Fish Microbiology and Quality Assurance (2+2)

Unit 4 - Food preservation techniques and microorganisms

4.4.1. Radiations types and their characters
1.UV rays
Ultraviolet radiations are powerful bactericidal agents. These non-ionizing radiations of < 450 nm wavelength are absorbed by proteins and nucleic acids leading to photochemical changes and subsequent cell death. The death of microorganisms results from the production of lethal mutations in nucleic acid preventing transcription and DNA replication.
UV rays are bactericidal/virucidal in the wave length between 200-290 nm, but are most effective at 260 nm (2600 A^o). These have poor penetration capacity (penetrate only 0.1 mm thickness) hence their application in food industry is limited to disinfection of air, and application on food surface especially the packaging material. However, some foods are also treated with UV radiation before wrapping. UV radiation can cause rancidity in high fat products. Workers have to be protected from UV rays as it can cause burning of skin and eye disorders.

2.Beta rays ( β- particles)
These ionizing radiation are a stream of high energy electrons emitted by radioactive substances or machine generated electrons using cathode ray tubes. Beta rays have poor penetrating power but better than UV radiations. These are known to induce radioactivity in some foods under high energy sterilization conditions (at upper limit of energy level). Hence, these are of have limited application in food preservation.

3.Gamma rays
These are uncharged electromagnetic radiations emitted from the exited nucleus of radioactive elements such as ⁶⁰Cobalt, ¹³⁷cesium etc. These ionizing radiations produced by the decay of radioactive isotopes are cheapest form of radiations for use in food preservation since source elements are available as byproducts of atomic waste.
Gamma rays have excellent penetration power and penetrate almost anything. These can penetrate food up to a depth of 20 cm and effective as bactericidal agents.

4.X – rays
These ionizing radiations are produced by bombarding suitable metal target with high velocity electrons. These are similar to Gamma rays in their behavior.

Radiation energy unit
The radiation dose or amount of radiation applied for food preservation purpose is expressive by REP unit, RAD unit or GY unit.

REP unit (Roentgen – Equivalent physical)
1 REP = absorption of 83 ergs /g of matter.
Ergs is unit of energy.

RAD unit
RAD is unit of measurement of radiation.
1 RAD = absorption of 100 ergs / g of matter
1 K rad = 1000 rads

GY unit (Gray unit)
A newer and recently used unit of radiation
1 Gray = 100 rads = 11 joules /kg
1 KGy = 105 rads

Radiation process
The radiation process given to food is of 3 types.

1. Radappertization
2. Radicidation
3. Radurization

1.Radappertization
This refers to radiation process bringing about total destruction of microorganisms. It is equivalent to commercial sterility of heat processed foods, also called radiation sterilization. This process uses radiation dose of 30~40 K gray and ensures sterile product of prolonged shelflife.

2.Radicidation
This is a radiation process used for the reduction of number of viable non – spore forming pathogens which are undetectable by any standard method. This process is equivalent to pasteurization. Radiation level of 2.5~10 KGy is used in this process.

3.Radurization
It is a process of irradiation given to minimize the load of spoilage organisms thus extending the shelflife of food. This may be considered equivalent to pasteurization. This ensures enhancement of the keeping quality of a food by causing substantial reduction in the number of viable specific spoilage microorganisms. It is suitable for extending shelflife of fresh meat, sea food, fruits, vegetable etc. Dose used is 0.75~2.5 KGy.