20 sterilization methods in the processing of flour rice and its baked goods
1. Ultra-high pressure sterilization technology
Food ultra-high pressure sterilization (high hydrostatic pressure sterilization) is that after the food materials are packaged in a certain way, they are put into a liquid medium (usually edible oil, glycerin, emulsion of oil and water), and act under a pressure of 100~1000MPa for a certain period of time to meet the requirements of sterilization.
The basic principle of sterilization is the lethal effect of pressure on microorganisms, mainly by destroying the cell membrane, inhibiting the activity of enzymes and affecting the replication of genetic material such as DNA. Under the pressure of 400~600MPa, it can kill bacteria, yeast and mold, and avoid the adverse changes caused by general high temperature sterilization, so it can better maintain the inherent color, aroma and taste of food, and achieve the effect of prolonging the shelf life.
2. Low-temperature bacillus
Low-temperature sterilization is a heating method that partially sterilizes microorganisms present in food. Temperatures below 100°C are usually used. Since there are many bacterial residues in food after low temperature sterilization, in order to extend the shelf life of products, processing technologies such as refrigeration, fermentation, additives, and deoxygenation are used.
This method is mainly suitable for acidic foods with pH below 4.5 and foods with strong heat treatment that will significantly lead to a decrease in quality. In recent years, this method has also been adopted for milk and products with a short shelf life.
3. Tomoe bacterium method
Pasteurization refers to a heat treatment method with a relatively low temperature, which is generally carried out at a temperature lower than the boiling point of water. It is an ancient technique pioneered by the French physician Pasteur in the 19th century, and it still has some application value today.
Pasteurization is the earliest sterilization method that uses hot water as a heat transfer medium. The sterilization condition is 61~63 °C, 30min, or 72~75 °C, 10~15min. When heating, it should be noted that the surface temperature of the material is 4~5 °C lower than the internal temperature;
In addition, when bubbles are generated on the surface, it is difficult for the foam part to meet the sterilization requirements. This sterilization method, due to the long time required, the production process is not continuous, and it is easy to change some heat-sensitive components when heated for a long time, and the sterilization is not ideal. At present, it is rarely used in large and medium-sized food factories.
4. Ultra-high temperature instant sterilization
Ultra-high temperature sterilization is referred to as UHT sterilization. Generally, the heating temperature is 125~150 °C, the heating time is 2~8s, and the sterilization process of the product to meet the requirements of commercial sterility after heating is called UHT sterilization. This sterilization method can achieve the purpose of sterilization in an instant, and the sterilization effect is particularly good, almost reaching or approaching the sterilization requirements, and the chemical changes caused are very small.
It has the ability to improve processing capacity, save energy, reduce the volume of equipment, stabilize product quality, and can implement in-situ non-disassembly cycle cleaning of equipment. High constant temperature sterilization: sterilization kettle, eight-treasure porridge, etc
5. Microwave sterilization technology
Microwave refers to electromagnetic waves with a wavelength of 0.001~1m (frequency 300~300000MHz). It can go straight forward at the speed of light, and when it encounters object obstruction, it can cause reflection, penetration, absorption and other phenomena, and the microwave frequency used for sterilization is 2450MHz. The results of the study generally concluded that there are two factors in the lethal effect of microwave on microorganisms, namely thermal effect and non-thermal effect.
The thermal effect refers to the absorption of microwave energy by the material, which increases the temperature to achieve the sterilization effect. The non-thermal effect refers to the strong rotation of polar molecules in the microwave field of organisms, which inactivates the vegetative cells of microorganisms or destroys the enzyme system in microbial cells, resulting in the death of microorganisms.
Microwave sterilization has the characteristics of strong penetration, energy saving, high heating efficiency, wide range of application, etc., and microwave sterilization is easy to control, heating is uniform, and the nutrients and color, aroma and taste of food are still close to the natural quality of food after sterilization. Microwave sterilization is currently mainly used for the sterilization of meat, fish, soy products, milk, fruits and beer.
6. Ultraviolet sterilization
The bactericidal effect of ultraviolet rays is to promote the denaturation of the cytoplasm. When microbial cells inhale ultraviolet rays, chemical changes occur in intracellular components, especially nucleic acids and plasma proteins, due to photochemical effects, resulting in cytoplasmic denaturation. In particular, it inhibits DNA replication and cell division, causing microbial cells to be injured or even die. Ultraviolet rays with a wavelength of 250~260nm have the strongest sterilization effect.
7. Ozone sterilization
Ozone is a light purple gas with a peculiar fishy smell that exists at room and freezing temperatures, which is partially dissolved in water and increases in solubility as the temperature decreases; It can self-degrade at room temperature to produce a large number of free radicals, the most significant being hydroxide radicals, so it has the characteristics of strong oxidation.
8. Magnetic sterilization technology
Magnetic sterilization is to put the food that needs to be sterilized in the magnetic field, and under the action of a certain magnetic field strength, the food can be sterilized at room temperature. Because this sterilization method does not need to be heated, it has a broad-spectrum sterilization effect, and the flavor and quality of the processed food are not affected, and it is mainly suitable for all kinds of beverages, liquid foods, condiments and other solid foods in various packages.
9. High-voltage electric field pulse sterilization technology
High-voltage electric field pulse sterilization is an instantaneous high-voltage electric field generated by placing food between two electrodes, because high-voltage electric pulse (HEEP) can destroy the cell membrane of bacteria, change its permeability, and thus kill the cells.
There are two ways to obtain a high-voltage pulsed electric field. One is to use the principle of LC oscillation circuit, first charge a group of capacitors with a high-voltage power supply, connect the capacitor with an inductance coil and the electrode of the processing room, and the high-frequency exponential pulse attenuation wave generated when the capacitor is discharged is added to the two electrodes to form a high-voltage pulse electric field.
Due to the fast discharge pole of the LC circuit, the electric field energy can be discharged within tens to hundreds of microseconds, and the automatic control device is used to continuously charge and discharge the LC oscillator circuit, and the sterilization process can be completed in tens of milliseconds. The other is to use a specific high-frequency and high-voltage transformer to obtain a continuous high-voltage pulsed electric field. The high-voltage pulse electric field strength for sterilization is generally 15~100kV/cm, the pulse frequency is 1~100kHz, and the discharge frequency is 1~20kHz.
High-voltage electric field pulse sterilization is generally carried out at room temperature, and the processing time is tens of milliseconds, and this method has two characteristics: first, due to the short sterilization time, the energy consumption in the treatment process is much less than that of the heat treatment method. Second, due to the fact that it is carried out at room temperature and pressure, the processed food has little change in physical properties, chemical properties and nutrients compared with fresh food, and there is no difference in flavor and taste. Moreover, the sterilization effect is obvious (N/No<10-9), which can meet the requirements of commercial sterility, especially suitable for heat-sensitive food, and has broad application prospects.
10. Ultrasonic sterilization technology
Ultrasound is a sound wave with a frequency greater than 10 kHz. Ultrasound waves are longitudinal waves like ordinary sound waves. The interaction between ultrasonic wave and sound medium contains a huge amount of energy, when it encounters the material, it produces a rapid alternating compression and expansion effect, which is enough to kill and destroy microorganisms in a very short period of time, and can also produce a variety of effects such as homogenization, aging, cracking macromolecular substances and other effects on food, with multiple effects that are difficult to obtain by other physical sterilization methods, so as to better improve food quality and ensure food safety. Zhu Shaohua used the ultrasonic generator as the sterilization equipment and the soy sauce as the sterilization object, and achieved good results.
11. Pulsed strong light sterilization technology
Pulsed light sterilization technology is sterilized by the method of intense white light flashing, which consists of a power unit and an inert gas lamp unit. The power unit is a component that can provide high voltage and high current pulses, which provides energy for the inert gas lamp, which can emit light from ultraviolet to near-infrared regions, and its spectrum is very similar to that of sunlight, but the intensity is thousands to tens of thousands of times stronger, and the optical pulse width is less than 800μs.
This technology can be used to extend the shelf life of food packaged in transparent materials and fresh foods because it only treats the surface of the food, so it has little impact on the flavor and nutritional content of the food. Zhou Wanlong et al. showed that pulsed light had a strong lethal effect on Bacillus subtilis and yeast, and the number of these bacteria could be reduced from 105 to 0 after more than 30 flashes. The band in which the pulsed light acts as a germicidal may be ultraviolet light, but other wavelengths may have a synergistic effect.
12. Membrane filtration and sterilization technology
With the development of materials science, various membranes that can be used for material separation have appeared one after another, and membrane separation technology has been widely used in industrial production such as food and biopharmaceutical, such as the extraction of biochemical substances, the preparation of pure water, and the concentration of fruit juice. The membrane separation process can be broadly divided into two types according to the different driving forces. One is membrane processes driven by pressure, such as ultrafiltration; The other type is a membrane process that is driven by electricity, called ion exchange, such as electrodialysis. Pressure-driven membrane processes can be divided into microporous filtration, ultrafiltration, and reverse osmosis according to the pore size and retention capacity used in the membrane.
Usually the pore size of the membrane is 0.0001~10μm, and the size of microbial particles in the material is generally 0.5~2μm.
13. Chemical fungus
There are many sterilization technologies in food engineering, such as: chlorine dioxide sterilization technology, chlorine sterilization technology, electronic sterilization technology, heating and pressurization combined sterilization technology, heating and chemical combined sterilization technology, heating and radiation combined sterilization technology, electrostatic sterilization technology, etc. These technologies are being researched and applied.
14. Far infrared sterilization technology
The use of infrared began in the 20th century, and in 1935, Groveny of Ford Motor Company of the United States first obtained a patent for the use of infrared for heating and drying. Many ingredients and microorganisms in food have strong absorption in the far-infrared region of 3~10μm. Far infrared heating sterilization does not require media, and the heat directly penetrates from the surface of the object to the inside, so it can be used not only for the sterilization of general powdered and lumpy foods, but also for the sterilization and mildew sterilization of nut foods such as coffee beans, peanuts and grains, as well as the direct sterilization of bagged foods.
15. Resistance heating sterilization technology
Resistance heating sterilization, also known as ohmic sterilization, is a new type of thermal sterilization method, which borrows the current to generate heat inside the food and achieves the purpose of sterilization, is a new technology for continuous sterilization of acidic and low-acid food and food with particles (particle size less than 25mm).
Resistance heating sterilization uses alternating current at a frequency of 50~60Hz, which uses electrodes to directly introduce current into food, and generates heat from the dielectric properties of food itself to achieve the purpose of sterilization. The applicability of resistance heating is determined by the conductivity of food materials, most of the food that can be transported by pumps, dissolved with salt ions and water content of more than 30% can be sterilized by resistance heating, and the effect is very good, while some non-ionized foods such as fat, sugar, oil, and treated water without added salt are not suitable for this technology.
16. Inert gas packaging sterilization, nitrogen sterilization.
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17. Vacuum packaging sterilization.
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18. Ethanol alcohol sterilization at 75 °C.
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19. Microbial preservative sterilization.
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20. Sterilization of industrial chemical preservatives.
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Dr. Du Dechun Bakery Process Technology Chief Engineer.