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University of Agriculture and Forestry Faculty of Food Science and Technology MILK AND DAIRY PRODUCTS LABORATORY REPORT Name: Nguyễn Thị Thanh Hằng Experiment 1: Freshness/ raw milk quality 1.1 Measurement of density a Objective The purpose of the first experiment are measure the density and identify samples: normal raw milk, skimmed milk and water-added milk b Materials and Methods  Apparatus and reagents - Milk: normal milk, skimmed milk and water-added milk - Lactometer used for measuring the density  Method: Put the lactometer into the flask, which contain milk inside, to measure the density Part of lactometer rises on the surface Record the value c Results Table The density value of milk samples Sample Density (Kg/ m3) Conclusion 1025 Water-added milk 1040 Normal raw milk 1050 Skimmed milk d Discussion - The density of milk depends on temperature The higher the temperature is, the more liquid expenses, the lower the density is - Density also depends on composition of milk: • Fat content decrease density • Solubilized components increase density, so Dskimmed milk > Dwhole milk • Addition of water leads to the decreasing of milk density e Conclusion - The identification of various types of milk can be considered based on its density value - The density of milk can be measured by using a lactometer The standard temperature for the measurement is normally 20 ⁰C However, this experiment was conducted at room temperature and because of some human mistake during value recording step, the results maybe not entirely accurate 1.2 Acidity a Objective This experiment was conducted to measure the acid value of milk as well as to evaluate milk quality based on this value b Materials and Methods  Apparatus and reagents - pH meter - Burret - Pipet 10 mL - Beaker 25 mL  1.2.1 1.2.2 - - Bottle of distilled water Erlen 50 or 100 mL NaOH 0.1 N solution Phenolphthalein sulution Method Procedure for pH reading Put several mL of the sample into the beaker Calibrate the pH meter before use Read pH with the pH meter (pay attention to the sample temperature) Procedure for titration Pipet 10 mL of the sample into the Erlen, add 10 mL of distilled water and drops of phenolphthalein solution (indicator) Titrate the content with NaOH 0.1 N to the appearance of pink color which remains for about 10 seconds Calculate into Dornic degree using the following conversion 1° Dornic is equal to 0.1ml of NaOH 1/9N necessary to neutralize 10 mL of sample c Results Table pH result, volume of titrated NaOH (mL) and Dornic degree of milk samples Sample pH Volume of titrated NaOH (mL) Dornic degree 6.1 2.6 23.4 6.28 2.3 20.7 6.25 2.3 20.7 6.29 2.3 20.7 6.43 2.2 19.8 d e 1.3 6.4 2.2 19.8 Discussion - Acidity is an important parameter to indicate the quality of milk as well as fermentation process Normal milk often has pH of 6.6 to 6.8 and is the result of dry matter, not lactic acid Conclusion - In conclusion, this is a quantitative method used to evaluate the quality of milk - The result cannot be exactly due to mistake from equipment (burret for titration) as well as human during conducting the experiment Alcohol test a Objective This experiment was conducted to evaluate the freshness of milk by observing its coagulation when adding alcohol b c Materials and Methods  Apparatus and reagents - Test tubes (e.g, 150 mm length x 16 mm diameter) - Test tube rack - 68% alcohol solution (68% of 96% alcohol + 28% of distilled water)  Method Single test Bring equal volumes of milk and 68% alcohol (2 mL each) into a test tube Invert the test tube several times which the thumb held tightly over the open end of the tube Examine the tube to determine whether the milk has coagulated If it has, fine particles of curd will be visible Double test Bring volume of milk and volumes of 68% alcohol into a test tube Proceed and examine similarity Results Table Phenomenon results of Alcohol test Sample Single test Double test Precipitation occurs Precipitation occurs No Precipitation No Precipitation No Precipitation Precipitation appear but less than single test experiment Precipitation appear but less than single test experiment No Precipitation 1.4 No Precipitation Precipitation occurs d Discussion - Milk that contains more than 0.21% lactic acid, or calcium and magnesium compounds in greater than normal amounts, will precipitate when alcohol is added - In case of this experiment, when alcohol was added, the two samples (1 and 2), did not coagulate when alcohol is added It shows negative alcohol test or both of samples had good quality e Conclusion - Milk quality can be tested by using alcohol test, especially for determining the sour milk - When using this test, the observer should differentiate the bubbles and curd particle so that the result in this test can be obtained exactly Clot-on-boiling and CaCl2 20% test a Objective The clot-on-boiling test is used to determine whether milk is suitable for processing, as it indicates whether milk is likely to coagulate during processing (usually pasteurisation) b Materials and Methods  Apparatus - One boiling water bath (a 60 mL beaker on a gas or electric heater is adequate) - Test tubes - Timer (a watch or clock is adequate)  Method - Place about ml of milk in a test tube (the exact amount is not critical) and place the test tube in boiling water for minutes - Carefully remove the test tube and examine for precipitate The milk is rejected if any curd forms - Place drops of CaCl2 in each test tube c Results - All samples give precipitate Milk in sample and showed with visible fine particles of curd around the test tube when heating With milk in the three sample (3,4,5 and 6), there was a lot of precipitate when heating so it not suitable for processing due to poor quality d Discussion - The experiment is based on the acidity of milk - When milk is stored for a long time at ambient temperature, the increased acidity will reduce the heat stability of milk caused by fermentation or growth of bacteria Heating will precipitate protein if milk is sour Therefore, that milk cannot be used e Conclusion Beside the other testing methods, clot-on-boiling test is one type of test that used to check the milk quality, especially the heat stability of milk during storing in long time - Results cannot be accurate due to human error and long execution time while conducting experiments 1.5 The methylene blue reduction test a Objective This experiment was carried out to determine the length of time milk takes to decolourise methylene blue is a good measure of its bacterial content and hence of its hygienic quality b Materials and Methods  Apparatus and reagents - A water bath at 37 – 38⁰C - Test tubes graduated at the 10 mL mark - A supply of mL pipettes (bacteriological type) - A thermometer, - 100⁰C graduated at 0.5⁰C intervals - A supply of sterilised rubber stoppers for closing the tubes when the samples have been put into them These can be conveniently sterilised before use by immersing in boiling water for 10 minutes - A methylene blue solution made up from standard methylene blue milk testing tablets which are available commercially - Test-tube racks Method When the test is used to grade suppliers’ milk the samples are taken in the milk reception area Mix the supplier’s milk thoroughly Take the sample with a clean, sterile dipper and fill the test tube to the 10 mL mark Stopper the test tube with a sterile rubber stopper and mark the milk supplier’s number on the tube Place the test tube in the test tube rack When sufficient samples to fill the test tube rack have been collected, add ml of the methylene blue solution to each test tube Replace the rubber stopper with aseptic precautions Invert each tube twice, to mix the milk and solution thoroughly, and place the tubes in the water bath at a temperature of 37 - 38⁰C Make a note of the time at which the tubes are put into the water bath Examine the samples after 30 minutes, note the numbers of decolourised samples and remove them from the bath Do not disturb partly decolourised tubes Invert all other tubes At half-hourly intervals examine again for decolourised samples and repeat as above  Control tubes Include two control tubes with each batch consisting of: • 10 mLof mixed milk plus ml of methylene blue solution • 10 mL of mixed milk plus ml of water Immerse both tubes in boiling water for minutes to destroy the natural reduction action of the milk Table Standard time for evaluating milk quailty Time taken to Grade decolorized milk < 30 Very bad 30 - 1h Bad - 2h Poor - 4.5h Fair > 4.5 h Good c Results 10h05’ 13h20’ Figure & Color changes during incubation time d Discussion - The time for blue color of methylene solution to disappear is related to amount of microorganisms in milk - In this experiment, milk takes more than hours to decolorize blue color of methylene and this indicate the low amout of microorganism in milk - The length of time milk takes to decolorize methylene blue is a good measure of its bacterial content and hence of its hygienic quality The time is governed by the activity of reducing bacterial present in milk and the oxygen content When oxygen has been used the methylene blue is reduced, changing in color from blue to white - e The temperature of incubation tank can cause error to this experiment If the temperature is too high, it can destroy all the bacteria and then the milk will not decolorize methylene blue Conclusion - This experiment can test the stability of milk but takes time - Incubation tank should be check carefully to ensure the result of this experiment - Results cannot be accurate due to human error and long execution time while conducting experiments Experiment 2: Fermented milks: yogurt a Objective This experiment was conducted to know how to make two types of yogurt, namely set yogurt and stirred yogurt Yogurt products were made from different recipes and than were evaluated by sensory test b Materials and methods  Apparatus and reagents - Raw milk - UTH –full fat milk - Skim milk powder - Starter culture (thermophilic bacteria) Method Yogurt was made by following the procedure below  Milk Standardize Homogenize 15 - 35 MPa, 50⁰C Heat 15 at 90⁰C Cool down To 43⁰C Innoculate Pour into pots Incubate 43⁰C or 37⁰C Cool down e.g by ice water Storage 4⁰C Explanation of main steps - Milk/ standardization Each group starts with one litter of milk and proceeds accordingly with the table below Table Types of milk and incubation temperatures for making yogurt Types of milk Incubation temperature 43⁰C UHT full-fat milk UHT full-fat milk + 2% Skim milk powder 43⁰C 37⁰C UHT full-fat milk UHT full-fat milk + 2% Skim milk powder 37⁰C Inoculation • Add appropriate amount of starter culture (0.25%) into milk • Mix the content after addition and then incubation - Incubation/ Fermentation • For set type of yogurt: Leave a small amount of sample for determination of pH and ⁰Dornic at the starting point Pour the rest into pots (e.g., 100 mL) and incubate at 43⁰C until a pH of 4.6 (90-120⁰D) The fermentation time is about - 5h, depending on the strength of the starter culture - • c For stirred type of yogurt: Leave a small amount of sample for determination of pH and ⁰Dornic at the starting point Incubate the rest directly at 37 ⁰C until a pH of 4.6 (14 - 16h) Cool down, stir and transfer into pots Results Table pH assessment during incubation time Time pH 5.3 5.0 4.8 4.6 - pH of the final yogurt is 4.6 - The final product after incubation and cool storage had these characteristics: white color, smooth texture, and slightly sour d Discussion - Heating step in the procedure is carried out to destroy the undesirable microorganism and inhibit the activity of enzyme - The final product belongs to stir type group, this group is incubated at 37 ⁰C so that it has longer fermentation time (14 - 16 hours) compare to at 43 ⁰C (4 - hours) e Conclusion - Yogurt is a fermented product that can help preserve milk for a long time - Each step in making process should be carried out carefully to obtain the final product Experiment 3: Making ice cream Objective - This experiment was carried out to show how to make ice cream product as well as factors affect to the processing I II - - Materials and methods  Apparatus and reagents chicken eggs egg yolks 250gr of sugar 20 mL of vanilla 800 mL of fresh milk without sugar 300 mL of whipping cream 10gr of cornstarch 30gr of cocoa powder 34gr of instant coffee powder passion fruits  Method Making ice cream • Beat eggs, egg yolk and sugar then adding milk  heat with a small flame (warm) and stir  boil the mixture at low temperature (warm), adding cornstarch (which should be stirred first with a little water) Lifted out of the kitchen • Whip the whipping cream and adding to the mixture • Incubate in the fridge for hours and then pour into the ice cream maker - • Get the final product Making chocolate ice cream: Take the same steps as making vanilla ice cream but instead of adding vanilla, we adding cocoa powder and instant coffee powder Results III Table Sensorial properties of the product Vanilla ice cream - Dark brown color - Cocoa, coffee and milk flavor - Not fatty - Sweet IV - V Chocolate ice cream Passion fruit ice cream - Light yellow color - Smoother and fattier than chocolate ice cream - Slightly sweet - Yellow color - Smoother - Slightly sour taste - Natural aroma of passion fruit Discussion When making ice cream, milk is not heated to very high temperature, if not, when add egg yolk, it will not curdle Some stabilizers (CMC, xanthan gum, corn starch) can be used to increase the thickness and help the product melt slowly Conclusion - Ice cream is one type of frozen dairy product that can be served as a dessert - The quality of ice cream varies depends on how it made so that every step in process should be taken out carefully Experiment 4: Tasting of milk and dairy products Consumption milk Table Sensorial evaluation results of milk product Yakult - Light yellow color Sweet and sour taste Steurilized milk - Smell of fresh milk White color Fatty taste Fermented milk products Table Sensorial evaluation results of fermented milk product Fermented milk products Characteristics Dink yogurt - Odorous, sweet taste Opaque yellow color Liquid form Yogurt - Sour taste White with slightly yellowish color The structure is smooth Yogurt - Slightly sour taste, sweet White color The structure is very smooth Yogurt - Sour taste White color Soft texture, smooth, no water separation Yogurt - Slightly sour taste Whiter color Soft texture, no smooth, water separation Cheese products Table 10 Sensorial evaluation results of cheese product Types of cheese product Smoked cheese Blue vein cheese LaVache quirit cheese Chracteristics - In the form of yarn, tough Yellow color outside, white inside Very salty taste, smell of smoke - White color with blue veins No smell of milk, horrible smell, salty - Salty taste Ivory white solor Soft and smooth texture
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