17_Milk_and_whey_powder

Dairy Processing Handbook/Chapter 17 375 Milk and whey powder The method of preserving foodstuffs by drying them, and thereby depriving micro-organisms of the water necessary for their growth, has been known for centuries. According to Marco Polo’s accounts of his travels in Asia, Mongolians produced milk powder by drying milk in the sun. Skim milk powder has a maximum shelf life of about three years, while whole milk powder has a maximum shelf life of six months. This is because the fat in the powder oxidises during storage, with consequent deterioration in flavour. However, the shelf life can be extended by using appropriate packaging technology, i.e. packing the powder under an inert gas such as N2. Dairy Processing Handbook/Chapter 17376 Drying Drying means that the water in a liquid product – in this case milk concentrate – is removed, so that the product takes on a solid form. The water content of milk powder ranges between 1,5 and 5 %. Micro- organisms are unable to reproduce at such a low water content. Drying extends the shelf life of the milk, simultaneously reducing its weight and volume. This reduces the cost of transporting and storing the product. Freeze-drying has been used to produce very high quality powder. In this process, the product is deep-frozen and the water frozen in the product is evaporated under vacuum. This ensures high product quality as, among other things, no protein denaturing takes place. When drying milk at higher temperatures, the proteins are denatured to a greater or a lesser extent. Freeze-drying is not widely used for the production of milk powder because of the high energy demand. Commercial methods of drying are based on heat being supplied to the product. The water evaporates and is removed as vapour. The residue is the dried product – the milk powder. Nowadays, spray drying is the method primarily used for drying in the dairy industry. Roller drying is also used for a few special products. In spray drying, the milk is first concentrated by evaporation and then dried in a spray tower. During the first stage of drying, the excess water – in free form between the particles of the dry solids – is evaporated. In the final stage, the water in the pores and capillaries of the solid particles is evaporated. The first stage is relatively quick, whereas the last stage demands more energy and time. The product will be significantly affected by the heat if this drying is carried out in such a way that the milk particles are in contact with the hot heat transfer surfaces – as in the case of roller drying. The powder may then contain charred particles, which impair its quality. Various uses of milk powder Dried milk can be used for various applications, such as: • Recombination of milk and milk products • In the bakery industry to increase the volume of bread and improve its water-binding capacity. The bread will then remain fresh for a longer period of time • Substitute for eggs in bread and pastries • Producing milk chocolate in the chocolate industry • Producing sausages and various types of ready-cooked meals in the food industry and catering trade • In baby foods • Production of ice cream • Animal feed, calf growth accelerator Skim milk powder Skim milk powder is by far the most common type of milk powder. Table 17.1 Typical properties of spray-dried milk powder Product Whole milk Skim milk powder powder Solubility, ml 0,1 – 0,5 0,1 – 0,5 Scorced particles, ADMI Disc A A Residual moisture, max. % 3,0 4,0 Bulk density, tapped, g/ml 0,45 – 0,57 0,45 – 0,6 Total spore count, max. n/g 10 000 10 000 Dairy Processing Handbook/Chapter 17 377 Each field of application makes its own specific demands of milk powder. If the powder is to be mixed with water for direct consumption (recombination), it must be readily soluble and have the correct taste and nutritive value. For this application, the product has to be dried very carefully in a spray dryer. Some degree of caramelisation of the lactose is beneficial in chocolate production. Here, the powder can be subjected to intensive heat treatment in a roller dryer. Two types of powder are therefore distinguishable: • Spray-dried powder • Roller-dried powder Depending on the intensity of the heat treatment, milk powder is classified into categories related to the temperature/time combinations to which the skim milk has been exposed prior to and during evaporation and drying. Heat treatment denatures whey proteins, the percentage denaturated increasing with the intensity of the heat treatment. The degree of denaturation is normally expressed by the Whey Protein Nitrogen Index (WPNI), i.e. in milligrams of undenatured whey protein nitrogen per gram of powder. Information about the various categories of spray dried skim milk powder is summarised in Table 17.2. Table 17.2 Categories of spray-dried skim milk powder. Category Temp/time WPNI mg/g Extra low-heat <70 °C *) Low-heat (LH) powder 70 °C/15 s > 6,0 Medium-heat (MH) powder 85 °C/20 s 5 – 6,0 ” 90 °C/30 s 4 – 5,0 ” 95 °C/30 s 3 – 4,0 Medium high-heat (HH) 124 °C/30 s 1,5 – 2,0 High-heat (HH) appr. 135 °C/30 s <1,4 High-heat high stable (HHHS) appr. 135 °C/30 s <1,4 (from selected milk) *) Not measurable Table by Sanderson N.Z., J. Dairy Technology, 2, 35 (1967) Whole milk powder Spray dried whole milk powder is normally produced from standardised milk. After standardisation of the fat content, the milk need not be homogenised, provided that it is thoroughly agitated, without air inclusion. Homogenisation is normally carried out between evaporation and spray drying. Fat standardised milk for the production of roller dried powder is normally homogenised. Whole milk powder, unlike skim milk powder, is not categorised. Milk intended for whole milk powder is pasteurised at 80 – 85 °C in order to inactivate most of the lipolytic enzymes that would otherwise degrade the milk fat during storage. Instant-milk powder Special methods for the production of both skim milk and whole milk powder with extremely good wettability and solubility – known as instant powder – are also available. This powder is agglomerated into larger particles. A number of particles are combined to form a larger grain (agglomerate). The average grain size of the product increases. This instant powder, as it is known, dissolves instantly, even in cold water. Dairy Processing Handbook/Chapter 17378 Bulk density When powders are shipped over long distances, it is important that they have a high bulk density as the reduced volume saves on transportation costs and packaging. However, in some instances, producers may be interested in low bulk density in order to supply visibly larger amounts of powder than their competitors. Low bulk density, as influenced by agglomeration, is also an important characteristic of instant powders. Definition Bulk density is the weight of a unit volume of powder; in practice it is expressed as g/ml, g/100 ml or g/l. Production of milk powder In the production of roller-dried powder, the pre-treated milk is fed to a roller dryer after evaporation and the whole drying process takes place in one stage. In the production of spray-dried powder, the milk is first evaporated in a vacuum evaporator to a DM content of 48 – 50 % and then dried in the spray tower. Spray-dried skim milk powder is manufactured in two basic qualities: • Ordinary product • Agglomerated (instant) milk powder by various spray drying processes Following roller or spray drying, the powder is packed in cans, paper bags, laminated bags or plastic bags, depending on the quality and the requirements of the consumers. Raw material Very strict demands are made of the quality of the raw material for the production of milk powder. Since spray powder production involves vacuum evaporation, it is very important to keep heat-resistant bacteria under control so that they cannot multiply during evaporation. Bactofugation or microfiltration of the milk can therefore partly be used to remove bacteria spores from the milk, thereby improving the bacteriological quality of the end product. Milk for powder production must not be subjected to excessive thermal impact prior to evaporation and drying. This could denature whey proteins and thereby impair the solubility, aroma and flavour of the milk powder. The milk is subjected to a peroxidase test or a whey protein test to determine the degree of heat treatment. General pre-treatment of the milk In the production of skim milk powder, the milk is clarified in conjunction with fat separation. This is also the case if the fat content is adjusted in a Strict demands are made on the quality of the raw material for production of milk powder. Table 17.3 Composition of milk powder Product Whole milk Skim milk Fat, % solids content 26 – 29 max. 1,25 Protein, % solids content 25 – 27 34 – 38 Lactose, % solids content 35 – 38 48 – 56 pH 6.6 – 6.7 6.6 – 6.7 Acid value, % < 0,16 < 0,16 Total spore count, max. n/ml 10 000 10 000 Dairy Processing Handbook/Chapter 17 379 direct standardisation system. Standardised milk used for producing whole milk powder is normally homogenised. Whole milk and skim milk are pasteurised at various temperatures depending on the powder quality requirements. Roller drying In roller drying, the milk concentrate is distributed as a film on rotating, steam-heated drums or rollers. The water in the concentrated milk evaporates, and the vapour is drawn off. The high temperature of the heating surfaces denatures the proteins, solubility deteriorates and brown discolouration may occur. On the other hand, this intensive heat treatment increases the water- binding properties of the powder. This characteristic is useful in the production of ready-cooked meals, sausages and bread, cakes and pastries. Roller powders are used in the chocolate industry in particular due to their special properties. The pre-treated milk is applied to the hot roller surface as a thin film. The dried milk is scraped off the rollers and removed by means of a screw conveyor, at which point the milk is broken down into flakes. The flakes are then transferred to a grinder which is used to create the desired grain size. After that, hard and burned particles are sifted off. Depending on the desired capacity, the roller dryer is 1 – 6 m long and has a roller diameter of 0,6 – 3 m. Its performance is dependent on film thickness, roller surface temperature, roller speed and the DM content of the supplied milk. Spray drying Powder production is carried out in two phases. In the first phase, the pre- treated milk is evaporated to a DM content of 48 – 52 %. Whey is concentrated to a DM content of 58 – 62 %. In the second phase, the concentrate is turned into powder in a spray tower. Likewise, drying is also a multiple-stage process: • Atomisation of the concentrate into very fine droplets in a hot air stream • Water evaporation • Separation of the powder from the drying air Evaporation is necessary to produce high-quality powder. Without prior concentration, the powder particles will be very small and have a high air content, poor wettability and a short shelf life. The process would then also be extremely uneconomical. Falling-film tubular evaporators are generally used for concentration, which is carried out in one or more effects. See Chapter 6.5. Fig. 17.1 Principle of the trough-fed roller dryer. Milk Heating medium Air for pneumatic transportation and cooling Fig. 17.2 Conventional spray dryer (one- stage drying) with conical base chamber. 1 Drying chamber 2 Air heater 3 Milk concentrate tank 4 Feed pump 5 Atomiser 6 Main cyclone 7 Transport system cyclone 8 Air suction fans and filters 1 2 3 4 5 6 7 8 Concentrated milk Air Powder Dairy Processing Handbook/Chapter 17380 Basic drying installations Single-stage drying The simplest installation for producing a powder consists of a drying chamber with an atomisation system, the air heater, a system for collecting the finished powder from the dry air and a fan which sucks the necessary amount of air through the entire system. An installation of this type is known as single-stage dryer as the entire drying process takes place in a single unit, the drying chamber. A powder with a small grain size and a high fines content is produced here. Two-stage drying The system described above is extended by means of a fluid bed dryer. The powder leaves the drying chamber with a higher residual moisture. In the fluid bed, drying ends at relatively low temperatures, and the powder may also be cooled. In terms of energy, this installation is better than a single- stage dryer as it is possible to work with considerably lower air exit temperatures. The quality of the powder can be improved by separation of the fine powder in the fluid bed. Three-stage drying Three-stage drying is an extension of the two-stage concept, developed to achieve greater savings in process costs and to meet various product quality demands more effectively. Operating principle of spray drying In all configurations, most of the air required for drying is sucked through the installation by centrifugal fans. The air for building up the fluidized layer in an integrated and/or external fluid bed and the air for optional recirculation of the fine powder is brought into the system via fresh air ventilators. The temperature of the outgoing air is the reference variable for the residual moisture in the powder. Attempts must be made to avoid high outgoing air temperatures and thus high product temperatures which have an adverse effect on the quality of the powder, e.g. deterioration of solubility. Single-stage drying Figure 17.2 shows the arrangement of a single-stage spray drying plant. The concentrate is fed via a high-pressure pump (4) to an atomisation system (5) integrated centrically in the roof of the chamber. This system produces very small droplets, Ø 40 – 125 µm. The drying air is normally sucked through a pre-filter and fine- filter and then passes an air heater. Depending on the desired air temperature, the air is steam-heated up to 190 – 200 °C. New installations are mostly provided with an indirect air heater which can be operated by means of a combined fuel burner for natural gas or, in an emergency, with light fuel oil. An indirect heat recovery system can be provided to improve energy economy. Residual heat from the outgoing air and the flue gas from the heater are used to pre-heat the incoming air. Depending on the product, the incoming air is heated to a temperature of 150 – 210 °C. The hot air flows through a distributor which ensures that the air is travelling at a uniform speed into the drying chamber, where it is mixed with the atomised product in the straight flow. The free water evaporates immediately when the atomised product enters the drying chamber. Surface water evaporates very quickly, as does the moisture from the inside of the droplets which quickly reach the surface by capillary action. Then heat is transferred into the particles by convection. This results in the evaporation of bound water, diffusing it onto the surface of the particles.Fig. 17.3 Multi-stage dryer set up. (CPS) Dairy Processing Handbook/Chapter 17 381 Because the heat content of the hot air is continuously consumed by evaporation of the water, the product heats up to a maximum temperature of only 15 – 20 °C less than the temperature of the air when it exits the drying chamber; under normal conditions 60 – 80 °C. The evaporation of the water from the droplets leads to a considerable reduction in weight, volume and diameter. Under ideal drying conditions, weight will decrease by about 50 % and the volume by about 40 %. The diameter is reduced to 75 % of the droplet size after leaving the atomiser, Figure 17.4. During the drying process, the powder settles in the bottom cone of the chamber and is discharged from the system. It is conveyed to a silo or packing station by a pneumatic conveyor which uses cold air to cool the hot powder. The powder is then separated from the transport air by means of a cyclone. Small, light particles may be sucked out of the drying chamber, mixed in with the air. This powder is separated in one or more cyclones (6, 7 in Figure 17.2) and fed to the main flow of powder. Atomisation The more finely the product is atomised, the larger their specific area will be and the more effective the drying process. One litre of milk in a spherical shape has a surface area of about 0,05 m2. If this quantity of milk is atomised in the spray tower, each of the small droplets will have a surface area of 0,05 – 0,15 mm2, i.e. atomising increases the specific area by a factor of about 700. The type of atomisation depends on the product, the desired particle size and the properties required of the dried product. These may include texture, grain size, bulk density, solubility, wettability and density. There is an important functional differentiate between nozzle and centrifugal atomisation. A stationary nozzle which sprays the milk in the same direction as the flow of air is shown in Figure 17.5. A centrifugal disc for atomisation is shown in Figure 17.6. The pressure at the nozzle determines the particle size. At high pressures, up to 30 MPa, (300 bar), the powder will be very fine and have a high density. At low pressures, 5 – 20 MPa, (50 – 200 bar), larger particles will be formed and the fines content will be lower. The pressure is built up by means of multi-plunger high-pressure pumps. These are mostly homogenisers, which are needed for many products and can also operate as high-pressure pumps with by-passed homogenisation devices. The centrifugal atomiser consists of an electric drive which rotates a disc with a number of horizontal passages. The product is fed into the middle of the disc and forced through the passages at high speed by centrifugal force. The discs rotate at speeds of 5 000 to 25 000 rpm depending on their diameter. Peripheral speeds of between 100 – 200 m/s are achieved. The flow of product is atomised into very fine droplets upon its exit from the passage due to the high speed. The size of the droplets – and thus also the grain size of the powder – can be influenced directly by changing the atomiser speed. A centrifugal pump is normally sufficient to feed this type of atomiser. Essentially, a larger grain size can be achieved by means of nozzle atomisation, 150 – 300 µm as compared to 40 – 150 µm with centrifugal atomisation. However, centrifugal atomisation is straightforward to operate and not sensitive to variations in product viscosity and quantity supplied. Two-stage drying Two-stage drying in the fluid bed dryer, which is divided into a number of zones, ensures that the desired residual moisture is achieved and that the powder is cooled. Final drying in the integrated fluid bed dryer ensures that the desired residual moisture is achieved. After final drying the powder is cooled in a pneumatic cooling and conveying duct. The installations can be operated with both nozzle and centrifugal atomisers. 100 90 80 70 60 50 40 30 20 10 0 50 60 70 80 90 10045 D W V 4 % H20 Percentage of initial value Droplet solid content (%) D = Diameter W = Weight V = Volume Fig. 17.4 Weight, volume and diameter decrease of droplet under ideal drying conditions down to 4 % H2O. Fig. 17.5 Stationary nozzles for atomis- ing the milk in a spray drying chamber. Fig. 17.6 Rotating disc for atomising milk in the spray drying chamber. Dairy Processing Handbook/Chapter 17382 The products consist mainly of individual particles but have a slightly lower fines content. The solubility index and content of air included are smaller in the case of powders dried using the two-stage method on account of the lower thermal impact overall, although the bulk density is higher. Air and the fine powder are transported out of