Food is processed products of animals and plants. Most foods are rich in nutrients and can easily breed microorganisms, leading to food spoilage. The essence of any food processing technology is a method of re-modulating the color, aroma, taste and shape of food raw materials using certain techniques to preserve food for a long time to satisfy people's different preferences. In recent years, my country's food processing technology has made great progress, but there are still many problems in the research and development of sterilization technology. Understanding the new trends in sterilization technology in the world will help improve my country's backwardness in this field, and is also an urgent need to develop my country's food processing technology.
Food sterilization technology is one of the core technologies in the food industry. In a sense, the development history of the food industry is a history of the development of food sterilization technology. Food sterilization technology is to use various means to kill various harmful microorganisms that are contaminated by the food itself, brought in from food packaging containers, introduced by operators and equipment during processing and preparation, and existing in the production environment, thereby A technology that maintains food quality and reaches a certain shelf life. Sterilization costs account for a considerable proportion of the processing costs of some foods, which directly affects the price and market competitiveness of the product. The quality of the sterilization process directly affects the quality of the product. Therefore, research on sterilization technology should be vigorously carried out. Sterilization technology can generally be divided into heat sterilization technology, chemical agent sterilization technology, radiation sterilization technology (γ-ray, microwave, infrared, etc.), filtration sterilization method, and sterilization technology that combines heating with other means according to the sterilization (removal) method. wait. Various sterilization technologies have different development histories and have their own characteristics and scope of application. The characteristics, research status and application fields of various new sterilization (removal) methods used in modern food engineering are now introduced.
1 Research status
1.1 Thermal sterilization technology
The method of using heat to kill harmful microorganisms in food is not only an ancient method, but also an extremely important one in modern times. sterilization technology. In 1804, the Frenchman Appert invented a method of bottling food and boiling it in boiling water for a period of time to preserve food for a longer period of time. In the 1850s, the Frenchman Pasteur explained the method of food preservation. The mechanism of microbial spoilage has laid a theoretical foundation for the development of sterilization technology.
Food thermal sterilization can be divided into low-temperature sterilization (pasteurization), high-temperature short-term sterilization and ultra-high temperature instant sterilization. The first two methods have a long history of application due to their stable sterilization effect, simple operation, and low equipment investment. They are now widely used in the production of various canned foods, beverages, alcohol, medicines, and dairy products. The latter method, due to its unique advantages, has been developed into a high-tech food sterilization technology.
1.2 Ultra-high temperature instant sterilization technology (UHT)
Ultra-high temperature sterilization came into being in 1949 with the emergence of the Stork device. Since then, many technologies have appeared in the world. Types of ultra-high temperature sterilization devices. Ultra-high temperature treatment can be divided into two types: indirect heating and direct heating. It quickly heats the material liquid to above 130°C and then maintains it for a few seconds, thereby achieving instant sterilization of the material liquid.
The sterilization effect of ultra-high temperature instant sterilization technology is particularly good. It can almost meet or approach the sterilization requirements. Moreover, the sterilization time is short, the nutrients in the material are less destroyed, and the nutrient preservation rate is over 92%, which is greatly superior. In the above two thermal sterilization methods. Ultra-high temperature sterilization devices combined with food aseptic packaging technology are developing rapidly at home and abroad. Currently, this sterilization technology has been widely used in the production of sterilized milk, juice, various beverages, soy milk, wine and other products.
1.3 Resistance heating sterilization technology
Resistance heating sterilization, also called Ohmic sterilization, is a new type of thermal sterilization method. It uses the incoming electric current to generate heat inside the food to achieve sterilization. The purpose is a new technology for continuous sterilization of acidic and low-acidic foods and foods with particles (particle size less than 25mm).
Resistance heating sterilization uses alternating current at a frequency of 50~60Hz. It uses electrodes to directly introduce current into the food, and generates heat from the dielectric properties of the food itself to achieve the purpose of sterilization. The applicability of resistance heating is determined by the conductivity of the food material. Most foods that can be transported by pumps, have dissolved salt ions and have a water content of more than 30% can be sterilized by resistance heating, and the effect is very good, while some fats This technology is not suitable for non-ionized foods such as sugar, oil, and processed water without added salt. Tests at the British APV Food Processing Center have shown that resistance heating has been successfully used to sterilize a variety of foods containing large particles and flake foods, such as potatoes, carrots, mushrooms, beef, chicken, sliced ??apples, pineapples, peaches, etc.
1.4 Ozone sterilization technology
Ozone is extremely unstable in water. Reduction reactions occur all the time, producing monoatomic oxygen with strong oxidation. At the moment of its generation, it interacts with the bacteria in the bacterial cell wall. Lipoproteins or phospholipids and proteins in the cell membrane react chemically, thereby damaging the bacterial cell wall and cell membrane, increasing the permeability of the cell membrane, causing intracellular substances to flow out, causing the bacteria to lose activity. At the same time, ozone can quickly diffuse into cells and oxidize enzymes, RNA, and DNA in cells, thereby killing the bacterial pathogens.
Ozone sterilization has the advantages of high efficiency, fast, safe and cheap. Since its discovery in 1785, it has been widely used in food processing, transportation and storage, tap water, purified water production and other fields.
1.5 Radiation Sterilization Technology
Since the peaceful use of atomic energy, after more than 40 years of research and development, people have successfully used atomic radiation technology for food sterilization and preservation. Irradiation is a cold sterilization method that uses X-rays, gamma rays or accelerated electron rays (the most common ones are Co60 and Cs137 gamma rays) to penetrate food to kill microorganisms and insect pests in food. Irradiated food or organisms will form ions, excited molecules or molecular fragments, and then these products will interact with each other to form compounds that are different from the original substances. On the basis of chemical effects, the irradiated materials or organisms will also A series of biological effects occur, causing the proteins, nucleic acids and enzymes that promote biochemical reactions in pests, insect eggs, microorganisms, and enzymes that promote biochemical reactions to be damaged and lose their vitality, thus ending the process of erosion and growth and aging of agricultural products and food, and maintaining stable quality.
In 1980, the Joint Expert Committee of the Food and Agriculture Organization of the United Nations (FAO), the International Atomic Energy Agency (IAEA) and the World Health Organization (WHO) proposed that "any food irradiated with a dose of less than 10KGY has no toxicological "There is no need to conduct toxicological tests", which has promoted the commercial application of irradiation in food production worldwide.
1.6 Microwave sterilization technology
Microwave refers to electromagnetic waves with a wavelength of 0.001~1m (frequency 300~300000MHz). It can move forward at the speed of light. When it encounters obstacles, it can cause reflection, penetration, absorption and other phenomena. The microwave frequency used for sterilization is 2450MHz. Research results generally believe that the lethal effect of microwaves on microorganisms has two factors, namely thermal effects and non-thermal effects. Thermal effect means that the material absorbs microwave energy and increases the temperature to achieve the sterilization effect. The non-thermal effect refers to the strong rotation effect of polar molecules in organisms in the microwave field. This strong rotation inactivates the vegetative cells of microorganisms or destroys the enzyme system in microbial cells, causing the death of microorganisms. Microwave sterilization has the characteristics of strong penetration, energy saving, high heating efficiency, and wide application range. Microwave sterilization is easy to control and heats evenly. The nutritional components, color, aroma, and taste of the food remain close to the natural quality of the food after sterilization. Microwave sterilization is currently mainly used for sterilizing meat, fish, soy products, milk, fruits, beer, etc.
1.7 Far-infrared sterilization technology
The use of infrared rays began in the 20th century. In 1935, Groveny of Ford Motor Company in the United States first achieved the use of infrared rays for heating and drying. patent.
Many ingredients and microorganisms in food have strong absorption in the far-infrared region of 3~10μm. Far infrared heating sterilization does not require a media. The heat penetrates directly from the surface of the object to the interior. Therefore, it can be used not only for the sterilization of general powdery and block foods, but also for the sterilization and sterilization of nut foods such as coffee beans, peanuts and grains. Direct sterilization of mold and packaged food.
The infrared aseptic packaging machine pioneered by Japan's Sanz Company is composed of an ML-501 packaging machine and an MS-801 channel-type infrared heat shrink machine. This machine can select heat shrinkable films of corresponding thickness and color according to the shape and size of the packaged objects, and sterilize them in heat radiation at the same time. Its sterilization procedure is simple, and the packaging quality is much better than manual packaging, and the packaging
The efficiency is increased by 6~8 times.
1.8 Ultraviolet sterilization technology
Ultraviolet rays can be divided into three segments according to their wavelengths: long-wavelength band (3200~4000), medium-wavelength band (2750~3200), and short-wavelength band (1800~ 2750). The ultraviolet sterilization power in the 2400~2800 range is strong, and the strongest wavelength is 2500~2650, with 2537 being the wavelength of ultraviolet sterilization. When a microorganism is irradiated by ultraviolet light, some of the amino acids and nucleic acids in its cells absorb ultraviolet light and produce photochemical effects, causing chemical changes in intracellular components, especially nucleic acids, protoplasmic proteins, and esters, denaturing the cytoplasm, and thus leading to the death of the microorganism. Ultraviolet light propagates in a straight line, its intensity weakens in proportion to the square of the distance, and can be reflected by different surfaces with weak penetrating power. It is widely used in air, water and food surfaces, food packaging materials, food processing workshops, equipment, appliances, Sterilization of workbench.
1.9 Magnetic sterilization technology
Magnetic sterilization is to place the food that needs to be sterilized in a magnetic field. Under the action of a certain magnetic field strength, the food can be sterilized at normal temperature. Since this sterilization method does not require heating and has a broad-spectrum sterilization effect, the flavor and quality of the treated food will not be affected. It is mainly suitable for various beverages, liquid foods, condiments and other packaged solid foods.
1.10 High-voltage electric field pulse sterilization technology
High-voltage electric field pulse sterilization is to place food in an instant high-voltage electric field generated between two electrodes. Since high-voltage electric pulse (HEEP) can destroy The bacterial cell membrane changes its permeability, thereby killing the cell.
There are two ways to obtain high-voltage pulsed electric fields. One is to use the principle of LC oscillation circuit, first use a high-voltage power supply to charge a set of capacitors, and connect the capacitors to an inductor coil and the electrodes of the processing chamber. The high-frequency exponential pulse decay wave generated when the capacitors discharge is added to the two electrodes. A high-voltage pulsed electric field is formed on the Since the LC circuit discharges very quickly, the electric field energy can be released within tens to hundreds of microseconds. Using an automatic control device to continuously charge and discharge the LC oscillator circuit, sterilization can be completed within tens of milliseconds. process. The other is to use a specific high-frequency and high-voltage transformer to obtain a continuous high-voltage pulse electric field. The high-voltage pulsed electric field intensity used 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 performed at room temperature, and the processing time is tens of milliseconds. This method has two characteristics: First, due to the short sterilization time, the energy consumption during the treatment process is much less than that of the heat treatment method. Second, because it is carried out at normal temperature and pressure, the physical properties, chemical properties, and nutritional content of the processed food are very small compared with fresh food, and there is no perceptible difference in flavor and taste. Moreover, the sterilization effect is obvious (N/Nolt; 10-9), which can meet the requirements of commercial sterility. It is especially suitable for heat-sensitive foods and has broad application prospects.
1.11 Ultrasonic sterilization technology
Ultrasound is a sound wave with a frequency greater than 10kHz. Ultrasonic waves, like ordinary sound waves, are longitudinal waves.
The interaction between ultrasonic waves and sound transmission media contains huge energy. When it encounters materials, it will produce rapid alternating compression and expansion effects. This energy is enough to kill and destroy microorganisms in a very short time. It can also produce multiple effects on food, such as homogenizing, aging, and cracking macromolecular substances. It has multiple effects that are difficult to achieve with other physical sterilization methods, thereby better improving food quality and ensuring food safety. Zhu Shaohua used an ultrasonic generator as the sterilization equipment and used soy sauce as the sterilization object, and achieved good results.
1.12 Pulse strong light sterilization technology
Pulse strong light sterilization technology uses strong white light flash for sterilization. It 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. It provides energy for inert gas lamps. Inert gas lamps can emit light from ultraviolet to near-infrared regions. Its spectrum is very similar to sunlight, but its intensity is thousands of times stronger. to tens of thousands times, the light pulse width is less than 800μs. Since this technology only treats the surface of the food, it has little impact on the flavor and nutritional content of the food. It can be used to extend the shelf life of food packaged in transparent materials and fresh food. Studies by Zhou Wanlong and others have shown that pulsed strong light has a strong lethal effect on Bacillus subtilis and yeast. After more than 30 flashes, the number of these bacteria can be reduced from 105 to 0; the band of pulsed strong light has a bactericidal effect. Probably UV, but other bands may have synergistic effects.
1.13 Ultra-high pressure sterilization technology
In recent years, Japan has taken the lead in developing a new type of food processing and preservation technology, which is ultra-high pressure sterilization technology. The so-called high hydrostatic pressure technology (HighHydrostaticPressure, referred to as HHP) is to seal food in elastic containers or place it in a sterile pressure system (often using water or other fluid media as the medium to transmit pressure), under high static pressure (generally above 100MPa) Process for a period of time to achieve the purpose of processing and preservation. Under high pressure, proteins and enzymes will be denatured, and the nuclear membrane of microbial cells will be crushed into many small fragments and the protoplasm will become mushy. This irreversible change can cause the death of microorganisms. The death of microorganisms follows first-order reaction kinetics. For most non-spore microorganisms, the sterilization effect is good at room temperature and 450MPa pressure; spore spores are pressure-resistant and require higher pressure for sterilization, and are often combined with other treatments such as heating to be more effective. Temperature, medium, etc. have a great influence on the mode and effect of ultra-high pressure sterilization of food. Intermittent repeated high-pressure treatments are a good way to kill pressure-tolerant spores.
The latest ultra-high pressure sterilizer developed in Japan has an operating pressure of 304~507MPa. The biggest advantage of ultra-high pressure sterilization is that it has no effect on flavor substances, vitamin C, pigments, etc. in food, and the loss of nutrients is very small. It is especially suitable for sterilizing juice and jam foods.
1.14 Membrane filtration sterilization technology
With the development of materials science, various membranes that can be used for material separation have emerged one after another. Membrane separation technology has been used in industrial production such as food and biopharmaceuticals. It has been widely used in applications such as extraction of biochemical substances, preparation of pure water, concentration of juice, etc. The membrane separation process can be roughly divided into two types according to the different driving forces. One type is a membrane process that uses pressure as the driving force, such as ultrafiltration; the other type is a membrane process that uses electricity as the driving force, called ion exchange, such as electrodialysis. Membrane processes driven by pressure can be divided into micropore filtration, ultrafiltration and reverse osmosis based on the pore size and retention capacity of the membrane.
Usually the pore size of the membrane is 0.0001~10μm, and the size of the microbial particles in the material is generally 0.5~2μm. If a membrane with a pore size smaller than the microorganisms is selected and the material liquid is filtered through the membrane filter, the bacterial particles will Being trapped is called filtration sterilization.
Membrane filtration sterilization technology has the advantages of low energy consumption, operating at room temperature, suitable for heat-sensitive materials, and strong process adaptability. It has broad application prospects and is now widely used in food, biochemistry, pharmaceuticals, Filtration and sterilization of water and air, dairy products, juices, etc.
There are many sterilization technologies in food engineering, such as: chlorine dioxide sterilization technology, chlorine sterilization technology, electronic sterilization technology, combined use of heating and pressure sterilization technology, combined use of heating and chemical sterilization technology, heating Use sterilization technology, electrostatic sterilization technology, etc. together with radiation. These technologies are being researched and applied.
2 Development Trends and Countermeasures
Contemporary food sterilization technologies are diverse and have their own characteristics and application scopes. People are also constantly exploring new sterilization methods. Modern food sterilization technology is gradually getting rid of the traditional heating sterilization method, or adopts low-temperature cold sterilization, or uses various sterilization methods, or uses various modern packaging technologies to closely cooperate with the sterilization process, or uses modern processing technologies such as freeze-drying , vacuum concentration, refrigeration, freezing, vacuum impregnation, etc., in order to minimize the loss of various nutrients in food, maintain the original flavor of food as much as possible, improve the economy and convenience of sterilization technology as much as possible, and improve food Improve the packaging and storage conditions to extend the shelf life of food to meet the growing material needs of consumers. Facing many problems such as global food resource shortage, energy depletion, environmental pollution, population explosion, etc., there is an urgent need for economical, convenient, practical, multi-functional high-tech food sterilization technology to be vigorously researched and developed rapidly to adapt to the needs of the food industry. modernization.
In recent years, my country's food industry has entered a period of rapid development, which has posed severe challenges to my country's relatively backward sterilization technology research. It urgently requires us to actively introduce and absorb foreign advanced technologies and conduct in-depth research on sterilization engineering technology. and development, deepen the reform of the scientific research system, increase investment in scientific research, implement the large-corps combat model, and scientific researchers from various disciplines such as machinery, chemical engineering, biochemistry, microbiology, advanced physics, electronics, etc. unite and cooperate closely to structure my country's sterilization engineering technology research as soon as possible and promotion system, which has promoted the rapid improvement of my country's sterilization engineering technology in recent years, narrowed the gap with the international advanced level, and promoted the further development of my country's food industry.