◆Yogurt development
In recent years, my country’s yogurt production has reached 500 tons per day, ranking first in my country’s fermented dairy products. As my country’s people’s consumption level gradually improves, yogurt production is increasing at a rate of 25% per year. While the varieties and nutritional value are increasing, the problem of short storage time of yogurt production has emerged, which has affected the yogurt industry to a certain extent.
The shelf life of yogurt is mainly related to two aspects. One is the contamination caused by yeast and mold, which causes the yogurt to develop bulges. These problems have been further controlled with the help of production equipment and improved sanitary conditions; the other is the changing flavor of yogurt. The key is the heavier sour taste, which affects the taste of consumers.
◆Yogurt fermentation mechanism
Lactic acid bacteria fermentation can be divided into two types: homolactic acid fermentation and heterotypic lactic acid fermentation. Homolactic acid fermentation only produces lactic acid, and heterolactic acid fermentation produces not only lactic acid but also carbon dioxide and other substances. In the process of making yogurt, Streptococcus bulgaricus subspecies and Streptococcus thermophilus subspecies are often used, both of which belong to homolactic acid fermentation. Lactose directly enters lactic acid bacteria cells through two different mechanisms. The first mechanism is through the permembrane enzyme system, and the second mechanism is through the lactococcus system.
The first mechanism: lactose directly enters the cells through the action of permembrane, and galactosidase decomposes it into galactose and glucose. Among them, Lactobacillus bulgaricus and Streptococcus thermophilus first metabolize glucose to produce lactic acid, and the free type The galactose cannot be used and is mainly accumulated by gums that exist outside the cells or have been converted into high molecular substances.
The second mechanism: mainly distributed in Lactococcus lactis, Lactobacillus casei and other bacterial genera. Lactose acts through phosphotransferase located on the cell membrane and is phosphorylated directly into the cell. Lactose phosphate is mainly decomposed into glucose and galactose phosphate through the action of phosphogalactosidase. The former is metabolized through the glycolysis system, and the latter is metabolized through tagatose phosphate.
◆Measures to control fermentation
❶Application of engineering genetic technology and artificial breeding and mutagenesis technology
In the work of making yogurt starter, through engineering genetic technology and artificial breeding and mutagenesis technology, strains with strong acid-producing ability at high temperatures and weak acid-producing abilities at low temperatures are mainly used as starter cultures. There are still many shortcomings in my country’s research on the genetic mechanism of lactic acid bacteria. Modified lactic acid bacteria still rely on artificial mutagenesis technology. The operation is very complicated, and the acquired traits cannot be stably inherited. The fermentation functions, protein decomposition functions, and flavor substance production functions of lactic acid bacteria discovered in current research are all inherited through plasmids.
❷Using ultra-high temperature and pasteurization technology
When the fermented yogurt matures, the yogurt is processed through techniques such as pasteurization and ultra-high temperature sterilization to obtain yogurt without live lactic acid bacteria. Although the fermentation problem of yogurt has been well solved in this way, the obtained pasteurized and ultra-high temperature sterilized yogurt cannot meet the yogurt requirements proposed by the International Dairy Federation because it does not contain live bacteria. Therefore, it is not recommended to use this method to control yogurt fermentation.
❸Adjusting the proportion of bacteria in yogurt starter
In the yogurt starter, the ratio of Lactobacillus bulgaricus and Streptococcus thermophilus is adjusted to increase the ratio of cocci to bacilli. During the fermentation of yogurt, Streptococcus thermophilus grows rapidly. When the pH value of the milk drops to 5.5, Lactobacillus bulgaricus begins to grow rapidly, but Streptococcus thermophilus slows down its growth; when the pH value of the milk drops to At 5.0, the fermentation of yogurt is controlled by Lactobacillus bulgaricus; when the pH value of milk decreases to 4.6, casein gradually begins to condense, and milk coagulation occurs. During the refrigeration process of yogurt, the dominant Lactobacillus bulgaricus begins to continuously ferment lactic acid and produce acid, which reduces the pH value of yogurt to 3.5. Therefore, increasing the scientific ratio of cocci to bacilli in the starter culture can weaken the degree of yogurt fermentation.
The symbiotic relationship between Lactobacillus bulgaricus and Streptococcus thermophilus is currently understood. For Lactobacillus bulgaricus, the promoting function of formic acid is necessary, but pyruvate and carbon dioxide are not necessary. In recent years, studies have shown that not all Lactobacillus bulgaricus and Streptococcus thermophilus will have an excellent symbiotic relationship, and whether a good symbiotic relationship is closely related to the growth characteristics of the strains. The most suitable growth temperature has minimal impact on the symbiotic relationship between the two bacteria. Streptococcus thermophilus grows very well between 35-42°C. The culture temperature has an impact on the ratio of cocci and bacilli. Studies have shown that the best quality yogurt can be produced when the ratio of cocci to bacilli is 1.5:1. Therefore, appropriately increasing the ratio of cocci to bacilli and changing the ratio of Lactobacillus bulgaricus and Streptococcus thermophilus in traditional starter cultures will help better maintain the symbiotic relationship between the two bacteria, shorten the fermentation time, and weaken post-fermentation.
❹Adding foreign substances to control bacterial growth and acid production
In the process of producing yogurt, glucose and other substances need to be added frequently. Adding these substances will change the water activity value used by lactic acid bacteria. The water activity value affects the enzyme activity by changing the enzyme reaction kinetic parameters and the induced substrate structure changes, further affecting the growth and production of the bacteria. Enzyme capacity is affected. The water activity value specifically refers to the ratio of the vapor pressure of water in food to the vapor pressure of pure water under the same conditions, reflecting the amount of water that microorganisms can actually use.
Relevant research reports indicate that adding 4% sucrose in the process of making yogurt can effectively inhibit the growth and acid production of lactic acid bacteria. When the concentration of sucrose is higher than 8%, the synthesis of acetaldehyde will be reduced. Research has proven that the concentration of sucrose inhibits Lactobacillus bulgaricus more than Streptococcus thermophilus. It can also be considered that Lactobacillus bulgaricus is more sensitive to reduced water activity, and the inhibitory effect of the total solid components in milk is stronger than that of simple sugar. Impact. Because fermented yogurt is mainly caused by the continuous acid production of Lactobacillus bulgaricus under acidic conditions, reducing the water activity reduces its growth, which can also reduce the acidification of yogurt.
❺Change cell membrane permeability
In the process of producing low-lactose milk, chemical mutagens have been used to change the permeability of the lactic acid bacteria cell membrane, further increasing the activity of galactosidase and further increasing the degree of lactose hydrolysis. The bacteria used in fermented yogurt have a pH value that is significantly different between the bacteria and the surrounding medium under normal growth conditions. Under near-neutral conditions, the bacteria have the ability to maintain the pH of the cytoplasm, thus performing metabolic and enzymatic changes on the bacteria. The reactions were very favorable. Relevant studies have shown that the pH value of the medium can indirectly affect the metabolism of lactic acid bacteria cells. In fermented yogurt, due to the rapidly declining pH value of the medium, Lactobacillus bulgaricus and Streptococcus thermophilus will be damaged by acid on their cell membranes, changing their permeability, causing the pH value in the body to also decrease, further inhibiting the normal growth and metabolism of lactic acid bacteria. effect. Lactobacillus bulgaricus has extremely poor sensitivity to acid. When the pH value of the medium is between 4 and 4.5, the pH value in the cells can remain neutral. When the pH value of the medium reaches 3.5, the pH value in the cells is 4.4. Only At this time, all metabolic activities can be inhibited.
❻Inhibition of yogurt fermentation using bacteriocins
In recent years, some people have used gramicidin secreted by propionic anhydrobactin in making yogurt, which effectively inhibits the acidification of yogurt. Among them, nisin is internationally allowed to be added to food as a preservative. Nisin is added to the milk and the milk is inoculated until coagulation occurs. Nisin inhibits the reproduction of lactic acid bacteria, reduces the total bacterial count, and prolongs the time required for fermentation. However, as long as the appropriate dose and scientific addition method are selected, the number of viable yogurt bacteria can be made to meet international requirements. It can also effectively inhibit the acidification of yogurt, extend the shelf life, and enhance the flavor of yogurt. Currently, some people point out that lactic acid bacteria bacteriocin can be wrapped in an acid-soluble medium without toxic side effects, and then a starter is added to the milk. When the milk fermentation meets a certain acidity, the acid-soluble medium is destroyed and the bacteria are released, further affecting the yogurt. Excessive acid production is suppressed.
Conclusion
Through the above various control measures, the fermentation of yogurt can be effectively inhibited or prevented, and the shelf life of yogurt can be further extended. It is foreseeable that the application of bioengineering technology in fermented dairy products will become more and more widespread.