Preface

   With the increasing awareness of people's health care and the development of science and technology related to microecology, consumers are becoming more and more aware of the importance of probiotics in promoting human health and preventing diseases. Probiotics can enrich human nutrition, improve health, and enhance the sensory characteristics of food. They can not only reduce the severity of dysentery, inhibit the growth of pathogenic bacteria in the intestinal tract, but also help infants and young children to prevent eczema. Likewise, they can improve colon health and nutrition, and help strengthen the body's immune system. Since the human origin of the probiotics in the product has been repeatedly demonstrated by researchers, consumers have a high degree of acceptance of the product.

   In recent years, probiotics in the food industry are usually put into production as additives. In many countries and regions, probiotics are only used in dairy products, such as yogurt and acidified milk. Probiotics are defined as "a kind of microorganisms that can adjust the intestinal micro-ecology by adding to food or pharmaceuticals, and have a beneficial effect on humans or animals." Probiotics are microorganisms that help the body absorb food components that are difficult to digest or have low digestibility through selective growth and stress activities of microorganisms in the colon. Now, the term "probiotics" refers to a product that contains a specific number of microorganisms that can grow and reproduce until the product accumulates to 106 per gram or milliliter and has a beneficial effect on health.

However, to produce the above two effects, probiotics must meet the following prerequisites: First, the probiotics are live when consumers consume the product; secondly, the number of live probiotics that can be used every day reaches 109. The probiotics that can be used by the human body are mainly Lactobacillus, Streptococcus and Bifidobacterium. There is a potential synergistic effect between prebiotics and probiotics, and foods that can produce this synthetic effect are usually called synbiotics.

1 The significance of probiotics for human nutrition and health

   Probiotics can have a beneficial effect on human health. As the human body gradually ages, it is more and more important to improve immunity against diseases. The main function of probiotics is to maintain the balance of the microbial community and play an important role in improving intestinal function. Probiotics have an advantage in the competition for colonization sites in the intestines, and their nutrient absorption capacity is stronger than that of pathogenic bacteria. In addition, another reason for the decrease of pathogenic bacteria in the intestinal tract is that the antibiotics produced by probiotics have an inhibitory effect on it. The main benefits of probiotics to human health include promotion of lactose digestion, antagonism to intestinal pathogenic bacteria, anti-intestinal cancer, inhibition of small intestinal bacterial overgrowth, immune regulation, slowing down allergic reactions, regulating blood lipids, preventing cardiovascular diseases, and anti-high Blood pressure, inhibition of genitourinary tract infections, inhibition of infections caused by Helicobacter pylori, etc.

   The physiological activities of probiotics can inhibit pathogenic bacteria, prevent high incidence of specific eczema in infants and young children, strengthen the immune system, improve colon health, and increase nutrient absorption. In addition, probiotics also have the following functions: 1. Prevent and adjust intestinal disorders; 2. Reduce food intolerance; 3. Prevent cardiovascular disease and cancer.

   Some studies have shown that the microorganisms in yogurt (Lactobacillus bulgaricus of the genus Lactobacillus and Streptococcus thermophilus of the genus Streptococcus) can improve lactose digestion and relieve the symptoms of lactose intolerance. Probiotics play a role in reducing food allergies and intolerances, and also help prevent bone and rheumatoid arthritis.

   Studies have shown that the use of probiotics in the treatment process can help prevent acute bacillary dysentery and antibiotic-induced dysentery in children. Similarly, it is useful for the reduction of serum cholesterol, diabetes control, blood pressure reduction and allergic symptoms. Certain help. In addition, probiotics also help prevent infection by pathogenic bacteria and prevent osteoporosis.

   It is confirmed that probiotics have a certain anti-cancer effect, especially for bowel cancer. This is because the inhibition of bacterial enzymes can lower intestinal pH, remove carcinogens and activate the immune system.

At present, the mechanism and pathway of probiotics are one of the hotspots of current research, including the adjustment mechanism of intestinal pH, the synthesis mechanism of resistant compounds, the mechanism of competition with pathogenic bacteria, the mechanism of immune cell stimulation, and the production of lactose decomposing enzymes. mechanism. Although the various benefits of probiotics for human health have been proved, the research objects of these results are patients with diseases, and further research is needed in healthy people. Finally, some scientific research institutions have adopted carcinogen binding technology and immune regulation mechanisms to study the effects of inactivated probiotics on the host.

2 Selection criteria for probiotics

   In the past, the selection criterion for probiotic strains was whether their use brought harmful side effects. Now, whether a microorganism can be used as a probiotic is determined by specific criteria. It is generally believed that it should have the following conditions: ①Beneficial to the host; ②No toxic side effects and no pathogenicity; ③Can survive in the digestive tract; ④Resistant to gastric acid and bile salts; ⑤Can be colonized on the surface of the digestive tract; Enzymes and metabolites; ⑦ can maintain activity during processing and storage. Among them, the most important thing is that probiotics must be safe for the human body (generally considered safe, GRAS for short), can pass through the intestines and remain active when they reach the position of action, and can colonize the intestines. Another important criterion is that they can survive in the acidic environment of the human intestine, that is, they should be able to tolerate the low pH environment and the action of hydrolytic enzymes in the intestine.

The functionality of probiotics also includes activity, survivability in the intestinal system, survivability under digestive pressure, immune regulation and resistance to mutagenesis. Several aspects must be considered in the process of selecting probiotic strains, such as acid resistance, bile and gastric juice tolerance, epithelial colonization ability, survivability in the human digestive system, resistance to pathogens, Anti-mutagenic and anti-cancer ability, immune regulation ability.

The screening of probiotics for the purpose of technical applicability can make the strains more suitable for production and consumption. The safety standards for probiotics successfully applied in production have been compiled and published. The safety standards include the human origin of the strain, the independence of the strain from healthy individuals, the non-pathogenic history of the strain, and the connection with disease and health disorders. In addition, the strain should also have bile salt tolerance or express antibiotic resistance genes.

Probiotics must meet certain standards to meet the requirements of the production process and products, such as low pH resistance, viability and stability during production and storage. In addition, as a product, it should also have good sensory properties. Although viability is the most important prerequisite mentioned earlier, some studies have shown that deactivated probiotics can also have a beneficial effect on host health.

3 probiotic yogurt

   Dairy products are an excellent source of calcium and are typical functional foods. Fermented milk and yogurt are good sources of vitamins and minerals, and can supply lactic acid bacteria with health benefits. The consumption of probiotic products can help improve human health. The survival rate of probiotics during the product shelf life is an important indicator for judging whether it can be used as a product to enter the market. Cultivation conditions, strain selectivity, inoculation amount, fermentation culture and composition, interaction between bacterial species, final acid, availability of nutrients, growth factors, inhibitors, sugar concentration, dissolved oxygen concentration, incubation period time, storage temperature, and fermentation Time will affect the survival rate of probiotics in fermented dairy products. The limitations of research methods have brought many adverse effects on laboratory feasibility. However, in the process of product storage, transportation and consumption, the viability of probiotics is very important.

There are many requirements for the management of probiotic yogurt, especially the survival rate and pH value of the microorganisms contained. First of all, probiotics are alive when they are eaten, and the benefit to humans or animals is the count of vigorous CFU. Studies have shown that probiotics have little effect on the sensory properties of the product and grow slowly in milk. Maragkoudakis et al. verified the tolerance of Lactobacillus plantarum and Lactobacillus caseinus as excipients in yogurt produced by the traditional process of Greece.

   Studies have shown that the survival rate of probiotics in yogurt is lower than the daily intake. A small number of Lactobacillus acidophilus and Bifidobacterium have been used in the production of yogurt.

Practice has proved that milk additives help to improve the survival and reproduction ability of probiotics during storage, and can improve the taste of products when they are eaten.

4 Probiotic cheese

   The integration of probiotics in cheese can improve the nutritional value and quality of the product. The probiotics used in the cheese making process are mainly Lactobacillus plantarum and Bifidobacterium. In addition, the probiotic properties in cheese are formed by the action of bacteria of the genus Propionibacterium and Enterococcus.

   Cheese has a higher pH value than fermented milk, can provide a more stable and good environment during maturation, and can produce an almost completely anaerobic environment. In addition, the high fat content in cheese protects the probiotics as they pass through the digestive system. For cheese, the laboratory can improve the viability of probiotics by changing the pH, oxygen content, auxin content and inhibitor content. For example, American yogurt strains use high oxygen consumption to improve their viability. In addition, the main fatty acids in milk such as butyric acid, palmitic acid, and stearic acid promote the growth of bifidobacteria, while lauric acid and myristic acid inhibit their growth. Generally speaking, it is technically and economically feasible to add bifidobacteria to cheese. Bifidobacterium bifidum and Bifidobacterium longum show good viability during cheese processing and storage. The viability of Bifidobacterium longum, Bifidobacterium breve, Bifidobacterium hornis, Bifidobacterium bifidum and Bifidobacterium infantis was compared after 14 years of cold storage in soft cheese. The results showed that although the loss of Bifidobacterium bifidum was great, it still maintained good viability, while Bifidobacterium breve and Bifidobacterium infantis showed relatively low viability. In another study, the outer layer of the soft cake will be richer when fermented by Bifidobacterium infantis.

Gomes et al. produced a Gouda cheese-style cheese, which produced a probiotic pair synthesized by Bifidobacterium and Lactobacillus acidophilus. During production and storage for 9 weeks, the number of living cells is more than the number of probiotics in the food (106 CFU/g). In the maturation period after 9 weeks, the viability of bifidobacteria is determined by the salt concentration, the position of the cheese and the addition of protein hydrolysates. Through observation, it can be seen that the location with the highest viability is the center of the cheese, because the salt concentration and oxygen concentration are the lowest here. Due to enzymatic coagulation, protein hydrolysate does not significantly improve the viability of probiotics. The report also pointed out the impact of undesirable flavors on viability.

pH is one of the most important indicators that affect the viability of probiotics. Bacteria require a higher pH environment than yeast. Other probiotic dairy products such as fermented milk and yogurt have this problem, with a pH range of 3.7 to 4.3, which is lower than cheese with a pH range of 4.8 to 5.6. Therefore, the pH of cheese is closer to the growth pH of probiotics, and probiotics are more likely to survive in cheese.

The most difficult part of probiotic cheese production is not the proliferation of probiotics but their survival. This is because maturation is a long-term process, usually several weeks or even months. During this period, bacteria not only have to survive in numbers, but also increase in number to bring benefits to human health. There are certain standards for the number of probiotics in the product, and different regions have different requirements for the minimum concentration. In Japan, it is stipulated that probiotic food should contain a minimum of 107 CFU/ml or g of live probiotics, which is much higher than the lowest dose that can be effective (105 CFU/ml or g). The most widely used probiotic concentration is 106 CFU/ml or g, and the industry generally implements this standard.

For the first time, a combination of Bifidobacterium and Lactobacillus casei was used to ferment Fresco cheese, aged for 60 days, and the colony count in the product showed that the number of Bifidobacterium colonies decreased slightly while Lactobacillus casei remained unchanged [79]. In the experiment of Cheddar cheese, after 250 days of maturation, the concentration of Enterococcus faecalis rose from 1.1 × 108 to 3.9 × 108 CFU/g. In the same experiment, after 430 days of maturation, the concentration of Enterococcus faecalis rose from 6.9×107 to 1.7×108 CFU/g.

   The first use of bifidobacteria produced a large amount of acetic acid and proteolytic enzymes, which had a bad influence on the sensory properties of the product. Now, this situation has been well improved, and the probiotic cheese has sensory properties similar to traditional products. Corbo et al. conducted sensory tests on the appearance, color, mechanical properties, taste, texture and texture of Canestrato cheese that had been aged for 56 days and found that there was no significant difference between probiotic cheese and traditional cheese.

In addition, it has been found that probiotics can even improve the organoleptic properties of cheese. Gouda cheese fermented with Bifidobacterium longum and Bifidobacterium acidophilus has better flavor and mouthfeel than traditional cheese, especially the flavor [81]. The technology of immobilizing lactobacilli on fruit pieces in the cheese production process has recently been applied, and the viability of probiotics during the cheese maturation process has also undergone a lot of research.

Current research focuses on the development of probiotic cheeses, such as Cheddar cheese, Crescenza cheese, soft cheese and fresh cheese. At the same time, several other studies have reported the low viability of probiotics during cheese maturation. With higher pH, fat content, mechanical consistency and lower oxygen levels, whey cheese is superior to other products in the passage of live probiotics. In addition, the characteristics of low technical requirements, immature properties, easier production and processing, forced storage at refrigerated temperatures, and typical short-term shelf life make whey cheese particularly suitable for the growth of probiotic strains.

5 Future development of probiotics

Product acidity (pH), dissolved oxygen level, oxygen penetration, oxygen penetration from packaging, post-acidification, etc. will reduce the viability of probiotics. Since thermophilic and heat-resistant microorganisms can withstand higher temperatures during processing and storage, it is more conducive to production. The production of cheese and milk requires relatively high temperatures (45°C or above), and these microorganisms are of high biotechnological importance.

According to reports, microencapsulation technology can enable sensitive laboratory strains to relieve the stress of oxygen, freezing and acid environments in the production and storage of products and the transportation of the digestive tract. Due to the effect of significantly improving the survival rate of probiotics, microencapsulation technology has received widespread attention. At present, several packaging materials have been used in dairy products to improve the viability of probiotic cells. Sealing packaging materials include alginate, starch, carrageenan, whey protein and their combination products. The thickening and gelling effect of modified starch can prevent dehydration and improve the organoleptic properties of yogurt. However, there are two problems in the process of microencapsulation of probiotics: size and viability. The latter is of great significance for the selection and application of microencapsulation technology.

Another way to improve the viability of probiotics is to standardize food production methods. It is vital to find new probiotic strains to improve functional foods, while also being able to meet the requirements of the Chinese food industry.

Source: China Dairy Industry Association