Physical methods of food preservation

PHYSICAL METHODS OF FOOD PRESERVATION:

Radiation, High Pressure, Pulsed Electrical Field, Light and Modified Atmosphere

Basura J. Jayasundara

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CONTENTS

Introduction

Use of ionizing radiation

Use of nonionizing radiation

Light energy in food preservation

High pressure processing

Pulsed electric fields

Modified atmosphere

Summary

References 2

INTRODUCTION

Food begins to deteriorate from the time it is

harvested or slaughtered.

Each foodstuff has a specific period after which it

begins to spoil.

Preservation is needed to slow down the spoilage

process, extend the shelf life and ensure the safety

and quality of food.

Food preservation methods can be divided into

physical, chemical and biological.

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INTRODUCTION

Physical methods of food preservation are those that utilize physical treatments to inhibit, destroy or remove undesirable microorganisms .

Heat treatment

Radiation

Light

High Pressure Processing (HPP)

Pulsed Electric Fields (PEFs)

Modified Atmosphere

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USE OF IONIZING RADIATION

“Food irradiation (the application of ionizing radiation to food) is a technology that improves the safety and extends the shelf life of foods by reducing or eliminating microorganisms and insects”(FDA, 2014)

There are three sources of radiation approved for use on foods.

Gamma rays

X rays

Electron beam

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Applications of food irradiation

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Application Commodity

Inhibition of sprouting Potatoes, onions, garlic

Decontamination of food Spices

Insect disinfestation Grains

Delay in fruit maturation Mangoes, papayas, strawberries

Inactivation of MO (Salmonella)

Poultry, eggs

Logo for irradiated foods


Mode of action:

Affects on bacteria, yeasts and moulds.

Main sites of damages: nucleic acid and the lipids of the cell membrane

Membrane lipid degradation

Change the permeability of the cell membrane

Leach out of cell components and

Inhibition of the DNA replication

Indirect effect: inhibitory effects of free radicals produced by the radiolysis of water.

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USE OF NONIONIZING RADIATION

Microwave Radiation

Two frequencies used in food processing are 2450 MHz and 915 MHz.

Domestic microwave ovens use 2450 MHz which is less penetrating than the lower frequency.

Mode of action :

Microwaves act indirectly on micro-organisms through the generation of heat.

Destruction of MO is accomplished through the denaturation of protein and nucleic acids.

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LIGHT ENERGY IN FOOD PRESERVATION

UV Radiation

Use to inactivate MOs on the surface of foods and thin films of liquid.

Use extensively in disinfection of equipment, glassware, and air.

The optimum wavelengths: 260 nm.

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Use of UV light UV light conveyer

LIGHT ENERGY IN FOOD PRESERVATION

Mode of action:

UV light is absorbed by proteins and nucleic acid, in which photochemical changes are produced.

It disrupt DNA molecules, produce lethal mutations and thereby prevent cell replication.

Degradation of the bacterial cell walls also cause the germicidal effect

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HIGH PRESSURE PROCESSING

Involving pressurization of a packed food in a water filled closed chamber for a short duration to inactivate microorganisms.

Effective for both liquid and solid foods (liquid food can be pressurized directly.

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Nonthermal method and covalent bonds are not broken, flavor is unaffected.

The process has 3 parameters;

Pressure

Temperature of pressurization

Time of pressurization

Gram (+) bacteria are more resistant than Gram(-).

Vegetative cells are inactivated by pressures between 400 and 600 MPa.

Spores may resist pressures higher than 1000 MPa at ambient temperatures.

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Mode of action:

Many vital structure and components adversely affected, causing viability loss and sub lethal injury.

Damages to cell membrane

Alter the permeability of membrane

Damages in cell wall

Inactivation of enzymes

Cell lysis

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PULSED ELECTRIC FIELDS

Consists of the application of short pulses of high electric fields to foods placed between two electrodes.

No significant detrimental effect on heat-labile components present in foods such as vitamins.

Major disadvantage: high initial investment.

Antimicrobial effects of PEF are functions of:

Electric field strength

Treatment time

Treatment temperature

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General features of PEF:

G- bacterial cells more sensitive than G+ or yeasts.

By increasing electric field intensity and number of pulses greater microbial destruction can be achieved.

Destruction of bacterial and fungal spores requires a higher voltage and longer period of time.

Temperature can be increased to 60 0C or higher to obtain a greater microbial destruction.

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Mode of action:

Cell death by PEF is due to disruption of cell membrane function.

When microbial cells in a suspension are exposed to PEP, a potential differences occurs between outside and inside cell membrane.

Because of this difference , pore formation in the membrane, cause destruction of membrane function and cell death.

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MODIFIED ATMOSPHERE

Three different procedures are used.

1) Modified Atmosphere Packaging (MAP)

Bulk or retail pack is flushed with a gas mixture usually containing combination of CO2, O2 and N2.

Does not require a control of gaseous environment during the entire storage period.

The composition of the gas atmosphere changes during storage as a result of product and microbial respiration.

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MODIFIED ATMOSPHERE

2) Controlled Atmosphere Packaging (CAP)

Atmosphere in a storage facility is altered, levels of the gases are continually monitored

The product environment is maintained constant throughout storage.

Used for long term storage of fruits and vegetables to maintain their freshness.

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MODIFIED ATMOSPHERE

3) Vacuum Packaging

Involves removal of air from the package and then sealing the package hermetically.

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Vacuum packed food products

MODIFIED ATMOSPHERE

Mode of action

The growth of aerobes (mold, yeast, aerobic bacteria) is prevented in products .

However, anaerobic and facultative anaerobic bacteria can grow unless other techniques are used to control their growth.

The inhibitory effect of CO2 on microbial growth occurs at the 10% level and increase with the increase in concentration.

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SUMMARY

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Preservation method Mode of action

Ionizing radiation Affect on nucleic acid & membrane lipids

Microwave radiation Denaturation of protein and nucleic acids

UV Light Disrupt DNA molecules, produce lethal mutations

High Pressure Damages to cell membrane

Pulsed Electric Fields Disruption of cell membrane function

Modified Atmosphere (Vacuum packaging)

Low oxygen level inhibit aerobes

Modified Atmosphere (MAP or CAP)

Inhibition of MO by CO2

REFERENCES

Adams, M.R., and Moss, M, O., 2008. Food Microbiology, third edition. RSC publishing, UK.

Ray, B., Fundamental Food Microbiology, third edition, 2004. CRC press.

Jay, J.M., Loessner, M.J., and Golden, D.A., Modern Food Microbiology, seventh edition, 2005. Springer.

Rahman, M.S., Handbook of Food Preservation, second edition, 2007. CRC press.

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THANK YOU

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