Research on food sterilization technology and its prospect

Humid heat sterilization: thermal sterilization is a heat treatment for the main purpose of killing microorganisms, and humid heat sterilization is one of the most important ways. It is steam, hot water as a hot medium, or direct steam jet type heating sterilization method. A thermal converter (such as a boiler) is used to convert the burning heat energy into hot water or steam as a heating medium, and then a heat exchanger is used to transfer the heat energy of hot water or steam to food, or steam is sprayed directly into food to be heated. There are some commonly used heating media in food heat treatment, such as steam, hot water, air, flue gas, gas and electricity. Different heating medium has different characteristics. For example, steam is easy to be transported by pipeline, heating is uniform, temperature is easy to be controlled, latent heat of condensation is large, but the temperature is not too high; The hot water is easy to be transported by pipeline and heated uniformly, but the heating temperature is not high. The air heating temperature can be very high, but its density is low, resulting in low heat transfer coefficient; The heating temperature of flue gas can be very high, but its density is low and heat transfer coefficient is low, which may contaminate food. Gas heating temperature can be very high, low cost, but may contaminate food; The electric heating temperature can be very high, the temperature is easy to control, but the cost is high. Different kinds of heating agents have different characteristics. In the food industry, manufacturers must choose different heating agents according to their own needs.

In the humid heat sterilization, the two most commonly used sterilization methods are pasteurization, high-temperature short-time sterilization (HTST) and ultra-high temperature instantaneous sterilization (UHT). The sterilization conditions of pasteurization are 61 degrees C -- 63 degrees C /30 minutes, or 72 degrees C -- 75 degrees C /15 minutes -- 20 minutes. The pasteurization technology is to kill the bacteria in the packaging container by keeping the temperature below 100 degrees Celsius for a certain period of time after the food is filled and sealed in the packaging container. Pasteurization can kill most pathogenic bacteria, but the killing ability of non-pathogenic spoilage bacteria and their spores is insufficient. If pasteurization is combined with other storage means, such as refrigeration, freezing, deoxidation and packaging, certain storage period can be achieved. Pasteurization technology is mainly used for the sterilization of citrus, apple juice drinks, because the pH value of juice food is below 4.5, there is no microbial growth, the object of sterilization is yeast, mold and lactobacillus, etc.. In addition, pasteurization is also used to sterilize jam, canned fruit in sugar water, beer, canned pickled vegetables, pickles and so on. For sealing of acidic food pasteurization has reliable acid resistance, for those who are not resistant to high wet processing of low acid food, as long as it doesn't affect spending habits, often by using acid or by means of microbial fermentation to produce acid, make the pH fell to the scope of the acidic food, can take advantage of the low temperature sterilization reach the purpose of preserving food quality and resistant storage. This method requires a long time, heat sensitive food should not be used.

High temperature short time sterilization: sterilization conditions of 85 ° C - 90 ° C /3 ° C - 5 minutes, or 95 ° C /12 minutes liquid material heated to close to 100 ° C, then quickly cooled to room temperature. This method takes shorter time and has better effect, which is beneficial to product quality. It can kill yeast, mold, lactobacillus and so on. These two methods are characterized by stable sterilization effect, simple operation, small equipment investment and long application history, and are now widely used in the sterilization of all kinds of canned food, beverage, wine, medicine and dairy packaging.

Ultra high temperature instantaneous sterilization came into being in 1949 with the appearance of Stoker apparatus, after which many kinds of ultra high temperature sterilization apparatus appeared in the world. Ultra high temperature short time sterilization is to instantly heat food to high temperature (130 degrees Celsius above) to achieve the purpose of sterilization, can be divided into direct heating and indirect heating two methods. The direct heating method is to spray high pressure steam directly into the food to make the food warm up at the fastest speed, reaching 140 ° C - 160 ° C within a few seconds, and maintain for a few seconds, and then remove the water in a vacuum chamber, and then cool it to room temperature with a sterile cooling machine. Indirect heating method is to choose plate heat exchanger, tube heat exchanger and scraper heat exchanger according to the viscosity and particle size of food. Plate heat exchanger is suitable for liquid food whose pulp content does not exceed 1% - 3%. The tubular heat exchanger has a wide range of adaptability to products and can process liquid food such as concentrated fruit and vegetable juice with high pulp content. Where plate heat exchanger will produce coking or obstruction, and viscosity is not enough to use scraper heat exchanger products, can be used tube heat exchanger. The scraper heat exchanger is equipped with a vane rotating machine, which squeezes on the heating surface to push the high viscosity food forward to achieve the purpose of heating and sterilization. Instantaneous ultra high temperature sterilizing effect is very good, almost completely the requirements of the sterilization can reach or close to, and sterilization time is short, the material of nutrients and less disruptive to the quality of the food is almost the same, nutrient retention rate above 92%, and the production efficiency is high, more than the other two kinds of thermal sterilization effect is excellent, with food aseptic packaging technology of ultra high temperature sterilization equipment in the domestic and foreign development soon, now has developed into a high-tech food sterilization technology. At present, this sterilization technology has been widely used in the sterilization of milk, soybean milk, wine, fruit juice and all kinds of drinks, etc., or food can be packed into bags and then impregnated in hot water at this temperature for sterilization.

Dry heat sterilization: bacterial reproduction in the dry state, 80℃ - 100℃ 1 hour can be killed; Spores need to be heated to 160℃ -- 170℃ for 2 hours before extermination.

Dry heat sterilization method is burning: burning is a thorough sterilization method, destructive, only applicable to waste items or animal carcasses; (2) Cauterization: direct flame sterilization, suitable for the laboratory of metal instruments (tweezers, scissors, inoculation ring, etc.), glass tube mouth and bottle mouth sterilization; Dry roasting: Conducted in a hot air oven, heated to 160℃ -- 170℃ for 2 hours, all microorganisms, including spores, can be killed. Suitable for high temperature resistant glassware, porcelain, glass syringe, etc. Infrared radiation: electromagnetic waves of 770nm -- 1000 m, the thermal effects of which were strongest at 1 m -- 10 m wavelengths. The thermal effect of infrared ray can only be produced on the irradiated surface, which cannot make the object evenly heated. It is often used to sterilize food utensils such as bowls and chopsticks. Microwave (MICROWaves) : Electromagnetic waves from 1mm to 1000mm -- described as microwaves -- will penetrate materials such as glass, plastic sheets and ceramics, but not metal surfaces. The distribution of heat effect of microwave oven is not uniform and the sterilization effect is not reliable. It is used for sterilization of non-metallic instruments and utensils.