Processing of Pasteurized Market Milk

Transcription

1 Processing of Pasteurized Market Milk Depending on legislations and regulations, the design of the process line for pasteurized market milk varies a great deal from country to country and even from dairy to dairy. For instance, fat standardization (if applied) may be pre standardization, post standardizatin or direct standardization. Homogenization may be total or partial, etc. The simplest process is to pasteurize the whole milk, such a process will include a pasteurizer, a buffer tank and a filling machine. The process becomes more complicated if it is designed to produce several types of market milk products, i.e., whole milk, skim milk, standardized milk of various fat contents, as well as cream of various fat contents. Figure (1) shows the layout of a market milk process line. The milk enters the plant via balance tank 1 and is pumped to pasteurizer 4, where it is preheated before it continues to separator 5 which produces skim milk and cream. The standardization of market milk takes place in an in-line system. The fat content of the cream from the separator is set to the required level and is then maintained at that level, regardless of variations in the fat 1

2 content and in the flow rate of the incoming milk. The fat content of the cream is usually set at 40 % for whipping cream, but may be set at other levels, e.g., for production of butter or other types of cream. Once set, the fat content of the cream will be kept constant by the control system, consisting of flow transmitter 7, density transmitter 8, regulating valve 9 and the control system for the standardization system. In the line shown in figure (1), partial homogenization is used, i.e., only the cream is treated. The remixing of the cream and the skim milk does not take place immediately after the separator, but in the skim milk pipes before flow transmitter 7'''. The cream from the separator passes through flow transmitter 7'. The control panel receives signals from flow transmitters 7'' and 7''' and on the basis of these signals valve 10 adjusts so that the correct quantity of cream is supplied to the remixing point to give the required fat content in the standardized milk. The surplus cream flows to a tank for further treatment. The cream to be remixed into skim milk flows to homogenizer 12. The fat content of the cream must be reduced to 10 12% before being supplied to the homogenizer in order to produce the desired result for the partial homogenization. This is attained by mixing the cream with skim milk from the separator. The cream has been preheated regeneratively to homogenizing temperature in heat exchanger 4. The quantity of skim milk supplied to the homogenizer is regulated by means of the homogenizer inlet pressure. The homogenizer is a positive displacement pump and will always provide the same quantity of mixture of cream and skim milk through the homogenizer head. The correct quantity of skim milk will always be sucked in and mixed with the cream in the pipe before the homogenizer, so that the fat content will be 2

3 correct. The rest of the skim milk, the major part, continues to the remixing point. After homogenization, the cream is remixed with the skim milk in the skim milk pipe before flow transmitter 7'''. The milk, with standardized fat content, is pumped to the heating section of the milk pasteurizer where it is pasteurized. The necessary holding time is provided by a holding section in the plate heat exchanger or, as shown in the figure, by means of a separate holding tube 14. If the pasteurization temperature is too low, this is sensed by a temperature transmitter. A signal activates flow diversion valve 15 and the milk flows back to the balance tank. The pasteurizing temperature is registered continuously. Pump 13 is a booster pump which increase the pressure of the product to a level at which the pasteurized product cannot be contaminated by untreated milk or by the cooling medium if there is a leak in the plate heat exchanger. After pasteurization, the milk continues to a cooling section in heat exchanger 4 where it is regeneratively cooled by the incoming untreated milk and then to the cooling section where it is cooled with water and ice water. The cold milk is then pumped to the filling machines. Standardization The purpose of standardization is to give the milk a defined and guaranteed minimum fat content. The level varies considerably from one country to another. Common values are 0.5% for low fat milk and 3% for regular grade milk. The fat is a very important economic factor. Consequently, the standardization of milk and cream must be carried out with great accuracy. 3

4 Comprehension Exercises A. Put "T" for true and "F" for false statement. Justify your answers. 1. The process line for pasteurized milk is the same in all countries. 2. The milk separator in the process line produces cream and skim milk. 3. The fat content of the cream is usually set at 40% for whipping cream. 4. The booster pump increases the pressure of the product in milk process line. 5. The fat content of pasteurized milk is 2.5%. B. Choose a, b, c, or d which best completes each item. 1. Fat standardization in milk processing may be.... a. direct standardization. b. pre-standardization. c. post-standardization. d. direc, pre- or post-standardization. 2. Milk entering the process line is preheated in the.... a. balance tank. c. heating tank. b. pasteurizer. d. holding tank. 3. The remixing of the cream and the skim milk takes place.... a. in the skim milk pipes b. in the folw transmitter c. immediately before the separator d. immediately after the separator 4. the control system for the fat content of cream consists of the.... 4

5 a. flow transmitter and the regulating tube b. density transmitter and the standardization system c. flow transmitter, the regulating valve, and the density transmitter d. standardization control system, the regulating valve, and the density and flow transmitters 5. The purpose of milk standardization is to get a.... a. regular grade milk b. milk with a defined and guaranteed minimum fat content c. high quality milk d. milk with a high fat content C. Answer the following questions orally. 1. What parts does the simplest process for pasteurizing whole milk include? 2. What is the fat content of cream before it enters the pasteurizer? 3. What are the fat contents of low fat and regular grade milks? 4. How is milk cooled after it is pasteurized? 5. Where in the process line is cream preheated to homogenizing temperature? Part 2. Language Practice A. Choose a, b, c, or d which best completes each item. 1. A building where milk is stored and processed in order to get milk product is called a.... a. farm c. milk house b. dairy d. pasteurizer 2. The cream in the process line is preheated in the.... 5

6 a. homgenizer c. heat exchanger b. displacement pump d. flow tansmitter 3. Milk with a 0.5% fat content is a.... a. low fat milk c. regular grade milk b. standard milk d. low grade milk 4. Mixing two substance e.g., cream and milk, after they have been separated from each other, is called.... a. regenerating c. homogenizing b. regulating d. remixing 5. the cream from the separator passes through a.... a. density transmitter c. flow transmitter b. flow diversion valve d. product pump B. Fill in the blanks with the appropriate of the words given. 1. whip a. Cream with a high fat content can be... in to a thick foam and used as a dessert. b. If you... cream, you will get a fine cream foam. c. Confectioners use... cream in baking cake and other sweets. 2. pasteurize a. It is recommended that people consume... milk. b. Milk... plants are found in areas where large quantities of milk are produced. c. One of the important pieces of machinery in a milk process line is the milk a. There are... machines in dairy farms. b. Dairy products are... products. 6

7 c. Well fed cows... well. 4. skim a. There is no fat in... milk. b. She... the milk before she gave it to her old mother. c. Milk from which the cream has been removed is called... milk. 5. homogenize a. A step in milk processing is.... b. Most people like... milk. c. Small... are cheaper than large ones. C. Fill in the blanks with the following words. pasteurizer whole milk low-fat cream regular-grade preheated remixed valve Milk is a perishable food. Therefore it should be pasteurized or sterilized as soon as possible after it enters the milk processing plant. It is possible to pasteurize..., that is, with out removing the fat from it. However, in many cases the... is removed partially before the milk is processed. The production line for pasteurized, homogenized milk comprises several steps. The milk entering the plant is pumped to the... to be..., and then flows to the separator. The separator removes the cream and produces skim milk. The fat content of the cream is kept constant by flow and density transmitters, a regulating..., and the standardization system control. The cream then flows to the homogenizer to be... into the skim milk. The remixed milk is pasteurized, cooled and, finally, pumped to the filling machine. 7

8 Standardization of the processed milk is required in many countries. Its purpose is to get a milk with a guaranteed minimum fat content. The level of the fat varies, but it is usually 0.5% for... milk and 3% for... milk. D. Put the following sentences in the right order to form a paragraph. Write the corresponding letters in the boxes provided. a. The standardized cream is collected in a holding tank and is fed into the tank from above. b. This makes it possible to collect small flows of surplus cream over a period of time in the holding tank. c. This ensures a minimum of mechanical treatment of cream. d. The optimum holding time in the tank is 30 minutes but it may be extended up to 3 hours. e. Pasteurization may then be carried out with the calculated flow through the pasteurizer