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POWDERED MILK | Characteristics of Milk Powders
POWDERED MILK/Characteristics of Milk Powders
Characteristics of Milk Powders M A Augustin, P T Clarke and H Craven, Food Science Australia, Weribee, Victoria, Australia Copyright 2003, Elsevier Science Ltd. All Rights Reserved.
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Milk powders are used by consumers as a substitute for fresh milk and as ingredients for the manufacture of a range of processed food products. In order to be acceptable to consumers and users of ingredients, it is essential that milk powders are of a good quality. Milk powders are manufactured to meet certain specifications and standards for composition. These have been developed for milk powders by authorities such as the American Dairy Products Institute, the International Dairy Federation, the Food and Agricultural Organization of the United Nations and national food authorities in individual countries. In addition, a range of other technical specifications have been developed for the characterization of milk powders to ensure that they have the required functional performance in specific target applications. Milk powders may be similar in composition but have different functional properties. There are many types of milk powders in the market place. This article focuses on the characteristics of skim and full-cream milk powders, which are the major types of milk powders produced. The microbiological quality, physical and chemical attributes of these milk powders, and their functional properties are discussed. Aspects of deteriorative changes that may occur in milk powders during transport and distribution that have an impact on the sensory properties of powders and their performance as food ingredients are included. The production, composition, and applications of various types of milk powders have been discussed elsewhere. (See Powdered Milk: Milk Powders in the Marketplace.)
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Microbiological Aspects Standards for Quality and Safety
Milk powder is a microbiologically stable product. It has a water activity of 0.3–0.4, which is too low to support the growth of microorganisms. However, after milk powder has been reconstituted, it is susceptible to microbial growth and spoilage in a similar manner to pasteurized milk. Provided milk powder is protected from moisture contamination before use, the numbers of microorganisms present generally decrease during storage, although the numbers of spores may remain constant. Although milk powder does not support the growth of microorganisms, the microbiological content is an important consideration in the subsequent use of the powder. For this reason, government bodies and customer groups have developed microbiological limits or specifications that apply to certain groups of microorganisms that may be present in milk powder. These specifications may relate to expectations of raw milk quality, hygiene during manufacture, microbial safety, or compatibility with the intended use of the milk powder. Common end-product standards relate to the total number of bacteria (mesophilic aerobes), coliforms, Salmonella, and Staphylococcus aureus. Criteria may also be applied for Bacillus cereus, Listeria, thermophiles, Enterobacteriaceae and spore-forming bacteria. The standards developed by the International Dairy Federation, for example, are shown in Table 1. Many countries have either adopted these standards or developed their own local specifications based on the principles of the International Commission on Microbiological Specifications for Foods (ICMSF). The microbiological count of milk powder is influenced by both the numbers and types of microorganisms in the raw milk and the processing conditions under which the milk powder is produced. In powders subjected to a high heat treatment, the microorganisms present will be predominantly spore-formers, belonging to the genus Bacillus. When heat treatment is less severe, vegetative cells
Table 1 Microbiological specifications for milk powder, as recommended by the International Dairy Federation Criteriaa
Total count (per gram)
Salmonella (per 25 g)
Coliforms (per gram)
Staphylococcus aureus (per gram)
m M n c
50 000 200 000 5 2
0 na 15 0
10 100 5 1
10 100 5 1
a For a production batch, n ¼ number of samples that must be tested, c ¼ number of samples that may exceed the microbiological limit specified as m, and M is the maximum allowable microbiological limit specified for any of the samples examined. na ¼ not applicable.
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of thermoduric bacteria will be present, with their proportion to spore-formers decreasing with the intensity of the heat treatment applied. Vegetative cells of pathogenic bacteria and Gram-negative milk spoilage bacteria are destroyed during the heat treatment. Coliforms, Salmonella, and other Enterobacteriaceae are killed when the milk is heated prior to evaporation; however, they may contaminate milk powder if conditions are not sufficiently hygienic during drying. These bacteria can enter the dryer through the intake air from the factory environment, or the equipment used to dry or transport the milk powder. Cracks in dryers have been shown to be a particularly significant source of Salmonella. Here, Salmonella are harbored in the insulation material. Although dryers operate at a high temperature, the concentrated milk offers protection to the bacteria, and they will survive heating at dryer air inlet and outlet temperatures. Salmonella spp. are significant pathogens, and several notable outbreaks of illness have been attributed to the presence of this organism in milk powder. Staphylococcus aureus is significant, as certain strains can produce a heat-stable toxin that is not destroyed during powder manufacture. Although Staphylococcus aureus is common in raw milk, it does not normally grow to produce toxin unless the milk is stored at a high temperature prior to processing. The risk of toxin production increases with temperature and storage time. Although the bacteria will be killed during the process, the toxin remains and can be detected only through specific tests. Large outbreaks of illness have been attributed to the presence of Staphylococcus aureus toxin in milk powder. Another bacterium of potential significance in milk powder is Bacillus cereus. This is commonly found in milk, and its spores may survive heat processing. Specialty powders such as infant formula often have specifications for this bacterium, owing to the potential risk of the growth of this organism in warmed milk and sensitivity of the target group of consumers. Sometimes, yeasts and molds or their toxins, and Listeria are included in powder specifications. Yeasts and molds may be significant spoilage organisms if powder is contaminated with moisture, and Listeria may contaminate powder from the factory environment, especially if the environment is not kept dry. In the milk powder process, milk is subjected to heat whilst concentrated under vacuum. Such conditions are conductive to the growth of thermophilic Bacillus species that may form biofilms in the process lines. When this occurs, the product may be contaminated with thermophiles that can reach more than 106 per gram in long production runs. Thermophiles may sporulate in the process, leading to the presence of large numbers of thermophilic spores in the
powder. The spores can be extremely heat-resistant and may not be completely destroyed when the reconstituted powder is used in ultrahigh-temperature (UHT) processes. They are significant because they may cause sterility failures or spoilage in other heated products. If not properly cleaned from the plant between production runs, residues of thermophiles will seed subsequent batches of milk powder. Although milk powder is a microbiologically stable product, the microbial quality of the raw milk may influence the shelf stability of the powder. Some bacteria present in raw milk, particularly Pseudomonas species, produce heat-stable spoilage enzymes, including proteases and lipases, that remain active in milk powder over many months. Experience has shown that lipase can act in full-cream milk powder to degrade milk fat to cause rancidity and other objectionable flavors. Proteases retain activity in milk powder and degrade milk proteins to cause objectionable flavors after the milk powder has been reconstituted. Proteases and lipases may be particularly detrimental in recombined milk products, or if milk powder is used to prepare UHT milk. Here, very low levels of protease and lipase may cause spoilage during long storage periods. (See Bacillus: Occurrence; Detection; Food Poisoning; Biofilms; Listeria: Properties and Occurrence; Pasteurization: Principles; Salmonella: Properties and Occurrence; Spoilage: Bacterial Spoilage; Fungi in Food – An Overview; Molds in Spoilage; Yeasts in Spoilage; Staphylococcus: Properties and Occurrence.)
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Control of Microorganisms
The manufacture of microbiologically sound milk powder is dependent upon processing good-quality raw milk under hygienic conditions. To ensure the supply of good-quality milk, farm milk should be tested regularly for microbial quality. Many countries now use the total count test to monitor levels of bacteria in farm supplies. Thermoduric counts are sometimes used also. Raw milk ideally should be stored at less than 5 C and used within 72 h of collection to minimize bacterial growth. The pasteurization of milk is important and is normally identified as a critical control point. An example of process criteria for pasteurization would be heating of the milk for at least 15 s at 72 C or 5 s at 80 C. Within the factory, application of good manufacturing practice is essential to minimize the risk of milk powder contamination with undesirable types or levels of microorganisms. To achieve this, consideration must be given to the design of the premises and control of staff or vehicular movement to separate raw materials from drying areas. Manufacturing
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equipment and the processing environment must be maintained, cleaned, and sanitized to ensure that microbial build-up and spread are prevented. Staff must be trained in practices to maintain high standards of hygiene. A supply of good-quality water and air for the process is also essential. Many factories now have ongoing monitoring systems for Salmonella and Listeria in place. If these bacteria are detected in the processing environment, special clean-up regimes and extra product testing are implemented. The modern approach to ensuring that milk powder is microbiologically safe involves preventative management to ensure manufacture under appropriate conditions of hygiene. Many factories now either have in place, or are moving towards, the hazard analysis critical control point (HACCP) system. Although end-product testing is still used to verify compliance and to detect gross process failures, it cannot be relied upon to ensure the safety of a batch of product. Testing can be labor-intensive and timeconsuming, taking up to 7 days to obtain final results. To overcome these problems, samples may be composited and rapid techniques for detection of pathogens based on ELISA or DNA methods applied. These methods have advanced efficiencies in testing, and product can now be cleared in 24–48 h. (See Hazard Analysis Critical Control Point; Quality Assurance and Quality Control.)
Physical Properties 0017
The physical properties of milk powders are governed by process variables, the type of dryer, and the composition of the milk. The physical properties of milk powders play an important role in their use as food ingredients. Their ability to be readily incorporated into products and to perform specific functions in a food formulation can be influenced by many physical properties. There are many physical attributes that must be taken into account when either evaluating a current product, setting specifications for new products or designing or modifying a drying system. Tighter and more demanding specifications have meant that powders are now often manufactured on specialist dryers designed specifically to produce the best possible product of defined specification. Moisture
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The final moisture content is critical for several reasons and is therefore defined in all powder specifications. It can affect functionality and microbiological quality, and is an economic consideration in the manufacture of powders. There are several factors during manufacture that can influence the moisture content of powders. These include the characteristics
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of the concentrate fed into the dryer, the type of atomization used, and the operating conditions during drying. Insolubility Index
The insolubility index of a powder is a measure of the degree to which it can be readily solubilized in water prior to use. It is related to the amount of sediment obtained under defined conditions of mixing milk powders. The main reason for loss of solubility is the temperature of the particles during the primary stage of the drying process where the majority of the moisture is removed. During this stage, an impermeable crust can form on the particle surface that severely restricts water removal, leading to the production of case-hardened particles and subsequent loss of solubility.
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Bulk Density and Particle Density
Bulk density is the amount of powder by weight that is present in a defined volume. It is usually expressed as g ml1 and is obtained by measuring the volume of a fixed weight of powder after it has been tapped for a defined number of times. A high bulk density is very important in packaging and transportation, and is desirable as it can significantly reduce costs. The bulk density is influenced by a range of factors. These include the amount of air entrapped in the powder particles (occluded air), the overall density of the particle (determined by the composition), the air between the individual powder particles (interstitial air), the particle size distribution and the particle shape. The bulk density of powders is influenced by dryer design and configuration (Tables 2 and 3). Particle density is the density of the solids (determined by the composition), which determines the particle density, together with the amount of occluded air.
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Particle Size Distribution
The individual particles produced during drying can vary greatly in size. The distribution of particle size then can be further altered by the degree of agglomeration or after grinding. An indication of the range of particle sizes obtained from different dryer configurations is given in Table 4.
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Interstitial Air and Occluded Air
Interstitial air is the amount of air that exists between particles or agglomerates as well as the air inside porous agglomerates. The sphericity of the particles, the particle size distribution, and the degree of agglomeration determine the amount of interstitial air. To obtain minimum interstitial air, the particles need to be smooth, have a range of particle sizes, and be in compact agglomerates.
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Table 2 Ability of various spray dryers to manufacture nonagglomerated and agglomerated skim milk powder with low or high bulk densities Type of drying process
Chamber configuration
Single stage
Conventional Tall form Conventional Tall form Conventional Tall form Compact Compact Multistage Integrated belt
Two stage
Three stage
Postprimary treatment
None None Cooling bed Cooling bed External bed External bed Integrated bed Integrated bed þ external bed Integrated bed þ external bed Integrated belt
Atomizer type
Rotary/nozzle Nozzle Rotary/nozzle Nozzle Rotary/nozzle Nozzle Rotary/nozzle Rotary/nozzle Rotary/nozzle Nozzle
Nonagglomerated skimmilk powder
Agglomerated skimmilk powder
Low BDa
High BDa
Low BDa
High BDa
Yes Yes Yes Yes Yes Yes Ideal Yes Yes Ideal
Yes Yes Yes Ideal Yes Ideal Ideal Yes Yes No
No No No No No No No No Ideal Yes
No No Yes Yes Yes Yes No Yes Ideal Yes
a BD ¼ bulk density; nonagglomerated skim milk powder: low bulk density 0.72 g ml1; high bulk density 0.72 g ml1; agglomerated skim milk powder: low bulk density 0.30–0.50 g ml1; high bulk density 0.45–0.55 g ml1. Adapted from Pisecky J (1997) Handbook of Milk Powder Manufacture, p. 79. Copenhagen: Niro A/S.
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Table 3 Ability of various spray dryers to manufacture nonagglomerated and agglomerated full-cream milk powder with low or high bulk densities Type of drying process
Single stage
Two stage
Three stage
Chamber configuration
Conventional Tall form Conventional Tall form Conventional Tall form Compact Compact Multistage Integrated belt
Postprimary treatment
None None Cooling Bed Cooling Bed External bed External bed Integrated bed Integrated bed þ external bed Integrated bed þ external bed Integrated belt
Atomizer type
Rotary/nozzle Nozzle Rotary/nozzle Nozzle Rotary/nozzle Nozzle Rotary/nozzle Rotary/nozzle Rotary/nozzle Nozzle
Nonagglomerated full-creammilk powder
Agglomerated full-creammilk powder
Low BDa
High BDa
Low BDa
High BDa
Yes Yes Yes Yes Yes Yes No Yes Ideal Yes
Yes Yes Yes Yes Yes No Ideal Yes Yes No
No No No No Yes Ideal Ideal Yes No No
No No Yes Yes No No No No Ideal Yes
a BD ¼ bulk density; nonagglomerated full-cream milk powder: low bulk density 0.63 g ml1; high bulk density 0.63 g ml1; agglomerated full-cream milk powder: low bulk density 0.30–0.50 g ml1; high bulk density 0.45–0.55 g ml1. Adapted from Pisecky J (1997) Handbook of Milk Powder Manufacture, p. 79. Copenhagen: Niro A/S.
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300–2000
Occluded air is the amount of air entrapped within the powder particles. It is affected by the preheat treatment of the original milk, with a higher pretreatment of milk resulting in less occluded air, and the amount of air incorporated in the concentrate. Higher total solids generally result in lower occluded air. Powders atomized by a nozzle contain less air than rotary atomized powders, despite improvements to the modern rotary atomizers. Gentle drying also reduces the level of occluded air, and therefore, the use of multistage dryers is recommended for the production of powders with low occluded air.
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Flowability
Table 4 Mean particle size obtained from dryers of different configuration Powder characteristics
Dryer configuration
Individual particles
Concurrent with pneumatic conveying Tall form – tower Roller dryer Mixed flow with integrated fluid bed Concurrent with integrated fluid bed Concurrent spray dryer with integrated belt Mixed flow with integrated fluid bed
Flakes Loose agglomerate – open structure Compact agglomerate – porous structure
Particle size (mm) 20–200 30–250 200–5000 100–400 100–200
From personal communication (E. Refstrup), Niro A/S, Denmark.
With the ever-increasing diversity of use of milk powders today, the need for properties such as
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flowability is increasing. Powders are used in applications ranging from dispensing machines through to the large-scale recombining operations that utilize mechanical handling and dosing. For both agglomerated and nonagglomerated powders, a better flowability can be obtained by producing larger powder particles with smooth and rounded particle surfaces within a narrow particle size distribution. Flowability is also influenced by other factors such as total fat in the powder and the amount of ‘free fat.’
include the mechanical stability of the powder, which influences the degree of agglomeration breakdown during transport and storage, hygroscopicity, which is related to the degree of water attraction a powder exhibits, and cakiness, an attribute that is a measure of the extent to which a powder adheres to itself, especially under compression. (See Agglomeration; Drying: Spray Drying; Rheological Properties of Food Materials.)
‘Free Fat’
Chemical Characteristics
‘Free fat’ in powder is defined as the fat fraction that is extractable by organic solvents under specific conditions of solvent type, time, and temperature of extraction. In most instances, ‘free fat’ is considered a defect. The exception is where ‘free fat’ is required for a specific application, e.g., chocolate manufacture. One of the most critical influences of ‘free fat’ is the moisture content of the powder. If the moisture is too low (< 2.5%), ‘free fat’ increases and then decreases as the moisture content is raised from 2.5 to 4–5% but increases again if the moisture content is > 6–7%.
The chemical properties of milk powders are determined by the composition of the milk and the heat treatment applied during powder manufacture.
Instant Properties 0026
Very fine powder particles are difficult to handle and have poor reconstitution properties. Agglomeration of powders allows water to permeate the powder particles more readily, breaking up the agglomerate and allowing the individual powder particles to dissolve. Instant milk powder is highly soluble and designed to reconstitute completely in water at both hot and cold temperatures. The other properties required in instant powders are wettability and dispersibility. The wettability of a powder is measured by determining the time taken for a given amount of powder to pass through the surface of water. Wettability may be enhanced by lecithination. The dispersibility of a powder is a measure of how completely a powder dissolves under controlled conditions. Other tests carried out on milk powders related to their instant properties include slowly dispersible particles, coffee test, white flecks number (minute particles that are seen on the surface of reconstituted milk), and the sludge test. Color
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The color of a powder is determined by composition, preheat treatment, drying conditions, and particle size distribution. Scorched particles can be a visual defect that will often show up as deposits on the bottom of mixing vats and in strainers. Other Properties
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Apart from the properties described above, there are others that influence a powder’s acceptability. These
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Chemical Composition
Skim and full-cream milk powders are obtained by dehydration of skim milk and full-cream milk to 4% moisture. Full-cream milk is usually standardized to a fat:solids-nonfat ratio of 1:2.67 to meet the 26% legal minimum fat content for this powder. The protein content of skim milk powders may be standardized also. Variations in milk composition owing to factors such as cow breed, feed, stage of lactation, and season are reflected in the composition of milk powders. The American Dairy Products Institute standards for skim and full-cream milk powder compositions are as follows: skim milk powder should have a maximum fat content of 1.25% and a maximum water content of 4.0%, whereas full-cream milk powder should have a minimum fat content of 26% and a maximum water content of 4.0%. Control of the moisture content of milk powders to a maximum of 4% is essential for good shelf-life stability. Table 5 shows the range of values observed in milk powder. Table 5 Composition of milk powdersa
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Constituent
Skim milk powder
Full-cream milk powder
Moisture (g per 100 g) Fat (g per 100 g) Crude protein (g per 100 g) Lactose (g per 100 g) Citric acid (g per 100 g) Ash (g per 100 g) Sodium (mg per 100 g) Potassium (mg per 100 g) Calcium (mg per 100 g) Magnesium (mg per 100 g) Phosphorus (mg per 100 g) Chloride (mg per 100 g)
3–5 0.7–1.3 35–37 49–52 1.8–2.1 7.5–8.0 400–550 1550–1750 1200–1300 110–140 950–1050 1100
2–4 25–28 25–27 36–38 1.3–1.4 6.0–7.0 370–420 1150–1350 900–1000 85–100 700–770 750–800
a Adapted from Walstra P and Jenness R (1984) Dairy Chemistry and Physics, pp. 418–419. New York: John Wiley.
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Another important indicator of milk powder quality is the titratable acidity of the reconstituted powder. This is an indicator of the microbiological quality of the milk. The American Dairy Products Institute sets a maximum of 0.15% for titratable acidity of skim milk powder. Heat-treatment Classification
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The characteristics of milk powder can be influenced by the heat treatment received by the milk powder during manufacture. The time and temperature of the preheat treatment affects the level of whey-protein denaturation. The whey-protein nitrogen index, which is a measure of the undenatured whey-protein nitrogen in the powder and was developed by the American Dairy Products Institute, is commonly used to classify powders into low-heat, medium-heat and high-heat milk powders. Typical preheat treatments used for the manufacture of these powders are listed in Table 6. As the composition of milk, including the initial level of whey proteins in milk, can vary with season, the same heat treatment can result in a different whey-protein nitrogen index. Other methods for heat classification of milk powders, such as the heat number, cystine number, and thiol number also may be used as a measure of the heat treatment given to the milk during powder manufacture. (See Heat Treatment: Ultra-high Temperature (UHT) Treatments.)
Functional Properties of Milk Powders 0033
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When milk powders are used as ingredients in food applications, they contribute to the physical attributes of the food. The ability of milk powders to impart desirable properties to food is related primarily to functional properties of milk-protein components in the powders. These functional properties include solubility, hydration, heat stability, viscosity, gelling, foaming, and emulsifying. In milk powders,
Table 6 Heat classification of skim milk powder Heat class
Low heat Medium heat High heat a
Whey protein nitrogenindexa (milligrams of undenatured whey protein Nper gram of powder)
Preheat treatment of milkb
Not less than 6 1.51–5.99 Not more than 1.5
72 C for 15 s 75 C for 3 min 90 C for 10 min 120 C for 2 min
Solubility
Solubility is a fundamental functional property that is a prerequisite for most other desired functionalities. The solubility of milk powders is dependent on pH. Proteins have a minimum solubility at the isoelectric pH, and solubility is increased on the acid and alkaline side of this pH. Caseins, the major proteins in milk, are least soluble at pH 4.6.
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Hydration
Hydration is related to the ability of the milk proteins to bind or entrap water. Caseins hold about 3.3 g of water per gram, whereas undernatured whey proteins hold 0.4 g of water per gram. Heat denaturation of whey proteins increases the water holding to 2.5 g of water per gram. Milk powder contains other components, such as lactose, that bind water in addition to the protein. Skim milk powders have a water sorption of 0.96–1.28 g water per gram, depending on the conditions used during powder manufacture.
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Heat Stability
Heat stability is an important property in certain applications such as the manufacture of recombined evaporated milk. Single-strength milks made from low-, medium- or high-heat milk powders have a similar heat stability to fresh milk. They are heatstable at the pH of milk (pH 6.7), being able to withstand coagulation for up to *20 min at 140 C. However, for adequate heat stability of evaporated milks under sterilization conditions (e.g., 120 C for 12 min), high-heat milk powders are necessary. Heat stability is affected by the pH of the milk, mineral content, and other components in the milk (e.g., lecithin, urea).
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Viscosity
From American Dairy Products Institute (1990) Standards for Grades of Dry Milks including Methods of Analysis, Bulletin 916. A range of other preheating conditions may be used to achieve a desired whey protein nitrogen index. b
the functional properties of the milk proteins may be modulated by heat, ions, and other components. Heat treatment of milk prior to concentration and drying is the most common method used to alter the functional properties of milk powders. Milk powders with the same composition given different preheat treatments prior to concentration and drying have different functional attributes when used as ingredients.
The viscosity of milks reconstituted from milk powders is dependent on their state of dispersion, concentration of solids, and temperature. Increasing the concentration of milk solids increases the viscosity. Decreasing the temperature increases the viscosity, but heating milk to a temperature that results in denaturation of whey proteins also increases the viscosity.
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POWDERED MILK/Characteristics of Milk Powders Gelation 0038
Milks reconstituted from milk powders have the ability to form gels under similar conditions to those required for the formation of gels from fresh milk, i.e., by rennet action for formation of rennet gels and by acidification of milk under quiescent conditions. Foaming and Emulsifying
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Milk powders can be used in applications where foaming and emulsifying properties are required. The ability of milk proteins in the milk powders to stabilize foams and emulsions may be exploited when these properties are required. (See Aerated Foods; Emulsifiers: Uses in Processed Foods; Mixing of Powders.)
Functional Requirements of Milk Powders in Major Food Applications 0040
For milk powders to have the desired performance in food applications, the functional characteristics of the powders have to be matched to the application. This requires an understanding of the required functional properties of the milk powder ingredients in the target application. Milk Powders for Recombined Dairy Products
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A significant amount of milk powder is used in the manufacture of reconstituted and recombined dairy products. In these applications, the milk powders are combined with water and milkfat to reestablish the fat:solids-nonfat:water ratio of milk or other dairy products. Some of the major applications of milk powders in the recombination industry are for the preparation of pasteurized fluid milk, UHT milk, cream, evaporated milk, sweetened condensed milk, yogurt and cultured dairy products, recombined cheese, and icecream. Different functionalities of the milk powder ingredients are needed in these various recombined dairy products. Table 7 lists the major functional requirements of milk powders for recombined dairy products. Pasteurized milks and UHT milks These products have a similar composition to fresh milk. For pasteurized milks, low-heat or medium-heat powders are used to obtain a flavor similar to milk and to minimize heat-induced flavors. In the case of UHT milks, any type of powder can be used, as single-strength milks made from low-, medium-, or high-heat powders are stable to UHT conditions. Evaporated milks It is essential to use high-heat powders for this application to obtain evaporated
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Table 7 Functional requirements of milk powders in recombined dairy products and selected processed foods Product
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Functionalproperties Heat treatment required inmilk of milk powder powder
Pasteurized milk Good flavor Emulsifying UHT milk Good flavor Heat stability Emulsifying Cream Good flavor Emulsifying Evaporated milk Heat stability Viscosity Sweetened Viscosity condensed milk Yogurt Water-binding Viscosity Gelling Cheese Rennetability Icecream Foaming/whipping Emulsifying Confectionery Water-binding Foaming/whipping Emulsifying Heat stability Bakery Water-binding Foaming/whipping Emulsifying Gelling
Low–medium heat Low–medium–high heat
Low–medium heat High heat Low–medium heat
Low heata
Low heat Low–medium–high heat High heat
High heat
a If a low-heat milk powder is used, the yogurt milk has to be given a highheat treatment during yogurt manufacture. Alternatively, a high-heat milk powder may be used, in which case, the yogurt milk requires only a lowheat treatment to pasteurize the milk during yogurt manufacture.
milk with the desired viscosity. A high-preheat treatment improves the heat stability of a recombined milk concentrate (typically 26% total solids; 18% solidsnonfat: 8% fat) to in-can sterilization conditions used in its manufacture. Additionally, high-heat powders are screened using heat-stability tests to ensure that they withstand sterilization without excessive thickening or coagulation. Sweetened condensed milk This is a traditional dairy product containing 74% total solids (20% milk solids nonfat: 8% fat: 46% sucrose). The most important physical attribute of this product is its viscosity. Low- and medium-heat powders are used in this application. Milk powders given a high-heat treatment (e.g., 85 C for 30 min) cannot be easily processed, because the high viscosity of concentrates made from these powders also results in rapid age thickening during storage of the product. There are a number of viscosity tests that may be used as indicators of suitability of powders for sweetened condensed milk manufacture.
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Yogurt Milk powders may be used as a partial or total replacement for fresh milk in this application. Viscosity development, gelling, and good waterbinding properties are necessary for the production of high-quality yogurts. These properties are obtained in yogurt by preheating the yogurt milk at a temperature that causes significant denaturation of whey proteins (e.g., 90 C for 10 min). Low-heat milk powder may be used if a high-heat treatment is given during yogurt manufacture. If a high-heat milk powder is used, the yogurt milk requires only a low-heat pasteurization treatment during yogurt manufacture. Cheese Only low-heat milk powders are suitable for recombined cheese manufacture. This ensures good rennetability of the reconstituted milk. With a highheat treatment of milk, there is association of the denatured whey proteins with the casein, which hinders the reaction of the rennet. Ice cream Milk powders contribute to the flavor and texture of ice cream. The milk powder aids in the emulsification of the ice cream mix and has a role in the development of an aerated matrix. (See Condensed Milk; Evaporation: Basic Principles; Uses in the Food Industry; Recombined and Filled Milks.) Milk Powders for Selected Food Applications
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Milk powders are used as functional ingredients in a number of processed foods. Chocolate and confectionery products Milk powders contribute to the flavor, color, and texture development in chocolate and confectionery applications. The emulsifying properties of the milk proteins influence the miscibility of the ingredients used in chocolate and confectionery, hence influencing flow properties and texture. In confectionery products such as toffee, good water-binding properties of milk proteins contribute to the texture of these products. The Maillard reaction, which is the reaction of the amino groups of the proteins with reducing sugars in the formulation, is responsible for color development and for the production of caramelized flavors; the lactose in milk powders participates in the Maillard browning reactions. Bakery products High-heat milk powders are useful in bakery applications. In addition to enhancing the nutritive value of cereal-based baked goods, milk powders contribute to the texture and flavor of these products. Their emulsification and foam-stabilization properties and their ability to participate in the
Maillard browning reaction are important requirements in bakery applications. Other applications The functional properties of milk powders also make them useful in a number of other applications, such as processed meat products, soups, gravies, and dips.
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Characteristics of Stored Milk Powders The characteristics of milk powder are dependent on the quality and composition of the raw milk and the manufacturing process used during its manufacture. However, even if milk powders are manufactured to meet the desired standards and specifications, changes in the properties of milk powders may occur during storage and distribution. The composition of the powder, the type of packaging material used, and the conditions of handling and storage influence the shelf-life of the powder. Deterioration of milk powders resulting from Maillard browning, lactose crystallization, and oxidation of fat may lead to flavor and physical defects in the powder. It may also affect the functionality of the milk powder when it is used in a food product. Some of the changes that may occur during storage include the development of a brown color, a reduction in pH, reduced solubility, development of off-flavors, and reduced heat stability of powders. See also: Biofilms; Condensed Milk; Emulsifiers: Uses in Processed Foods; Evaporation: Basic Principles; Uses in the Food Industry; Heat Treatment: Ultra-high Temperature (UHT) Treatments; Mixing of Powders; Pasteurization: Principles; Rheological Properties of Food Materials; Powdered Milk: Milk Powders in the Marketplace; Quality Assurance and Quality Control; Recombined and Filled Milks
Further Reading American Dairy Products Institute (1990) Standards for Grades of Dry Milks Including Methods of Analysis, Bulletin 916, rev ed. Chicago, IL: ADPI. Early R (1998) Milk concentrates and milk powders. In: Early R (ed.) The Technology of Dairy Products, 2nd edn, pp. 228–300. London: Blackie Academic & Professional. International Commission on Microbiological Specifications for Foods (1998) Microorganisms in Foods 6, Microbial Ecology of Food Commodities. London: Blackie Academic & Professional. International Dairy Federation (1984) General Code of Hygienic Practice for the Dairy Industry and Advisory Microbiological Criteria for Dried Milk, Edible Rennet
0052
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POWER SUPPLIES/Use of Electricity in Food Technology Casein and Food Grade Whey Powders, Bulletin No. 178. Brussels: International Dairy Federation. International Dairy Federation (1990) Recombination of Milk and Milk Products, Special Issue No. 9001. Brussels: International Dairy Federation. International Dairy Federation (1991) IDF Recommendations for the Hygienic Manufacture of Spray Dried Milk Powders, Bulletin No. 267. Brussels: International Dairy Federation. International Dairy Federation (1999) 3rd International Symposium on Recombined Milk and Milk Products, Special Issue No. 9902. Brussels: International Dairy Federation. Knipschildt ME and Andersen GG (1994) Drying of milk and milk products. In: Robinson KR (ed.) Modern Dairy Technology, 2nd edn., vol. 1, pp. 159–254. London: Chapman & Hall.
4711
Masters K (1997) Spray dryers. In: Baker CGJ (ed.) Industrial Drying of Foods, pp. 90–112. London: Blackie Academic & Professional. Mettler AE (1994) Present day requirements for effective pathogen control in spray dried milk powder production. Journal of the Society of Dairy Technology 47: 95–107. Pisecky J (1986) Standards, specifications, and test methods for dry milk products. In: MacCarthy D (ed.) Concentration and Drying of Foods: Proceedings of the Kellogg Foundation 2nd International Food Research Symposium (Cork: 1985), pp. 203–220. London: Elsevier Applied Science. Pisecky J (1997) Handbook of Milk Powder Manufacture. Copenhagen: Niro A/S. Walstra P and Jenness R (1984) Dairy Chemistry and Physics, pp. 418–419. New York: John Wiley.
POWER SUPPLIES Use of Electricity in Food Technology D Graham, R and D Enterprises, Walnut Creek, CA, USA Copyright 2003, Elsevier Science Ltd. All Rights Reserved.
Introduction 0001
0002
0003
The food industry in the USA consumed 17.4 billion kcal in 1999. Fifty-eight percent of total food industry energy costs are from electricity. The industry purchases 64.0 billion kWh of electricity annually at a cost of $3.36 billion. Purchased energy is 1.3% of the value of product shipments for the food industry compared with 1.7% for the total US industry. Major uses of electricity include freezer and refrigeration compressors, conveyors, air handling, pumping, lighting, process controllers and monitors, and packaging forming and sealing. Natural gas is the primary source of thermal energy for ovens, fryers, dryers, evaporators, and boilers, with relative costs being the main reason. Closely related uses of electricity in agricultural food production include crop irrigation, pest control, produce disinfection, grain harvesting and storage, weed reduction, livestock waste management, and fish farming. Irradiation, ohmic heating, microwave processing, ozonation, freeze concentration, nonthermal pasteurization, and the use of heat pumps are examples of relatively new electrotechnologies that the food industry may apply increasingly in the future. Application of
these technologies provides opportunities for food processors to improve operating efficiencies and helps insure the quality and safety of processed food products. Electricity Use by Food Industry Sectors
Food energy costs rose steadily over the past two decades. In 1996, purchased electricity comprised over half of the total energy used in food processing. Electrical consumption by grain milling, meat processing, preserved fruits and vegetables accounts for 23, 19, and 13%, respectively, of the total food industry use of electricity. Some food processors operate cogeneration facilities using surplus heat to produce electricity, but approximately 92% of total electricity used by the food industry in the USA is purchased from utility companies. Approximately 87% of the electricity is used by motor-driven equipment such as compressors, pumps, mixers, grinders, fans, etc. Table 1 compares the cost of electricity with total energy costs in food processing. Electricity consumption has increased approximately 3% annually from 48.9 billion kWh in 1986 to 69.1 billion kWh in 1999. Table 2 shows the trends in electric energy cost from 1980 to 1996.
0004
New Electrotechnologies for Food Processing and Preservation Electron beam irradiation, X-ray, microwave processing, membrane separation technology, ozonation, ohmic heating, high pressure pasteurization, infrared
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Characteristics of Milk Powders M A Augustin, P T Clarke and H Craven, Food Science Australia, Weribee, Victoria, Australia Copyright 2003, Elsevier Science Ltd. All Rights Reserved.
Background 0001
0002
tbl0001
Milk powders are used by consumers as a substitute for fresh milk and as ingredients for the manufacture of a range of processed food products. In order to be acceptable to consumers and users of ingredients, it is essential that milk powders are of a good quality. Milk powders are manufactured to meet certain specifications and standards for composition. These have been developed for milk powders by authorities such as the American Dairy Products Institute, the International Dairy Federation, the Food and Agricultural Organization of the United Nations and national food authorities in individual countries. In addition, a range of other technical specifications have been developed for the characterization of milk powders to ensure that they have the required functional performance in specific target applications. Milk powders may be similar in composition but have different functional properties. There are many types of milk powders in the market place. This article focuses on the characteristics of skim and full-cream milk powders, which are the major types of milk powders produced. The microbiological quality, physical and chemical attributes of these milk powders, and their functional properties are discussed. Aspects of deteriorative changes that may occur in milk powders during transport and distribution that have an impact on the sensory properties of powders and their performance as food ingredients are included. The production, composition, and applications of various types of milk powders have been discussed elsewhere. (See Powdered Milk: Milk Powders in the Marketplace.)
4703
Microbiological Aspects Standards for Quality and Safety
Milk powder is a microbiologically stable product. It has a water activity of 0.3–0.4, which is too low to support the growth of microorganisms. However, after milk powder has been reconstituted, it is susceptible to microbial growth and spoilage in a similar manner to pasteurized milk. Provided milk powder is protected from moisture contamination before use, the numbers of microorganisms present generally decrease during storage, although the numbers of spores may remain constant. Although milk powder does not support the growth of microorganisms, the microbiological content is an important consideration in the subsequent use of the powder. For this reason, government bodies and customer groups have developed microbiological limits or specifications that apply to certain groups of microorganisms that may be present in milk powder. These specifications may relate to expectations of raw milk quality, hygiene during manufacture, microbial safety, or compatibility with the intended use of the milk powder. Common end-product standards relate to the total number of bacteria (mesophilic aerobes), coliforms, Salmonella, and Staphylococcus aureus. Criteria may also be applied for Bacillus cereus, Listeria, thermophiles, Enterobacteriaceae and spore-forming bacteria. The standards developed by the International Dairy Federation, for example, are shown in Table 1. Many countries have either adopted these standards or developed their own local specifications based on the principles of the International Commission on Microbiological Specifications for Foods (ICMSF). The microbiological count of milk powder is influenced by both the numbers and types of microorganisms in the raw milk and the processing conditions under which the milk powder is produced. In powders subjected to a high heat treatment, the microorganisms present will be predominantly spore-formers, belonging to the genus Bacillus. When heat treatment is less severe, vegetative cells
Table 1 Microbiological specifications for milk powder, as recommended by the International Dairy Federation Criteriaa
Total count (per gram)
Salmonella (per 25 g)
Coliforms (per gram)
Staphylococcus aureus (per gram)
m M n c
50 000 200 000 5 2
0 na 15 0
10 100 5 1
10 100 5 1
a For a production batch, n ¼ number of samples that must be tested, c ¼ number of samples that may exceed the microbiological limit specified as m, and M is the maximum allowable microbiological limit specified for any of the samples examined. na ¼ not applicable.
0003
0004
0005
0006
4704 POWDERED MILK/Characteristics of Milk Powders
0007
0008
0009
0010
0011
of thermoduric bacteria will be present, with their proportion to spore-formers decreasing with the intensity of the heat treatment applied. Vegetative cells of pathogenic bacteria and Gram-negative milk spoilage bacteria are destroyed during the heat treatment. Coliforms, Salmonella, and other Enterobacteriaceae are killed when the milk is heated prior to evaporation; however, they may contaminate milk powder if conditions are not sufficiently hygienic during drying. These bacteria can enter the dryer through the intake air from the factory environment, or the equipment used to dry or transport the milk powder. Cracks in dryers have been shown to be a particularly significant source of Salmonella. Here, Salmonella are harbored in the insulation material. Although dryers operate at a high temperature, the concentrated milk offers protection to the bacteria, and they will survive heating at dryer air inlet and outlet temperatures. Salmonella spp. are significant pathogens, and several notable outbreaks of illness have been attributed to the presence of this organism in milk powder. Staphylococcus aureus is significant, as certain strains can produce a heat-stable toxin that is not destroyed during powder manufacture. Although Staphylococcus aureus is common in raw milk, it does not normally grow to produce toxin unless the milk is stored at a high temperature prior to processing. The risk of toxin production increases with temperature and storage time. Although the bacteria will be killed during the process, the toxin remains and can be detected only through specific tests. Large outbreaks of illness have been attributed to the presence of Staphylococcus aureus toxin in milk powder. Another bacterium of potential significance in milk powder is Bacillus cereus. This is commonly found in milk, and its spores may survive heat processing. Specialty powders such as infant formula often have specifications for this bacterium, owing to the potential risk of the growth of this organism in warmed milk and sensitivity of the target group of consumers. Sometimes, yeasts and molds or their toxins, and Listeria are included in powder specifications. Yeasts and molds may be significant spoilage organisms if powder is contaminated with moisture, and Listeria may contaminate powder from the factory environment, especially if the environment is not kept dry. In the milk powder process, milk is subjected to heat whilst concentrated under vacuum. Such conditions are conductive to the growth of thermophilic Bacillus species that may form biofilms in the process lines. When this occurs, the product may be contaminated with thermophiles that can reach more than 106 per gram in long production runs. Thermophiles may sporulate in the process, leading to the presence of large numbers of thermophilic spores in the
powder. The spores can be extremely heat-resistant and may not be completely destroyed when the reconstituted powder is used in ultrahigh-temperature (UHT) processes. They are significant because they may cause sterility failures or spoilage in other heated products. If not properly cleaned from the plant between production runs, residues of thermophiles will seed subsequent batches of milk powder. Although milk powder is a microbiologically stable product, the microbial quality of the raw milk may influence the shelf stability of the powder. Some bacteria present in raw milk, particularly Pseudomonas species, produce heat-stable spoilage enzymes, including proteases and lipases, that remain active in milk powder over many months. Experience has shown that lipase can act in full-cream milk powder to degrade milk fat to cause rancidity and other objectionable flavors. Proteases retain activity in milk powder and degrade milk proteins to cause objectionable flavors after the milk powder has been reconstituted. Proteases and lipases may be particularly detrimental in recombined milk products, or if milk powder is used to prepare UHT milk. Here, very low levels of protease and lipase may cause spoilage during long storage periods. (See Bacillus: Occurrence; Detection; Food Poisoning; Biofilms; Listeria: Properties and Occurrence; Pasteurization: Principles; Salmonella: Properties and Occurrence; Spoilage: Bacterial Spoilage; Fungi in Food – An Overview; Molds in Spoilage; Yeasts in Spoilage; Staphylococcus: Properties and Occurrence.)
0012
Control of Microorganisms
The manufacture of microbiologically sound milk powder is dependent upon processing good-quality raw milk under hygienic conditions. To ensure the supply of good-quality milk, farm milk should be tested regularly for microbial quality. Many countries now use the total count test to monitor levels of bacteria in farm supplies. Thermoduric counts are sometimes used also. Raw milk ideally should be stored at less than 5 C and used within 72 h of collection to minimize bacterial growth. The pasteurization of milk is important and is normally identified as a critical control point. An example of process criteria for pasteurization would be heating of the milk for at least 15 s at 72 C or 5 s at 80 C. Within the factory, application of good manufacturing practice is essential to minimize the risk of milk powder contamination with undesirable types or levels of microorganisms. To achieve this, consideration must be given to the design of the premises and control of staff or vehicular movement to separate raw materials from drying areas. Manufacturing
0013
0014
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POWDERED MILK/Characteristics of Milk Powders
0016
equipment and the processing environment must be maintained, cleaned, and sanitized to ensure that microbial build-up and spread are prevented. Staff must be trained in practices to maintain high standards of hygiene. A supply of good-quality water and air for the process is also essential. Many factories now have ongoing monitoring systems for Salmonella and Listeria in place. If these bacteria are detected in the processing environment, special clean-up regimes and extra product testing are implemented. The modern approach to ensuring that milk powder is microbiologically safe involves preventative management to ensure manufacture under appropriate conditions of hygiene. Many factories now either have in place, or are moving towards, the hazard analysis critical control point (HACCP) system. Although end-product testing is still used to verify compliance and to detect gross process failures, it cannot be relied upon to ensure the safety of a batch of product. Testing can be labor-intensive and timeconsuming, taking up to 7 days to obtain final results. To overcome these problems, samples may be composited and rapid techniques for detection of pathogens based on ELISA or DNA methods applied. These methods have advanced efficiencies in testing, and product can now be cleared in 24–48 h. (See Hazard Analysis Critical Control Point; Quality Assurance and Quality Control.)
Physical Properties 0017
The physical properties of milk powders are governed by process variables, the type of dryer, and the composition of the milk. The physical properties of milk powders play an important role in their use as food ingredients. Their ability to be readily incorporated into products and to perform specific functions in a food formulation can be influenced by many physical properties. There are many physical attributes that must be taken into account when either evaluating a current product, setting specifications for new products or designing or modifying a drying system. Tighter and more demanding specifications have meant that powders are now often manufactured on specialist dryers designed specifically to produce the best possible product of defined specification. Moisture
0018
The final moisture content is critical for several reasons and is therefore defined in all powder specifications. It can affect functionality and microbiological quality, and is an economic consideration in the manufacture of powders. There are several factors during manufacture that can influence the moisture content of powders. These include the characteristics
4705
of the concentrate fed into the dryer, the type of atomization used, and the operating conditions during drying. Insolubility Index
The insolubility index of a powder is a measure of the degree to which it can be readily solubilized in water prior to use. It is related to the amount of sediment obtained under defined conditions of mixing milk powders. The main reason for loss of solubility is the temperature of the particles during the primary stage of the drying process where the majority of the moisture is removed. During this stage, an impermeable crust can form on the particle surface that severely restricts water removal, leading to the production of case-hardened particles and subsequent loss of solubility.
0019
Bulk Density and Particle Density
Bulk density is the amount of powder by weight that is present in a defined volume. It is usually expressed as g ml1 and is obtained by measuring the volume of a fixed weight of powder after it has been tapped for a defined number of times. A high bulk density is very important in packaging and transportation, and is desirable as it can significantly reduce costs. The bulk density is influenced by a range of factors. These include the amount of air entrapped in the powder particles (occluded air), the overall density of the particle (determined by the composition), the air between the individual powder particles (interstitial air), the particle size distribution and the particle shape. The bulk density of powders is influenced by dryer design and configuration (Tables 2 and 3). Particle density is the density of the solids (determined by the composition), which determines the particle density, together with the amount of occluded air.
0020
Particle Size Distribution
The individual particles produced during drying can vary greatly in size. The distribution of particle size then can be further altered by the degree of agglomeration or after grinding. An indication of the range of particle sizes obtained from different dryer configurations is given in Table 4.
0021
Interstitial Air and Occluded Air
Interstitial air is the amount of air that exists between particles or agglomerates as well as the air inside porous agglomerates. The sphericity of the particles, the particle size distribution, and the degree of agglomeration determine the amount of interstitial air. To obtain minimum interstitial air, the particles need to be smooth, have a range of particle sizes, and be in compact agglomerates.
0022
4706 POWDERED MILK/Characteristics of Milk Powders tbl0002
Table 2 Ability of various spray dryers to manufacture nonagglomerated and agglomerated skim milk powder with low or high bulk densities Type of drying process
Chamber configuration
Single stage
Conventional Tall form Conventional Tall form Conventional Tall form Compact Compact Multistage Integrated belt
Two stage
Three stage
Postprimary treatment
None None Cooling bed Cooling bed External bed External bed Integrated bed Integrated bed þ external bed Integrated bed þ external bed Integrated belt
Atomizer type
Rotary/nozzle Nozzle Rotary/nozzle Nozzle Rotary/nozzle Nozzle Rotary/nozzle Rotary/nozzle Rotary/nozzle Nozzle
Nonagglomerated skimmilk powder
Agglomerated skimmilk powder
Low BDa
High BDa
Low BDa
High BDa
Yes Yes Yes Yes Yes Yes Ideal Yes Yes Ideal
Yes Yes Yes Ideal Yes Ideal Ideal Yes Yes No
No No No No No No No No Ideal Yes
No No Yes Yes Yes Yes No Yes Ideal Yes
a BD ¼ bulk density; nonagglomerated skim milk powder: low bulk density 0.72 g ml1; high bulk density 0.72 g ml1; agglomerated skim milk powder: low bulk density 0.30–0.50 g ml1; high bulk density 0.45–0.55 g ml1. Adapted from Pisecky J (1997) Handbook of Milk Powder Manufacture, p. 79. Copenhagen: Niro A/S.
tbl0003
Table 3 Ability of various spray dryers to manufacture nonagglomerated and agglomerated full-cream milk powder with low or high bulk densities Type of drying process
Single stage
Two stage
Three stage
Chamber configuration
Conventional Tall form Conventional Tall form Conventional Tall form Compact Compact Multistage Integrated belt
Postprimary treatment
None None Cooling Bed Cooling Bed External bed External bed Integrated bed Integrated bed þ external bed Integrated bed þ external bed Integrated belt
Atomizer type
Rotary/nozzle Nozzle Rotary/nozzle Nozzle Rotary/nozzle Nozzle Rotary/nozzle Rotary/nozzle Rotary/nozzle Nozzle
Nonagglomerated full-creammilk powder
Agglomerated full-creammilk powder
Low BDa
High BDa
Low BDa
High BDa
Yes Yes Yes Yes Yes Yes No Yes Ideal Yes
Yes Yes Yes Yes Yes No Ideal Yes Yes No
No No No No Yes Ideal Ideal Yes No No
No No Yes Yes No No No No Ideal Yes
a BD ¼ bulk density; nonagglomerated full-cream milk powder: low bulk density 0.63 g ml1; high bulk density 0.63 g ml1; agglomerated full-cream milk powder: low bulk density 0.30–0.50 g ml1; high bulk density 0.45–0.55 g ml1. Adapted from Pisecky J (1997) Handbook of Milk Powder Manufacture, p. 79. Copenhagen: Niro A/S.
tbl0004
300–2000
Occluded air is the amount of air entrapped within the powder particles. It is affected by the preheat treatment of the original milk, with a higher pretreatment of milk resulting in less occluded air, and the amount of air incorporated in the concentrate. Higher total solids generally result in lower occluded air. Powders atomized by a nozzle contain less air than rotary atomized powders, despite improvements to the modern rotary atomizers. Gentle drying also reduces the level of occluded air, and therefore, the use of multistage dryers is recommended for the production of powders with low occluded air.
100–400
Flowability
Table 4 Mean particle size obtained from dryers of different configuration Powder characteristics
Dryer configuration
Individual particles
Concurrent with pneumatic conveying Tall form – tower Roller dryer Mixed flow with integrated fluid bed Concurrent with integrated fluid bed Concurrent spray dryer with integrated belt Mixed flow with integrated fluid bed
Flakes Loose agglomerate – open structure Compact agglomerate – porous structure
Particle size (mm) 20–200 30–250 200–5000 100–400 100–200
From personal communication (E. Refstrup), Niro A/S, Denmark.
With the ever-increasing diversity of use of milk powders today, the need for properties such as
0023
0024
POWDERED MILK/Characteristics of Milk Powders
0025
flowability is increasing. Powders are used in applications ranging from dispensing machines through to the large-scale recombining operations that utilize mechanical handling and dosing. For both agglomerated and nonagglomerated powders, a better flowability can be obtained by producing larger powder particles with smooth and rounded particle surfaces within a narrow particle size distribution. Flowability is also influenced by other factors such as total fat in the powder and the amount of ‘free fat.’
include the mechanical stability of the powder, which influences the degree of agglomeration breakdown during transport and storage, hygroscopicity, which is related to the degree of water attraction a powder exhibits, and cakiness, an attribute that is a measure of the extent to which a powder adheres to itself, especially under compression. (See Agglomeration; Drying: Spray Drying; Rheological Properties of Food Materials.)
‘Free Fat’
Chemical Characteristics
‘Free fat’ in powder is defined as the fat fraction that is extractable by organic solvents under specific conditions of solvent type, time, and temperature of extraction. In most instances, ‘free fat’ is considered a defect. The exception is where ‘free fat’ is required for a specific application, e.g., chocolate manufacture. One of the most critical influences of ‘free fat’ is the moisture content of the powder. If the moisture is too low (< 2.5%), ‘free fat’ increases and then decreases as the moisture content is raised from 2.5 to 4–5% but increases again if the moisture content is > 6–7%.
The chemical properties of milk powders are determined by the composition of the milk and the heat treatment applied during powder manufacture.
Instant Properties 0026
Very fine powder particles are difficult to handle and have poor reconstitution properties. Agglomeration of powders allows water to permeate the powder particles more readily, breaking up the agglomerate and allowing the individual powder particles to dissolve. Instant milk powder is highly soluble and designed to reconstitute completely in water at both hot and cold temperatures. The other properties required in instant powders are wettability and dispersibility. The wettability of a powder is measured by determining the time taken for a given amount of powder to pass through the surface of water. Wettability may be enhanced by lecithination. The dispersibility of a powder is a measure of how completely a powder dissolves under controlled conditions. Other tests carried out on milk powders related to their instant properties include slowly dispersible particles, coffee test, white flecks number (minute particles that are seen on the surface of reconstituted milk), and the sludge test. Color
0027
The color of a powder is determined by composition, preheat treatment, drying conditions, and particle size distribution. Scorched particles can be a visual defect that will often show up as deposits on the bottom of mixing vats and in strainers. Other Properties
0028
4707
Apart from the properties described above, there are others that influence a powder’s acceptability. These
0029
Chemical Composition
Skim and full-cream milk powders are obtained by dehydration of skim milk and full-cream milk to 4% moisture. Full-cream milk is usually standardized to a fat:solids-nonfat ratio of 1:2.67 to meet the 26% legal minimum fat content for this powder. The protein content of skim milk powders may be standardized also. Variations in milk composition owing to factors such as cow breed, feed, stage of lactation, and season are reflected in the composition of milk powders. The American Dairy Products Institute standards for skim and full-cream milk powder compositions are as follows: skim milk powder should have a maximum fat content of 1.25% and a maximum water content of 4.0%, whereas full-cream milk powder should have a minimum fat content of 26% and a maximum water content of 4.0%. Control of the moisture content of milk powders to a maximum of 4% is essential for good shelf-life stability. Table 5 shows the range of values observed in milk powder. Table 5 Composition of milk powdersa
0030
tbl0005
Constituent
Skim milk powder
Full-cream milk powder
Moisture (g per 100 g) Fat (g per 100 g) Crude protein (g per 100 g) Lactose (g per 100 g) Citric acid (g per 100 g) Ash (g per 100 g) Sodium (mg per 100 g) Potassium (mg per 100 g) Calcium (mg per 100 g) Magnesium (mg per 100 g) Phosphorus (mg per 100 g) Chloride (mg per 100 g)
3–5 0.7–1.3 35–37 49–52 1.8–2.1 7.5–8.0 400–550 1550–1750 1200–1300 110–140 950–1050 1100
2–4 25–28 25–27 36–38 1.3–1.4 6.0–7.0 370–420 1150–1350 900–1000 85–100 700–770 750–800
a Adapted from Walstra P and Jenness R (1984) Dairy Chemistry and Physics, pp. 418–419. New York: John Wiley.
4708 POWDERED MILK/Characteristics of Milk Powders
0031
Another important indicator of milk powder quality is the titratable acidity of the reconstituted powder. This is an indicator of the microbiological quality of the milk. The American Dairy Products Institute sets a maximum of 0.15% for titratable acidity of skim milk powder. Heat-treatment Classification
0032
The characteristics of milk powder can be influenced by the heat treatment received by the milk powder during manufacture. The time and temperature of the preheat treatment affects the level of whey-protein denaturation. The whey-protein nitrogen index, which is a measure of the undenatured whey-protein nitrogen in the powder and was developed by the American Dairy Products Institute, is commonly used to classify powders into low-heat, medium-heat and high-heat milk powders. Typical preheat treatments used for the manufacture of these powders are listed in Table 6. As the composition of milk, including the initial level of whey proteins in milk, can vary with season, the same heat treatment can result in a different whey-protein nitrogen index. Other methods for heat classification of milk powders, such as the heat number, cystine number, and thiol number also may be used as a measure of the heat treatment given to the milk during powder manufacture. (See Heat Treatment: Ultra-high Temperature (UHT) Treatments.)
Functional Properties of Milk Powders 0033
tbl0006
When milk powders are used as ingredients in food applications, they contribute to the physical attributes of the food. The ability of milk powders to impart desirable properties to food is related primarily to functional properties of milk-protein components in the powders. These functional properties include solubility, hydration, heat stability, viscosity, gelling, foaming, and emulsifying. In milk powders,
Table 6 Heat classification of skim milk powder Heat class
Low heat Medium heat High heat a
Whey protein nitrogenindexa (milligrams of undenatured whey protein Nper gram of powder)
Preheat treatment of milkb
Not less than 6 1.51–5.99 Not more than 1.5
72 C for 15 s 75 C for 3 min 90 C for 10 min 120 C for 2 min
Solubility
Solubility is a fundamental functional property that is a prerequisite for most other desired functionalities. The solubility of milk powders is dependent on pH. Proteins have a minimum solubility at the isoelectric pH, and solubility is increased on the acid and alkaline side of this pH. Caseins, the major proteins in milk, are least soluble at pH 4.6.
0034
Hydration
Hydration is related to the ability of the milk proteins to bind or entrap water. Caseins hold about 3.3 g of water per gram, whereas undernatured whey proteins hold 0.4 g of water per gram. Heat denaturation of whey proteins increases the water holding to 2.5 g of water per gram. Milk powder contains other components, such as lactose, that bind water in addition to the protein. Skim milk powders have a water sorption of 0.96–1.28 g water per gram, depending on the conditions used during powder manufacture.
0035
Heat Stability
Heat stability is an important property in certain applications such as the manufacture of recombined evaporated milk. Single-strength milks made from low-, medium- or high-heat milk powders have a similar heat stability to fresh milk. They are heatstable at the pH of milk (pH 6.7), being able to withstand coagulation for up to *20 min at 140 C. However, for adequate heat stability of evaporated milks under sterilization conditions (e.g., 120 C for 12 min), high-heat milk powders are necessary. Heat stability is affected by the pH of the milk, mineral content, and other components in the milk (e.g., lecithin, urea).
0036
Viscosity
From American Dairy Products Institute (1990) Standards for Grades of Dry Milks including Methods of Analysis, Bulletin 916. A range of other preheating conditions may be used to achieve a desired whey protein nitrogen index. b
the functional properties of the milk proteins may be modulated by heat, ions, and other components. Heat treatment of milk prior to concentration and drying is the most common method used to alter the functional properties of milk powders. Milk powders with the same composition given different preheat treatments prior to concentration and drying have different functional attributes when used as ingredients.
The viscosity of milks reconstituted from milk powders is dependent on their state of dispersion, concentration of solids, and temperature. Increasing the concentration of milk solids increases the viscosity. Decreasing the temperature increases the viscosity, but heating milk to a temperature that results in denaturation of whey proteins also increases the viscosity.
0037
POWDERED MILK/Characteristics of Milk Powders Gelation 0038
Milks reconstituted from milk powders have the ability to form gels under similar conditions to those required for the formation of gels from fresh milk, i.e., by rennet action for formation of rennet gels and by acidification of milk under quiescent conditions. Foaming and Emulsifying
0039
Milk powders can be used in applications where foaming and emulsifying properties are required. The ability of milk proteins in the milk powders to stabilize foams and emulsions may be exploited when these properties are required. (See Aerated Foods; Emulsifiers: Uses in Processed Foods; Mixing of Powders.)
Functional Requirements of Milk Powders in Major Food Applications 0040
For milk powders to have the desired performance in food applications, the functional characteristics of the powders have to be matched to the application. This requires an understanding of the required functional properties of the milk powder ingredients in the target application. Milk Powders for Recombined Dairy Products
0041
0042
0043
A significant amount of milk powder is used in the manufacture of reconstituted and recombined dairy products. In these applications, the milk powders are combined with water and milkfat to reestablish the fat:solids-nonfat:water ratio of milk or other dairy products. Some of the major applications of milk powders in the recombination industry are for the preparation of pasteurized fluid milk, UHT milk, cream, evaporated milk, sweetened condensed milk, yogurt and cultured dairy products, recombined cheese, and icecream. Different functionalities of the milk powder ingredients are needed in these various recombined dairy products. Table 7 lists the major functional requirements of milk powders for recombined dairy products. Pasteurized milks and UHT milks These products have a similar composition to fresh milk. For pasteurized milks, low-heat or medium-heat powders are used to obtain a flavor similar to milk and to minimize heat-induced flavors. In the case of UHT milks, any type of powder can be used, as single-strength milks made from low-, medium-, or high-heat powders are stable to UHT conditions. Evaporated milks It is essential to use high-heat powders for this application to obtain evaporated
4709
Table 7 Functional requirements of milk powders in recombined dairy products and selected processed foods Product
tbl0007
Functionalproperties Heat treatment required inmilk of milk powder powder
Pasteurized milk Good flavor Emulsifying UHT milk Good flavor Heat stability Emulsifying Cream Good flavor Emulsifying Evaporated milk Heat stability Viscosity Sweetened Viscosity condensed milk Yogurt Water-binding Viscosity Gelling Cheese Rennetability Icecream Foaming/whipping Emulsifying Confectionery Water-binding Foaming/whipping Emulsifying Heat stability Bakery Water-binding Foaming/whipping Emulsifying Gelling
Low–medium heat Low–medium–high heat
Low–medium heat High heat Low–medium heat
Low heata
Low heat Low–medium–high heat High heat
High heat
a If a low-heat milk powder is used, the yogurt milk has to be given a highheat treatment during yogurt manufacture. Alternatively, a high-heat milk powder may be used, in which case, the yogurt milk requires only a lowheat treatment to pasteurize the milk during yogurt manufacture.
milk with the desired viscosity. A high-preheat treatment improves the heat stability of a recombined milk concentrate (typically 26% total solids; 18% solidsnonfat: 8% fat) to in-can sterilization conditions used in its manufacture. Additionally, high-heat powders are screened using heat-stability tests to ensure that they withstand sterilization without excessive thickening or coagulation. Sweetened condensed milk This is a traditional dairy product containing 74% total solids (20% milk solids nonfat: 8% fat: 46% sucrose). The most important physical attribute of this product is its viscosity. Low- and medium-heat powders are used in this application. Milk powders given a high-heat treatment (e.g., 85 C for 30 min) cannot be easily processed, because the high viscosity of concentrates made from these powders also results in rapid age thickening during storage of the product. There are a number of viscosity tests that may be used as indicators of suitability of powders for sweetened condensed milk manufacture.
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4710 POWDERED MILK/Characteristics of Milk Powders
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Yogurt Milk powders may be used as a partial or total replacement for fresh milk in this application. Viscosity development, gelling, and good waterbinding properties are necessary for the production of high-quality yogurts. These properties are obtained in yogurt by preheating the yogurt milk at a temperature that causes significant denaturation of whey proteins (e.g., 90 C for 10 min). Low-heat milk powder may be used if a high-heat treatment is given during yogurt manufacture. If a high-heat milk powder is used, the yogurt milk requires only a low-heat pasteurization treatment during yogurt manufacture. Cheese Only low-heat milk powders are suitable for recombined cheese manufacture. This ensures good rennetability of the reconstituted milk. With a highheat treatment of milk, there is association of the denatured whey proteins with the casein, which hinders the reaction of the rennet. Ice cream Milk powders contribute to the flavor and texture of ice cream. The milk powder aids in the emulsification of the ice cream mix and has a role in the development of an aerated matrix. (See Condensed Milk; Evaporation: Basic Principles; Uses in the Food Industry; Recombined and Filled Milks.) Milk Powders for Selected Food Applications
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Milk powders are used as functional ingredients in a number of processed foods. Chocolate and confectionery products Milk powders contribute to the flavor, color, and texture development in chocolate and confectionery applications. The emulsifying properties of the milk proteins influence the miscibility of the ingredients used in chocolate and confectionery, hence influencing flow properties and texture. In confectionery products such as toffee, good water-binding properties of milk proteins contribute to the texture of these products. The Maillard reaction, which is the reaction of the amino groups of the proteins with reducing sugars in the formulation, is responsible for color development and for the production of caramelized flavors; the lactose in milk powders participates in the Maillard browning reactions. Bakery products High-heat milk powders are useful in bakery applications. In addition to enhancing the nutritive value of cereal-based baked goods, milk powders contribute to the texture and flavor of these products. Their emulsification and foam-stabilization properties and their ability to participate in the
Maillard browning reaction are important requirements in bakery applications. Other applications The functional properties of milk powders also make them useful in a number of other applications, such as processed meat products, soups, gravies, and dips.
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Characteristics of Stored Milk Powders The characteristics of milk powder are dependent on the quality and composition of the raw milk and the manufacturing process used during its manufacture. However, even if milk powders are manufactured to meet the desired standards and specifications, changes in the properties of milk powders may occur during storage and distribution. The composition of the powder, the type of packaging material used, and the conditions of handling and storage influence the shelf-life of the powder. Deterioration of milk powders resulting from Maillard browning, lactose crystallization, and oxidation of fat may lead to flavor and physical defects in the powder. It may also affect the functionality of the milk powder when it is used in a food product. Some of the changes that may occur during storage include the development of a brown color, a reduction in pH, reduced solubility, development of off-flavors, and reduced heat stability of powders. See also: Biofilms; Condensed Milk; Emulsifiers: Uses in Processed Foods; Evaporation: Basic Principles; Uses in the Food Industry; Heat Treatment: Ultra-high Temperature (UHT) Treatments; Mixing of Powders; Pasteurization: Principles; Rheological Properties of Food Materials; Powdered Milk: Milk Powders in the Marketplace; Quality Assurance and Quality Control; Recombined and Filled Milks
Further Reading American Dairy Products Institute (1990) Standards for Grades of Dry Milks Including Methods of Analysis, Bulletin 916, rev ed. Chicago, IL: ADPI. Early R (1998) Milk concentrates and milk powders. In: Early R (ed.) The Technology of Dairy Products, 2nd edn, pp. 228–300. London: Blackie Academic & Professional. International Commission on Microbiological Specifications for Foods (1998) Microorganisms in Foods 6, Microbial Ecology of Food Commodities. London: Blackie Academic & Professional. International Dairy Federation (1984) General Code of Hygienic Practice for the Dairy Industry and Advisory Microbiological Criteria for Dried Milk, Edible Rennet
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POWER SUPPLIES/Use of Electricity in Food Technology Casein and Food Grade Whey Powders, Bulletin No. 178. Brussels: International Dairy Federation. International Dairy Federation (1990) Recombination of Milk and Milk Products, Special Issue No. 9001. Brussels: International Dairy Federation. International Dairy Federation (1991) IDF Recommendations for the Hygienic Manufacture of Spray Dried Milk Powders, Bulletin No. 267. Brussels: International Dairy Federation. International Dairy Federation (1999) 3rd International Symposium on Recombined Milk and Milk Products, Special Issue No. 9902. Brussels: International Dairy Federation. Knipschildt ME and Andersen GG (1994) Drying of milk and milk products. In: Robinson KR (ed.) Modern Dairy Technology, 2nd edn., vol. 1, pp. 159–254. London: Chapman & Hall.
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Masters K (1997) Spray dryers. In: Baker CGJ (ed.) Industrial Drying of Foods, pp. 90–112. London: Blackie Academic & Professional. Mettler AE (1994) Present day requirements for effective pathogen control in spray dried milk powder production. Journal of the Society of Dairy Technology 47: 95–107. Pisecky J (1986) Standards, specifications, and test methods for dry milk products. In: MacCarthy D (ed.) Concentration and Drying of Foods: Proceedings of the Kellogg Foundation 2nd International Food Research Symposium (Cork: 1985), pp. 203–220. London: Elsevier Applied Science. Pisecky J (1997) Handbook of Milk Powder Manufacture. Copenhagen: Niro A/S. Walstra P and Jenness R (1984) Dairy Chemistry and Physics, pp. 418–419. New York: John Wiley.
POWER SUPPLIES Use of Electricity in Food Technology D Graham, R and D Enterprises, Walnut Creek, CA, USA Copyright 2003, Elsevier Science Ltd. All Rights Reserved.
Introduction 0001
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The food industry in the USA consumed 17.4 billion kcal in 1999. Fifty-eight percent of total food industry energy costs are from electricity. The industry purchases 64.0 billion kWh of electricity annually at a cost of $3.36 billion. Purchased energy is 1.3% of the value of product shipments for the food industry compared with 1.7% for the total US industry. Major uses of electricity include freezer and refrigeration compressors, conveyors, air handling, pumping, lighting, process controllers and monitors, and packaging forming and sealing. Natural gas is the primary source of thermal energy for ovens, fryers, dryers, evaporators, and boilers, with relative costs being the main reason. Closely related uses of electricity in agricultural food production include crop irrigation, pest control, produce disinfection, grain harvesting and storage, weed reduction, livestock waste management, and fish farming. Irradiation, ohmic heating, microwave processing, ozonation, freeze concentration, nonthermal pasteurization, and the use of heat pumps are examples of relatively new electrotechnologies that the food industry may apply increasingly in the future. Application of
these technologies provides opportunities for food processors to improve operating efficiencies and helps insure the quality and safety of processed food products. Electricity Use by Food Industry Sectors
Food energy costs rose steadily over the past two decades. In 1996, purchased electricity comprised over half of the total energy used in food processing. Electrical consumption by grain milling, meat processing, preserved fruits and vegetables accounts for 23, 19, and 13%, respectively, of the total food industry use of electricity. Some food processors operate cogeneration facilities using surplus heat to produce electricity, but approximately 92% of total electricity used by the food industry in the USA is purchased from utility companies. Approximately 87% of the electricity is used by motor-driven equipment such as compressors, pumps, mixers, grinders, fans, etc. Table 1 compares the cost of electricity with total energy costs in food processing. Electricity consumption has increased approximately 3% annually from 48.9 billion kWh in 1986 to 69.1 billion kWh in 1999. Table 2 shows the trends in electric energy cost from 1980 to 1996.
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New Electrotechnologies for Food Processing and Preservation Electron beam irradiation, X-ray, microwave processing, membrane separation technology, ozonation, ohmic heating, high pressure pasteurization, infrared
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