Basic requirements for making fermented dairy products
Converting lactose into lactic acid can protect milk, and the low pH value of fermented dairy products inhibits the growth of harmful microorganisms such as spoilage bacteria, thus prolonging the shelf life of products. On the other hand, yogurt provides a good growth environment for yeast and mold, and being polluted by these microorganisms will cause bad flavor of the product.
Some people's digestive system lacks lactase, so lactose cannot be broken down into monosaccharides during digestion. These people can only drink a small amount of milk, but they can eat fermented milk because some lactose has been broken down by bacterial enzymes. In the production of fermented dairy products, it is necessary to create the best growth conditions for starters. The heat treatment of milk can meet this requirement, which destroys any microorganisms in raw milk. In addition, milk must be kept at the optimum temperature of the corresponding starter. When fermented milk gets the best taste and fragrance, it must be cooled quickly to stop fermentation. If the fermentation time is too long or too short, the flavor and consistency will deteriorate accordingly. Besides flavor and fragrance, good appearance and coagulation are also important aspects of fermented dairy products, which are determined by pretreatment parameters. In order to harden the curd structure of yogurt in production, it is necessary to fully heat treat and homogenize the milk, sometimes by increasing the solid content of skim milk. The production process requirements of some important fermented dairy products are as follows: there are many similarities with other fermented products, such as the pretreatment of milk is almost the same, so the description of other products mainly focuses on different production stages from yogurt production.
Yogurt production process
The technological process of yogurt is as follows:
Pure culture of lactic acid bacteria → mother koji → production koji.
↓
Pretreatment of raw milk → standardization → batching → homogenization → sterilization → cooling → adding starter.
Filling into a retail container → fermentation in a fermentation chamber → cooling → post-ripening → solidified yogurt.
→
Fermentation in fermentor → cooling → adding fruit materials → stirring → filling → ripening → stirring yogurt.
Preparation of starter cultures
(I) Concept of starter A starter is a specific microbial culture that can promote the acidification process of milk and contains high concentration of lactic acid bacteria.
(2) Types of starter cultures
1. According to the preparation process of starter.
(1) The pure culture of lactic acid bacteria, that is, the culture of first-class strains, is usually inoculated in skim milk, whey, gravy or other culture media, or frozen and sublimated to make freeze-dried vaccines.
(2) The mother koji, that is, the first-class strain, is expanded and re-cultured, which is the basis of producing koji.
(3) The production of starter, that is, the expanded culture of mother starter, is a kind of starter for practical production.
2. According to the purpose of using the starter.
(1) mixed starter This kind of starter contains two or more kinds of bacteria. For example, yogurt starter is made by mixing Lactobacillus bulgaricus and Streptococcus thermophilus in the ratio of 1: 1 or 1:2, and the smaller the change in the ratio of the two bacteria, the better.
(2) Single starter This kind of starter contains only one kind of bacteria.
3. Classification according to usage methods
(1) Sub-culture koji This kind of koji is made according to the process of pure culture-mother koji-koji production, and can only be used for production after culture.
(2) Direct-throw starter This kind of starter is a kind of starter which mixes and cultivates lactic acid bacteria in a certain proportion and then directly uses it for production without subculture. DVS Directed Vat Set refers to a series of highly concentrated and standardized freeze-dried starter strains, which can be directly added to the heat-treated raw milk for fermentation without other pretreatment such as activation and expansion. Generally, the number of viable bacteria in direct-cast yogurt starter is101012 cfu/g. Because direct-cast yogurt starter has strong vitality and many kinds, yogurt producers can choose it at will according to their needs, which enriches the varieties of yogurt products, omits the strain workshop and reduces personnel, investment and investment. Direct-throw yogurt starter can be used directly, and it is easy to manage without expanding culture. The production and application of direct-throw yogurt starter can make the production of starter professional, socialized, standardized and unified, thus standardizing the production of yogurt, improving the quality of yogurt and ensuring the interests and health of consumers.
(3) The main function of starter and the selection of strains
The main functions of starter are: ① decomposing lactose to produce lactic acid; (2) Volatile substances, such as butanedione and acetaldehyde, are produced, so that yogurt has a typical flavor; ③ It can degrade fat and protein to a certain extent, thus making yogurt more conducive to digestion and absorption; ④ Acidification inhibited the growth of pathogenic bacteria.
The selection of strains plays an important role in the quality of starter cultures, and suitable strains should be selected according to different production purposes. The main technical characteristics of the product, such as aroma production, acid production, viscosity production and proteolytic activity, are used as the basis for selecting starter strains.
(4) Preparation of starter cultures
1. Resurrection and preservation of strains Pure lactic acid bacteria cultures purchased from strain preservation units are usually packed in test tubes or ampoules. Due to the influence of preservation and delivery, the vitality is weakened and needs to be restored. This process needs to be inoculated into sterile skim milk test tubes under aseptic operation conditions for multiple subcultures. Then stored in the refrigerator at 0 ~ 4℃ and transplanted once every 1 ~ 2 weeks. However, in the process of long-term transplantation, there may be mixed bacteria pollution, which will lead to strain degradation or strain aging and cracking. Therefore, it is necessary to carry out irregular purification treatment to remove polluting bacteria and improve vitality.
2. preparation of moth starter: inoculate that fully activated strain with skim milk content of 1% ~ 2% into a triangular bottle filled with sterilized skim milk, mix well, and culture in a constant temperature box. After coagulation, it is transferred into sterilized skim milk, and this is repeated for 2 ~ 3 times to keep the lactic acid bacteria alive, and then the starter is prepared.
3. Preparation of production starter (working starter)
Skim milk, fresh whole milk or reconstituted milk (total solid content 10% ~ 12%) are heated to 90℃ for 90℃30 ~ 60min, then cooled to 42℃ (or temperature required by strains), inoculated with mother starter, fermented until the acidity is >: 0.8%, and then cooled to 4℃. At this time, the number of viable koji-producing bacteria reached/kloc-0 /×108 ~/kloc-0 /×109 cfu? mL- 1 .
It is best that the culture medium for producing starter is the same as the raw material of the finished product, so that the living environment of the strain will not change sharply and affect the vitality of the strain. The dosage of the produced starter is 1% ~ 2% of the fermented milk, which can be increased to 3% ~ 4% with a maximum of 5% in order to shorten the production cycle.
Pretreatment method of yogurt production
(1) Pretreatment of raw materials
Firstly, the milk is pretreated, standardized according to the required fat and dry matter content, and degassed if necessary. The lower the air content in milk used to ferment dairy products, the better. However, if milk powder is added to increase the solid content of skim milk, it will inevitably mix some air, so it should be degassed after adding milk powder. When the solid content of skim milk is increased by evaporation, degassing is part of the process. Degassing has the following advantages:
● Improve the working conditions of the homogenizer.
● Reduce the risk of precipitation during heat treatment.
● The viscosity and stability of yogurt are improved.
● Remove volatile odor (deodorize).
Followed by homogenization and pasteurization. Then add a starter to the milk, ferment to the required PH value, and finally add spices before cooling and packaging. For yogurt with long shelf life, UHT treatment was performed on fermented yogurt before aseptic packaging. Yogurt can be produced with liquid milk or spray-dried high-quality milk powder without low-heat treatment. The fat content of milk is standardized between 0. 1%-3.5%. If liquid milk is used, the dry matter content of milk protein must reach 3% (weight/volume) by evaporation (10-20% milk volume) or adding milk powder. The higher the content of casein and whey protein, the more stable the yogurt clot, which can reduce whey precipitation. Milk powder is dissolved in a mixing device, which consists of a rotor and a perforated stator. This slurry circulates around a jar to produce milk without caking and ensure the optimal processing. The vacuum system sucks milk powder from the storage bin into the mixing tank near the top of the mixing unit, and effectively removes the air in the milk through vacuum, which is beneficial to the longer production process and reduces the adhesion of hot surfaces. The increase of dry matter can also be achieved by ultrafiltration or adding concentrated milk. Reduce air, because milk powder is often used as raw material, yogurt will contain a lot of air, which will have a negative impact on all fermented products. Removing air will improve the stability and viscosity of the final product and prolong the eating time.
(2) batching
Dry-mix sugar or other sweeteners with stabilizer, then add it into milk at 60-70℃, and keep stirring for more than 10 minutes to completely dissolve the stabilizer. Other auxiliary materials should be processed and added at this time according to different requirements, but some auxiliary materials of stirred yogurt should be added at the later stage of fermentation.
(3) batch pretreatment
1. Homogeneous
The main purpose of homogenizing milk used to make fermented dairy products is to prevent cream from floating and ensure the uniform distribution of milk fat. Even if the fat content is very low, homogenization can improve the stability and consistency of yogurt. Homogenization and subsequent heat treatment (usually 90-95℃ for about minutes) have a good influence on the viscosity of fermented milk.
Viscosity can be measured by a simple viscometer (SMR viscometer). The determination result is expressed by the time (seconds) when 100ml product passes through a nozzle with a certain diameter at 20℃. The viscosity of fermented milk made of full-flow homogenized milk is directly proportional to the homogenization pressure, regardless of whether the milk is subjected to ordinary heat treatment or not. The table also shows that high-temperature heat treatment can also make the product sticky. According to the general requirements, in order to obtain the best physical state of the product, the homogenization pressure and temperature of milk should be 20-25MPa and 65-75℃ respectively, and the homogenization process is often used for low-fat fermented milk. The primary or secondary homogenization problem is usually considered as the design problem of homogeneous system, especially the design of homogeneous head.
The raw materials are mixed and homogenized. Homogenization can fully mix the raw materials, prevent the formation of cream during culture and ensure the uniform distribution of milk fat. It is beneficial to improve the stability and consistency of yogurt and make it delicate and delicious.
2. Heat treatment
Milk needs to be heat-treated before inoculation with starter cultures in order to:
Improve the performance of milk as a bacterial culture medium.
● Ensure that the clots of finished yogurt are firm.
● Prevent whey precipitation.
Mainly to kill miscellaneous bacteria in raw milk and ensure the normal growth and reproduction of lactic acid bacteria; Passivating natural inhibitors in raw milk that have inhibitory effect on fermentation bacteria; Heat treatment denatures whey protein in milk, thus improving tissue state, increasing viscosity and preventing whey precipitation of finished products. It is best to heat treat milk at 90-95℃ for 5 minutes, because under such conditions, whey protein denatures by 70-80%, especially whey protein-lactoglobulin for 5 minutes will interact with κ -casein to make yogurt a stable solidified body. However, the milk used for fermentation after UHT treatment can not achieve the same best effect in viscosity, and the reason is not very clear at present.
Inoculation fermentation
After heat treatment, milk should be cooled to the optimal growth temperature of starter strains immediately. Inoculation amount depends on strain activity, fermentation mode, production time and proportion of mixed strains. The acid-producing activity of starter is generally between 0.7% and 65438 0.0%, and the inoculation amount should be 2% to 4%. If the activity is lower than 0.6%, it should not be used in production. The added starter should be stirred into a uniform and delicate state in advance under aseptic operation conditions, and there should be no large clots, so as not to affect the quality of finished products.
The commonly used starter in yogurt production is a mixed strain of Streptococcus thermophilus and Lactobacillus bulgaricus. If the proportion of Bacillus decreases, yogurt will produce acid smoothly during the shelf life to prevent excessive acidification. For example, in the production of ordinary yogurt with short shelf life, the ratio of cocci and Bacillus in the starter should be adjusted to 1:l or 2: l; When producing ordinary yogurt with shelf life of 14 ~ 2 1d, the ratio of cocci to bacilli should be adjusted to 5:1; When making fruit yogurt, the ratio of two kinds of bacteria can be adjusted to 10: 1. At this time, the aroma-producing performance of Lactobacillus bulgaricus is not important, and the flavor of this kind of yogurt mainly comes from the added fruit.
Cooling, filling and post-fermentation.
After the fermentation, it should be quickly cooled to below 65438 00℃ in time. For stirred yogurt, demulsification can be carried out when it is cooled below 30℃. At this time, juice jam or fruit particles can be added (without online addition), and at the same time, it is cooled to below 10℃ by plate heat exchanger for filling.
In fact, this product can't be called finished yogurt. It is generally believed that solidified yogurt is cooled after fermentation, and stirred yogurt is the finished product after cooling and filling. In fact, post-fermentation plays a key role in the aroma generation of yogurt and the viscosity recovery of stirred yogurt. Post-fermentation is to keep such yogurt at a temperature below 10℃ for 8- 12 hours, and the whole process can be regarded as the completion of yogurt production.