As we all know, it is the most important thing that no matter what food must contain pathogenic microorganisms and other toxic substances. On the premise of meeting this requirement, try to meet other requirements for food that people want. Although a small amount of non-pathogenic microorganisms will not cause any harm to the human body after entering the human body through the oral cavity, if there are a small amount of non-pathogenic microorganisms in soybean milk, they will multiply and grow in large numbers under suitable temperature conditions, which will eventually lead to rancidity and precipitation of soybean milk, which is not conducive to preservation. If there are a large number of non-pathogenic microorganisms in soybean milk, it is not only difficult to preserve, but also a large number of miscellaneous bacteria can cause gastrointestinal infections and diseases. From the point of view of production hygiene, soybean milk contains a large number of miscellaneous bacteria, which is the result of serious pollution or long-term reproduction and growth in the production process and is the symbol of unqualified products. Therefore, thermal sterilization of soybean milk is necessary; However, in order to ensure the flavor quality of soybean milk, the sterilization temperature and time must be selected. Long-term high-temperature sterilization will promote Maillard reaction, which will not only cause nutritional loss of soybean milk, but also cause bad color and flavor deterioration. In severe cases, additives will fail and the stability of soybean milk will decrease.
How to achieve the purpose of sterilization and make soybean milk meet people's nutritional and flavor requirements requires the selection of sterilization methods according to product characteristics. Common thermal sterilization methods are as follows:
1. LTLT: LTLT is a long-used pasteurization method. Usually soybean milk is heated to 60-70℃ for 30 minutes, so it is also called heat preservation sterilization. This sterilization method can kill all pathogenic bacteria, but its sterilization effect is generally less than 99%, and it is not easy to kill thermophilic bacteria and spores, and some lactic acid bacteria will remain.
2. HTST: HTST is a rapid pasteurization method, which adopts
The sterilization method at 80-85℃ 10- 15 seconds, or at 75-78℃ 15-40 seconds, has better sterilization effect than the method at low temperature for a long time, and has little damage to soybean milk components, but it can't kill all microorganisms to remove pathogenic bacteria. Low-temperature long-time sterilization is an intermittent process, while high-temperature short-time sterilization is usually carried out in plate heat exchangers, which are widely used in the production of heat-sensitive foods. The products obtained by these two methods still contain microorganisms and need to be refrigerated during storage and treatment.
Soybean milk sterilized by the above two methods is generally called sterilized soybean milk. Enterprises that have the conditions to produce and sell fresh milk can produce sterilized soy milk. Because the production of sterilized soybean milk can simplify the production process and packaging materials, and additives such as stabilizers and emulsifiers are not needed, the cost is greatly reduced and the damage to the quality of soybean milk caused by high-temperature sterilization is avoided. However, soybean milk is a kind of food with extremely rich nutrition and is in a neutral state.
It grows and reproduces easily. As long as a small amount of microorganisms remain in soybean milk, it will multiply and grow rapidly at a suitable temperature. In most cases, protein in soybean milk will solidify first and then rot. Qualified sterilized soybean milk can only be stored for 1 day (24 hours) at normal temperature (20℃) and for 3-5 days at low temperature (4-8℃). Therefore, sterilized soybean milk must be sold and produced on the same day and sold out on the same day. It must be stored in the cold storage (5- 10℃) before leaving the factory, and special attention should be paid in summer.
3. High-temperature sterilization method: The sterilization temperature is 12 1℃, the steam pressure is 0. 14 MPa, and the sterilization time is 20-30 minutes (packaging soybean milk). In fact, it can be completed in121℃ for 4-5 minutes. Keeping sterilization kills all microorganisms that can grow, and the obtained product is commercially sterile, that is, it does not contain toxins, pathogenic bacteria and microorganisms that can reproduce under normal storage and distribution conditions. Therefore, its storage, transportation and sales do not need refrigeration.
4.UHT sterilization method: the sterilization temperature of this method is 138- 150℃, and the steam pressure is
0.5 MPa, and the sterilization time only takes a few seconds. Ultra-high temperature sterilization is to make the product reach the commercial aseptic level through high-intensity heating for a short time.
The latter two methods are high temperature and pressure sterilization, also known as steam sterilization, which is characterized by killing all microorganisms. Sterile soybean milk can be produced if it is combined with aseptic packaging or sterilized with packaging after packaging. Choose packaging materials with good thermal insulation, which can be stored at room temperature for 3-5 months without rancidity and deterioration.
Every sterilization will have a negative impact, and the product will change, thus affecting its nutritional value and sensory characteristics. The influence of heat treatment on protein has changed its properties more or less (the denaturation process is only related to the physical state of protein), and its nutritional value has not changed much. Heating has little effect on amino acids in protein, mainly the loss of lysine, as follows: pasteurization 1%-2%, sterilization 6%- 10%.
Ultra-high temperature treatment 3%-4%; Excessive heat treatment will lead to browning, and reducing sugar involved in browning reaction will be lost. Pasteurization and ultra-high temperature instantaneous sterilization will not produce this change. But at higher storage temperature (35-40℃), browning may also occur; Fat is generally unaffected by different sterilization processes.
Effect of heating; Minerals are basically unaffected by sterilization heat treatment, and the contents of soluble calcium and phosphate may decrease; Among all kinds of vitamins, vitamins are basically unaffected by pasteurization and ultra-high temperature treatment; Vitamins remain stable during sterilization; Vitamins are considered to be affected by heating, and their loss depends on the degree of heat treatment; There is little difference in vitamin loss between pasteurization and ultra-high temperature treatment. Vitamins are easily affected by heating, especially during sterilization. Folic acid is also easily affected by heating, and the loss is basically the same as that of vitamins. Biotin and nicotinic acid are relatively stable, and the heat loss during sterilization is limited.
In the process of sterilization, low sterilization temperature takes a long time, and the loss rate of heat-sensitive components in food increases with the increase of temperature and time. With the increase of sterilization temperature, the time required for sterilization decreases rapidly. Although the loss rate of heat-sensitive components in food will increase with the increase of sterilization temperature, it will decrease with the shortening of sterilization time. Due to the short time required for high-temperature sterilization, with the increase of sterilization temperature, the loss rate of heat-sensitive components in food tends to decrease. Some data show that when the sterilization temperature is 12 1℃, the spore killing time is 4 minutes, and the retention rate of some components is 70%. However, with the increase of temperature, the killing time of spores decreased sharply. On the contrary, the retention rate of ingredients has risen sharply. When the sterilization temperature is 132℃, the retention rate of this component is 90%, and when the sterilization temperature reaches 150℃, the retention rate is above 98%.
High-temperature short-time sterilization must really be a short time in the whole high-temperature stage, not a short time at the highest temperature. Therefore, in the choice of equipment, we should be able to instantly raise the temperature of soybean milk to the highest sterilization temperature, and then cool it down to direct heating and direct heating after sterilization.
The method of direct steam heating is generally to preheat soybean milk to above 70℃, and then mix it with high-temperature steam instantly to raise the temperature of soybean milk to sterilization requirements. Immediately after the arrival time, it is sucked into a vacuum tank for flash evaporation. Due to the heat absorption of evaporation, the temperature of soybean milk is reduced to below 85℃ in a short time (10 second), and the condensed water mixed with soybean milk is evaporated, and the bad flavor in soybean milk can be effectively removed. This is by far the most effective way to combine sterilization and deodorization of soybean milk.
The method of direct heating with current is to let the soybean milk pass through the resistance tube while passing the current, and try to let the current mainly pass through the soybean milk in the tube instead of the outer wall, so that the soybean milk can be heated to 140℃ in a very short time. This patented method overcomes the scaling problem caused by high pipe wall temperature caused by indirect heating, and the heat transfer and cleaning difficulties caused by it, and the thermal efficiency is extremely high, reaching more than 80%.