Food Microbiology

Microbial Food Spoilage Causes, Classification, Process, Factors

Food spoilage is the result of a metabolic process which causes food products to become unpalatable or inedible for human consumption because...

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Microbial Food Spoilage Causes, Classification, Process, Factors
Microbial Food Spoilage Causes, Classification, Process, FactorsMicrobial Food Spoilage Causes, Classification, Process, Factors

What is food spoilage?

Food spoilage is the result of a metabolic process which causes food products to become unpalatable or inedible for human consumption because of changes in the sensory attributes.

Foods with sporilage are identified through off-colors, off odors, softening of fruits, vegetables and the production of slime. Spoilage could be caused by insects, physical damages as well as the activity of enzymes indigenous to in food, or caused by microorganisms (bacteria or viruses, fungi, etc.). The microbes that cause spoilage are the soil, water or the intestinal tracts of animals , or they can be dispersed throughout the air and in the water.

The microorganisms responsible for food spoilage depend on the intrinsic (pH water activity, pH nutrients, oxidation reduction potential antimicrobial property) and extrinsic elements (temperature and relative humidity pressure). Different microorganisms that cause spoilage have different requirements for nutrients. Microorganisms are the biological agent that cause foodborne illnesses when eaten, however, the microorganisms are not just responsible for food spoilage, but some can be beneficial to food fermentation.

The food spoilage caused by microbial organisms is determined by these changes:

  • Change in appearance: It is identified by the appearance of cloudiness and the fluid formation in the food.
  • Change in texture: The texture changes are due to the increase in microbial cells as well as tissue degrading, leading to slime creation and the rotting process.
  • Colour change: This usually is caused by mycelial growth and breakdown.
  • Change in taste and odour: Odour and taste of the food is affected because of the oxidation process that occurs in sulfurous nitrogenous substances and organic acids, etc. through the enzymes of microbial life.

What causes food spoilage?

Two common elements favor the expansion and multiplicity of microorganisms.

  1. Storage Condition
  2. Chemical Properties

1. Storage Conditions of the Food

The conditions of storage generally involve elements of the environment like pH, temperature and oxygen, which favor growth of microbial colonies.

a. Temperature

The temperature that is psychrophilic, i.e. -17, is believed to be safe, as it will stop any growth in microorganisms. The temperature that is above this is known as mesophilic and is the most favorable for the growth of microbial. The mesophilic temperature is the range of 20-40 deg Celsius. So, the temperature that is warm is the ideal temperature for microbial growth such as thermophilic and mesophilic microorganisms.

b. Oxygen

There are anaerobic and aerobic microorganisms which attack food products in the storage environment in the absence or presence of oxygen, or both. The conditions of storage that favor aerobics favour molds and aerobic bacteria. If there is anaerobic storage conditions, they favors anaerobic bacteria such as Clostridium species.

2. Chemical Properties of the Food

Food spoilage happens by the degrading of food ingredients by the microorganisms’ enzymatic activities. The chemical properties of food products that impact the growth of microbes comprises:

  • Chemical Composition of  foods
  • Acidity o food
  • Moisture content of food

a. The chemical composition of the food

In the food industry there are certain organic biomolecules such as carbohydrate, protein and fats are required to promote the growth of microbial colonies.

(i) Protein-rich foods

In foods that are rich in protein microorganisms that attack are known as proteolytic microorganisms. The proteolytic enzyme triggers the degrading of proteins into simpler forms, such as amino acids, amino amines and so on. The proteolytic microorganisms are the gram-negative bacteria that produce spores.

(ii) Carbohydrate-rich foods

In carbohydrate-rich food the microorganisms that attack are known as carbohydrate fermenting microorganisms. Microorganisms that ferment carbohydrate cause the degrading of carbohydrate to the fermentative substances by producing gases, acids, and alcohols. The microorganisms that ferment carbohydrate comprise yeast, moulds as well as the bacteria (Micrococcus Sp., Streptococcus sp. etc. ).

(iii) Fat rich foods

In fat-rich food the microorganisms that attack are known as lipolytic microorganisms. Lipolytic enzymes result in the degradation from fat to simpler form, such as Glycerol, fatty acids and so on. The microorganisms that lipolyze include moulds as well as gram-negative bacteria.

b. The acidity of the food

The pH that is below 4.5 is not conducive to the growth of bacterial colonies which favors moulds and yeasts that mostly impact the acidic food items (like fruit and vegetable juices). The higher pH encourages bacteria that are most prevalently affected by the non-acidic food items.

c. Moisture and osmotic concentration of food

In food 13% of the free water encourages microbial growth. A high sugar and salt content stop the growth of microorganisms. To prevent for the development of molds the sugar concentration required is 65-70 percent. In order to grow microorganisms like yeast and bacteria, the recommended sugar content is 50%.

Classification Of Foods By Ease Of Spoilage

Based on the degree of their ability to withstand spoilage, food items are categorized into three categories:

  1. Stable or nonperishable foods: They are those that are not prone to spoiling if handled with care, include items as flour, sugar dry beans, and sugar.
  2. Semiperishable foods: When they are properly handled and stored, they’ll remain unspoiled for a lengthy time period, e.g., potatoes and various varieties of apples waxed rutabagas and nutmeats.
  3. Perishable foods: This class includes the majority of daily meals which are prone to spoiling unless specific preservation methods are employed. Meats and poultry, fish, most fruit and vegetable, egg and milk are included in this class.

Most food items fall in one of the three categories Some are close enough to borderline food items to be difficult to classify.

The Process of Microbial Food Spoilage

Food spoilage caused by microbes can be summarized in these steps

  1. Microbes first attack the food: The microbes attack first the food as it contains all the nutrients needed by microorganisms, they can be found with a suitable temperature, pH, humidity and oxygen, etc.
  2. Food degradation: Microorganisms break down the food ingredient by using the nutrients present in the food, and also decompose the material.
  3. Decomposition: The enzymatic reaction occurs between the food components like protein, lipid, fat, carbohydrates etc., and the microbial enzymes carry out some chemical changes.
  4. Changes as a result of food decomposition: The changes are as a result of texture, appearance color, taste, odour and so on. because of spoilage.

Factors Affecting Kinds And Numbers Of Microorganisms In Food

The extent of spoilage caused to food items by enzymes and microorganisms will be based on the types and quantities of these agents that are present as well as the surrounding surroundings around them. Most raw food products include a range of yeasts, bacteria and molds. They may also contain animal or plant enzymes, as the case might be. Due to the unique environment there is a very small percentage of the microorganisms that are present can grow quickly and cause spoilage. Usually, it is only one kind of organism, however there are times when it is three or four types. They might not have been the predominant ones in the first food. If the spoilage caused by the initial organism is allowed to continue in a different way, other kinds of organisms are more likely to cause secondary spoilage or a new variety of organisms and/or changes might be involved.

The variety and quantity of microorganisms present in food items or food products will be affected by the nature and severity of contamination, the previous possibilities for the growth of specific varieties, and the types of pretreatments the food was subjected to. The contamination can increase the number of microorganisms present in food, and could even introduce new varieties. Wash water could also contain contaminants that can taint the surface of butter. Plant equipment could introduce spoilage organisms into food items when they are processed; washing machines can include them in eggs; and dirty boats could introduce them into fish.

The increasing “bioburden” of microorganisms especially those that can are responsible for spoilage, make preservation more difficult. i.e. spoilage is more likely to occur and faster and may take an entirely different form than that could have been seen in the absence of contamination. In the case of microorganisms, their growth on food items will increase the numbers or bioburden of microorganisms in all foods, can cause the largest rise in the bacteria that are most likely to participate in spoilage. The higher burden of bioburden can make it more difficult in preventing spoilage of food items and could affect the type of spoilage that can be expected.

Foods that have been pretreated may eliminate or destroy certain kinds of microorganisms. They may also add new organisms, or alter the proportions of these present, or even inactivate a portion or all the enzymes in food and limit the amount of spoilage ingredients and thus the types of spoilage that can occur. For instance, washing could remove microorganisms off the surface or bring them into the water that is used to wash. If washing is made by the use of an antiseptic or germicidal solution the amount of bacteria present could be reduced significantly and certain species might be completely eliminated.

Treatment with rays, oxygen sulfur dioxide, rays or germicidal gases will decrease the number of organisms and make them more selective in the kinds of organisms. Temperatures that are high will cause the death of increasing numbers of organisms and result in fewer and less types of organisms that the treatment with heat is enhanced. Storage conditions under different conditions can be able to increase or decrease the types and quantities. Each of these strategies along with other methods not listed, can affect the number of types, sizes and the health of microorganisms.

Factors Affecting The Growth Of Microorganisms In Food

1. Associative Growth

Microorganisms that interact with one and with each other can be found in the fermentations or spoilage of many types of food. There is a competition among the various kinds of yeasts, bacteria, and other molds that are present in food is usually the determining factor in which can outgrow the other and result in its particular type of spoilage. If the conditions are favorable for all, the bacteria tend to develop faster than yeasts, and yeasts more quickly than molds.

So, yeasts are able to outcompete bacteria only when they are dominant at the beginning, or in conditions that are favorable to inhibit the growth of bacteria. The molds will dominate when the conditions are more favorable for them than the yeasts, or for bacteria. Different types of bacteria that are present fight for dominance with one species generally outperforming the other. If yeasts are popular by one type, it is likely that they will outdo others, and within the molds, one type will have a better chance of surviving than other types. Microorganisms may not be always antagonistic or anti-biotic against one another However, they can occasionally be symbiotic i.e. mutually beneficial or even be growing simultaneously, but not seem to help or hinder one the other.

Two types of microorganisms can be synergistic i.e. when they grow together they might be able to create changes, like fermentations, which neither of them could create on their own. Pseudomonas syncyanea* that is growing by itself in milk only produces an ethereal brown hue, while Streptococcus lactis does not cause any change in milk’s color; however when the two organisms develop together, a vivid blue hue develops. This is resulting from the pH effects upon the pigment brown made from P. Syncyanea. One of the most significant effects of a microorganism’s effect on another is the metabiotic effect which happens when one organism creates conditions that encourage growth of the other. Both organisms can be growing simultaneously and more often, one will outgrow over the other. The majority of natural decompositions or fermentations of food items show metabi-osis.

Room temperature raw milk usually starts to support an acid fermentation caused by Streptococcus lactis, as well as the coliform bacteria , until the bacteria are slowed down by the acid they’ve created. The acid-tolerant lactobacilli then enhance the acidity even more until they stop. Film yeasts and molds develop over the toplayer, eventually reducing the acidity to allow proteolytic bacteria be active. The normal sequence of bacteria is first diverse bacteria, mainly Coliform, followed by Leuconostoc mesenteroides, third Lactobacillus plantarum and, lastly Lactobacillus brevis.

2. Effect of Environmental Conditions

The environment determines which the microorganisms that are present in food will be more dominant than others and create their own sort of alteration or spoilage. The elements that comprise this environment are interconnected and together determine the species that develop and the results that will be produced. Most important among these elements are the chemical and physical characteristics of food items, as well as the availability of oxygen and temperature.

A. Physical State and Structure of the Food 

The physical condition of the food and its colloidal nature and whether it’s been frozen or heated, moistened or dried, in conjunction with its biochemical structure can have a significant impact on the likelihood that a food item is rotten and what kind of spoilage it will experience.


The presence of water in food items the location of it, as well as its availability are the main factors that influence the growth of microbial species. Water can be viewed as a chemical substance that is essential for growth as well as as an element in the structure and physical makeup of food. The requirements for moisture are met by molds yeasts, bacteria, and molds.

It is emphasized how all microorganisms need a source of moisture for growth , and that they all thrive when they have plenty of. The moisture should be accessible to the microorganisms, i.e. it must not be tied up in any way like by solutes, or a hydrophilic collloid like Agar. Solutes like salt and sugar dissolved in water can cause an osmotic force which causes water to be drawn out of cells when the amount of dissolving materials is higher outside the cell than within. It is imperative to note that if it is determined that the relative moisture of air around the food is in line with the available water or moisture of the food, then the food and the air around it will be in equilibrium with relation to the moisture.

A food item that is dry like bread is more likely to become spoilt by molds, sirups and honey, due to their high sugar content, which in turn lower aw levels, favor the development of yeasts that are osmophilic; and neutral and moist food items like fish, meats, milk eggs and fish are usually affected by bacteria. But, other environmental factors than moisture must be kept in mind when determining the kind of microorganism most likely to lead to spoilage. Grape juice, as an example is a good source of yeast due to its high sugar content and low pH , but can encourage an increase in bacteria when the temperatures of incubation are either too high or low for yeasts that ferment. Foods stored in refrigerators can be prone to mold when they are in contact with air, but will be spoiled by bacterial growth in the absence of air. Honey, even though the sugar levels are high for many yeasts however, it is not suitable for all kinds of molds seldom is damaged by molds due to the presence of the presence of fungistatic ingredients.

A w of that is as low as 0.70 is unlikely to cause any loss due to microorganisms in food stored at ambient temperature. This is about the amount of moisture present in dry milk, which is 8 percent total moisture. dried whole eggs between 10 and 11 percent flour, between 13 and 15 percent dry milk with nonfat content of 15 percent. Dehydrated fat-free animal with 15 percent the seeds of leguminous crop with 15 percent moisture, dehydrated vegetable with 14 to 20 percent, fruits dehydrated with 18 to 25 percent as well as starch with 18% (Mossel and Ingram 1955).

There is a possibility for the microorganisms that grow in food sources to alter the amount of moisture available by releasing metabolic water or by altering the substrate to release water. When they produce bread’s ropiness for instance it is thought that Bacillus subtilis is responsible for the release of water as a result of the breakdown of starch and, in turn, creates conditions favorable to its growth. The destruction of the tissues that hold moisture, such for example, in fruit by molds, could allow water to yeasts or other bacteria.

Heat processing

The process of heating can alter not just the compositional chemical of food, but also its structure through softening tissues; the release of or the tying up of water; degrading or forming the colloidal suspensions and gels or emulsions, and altering the food’s ability to penetrate to oxygen and moisture. Proteins may denature and, consequently, more readily available to certain organisms than in its original state. Protein or starch can become gelled, releasing moisture and allowing for a faster decomposition. Because of the reasons mentioned cooked food is typically more readily decomposed than fresh food.

Changes in the colloidal constituents

Changes in the colloidal component of food products could result from other causes that the heating or freezing processes, e.g., by sound waves, yet the effects are comparable. Emulsions of water and fat will be less likely to spoil and spoilage can spread more quickly in the case of water being the constant phase , and fat the discontinuous such as in French dressing, in contrast to butter, even when the reverse is the case.

B. Chemical Properties of the Food 

Chemical composition food is a determinant of how effective it can be as a suitable medium for cultivation for microorganisms. Every organism is unique and has the ability to use certain substances as sources of energy, carbon sources or a fuel source for nitrogen. The properties of food determine the number and kind of organisms that can be present in and potentially spoil the food. They are (1) pH or hydrogen-ion content, (2) nutritional content, (3) moisture availability, (4) O-R potential and (5) potential presence of inhibiting substances.


Food that is not sterile is more likely to spoil over time if it’s damp enough and is not frozen. The risk of spoilage is high at temperatures ranging from -5 to 70 C. Because microorganisms vary significantly in their ideal minimum, maximum, and optimal temperatures to grow, it’s evident that temperatures at which food is kept can have a significant impact on the nature, speed and quantity of microbially-induced changes that takes place. Any slight change in temperature can favor a different type of organism, resulting in different types of spoilage.

The yeasts and molds, for the majority of the time, don’t develop well over 35 to 37 C and , therefore, are not a factor in food items that are stored at temperatures that are high. However molds and yeasts thrive easily at normal room temperature, and a lot of them thrive at lower temperatures, with some even thrive at temperatures that are just a little below. While most bacteria thrive at room temperature certain (thermophiles) thrive at higher temperatures, while others (psychrophiles and psychotrophs) in cold temperatures.

Thus, molds are often found in refrigerated foods. Likewise, thermophilic bacteria thrive in heated pea blanchers. Raw milk stored at various temperatures encourages the development of different bacteria. When temperatures are near freezing cold-tolerant organisms, e.g., species of Pseudomonas and Alcaligenes are preferred in room temperatures. Streptococcus lactis , coliform and other bacteria are the most prevalent and at 40 to 45 C temperatures, thermoduric organisms like e.g., S. thermophilus and S. Faecalis, will grow first. At 55-60 C thermophilic bacteria, such as Lactobacillus thermophilus* are likely to grow.

It is essential to keep on your toes that temperature of raw food item is stored can influence its self-decomposition, and consequently the likelihood of microbial spoilage. Incorrect storage temperatures for fruits reduce their vigor and make them more susceptible to spoilage. The temperature used to handle and storing food items, particularly at the grocery store or at home, are distinct in different countries. In the United States refrigeration of perishable food items is the norm and keeping them at ambient temperatures for extremely long is not the norm however the opposite is the case in many other countries. So, the most frequently seen type of spoilage that occurs in an item could be completely different across different countries.

The United States, concern with spoilage of the most perishable food items typically concerns changes during temperatures of chilling, as well as the psychotrophic forms that are found in Pseudomonas, Flavobacterium, Alcaligenes and others genera as well as specific yeasts and molds will be significant. If food items are not typically refrigerated, the typical temperature of the air could be significant and the variations in temperature during the different seasons must be taken into account. When it is time for the seasons to change or when the temperature is cool the meso-philic bacteria that are common to all molds and yeasts would take on greater significance.

In hot weather, 26.7 to 43 C and above, the organisms favoured by these temperatures are crucial, including the coliform bacteria as well as species of Bacillus Clostridum Streptococccus, Lactobacillus, and other genera. The extreme temperatures are detrimental to the majority of moulds, yeasts and other yeasts. In extreme conditions, food items particularly canned ones often are stored in temperatures that favor thermophiles like throughout World War II, when canned food was stored under tarpaulins during the tropical heat.

The totality of the above mentioned factors and the types number, sizes, and proportions of microorganisms that are present and their environment , as controlled by the physical and chemical characteristics of the food oxygen tension, oxidation reduction potential, as well as temperature, will determine the types of microorganisms that are most likely to develop in the food and the changes that will be created. All these variables are to be considered when making predictions on the shelf-life stability. Table 4.1 lists the most frequently used processing changes and their impact on the quality of keeping as well as the microbial diversity of the most important foods.

Chemical Changes Caused By Microorganisms During food spoilage

Due to the range of organic compounds present in food products and the many types of microorganisms able to decompose them, numerous chemical transformations are possible, and various kinds of products are possible to result. This article is focused exclusively with the major types of breakdown of the principal constituents of food and the main products that are produced.

Changes in Nitrogenous Organic Compounds

The majority of nitrogen present in food comes made up of proteins, which need to be hydrolyzed by enzymes from microorganisms, or of food chain to polypeptides, more simple amino acids or peptides before they are able to function as nitrogenous food sources for the majority of organisms. Proteinases facilitate the hydrolysis process of proteins into peptides which can impart the food a bitter taste. Peptidases are responsible for the hydrolysis of polypeptides into simpler peptides and then to amino acids. These give flavor either undesirable or desirable to certain foods; e.g. amino acids are responsible for the taste of cheeses that have been ripened.

In the majority of cases, these hydrolyses don’t create products that are particularly offensive. Anaerobic breakdown of peptides, proteins or amino acids can result in the generation of odors that are unpleasant and is referred to as putrefaction. It produces foul-smelling, sulfur-containing substances, like hydrogen, methyl and ethyl sulfides , mercaptans and along with ammonia and amines (e.g. piperidine, histamine, tyramine putrescine, cadaverine, and histamine) Indole as well as skatole and other fat acids.

Microorganisms that act on amino acids, they could dissociate them, decarboxylate them or both, resulting in the products mentioned in Table 4.2. Escherichia coli, for instance produces glyoxylic acids, ammonia, and acetic acids from glycine. Pseudomonas also produces carbon dioxide and methylamine and clostridia produce ammonia, acetic, and methane. From alanine , these three organisms generate (1) an ammonia, a-keto acids, as well as carbon dioxide (2) ammonia, acetic acid ammonia, acetic acid, and carbon dioxide and (3) propionic acid ammonia, acetic acid and carbon dioxide, respectively. Serine is the main source. E. coli produces pyruvic acid and ammonia. Other Clostridium species give propionic acidand formic acid and ammonia. As mentioned previously that sulfur in sulfur-containing amino acids could be reduced to sulfides with foul odors or Mercaptants.

Changes in Nitrogenous Organic Compounds
Changes in Nitrogenous Organic Compounds

Desulfotomaculum Nirificans (formerly C. nigrificans) is an anaerobe that reduces sulfate into sulfuric acid and create hydrogen sulfide out of cystine. Other nitrogenous compounds that are decomposed comprise (1) imides, amides and urea, of where ammonia is the primary result, (2) the creatine and guanidine, both of which produce ammonia and urea, and (3) purines, amines and pyrimidines that could yield carbon dioxide, as well as organic acids (chiefly the acetic or lactic).

Changes in Nonnitrogenous Organic Compounds

The most important nonnitrogenous food sources for microorganisms that are mostly used for energy production, but also serving as carbon sources are organic acids, carbohydrates aldehydes and ketones glycosides, alcohols, organic compounds, and lipids.


Carbohydrates, when they are available generally are the preferred food of microorganisms over other foods that are energy-producing. Complex di-, tri-or polysaccharides generally are hydrolyzed into simple sugars prior to their use. A monosaccharide, such as glucose, aerobically would be oxidized to carbon dioxide and water and anaerobically would undergo decomposition involving any of six main types of fermentation: (1) an alcoholic fermentation, as by yeasts, with ethanol and carbon dioxide as the principal products, (2) a simple lactic fermentation, as by homofermentative lactic acid bacteria, with lactic acid as the main product, (3) a mixed lactic fermentation, as by heterofermentative lactic acid bacteria, with lactic and acetic acids, ethanol, glycerol, and carbon dioxide as the chief products, (4) the coliform type of fermentation, as by coliform bacteria, with lactic, acetic, and formic acids, ethanol, carbon dioxide, hydrogen, and perhaps acetoin and butanediol as likely products, (5) the propionic fermentation, by propionibacteria, producing propionic, acetic, and succinic acids and carbon dioxide, or (6) the butyric-butyl-isopropyl fermentations, by anaerobic bacteria, yielding butyric and acetic acids, carbon dioxide, hydrogen, and in some instances acetone, butylenes glycol, butanol, and 2-propanol. Other substances can be produced from sugars when various microorganisms are in active use, such as higher fatty acids, additional organic acids and aldehydes and ketones.

Organic Acids 

A lot of organic acids found in food as salts are converted by organisms into carbonates, which causes the medium to become alkaline. Aerobically, organic acids can be completely oxidized in water and carbon dioxide like film yeasts. Acids could be oxidized simpler acids or other substances like those produced by sugars. Saturated fatty acids, or lower ketonic derivatives are reduced to acetic acid, which is two carbons at once and assisted by coenzyme. Acids that are hydroxy or unsaturated acids are degraded in the same manner, but need to be converted into an acid saturated (or ketonic derivative) to full beta oxygenation.

Other Compounds 

Alcohols are usually converted to the appropriate organic acid e.g. the acetic acid from ethanol. Glycerol can be dissimilated into substances similar to those produced by glucose. Glycosides, following hydrolysis to release sugar, will exhibit the sugar dissimilated as it is. Acetaldehyde can be oxidized to acetic acid , or reduced to the ethanol. Cyclic compounds cannot be destroyed.


Fats are hydrolyzed to the fatty acids and glycerol, that are dissimilated in the manner previously mentioned. Microorganisms can participate in the oxidation process of fats, however autooxidation is the most common. Phospholipids can be degraded into their constituents, namely glycerol, phosphate fat acids, nitrogenous bases, e.g., choline. Lipoproteins are composed of cholesterol esters, protein and phospholipids.

Pectic Substances 

Protopectin is the water-insoluble plant pectic compound transforms into pectin, a polymer that is water-soluble of galacturonic acids that has linkages between methyl ester as well as varying levels of neutralization through various Cations. It can be gelled with acid and sugar. Pectinesterase causes hydrolysis to the methyl ester linkage in pectin in order to produce pectic acids and Methanol. Polygalacturonases sever connections between units of galacturonic acids in pectic acid or pectin to create smaller chains, and, ultimately, free D-galacturonic acids which can be reduced into simple sugars.

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