergo fermentation for which desirable microorganisms must grow. If this fermentation exceeds the required limits, it causes souring.

Excessive growth of proteolytic bacteria reduces the gas holding capacity which is otherwise required for dough rising. Spoilage of bread is usually of two types namely: moldiness and ropiness. During bread making, it is baked at very high temperature thereby leaving less chances of survival of microorganisms. Thus, the contamination usually occurs when cooling is done as well as during packing, handling and from the environment.

The molds which are prevalent are Rhizopus stolonifer (referred as bread mold), Penicillium expansum, Aspergillus niger. Mucor and Geotrichum also develop. The ropiness in bread is usually due to bacterial growth and is considered more prevalent in homemade breads. The main causative organism is Bacillus subtilis or B. licheniformis.

These are spore forming bacteria with their spores surviving baking temperatures. These spores can germinate into vegetative cells, once they get suitable conditions as heat treatment activates them.

In ropiness, the hydrolysis of bread flour protein takes place by proteinases. Starch is hydrolysis by amylases encourages ropiness. The manifestation of ropiness is development of yellow to brown color and soft and sticky surface accompanied by odor.

Another type of spoilage of bread is chalky bread which is caused by growth of yeast like Endomycosis fibuligera and Trichosporon variable and characterized by development of white chalk like spots.

An unusual spoilage of bread is Red or Bloody bread, which is due to the growth of the bacteria Serratia marcescens. This organism produces brilliant red color on starchy foods giving blood like appearance. Neurospora and Geotrichum may also be involved in imparting pigmentation during spoilage of bread.

Contamination and Spoilage Processes in Vegetables

Contamination and Spoilage of Vegetables Vegetables form an integral part of diet due to their role in providing various types of vital nutrients such as carbohydrates, minerals, vitamins, roughages etc.

Vegetables being a part of fresh produce, contain high moisture which makes them highly perishable foods and hence more prone to spoilage. Microorganisms gain entry into vegetables from various sources

Harvesting and processing equipment Handlers Packaging and packing material Contact with spoiled vegetables The conditions in which vegetables are stored and transported after harvesting also contribute to rate of spoilage.

Types of Microbial Spoilage in Vegetables

Types of Spoilage in Vegetables The microbial spoilage of vegetables is predominately of following types

Spoilage due to pathogens The plant pathogens which infect stems, leaves, roots, flowers, fruits, and other parts of the plant causing different plant diseases.

Spoilage due to saprophytes Vegetables have general microflora inhabiting them which grows under certain conditions and spoil them. There are certain secondary invaders which may enter the healthy food or grow after growth of pathogens. It is well known that plant diseases are mostly caused by fungi.

Thus, most of the spoilage causing pathogens in vegetables is fungi. Fungi have specific characteristics when spoiling food as it leads to mushy areas which may be water soaked. The fungi produce characteristic spores which may be pigmented. The pigmentation helps in identification of the type of spoilage by fungi.

The bacterial diseases too cause spoilage of vegetables but to a lesser extent. Spoilage in vegetables is largely affected by composition of vegetable. The non-acidic foods are spoiled by bacterial rot while acidic foods with dry surfaces are more prone to mold spoilage. The product on which organism grows and types of organisms growing largely determine the character of spoilage.

Bacterial Soft Rot Spoilage in Vegetables

Bacterial Soft Rot Caused by Erwinia carotovora and Pseudomonas such as P. marginalis. Bacillus and Clostridium species are also implicated.

Breaks down pectin, giving rise to a soft, mushy consistency, sometimes a bad odour and water-soaked appearance. Vegetables affected include onions, garlic, beans, carrot, beets, lettuce, spinach, potatoes, cabbage, cauliflower, radishes, tomatoes, cucumbers and watermelons.

Fungal Spoilage Mechanisms in Vegetables

Fungal spoilage of vegetables Penicillium, Cladosporium, Rhizopus and Aspergillus species are responsible for various defects in vegetables. Gray mold rot caused by Botrytis cinera in vegetables is favoured by high humidity and warm temperature

Contamination and Spoilage Processes in Fruits

Contamination and Spoilage of Fruits Fruits are natural sources of minerals, vitamins besides carbohydrates and other essential substances. Naturally fresh fruits and juices made out of them contain high amount of water thereby making them hig

hly prone to attack by microorganisms. Most of the fruits are naturally provided with coatings and coverings in the form of skins but these are fragile enough to be easily disturbed by various biological and mechanical factors.

Like vegetables, fruits being foods of plant origin get contaminated through different sources by a variety of microorganisms which may play significant role in their spoilage. These are soil, water, diseased plant, harvesting and processing equipment, handlers, packaging and packing material and contact with spoiled fruits.

Microorganisms Associated with Spoilage in Fruits and Juices

Microorganisms associated with spoilage in fruits and juices The microorganisms associated with fruits depend on the structure of fruit. The fruits contain different organic acids in varying amounts and the predominately found of acids are citric acid, malic acid and tartaric acid.

The low pH of fruits restricts the proliferation of various types of organisms. Due to the low pH, a large number of microorganisms are restricted to grow on fruits. Fungi are most dominating organisms to grow on fruits because of the ability of yeasts and molds to grow under acidic conditions.

A small number of bacteria which are aciduric grow on fruits. The dry conditions prevailing on the skin and surface do not allow the growth of certain microorganisms and these plants also produce certain antimicrobial components too. Despite the high water activity of most fruits, the low pH leads to their spoilage being dominated by fungi, both yeasts and molds.

Yeasts Involved in Fruit Spoilage

Yeasts Yeasts are unicellular fungi which normally reproduce by budding. Of the 215 species important in foods, about 32 genera are associated with fruits and fruit products. Only a few species of yeasts are pathogenic for man and other animals but none of the pathogenic species are common contaminants of fruits and fruit products.

Fruit that has been damaged by birds, insects, or pathogenic fungi usually contain very high yeast populations. The yeasts are introduced into the exposed tissue, often by insects, and are able to use the sugars and other nutrients to support their growth.

Types of yeasts growing in fruits depend upon the nature of the fruit and the strain of yeast. Growth of a strongly fermentative type such as certain strains of Saccharomyces cerevisiae may produce sufficient CO2 to burst the container. Growth of some species in a clear fruit juice may produce only slight haze and sediment.

Carbon dioxide and ethanol are the predominant metabolic products of yeasts but other products such as glycerol, acetaldehyde, pyruvic acid, and a – ketoglutaric acid are also formed.

Oxidative yeasts such as species of Brettanomyces produce acetic acid in wines and other fruit products. Although yeasts produce hydrolytic enzymes which degrade pectins, starch, and certain proteins, enzymatic activity is usually much less than that exhibited by other aciduric microorganisms (molds in particular).

Moulds Involved in Fruit Spoilage

Moulds Moulds are filamentous fungi which are important group of microflora of fruits and fruit products. Some of the members are xerophilic, thereby having potential to spoil foods of low water activity such as dried fruits and fruit juice concentrates. Some of the species have heat resistant spores such as ascospores which can survive the commercial pasteurization treatments that are given to most fruit products.

Growth of moulds on processing equipment such as wooden tanks can result in the generation of off-flavors in wines, juices, and other fruit products. Mould- infected raw fruit may become soft after processing because pectinases will not inactivated by the thermal treatment.

The metabolic products of many molds are toxic to humans, e.g. mycotoxins. Moulds are aerobic microorganisms, but many of them are very efficient scavengers of oxygen. Due to this property of molds, processed fruits, including those hermetically sealed in cans or glass, are susceptible to spoilage.

In case of limited vegetative growth, evidence of spoilage may be the changes produced by fungal enzymes such as the breakdown of starch or pectins while in case of heavy growth, colonies develop in the headspace or as strands throughout a beverage or similar product.

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Specific Types of Fungal Spoilage in Fruits

Some types of spoilage by fungi Rhizopus stolonifer cause soft and mushy food, cottony growth of mold.

Anthracnose Colletotrichum lindemuthianum, cause spotting of leaves and fruits and seed pods

Downy mildew: Initially, the lesions tend to be small and confined to the upper surface of wrapper leaves. As the areas enlarge, they turn from light green or yellowish to brown and become soft. It is caused by Phytophthora. Watery soft rot This rot occurs on the lower part of water soaked and light or pinkish brown called leads. A white cottony mold spreads over the decayed tissue and the lead eventually becomes a watery mall.

Bacteria Involved in Fruit Spoilage

Bacteria:

Various groups of bacteria have ability to grow on fruits and its juices. These bacteria by virtue of their diversity in metabolism grow on fruits and produce different types of compounds. The major groups of bacteria of bacteria that grow on the fruits: Lactic acid bacteria Acetic acid bacteria Spore forming bacteria

1. Lactic Acid Bacteria in Fruit Spoilage
Lactic acid bacteria The lactic acid bacteria are Gram-positive, catalase negative, rod-shaped (lactobacilli), or cocci organisms which can grow under anaerobic conditions. The homo fermentative species produce mainly lactic acid from hexose sugars; the hetero fermenters produce one molecule of lactic acid, one molecule of carbon dioxide, and a two-carbon compound, which is usually acetic acid or ethanol or a combination of the two.

Growth of lactic acid bacteria in juices and other fruit products cause the formation of haze, gas, acid, and a number of other changes. Certain hetero-fermentative lactobacilli lead to slime in cider.

The lactobacilli and leuconostocs that are present in citrus juices generate acetylmethylcarbinol and diacetyl, compounds that give the juices an undesirable, butter-milk- like flavor. Lactic acid bacteria have the ability to decarboxylate malic acid to lactic acid.

This malo-lactic fermentation is often desirable in high-acid wines because the acidity is reduced and desirable flavors are produced. Oenococcus oenos is the most acid and alcohol-tolerant species often isolated from wines that are undergoing a malo-lactic fermentation.

2. Acetic Acid Bacteria in Fruit Spoilage
Acetic acid bacteria These are Gram negative, aerobic rods having two genera, namely: Acetobacter and Gluconobacter. These species oxidize ethanol to acetic acid under acidic condition, Acetobacter species can oxidize acetic acid to carbon dioxide thus, the genus is called over oxidizer.

The bacteria are obligate aerobes, so, juices, wines, and cider are most susceptible to spoilage while held in tanks prior to bottling. Some strains of Acetobacter pasteurianus and Gluconobacter oxydans produce microfibrils composed of cellulose, which leads to formation of flocs in different fruit juice beverages.

Spore Forming Bacteria in Fruit Spoilage

Spore forming bacteria Spores are heat resistant, so, the role of organisms producing spores is important in heat treated of juices and beverages. Various spore formers such as Bacillus coagulans, B. subtilis, B. macerans, B. pumilis, B. sphaericus , and B. pantothenticus have been found to grow in different types of wines.

Some of these organisms have also been involved in canned fruits. Sporeforming bacilli that actually prefer a low pH have been responsible for spoilage of apple juice and a blend of fruit juices.

Contamination and Spoilage in Meat and Meat Products

Contamination and Spoilage of Meat and Meat Products The microbiological profile of meat products presented to the consumers is the sum total of the slaughtered animal health, conditions under which it was reared, quality of slaughtering, processing, packaging and conditions under which the meat was stored.

Meat pathogens can cause self-limiting human enteric diseases or systemic and fatal infections of the immunocompromised, the elderly and the young. Meat can act as an ideal substrate for microbial proliferation. Major meat associated pathogenic bacteria include Clostridium perfringens,

Staphylococcus aureus, Salmonella spp, pathogenic strains of Escherichia coli, Campylobacter spp, Yersinia enterocolitica, Listeria monocytogenes and Aeromonas hydrophila. Microorganisms Associated With Meat during Processing Meat spoilages .

Salmonella is the primary microbial challenge of poultry and Escherichia coli O157: H7 is the primary microbial to the beef industry. Listeria, which is an adulterant with zero tolerance, is the major problem for ready to eat meat products.

Treatment with organic acids, hot water steam, carcass pasteurization and steam carcass vacuuming, trisodium phosphate, acidified sodium chlorite, chlorine dioxide, lactoferrins, peroxyacetic acid, sodium lactate, sodium acetate and sodium diacetate, ozone and radiation have been used as microbial decontaminants during meat processing operations.

Carcass washing with hot water of 80°C for 10 seconds can reduce microbial loads by two logs. Current regulatory policies and inspection in the meat industry include the HACCP (Hazard Analysis Critical Control Point) food safety system with an objective to provide safe food for consumption and prevent chemical, physical and biological hazards.

Gram-negative bacteria (Aerobes): Neisseriaceae: Psychrobacter immobilis, P. phenylpyruvica, Acinetobacter spp., A. twoffii, A. Johnsonii, Pseudomonadaceae: Pseudomonas fluorescens, P. lundensis, P. fragi, and P. putida Gram-positive bacteria: Brochothrix thermosphacta, Kurthia zophii, Staphylococcus spp., Clostridium estertheticum, Clostridium frigidicarnis, Clostridium casigenes, and Clostridium sp.

Frequently Asked Questions (FAQs)

1. What are the main factors influencing the spoilage of cereals and cereal products?
The main factors include physical factors such as spillages from faulty packaging, physiological factors like respiration, heating, temperature, humidity, and oxygen, and biological factors involving microorganisms, insects, rodents, and fungi.

2. Which microorganisms are commonly responsible for mold spoilage in stored grains and flours?
The most common species of molds are Aspergillus, Rhizopus, Mucor, and Fusarium, which can produce mycotoxins and thrive when moisture content exceeds 12-13%, especially under high storage temperatures or with physical damage to the grains.

3. What causes ropiness in bread and how does it manifest?
Ropiness in bread is usually due to bacterial growth, primarily from Bacillus subtilis or B. licheniformis, whose spores survive baking temperatures. It manifests as development of yellow to brown color, a soft and sticky surface, and an accompanying odor, resulting from hydrolysis of proteins and starch.

4. Why are vegetables highly prone to microbial spoilage?
Vegetables contain high moisture, making them perishable, and microorganisms enter from sources like soil, water, diseased plants, harvesting equipment, handlers, packaging, and contact with spoiled vegetables. Storage and transport conditions after harvesting also contribute to the spoilage rate.

5. What types of fungi cause specific spoilage in fruits, such as soft and mushy textures?
Rhizopus stolonifer causes soft and mushy food with cottony mold growth, while other types include anthracnose by Colletotrichum lindemuthianum (spotting of leaves, fruits, and seed pods), downy mildew by Phytophthora (lesions turning brown and soft), and watery soft rot (water-soaked areas with white cottony mold).

6. How do lactic acid bacteria affect fruit juices and products?
Lactic acid bacteria cause formation of haze, gas, acid, and other changes in juices; hetero-fermentative types can lead to slime in cider, while in citrus juices, they generate acetylmethylcarbinol and diacetyl, giving an undesirable buttermilk-like flavor. They also perform malo-lactic fermentation in high-acid wines, reducing acidity.

7. What are the major pathogenic bacteria associated with meat spoilage and contamination?
Major meat-associated pathogenic bacteria include Clostridium perfringens, Staphylococcus aureus, Salmonella spp., pathogenic strains of Escherichia coli, Campylobacter spp., Yersinia enterocolitica, Listeria monocytogenes, and Aeromonas hydrophila, which can cause enteric diseases or fatal infections in vulnerable groups.

8. What methods are used to decontaminate meat during processing to reduce microbial loads?
Methods include treatment with organic acids, hot water steam, carcass pasteurization, steam carcass vacuuming, trisodium phosphate, acidified sodium chlorite, chlorine dioxide, lactoferrins, peroxyacetic acid, sodium lactate, sodium acetate, sodium diacetate, ozone, and radiation. Carcass washing with hot water at 80°C for 10 seconds can reduce microbial loads by two logs.

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