Food preservation plays a significant role in maintaining foods at the required quality levels. While following conventional food preservation methods, wherein, foods are subjected to high temperatures, there is certainly a reduction in contamination or microbial load from the food, but it also causes some unfavourable changes in the food, such as the loss of nutritional components that are temperature-sensitive, changes in the texture of the food brought on by heat and modifications to the organoleptic properties of the food.
During thermal processing, food is subjected to heat treatment for a long duration of time, which affects food quality and results in the production of toxicants as well as low-grade food produce . The method of food preservation starts with the complete analysis and understanding of the whole food chain, including growing, harvesting, processing, packaging and distribution; thus an integrated approach is required to be applied. It lies at the heart of food science and technology and it is the main purpose of food processing.
First, we must identify the properties or characteristics of the property that is required to be maintained. Foods are made from natural materials and like any living matter, will deteriorate over time. The deterioration of food or food spoilage is the natural way of recycling, restoring carbon, phosphorus and nitrogenous matters to the good earth. The flaws of spoilage are usually that it changes the quality of the food from good to not so good by altering the appearance, smell and taste, thus making it less than desirable. Food spoilage could be caused by several factors, chiefly sickness and even death. Thus, food safety is the major concern in spoiled foods . To overcome or minimize such disadvantages, the concept of non-thermal treatment was born. Non-thermal methods allow processing of foods below the temperatures used during thermal pasteurization with flavours, essential nutrients and vitamins undergoing minimal or no changes during processing. There are several new non-thermal technologies of potential interest to the industry, including High-Pressure Processing (HPP), Pulsed Electric Fields (PEF), High-Intensity Pulsed Light, Irradiation, Ultrasound Cold Plasma, Ozone, Oscillating Magnetic Field that are intended to be utilized during the production of food as processes for the inactivation of microorganisms.
1. High-Pressure Processing
In High-Pressure Processing (HPP), food materials are subjected to elevated pressures (up to 87,000 pounds per square inch or approximately 6,000 atmospheres), with or without the addition of heat, to effect microbial inactivation or to alter the attributes of the food, so that consumers can have the desired attributes. A pressure of more than 60,000 pounds per square inch is sufficient to inactivate most vegetative bacteria. In the range of 1-8 kbar (100-800 MPa) of hydrostatic pressure, vegetative cells of pathogenic and spoilage microorganisms can be inactivated without damaging the quality of sensitive foods by excessive heat. Pasteurization and minimal processing can both be conducted through High-Pressure Processing (HPP). 
Typically, the HPP process involves packaging the product in a flexible container (usually a pouch or plastic bottle) and loading the pouch or bottle into a high-pressure chamber filled with a fluid that can transmit pressure (hydraulic fluid) during the HPP process. It is necessary to pressurize the hydraulic fluid (normally water) in the chamber with the assistance of a pump and this pressure is transmitted to the food inside the package with its pressure. Pressure is applied for a specific time (3-5 min). Afterwards, a conventional manner of storing/distributing the processed product is carried out to remove the finished product. Since, the pressure is transmitted uniformly (in all directions simultaneously), food retains its shape even at extreme pressures. The sensory characteristics of the food are retained without the need for heat and the microbial safety of the food is not compromised by this process as compared to conventional processing methods.
High-Pressure Processing has minimal adverse effects on freshness, as it eliminates any thermal decomposition of the food . This processing technology is used for fruits, vegetables, meat products, sea products, etc. This technology is eco-friendly and does not generate any waste, but the cost of equipment is extremely high. The operating principles of High-Pressure Processing, packaging requirements, effects on food quality and effects of microorganisms are reviewed. 
2. Pulse Electric Field
Pulsed Electric Field (PEF) processing is a non-thermal method of food preservation that employs brief bursts of electricity to inactivate bacteria, while having little to no effect on food quality. PEF may be used to prepare both liquid and semi-liquid foods. PEF processing produces high-quality fresh-like liquid meals with superior flavour, nutritional content and shelf life. Food preservation that takes place in this manner help to maintain their freshness, flavour, taste and look, as they are preserved without the use of heat. PEF processing is creating foods that are put between electrodes with high voltage pulses ranging from 20 to 80 kV (usually for a couple of microseconds). The high voltage used creates an electric field that induces microbial inactivation. The electric field can be applied as exponentially declining, square wave, bipolar or oscillatory pulses at ambient, sub-ambient or slightly above-ambient temperatures.
Following treatment, the food is packaged aseptically and stored under refrigeration. The use of PEF technology for the pasteurization of foods such as juices, milk, yogurt, soups and liquid eggs has been successfully shown. PEF processing is limited to food products with minimal air bubbles and low electrical conductivity. To guarantee effective treatment, the maximum particle size in the liquid must be lower than the gap of the treatment zone in the chamber. PEF is a continuous processing technology that is incompatible with solid food items that cannot be pumped. PEF is also used to improve sugar and other cellular content extraction from plant cells, such as sugar beets. PEF has also been used to reduce the solid volume (sludge) of wastewater. PEF treatment kills a variety of vegetative bacteria, mould and yeast. The effectiveness of spore inactivation by PEF in conjunction with heat or other barriers is currently being studied. 
PEF treatment might be useful for heat-sensitive liquid meals, when thermal pasteurization is not an option (owing to taste, texture or colour changes). PEF pasteurized goods are currently kept chilled. This is required for safety in some circumstances (for example, milk) (to prevent the growth of spores in low-acid foods). Refrigeration is not required for microbiological stability in acid foods, but it is utilized to retain taste quality for extended periods.
There are three types of factors that influence microbial inactivation with PEF: factors based on the process (electric field intensity, pulse width, treatment time and temperature, and pulse wave shapes), factors based on the microbial entity (type, concentration and growth stage of microorganism) and factors based on the treatment media (pH, antimicrobials and ionic compounds, conductivity and medium ionic strength) .
The various advantages of PEF over convectional processing such as increased mass transfer, improved extraction yield, decreased processing time, decreased intensity of the conventional extraction parameter, reduction of heat-sensitive compounds degradation, facilitation of purified extract, reduction of energy costs and environmental impact. 
3. Ohmic Heating
Ohmic heating is an advanced thermal processing method, wherein, the food material which serves as electrical resistor is heated by passing electric through it. Heating occurs due to the transforming of electrical energy into heat inside the food material, which causes rapid and uniform heating. A heating rate in the region of 1-10ºC/s can be observed during ohmic heating. It is also called Joule heating, Electrical Resistance heating or Electro heating. Ohmic heating is comparable to Microwave heating, however, the frequencies are significantly different.
The benefit of Ohmic heating is that it consistently warms up items of varying densities, such as chicken noodle soup . Ohmic heating is useful for heating liquid meals with big particles, such as soups, stews and fruit slices in syrups and sauces as well as heat-sensitive liquids. The method may be used to cure proteinaceous foods, which tend to denature and coagulate when heated. Liquid eggs, for example, may be ohmically heated in a fraction of a second without coagulating and juices can be treated to inactivate enzymes without compromising on the flavour.
Blanching and thawing, online measurement of starch gelatinization, fermentation, peeling, dehydration and extraction are all possible applications for ohmic heating. The shelf life of ohmically prepared foods is comparable to canned and sterile, aseptically processed products. Ohmic heating saves time and energy in hot air and freeze drying of foods as well as improving extraction yields in specific processing activities. The factors employed during ohmic heating, such as alternating current frequency, applied voltage and sample temperature, all have a substantial impact on its performance. The electrical conductivity of the meal or food combination is also an important consideration. Ohmic heating is a valuable tool for value-added processing and it has a wide range of applications in food processing activities requiring heat and mass transfer .
4. Osmotic Dehydration
Osmotic dehydration (OD) is a valuable technique for producing safe, stable, nutritious, flavourful, inexpensive and concentrated food by immersing the solid food, either whole or in slices in high osmotic pressure of sugar or salt aqueous solutions. Water diffuses from dilute solution (Hypotonic solution) to concentrated solution (Hypertonic solution) over a semi-permeable membrane until equilibrium is reached. The concentration gradient between the solution and the intracellular fluid is the driving factor for water elimination. The solute cannot diffuse through the membrane into the cells if the membrane is fully semi-permeable. However, due to the complicated internal structure of food systems, it is difficult to achieve a perfect semi-permeable barrier and there is always some solid diffusion into the food, implying that osmotic dehydration is a mix of simultaneous water and solute diffusion processes.
The Osmotic Dehydration method is mostly used for salting in the fruit and vegetable sectors as well as in the Seafood industry. Solute penetration during osmosis begins slowly and gradually increases with time. If the osmotic dehydration time is prolonged, a large quantity of solute penetration occurs. Many fruits and vegetables have been osmotically dehydrated, including apple, apricot, banana, carrot cherry, citrus fruits, grapes, guava, papaya, mango, potato and others .
Food irradiation is a physical therapy in which food is subjected to ionizing radiation, which is high-energy radiation that is used to expel electrons from atoms and ionize molecules. It is employed in the destruction of germs and parasites that cause human sickness. It can also make food last longer by eliminating or inactivating spoiling insects, moulds and yeasts, delaying fruit and vegetable ripening and preventing undesired sprouting of potatoes and other similar vegetables. Food irradiation is safe and keeps the food fresh .
Food Irradiation is a physical procedure, similar to drying, freezing and thermal processing (canning and pasteurization) that can be used to decontaminate, sterilize and preservation of food. Gamma rays are very short wavelength electromagnetic radiations that belong to the same family as x-rays, ultraviolet, visible, infrared, microwaves, and radio waves. The electromagnetic energy utilized in food irradiation is comparable to x-rays but has a shorter wavelength that produces more energy and penetration. So, regardless of the dose received or the amount of time the food is exposed to, food treated by irradiation is not rendered radioactive.
For food treatment, three types of ionizing radiation can be used: (1) gamma rays from Cobalt-60 (Co60) or Cesium-137 (Cel37) with respective energies of 1.33 and 0.67 million electron volts, (2) electronic generated from machine sources operating at a maximum energy of 10 MeV, and (3) electronic generated from machine sources operating at a maximum energy of 5 MeV.
According to the Joint FAO/IAEA/WHO Expert Committee on Food Irradiation (JECFI), irradiating foods up to an overall average absorbed dose of 10 kilos Gray (kGy) = 1000Gy (grey), which is in SI unit of energy absorbed (1joule/kg) from ionizing radiation Gy = 100 rad (1 rad = 100 erg/g), and 1kGy = 100 krad present no toxicological hazards and ionizing radiation might be used in food processing and other related processes .
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About the Authors:
1. Shivani Motegaonkar
Department of Food Technology,
Faculty of Engineering & Technology,
Jain (Deemed-to-be University), Bangalore, India.
2. Mahendra Gunjal*
Department of Food Technology and Nutrition,
Lovely Professional University, Punjab, India.
*Corresponding Author Email ID: firstname.lastname@example.org
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