Pasteurization, also known as cryogenic sterilization, is a low-temperature sterilization method invented by French microbiologist Louis Pasteur. It uses a lower temperature to kill germs and maintain nutrition in items. Sterilization methods with constant material flavor are often used in a broad sense to define heat treatment methods that need to kill various pathogenic bacteria.
Adopting a lower temperature (generally 60 ~ 82 ℃) and heating the food within a specified time to achieve the purpose of killing microbial nutrients, is a method that can achieve the purpose of disinfection without compromising food quality. Invented by French microbiologist Pasteur. The degree of pasteurization heat treatment is relatively low. Generally, the heating is performed at a temperature lower than the boiling point of water, and the heating medium is hot water.
Pasteurization is to heat the mixed raw materials to 68 ~ 70 ° C, and maintain the temperature for 30min, then quickly cool to 4-5 ° C. Because the lethal point of general bacteria is 68 ℃ and time less than 30min, the mixed raw materials can kill the pathogenic bacteria and most non-pathogenic bacteria after processing this method; the mixed raw materials are suddenly heated after being processed. Cooling, rapid heat and cold changes can also promote bacterial death.
Within a certain temperature range, the lower the temperature, the slower the bacterial reproduction; the higher the temperature, the faster the reproduction (generally suitable temperature for microbial growth is 28 ° C-37 ° C). But if the temperature is too high, the bacteria will die. Different bacteria have different optimum growth temperatures and heat and cold resistance. In fact, pasteurization is to take advantage of the characteristics of pathogens that are not very heat resistant, and treat them with appropriate temperature and holding time to kill them all. However, after pasteurization, a small number of harmless or beneficial, heat-resistant bacteria or bacterial spores are still retained. Therefore, pasteurized milk should be stored at a temperature of about 4 ° C and can only be stored for 3 to 10 days. Up to 16 days.
A wide variety of pasteurization procedures are used today. "Low temperature long time" (LTLT) processing is a batch process and is now only used by small dairy plants to produce some cheese products. "High temperature short time" (HTST) processing is a "flowing" process, usually performed in a plate heat exchanger, and is now widely used in the production of drinking milk. The products obtained in this way are not sterile, that is, they still contain microorganisms, and need to be refrigerated during storage and handling. "Fast pasteurization" is mainly used in the production of yogurt and dairy products. There are two main types of pasteurization methods used internationally:
One is to heat the milk to 62-65 ° C for 30 minutes. This method can kill various growth-type pathogenic bacteria in milk, and the sterilization efficiency can reach 97.3% ~ 99.9%. After sterilization, only some thermophilic bacteria, heat-resistant bacteria, and spores remain, but these Most of the bacteria are lactic acid bacteria, which are not only harmless to people but beneficial to health.
The second method is to heat the milk to 75 ~ 90 ℃ and keep it for 15 ~ 16s. The sterilization time is shorter and the work efficiency is higher. However, the basic principle of sterilization is that it can kill the pathogenic bacteria. If the temperature is too high, there will be more nutrition loss.
For PU, holding at 60 ℃ for one minute is called the sterilization strength. It is a PU value.
The degree of pasteurization heat treatment is relatively low. Generally, the heating is performed at a temperature lower than the boiling point of water, and the heating medium is hot water. Different foods use pasteurization for different purposes. In some foods, especially cow's milk, whole eggs, egg whites, and egg yolks, pasteurization is mainly to destroy possible pathogens such as Mycobacterium tuberculosis and Salmonella. In addition, most foods, such as beer, fruit wine (wine), and fruit juice, use pasteurization to extend the shelf life of the product from the perspective of microorganisms and enzymes. Generally pasteurized foods still contain many growing microorganisms, usually thousands of live bacteria per milliliter or gram, which is shorter than the shelf life of commercially sterilized products. In addition to pasteurization technology used in liquid foods (fruit juice, milk), acid foods, and canned jams, it also penetrates into other areas. For example, steam treatment when shelled oysters are used will reduce the oyster's bacterial count. Its advantage is that the food is processed at a lower temperature and in a shorter period of time, which maximizes the color, aroma, taste and nutritional components of the food from high temperature and long-term damage.
A sterilization method mainly for milk, which can kill pathogens harmful to health and make milk quality change as little as possible. That is, based on the difference between the heat-lethalation curve of tuberculosis bacteria with extremely high temperature resistance and the cream-separating heat-disruption curve that is most susceptible to heat in milk, a kind of heat treatment at low temperature for a long time or high temperature for a short time method. Among them, the method of heating at 60 ° C or less for 30 minutes has been widely used as a standard for low-temperature sterilization. The use of high temperature treatment may have a slight effect on milk quality, but it can enhance the sterilization effect. This method is called high temperature sterilization, that is, heating at 95 ° C for 20 minutes. In addition to milk, pasteurization can also be applied to fermented products.
Usually, milk bags in the market are produced by pasteurization. Fresh milk is collected at the factory, processed at low temperature, and then sterilized by pasteurization. Milk produced in this way can usually be stored for a longer period of time. Of course, the specific processes and processes are much more complicated, but the general principle is this.
It should be pointed out that drinking fresh milk (referring to freshly squeezed milk) is not safe because it may contain bacteria that are harmful to our body. Another point is that the pasteurization method is not a panacea. The milk processed by the pasteurization method should still be stored at a lower temperature (generally <4 ° C), otherwise there is still the possibility of deterioration. Therefore, many methods of selling bagged milk on the market are very irregular.
Pasteurized milk is the most consumed milk variety in the world. The consumption of pasteurized milk in the United Kingdom, Australia, the United States, Canada and other countries accounts for more than 80% of liquid milk. The varieties are fully skimmed, semi-skimmed or full-fat. In the US market, practically all pasteurized milk is available in large packages (1 liter, 2 liters, 1 gallon). Citizens in the supermarket can buy fresh milk for a week. Few sterilized pure milk is sold in the market, and some small towns simply cannot buy it.
Pasteurized pure fresh milk better preserves the nutrition and natural flavor of milk, and is the best of all milk varieties. In fact, as long as pasteurized milk is stored at a temperature of about 4 ° C, bacteria multiply very slowly, and the nutrition and flavor of the milk can remain unchanged for a few days.
Pasteurization solves the problem of sour wine. At that time, the French wine industry was faced with a headache, that is, the wine became sour after it was made, and could not be drunk at all. And this sour phenomenon often happens. Pasteur was invited to study this issue. After a long period of observation, he found that the culprit that acidified the wine was Lactobacillus. Nutrient-rich wines are simply paradise for lactobacillus growth. Lactobacillus can be killed by a simple boiling method, but the wine is then boiled. Pasteur has tried using different temperatures to kill lactic acid bacteria without damaging the wine itself. In the end, Pasteur's research concluded that heating wine at 63.5 ° C for half an hour can kill lactic acid bacteria in wine without having to boil. This method saved the French wine industry.
With the advancement of technology, people also use ultra-high temperature sterilization (UHT ultra-high temperature instant sterilization, 135 ℃ -150 ℃, 2 seconds, little damage to nutritional components) to process milk. The shelf life of milk treated in this way will be longer. This kind of milk is mostly used in cartons.
Sterile storage technology
In the liquid medium, the biological effect of pulse discharge is used for sterilization. The sterilization equipment generally consists of two parts: a pulse discharge power supply that supplies energy and a sterilization room for direct sterilization of liquid materials. The working principle: during pulse discharge sterilization, liquid materials are placed as dielectric Within the gap between the two electrodes of the sterilization chamber or continuously flowing through the gap between the two electrodes of the sterilization chamber, when a pulsed electric field of a certain intensity and frequency is added to the two electrodes, a very strong biological effect in the liquid material kills the bacteria in it.
In a conductive solution, the oxidation-reduction reaction caused by the exchange of electrons at the electrode and the solution interface is divided into two types. The reaction occurs under the action of external electrical energy, and the other is without an external power source. It is excited by external light energy, thermal energy or other forms of energy.
Generally refers to the method of killing microorganisms and bacteria by low-frequency alternating current below hundreds of Hz in liquid materials such as fruit and vegetable juices. One of them is an extended use method that uses the thermal effect of alternating current, also known as resistance heating technology. It uses continuous flow The conductive liquid is heated by the resistance heat effect to achieve the purpose of sterilization.
A series of reactions such as cavitation effect, mechanical effect, chemical effect, thermal effect, dispersion effect, sound flow effect, capillary effect, thixotropic effect when ultrasonic waves propagate in solid, liquid, and gas are used to achieve the purpose of sterilization.
Laser is an electromagnetic wave and a kind of energy flow (photon flow). It uses some special biological effects (chemical reactions, thermal effects, electronic effects, pressure effects, biological stimulation effects) to produce sterilization when it acts on living organisms.
Pulsed light sterilization
Sterilization is performed by using a strong white light flash technology. This technology is generally used to sterilize the surface of foods, which can extend the shelf life of prepackaged foods made of transparent materials.
Magnetic field sterilization
Whether it is a constant magnetic field or a pulsed magnetic field, it can effectively inhibit the growth of certain microorganisms and bacteria. When a certain magnetic field strength is reached, the enzymes, amino acids, nucleic acids, and proteins are inhibited by inhibiting the free radical activity of food and affecting the activity of proteins and enzymes. And other biological macromolecules lose their activity and kill bacteria.
Thermal effect of microwave energy: Under the action of a certain intensity microwave field, insects and bacteria in food will absorb microwave energy to heat up due to molecular polarization phenomenon, thereby denaturing their proteins and losing biological activity. The thermal effect of microwave mainly plays a role of rapid heating and sterilization; the non-thermal effect of microwave energy: high-frequency electric field also changes its membrane potential and polar molecular structure; it mutates proteins and physiologically active substances in microorganisms, and loses vitality or death. In sterilization, it plays a special role that conventional physical sterilization does not have, and it is also one of the causes of bacterial death.