Microbial growth and basic understandings of Food Preserving Technique.

Food preservation basics

Food preservation is a method to protect the food from unwanted microorganisms. Unwanted microorganisms spoil the food and may produce microbial toxin after proliferation into food materials. Food for human consumption can be divided into eight primary groups  which are derived from plants and animals.

Foods from Animal origin

Foods that are produced from animal origin are-

Milk and Milk Products

Meat and Meat products

Poultry meat and eggs

Fish and other seafoods

Foods from Plant origin

Foods that are produced from plant origin are-

Cereals and cereal products

Vegetables and vegetable products

Sugar and sugar products

Fruits and fruits products

Food preservation method for each type of food product is different.

Principles of Food preservation

To preserve foods by various methods following principles are involved.

1. Prevention of self decomposition of food

Prevention of chemical reaction such as oxidation by use of antioxidants. 

Destruction of enzymes to prevent enzymatic reaction in food material. Use of blanching method to deactivate various enzymes. Before frozen most of vegetables are blanched or heated to inactivate enzymes present in vegetables. 

2. Prevention of microbial decomposition 

To keep safe from microorganisms, also called asepsis. 

Prevention of growth and activity of microorganisms by means of drying, anaerobic condition, low temperature or use of chemicals. Also killing of micro organisation by heating or use of radiation. 

3. Prevention or damaged by animals, insects, pests or any mechanical cause.

Drying or low temperature preservation however allow to self decomposition of food by enzymatic activity. Destruction of micro organisms present in food control enzymatic decomposition of foods. 

Various methods of food preservation 

The methods which are applied to preserve foods are as follows – 

1. Asepsis or keeping out of microorganisms 

2. Use of high temperature 

3. Use of low temperature 

4. Use of preservatives. It may added or produced by microorganisms. 

5. Drying of foods using hydrophilic colloids which tying up water molecules present in food materials. 

6. By creating anaerobic condition in sealed container. Or filling of inert gas inside the container.

7. Irradiation of foods which kill microorganisms. 

8. Mechanical destruction of microorganisms by use of high pressure or grinding operation. 

9. Lastly combination of one or two methods simultaneously. 

Canned food: heat processing + evacuation (in sealed can) 

Fruit juice: sodium benzoate + less heat for sterilisation 

Pickles / Catchup: salt, sugar and vinegar. If we use all three preservatives simultaneously then lower concentration required for each preservative. 

Food treated with irradiation by gama rays or antibiotic treatment require lesser heat for sterilisation. If we do not treat with radiation or antibiotics then higher temperature is required for sterilisation. 

Microbial Growth Curve

When microorganisms are added to food material and condition of growth becomes favourable the organisms present in food will begin to multiply. There are different phases of microbial growth time to time. It would be easily understandable if we see the curve of microbial growth. 

Microbial Growth Curve

A to B = lag phase

B to C = phase of acceleration (positive) 

C to D = exponential growth phase or logarithmic growth phase

D to E = phase of acceleration (negative)

E to F = prolong stationary phase

F to G = accelerated death phase

G to H = death phase

H to I = survival phase

A to B = Lag Phase

During lag phase, micro organisms adapt themselves in growth environment. In this period micro organisms try to become matured. During this phase of growth cycle enzyme synthesis, RNA synthesis and synthesis of other molecules occurred. In lag phase microbial cells are not dormant and it will take around 1 hr to several days to multiply, little bit cell division may occurred.

B to C = Acceleration Phase (positive)

During this period microbial growth is increased continuously

C to D = Log Phase

In this phase growth of micro organisms are rapid and constant. It is also called exponential growth phase or logarithmic phase. Microbial cell multiplication or doubling continues at a constant rate. In log phase the number of cells and population rate increase doubles with each consecutive time. However actual growth rate depends upon the condition of growth environment. The growth of micro organisms could not be continued indefinitely because nutrients present in growth medium will be depleted after some time and whole media will be deposited or enriched with wastes or death cell of micro organisms.

D to E = Negative Acceleration Phase

In this phase rate of doubling or multiplication decreased.

E to F = Stationary Phase

In this phase the number of micro organisms remains constant. The growth-limiting factor such as inadequacy of essential nutrient, and the formation of an inhibitory product such as an organic acids limit and inhibit growth. In the stationary phase death rate and growth rate micro organisms is equal. Mutation of micro organisms occur in this phase.

F to G = Accelerated Death Phase

The rate of death of micro organisms accelerated in this phase.

G to H = Death Phase

This is also called phase of decline. Death of micro organisms caused by the lack of nutrients, unfavourable environmental condition i.e. temperature above or below the tolerance level and other injurious condition. 

H to I = Survival Phase

During this phase no cell division occurs but remaining micro organisms survive on endogenous (own decomposed) nutrients

What is exponential growth or logarithmic growth? 

If we put one (1 no.) organism in food material it will grow like 1 to 2, 2 to 4, 4 to 8, 8 to 16. Its called exponential growth. 

Basic methods of food preservation depends on destruction or removal of microorganisms from food materials. So delay in the initial growth of microorganisms matters. 

If we classified various preservation factors we will definitely see three main things. 

1. Mode of action 

A. Inhibition or prevention of growth of microorganisms in food materials. 

B. Restrictions of access of microorganisms into food materials 

C. Destruction or removal of microorganisms from food materials 

2. Factors of preservation 

3. How to achieve or mode of achievement. 

Table showing the classification of preservation factors. 

Mode of action Factors of preservation How to achieve
Prevention of growth of microorganismsi) Oxygen restriction ii) Water restriction or reduce water activity iii) Cooling iv) CO2 pack v) Alcohol vi) Addition of preservativesi) nitrogen pack, vacuum pack ii) drying, addition of sugar, salt, glycerol or combination of other solutes  iii) chilling and freezing iv) to increase carbon di oxide v) fermentation of food  vi) benzoate, sorbate as organic substance. Nitrite, sulphite as inorganic substance. Nisin as antibiotic. Smoking of foods etc.   
Restrictions of access of microorganismsi) Control of microstructures ii) Decontamination iii) Aseptic handling iv) Packaging  i) emulsion formation ii) packaging materials and ingredients using H2O2, heat, irradiation etc.  iii) aseptic processing  iv) Aseptic packaging   
Destruction or removal of microorganismsi) Radiation ii) Heatingi) radappertization, radicidation, radurization ii) sterilisation, pasteurisation 

What is water activity (aw)? 

Water Activity of the food is the ratio of partial vapor pressure of food itself and the partial vapour pressure of pure water under identical condition.

A water activity of 0.70 means the vapor pressure is 70 percent of that of pure water. Most foods have a water activity above 0.95 and that will provide sufficient moisture to support the growth of bacteria, yeasts, and mold. The water activity increases with the temperature.

Water migrates from high aw areas to low aw area. If we expose salami (having aw=0.87 approx.) to dry air (having aw=0.5 approx.), salami will dry out. Similarly if we expose milk powder (having aw=0.60 approx.) to humid air (having aw=0.70 approx.), milk powder will absorb water from humid air.

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