Thermal processing generally involves heating of food for a predetermined time at a pre-selected temperature to eliminate the pathogenic microorganisms that endanger the public health as well as those microorganisms and enzymes that deteriorate the food during storage. But due to very high temperature, the nutritional factors of food such as vitamins, minerals, proteins, fats etc which are highly sensitive to high temperature, deteriorates during processing which leads to reduction of nutritional quality and generation of off flavour, off colour and other sensory properties of food products. Therefore, in order to improve food quality and sensory properties, novel non-thermal food processing technologies are the need of food engineers, food processors and product developers. The various non-thermal food technologies used now a day in food industries are discussed below.
High Hydrostatic Pressure (HPP)
By subjecting food to high pressure in the range of 5000-8000 bars, microorganisms and enzymes can be inactivated without degradation in flavour and nutrition. It is an effective non-thermal sterilization/pasteurization treatment for liquid and solid foods which permits microbial inactivation at low or moderate temperature with minimum degradation. The energy required for high-pressure processing is also less than that required in thermal processing and it can be applied to a very wide range of food products like milk, juice, meat, seafood and many other solid and liquid foods.
In typical HPP, the product is packaged in a flexible container usually a pouch or plastic bottle and is loaded into a high-pressure chamber filled with a pressure transmitting (hydrodynamic) fluid ex-oil or water. The hydrodynamic fluid in the chamber is pasteurized with pump and this pressure is transmitted uniformly through the package into the food itself. Pressure is applied for specific time usually 3 to 5 minutes. The processed product is then removed and stored in a conventional manner. Because the pressure is transmitted uniformly in all directions simultaneously, food retains its shape even at extreme pressure and because no heat is needed, the sensory characteristics of food are retained without compromising the food quality and microbial safety. It only affects non-covalent chemical bonds; leaving covalent bonds which permit the destruction of microorganisms without significantly affecting food molecule that contributes texture or flavour of food.
The basic principle of PEF includes application of short pulses of high electric field with duration of microseconds and intensity in the order of 20-80 kV/cm. the processing time is calculated by multiplying the number of pulse time with effective pulse duration. The process is based on pulsed electric current delivered to a product placed between a set of electrodes, the distance between electrodes is termed as treatment gap of PEF chamber.
The applied high voltage results in an electric field that causes microbial inactivation. The PEF equipment consists of a high voltage pulse generator, a treatment chamber and monitoring and controlling devices. The food product is placed in a treatment chamber either in static or continuous manner where two electrodes are connected with the non-conducting material.
Generally high electrical pulses applied to electrodes which then conduct the high-intensity electric pulses to the product placed between two electrodes. The food product experience a force per unit change so-called electric field which is responsible for irreversible cell membrane breakdown. This leads to dielectric breakdown of microbial cell membrane and to interaction with charged molecule of food.
Advantage of PEF
- Less treatment time
- Low treatment temperature
- Increased shelf life of product
- Maintain food safety with low processing cost
- Inactivate vegetative microorganisms including yeasts, spoilage microorganisms and pathogens.
- Reduces microorganism by 4-6 log cycle
Disadvantages of PEF
- High capital cost
- Effective for Inactivate vegetative microorganisms only
- Refrigeration is required to extend shelf life
- Not suitable for solid food products that are not pumpable
Irradiation
Irradiation can be defined as exposing food to gamma rays, x- rays or electrons to improve shelf life and safety. It has a range of effects including killing bacteria, moths and insect pests, reduces ripening and spoilage of fruits and at higher doses, it can be used for sterility. It is sometimes called as cold pasteurization as the product is not heated. It is also known as Ionizing radiation, Surface pasteurization, Electronic pasteurization or E-beam sterilization/pasteurization. In actual processing the food is packed and moved by conveyer belt into shielded room. Food is exposed briefly to a radiant energy source. Food is left virtually unchanged but the number of harmful bacteria, parasite and fungi are reduced or eliminated. Irradiation can decrease the loss of food due to insect infestation, foodborne pathogens and spoilage. It can be used for preservation, sterilization, control sprouting, ripening, foodborne illness and insect damage.
Radiation dose: The dose is an amount of radiation used to expose food. It is measured by unit called kilo grays (kGy). The dose permitted for use in food varies according to type of food and desired action like low dose (< 1 kGy), medium dose (1-10 kGy) and high dose (>10 kGy)
Advantage of irradiation:
- The nutritional value is unchanged
- No harmful chemical changes
- Food does not become radioactive
- The appearance of food in unchanged
Disadvantages of irradiation:
- Can be used only for limited range of foods, not all fresh produce is suitable for irradiation
- Some treated food may taste different
- Some chemical changes occur
It is an alternative to thermal treatment for killing pathogenic and spoilage microorganism in foods including bacteria, yeasts, moulds and viruses. The treatment consists of applying a series of very short high power pulses of broad spectrum light. It is very effective on product surface and marginally effective at penetrating to depths in food. It also reduces the need for chemical disinfectants and preservatives. Pulses of light for food processing applications typically emit 1-20 flashes per seconds of electromagnetic energy.
The key component of pulse light unit is a flash lamp filled with inert gas such as xenon which emits broadband radiations that ranges from the ultraviolet cutoff to near infrared cutoff (200 nm -1000 nm with peak 400-500 nm). A high voltage, high current electric pulse is applied to the inert gas in the lamp and the strong collision between electron and gas molecule cause excitation of latter which then emits an intense very short light pulse.
The treatment is most effective on smooth, nonreflecting surfaces or in liquids that are free of suspended particles. Rough surface hinder inactivation of microorganisms due to cell hiding. The main limitation of pulsed light treatment is its limited penetration depth. It has a very wide range of applications like decontamination of vegetables, dairy products, microbial inactivation of water, and sanitation of packaging material and disinfection of equipment surface.
Food processing is very important for value addition. Vegetable and fruit growers sometimes get a very low price because of overproduction and glut in the market. Value addition through processing can help to boost the price of these products and help the growers to get a high price. The new technologies of food processing are expected to give a better result than the traditional methods.
By S. Shivasankar
Ph. D. Scholar, Department of Post Harvest Process and Food Engineering
