Food preservation from microbial spoilage: Principle and methods

August 25, 2020 Gaurab Karki Food Microbiology 0




Food preservation from microbial spoilage: Principle and methods
Food preservation from microbial spoilage: Principle and methods

Principle of food preservation:

  • Various chemical and physical methods are available for preservation of food. But selection of appropriate method is very important to protect the quality and nutritional value of the food.
  • During preservation by these methods, following principle are involved.
  • Prevention or delay of microbial decomposition of food by:
    • Preventing contamination of food by spoilage organisms (Asepsis).
    • Removing micro-organisms from contaminated food. For.eg. by washing.
    • Slowing down the growth and activity of micro-organisms in food. For.eg. by freezing the food.
    • Killing micro-organisms present in the food. For.eg. by heating the food or by adding some chemicals.
  • Prevention of self-decomposition of food by:
    • Damaging self-enzyme of food that cause self-decomposition. For.eg. damaging of ripening enzyme of fruits by washing it in hot water.
    • Preventing purely chemical reaction occurring in food that damage the food. For.eg. prevention of oxidative rancidity of lipid by addition of anti-oxidant.
    • Prevention of mechanical damage of food caused by insects, bird, food handling device etc.

Methods for prevention of microbial spoilage of food:

  • Various methods of food preservation include:

1. By preventing food contamination (Asepsis):

  • Process of preventing contamination of food by spoilage micro-organisms is called asepsis.
  • Inner tissue of healthy animal and plants do not contain micro-organisms and if present they usually do not spoil food.
  • Most spoilage micro-organisms enter into the food by contamination from external sources like water, air etc.
  • Therefore, microbial decomposition of food can be prevented by preventing contamination of food by spoilage organisms.
  • Several foods like egg contains natural covering around food that prevent entry of spoilage organism into the food.
  • Nowadays, variety of such artificial barrier are made around the food to prevent contamination.
  • For.eg. fruits and vegetables are stored for years by coating the wax around them.
  • Some other artificial barriers that prevent the contamination includes wrappers, cartoons and cans.
  • Safe handling of food and good personal hygiene of food handler are important method to prevent or minimize contamination of food by spoilage organisms.

2. By removing microorganisms from contaminated food:

  • It is second level of preservation of microbial decomposition of food.
  • If contamination of food by spoilage organism is not prevented, spoilage of food still can be prevented by removing contaminated micro-organisms from the food.
  • Some techniques that preserve food by removing micro-organisms include:
  • Filtration:
    • It is only one successful method for removing micro-organisms from contaminated food.
    • Unfortunately, this method is applicable only for clear liquid like clear fruit juice, beverages etc.
    • To remove micro-organisms, liquid food is filtered through sterile bacterial filter.
  • Centrifugation/ sedimentation:
    • Centrifugation removes some but not all micro-organisms from contaminated liquid food.
    • When liquid food is centrifuged suspended particle, debris and some micro-organisms settle to the bottom which is later separated and remove from the food.
    • This technique is commonly used for milk in dairy industries.
  • Washing:
    • This method is commonly used for vegetable and fruits. Dusts and micro-organisms present on the surface of whole vegetable and fruits are removed by washing with sterile water.
    • One limitation of washing is that it increases water activity of some food and hence facilitates growth of remaining spoilage organisms.
    • Furthermore, if washing water isnot sterile, it adds spoilage organisms in food.
  • Trimming:
    • Trimming is the process of removing spoiled parts of food like vegetables and fruits by cutting.
    • Trimming prevents spread of micro-organisms from spoiled part into inner healthy tissue.

3. By inhibiting or killing microorganisms:

  • Different methods that preserve food either by killing micro-organisms or by slowing down their growth and activity includes:
    • Preservation of High temperature (Heat)
    • Preservation by low temperature
    • Preservation by Drying
    • Preservation by Radiation (irradiation)
    • Chemical agents used in preservation of food

I. Preservation of food by High temperature (Heat):

  • Heat is the most commonly used method of food preservation.
  • Heat kills spoilage of micro-organisms by denaturation of cytoplasmic protein and enzyme.
  • Two types of heat i.e. dry heat and moist heat are used in preservation of food.
  • Moist heat is more effective in killing micro-organisms than dry heat.
  • Heat causes denaturation of protein and enzyme by breaking down their bonds.
  • Breaking of bond occurs by two mechanisms i.e. hydrolysis and oxidation.
  • Hydrolysis require water but oxidation does not.
  • Hydrolysis require less energy (heat) for breaking of bond than oxidation.
  • Therefore, if moisture is present in food, bonds of protein and enzymes are broken down by hydrolysis even at lower temperature.
  • This is the reason why moist heat can kill micro-organisms completely at condition of 121.5oC for 15 min but dry heat requires 160oC for 1-2hrs to kill the same organisms.
  • Various factors affect killing of micro-organisms in food by heat.
  • Some such factors include:
    • Time-temperature relationship
    • Density of micro-organisms in food, if density is high, higher temperature is needed.
    • Types of micro-organisms present in food, e.g. Spore are difficult to kill than vegetative cells
    • Growth stage of micro-organisms, e.g. micro-organisms present in log phase are easily killed but micro-organisms present in late log phase and stationary phase are somewhat heat resistant.
    • Micro-organisms in dry food are difficult to kill than in moist food.
    • If food is acidic or alkaline, micro-organisms present in log phase are easily killed by heat. On the other hand, if food is neutral, high temperature is needed to kill the same organisms.
    • Penetration of heat into inner layer of food also affects killing of micro-organisms.
    • Heat penetration into the food is affected by various factors as given below:
      • Nature of material by which container of food is made, affects heat penetration. E.g. heat penetrates easily into smaller and slim can than through larger and wider can.
      • In case of open heating of food, agitation or stirring of food facilitates heat penetration.

Differences between dry heat and moist heat:

Dry heatMoist heat
It is less effective in killing micro-organisms.It is more effective in killing micro-organisms
It damages protein and enzyme by oxidation.It damages protein and enzyme by hydrolysis or coagulation.
It is especially valuable to sterilize the food whose quality is damaged by moisture (e.g. flour) and for lipids foods.Moisture cannot penetrate into hydrophilic lipid food and moisture damage the quality of powder foods like flour.
Dry heat is applied to the food by spraying moisture free heated air over the food and by hot air oven.Moist heat can be applied by boiling the food in water or by spraying heated steam over the food.
Conditions for sterilization is 160oC for 1-2 hrs.Conditions for sterilization is 121.5oC for 15 minutes.

Some technique of food penetration by heat includes:

i. Pasteurization:

  • Pasteurization is a selective or gentle heating technique originally developed by Lewis Pasteur for milk and milk products.
  • Nowadays, pasteurization is also used for preservation of other beverages like beer, fruit juice etc.
  • Pasteurization is a selective heating technique and it kills some but not all micro-organisms present in milk.
  • If milk is contaminated by some disease causing micro-organisms, milk cannot be heated at boiling temperature to kill these micro-organisms.
  • It is because heating at high temperature brings several undesirable changes in milk. E.g. it kills beneficial lactic acid bacteria present in milk.
  • Similarly, casein protein precipitate, lactose caramelizes, flavor of milk changes.
  • Such milk becomes unsuitable for production of milk products like icecream, cheese.
  • Similarly, beverages like beer and fruit juice cannot be heated at higher temperature because it brings several undesirable changes in flavor, taste etc.
  • In pasteurization, milk and beverages are heated at lower temperature that kills spoilage or pathogenic organisms without bringing undesirable changes in the product.
  • There are three techniques of pasteurization of milk which differ in their time-temperature relationship.
  • Time-temperature relation of pasteurization are selected on the basis of three bacteria which are transmitted through milk (Mycobacterium bovis, Coxiella buretii and Brucella abortus).
  • Low temperature holding (LTH) or Vat pasteurization:
    • In this method, milk is heated at 62.8oC for 30 minutes.
  • High temperature short time (HTST) method:
    • In this method, milk is heated at 71.7oC for 15 seconds.
  • Ultra-pasteurization:
    • In this method, milk is heated at about 137.8oC for 2 seconds.

ii. Canning:

  • Canning is the process of preservation of food by keeping them in sealed container and then heating the container.
  • Canning techniques was originally developed by Nicoles Appert.
  • There are two types of canned food i.e. heated and unheated canned food.
  • During canning at first raw food is washed and then processed.
  • Then processed food is placed into a suitable can and the can is then ehated.
  • Sealed cans can be heated either by immersing it into boiling water or by spray of super-heated steam or heated dry air.
  • There are two system of canning,
    • i.e. hot pack system and cold pack system.
  • In hot pack system, raw food is preheated before packing into the can and in cold pack system, food is not preheated.
  • Canning is commonly used for preservation of many foods like vegetable, meat, fish etc.
  • Canned food is microbiologically more stable and remain unspoiled for long time.
  • It is because any micro-organism present in raw food are killed during heating the can and sealed container prevents entry of spoilage organism into the food.
  • Defective canning is not safe for preservation of food. For.eg. if canned food is not properly heated, some thermophilic organism and endospore of bacteria present in raw food are not killed.
  • Such spores then germinate and cause spoilage of the food even if the can is intact.
  • Furthermore, can must be completely filled with food.
  • If any space or air is present inside the can, this aerobic condition facilitates germination of surviving spores.
  • Similarly, can used for packaging the food must be completely sealed.
  • If any breaks or leaks are present in can many spoilage organisms enter into the food and spoil the food.
  • Furthermore, air enters into the can through leaks that facilitates germination of surviving spores.

iii. Blanching or scalding:

  • Blanching is the process of washing vegetables or fruits in warm water before storage.
  • Advantages of blanching includes:
    • It inactivates enzyme of food that cause self-decomposition.
    • It reduces number of micro-organisms present on the surface of food.
    • It brings wilting of leafy vegetables and helps in packaging.
    • It enhances fixing of intense green color of fruits and vegetables.

iv. Roasting:

  • It is used for meat and meat products.
  • During roasting internal temperature of meat reaches (80-85)oC.

v. Baking:

  • It is used for bread, cake and other bakery products.
  • During baking temperature maximum of 97oC is reached.

vi. Frying:

  • It is used for vegetable, meat, fish etc.
  • During frying outer surface of food becomes very hot but internal temperature of food never exceed 100oC.

vii. Cooking:

  • During cooking temperature around 100oC is reached.

viii. Simmering:

  • It is the process of gentle boiling of food with the temperature of about 100oC.

II. Preservation of food by low temperature:

  • Low temperature is commonly used physical method of preservation of food.
  • Low temperature preserves food by:
    • Slowing down growth and activity of spoilage organism in food.
    • By slowing down the rate of chemical reaction or other enzymatic activity that cause self-decomposition of food.
    • Sometimes low temperature kills spoilage organism in food. E.g. in case of deep freezing.
  • Different methods of food preservation by low temperature:

ii. Common or cellar storage:

  • Common storage is the technique of preservation of certain foods like potato, onion etc. in home by spreading them in cold room.
  • In common storage, storage temperature is slightly lower than that of outside air.
  • But this technique food is not stored for long time but it is still used where cold storage and facilities are not available.

ii. Chilling or cold storage:

  • In chilling or cold storage, storage temperature is usually between (0-10)oC.
  • At this temperature, growth and activity of most bacteria except psychrophiles is significantly slowed down.
  • Therefore, by chilling storage, food can be stored for long time than common storage.
  • However, if psychrophilic organisms are present in food, they grow in food and cause spoilage.
  • Selection of appropriate temperature is very important because quality of some food is damaged at lower temperature storage temperature of some food are (10-12.8)oC for sweet potato, (13.3-16.7)oC for banana etc.
  • This method is most commonly used for short term storage of highly perishable foods like meat, milk, fish etc.
  • Fish is stored by placing it in cold ice.

iii. Freezing or frozen storage:

  • In freezing storage, storage temperature is between (-15 to -29)oC.
  • At this temperature, growth of even psychrophilic bacteria is completely inhibited.
  • Therefore, food kept in frozen storage remain fresh in long time without undergoing microbial decomposition.
  • Furthermore, self-decomposition of food occurs very rarely or not at all at this temperature.
  • Sometimes, freezing also kills spoilage organisms present in food.
  • Therefore, frozen storage is very effective method of food preservation among these three techniques.
  • Killing mechanism of freezing is due to formation of ice-crystal and dehydration of microbial cell.
  • When food is frozen, surrounding water freezes to form ice-crystals.
  • Then cytoplasmic water from microbial cell continuously diffuses out and is added in surrounding ice-crystal.
  • The remaining cytoplasm in microbial cell gradually becomes more and more concentrated.
  • High salt concentration in remaining cytoplasm kill the microbial cell by causing mechanical damage to the cell.
  • There are two methods of freezing the food i.e. quick freezing and slow freezing.
  • Slow freezing is more effective in killing microbial cell but quick freezing is considered more appropriate for preservation of food.
  • When food is frozen very slowly, there is sufficient time for cytoplasmic water to diffuse out.
  • In this case microbial cell is dehydrated to greater extent.
  • Similarly, larger ice-crystal are formed due to continuous addition of diffused water into ice.
  • Larger ice crystal gives more mechanical damage to microbial cell.
  • However, if larger ice-crystal are formed in food, it gives more mechanical damage (causes cracking of food) to the food.
  • Therefore, quick freezing is commonly used for food preservation.
  • Although, frozen storage completely prevents microbial decomposition, it brings several undesirable changes in some food.
  • Some such examples include:
    • Protein of food e.g. meat may become dehydrated so that the food becomes very tough and rigid.
    • Myoglobin present in meat may be oxidized into brown colored meat-myoglobin.
    • Fat present in food may be hydrolyzed or oxidized.
    • Ice-crystals formed on surface of food evaporate and give mechanical damage on the surface of food. This damage is called freeze-burn.
  • Methods of freezing:
  • Quick freezing:
    • In quick freezing, temperature of the food lowered very quickly.
    • Food is frozen quickly by immersing food into solution of refrigerant. For.eg. fish frozen by immersing it into frozen solution of NaCl.
    • By blowing frigid air (at -17.8 to -34.4oC) over the food to be frozen.
  • Slow freezing:
    • In this process, temperature of the food is lowered very slowly.
    • Food is frozen slowly by placing it inside the mechanical refrigerator.
    • This technique is commonly used to freeze food in home.
    • However, slow freezing forms larger ice-crystal in food and give more mechanical damage.
    • Therefore, quick freezing is preferred over slow freezing.

Differences between slow freezing and quick freezing

Slow freezingQuick freezing
It is more effective in killing micro-organisms in food.It is less effective in killing micro-organisms in food.
Larger ice-crystals are formed.Smaller ice-crystals are formed.
Microbial cell is dehydrated to greater extent.Microbial cell is dehydrated to lesser extent.
It gives more mechanical damage due to formation of larger ice-crystal.It gives less mechanical damage due to formation of larger ice-crystal.
Micro-organisms may adopt slowly to lower temperature.There is no time for adaptation.
Cold shock does not occur.Microorganisms may be killed by cold shock.

III. Preservation of food by Drying:

  • Many spoilage micro-organisms are highly susceptible to drying.
  • Some spoilage organisms are killed rapidly by drying and growth of most other organism is lowered or, inhibited by drying.
  • Therefore, microbial decomposition of food can be prevented or at least minimized by drying the food.
  • There are different processes of drying food.
  • Drying is performed by either evaporation of water from the food or by lowering Aw value of food by addition of salt and sugar.
  • During drying of food by evaporation, several parameters like time of drying, humidity of air, temperature of the air etc. should be controlled.
  • Improper control of this parameter brings several undesirable changes on food.
  • For.eg. if food is dried quickly by placing it in very hot environment, moisture is evaporated only from the surface but not from inner layer of food.
  • In this case, very hard layer is formed on surface that prevents further dehydration of food.
  • This defect is called case hardening.
  • Different methods of drying food include:

i. Solar drying:

  • In this method, food is dried by placing it in hot atmosphere of sun.
  • Many foods like grains and chopped pieces of vegetables used for making pickles are dried by this method.

ii. Drying by mechanical drier:

  • In this method, food is dried with the help of some mechanical drier.
  • Liquid food like milk are dried by spray drying and drum drying method.
  • Some liquid foods are dried by placing them in vacuum chamber of lower temperature.

iii. Drying by addition of salt and sugar:

  • It is indirect method of drying.
  • When salt or sugar is added in food, it binds water in the form of shell of hydration and make it unavailable for micro-organisms.
  • Therefore, they decrease the amount of available water (Aw) of food.
  • Example of food dried by this method includes preservation of food by adding high salt concentration and preservation of milk by adding sugar.

IV. Preservation of food by Radiation (irradiation):

  • Two types of radiation are used in food preservation. They include:

i. Non-ionizing radiation:

  • UV light is an example of non-ionizing radiation.
  • When microbial cells are exposed to UV light, it is absorbed by nitrogenous bases of DNA.
  • After absorption, UV light catalyze the formation of thymine dimer by linking ring of adjacent thymine.
  • When such DNA replicates rate of error of replication is very high at the position of thymine dimer.
  • Therefore, UV light kills microbial cell by inducing mutation.
  • One major limitation of UV light in food preservation is its lower penetration power.
  • Therefore, it cannot penetrate into inner layer of solid food and viscous liquid food.
  • It is applicable only for clear liquid food like clear fruit juice.
  • It can also be used to kill micro-organisms on surface of solid food.
  • UV light of wavelength around 254nm has maximum antimicrobial activity.
  • In food industry germicidal lamp that emits UV light mainly in the range of 254nm are used as a source of UV light.
  • Application of UV light in food preservation:
    • UV light is used to kill micro-organisms present in clear liquid foods. For.eg. it is used to kill yeast cells and other micro-organisms in fruit juice.
    • It is used to inhibit growth of yeast and mold on surface of some foods like pickles, vinegar, cheese etc. UV light is commonly used to prevent surface spoilage of various foods including pickles, vinegar etc.
    • UV light is used to disinfect water used for production of beverages.
    • It is used to kill micro-organisms present in air of food processing room.
    • It is also used to kill micro-organisms on various food processing instruments like knife, eating utensils, etc.
    • UV light is used in rapid ageing of meat in meat industries. Ageing is the process of drying meat by hanging pieces of meat in controlled environment. Usually meat is aged by hanging it in room at a temperature of 2.2-3.3oC. At this temperature, ageing occurs in several months. Temperature is maintained lower to prevent growth of micro-organisms in meat during ageing. If ageing is done at higher temperature micro-organisms grow in meat and spoil it before ageing takes place. In rapid ageing technique temperature of room is maintained around 18oC and UV light is used to inhibit growth of micro-organisms in meat during ageing. At this temperature ageing occurs in 2-3 days.

ii. Ionizing radiation: (Cold sterilization)

  • X-ray, γ-ray and cathode ray are examples of ionizing radiation.
  • When microbial cells are exposed to ionizing radiation, they cause ionization of cytoplasmic water and generate very toxic compound like H2O2, hydroxyl free radical (OH*) and super oxide ion (O2–).
  • They are powerful oxidizing agents and kill the microbial cell by oxidizing various cellular materials.
  • Some common ionizing radiation used in food industry include:
  • X-ray:
    • Production of X ray is expensive.
    • Furthermore, its penetration power is lower. Therefore, it is not commonly used in food preservation.
  • γ-ray:
    • It has good penetration power and it can penetrate upto 20cm even in solid food.
    • γ-ray is emitted in all direction from the source. Therefore it is difficult to make γ-ray to hit the food.
    • So, efficiency of γ-ray in food preservation is only 10-25%.
  • Cathode ray:
    • Cathode ray is directional and it is emitted in only one direction.
    • Therefore, efficiency of cathode ray in food preservation is high (upto 40-80%).
    • Unfortunately, it has lower penetration power and can penetrate only upto 0.5cm in solid.

Applications of ionizing radiation in food processing:

  • Some applications of ionizing radiation in food preservation includes:
  • Low level of ionizing radiation is used to kill microorganisms on fresh fruits and vegetables. It is used to kill insect as well as microorganisms on vegetable and fruits.
  • High level of ionizing radiation is used to kill microorganisms and insect on dry vegetable and fruits.
  • Ionizing radiation are commonly used to preserve meat and meat product. For.eg. irradiation is commonly used to inhibit microorganisms, nematodes and other parasites present in meat and meat products.

Undesirable changes of ionizing radiation:

  • In meat, it increases pH, damage glutathione, increase carbonyl compound and generate bad smelling compound like H2S and methylmercaptan.
  • In fatty food, it damages natural anti-oxidant and increases chance of oxidative rancidity.
  • It causes loss of several vitamins like pyridoxine, thiamine, vit. C, vit. D, vit. A, vit. B12. Therefore, it reduces nutritional value of food.

V. Chemical agents used in preservation of food:

  • Different types of chemical preservative are used to preserve variety of food.
  • Chemical preservatives are utilized in food preservation in various ways as given below:
    • Some food preservatives are directly mixed with the food to be preserved.
    • Some chemical preservative are used in wrapper or container of food rather than in food itself.
    • Some chemical preservative are placed in air of storage environment of food.
    • Some chemical preservative are used for treatment of equipment used in food processing.
  • Not all food preservative can be used to preserve ah types of food.
  • It is because some chemical preservative brings some undesirable changes for.eg. change in taste, smell etc. in some food. For.eg. spices are usually not considered suitable for preservation of milk and milk product because taste of spices in milk product is not considered desirable.
  • Some chemical food preservatives used in preservation of food include:
  • a) Organic acids: Examples
  • i) Propionic acid and propionate:
    • Propionic acid is naturally found in Swiss cheese as a developed preservative up to a level of 1% .
    • Na and Ca propionate are commonly used to inhibit growth of mold in bakery product like bread.
    • Propionic acid and propionate are effect against mold, with very little or no effect against bacteria and yeast.
    • Therefore they are added in food which usually undergo mold spoilage 
  • ii) Benzoic acid and benzoate
    • Sodium benzoate us used to preserve variety of food like jam, jellies, carbonated beverages, fruits etc.
    • It is most effective at power pH and it’s anti-microbial activity decreases with increase in pH.
    • Therefore, it is usually used to preserve acidic and slightly acidic food. At pH of 2.5-4 it is effective against most bacteria, some yeast and mold.
    • Methyl paraben and propyl paraben which are derivative of benzoic acid are also commonly used to preserve variety of food.
    • Their advantages over benzoate is that they are more effective at higher pH.
  • iii) Sorbic acid and sorbates:
    • Sorbic acid and its Ca, Na and K salt are used as antimicrobial agent to preserve various food.
    • They are effective against yeast and mold but less effective against bacteria.
    • Their antimicrobial activity is high at lower pH.
    • Therefore they are used in slightly acidic food like pickles, fruit juice, syrup and other foods like cheese, bakery product etc. to inhibit growth of mold and yeast.
  • iv) Acetic acid and acetates:
    • Acetic acid and its derivatives like monochloroacetic acid, peracetic acid, dehydroacetic acid and sodium diacetate are used as food preservatives.
    • Some examples include;
    • Dehydroacetic acid is used to treat wrappers of cheese to inhibit growth of mold on the surface of cheese.
    • Acetic acid in the form of vinegar is added in the form of vinegar is added in pickles to inhibit growth of microorganisms. Its activity increases with decrease in pH.
    • Sodium diacetate is used in malt syrup. It is also used to treat wrapper of butter.
  • v) Nitrites and nitrates:
    • Nitrite and nitrate of Na and K are used in curing solution or curing mixture of meat.
    • They have slightly bacteriostatic action, but they are added in meat mainly to fix red colour.
    • In acid environment, nitrite decomposed into nitric acid which reacts with haeme pigment of meat to form nitrosomyoglobin that gives attractive red colour to the meat.
    • Nitrite is a real colour fixative and nitrate is used as a source of nitrite.
    • Nitrite can react with secondary and tertiary amine to form nitrosamine which is carcinogenic.
    • Nitrite and nitrate are mainly used to fix red colour to meat but not to preserve it.
  • b) Smoking (wood smoke):
    •  Smoking is a method of preservation of meat by exposing it to wood smoke.
    • Two main purpose of smoking of meat are to add desirable flavour and to preserve the meat.
    • Some other desirable changes include improvement in colour, drying, and tenderization of meat.
    • Wood smoke contains large no of volatile compounds having bacteriostatic and bactericidal activity that help in preservation of meat.
    • Some such compound include formaldehyde, phenol, cresol, aliphatic acid, acetaldehyde, ketones and others.
    • Wood smoke also contains variety of flavoring compound that give desirable flavor to the meat.
  • c) SO2 and sulphites:
    • SO2 and sulphites are used in wine industry to sanitize equipments and to reduce number of normal flora of grapemust.
    • In aqueous solution, SO2 and sulphites form sulfurous acid which is anti-microbial.
    • Fume of burning sulfur are used to treat fruits and vegetables.
    • SO2 is also added in fruit juice, syrup and various wine.
  • d) Sugar and salt:
    • Sugar and salt are added in variety of food.
    • For.eg. milk is preserved for long time by adding high concentration of sugar.
    • Similarly, foods like pickles, fish etc. are preserved by adding high concentrations of NaCl.
    • Preservative action of salt and sugar include:
    • They bind water and make it unavailable for growth of microorganisms i.e. reduce Aw value of food.
    • Sugar increases osmotic pressure and causes osmotic lysis of microbial cell.
    • They draw out moisture from food and help in drying of the food.
    • NaCl ionizes to give chloride ion which is antimicrobial. NaCl reduces the solubility of O2 in liquid food and hence indirectly interfere with growth of aerobic microorganisms.
  • e) Formaldehyde:
    • It is highly poisonous and is not permitted in food except as a minor component of wood smoke.
    • In food industry, it is used to sterilize equipment and to kill microorganisms in air of food processing room.
    • It is highly antimicrobial and can kill fungi, bacteria, spore and viruses.
  • f) Alcohol:
    • Ethanol is anti-microbial.
    • Excellent keeping quality of some food is due to their alcohol content.
    • Ethanol content of soft drinks like beer is not sufficient to completely kill microorganisms but it slows down growth and activity of spoilage organism.
    • Alcoholic content of distilled alcoholic beverages is sufficient to prevent microbial spoilage.
    • Some food products like lemon extract (eg. Jolley) are also preserved by addition of ethanol.
  • g) Ethylene and propylene oxide:
    • Ethylene and propylene oxide are gaseous sterilizing agents.
    • They are used to sterilize packaged food like dried food.
    • Furthermore, ethyleneoxide is also used to kill microorganisms in air of food processing room.
  • h) Spices and other condiments:
    • Spices and other condiments added in food for taste and flavour are anti-microbial and help in food preservation.
    • Antimicrobial activity differs with type of spices. For.eg. mustard, flour and mustard oil are effective against like Saccharomyces but are less effective against bacteria.
    • Cinnamon and cloves is added in food are very effective against bacteria.
    • Similarly, garlic and onion mixed in food are bacteriostatic or bacteriocidal.
    • However, not all spices can be added in all types of food products because their taste and flavour maybe considered undesirable in certain food.
  • i) Antibiotics:
    • Use of antibiotics in food preservation is not permitted due to risk of development of antibiotic resistant microorganisms.

Food preservation from microbial spoilage: Principle and methods