handbook 1.pdf

Introduction Milk is a normal secretion of the mammary glands of female mammals. The U.S. Public Health Service defines milk as “the lacteal secretion, practically free of colostrum, obtained by the complete milking of one or more healthy cows which contains not less than 8.25% milk-solids-not-fa...

Introduction Milk is a normal secretion of the mammary glands of female mammals. The U.S. Public Health Service defines milk as “the lacteal secretion, practically free of colostrum, obtained by the complete milking of one or more healthy cows which contains not less than 8.25% milk-solids-not-fat and not less than 3.25% fat.” The term milk is understood as referring to cow’s milk unless other species are mentioned specifi- cally. For most of the world, particularly the west, milk from cattle accounts for nearly all the milk processed for human consumption. However, other milking animals are very important to some pop- ulations because their milk provides an excellent and cheap source of highly valuable animal protein and other constituents. For example, sheep followed by goat make a major contribution to the milk produc- tion of the Mediterranean countries and also in large areas of Africa and Asia. Worldwide, the dairy industry produces milk as a fluid product and is processed into a variety of manufactured dairy products using a range of advanced processing technologies. The family of dairy products manufactured from milk is shown in Figure 8.1. 8.2 Composition and Structure Milk is a polyphasic normal secretion of the mammary glands. Milk consists of (i) an oil-in-water emul- sion with the fat in the form of droplets or globules dispersed in the continuous milk serum known as whey, (ii) a colloidal suspension of proteins of various sizes in milk serum, consisting mostly of casein micelles, globular proteins, and lipoprotein particles, and (iii) a solution of lactose, soluble proteins, min- erals, vitamins, and other components. In addition, milk is a very complex food with over 100,000 different molecular species found, but most have not been identified. The main component of milk is water. The remaining compounds are mainly fat (3.9%), protein (3.3%), lactose (5%), and minerals (0.7%). Milk also contains vitamins (e.g., vitamins A and C), enzymes (e.g., lactoperoxi- dases (LP) and acid phosphatase), and somatic cells. The average composition of milk with respect to the major classes of compounds and a range of average values for milks of western breeds are shown in Table 8.1. There can be con- siderable compositional differences between species and even between breeds of a single species. The lipid content is the most variable fraction. Lipid is present mainly in the form of triglyceride, which makes up about 98% of milk fat. The remaining 2% consists of diglycerides, monoglycerides, cholesterol, phospholipids, free fatty acids, cerebrosides, and gangliosides. The major fatty acids of milk are C 14 , C16 , C 18 , and C 18:1 fatty acids. The fat is present in fresh milk mainly in the form of fat globules surrounded by a phospholipid-rich layer known as the milk fat globule membrane. Milk proteins are fractionated into two main groups: the casein fraction and the whey proteins. Caseins precipitate out of solution upon acidification of milk to pH 4.6 at 20°C, while whey proteins remain soluble under these conditions. Caseins can be fractionated into four main proteins s1 -, s2 -, -, and -caseins. Whey proteins include mainly -lactoglobulin, -lactalbumin, serum albumin, lactotransferrin, immunoglobulins, and 2 -microglobulin. Lactose is the predominant sugar in milk; other carbohydrates are present in trace amounts and are mainly galactose and glucose. The most important function of lactose is as a fermentation substrate for lactic acid bacteria. Quality Criteria for Milk Milk is an important raw material for the production of a variety of dairy products. It is therefore important that the milk used for processing has acceptable quality characteristics. Quality characteristics for raw milk include compositional quality, microbial contamination levels, somatic cell count, freedom from inhibitory substances, and reception temperature. The most common grades of raw milk are Grade A and Manufacturing Grade. The dairy farmer must meet state and federal standards to produce Grade A milk. In addition to the state requirements, a few municipal governments also have raw milk regulations. The dairy farmer must have healthy cows, adequate facilities (barn, milk house, and equipments), and must maintain satisfactory sanitation of these facilities. The Food and Drug Administration’s Pasteurized Milk Ordinance (PMO ) requires that Grade A milk must not exceed 100,000 cfu/mL standard plate count (SPC) for an individual milk producer, 300,000 cfu/mL SPC as commingled milk and 750,000 cells/mL somatic cell count (SCC). In addition, good-quality milk must not contain pesticides, antibiotics, sanitizers, drug residues, and other abnormalities. The storage temperature should not exceed 7°C within 2 h of milking. It is also important that the milk used for processing have acceptable flavor characteristics. Various weed, feed, and cowy flavors can be transmitted to milk by the cow’s respiratory or digestive system. These are considered normal and acceptable up to a certain level, although excessive amounts can cause off- flavors that are difficult to remove by processing. A salty flavor can arise from cows in late lactation and those infected with mastitis. Milk diluted with water can taste flat and can lack typical flavor. 8.4 Microflora of Raw Milk Milk is an excellent medium for the growth of a variety of microorganisms owing to its high water content, neutral pH (6.4–6.6), and ample supply of nutrients. Aseptically collected milk from clean, healthy cows typically has an SPC less than 1000. Higher SPCs indicate that milk was subject to contamination. Microbial contamination generally occurs from three main sources: from within the udder, from the exterior of the udder, and from the surface of milk handling and storage equipment. The contribution of some sources of contamination on the colony count of raw milk is shown in Table 8.2. Bacterial contamination from within the udder is frequently a result of mastitis, an inflammatory disease of the mammary tissue. Many microorganisms can cause mastitis, the most important being Staphylococcus aureus, Escherichia coli, Streptococcus agalactiae, Streptococcus uberis, Pseudomonas aeruginosa, and Corynebacterium pyogenes. The first three of these are all potential human pathogens. Sources of contamination from the exterior of the udder include water, soil, vegetation, and bedding material. In general, contamination with psychrotrophic bacteria has been associated with bedding material, untreated water, soil, and vegetation; coliform contamination with soil; and spore formers with bedding material [8,30]. Therefore, milk is susceptible to contamination by two types of microorganisms: the pathogenic bacteria and the spoilage bacteria. The presence of pathogenic microorganisms in milk may result in infection and threat to the consumer’s health. The growth of the spoilage bacteria is more detrimental to the shelf life of milk than that of the pathogenic flora. The spoilage bacteria degrade the milk through the production of enzymes. Four types of enzyme activity are encountered : (i) lactose may be fermented to lactic acid resulting in a soured product, (ii) lipids are hydrolyzed by lipase—both microbial and the native milk enzyme—and, as a result, rancidity develops, (iii) proteinase activity results in the break-down of milk proteins with both physical and organoleptic effects, principally gelation and the develop-ment of intense bitter flavors, and (iv) phospholipases can attack the milk fat globule membrane that stabilizes the native emulsion of milk fat. Once milk leaves the cow, the retention or preservation of milk quality requires cleanliness, sanitation, and careful handling. Undesirable changes in raw milk are initiated by microbiological growth and metabolism or by chemical or enzymatic reactions. Temperature is critical for dairy food quality and shelf life. Cold temperatures are used to minimize microbial growth in raw milk until it can be processed and extend the shelf life of nonsterile dairy foods. A reduction in temperature below the minimum necessary for microbial growth extends the generation time of microorganisms and in effect prevents or retards reproduction. This is clearly shown in Figure 8.2, which illustrates the likely effect of temperature on milk having an initial SPC of 50,000 cfu/mL. The microorganisms in raw milk just prior to pasteurization may include heat-susceptible pathogens as well as spoilage types. Psychrotrophs became an escalating problem for the dairy industry during the introduction of refrigerated storage of raw milk. Psychrotrophs are of primary concern to the dairy industry since they can grow and cause spoilage in raw and processed dairy products commonly held under refrigeration. Psychrotrophic microorganisms capable of growing in milk at temperatures close to 0°C are represented by both Gram- negative and Gram-positive bacteria. For example, the Gram-negative bacteria are Pseudomonas, Achromobacter, Serratia, Alcaligenes, Chromobacterium, and Flavobacterium; and Gram-positive bacteria are Bacillus, Clostridium, Corynebacterium, Streptococcus, Lactobacillus, and Microbacterium [29,17]. In aerated milk at 4°C, many strains of Pseudomonas spp. can produce sufficient proteinases to hydrolyze all the available casein into soluble peptides [21,28]. The enzyme activity from psychrotrophs stimulates the growth of starter lactic acid bacteria in milk. Most psychrotrophs normally would not be a serious problem in milk because they are eliminated by pasteurization or Ultra High Temperature (UHT) treatment. However, psychrotrophs produce thermostable proteolytic enzymes, most of which attack -CN, resulting in a destabilization of the casein micelles and coagulation of the milk in a manner that is analogous to chymosin. The quality of milk may be affected by heat- resistant enzymes secreted by psychrotrophs in raw milk before heat treatment or other enzymes and metabolites that are produced by microflora during cold storage. Some of these enzymes are not inactivated by pasteurization or by other heat treatments and may continue to degrade milk products, even when the bacterium is destroyed. The shelf life of various dairy products is given in Table 8.3. The spoilage of milk and dairy products is characterized by taste and odor changes, such as sour, putrid, bitter, malty, fruity, rancid, and unclean. The type of spoilage may also cause undesirable body, texture, and functional changes. In milk, about 40% of the milk solids is lactose, a major substrate for microbial fermentation in milk. Microorganisms use one of the two following methods to start fermentation: by the lactase enzyme (-D -galactosidase) or by hydrolyzing the phosphorylated lactose by -D -phosphogalactoside galactohydrolase. Microorganisms containing the lactase enzyme include Escherichia coli, Streptococcus thermophilus, Lactococcus lactis, Lactobacillus bulgaricus, Lactobacillus plantarum, and Bacillus subtilis. Lactic acid bacteria convert lactose to lactic acid and other by-products. Milk with a detectable acid/sour flavor is considered unacceptable commercially. Cold storage temperatures and sanitary storage and processing conditions for raw milk and cream can prevent the development of high acid/sour flavors. A malty flavor or odor can occur in milk if Streptococcus lactis var. maltigenes grows and metabolizes amino acids in milk to aldehyde and alcohols. The fruity flavors in dairy foods can be caused by the metabolic activity of lactic acid and psychrotrophic bacteria with the formation of esters. Flavor defects in milk described as putrid, itter, and unclean may be caused by the growth and metabolism of psychrotrophic bacteria. The lipase enzyme is often active at low temperatures, causing lipolyzed flavor. Gram-negative psychrotrophic bacteria have lipolytic activity. Table 8.4 summarizes the most important types of spoilage and the microorganisms responsible. Type of Spoilage Microflora Souring Lactic acid bacteria Casein precipitation Lactic acid bacteria producing enough acid to drop the pH below 4.6 Gas production Clostridium, Bacillus, yeasts, coliform bacteria, heterofermentative lactics, and propionics Proteolysis Psychrotrophic bacteria: Streptococcus faecalis var liquefaciens, Bacillus cereus, Micrococcus, Pseudomonas, Flavobacterium, Acinetobacter, Aeromonas Thermophilic organisms: Streptococcus and Lactobacillus Sporeforming organisms: Bacillus Lipolysis Psychrotrophs: Pseudomonas spp., Achromobacter spp., Alcaligenes spp., Acinetobacter spp. Thermoduric organisms: Streptococcus and Lactobacillus. Sporeforming bacteria: Bacillus Ropiness Alcaligenes viscolactics, Enterobacter, lactics Changes in butterfat Pseudomonas, Proteus, Alcaligenes, Bacillus, Micrococcus Numerous off-flavors Pseudomonas, Actinomyces, Flavobacterium, Alcaligenes, Acinetobacter, Proteus, Lactococcus lactis var matigenes, molds, yeasts, coliforms, and mastitis-causing organisms Color changes Pseudomonas syncyanera, P. synxantha, Serratia marcescens, P. fluorescen Control of Microorganisms in Raw Milk Microbial growth and contamination can be prevented, slowed, or reduced by many means: (1) cleaning and sanitizing of the milk-handling equipment and the environment, (2) holding milk at low temperature, (3) use of antimicrobial systems, (4) thermization, and (5) clarification. 8.5.1 Cleaning and Sanitizing Hygienic processing of food requires that the equipment are cleaned frequently and thoroughly to restore them to the desired degree of cleanliness. The degree of cleanliness of the milking system probably influences the total bulk milk bacterial count as much, if not more than any other factor. Since proper cleaning and sanitizing of dairy equipment are important for production of milk with acceptable microbial quality, control of psychrotrophs should begin at the farm level. Psychrotrophic bacteria tend to be present in higher count milk and are often associated with occasional neglect of proper cleaning and sanitizing procedures. Cleaning of dairy facilities involves removing soil from all surfaces that come into contact with milk and using a sanitizer after each processing period. Soil in the dairy industry is mainly minerals, lipids, carbohydrates, proteins, and water. Soil may also contain dust, lubricants, microorganisms, cleaning compounds, and sanitizers. Microbial cleaning, also known as sanitizing or disinfection, is used to reduce the load of microbial contaminants that may be present on milk contact surfaces. Most chemical sanitizers used in the dairy industry kill a broad spectrum of microorganisms provided that they are used properly. Sanitizers commonly used in the dairy industry include chlorine compounds, iodophors, quaternary ammonium compounds (QUATs), acid anionic surfactants, and peroxyacetic acid. All disinfectants are deactivated to some extent by organic matter. This is why they are best used after thorough cleaning has removed most of the soil. Many dairy plants use hot water as a common method of sanitation. This can be achieved by circulating water at 76°C–85°C for at least 5 min, followed by a cooling chemical sanitizer rinse. Hot water sanitation requires careful control to ensure that the required temperature is maintained long enough for it to be effective. This can be achieved by the use of thermostat-controlled tanks, which will circulate the water and maintain the desired temperature [2,6,13]. Hot water will often provide greater kill and longer milk shelf life than can be achieved with chemical sanitizer alone. 8.5.2 Cooling of Milk Milk leaves the udder at a temperature of about 37°C, which is favorable for the growth of a large number of microorganisms, mainly mesophiles. Milk should therefore be quickly cooled down after leaving the udder. Cooling is the main means of slowing down the growth of bacteria in milk. The maximum storage time of milk is closely related to the storage temperature. Low-temperature storage can reduce the frequency of raw milk collection from dairy farms to just two or three times a week, and enable further storage of milk in the dairy plant over weekends. Spray and immersion coolers are commonly used on farms, which deliver milk to the dairy in cans. In spray cooling, circulating chilled water is sprayed onto the outsides of the cans to keep the milk cool. The immersion cooler consists of a coil, which is lowered into the can. Chilled water is circulated through the coil to keep the milk at the required temperature. Where milking machines are available, bulk milk tanks, usually ranging from 0.8 to 19 m3 , are used to receive, cool, and hold the milk. As the cows are mechanically milked, the milk flows through sanitary pipelines to an insulated stainless-steel bulk tank. An electric agitator stirs the milk, and mechanical refrigeration begins to cool it even during milking, from 32.2°C to 10°C within the first hour, and from 10°C to 4.4°C within the next hour. Some large dairy farms and collecting centers may use a plate or tubular heat exchanger to rapidly cool the milk. In these cases, the tank is mainly to maintain the required storage temperature. The temperature of the blended milk must be below 7.2°C during the second and subsequent milkings. Since the milk is picked up from the farm tank daily or every alternate day, cooled milk may be stored in an insulated silo tank. Milk in the farm tank is pumped into a stainless-steel tank on a truck for delivery to the plant or receiving station. The tanks are well insulated, and the temperature rise should not be more than 1.1 K in 18 h when testing the tank full of water and the average gradient between the water and the atmosphere surrounding the tank is 16.7°C. Most dairy processing plants either receive raw milk in bulk from a producer or arrange for pickup directly from the dairy farms. Storage tanks, from 4 to 230 m 3 made of stainless-steel lining and well insulated, may be required for nonprocessing days and emergencies. The average 18-h temperature change should be no more than 1.6°C in the tank filled with water, and the gradient to the surrounding air 16.7°C. For horizontal storage tanks, the allowable temperature change under the same conditions is 1.1°C. The tank may need cooling depending on the initial milk temperature and holding time. A plate heat exchanger may be connected or the tank surface, around the lining, may be cooled by passing a refrigerant or by circulation of chilled water or glycol solution. Agitation is essential to maintain uniform milk fat distribution. Milk held in large tanks, such as the silo type, is continuously agitated with a slowspeed propeller driven by a gearhead electric motor or with filtered compressed air. Antimicrobial Constituents The Lactoperoxidase System There are some naturally occurring antimicrobial systems present in raw milk that might improve its shelf life. The main representative of these systems is LP. The milk enzyme LP catalyzes the oxidation of thiocyanate by hydrogen peroxide to produce antimicrobial substances. The inhibitory substances are claimed to be short-lived intermediary compounds, such as hypothiocyanate, cyanosulfurous acid, and cyanosulfuric acid. Hypothiocyanate can kill Gram-negative bacteria and inhibit Gram-positives, possibly by damaging the bacterial cytoplasmic membrane. The LP system consists of three components: LP, thiocyanate, and hydrogen peroxide. All three components are required for antimicrobial activity. The enzyme is available in milk in abundance; however, the availability of thiocyanate in milk for the proper LP preservation is not sufficient. Certain bacteria in milk produce small quantities of hydrogen peroxide, but the quantity of oxygen that can be provided is too small for the oxidation process in LP system. Stimulation of LP activity through the addition of exogenous thiocyanate and hydrogen peroxide has been investigated as a means of preserving raw milk in developing countries where ambient temperatures are high and refrigeration is not often available. For proper LP preservation, very small quantities of thiocyanate (0.00015%) and hydrogen peroxide (0.00085%) must be added to milk. These quantities are sufficient to preserve milk at tropical temperatures for about 8h, while the preserved milk can be easily kept overnight at temperatures of 15°C–20°C; at temperatures of 4°C, the milk can be kept for a few days without spoilage. Similarly, Bjorck et al. studied the effect of this system on the quality of raw milk in developing countries. Their results showed that the quality of treated milk was significantly improved over that of the untreated control. Furthermore, they demonstrated that the length of bacteriostasis is temperature dependent: 7–8 h at 30°C, 11–12 h at 25°C, 15–16 h at 20°C, and 24–26 h at 15°C. The IDF recommended the addition of hydrogen peroxide and thiocyanate at concentrations of about 10–15 ppm to activate the LP system and extend the shelf life of raw milk. 8.5.3.2 Hydrogen Peroxide Hydrogen peroxide is a preservative that has been used for a long time to preserve raw milk, under conditions where it may be difficult to cool the milk quickly. The concentrations required (300–800 ppm) are much higher than those required to activate the LP system. For milk of reasonably good quality, 0.03%–0.05% of pure hydrogen peroxide may be used to extend the keeping quality by at least 5 h, depending on a number of conditions such as temperature, catalase content of the milk, presence of heavy metals, and type of contaminating microorganisms. In one trial in Africa, addition of hydrogen peroxide increased the proportion of samples passing the 10-min resazurin quality test from 26% to 88%. Treatment levels of 0.115% completely inactivated Mycobacterium tuberculosis. Hydrogen peroxide is more effective at increased temperature. A level of 0.8% by weight combined with a temperature of 49°C–55°C for 30 min has been suggested as a substitute for pasteurization. Anaerobic and coliform bacteria are more resistant than lactic acid and anaerobic bacteria. Gram-positive bacteria are not inactivated by hydrogen peroxide to the same extent as Gram-negative bacteria Thermization (Thermalization) Often, the dairy is unable to process all milk supplies within 4 days of milking. Consequently, measures must be taken to keep the raw milk for a longer time. Dairy processors in European countries use a process called thermization to prevent psychrotrophs from growing in milk [6,39]. Thermization is a mild thermal process applied to milk that may need to be stored over a long period prior to use. Thermization has now been defined as a heat treatment that uses temperatures between 57°C and 68°C for 15s. The purpose of this treatment is to protect against microorganisms that may grow during storage of raw milk, especially Gram-negative psychrotrophic bacteria. These bacteria produce heat-resistant lipases and proteinases that may eventually cause deterioration of milk products. Thermization should be applied soon after milk treatment and it is only effective if thermized milk is kept cool (4°C). Thermization is not only a far better method of controlling the quality of dairy products than merely cooling the raw milk, but it is also more expensive. Except for the killing of many vegetative microorganisms, thermization causes almost no irreversible changes in milk. Some problems associated with thermization were reported however by Muir. One problem is associated with the contamination of thermized milk with Gram- positive cocci such as Streptococcus thermophilus as a result of a build up in the regeneration section of a commercial thermization unit. Thermization may also slightly affect the flavor and texture of cheese, but not the yield. 8.5.5 Clarification Clarification is a commonly employed pretreatment of milk prior to its storage/manufacture into other products. The shelf life of milk can be extended by clarification. Clarification may be as simple as filtration or may include high-speed centrifugation to remove microbial cells and spores. Filtration is usually carried out by pumping milk through specially woven cloth. This results in the removal of debris and all extraneous matter. Bactofugation refers to a high-speed centrifugation process carried out in a specifically designed separator called a clarifier. The purpose of bactofugation is to separate bacterial cells and spores. The process is particularly important in Europe where it has been used in the cheese industry to remove spores from cheese milk that could cause latent fermentation in some types of cheeses. Bactofugation has also been adapted to processing drinking milks where it succeeds in prolonging the shelf life of fresh, pasteurized milk by 2–5 days as a result of a reduction in the microbial population. In addition, a reduction in the microbial population induces a reduction in the pasteurization temperature and consequently the manufacture of a product with improved flavor.