Thermal Food Engineering Operations. NITIN KUMAR

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СКАЧАТЬ undesirable for consumption so it becomes the prime factor to study the rate of variations of quality attributes which indicates its shelf life; this is a very important parameter to consider. The quality of products can rely on appearance, yield, eating characteristics, microbial characteristics, and the consumer’s overall experience. The deterioration of food depends upon mechanical, chemical, physical, and microbial reactions. The quality of the product was maintained at every stage of food production and overall processing chains such as manufacture, storage, distribution, and sale. Additionally, the need for preservation must depend on its purpose and use, and consider the population for whom the preservation is to be done as the nutritional requirement and food restriction apply differently to different sections of groups.

      There are many measures for food preservation; inhibition, inactivation, and avoiding recontamination are the common ones. Each method contains several processes of preservation such as inhibition, which includes a decrease of oxygen, adding preservatives, control of pH, freezing, drying, surface coating, gas removal, fermentation, and many others. Inactivation includes irradiation, sterilization, extrusion, and others; avoiding contamination involves packaging, hygienic processing, aseptic processing, and others.

      Thermal technology has been the backbone of food production and preservation for many years. In this technology, the temperature is assumed to be the major parameter for preservation and processing mechanism to make food commercially sterile, i.e., to get rid of pathogens and microorganisms which usually grow in the normal shelf life of the food product. Thermally processing the food provides real importance to the food by increasing and preserving its shelf life longer than the chilled food processing technologies.

In novel thermal technologies, preservation is done by the use of electricity. Various forms of electrical energy are utilized for food preservation such as ohmic heating, high intensity pulsed electric field, high-voltage arc discharge, microwave heating, and low electric field stimulation. Ohmic heating is the most common and is based on volumetric heating which prevents the overheating of food, provides uniform and quick heating; it depends on the principle that generation of heat in the food is an outcome of electrical residence when an electric current is moved through the food product. Furthermore, ohmic heating prevents thermal damage and promotes the efficiency of energy. Similarly, microwave heating is also very common and utilized in almost every household and the food industry but its low penetration depth of microwave into solid provides thermal non-uniformity. The other available methods utilizing electric energy are also very versatile, useful, and efficient for the preservation of food. The current electro heating can be used to produce to form new and up-to-date products with diversified functionality.

1.3 Types of Thermal Technologies

      Thermal processings are perhaps most essential in the food sector which has been used for the past many years; it has been discovered to increase the quality and shelf life of food with heat treatments. Thermal processing is the heating of foods at a particular temperature for a specified period of time. There are several techniques available in thermal technologies such as radiofrequency heating, ohmic heating, blanching, drying, frying, chilling, infrared heating, freezing and microwave heating, and extrusion. Combined high-pressure thermal treatment of food is also a very efficient prospect for the processing and preservation of food. The most common among them are microwave heating, ohmic heating, combined microwave vacuum-drying, radiofrequency processing, and new hybrid drying technologies.

      Hybrid technologies are the recent development in engineering in the operation and design of the dryers to attain dried products with desired characteristics. In hybrid technologies, the drying technologies are combined with the new drying techniques to achieve a new age drying process to reduce energy consumption and enhance product quality. New age drying technologies would be very helpful for the bioproducts in agricultural sectors for all economic, environmental, and product quality aspects. Instant infusion is another new process for the heat treatment of food, depending upon the product requirement providing mild pasteurization and sterilization. For effective and efficient pasteurization and sterilization, the following are the needs: rapid and small heating time, accurate, and small residence time at sterilizing temperature, and rapid cooling time.

For examining the food when it goes under thermal process, nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) can be used as they possess some unique properties for the same. Both of them can be used to investigate variation in the food during processing. Both of them are non-invasive and able to detect water mobility. NMR is the most versatile analytical technique used in modern times. It is capable of revealing complicated multivariate information inside the optically opaque and complex food matrix, also the thermal transformation in liquid, suspension, and gels regarding food samples. NMR and MRI are based on the magnetic properties of atomic nuclei and many elements have isotopes with such properties. Both of them are superior to any other instrumental methods because both are non-invasive, non-destructive, both measure volumes instead of surfaces, and are able to extract both physical and chemical information. By both techniques, it can extract knowledge about diffusion, flow, water distribution, and others. Furthermore, processes such as heating, freezing, hydration, dehydration, and salting can be detected and monitored non-invasively.

      1.3.1 Infrared Heating

       1.3.1.1 Principal and Mechanism

      Infrared was discovered by William Herschel in the 1800s. Infrared depicted below red (infra: below); red is the longest wavelength of visible light. IR heating is the transmission of thermal energy in the form of electromagnetic waves. Wavelength between 0.7 and 1000 micrometer, wavelength larger than visible light but smaller than those of radio waves are the infrared waves. Three major types of infrared waves are [39]:

      1 Short waves 0.76-2 μm (near IR waves), temperature above 1000°C

      2 Medium waves: 2-4 μm (medium IR waves), when the temperature is above 400 - 1000°C

      3 Long waves: 4-1000 μm (far IR waves), when the temperature is below 400°C

      The working principle of infrared waves includes: IR energy is electromagnetic radiation emitted by hot objects (quartz lamb, quartz tubes, or metal body) by vibrations and rotation of molecules. When it is absorbed, the radiation provides up its energy to heat materials. An object is a “black-body”, if it absorbs (or emits) 100% of incident IR radiation. The quantity of heat emitted from a perfect radiator (blackbody) is expressed by Stefan-Boltzmann law equation:

      Where Q is defined as the rate of heat emission, σ_SB is the Stefan-Boltzmann constant, T is defined as the absolute temperature, and A is defined as the surface area.

      When radiant heaters and food products are not perfect absorbers, the Stefan-Boltzmann equation was modified and the concept of “grey body” was found:

Where ɛ defined as the emissivity of the grey body (ranging from 0 to 1).

      This property changes with the wavelength of emitted radiation and temperature of the grey body.

      The heating level depends on the absorbed energy, which then rely on the composition of food and the radiation frequency.

      Mathematically, the transfer of heat rate to food is expressed as,

      Where T₁ depicts the temperature of the emitter and T₂ depicts the temperature of the absorber. Heat СКАЧАТЬ