Effect of Oxygen Concentration on Food Microbes

All living beings do respire. To respire oxygen is taken up by the body. Respiration is the oxygenation break down of the complex large molecules into simple small molecules. Microbes also do respiration in order to gain energy and also to survive. Hence in the presence of adequate oxygen levels microbes carry out their body functions efficiently. Microbes vary in their oxygen requirement. Some may need high oxygen levels where as for some it may be very low or they may grow in the absence of oxygen. Hence if we can control the oxygen level in the food environment then we are able to prevent food spoilage and microbial interactions with the food.

Aerobic microorganisms need Oxygen to do respiration. Hence aerobes grow on the top of the medium. Because top surface of the medium touches the atmosphere where oxygen is present. Hence aerobes grow near to the atmosphere in the medium.

Ex: Penicillium sp. ,Pseudomonas fluorescens

Facultative anaerobes can survive both in the presence of oxygen and in the absence of oxygen. Hence they can grow all over the medium even at the bottom of the test tube.

Ex: Escherichia coli, Salmonella sp., Staphylococcus aureus

Microaerophiles need oxygen for respiration but they need lesser amount of oxygen than aerobes. Hence they grow somewhat below to the top surface where oxygen is present in lower levels.

Ex: Campylobacter sp.

Obligate anaerobes are the microorganisms that do not need oxygen or survive without oxygen. They only survive in oxygen absent environments.

Ex: Clostridium botulinum

The test tube which contains obligate anaerobes is sealed with a plug. That is to ensure no oxygen is gone inside the test tube hence this confirms that it contains only obligate anaerobes. Whereas other 3 tubes are covered with a cotton wool. Thus it facilitates the air movement into the medium.

Effect of Interactions on Food Microbes

Microorganisms in a medium have lots of interactions with the medium as well as with other microorganisms. It can be positive interactions or it can be negative ones too. Positive interactions may increase the effect on the substrate which may cause harm or as well as a good improvement. Negative interaction may decrease the effect. These interactions are divided into two forms;

• Antagonistic
• Synergistic

Antagonistic interactions drive apart each microorganism. Which means it may reduce the activity of another microorganism.

Ex: The production of lactic acid in milk at room temperature by lactic acid bacteria inhibits gram negative rod shaped microorganisms. Gram negatives are mentioned in the 1st section-Basics in microbiology.
Synergism is the type of interactions that make two types of microorganisms to come closer each other.

Ex: Yeast present in certain grains produces vitamin B which stimulates the growth of lactic acid bacteria.

Antagonism of O. ulmi with P.syringae (top left) and other Pseudomonas strains (lower panels). Control top right.This explains that O.ulmi’s cells get degraded with the introduction ofPseudomonas strains. This means both together reduce the activity of a particular function where that particular function was positive only in the presence of O.ulmi.

Food Spoilage

A spoiled food is a food that is unacceptable to the consumer. When the suitable environment is present for the growth of microbes, they usually cause this phenomenon. Microbes’ uptake nutrients from the food which is present in a suitable environment thus deteriorate the food. This may include secreting various chemicals which makes the food appearance a slimy in nature. And these chemicals are responsible for different odors in the food which may be reducing appetite to eat that food. Also due to chemicals, and the changes in the structure of molecules occur with the deterioration may cause different tastes which is not acceptable for the consumption. Due to the deterioration of food substrate the texture may also change.

Food spoilage is not only due to microbial activity. There are other reasons too.

• Microbial activity is the main reason.
• Chemical changes.
• Physical damage.
• Freezer burn.
• Staling (changes in water content, ex: stale biscuits).
• Contamination with chemicals and other foreign bodies.

Microbial Activity on Food Spoilage

The main causation for the spoilage of food is by microbes. There are three types of microorganisms that cause food spoilage. Those are bacteria, yeast and moulds.
Yeasts are popular in the fermentation process. They grow on the food substrate and through their metabolic activity yeast cause fermentation. But their activity is useful in many industrial applications such as preparation of bread, buns, biscuits, cakes, beverages, various confectionaries and so on. Although this is food spoilage it is economically useful.
Mould growth is visible to naked eye. They grow in tough masses which are made out of their filaments. They form spores which are dispersed through air. When a suitable substrate is found they grow on that substrate. Moulds may be allergic to various people, hence causing severe illness.

Both yeast and moulds can survive in highly acidic foods like fruits, jams, jellies, pickles and tomatoes. But both can be easily destroyed by heat.

Ex:

• Storage rots in grapes and strawberry by Botrytis cinerea
• Blue mould rot in tomato caused by Penicillium

There are many types of bacteria that cause food spoilage. Bacteria generally like low acidic foods like vegetables and meat. Food poisoning can cause by eating spoiled food caused by bacteria.

Chemical Changes on Food Spoilage

Nuts are regularly prone to chemical changes. Fats and oils are the major constituents of nuts. Rancidity causes due to high content of oil and fat substitutes. Rancidification is the chemical decomposition of fats, oils and other lipids. There are three types of rancidity reactions.

Hydrolytic Rancidification is splitting of fats by water. Water splits fatty acid chains and this splitting occurs away from the glycerol backbone in triglycerides.
Oxidative Rancidification is cleavage of double bonds of unsaturated fatty acids caused through oxygen degradation in air. Oxidative rancidity in fat depends on initiators, oxygen, fatty acid composition, temperature, antioxidant, comminution and cooking.

Microbial rancidity is breaking of fats using enzymes of microorganisms such as lipase.
Rancidity is found to be happened due to disorganized storage conditions. This makes various chemical formulas in the food to change, ultimately derived in a spoiled food.

Browning is the process of becoming brown coloration in food items. Enzymatic browning is a chemical process. It involves enzymes. The result is creation of melanin and other compounds which cause brown colour. The enzymatic browning normally requires exposure to oxygen. Not only enzymes cause browning, the variations of water activity too cause non-enzymatic browning.

Effect of Physical Damage on Food Spoilage

Physical damages on food items is caused through wounds, Stress conditions such as water stress, nutrimental stresses etc., extreme conditions like high temperature, low temperature, pathogen and pest attacks, chemicals and radiation.

Ethylene is the ripening hormone also called as senescence hormone. Ethylene concentration is pronouncedly increased during the ripening stage of climacteric fruits. Ethylene causes fruit ripening as well as aging or senescence and eventually deterioration. Due to the stress conditions Ethylene is produced hence deterioration may occur. Hence through physical injuries the production of ethylene is rapidly increased. When a fruit or a crop is ripen its amount of nutritional components available for microbes are increased. Hence a ripe fruit or crop is highly prone to damage by microorganisms. Thus physical damage to food items indirectly causes food spoilage.

Freezer Burn in Food Spoilage

Freezer burn is a condition which is usually occurs due to dehydration and oxidation in foods. This results in frozen foods. This situation occurs due to the air contamination and air reaching into the food. Freezer burn condition generally occurs in non-air tight packaging. This dries the product and appears in grayish-brown spots on the frozen food. This also changes the colour of the food. But these foods are not unsafe to consume. Food freezing occurs due to the evaporation of water; hence it leads to formation of ice crystals. The water molecules vibrate and it makes the water molecules to come to the surface of the frozen food. Which is then appears as ice crystals. Ice crystal formation makes the food to shrivel and dry which in turn appears in a burnt look. Prevention can be done by using open air-tight plastic containers or with polythene covers. This helps to maintain the humidity.

Staling Effect for Food Spoilage

Staling is a process occurs in combination of chemical and physical reactions. Staling found to happen rapidly at temperature just above freezing. Bread is more susceptible to stale. Bread stored in a refrigerator usually has increased rates of staling. Therefore it should be kept in room temperature. Biscuits too get staled easily due to the absorption of water. Food which staled turns its texture into a spongy nature and also taste too changes and cannot eat anymore since the unappealing nature.

Contaminating Micro Flora Composition Factors

These are the interactions involved in the selection of a spoilage micro flora.Spoilage of a food substance which caused by microorganisms are a type of interaction. The particular interaction between the food and the organism may alter by the external environmental factors (Extrinsic factors) and also internal environmental factors (Intrinsic factors). Due to these various factors the particular microorganism causing the spoilage, will grow. The growth of microorganism will lead to various secretions and other chemical compounds. This will lately spoil the food also we may able to observe that through the spoilage symptoms.

Factors Determining the Composition of the Contaminating Micro Flora

The interaction between microorganisms and the food mainly based on two factors. Those are intrinsic factors and extrinsic factors. Other than these two there are many more and essentials are implicit factors, Contaminating micro flora growth of the specific micro flora and spoilage symptoms.

Extrinsic factors include storing temperature, gaseous atmosphere, relative humidity and duration. Depending on different storing temperatures and relative humidity ranges there are different kinds of microbes. According to the types of gases present in atmosphere microbes vary such as anaerobes, aerobes, facultative anaerobes and microaerophiles.

Intrinsic factors include nutrient content of the food, anti microbial components present, pH, water activity, gases in food such as oxygen, mechanical barriers for the microbial invasion. When nutrients, water, oxygen are high then composition and number of different types of microbes increase. Mechanical barriers and anti microbial substances prevent microbe’s arrival.
Contaminating micro flora: different foods have different microbial composition. Foe an example in meat usually gram negatives present.

Implicit factors include the microbial succession. (Will be discussed in practicals).

Effect of Nutrient Compositionon Food Microbes

Microbes also need nutrients for their growth. Culture medium usually contains nutrients which are essential for their well growth. In a culture medium there are carbon source, nitrogen source (organic source), macronutrients, micronutrients, growth factors. Gelling agents, antibiotics and dyes usually need in order toculture microorganisms.

Especially for heterotrophs a carbon source is an essential requirement. Hence depending on the microorganism that is growing in the culture medium, we must adjust the nutrient requirement. Nitrogen source can be inorganic or organic. Organic source include starch, glucose, sucrose, fructose etc. Macronutrients include Sulfur (S), Magnesium (Mg), Calcium (Ca), Potassium (K), Chlorine (Cl). Some of the micronutrients are Zinc (Zn), Manganese (Mn), Copper (Cu). Growth factors usually added are vitamins, pirins, pirimidins etc.

Gelling agents are agar and gelatin.Agar is made out of red algae. Melting point is 850C. It is a complex mixture of polysaccharides extracted from red algae. Red algae are a group of microorganisms. Usually used red algae are Gelidium andGracillaria. Agar is insoluble in cold water and dissolved in hot water. It usually sets in between 320C to 390C.

Gelatin is made out of hooves of animals. Therefore this is high in protein. Melting point is very low compared to agar. Melting point is 300C to350C. Integrating temperature is around 200C to 250C. Thus it limits the usage. This is readily digested by some bacteria.

Antibiotics generally included before pouring into plates to inhibit specific groups of microorganisms. Streptomycin is heat resistance and thus added before heating. Penicillin is heat sensitive, hence added after heating.


Dyes are generally thermo stable. Usually added before autoclaving. Rose Bengal, Brilliant green, Crystal violet are some of the dyes used in culturing.

Spoilage of Canned Foods in Microbiology Aspect

Canned foods are the foods that are stored in cans adding preservative for the future consumption. They can be spoiled due to three reasons.

1. Under processing
2. Survival of thermophilic bacteria
3. Leaker spoilage

Under processing - During the processing period mainly spore formers or facultative anaerobes can spoil the foods inside can. If the canned food medium is slight acidic ( low acidic ) then usually mesophyllic spore formers make off odours, CO2 and H2 which causes the can to swell and burst. Eg: Clostridium sporogenes ,  If the medium containing the food are high in acidicity then microbes such as Bacillus coagulans do not form any gas but increase the acidic nature of the food. Hence this type of acidicity do not swell the can.

Survival of thermophilic bacteria - Thermophilic bacteria can produce lactic acid which may sour the food leading to spoilage. Since they mainly inhabit in elevated temperatures the cans should be rapidly cooled after processing. Eg: Bacillus steareothermophilus

Leaker spoilage - When hot cans are instantly cooled down, it develops water molecules inside the can which will lead to spoil the food.