SAMPLES 5.1 INTRODUCTION 5.2 MILK QUALITY SPECIFICATIONS

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1 PRACTICAL 5 EVALUATION OF MILK SAMPLES Structure 5.1 Introduction 5.2 Milk Quality Specifications 5.3 Basic Tests for Milk Analysis Taste, Flavour and Appearance Milk ph Alcohol Test Presence of Additives Fat Content in Milk Total Solids in Milk Solid Non-Fat (SNF) Proteins in Milk Activity 1: Determination of Physical Characteristics and Presence of any Additives Activity 2: Determination of the Fat Content Activity 3: Determination of the Percentage of Total Solids Activity 4: Determination of the Solid Non-Fat (SNF) Percentage Activity 5: Determination of the Protein Content 5.1 INTRODUCTION Milk is one of the most common articles of food throughout the world. It is regarded as a complete food. Why? Simply, because it contains protein, fat, carbohydrates, all the known vitamins, various minerals and all the food ingredients considered essential for sustaining life and maintaining health. The protein of milk is of the highest biological value and it contains all the amino acids essential for body building and repair of body cells. Various tests are available for milk composition and milk quality analysis. Practical 5 focus on these tests. Objectives After completing this practical and the activity given herewith, you will be able to: enlist the different types of milk available based on their fat percentages, distinguish between synthetic milk and pure milk, determine the total fat and protein content in a given sample of milk, detect the presence of additives in a sample of milk, and detect the total solids and solids not-fat percentage in a sample of milk. 5.2 MILK QUALITY SPECIFICATIONS Milk is a whole, clean, fresh lacteal secretion obtained by complete milking of one or more healthy milch animals. Milking is not done 15 days before or 5 days after calving or such periods as may be necessary to render the milk practically colostrum free containing minimum prescribed percentage of milk fat and SNF (solids non-fat). The components of milk are water, which is around 87 per cent, fat, present in the form of an emulsion, protein (3.5%), lactose, vitamins and minerals. Milk is classified on the basis of SNF and fat content. The PFA standards prescribed for milk is given in Table

2 Table 5.1: PFA standards prescribed for milk Evaluation of Milk Samples Types of milk SNF (percentage) Fat (percentage) Skimmed milk 8.7 minimum 0.5 maximum Double toned milk 9.0 minimum 1.5 maximum Toned milk 8.5 minimum 3.0 maximum Standard milk 8.5 minimum 4.5 Full cream 9.0 minimum 6.0 The quality of milk and milk products is rigorously controlled by laboratory tests at each stage till the milk is delivered to the final consumer. Thus, some tests are regarded as the platform tests of milk, which results in acceptance, or rejection of the milk samples collected. Milk is evaluated for its fat, milk solids non-fat and various adulterants in milk. The tests can be regarded as physical tests, chemical tests and microbiological analysis. In this chapter we will limit ourselves to the physical and chemical tests for milk analysis. We shall study about the microbiological analysis of milk in the Food Microbiology and Safety Practical (Course MFNL-003). Let us start with the basic tests first. 5.3 BASIC TESTS FOR MILK ANALYSIS The Federation R&D has developed these tests stepwise and these should be done in the same sequence along with the blank, good milk. Let us learn about these tests Taste Flavour and Appearance Milk should have its own mild sweet flavour. A soapy or chemical flavour and slight pale colour or extra white colour may give indication regarding adulteration. Rancid flavours in milk are also easily detectable through perception of smell Milk ph Milk has a ph between 6.6 to 6.8. The ph of the given sample of milk can be tested by ph strip prepared with phenol red, which gives a change in colour exactly at ph 7.0. Thus this strip is sensitive to even slight levels of neutralization Alcohol Test Alcohol test is based on the principle of curdling of milk in presence of alcohol. This forms the principle for testing of synthetic milk, which is composed of neutralizers, stabilizers and detergents. Take 0.5 ml of milk sample in a test tube along with a blank (sample of a good milk). Add to each tube 0.5 ml of 95% alcohol, mix, heat and observe for clotting Presence of Additives The additives such as starch, urea, sugar and soda are added to milk to maintain its colour, flavour, sweetness and ph. For the analysis of these adulterants, standard methods are used. These are highlighted herewith: For Starch: Iodine test is used. For Soda: Rosalic acid test is used for determination of added soda. For Sugar: Resorcinol test is used for determination of sugar. For Urea: Dimethyl amino benzaldehyde test is used for urea determination. 85

3 Principles of Food Science The procedure for detecting the presence of these additives is presented in Table 5.2. Table 5.2: Procedure for detecting the presence of starch, soda, starch, urea in a sample of milk Starch Soda Sugar Urea 1. Take 1 ml of the milk sample 2. Add 0.5 ml of iodine solution 3. Blue colour in the solution indicates starch aduteration 1. Take 5 ml of milk sample 2. Add equal quantities of rosalic acid solution. 3. If solution turns pink it indicates that soda has been added. 1. Take 0.1 ml of the milk sample 2. Add 0.2 ml of resorcinol solution 3. Boil for 30 seconds 4. The colour change to pink indicates the presence of added sugar 1. Take 0.5 ml of the test milk sample and one control sample in each test tube 2. Add 0.5 ml of the prepared DAMB solution in each test tube 3. Observe for the colour changes. 4. Deep yellow indicates the presence of added urea We shall apply these procedures in experiment 1 to determine the presence of these additives in a given sample of milk. Next, let us get to know about the different tests used to determine the milk composition. We start with the procedures for fat content in milk Fat Content in Milk The fat content in milk can be determined through various procedures as: Direct determination of fat by extraction with ether Adam's coil method Separation of fat by acid/ alkali treatment and its subsequent removal by ether Werner-Schimdt and Gottleib method Separation of fat by chemical means followed by centrifugation Gerber method The main principle behind the fat determination is protein precipitation, which is further dissolved in acid to free fat globules. In some methods, amyl alcohol is also added to produce difference in the surface tension of fat globules and the supporting liquid, thus causing the aggregation and separation of fat. As Gerber method gives fast results, thus, it is used in the dairy plants as a platform test for checking the quality of milk. In this test, fat is dissolved in hot amyl alcohol and its separation from heavy acid solution is affected by subjecting the mixture to centrifuging. In Gerber method, 10 ml of conc. H 2 SO 4 is added into the gerber tube. 1 ml of amyl alcohol is added in this tube ml of the milk sample is added and the tube is stoppered. The tube is then inverted twice or thrice and shaken briskly to mix the ingredients. The tube is then centrifuged for 3 min at 1000 rpm to get the reading of fat in the stem of the tube. However, as this method is requiring special equipments like gerber tube, as highlighted in Figure 5.1(a), and milk centrifuge system, we will in this exercise limit ourselves to the Rose-Gottleib method which is a gravimetric extraction method. A rose-gottleib tube is illustrated in Figure 5.1(b). 86

4 Evaluation of Milk Samples Total Solids in Milk Figure 5.1: Apparatuses for fat analysis in milk Total solids determination is a common procedure in many manufacturing plants using dairy products. The total solids in milk can be calculated from the specific gravity and fat percentage from the table wherein lactometer reading is taken at 60 F. Table 5.3 gives the lactometer reading for a given percentage of fat. You can refer to this table to calculate the total solids in milk. Besides carrying out the total solids percentage from the indirect method of taking lactometer reading, a direct method of gravimetric analysis can also be used. This method involves accurately weighing a few grams of the material and subjecting it to heat until all moisture has been driven off on a water bath. The dry residue is weighed, its percentage calculated as total dry solids. We would be analyzing the total solids percentage in the given sample of milk in activity 3 of this practical Solids Non-Fat (SNF) Solid non-fat is an important criterion of milk selection for further processing. Milk solids non-fat would include the nitrogenous substances, milk sugar and mineral matter. The determination of solid non-fat is done by taking lactometer reading at 40 C. The method is fully explained in experiment 4 of this practical. You shall be applying this procedure to determine the SNF of a given sample of milk Proteins in Milk Milk comprises of casein, lactoalbumins and lactoglobulins. About 82 per cent of the protein in milk is casein and the remaining proteins are whey proteins, which are lactoalbumin and lactoglobulin. Casein binds with calcium in milk and forms the calcium-caseinate complex, which is present in the colloidal form. Acid, rennet, alcohol and heat can precipitate this complex. 87

5 Principles of Food Science Percentage of fat Table 5.3: Lactometer reading and the percentage of fat Lactometer reading at 60ºF (15.6ºC) (Quevenne degrees)

6 The proteins in milk are determined by the Kjeldahl procedure, which is defined as the amount of nitrogen experimentally found and multiplied by an approximate conversion factor. In this procedure, as you may have already studied about in the Nutritional Biochemistry practical (Course MFNL-002), the sample of milk is oxidized in the presence of sulphuric acid and nitrogenous compounds are converted into ammonium sulphate. Ammonia is liberated by adding an excess of alkali and is quantitatively distilled into a measured volume of standard hydrochloric acid or sulphuric acid. The acid not neutralized by ammonia is back-titrated with standard alkali. The conversion factor used for milk and milk products as given by BIS is Although Kjeldahl method gives the accurate estimation milk proteins but as the digestion of milk poses the problem at the laboratory scale thus we would carry out the estimation of milk proteins through Pyne's method or aldehyde number method. This method gives the approximate estimation of the total protein present in milk and is also used as the platform test. Let us now start with the various tests involved in the evaluation of the given sample of milk. Evaluation of Milk Samples 89

7 Principles of Food Science ACTIVITY 1 Date:. DETERMINATION OF THE PHYSICAL CHARACTERISTICS AND PRESENCE OF ANY ADDITIVES Aim: To determine the physical characteristics and presence of any additives in the given samples of milk. Objectives After undertaking this activity, you will be able to: identify the physical characteristics of milk, determine any additives present in milk, and distinguish between synthetic milk and pure milk. Principle The principle is discussed in sub-section of this practical. We suggest you look up the sub-section once again and write the principle here in the space provided. Materials Required 1) Sample of milk 2) Iodine solution 3) 0.01% alcoholic solution of rosalic acid 4) Resorcinol solution (0.05 g resorcinol in 100 ml of dilute HCl (1:2) 5) DAMB solution- dissolve 0.4 g of p-dimethyl amino benzaldehyde in 250 ml alcohol and add 23 ml of conc. HCl 6) ph strip prepared with phenol red 7) 95% alcohol Procedure You may recall studying about the procedure for determining the physical characteristics in sub-section earlier in this practical. Write the procedure for carrying out the adulteration tests in the format given herewith: Starch Soda Sugar Urea 90

8 Next carry out the ph test and alcohol test as indicated in the sub-section and earlier. Observations Based on the experiment you have just conducted, write your observations in the format given herewith: Evaluation of Milk Samples Characteristics/adulterants Sample Taste, flavour and appearance ph Alcohol test Presence of starch Presence of soda Presence of sugar Presence of urea Inference The given samples of milk were found to be of.. quality on the parameters of taste, flavour and appearance and ph. The samples tested for presence of additives gave the following results: Additives Starch Soda Sugar Urea Conclusion (Comment regarding the acceptability of the sample) Sample Submit the activity for evaluation... Counsellor Signature 91

9 Principles of Food Science ACTIVITY 2 Date:. DETERMINATION OF THE FAT CONTENT Aim: To determine the fat content in the given sample of milk. Objectives After undertaking this activity, you will be able to: explain the technique of determination of the total fat content in the samples of milk and milk products, and differentiate between the different types of milk available varying in their fat percentages. Materials Required Samples of milk (toned/double toned and full cream milk) The rose gottlieb tube/ separating funnels Tared beakers Ammonia Diethyl ether Petroleum ether (40-60ºC) 95% alcohol Air-oven maintained at 100ºC Principle The principle of the fat extraction has been discussed in sub-section earlier in this practical. Recapitulate what you learnt and write down the principle here in the space provided. 92 Procedure Now carry out the experiment step-by-step as enumerated herewith: 1) Take 10 ml of the milk sample in the rose-gottlieb tube. Alternatively, you can take the contents in a beaker. 2) Add 2 ml of ammonia. 3) Cork the rose-gottlieb tube and mix the contents by shaking, with slight warming, until the contents present an entirely homogenous appearance and are free from flakes of protein. In case the beaker is used, mix contents with a glass rod and pour contents into the separating funnel. 4) Add 10 ml of 95 per cent alcohol with further shaking. 5) Now add 18 ml of ethyl ether and mix the contents of the tube/separating funnel thoroughly by repeated inversions. 6) Add 18 ml of petroleum ether and mix again.

10 7) Let it stand for a few minutes till the separation of the ether solution containing fat is obtained. 8) Transfer the ether layer in a small tared/ previously weighed beaker by means of wash bottle tube. Alternatively, do the mixing of the contents in a separating funnel and separate the ether layer in a tared beaker. 9) Repeat the extraction twice with small quantities of mixture of equal parts of ethyl ether and petroleum ether and add to the main volume of ether solution (already collected in the tared beaker). 10) Evaporate the ether by immersing the beaker in hot water bath. 11) Heat it in the air-oven at 100 ºC. Cool and weigh the beaker. 12) Calculate the fat content as given below. Calculations 100(W1 W) Total fat, percent by weight = g where, W1 = weight in g of the beaker with the fat W = weight in g of the empty beaker g = weight in g of the milk sample taken for analysis Results/ Findings Record you observations/ findings here and calculate the total fat percent. Milk sample = Weight of the milk sample used for analysis (g) =... Weight of the beaker (W) =... The final weight of the beaker containing fat (W1) =... Total fat percent by weight can be calculated as: Evaluation of Milk Samples Inference The given sample of milk is having fat percent by weight and thus can be classified as toned/ double toned/ skim/ full cream milk. Conclusion (Comment regarding the acceptability of the sample) Submit the activity for evaluation.. Counsellor Signature 93

11 Principles of Food Science ACTIVITY 3 Date:. DETERMINATION OF THE PERCENTAGE OF TOTAL SOLIDS Aim: To determine the percentage of total solids in the given samples of milk. Objectives After undertaking this activity, you will be able to: explain the technique of determination of the total solids in the samples of milk and milk products, and analyze the percentage total solids in different types of milk available. Materials Required Two samples of milk (toned/double toned and full cream) Shallow flat bottom dishes of aluminium alloy, nickel, stainless steel, porcelain/silica about 7-8 cm in diameter and 1.5 cm in height. Air oven maintained at temperature 100 C Weighing balance Principle The knowledge of the fat content and the specific gravity of the milk can determine total solids in milk. The percentage of total solids can be measured by the application of a simple formula as given below: T = 0.25 G F where, T represents the percentage of total solids G represents the specific gravity (in degrees) F represents the percentage of fat The gravimetric method as discussed in sub-section earlier in this practical would be used in this exercise for the determination of total solids. Write down the principle of gravimetric method here in the space provided: 94 Procedure Now carry out the experiment step-by-step as enumerated herewith: 1) Weigh accurately the clean, dry empty dish with the lid. 2) Pipette into the dish about 5 ml of the prepared sample of milk and weigh quickly with the lid on the dish. 3) Place the dish uncovered on a boiling water bath. 4) Keep the base of the dish horizontal to promote uniform drying and protect it from direct contact with the metal of the water bath. 5) After at least 30 minutes remove the dish, wipe the bottom and transfer to an air oven maintained at 100 C ± 2 C.

12 6) At the end of one hour, remove the dish to a dessicator and allow to cool completely before weighing again. 7) To ensure the constancy of weight, the dish must be replaced in the air-oven for successive periods of 30 minutes, with intermediate weighing, until two consecutive weighing differ by less than one milligram. 8) Note the lowest weight. 9) Calculate the total solid percent by weight using the formula given herewith. Calculations Total solids, percent by weight = 100 W1 W Evaluation of Milk Samples where, W1 = weight in g of the residue after drying. W = weight in g of the prepared sample taken for test. Precautions 1) The dish should be kept on a water bath for the evaporation of water. 2) In the oven, do not place the dish near the walls of the oven. Results/Findings Record you observations/findings here in the format provided and calculate the total fat percent. Weight of the milk sample used for analysis Milk sample 1 Milk sample 2 Weight of the beaker (W) The final weight of the beaker containing residue (W1) Total solids, percent by weight, can be calculated as- Sample 1 Sample 2 95

13 Principles of Food Science Inference The given sample no.1 is having total solids percent by weight. The given sample no.2 is having total solids percent by weight. Conclusion (Comment regarding the acceptability of the sample) Submit the activity for evaluation.. Counsellor Signature 96

14 DETERMINATION OF THE SOLID NON-FAT (SNF) PERCENTAGE Evaluation of Milk Samples ACTIVITY 4 Aim: To determine the solids non-fat percentage in the given samples of milk. Objectives After undertaking this activity, you will be able to: explain the technique of determination of the solids not-fat in the samples of milk, check the variations in percentage solids non-fat in various samples of milk, and check the given sample for conformance to the standard for SNF content. Principle Solid non-fat is an important criterion of milk selection for further processing. Milk solids non-fat would include the nitrogenous substances, milk sugar and mineral matter. The determination of solid non-fat is done by taking Lactometer reading at 40 C. Also refer to section 5.4 Materials Required Sample of milk Lactometer The rose gottlieb tube/ separating funnels Tared beakers Ammonia Diethyl ether Petroleum ether (40-60ºC) 95% alcohol Air-oven maintained at 100ºC Weighing balance Procedure Determine the fat content in the given sample of milk as explained in experiment 2 of this practical. For taking the Lactometer reading, carry out the following steps: 1) Warm the milk sample to 40 C. 2) Mix the contents by rotating and inverting the bottle, taking care to avoid air bubbles. Alternatively, mix the contents in a beaker. 3) Pour about 50 ml of milk in the measuring cylinder. 4) Insert the lactometer gently to wet the stem not more than a short length, about 3 mm beyond the position of equilibrium. 5) Note the reading regarding top of the meniscus on the stem. 6) Calculate the SNF percent using the formula given herewith: Calculations SNF, percent = Lactometer reading F where, F = Fat percent. Date:. 97

15 Principles of Food Science Precautions 1) Milk should be mixed gently and not vigorously. Vigorous shaking causes the milk to froth which leads to a fallacious reading. 2) Lactometer should float freely and not touch the sides of the cylinder. Results/Findings Record your observations/ findings in the format given herewith and calculate the total fat percent. Lactometer reading Fat percent SNF percent by weight can be calculated as- Lactometer reading + 0.2F Sample 1: SNF per cent = Milk sample 1 Milk sample 2 Sample 2: SNF per cent = Inference The given sample no.1 is having SNF content and fat percent and thus can be classified as (toned/ double toned/ skim/ full cream milk). The given sample no.2 is having SNF content and fat percent and thus can be classified as (toned/ double toned/ skim/ full cream milk). Conclusion (Comment regarding the acceptability of the sample) Submit the activity for evaluation. 98 Counsellor Signature

16 DETERMINATION OF THE PROTEIN CONTENT Evaluation of Milk Samples ACTIVITY 5 Aim: To determine the protein content in the given sample of milk. Objectives After undertaking this activity, you will be able to: explain the technique of determination of protein content in the given samples of milk, discuss the various factors in determination of proteins in milk, and check the given sample for conformance to the standard for protein content. Principle Pyne's method or aldehyde number method gives the approximate estimation of the total protein present in milk and is also used as the platform test. Milk proteins contain amino groups which react with formalin and thereby increasing its acidity due to formation of an NH 3 + ion and lowers the alkalinity of the NH 2 group Date:. CH-NH 2 COOH Amino acid H + C = O H 2 N + CH 2 COO H Formaldehyde Aldehyde number is expressed as the amount of 1N NaOH required to neutralize the acidity generated in 1 lt. of milk through the addition of 2 ml of formaldehyde (HCHO). Materials Required 1) Sample of milk 2) Formaldehyde 40% concentrated and neutral to phenolphthalein 3) Phenolphthalein indicator solution 4) Potassium oxalate solution- saturated and neutral to phenolphthalein 5) Standard NaOH solution (0.1 N) Procedure Now carry out the experiment step-by-step as enumerated herewith: 1) Take 10 gram of sample accurately weighed in a conical flask. 2) Add 1 ml of phenolphthalein indicator. 3) Add 0.4 ml of saturated potassium oxalate solution. 4) Set aside for 2 minutes. 5) Titrate against 0.1 N NaOH till end point (presence of light pink colour) 6) Add 2 ml formaldehyde. 7) Titrate against 0.1 N NaOH till end point. 8) Calculate the protein percent using the formula given herewith. Calculations Protein percent is calculated as = Volume of NaOH

17 Principles of Food Science Results/ Findings Record your readings in the observation table given below: S. No. Initial reading Final reading Difference (v) Pilot V = ml V= volume of NaOH used The percentage protein found in the given sample of milk is calculated as: Volume of NaOH used =. ml. Protein percentage is = Valume of NaOH 1.7 Now calculate the protial percentage in the space provided: Inference The given sample of milk had percentage of proteins. Conclusion (Comment regarding the acceptability of the sample) Submit the activity for evaluation.. Counsellor Signature 100