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Introduction to dairy technology
University of Eldoret
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DAIRY TECHNOLOGY (ANS 324) COURSE OUTLINE Credit hours: 3 Pre-requisite: None Purpose of the course To teach animal science students about milk: From farm through reception, analysis, manufacturing and final product quality and storage. Students gain hands on experience with respect to production of selected dairy foods and through industry visits and practicals, become familiar with raw material handling, production procedures and quality control of the finished product. Course content Milk and milk products as food; sources and composition of milk, factors affecting milk composition. Milk reception and analysis. Milk Chemistry. Milk microbiology; milk borne diseases, pasteurization and its effect on milk, sterilization, sanitizing and sterilizing dairy equipment. Milk value addition; separation and standardization, fermentation, butter, cheese. Residues and additives in milk and its products. Quality control in milk. N/B The highlighted content is this semester`s assignment (10 mks) Expected learning outcomes At the end of the course, the student should be able to: Understand the milk composition and factors affecting it. Explain processes involved in production of milk and milk products. Be able to explain the quality operations in the milk processing unit. Gain the ability to think critically about problems and issues in food processing. Gain an appreciation for the dairy industry’s role in the society. Mode of delivery Lecturers, field practicals, field trips Instructional materials/equipment Textbooks Handouts on relevant topics Course assessment Assignment 10% Continuous assessment test 20% Final exam 70% Total 100% Core reading materials Karanja, A. M. (2003). The dairy industry in Kenya: Post-liberalization agenda. Tegemeo Agricultural Monitoring and Analysis (TAMPA II) project. Tegemeo Institute of Agricultural Policy and Development. O’Mahony, F. (1998). Rural dairy technology. ILCA manual no. 4. ILCA Helvetica, Ethiopia. Walstra, P. (1999). Dairy technology: Principles of milk properties and processes. Marcel Dekker Inc., 270 Madison Avenue, New York Regular milking- The composition of milk is influenced by the milking interval. The ideal milking is an interval of 12 hours. The longer the interval the more the influence especially on the fat content of milk. Complete uniterrupted milking- The fat content of milk in the udder are obtained in one milking Composition Milk will contain its natural substances and those that come from the conditions and environment from which it is obtained. Table 1 shows the substances found in milk. Table 1: Substances found in milk MILK COMPONENET NATURAL SUBSANCES Major components Water-87% (range of 85.5-88%) FOREIGN SUBSTANCES Minor components Salts (cations and anions) Acids-0%-citrate, formate, acetate, lactate, oxalate Fat-3% (range of 2.4-5%) Phosphatides Proteins-3% Sterines Minerals-0%-(Ca, P, citrate, Mg, Enzymes-peroxidase, catalase, K, Na, Zn, Cl, Fe, Cu, etc. phosphatase, lipase e.t Vitamins- A, C,D thiamin, riboflavin, others Gases-oxygen, nitrogen The following terms are used to describe milk fractions: Antibiotics Lactose-4% Herbicides insecticides radionucleotides Plasma = milk –fat (skim milk) Serum/Whey = plasma – casein micelles (Whey contains whey proeins, minerals, lactose, other mineral components) Solids-not-fat (SNF) = proteins, lactose, minerals, acids, enzymes, vitamins Total milk solids = fat + SNF Physical state of milk as a polydisperse system In addition to the composition of milk, its physical structure is also important in determining its properties. In terms of physical chemistry, milk is an opaque, whitish fluid of polydisperse phases. Due to its role in nature, milk is in liquid form. It is a complex liquid. An oil-in-water emulsion with the fat globules dispersed in the continuous serum phase . (An emulsion is a mixture of two or more immisible liquids, one in dispersed and the other in continous phase). The fat is found in the fat globules with a diameter ranging from 0-20 µm and the fat on milking is in liquid form. A colloid suspension of casein micelles, globular proteins, and lipoprotein particles. (A colloid is a solid in dispersed phase suspended in water and generally maintained in suspension by brownian motion). The proteins are found in colloidal form and their size does not exeed 0 µm. A solution of lactose, soluble proteins, minerals, vitamins and other components. Composition of milk from different animals Milk in nature is not meant for processing but for the nutrition of the newly born. The milk from various species is directly suited for the nutrition of the young and the natural environmental conditions of the animal. Thus, the milk from various species will differ in composition. All milks contain the same gross constituents but in widely varying amounts. The main differences are in total solids and the composition of total solids (Table 2). Protein, calcium and phosphorous content show the widest variation and is related to the initial rate of growth of the animal (Table 3). The fat content serves as an energy source and is high in those animals in cold climates. The fat not only changes in percent but also increases in the total solids as shown in table 4. Table 4: Fat and total solids content in milk of selected animals Animal % Fat % fat in total solids Cow 3 30 Reindeer 16 50 Dolphin 18 60 Table5: Lactose and ash in milk of selected animals Milk % lactose % ash Human Reindeer Mare Cow Sheep Fin whale 7 2 6 4 4 1 0 0 0 0 1 The soluble substances like lactose and minerals regulate the osmotic pressure and there is an inverse relationship between them in milks of different species as shown in table 5. CASEIN MILK Cow Buffalo Goat Sheep Camel 2 3 2 4 2 80 0 85 0 85 0 83 0 75 1 ALBUMIN/GLOBULIN MILK Horse 1 50 1 Human 0 40 0 Albumin globulin milk are not suited for cheese making FACTORS INFLUENCING MILK COMPOSITION The factors that affect the composition of milk can be grouped as below: 1. Genetic factors Breed Selection Disease resistance 2. Physiological factors Stage of lactation Age Udder capacity-secretion disturbances Sexual cycle 3. Environmental factors Milking Season Feed Storage Hygiene Contamination Some of the above factors shall be discussed further. Breed 20 15 15 17 25 50 60 slightly. The problems that are likely to be experienced in this phase are; drop in milk production, low fat content in milk, silent heat and ketosis. Artificial insemination is normally done in this phase (around day 90). Mid to late lactation (140-300 days post-partum) During this phase, milk production and DMI declines and cows are in positive energy balance. The cow should be in calf and nutrient intake will easily meet or exceed requirements. Concentrate feeding should be synchronized to meet milk production and spare some for body condition (indicates energy status of dairy cattle in terms of fat cover), although not to be over conditioned. Dry period Milk production continues to decline. However, a sound dry cow management skill can increase milk yield in the next lactation and minimize metabolic problems immediately after calving. Dry cows’ diets should be formulated to meet nutrient requirements for body maintenance, foetal growth and maintenance of body condition. Transition period Nutrition and management is necessary during this period and determines the profitability of a cow for the rest of the lactation. The cows are adjusting to lactation diets fed two weeks to calving (Introduction of concentrates is necessary for changing the rumen bacteria from all roughage to concentrate digesters) Age of the cow (Number of lactations) Generally, the fat content increases up to the 4th to 5th lactation then tends to decrease. The decrease however is small and not more than 0.2-0%. A gradual decrease in lactose and protein is also noted. Secretion disturbances Udder inflammations cause secretion disturbances that affect the composition of milk. Mastitis is the most frequent cause of udder inflammation and can be defined as; An inflammatory disease of the milk glands, which apart from causing physical and microbiological changes in the milk by excretion of somatic cells (leukocytes), also causes pathological changes in the milk glands. Through mastitis, changes occur in milk which causes various problems during processing. There is impaired ability to synthesize casein, lactose and fats and the salts from the blood pass into the milk to balance the osmotic pressure. The tissue also becomes permeable to blood proteins (globulins). This results to a decrease in casein, lactose and fats and an increase in salt and serum proteins. The whole udder need not be affected and the quarters can be affected independently. Mastitis milk will normally be less heat stable, does not coagulate properly and because of reduced casein, cheese yields are affected. It is important therefore that milk cows are screened regularly for mastitis for the above reasons and for the health and maintenance of the milk animals. Mastitis can also be as a result of milk accumulation in the udder for a long time. Sexual cycle-Heat/Oestrous Heat can sometimes influence the composition of milk. There can be a decrease in yield. This can be attributed to increased nervousness and excitability causing the cow to hold some of the milk. Milking A good milker gets not only more milk, but milk of good composition. Milking factors that influence composition are; Completeness of milking Milking interval and frequency Differences between morning and evening milk. Changes in composition during milking During milking, the fat content increases with hardly any change in the SNF. This is because the big fat globules tend to stick to the alveolar walls after leaving the epithelium cells. As milking continues, these globules are loosened from the walls. Milk which deliberately omits the last milking is considered adulterated. Milk yield determination The total yield in a lactation is determined by; 1. Starting yield 2. Peak yield 3. Length of lactation Starting yield This is the average daily amount produced in the first month of lactation. The total amount for the whole lactation period can be estimated from the starting yield. Starting yield multiplied by 200 will give the approximate lactation yield. Good starting yields are only possible when The cow is in good condition after calving The feed available after calving has enough energy and protein The milk is efficiently removed from the udder. Peak yield Provided the cow is in good condition at calving and is given good-quality feed, she wiil increase production from her starting yield to peak. When post-calving feeding or body condition is inadequate, the peak yield may be same as starting yield (the cow does not peak). Length of lactation Calving at not more than 12 months sets the lactation length to around 300 days. Cross-bred and poorly fed cows will often be dried off earlier as milk yield and/or body condition drop too much. A gradual decline of 10% per month in yield is considered normal after peak yield is reached. MILK RECEPTION AND ANALYSIS Milk Reception When milk is brought from the farm to the dairy for processing, the following information is required: i. Quality Before weighing the milk, its quality should be checked. This is done using organoleptic tests. The organoleptic test permits rapid segregation of poor quality milk at the milk receiving platform. No equipment is required, but the milk grader must have good sense of sight, smell and taste. The result of the test is obtained instantly, and the cost of the tests are low. Milk which cannot be adequately judged organoleptically must be subjected to other more sensitive and objective tests. Procedure: Open a can of milk. Immediately smell the milk. Observe the appearance of the milk. If still unable to make a clear judgement, taste the milk, but do not swallow it. Spit the milk sample into a bucket provided for that purpose or into a drain basin, flush with water. Look at the can lid and the milk can to check cleanliness. Judgement: Abnormal smell and taste may be caused by: Atmospheric taint (e. barny/cowy odour). Physiological taints (hormonal imbalance, cows in late lactation- spontaneous rancidity). Bacterial taints. Chemical taints or discolouring. Advanced acidification (pH < 6). If the person receiving the milk suspects that it is of poor quality, the following tests can be carried out: acidity, pH, alcohol and clot-on-boiling. These will determine the quality of the milk. Once the person receiving the milk is satisfied with its quality, it can be weighed and the weight recorded. temperature between 15 and 32°C. If the cream is too cool it will be thick and viscous and will be difficult to sample. Sour milk or cream, in which casein has coagulated, must be sampled frequently. Sampling sour milk follows the same procedure as for fresh milk. If the milk or cream has been standing for a long time and a deposit has formed on the surface and sides of the container, it should be warmed while agitating before a sample is removed. For certain analyses, milk samples can be preserved and stored to await analysis. Samples of milk or cream for butterfat analysis can be preserved using formalin, corrosive sublimate or potassium dichromate. For general analyses, formalin is preferred, because the other two increase the solids content of the milk, influencing total solids determination. ii. Estimation of milk pH by indicator A rough estimate of pH may be obtained using paper strips impregnated with an indicator. Paper strips treated with bromocresol purple and bromothymol blue are sometimes used on creamery platforms as rejection tests for milk. Bromocresol purple indicator strips change from yellow to purple between pH 5 and 6, while bromothymol blue indicator papers change from straw yellow to blue-green between pH 6 and 6. iii. Electrometric measurement of pH Electrometric determination of pH depends on the potential difference set up between two electrodes when they are in contact with a test sample e pH meters. The test sample is placed in position and the electrodes dipped into the solution and the pH read. iv. Determination of milk titratable acidity The production of acid in milk is normally termed "souring" and the sour taste of such milk is due to lactic acid. The percentage of acid present in dairy products at any time is a rough indication of the age of the milk and the manner in which it has been handled. Total titratable acidity (natural and developed acidity may be found in titration. Standard NaOH solutions, using phenolphthalein as an indicator are used. When a faint but definite pink colour persists, the endpoint has reached. v. Alcohol test The alcohol test, together with the acidity test, is used on fresh milk to indicate whether it will coagulate on processing. The test is quick and simple. It is based on instability of the proteins when the levels of acid (> 0%) and/or rennet are increased and acted upon by the alcohol. Also increased levels of albumen (colostrum milk) and salt concentrates (mastitis) results in a positive test. Such milk will coagulate when 68-75% alcohol solution is added in a test tube. vi. Clot-on-boiling test Acidity decreases the heat stability of milk. 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 pasteurization). It is performed when milk is brought to the processing plant. If the milk fails the test it is rejected. The test measures the same characteristics as the alcohol test but is somewhat more lenient (0 to 0% acidity, as opposed to 0 % for the alcohol test). It has the advantage that no chemicals are needed. vii. Butterfat determination The main tests used to separate and determine the fat content of milk and milk products are either by centrifugal force (gerber method) or by use of solvents. The Gerber test The solids in milk other than fat are dissolved by means of concentrated sulphuric acid and the lighter fat separated from the heavier acid solution by centrifuging in special tubes known as butyrometers. Addition of Amyl alcohol makes a clear separation of acid and fat. These butyrometers are calibrated so that the percentage of fat can be read off directly on the graduated part of the tube. There are different butyrometers for cream, skim milk, raw milk and cheese. The area of cross-section is related to the volume of fat to be measured, being greater for cream and cheese than milk and skim milk. Butyrometer The only accurate way to determine TS is by evaporating the water from an accurately weighed sample. However, TS can be estimated from the corrected lactometer reading. Resazurin test. Resazurin test is the most widely used test for hygiene and the potential keeping quality of raw milk. Resazurin is a dye indicator. Under specified conditions it is dissolved in distilled boiled water. The solution can then later be used to test the microbial activity in a given milk sample. Resazurin can be carried out as: i. 10 min test. ii. 1 hr test. iii. 3 hr test. The 10 min Resazurin test is useful and rapid, screening test used on milk reception. The 1 hr test and 3 hr tests provide more accurate information about the milk quality, but after a fairy long time. They are usually carried out in the laboratory. Apparatus and reagents: Resazurin tablets Test tubes with 10 mls mark 1 ml pipette or dispenser for Resazurin solution. Water bath thermostatically controlled Lovibond comparator with Resazurin disc 4/9 Apparatus used in 10 min. Resazurin Test The solution of Resazurin as prepared by adding one tablet to 50 mls of distilled sterile water. Rasazurin solution must not be exposed to sunlight, and it should not be used for more than eight hours because it losses strength. Milk (10 mls) is put into a sterile test tube and one ml of resazurin solution added. The test tube is stoppered with a sterile stopper mixed gently and incubated in a water bath .The test tubes are then is placed in a Lovibond comparator with Resazurin disk and compared colourimetrically with a test tube containing 10 ml milk of the same sample, but without the dye (Blank). In some cases the the Lovibond comparator is not used thus visual assessments are made. Readings and results (10 minute resazurin test) Resazurin disc No. Colour Grade of milk Action 6 Blue Excellent Accept 5 Light blue v. good Accept 4 Purple Good Accept 3 Purple pink Fair Separate 2 Light pink Poor Separate 1 Pink Bad Reject 0 White Very bad Reject
Introduction to dairy technology
Course: Dairy Technology (ANS 324)
University: University of Eldoret
