1. Drying 

It is the most primitive methods of food preservation. Water activity is the ratio of vapour pressure of water in the product to the vapour pressure of pure water at same temperature. It is the amount of water available for growth of the contaminating microorganisms. The water activity of the food may be reduced and its microbial stability can be enhanced by the addition of solutes.
     
Process involves the addition of sodium chloride to meat, fish, and vegetables, sugar (syrup) to fruits and fruit products. Drying of foods increases the concentration of solutes in foods and also greatly reduces the mass and volume of the food product

Disadvantage of this process is that it results in a change in the physical and sensorial attributes of food, which is not always desirable.


2. Fermentation


It is one of the oldest methods of food preservation. It is one of the most common method of preparation and preservation technique of food. Fermentation process controls spoilage microorganisms by the release of metabolites (organic acids, bacteriocins, alcohols, and enzymes) and alteration of the conditions in the food (lowering of redox potential or water activity or removal of nutrients and growth factors).
         
In today’s world, demand for  minimally processed foods, “natural foods” or foods devoid of preservatives and chemical additives is growing. One approach to deal with this trend is fermentation. Fermentation fulfils this need of the consumers. Recently it has established that some of the compounds produced by fermenting cultures can be extracted and purified for the use as additives in food preservation.


3. Low temperature storage

All microorganisms have a defined temperature range in which they grow, with a minimum, optimum and maximum temperature. So, their growth can be controlled by lowering the temperature of food. Based on the temperature requirement microorganisms are classified into. 

       Classification

    Temperature range

   Optimum temperature

       Thermophiles

         55°C – 90°C

                  65°C

       Mesophiles

         20°C – 45°C

                  35°C

       Psychrophiles

           0°C – 20°C

                  12°C

       Psychrotrophs

             Variable

                  30°C

Source: ICMSF 1980

         
When the temperature is lowered below the optimum for the growth of microorganism, the lag time and generation time increases, growth rate decreases. As the temperature approaches the minimum for growth, cell division ceases. This is due to reduction in the reaction rate of enzyme-catalyzed reaction and reduction in the fluidity of the cytoplasmic membrane, thus interfering with the transport mechanisms leading to decreased growth rates.

Organisms that are commonly known to cause spoilage in frozen foods are:

  • Bacteria - Listeria monocytogenes, Clostridium botulinum, Yersinia enterocolitica
  • Yeast - Candida species, Cryptococcus laurenti, Trichosporon pullulans
  • Molds - Cladosporium herbarum, Penicillium hirsutum, Thamnidium elegans

 Disadvantages of this process

  • At any given temperature, growth rates differ according to the species of microorganism and minimal growth temperature of each organism is different.
  • Cytotoxigenic E.coli, Salmonella, E.coli 0157:H7 are known to survive the low temperature conditions.
  •  Relatively small increase in the storage temperature can cause a significant increase in growth rate of few pathogens

The present day requirement for long shelf life of chilled and frozen foods could mean that pathogens might multiply significantly during long storage of a product. Hence it is important to find an alternative method or a method in combination with this technique to make the product sterile and thus safe for long term storage.


4. Preservative

Chemical preservative helps in preserving food through different mode of action. Based on chemical nature,  it can be classified as

  • Antibiotics (Nisin, Natamycin).
  • Acidulants
    • organic acids (acetic acid, lactic acid, citric acid, malic acid) and
    • inorganic acids (hydrochloric acid, phosphoric acid)
    • lipophilic organic acids (propionic acid, sorbic acid, benzoic acid)
    • weak inorganic acids (sulfites, nitrites),

The most commonly used chemical preservatives are nitrites, sulfites and benzoates. These are most effective at low pH values. Nitrites are mainly used in meat curing and sulfites are used in variety of acidified products. Benzoates are used in preservation of fruit and vegetable products. They are used in alcoholic and non-alcoholic drinks where the pH is not low enough for an anti-microbial effect, mainly to prevent the growth of yeast and molds.

Disadvantages: They are less efficient at high pH values. These chemical preservatives are associated with undesirable potential toxicological effects. So, their input limits are restricted.


5. Thermal processing

Canning is one the most exploited thermal processing technique for preservation of food. The process of sealing food products hermetically in containers and sterilizing them by heat to enhance their shelf life is called canning. Canned foods are ‘commercially sterile’. Commercial sterility of thermally processed food implies to the condition achieved by the application of heat, alone or in combination with other treatments, to render the food products free from microorganisms capable of growing in food under normal conditions of distribution and storage.

Microbes like different species and strains of bacteria, yeast and molds, contaminating the foods, are present at different stages of growth. Among these, spores are more difficult to inactivate. The heat treatment results in destruction of microorganisms and also inactivates the enzymes in food which may cause spoilage. The heat resistance of microorganisms is closely related to pH of foods. From the thermal processing point of view, foods may be classified into three groups based on which the degree of heat treatment given varies.

Product

Product pH

Heat treatment

Spoilage microorganism

Low acid foods

³ 4.5

Pressure process,

Fo =3

Mesophilic spore forming bacteria

Acid foods

4.3 – 4.5

Flash sterilization

95°C

Bacillus coagulans, Clostridium butyricum, Clostridium pasteurianum

High acid foods

£ 4.5

Hot filled or minimum product temperature of 87.7° C

Yeasts, molds, Lactic acid bacteria


Disadvantages

  • Canned foods that are commercially sterile may contain viable bacteria in low numbers.
  • Canned low acid foods may contain spores of thermophilic bacteria that have very high heat resistance but are unable to germinate at temperature below 30°C. Such foods are stable at temperate climate but may spoil in tropical climate.
  • Leaker spoilage may occur due to improper sealing, under processing of cans and damage during transportation etc.


References


Institute of Food Technologists (IFT), Department of Science and Technology Projects (2000) – Special supplement: Kinetics of microbial inactivation for alternative food processing technologies. Barach, J.T., Barbosa-Canovas, G.V., Busta, F.F., Datta, A.K., Davidson, P.M., Farkas, D.F., Heldman, D,R., Hoover, D.G., Kokini, J.L., Pflug, I.J., Pierson, M.D., Sastry, S.K., Schaffner, D.W., Zhang, Q.H., editors. Chicago: IFT.p180. (Journal of food science; Vol. 65, no.8, Suppl)

Jay, J.M (2000) – Modern Food Microbiology, 6th edition, Gaithersburg (MD): Aspen. p 679

Olson, J.C., Jr., Nottingham, P.M (1980) - Temperature. In: ICMSF, Microbial ecology of foods, Volume 1, p1- 37. Academic press, London

Shapton, D.A and Shapton, N.F (1991) – Principles and practices for the safe processing of foods. Butterworth – Heinemann, Oxford

Olson, J.C., Jr., Nottingham, P.M (1980) - Temperature. In: ICMSF, Microbial ecology of foods, Volume 1, p1- 37. Academic press, London

ICMSF: International Commission of Microbiological Specification for Foods (1980) – Microbial ecology of foods, Volume 1: Factors affecting life and death of microorganisms. Orlando: Academic Pr. p. 311

Gill, C.O (1996) – Cold storage temperature fluctuations and predicting microbial growth. J. Food Prot 59 (Suppl): 43 – 47

Hagen, P.O (1997) - The effect of low temperatures on microorganisms: conditions under which cold becomes lethal. In W B Hugo (1997), ed., Inhibition and destruction of the microbial cell.p39 – 76, Academic Press, London

Jay, J.M (2000) – Modern Food Microbiology, 6th edition, Gaithersburg (MD): Aspen. p 679

Tags: Drying, Fermentation, Low, storage, temperature

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