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MILK AND MILK PRODUCTS | Microbiology of Dried Milk Products
Microbiology of Dried Milk Products P Schuck, INRA, Rennes, France; and Agrocampus Ouest, Rennes, France Ó 2014 Elsevier Ltd. All rights reserved. This article is a revision of the previous edition article by Donald Muir, volume 2, pp 1441–1445, Ó 1999, Elsevier Ltd.
The microbiology of dried milk products is governed by the quality of the raw material, the conditions employed during manufacture of the product, and any postprocessing contamination. Most dried milk now is produced by spray-drying, and therefore, other drying methods (e.g., roller-drying or freezedrying) are excluded from consideration in this chapter. Because of the complexity of the manufacturing process, each step is considered in turn.
Manufacturing Processes The purpose of dehydration of milk is to stabilize the milk constituents for their storage and later use. The industrial application of concentration and fractionation by membrane processes (e.g., microfiltration, ultrafiltration, nanofiltration, and reverse osmosis), electrodialysis, or ion exchange provides opportunities and versatility to dry dairy technology, processing not only milk but also whey and its components. Dry milk products currently include milk powder, skim milk powder, whey powder, various whey protein powders, dry dairy–based beverages, casein, caseinates, coprecipitates, baby foods and cheese products, lactose, coffee whiteners, dry ice cream mix, and single-cell protein. The world production of dry dairy products has increased consistently in recent years, mainly due to the advantages of powders, which are as follows: l l l l l l
Raw Milk The raw milk used for powder production must be of high chemical, sensory, and bacteriological quality, which is regulated by standards. Most of the dried milk in the world is produced from cow’s milk, although small quantities of caprine, ovine, and camel milk are converted into powder. In the case of bovine milk, there are closely controlled schemes in which payment is related to milk quality. Among the quality indices, measurement of total viable bacteria count is used as a basis for payment. A clear distinction is made between two classes of organisms found in raw milk – i.e., pathogens and potential pathogens, and spoilage bacteria. Bulk milk spoilage bacteria are prevalent in refrigerated milk, and pathogens are seldom present at high levels. Nevertheless, the presence of pathogens in dried milk must be avoided. Processing conditions, therefore, must ensure that this is achieved. The pertinent properties of pathogens found in milk are summarized in Table 1. Only three of these organisms survive pasteurization: Bacillus cereus, Clostridia spp., and, to a limited extent, Mycobacterium paratuberculosis. None of these bacteria present a major risk in dried milk. The other pathogens
Retain high quality, without special storage conditions Reduce mass and volume compared with fluid products Provide balance between milk supply and consumption Provide an irreplaceable food component in hot climates Are a valuable food reserve for emergencies Are suitable for various tailor-made food products
Drying is defined as the removal of a liquid (usually water) from a product by evaporation, leaving the solids in an essentially dry state. A number of different drying processes, such as spray-drying, fluid bed-drying, roller-drying, freeze-drying, microwave drying, and superheated steam-drying, are in use in the dairy, food, chemical, and pharmaceutical industries. In special circumstances, roller-drying is used for the production of milk powder for particular applications (e.g., confectionery and feed blends). Direct contact of concentrated milk with rotating steam-heated rollers adversely affects the components of milk, especially proteins and lactose. Certain reactions, such as protein denaturation, Maillard reactions, and lactose caramelization are irreversible. Due to factors related to drying economics and final product quality, the only processes of significance in milk and dairy powder manufacture are spray-drying and fluid bed-drying (most often in combination). Only the combination of these two drying processes will be discussed in this section. A flow chart of milk powder production, consisting of reception, clarification, cooling, standardization, heat treatment,
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evaporation, homogenization, drying, and packaging is shown in Figure 1.
Milk Receiving and selection Clarification Sediment Cooling Standardization – Skimming Fat Heat treatment Vacuum evaporation Water Homogenization Roller drying / Spray drying Water Package Packaging Storage Milk powder Figure 1
Flow chart for milk powder production.
Encyclopedia of Food Microbiology, Volume 2
http://dx.doi.org/10.1016/B978-0-12-384730-0.00220-2
MILK AND MILK PRODUCTS j Microbiology of Dried Milk Products Table 1
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Pathogens and potential pathogens found in raw milk
Organism
Growth at <6 C
Survives pasteurization a
Bacillus cereus Campylobacter jejuni Clostridium spp. Escherichia coli Listeria monocytogenes Mycobacterium paratuberculosis Salmonella spp. Staphylococcus aureus Yersinia enterocolitica
Yesb No (No)c – Yes – No No Yes
Yes (spores) No Yes (spores) No No Yes (limited) No No No
Heat treatment at 72 C for 15 s. Some species only. c Some proteolytic species can grow at low temperature. a
b
listed in Table 1 do not survive heat treatment and thus can find their way into dried milk only by postprocessing contamination from the environment. In contrast, the main spoilage organisms in refrigerated, bulk raw milk are Gram-negative, psychrotrophic bacteria (Table 2) Pseudomonas spp. of both fluorescing and nonfluorescing types predominate. The Gram-negative organisms are killed readily by pasteurization and pose no threat to the quality of milk or products manufactured from it per se. Many of the Gram-negative psychrotrophs, however, produce extracellular enzymes with the potential to degrade the milk constituents. Lipase, protease, and combined lipase and protease activity are found in a substantial proportion of bacteria isolated from refrigerated, raw bulk milk (Table 2). Moreover, these enzymes are noted for their heat tolerance. Substantial proportions of activity remain after pasteurization (Table 3) and, surprisingly, after ultra-high-temperature treatment (UHT) at 140 C for 5 s (Table 4). The corollary to this is that, to avoid breakdown of milk constituents in products, psychrotrophic bacteria numbers must not be allowed to reach the critical level at which there is sufficient activity to initiate degradation (Muir, 1999). Several studies have sought to define this critical level. Rancidity, caused by lipase breaking down milk fat to liberate free fatty acid, has been detected in Cheddar cheese made from milk in which the psychrotrophic bacteria count exceeded
Table 2
7 106 colony forming units (cfu) per milliliter (ml). Lipase activity also has been suggested to be the cause of the soapy character in chocolate. The offending ingredient was dried milk made from raw milk of poor quality. Parallel research has determined that protease activity, expressed by gelation in UHT milk, can be exacerbated when the psychrotrophic bacteria count in the raw material exceeds 3 106 cfu ml1. Product quality, therefore, must be safeguarded by not using milk in which the count of psychrotrophic bacteria exceeds 106 cfu ml1. The psychrotrophic bacteria in milk grow remarkably quickly in refrigerated milk, with typical generation times in the range of 4–12 h. In addition, growth is sensitive to small (1–2 C) differences in storage temperature. Typically, raw silo milk with an initial psychrotrophic count of 5 104 cfu ml1 has an expected ‘safe’ shelf life of 36 h during storage at 6 C. If the milk has been deep cooled to 2 C on reception at the factory, an extension of the ‘safe’ storage period by 24 h might be anticipated. When further extensions of ‘safe’ storage time are required, more drastic treatment of the raw milk is necessary. The most useful technique is thermization. Thermization is the generic description of a range of subpasteurization heat treatments that kill most spoilage bacteria found in raw milk (but not all pathogens) with minimum collateral heat damage. Thermization of good quality raw milk at 65 C for 15 s, followed by prompt cooling to 2 C, offers a ‘safe’ shelf life of 72 h.
Spoilage bacteria in raw milk and associated extracellular enzyme activity Pseudomonas
Proportion of population (%) Creamery silo Farm bulk tank
Fluorescing
Nonfluorescing
Other Gram-negative flora a
33.5 50.5
44.1 31.5
22.4 18.0
32 1 11
0–25 0–9 24–92
Proportion of isolates with stated activity (%) Lipase only 5 Protease only 2 Lipase and protease 71 a
Includes bacteria classified as Enterobacteriaceae, Aeromonas, Pasteurella, or Vibrio; Acinetobacter, Moraxella, or Brucella; Flavobacterium; Chromobacterium; Alcaligenes.
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MILK AND MILK PRODUCTS j Microbiology of Dried Milk Products Table 3 Residual activity of extracellular enzymes after pasteurization at 72 C for 15 s Enzyme activity
Residual activity (%)
Lipase Protease Phospholipase C
59 66 30
Table 4 Residual enzyme activity after heat treatment of cell-free supernatant at 140 C for 5 s Enzyme activity (%) Bacterial type
Protease
Lipase
Phospholipase C
Pseudomonas Fluorescent Nonfluorescent Other Gram-negativea Bacillus cereus Bacillus firmus
17–50 5–48 0–57 <5 <5
14–51 0–73 0–82 – –
31–57 5 0–40 – –
a
Includes bacteria classified as Enterobacteriaceae, Aeromonas, Pasteurella, or Vibrio; Acinetobacter, Moraxella, or Brucella; Flavobacterium; Chromobacterium; Alcaligenes.
Milk Processing Clarification and Fat Standardization After reception, milk is clarified, usually by centrifugal separators, and cooled to 4 C in plate heat exchangers, followed by storage at the same temperature. The next operation is standardization, which is used to adjust the ratio of milk fat to total solids as required in the final product. Cream is separated from skim milk using a high-speed centrifugal separator. This device separates the milk on the basis of the density difference between the ‘light’ milk fat globules and the relatively dense serum. The two liquid streams then are recombined to yield a product with the required fat content. The overall effect of fat standardization on the microbial population of milk is modest. Another subtly different separation process – clarification, sometimes called bactofugation – also may be applied. Clarifiers or bactofuges are special separators, which remove microorganisms from milk on the basis of the density difference between the bacterium and the serum phase of the milk. This density difference is greatest in the case of the spores of spore-forming bacteria found in raw milk (e.g., Bacillus spp. and Clostridium spp.). A modern clarifier can achieve a 90% reduction in spore count in a single pass. Clarification is particularly valuable because, although the vegetative cells of spore-forming bacteria are inactivated by modest heat treatment, the spores can resist fairly severe heating (see section Heat Treatment). Thus, clarification offers an alternate method of controlling the spore count of the finished product.
Heat Treatment Heat treatment is commonly performed using the indirect method in a tubular or plate heat exchanger at 88–95 C for 15–30 s, the aims being to destroy pathogenic bacteria and most of the saprophytic microorganisms, to inactivate
enzymes (especially lipase), and to activate SH-groups in blactoglobulin, resulting in an antioxidative effect. Heat treatment during the production of milk powder serves another distinct purpose. It not only controls microbial quality but also influences functionality. For example, it is usual to apply severe heat treatment to milk destined for manufacture into whole milk powder. Such heating results in denaturation of whey protein. The presence of denatured protein reduces the rate of lipid oxidation during subsequent storage. Pasteurization (63 C/30 min or 72 C/15 s) kills most pathogens (Table 1) and all Gram-negative, psychrotrophic spoilage bacteria. A residual population of heat-resistant bacteria remains, however. These bacteria are called thermoduric and include members of the coryneform group, heat-resistant streptococci, micrococci, and spore-forming bacteria. The predominant spore-forming organisms found in heated milk are Bacillus spp., which survive heat treatment in the spore form (Table 5). Bacillus spp. degrade milk readily and are noted for their phospholipase activity. Spoilage due to these organisms often is associated with damage to the milk fat globule membrane and is characterized by the defect known as ‘bitty cream.’ Two species of Bacillus, Bacillus stearothermophilus and Bacillus thermodurans, pose particular threats because of their extreme heat resistance. As described previously, the population of spores in milk can be reduced by clarification. If very low spore counts are required in the product, then severe heat treatment must be applied to the milk: typically 110–120 C for 30 s.
Concentration and Homogenization Concentration by vacuum evaporation is used to concentrate milk before drying and can be combined beforehand with reverse osmosis. Evaporation is performed in multiple effect vacuum evaporators with mechanical or thermal steam recompression, where energy consumption is about 10–30 times lower than in spray-drying. The differences in the degree of concentration are due to the drying technique used: 30–35% total solid (TS) content for roller-drying, and 45–50% TS for spray-drying. Concentrating milk prior to drying has a positive effect on milk powder quality: Milk powder produced from concentrated milk consists of larger powder particles containing less occluded air and therefore results in better storage stability. Homogenization is not an obligatory operation, but it is usually applied with the aim of reducing the free fat content,
Table 5 Heat-resistant bacteria in milk and their associated enzyme activity Bacillus spp. Coryneform group Proportion of isolates (%) 63 C/30 min 54 61 80 C/10 min
46 37
Isolates with enzyme activity (%) Protease þ lipase 37 Protease only 34 Phospholipase 80 Inactive 12
10 3 0 67
MILK AND MILK PRODUCTS j Microbiology of Dried Milk Products which has a negative effect on powder solubility and its susceptibility to oxidation. Apart from the expected increase in count caused by the concentration process, there is an additional potential hazard. In multistage evaporators, typical of modern dairy plants, a concentrate may be held for extended periods at the temperature range 45–55 C. Some heat-resistant bacteria can grow under these conditions and, as a result, the bacterial content of the concentrate can increase disproportionately. After concentration, the product is homogenized to reduce fat globule size and inhibit creaming. Homogenization may cause an increase in bacterial count as a result of the disaggregation of bacterial clusters.
Spray-Drying, Coating, and Agglomeration The basic principle of spray-drying is the exposure of a fine dispersion of droplets created by means of atomization of preconcentrated milk products over a hot air stream in a drying chamber. Some authors defined spray-drying as an industrial process for the dehydration of a liquid by transforming the liquid into a spray of small droplets and exposing these droplets to a flow of hot air. The very large surface area of the spray droplets causes water evaporation to take place very quickly, converting the droplets into dry powder particles. The small droplet size created, and hence the large total surface area, results in very rapid evaporation of water at a relatively low temperature, thus minimizing heat damage to the product. The main advantages of spray-drying over other drying techniques are as follows: l
The process is rapid, residence time in the chamber being less than 30 s. l The product has a fine structure and excellent properties, with no adverse effects of heat (as occur for freeze-drying), as drying is accomplished in a very short time and at a low temperature. l The process is fully automatic with complete control of drying parameters and minimal labor of any type. l The product comes into contact with the drying chamber wall only in the powder form, so there is no problem of equipment maintenance or the microbiological quality of the final product. The investment cost, however, for a spray-drying plant is high and is economically justified only for large quantities or for products with high added value. Single-stage spray-driers are now considered outdated. The residence time is not long enough to obtain a real equilibrium between the relative humidity of the outlet air and water activity (aw) of the powder. The outlet temperature of the air therefore must be high, reducing the thermal efficiency of the single-stage spray-dryer. The two-stage and three-stage drying systems consist of limiting the spray-drying to a process with a longer residence time (several minutes) to provide a better thermodynamic balance. A second or a third final drying stage is necessary to optimize the moisture content by using an integrated fluid bed (static) or an external fluid bed (vibrating). Such dryers currently dominate the dairy powder industry. Though two-stage and three-stage drying may produce both nonagglomerated and agglomerated powders, their main products are instant milk powders.
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The ideal water content for optimal preservation of a given powder can be determined on the basis of sorption isotherms. Thus, the water content of milk powder would be 4% (regulated), between 2% and 3% for whey, and 6% (regulated) for casein, for aw of 0.2. Provided the ultimate moisture content of the powder is at an aw close to 0.2, bacteriostasis is ensured. In the case of dried whole milk, the moisture content may be as low as 2%, to inhibit fat oxidation during extended storage.
Skim Milk Powder The procedure for the manufacture of skim milk powder differs in several features from the process for full-fat milk powder: fat standardization leads to very low fat content in skim milk – that is, 0.05–0.10%, heat treatment may be more intense compared with whole milk, and no homogenization is required. The skim milk heat treatment regime depends on the type of skim milk powder being produced. Skim milk powder produced by a ‘low-heat method’ is only pasteurized, whereas the ‘high-heat method’ requires an additional heat treatment at 85–88 C for 15–30 min. Such intensive heat treatment is necessary for the production of skim milk powders intended for use in the bakery industry, where a high degree of protein denaturation (low whey protein nitrogen index, WPNI) is desired. Figure 2 shows an alternative to heat treatment. Treatment Ò of raw skim milk by the Bactocatch procedure (microfiltration 1.4 mm) before concentration by vacuum evaporation and spray-drying leads to a high-quality milk powder. No heat
Milk Receiving and selection Clarification Sediment Cooling Skimming Fat Microfiltration 1.4 µm – Bactocatch ® Retentate Vacuum evaporation Water Spray-drying Water Package Packaging Storage Ultra-low-heat skim milk powder Figure 2
Flow chart for ultra-low-heat skim milk powder production.
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MILK AND MILK PRODUCTS j Microbiology of Dried Milk Products Table 6
Suggested microbiological standards for dried milk products
Contaminant
Skim/whole milk powder
Total viable count Coliforms Escherichia coli Staphylococcus aureus Salmonella spp. Yeasts and molds Thermophilic spores
3 10 cfu g <10 cfu g1 Absent in 25 g Absent in 25 g Absent in 200 g <10 per g <30 per 2 g 4
1
treatment is required to obtain an ultra-low-heat powder with a high WPNI (9 mg N g1 powder) and a maximum bacterial count of 3000 cfu g1. Such a powder has the same renneting time after water reconstitution as the original raw milk and it can be used as a reference powder for either industrial or scientific purposes. Instantization is a drying procedure that produces milk powder with better rehydration properties. By using two-stage or three-stage drying, this procedure significantly improves the quality and economics of drying technology. The rehydration properties (e.g., wettability, sinkability, dispersibility, solubility, and rate of dissolution) are enhanced, with optimal equilibrium between them. Instantization is based on agglomeration, which enables a larger volume of air to be incorporated between the powder particles, resulting in a characteristic coarse, clusterlike, agglomerated structure. Operations on the powder downstream from the drier have little further effect on bacterial load per se. Nevertheless, serious deterioration of powder quality can occur from environmental contamination.
Environmental Contamination Serious problems can arise when powder comes into contact with contaminated air surfaces. For example, a crack in a spraydrier wall can result in a reservoir of active bacteria within the material insulating the spray-drier. Such reservoirs are resistant to normal cleaning and disinfection procedures and can harbor pathogenic or spoilage bacteria. In addition, the air used for conveying powder must be sterile. The modern strategy to prevent powder recontamination involves careful separation of raw from heated products, tight control of environmental hazards, and scrupulous attention to cleaning and disinfection of surfaces that come into contact with the dried milk.
Casein/caseinates 1
3 10 cfu g Absent in 0.1 g Absent in 25 g Absent in 25 g Absent in 200 g <10 per g 4
Whey powder 5 104 cfu g1 <10 cfu g1 Absent in 25 g Absent in 25 g Absent in 200 g <10 per g
permeate), (5) powder exiting (the primary cyclone), and (6) packed product are monitored. It is prudent to include routine swabs from drains and walls in the monitoring operation because these can be valuable indicators of potential hazards.
Suggested Standards There is no single standard for the microbial status of dried milk products. Specifications vary from country to country and from customer to customer within countries. Nevertheless, there is an overall measure of agreement, and this is reflected in the suggested values proposed in Table 6. No account generally is taken of the potential threat of residual enzyme activity derived from psychrotrophic bacteria in the raw material from which the powder has been made. Protection from this undesirable occurrence could be ensured by the specification that the total viable count should not exceed 1 106 cfu ml1 at the point of manufacture.
See also: Bacillus: Introduction; Bacillus: Bacillus cereus; Cheese: Microbiology of Cheesemaking and Maturation; Clostridium; Dried Foods; Enterobacteriaceae: Coliforms and E. coli, Introduction; Enterobacteriaceae, Coliform, and Escherichia coli: Classical and Modern Methods for Detection and Enumeration; Fermented Milks: Range of Products; Fermented Milks and Yogurt; Heat Treatment of Foods: Ultra-High-Temperature Treatments; Heat Treatment of Foods – Principles of Pasteurization; Milk and Milk Products: Microbiology of Liquid Milk; Microbiology of Cream and Butter; Mycobacterium; Designing for Hygienic Operation; Process Hygiene: Overall Approach to Hygienic Processing; Process Hygiene: Modern Systems of Plant Cleaning; Process Hygiene: Risk and Control of Airborne Contamination; Pseudomonas: Introduction; Cronobacter (Enterobacter) sakazakii; Water Activity.
Process Monitoring It is apparent that limited information on the microbiological status of a spray-drying plant can be deduced from examination of the quality of the powder alone. Multipoint sampling is the most effective, especially if the bacterial load of (1) the raw milk, (2) stored milk from the balance tank of the heat exchanger, (3) vacuum evaporator, (4) crystallizer (used to crystallize the lactose on whey and
Further Reading Efstathiou, T., Feuardent, C., Méjean, S., Schuck, P., 2002. The use of carbonyl analysis to follow the main reactions involved in the process of deterioration of dehydrated dairy products: prediction of most favourable degree of dehydration. Lait 82, 423–439.
MILK AND MILK PRODUCTS j Microbiology of Dried Milk Products Masters, K., 2002. Spray Drying in Practice. SprayDryConsult International ApS, Charlottenlund. Muir, D., 1999. Milk and milk products j Microbiology and dried milk products. In: Robinson, R.K., Bath, C.A., Patel, P.D. (Eds.), Encyclopedia of Food Microbiology, first ed. Elsevier, Oxford, pp. 1441–1445. Pisecky, J., 1997. Handbook of Milk Powder Manufacture. Niro A/S, Copenhagen.
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Refstrup, E., 2003. Drying of milk. In: Roginsky, H. (Ed.), Encyclopedia of Dairy Sciences. Academic Press, London, pp. 860–871. Schuck, P., 2011. Dehydrated dairy products j milk powder: types and manufacture. In: Fuquay, J.W., Fox, P.F., McSweeney, P.L.H. (Eds.), Encyclopedia of Dairy Sciences, second ed., vol. 2. Academic Press, San Diego, pp. 108–116. Schuck, P., Dolivet, A., Jeantet, R., 2012. Analytical Methods for Food and Dairy Powders. Wiley-Blackwell, Oxford.
The microbiology of dried milk products is governed by the quality of the raw material, the conditions employed during manufacture of the product, and any postprocessing contamination. Most dried milk now is produced by spray-drying, and therefore, other drying methods (e.g., roller-drying or freezedrying) are excluded from consideration in this chapter. Because of the complexity of the manufacturing process, each step is considered in turn.
Manufacturing Processes The purpose of dehydration of milk is to stabilize the milk constituents for their storage and later use. The industrial application of concentration and fractionation by membrane processes (e.g., microfiltration, ultrafiltration, nanofiltration, and reverse osmosis), electrodialysis, or ion exchange provides opportunities and versatility to dry dairy technology, processing not only milk but also whey and its components. Dry milk products currently include milk powder, skim milk powder, whey powder, various whey protein powders, dry dairy–based beverages, casein, caseinates, coprecipitates, baby foods and cheese products, lactose, coffee whiteners, dry ice cream mix, and single-cell protein. The world production of dry dairy products has increased consistently in recent years, mainly due to the advantages of powders, which are as follows: l l l l l l
Raw Milk The raw milk used for powder production must be of high chemical, sensory, and bacteriological quality, which is regulated by standards. Most of the dried milk in the world is produced from cow’s milk, although small quantities of caprine, ovine, and camel milk are converted into powder. In the case of bovine milk, there are closely controlled schemes in which payment is related to milk quality. Among the quality indices, measurement of total viable bacteria count is used as a basis for payment. A clear distinction is made between two classes of organisms found in raw milk – i.e., pathogens and potential pathogens, and spoilage bacteria. Bulk milk spoilage bacteria are prevalent in refrigerated milk, and pathogens are seldom present at high levels. Nevertheless, the presence of pathogens in dried milk must be avoided. Processing conditions, therefore, must ensure that this is achieved. The pertinent properties of pathogens found in milk are summarized in Table 1. Only three of these organisms survive pasteurization: Bacillus cereus, Clostridia spp., and, to a limited extent, Mycobacterium paratuberculosis. None of these bacteria present a major risk in dried milk. The other pathogens
Retain high quality, without special storage conditions Reduce mass and volume compared with fluid products Provide balance between milk supply and consumption Provide an irreplaceable food component in hot climates Are a valuable food reserve for emergencies Are suitable for various tailor-made food products
Drying is defined as the removal of a liquid (usually water) from a product by evaporation, leaving the solids in an essentially dry state. A number of different drying processes, such as spray-drying, fluid bed-drying, roller-drying, freeze-drying, microwave drying, and superheated steam-drying, are in use in the dairy, food, chemical, and pharmaceutical industries. In special circumstances, roller-drying is used for the production of milk powder for particular applications (e.g., confectionery and feed blends). Direct contact of concentrated milk with rotating steam-heated rollers adversely affects the components of milk, especially proteins and lactose. Certain reactions, such as protein denaturation, Maillard reactions, and lactose caramelization are irreversible. Due to factors related to drying economics and final product quality, the only processes of significance in milk and dairy powder manufacture are spray-drying and fluid bed-drying (most often in combination). Only the combination of these two drying processes will be discussed in this section. A flow chart of milk powder production, consisting of reception, clarification, cooling, standardization, heat treatment,
738
evaporation, homogenization, drying, and packaging is shown in Figure 1.
Milk Receiving and selection Clarification Sediment Cooling Standardization – Skimming Fat Heat treatment Vacuum evaporation Water Homogenization Roller drying / Spray drying Water Package Packaging Storage Milk powder Figure 1
Flow chart for milk powder production.
Encyclopedia of Food Microbiology, Volume 2
http://dx.doi.org/10.1016/B978-0-12-384730-0.00220-2
MILK AND MILK PRODUCTS j Microbiology of Dried Milk Products Table 1
739
Pathogens and potential pathogens found in raw milk
Organism
Growth at <6 C
Survives pasteurization a
Bacillus cereus Campylobacter jejuni Clostridium spp. Escherichia coli Listeria monocytogenes Mycobacterium paratuberculosis Salmonella spp. Staphylococcus aureus Yersinia enterocolitica
Yesb No (No)c – Yes – No No Yes
Yes (spores) No Yes (spores) No No Yes (limited) No No No
Heat treatment at 72 C for 15 s. Some species only. c Some proteolytic species can grow at low temperature. a
b
listed in Table 1 do not survive heat treatment and thus can find their way into dried milk only by postprocessing contamination from the environment. In contrast, the main spoilage organisms in refrigerated, bulk raw milk are Gram-negative, psychrotrophic bacteria (Table 2) Pseudomonas spp. of both fluorescing and nonfluorescing types predominate. The Gram-negative organisms are killed readily by pasteurization and pose no threat to the quality of milk or products manufactured from it per se. Many of the Gram-negative psychrotrophs, however, produce extracellular enzymes with the potential to degrade the milk constituents. Lipase, protease, and combined lipase and protease activity are found in a substantial proportion of bacteria isolated from refrigerated, raw bulk milk (Table 2). Moreover, these enzymes are noted for their heat tolerance. Substantial proportions of activity remain after pasteurization (Table 3) and, surprisingly, after ultra-high-temperature treatment (UHT) at 140 C for 5 s (Table 4). The corollary to this is that, to avoid breakdown of milk constituents in products, psychrotrophic bacteria numbers must not be allowed to reach the critical level at which there is sufficient activity to initiate degradation (Muir, 1999). Several studies have sought to define this critical level. Rancidity, caused by lipase breaking down milk fat to liberate free fatty acid, has been detected in Cheddar cheese made from milk in which the psychrotrophic bacteria count exceeded
Table 2
7 106 colony forming units (cfu) per milliliter (ml). Lipase activity also has been suggested to be the cause of the soapy character in chocolate. The offending ingredient was dried milk made from raw milk of poor quality. Parallel research has determined that protease activity, expressed by gelation in UHT milk, can be exacerbated when the psychrotrophic bacteria count in the raw material exceeds 3 106 cfu ml1. Product quality, therefore, must be safeguarded by not using milk in which the count of psychrotrophic bacteria exceeds 106 cfu ml1. The psychrotrophic bacteria in milk grow remarkably quickly in refrigerated milk, with typical generation times in the range of 4–12 h. In addition, growth is sensitive to small (1–2 C) differences in storage temperature. Typically, raw silo milk with an initial psychrotrophic count of 5 104 cfu ml1 has an expected ‘safe’ shelf life of 36 h during storage at 6 C. If the milk has been deep cooled to 2 C on reception at the factory, an extension of the ‘safe’ storage period by 24 h might be anticipated. When further extensions of ‘safe’ storage time are required, more drastic treatment of the raw milk is necessary. The most useful technique is thermization. Thermization is the generic description of a range of subpasteurization heat treatments that kill most spoilage bacteria found in raw milk (but not all pathogens) with minimum collateral heat damage. Thermization of good quality raw milk at 65 C for 15 s, followed by prompt cooling to 2 C, offers a ‘safe’ shelf life of 72 h.
Spoilage bacteria in raw milk and associated extracellular enzyme activity Pseudomonas
Proportion of population (%) Creamery silo Farm bulk tank
Fluorescing
Nonfluorescing
Other Gram-negative flora a
33.5 50.5
44.1 31.5
22.4 18.0
32 1 11
0–25 0–9 24–92
Proportion of isolates with stated activity (%) Lipase only 5 Protease only 2 Lipase and protease 71 a
Includes bacteria classified as Enterobacteriaceae, Aeromonas, Pasteurella, or Vibrio; Acinetobacter, Moraxella, or Brucella; Flavobacterium; Chromobacterium; Alcaligenes.
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MILK AND MILK PRODUCTS j Microbiology of Dried Milk Products Table 3 Residual activity of extracellular enzymes after pasteurization at 72 C for 15 s Enzyme activity
Residual activity (%)
Lipase Protease Phospholipase C
59 66 30
Table 4 Residual enzyme activity after heat treatment of cell-free supernatant at 140 C for 5 s Enzyme activity (%) Bacterial type
Protease
Lipase
Phospholipase C
Pseudomonas Fluorescent Nonfluorescent Other Gram-negativea Bacillus cereus Bacillus firmus
17–50 5–48 0–57 <5 <5
14–51 0–73 0–82 – –
31–57 5 0–40 – –
a
Includes bacteria classified as Enterobacteriaceae, Aeromonas, Pasteurella, or Vibrio; Acinetobacter, Moraxella, or Brucella; Flavobacterium; Chromobacterium; Alcaligenes.
Milk Processing Clarification and Fat Standardization After reception, milk is clarified, usually by centrifugal separators, and cooled to 4 C in plate heat exchangers, followed by storage at the same temperature. The next operation is standardization, which is used to adjust the ratio of milk fat to total solids as required in the final product. Cream is separated from skim milk using a high-speed centrifugal separator. This device separates the milk on the basis of the density difference between the ‘light’ milk fat globules and the relatively dense serum. The two liquid streams then are recombined to yield a product with the required fat content. The overall effect of fat standardization on the microbial population of milk is modest. Another subtly different separation process – clarification, sometimes called bactofugation – also may be applied. Clarifiers or bactofuges are special separators, which remove microorganisms from milk on the basis of the density difference between the bacterium and the serum phase of the milk. This density difference is greatest in the case of the spores of spore-forming bacteria found in raw milk (e.g., Bacillus spp. and Clostridium spp.). A modern clarifier can achieve a 90% reduction in spore count in a single pass. Clarification is particularly valuable because, although the vegetative cells of spore-forming bacteria are inactivated by modest heat treatment, the spores can resist fairly severe heating (see section Heat Treatment). Thus, clarification offers an alternate method of controlling the spore count of the finished product.
Heat Treatment Heat treatment is commonly performed using the indirect method in a tubular or plate heat exchanger at 88–95 C for 15–30 s, the aims being to destroy pathogenic bacteria and most of the saprophytic microorganisms, to inactivate
enzymes (especially lipase), and to activate SH-groups in blactoglobulin, resulting in an antioxidative effect. Heat treatment during the production of milk powder serves another distinct purpose. It not only controls microbial quality but also influences functionality. For example, it is usual to apply severe heat treatment to milk destined for manufacture into whole milk powder. Such heating results in denaturation of whey protein. The presence of denatured protein reduces the rate of lipid oxidation during subsequent storage. Pasteurization (63 C/30 min or 72 C/15 s) kills most pathogens (Table 1) and all Gram-negative, psychrotrophic spoilage bacteria. A residual population of heat-resistant bacteria remains, however. These bacteria are called thermoduric and include members of the coryneform group, heat-resistant streptococci, micrococci, and spore-forming bacteria. The predominant spore-forming organisms found in heated milk are Bacillus spp., which survive heat treatment in the spore form (Table 5). Bacillus spp. degrade milk readily and are noted for their phospholipase activity. Spoilage due to these organisms often is associated with damage to the milk fat globule membrane and is characterized by the defect known as ‘bitty cream.’ Two species of Bacillus, Bacillus stearothermophilus and Bacillus thermodurans, pose particular threats because of their extreme heat resistance. As described previously, the population of spores in milk can be reduced by clarification. If very low spore counts are required in the product, then severe heat treatment must be applied to the milk: typically 110–120 C for 30 s.
Concentration and Homogenization Concentration by vacuum evaporation is used to concentrate milk before drying and can be combined beforehand with reverse osmosis. Evaporation is performed in multiple effect vacuum evaporators with mechanical or thermal steam recompression, where energy consumption is about 10–30 times lower than in spray-drying. The differences in the degree of concentration are due to the drying technique used: 30–35% total solid (TS) content for roller-drying, and 45–50% TS for spray-drying. Concentrating milk prior to drying has a positive effect on milk powder quality: Milk powder produced from concentrated milk consists of larger powder particles containing less occluded air and therefore results in better storage stability. Homogenization is not an obligatory operation, but it is usually applied with the aim of reducing the free fat content,
Table 5 Heat-resistant bacteria in milk and their associated enzyme activity Bacillus spp. Coryneform group Proportion of isolates (%) 63 C/30 min 54 61 80 C/10 min
46 37
Isolates with enzyme activity (%) Protease þ lipase 37 Protease only 34 Phospholipase 80 Inactive 12
10 3 0 67
MILK AND MILK PRODUCTS j Microbiology of Dried Milk Products which has a negative effect on powder solubility and its susceptibility to oxidation. Apart from the expected increase in count caused by the concentration process, there is an additional potential hazard. In multistage evaporators, typical of modern dairy plants, a concentrate may be held for extended periods at the temperature range 45–55 C. Some heat-resistant bacteria can grow under these conditions and, as a result, the bacterial content of the concentrate can increase disproportionately. After concentration, the product is homogenized to reduce fat globule size and inhibit creaming. Homogenization may cause an increase in bacterial count as a result of the disaggregation of bacterial clusters.
Spray-Drying, Coating, and Agglomeration The basic principle of spray-drying is the exposure of a fine dispersion of droplets created by means of atomization of preconcentrated milk products over a hot air stream in a drying chamber. Some authors defined spray-drying as an industrial process for the dehydration of a liquid by transforming the liquid into a spray of small droplets and exposing these droplets to a flow of hot air. The very large surface area of the spray droplets causes water evaporation to take place very quickly, converting the droplets into dry powder particles. The small droplet size created, and hence the large total surface area, results in very rapid evaporation of water at a relatively low temperature, thus minimizing heat damage to the product. The main advantages of spray-drying over other drying techniques are as follows: l
The process is rapid, residence time in the chamber being less than 30 s. l The product has a fine structure and excellent properties, with no adverse effects of heat (as occur for freeze-drying), as drying is accomplished in a very short time and at a low temperature. l The process is fully automatic with complete control of drying parameters and minimal labor of any type. l The product comes into contact with the drying chamber wall only in the powder form, so there is no problem of equipment maintenance or the microbiological quality of the final product. The investment cost, however, for a spray-drying plant is high and is economically justified only for large quantities or for products with high added value. Single-stage spray-driers are now considered outdated. The residence time is not long enough to obtain a real equilibrium between the relative humidity of the outlet air and water activity (aw) of the powder. The outlet temperature of the air therefore must be high, reducing the thermal efficiency of the single-stage spray-dryer. The two-stage and three-stage drying systems consist of limiting the spray-drying to a process with a longer residence time (several minutes) to provide a better thermodynamic balance. A second or a third final drying stage is necessary to optimize the moisture content by using an integrated fluid bed (static) or an external fluid bed (vibrating). Such dryers currently dominate the dairy powder industry. Though two-stage and three-stage drying may produce both nonagglomerated and agglomerated powders, their main products are instant milk powders.
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The ideal water content for optimal preservation of a given powder can be determined on the basis of sorption isotherms. Thus, the water content of milk powder would be 4% (regulated), between 2% and 3% for whey, and 6% (regulated) for casein, for aw of 0.2. Provided the ultimate moisture content of the powder is at an aw close to 0.2, bacteriostasis is ensured. In the case of dried whole milk, the moisture content may be as low as 2%, to inhibit fat oxidation during extended storage.
Skim Milk Powder The procedure for the manufacture of skim milk powder differs in several features from the process for full-fat milk powder: fat standardization leads to very low fat content in skim milk – that is, 0.05–0.10%, heat treatment may be more intense compared with whole milk, and no homogenization is required. The skim milk heat treatment regime depends on the type of skim milk powder being produced. Skim milk powder produced by a ‘low-heat method’ is only pasteurized, whereas the ‘high-heat method’ requires an additional heat treatment at 85–88 C for 15–30 min. Such intensive heat treatment is necessary for the production of skim milk powders intended for use in the bakery industry, where a high degree of protein denaturation (low whey protein nitrogen index, WPNI) is desired. Figure 2 shows an alternative to heat treatment. Treatment Ò of raw skim milk by the Bactocatch procedure (microfiltration 1.4 mm) before concentration by vacuum evaporation and spray-drying leads to a high-quality milk powder. No heat
Milk Receiving and selection Clarification Sediment Cooling Skimming Fat Microfiltration 1.4 µm – Bactocatch ® Retentate Vacuum evaporation Water Spray-drying Water Package Packaging Storage Ultra-low-heat skim milk powder Figure 2
Flow chart for ultra-low-heat skim milk powder production.
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MILK AND MILK PRODUCTS j Microbiology of Dried Milk Products Table 6
Suggested microbiological standards for dried milk products
Contaminant
Skim/whole milk powder
Total viable count Coliforms Escherichia coli Staphylococcus aureus Salmonella spp. Yeasts and molds Thermophilic spores
3 10 cfu g <10 cfu g1 Absent in 25 g Absent in 25 g Absent in 200 g <10 per g <30 per 2 g 4
1
treatment is required to obtain an ultra-low-heat powder with a high WPNI (9 mg N g1 powder) and a maximum bacterial count of 3000 cfu g1. Such a powder has the same renneting time after water reconstitution as the original raw milk and it can be used as a reference powder for either industrial or scientific purposes. Instantization is a drying procedure that produces milk powder with better rehydration properties. By using two-stage or three-stage drying, this procedure significantly improves the quality and economics of drying technology. The rehydration properties (e.g., wettability, sinkability, dispersibility, solubility, and rate of dissolution) are enhanced, with optimal equilibrium between them. Instantization is based on agglomeration, which enables a larger volume of air to be incorporated between the powder particles, resulting in a characteristic coarse, clusterlike, agglomerated structure. Operations on the powder downstream from the drier have little further effect on bacterial load per se. Nevertheless, serious deterioration of powder quality can occur from environmental contamination.
Environmental Contamination Serious problems can arise when powder comes into contact with contaminated air surfaces. For example, a crack in a spraydrier wall can result in a reservoir of active bacteria within the material insulating the spray-drier. Such reservoirs are resistant to normal cleaning and disinfection procedures and can harbor pathogenic or spoilage bacteria. In addition, the air used for conveying powder must be sterile. The modern strategy to prevent powder recontamination involves careful separation of raw from heated products, tight control of environmental hazards, and scrupulous attention to cleaning and disinfection of surfaces that come into contact with the dried milk.
Casein/caseinates 1
3 10 cfu g Absent in 0.1 g Absent in 25 g Absent in 25 g Absent in 200 g <10 per g 4
Whey powder 5 104 cfu g1 <10 cfu g1 Absent in 25 g Absent in 25 g Absent in 200 g <10 per g
permeate), (5) powder exiting (the primary cyclone), and (6) packed product are monitored. It is prudent to include routine swabs from drains and walls in the monitoring operation because these can be valuable indicators of potential hazards.
Suggested Standards There is no single standard for the microbial status of dried milk products. Specifications vary from country to country and from customer to customer within countries. Nevertheless, there is an overall measure of agreement, and this is reflected in the suggested values proposed in Table 6. No account generally is taken of the potential threat of residual enzyme activity derived from psychrotrophic bacteria in the raw material from which the powder has been made. Protection from this undesirable occurrence could be ensured by the specification that the total viable count should not exceed 1 106 cfu ml1 at the point of manufacture.
See also: Bacillus: Introduction; Bacillus: Bacillus cereus; Cheese: Microbiology of Cheesemaking and Maturation; Clostridium; Dried Foods; Enterobacteriaceae: Coliforms and E. coli, Introduction; Enterobacteriaceae, Coliform, and Escherichia coli: Classical and Modern Methods for Detection and Enumeration; Fermented Milks: Range of Products; Fermented Milks and Yogurt; Heat Treatment of Foods: Ultra-High-Temperature Treatments; Heat Treatment of Foods – Principles of Pasteurization; Milk and Milk Products: Microbiology of Liquid Milk; Microbiology of Cream and Butter; Mycobacterium; Designing for Hygienic Operation; Process Hygiene: Overall Approach to Hygienic Processing; Process Hygiene: Modern Systems of Plant Cleaning; Process Hygiene: Risk and Control of Airborne Contamination; Pseudomonas: Introduction; Cronobacter (Enterobacter) sakazakii; Water Activity.
Process Monitoring It is apparent that limited information on the microbiological status of a spray-drying plant can be deduced from examination of the quality of the powder alone. Multipoint sampling is the most effective, especially if the bacterial load of (1) the raw milk, (2) stored milk from the balance tank of the heat exchanger, (3) vacuum evaporator, (4) crystallizer (used to crystallize the lactose on whey and
Further Reading Efstathiou, T., Feuardent, C., Méjean, S., Schuck, P., 2002. The use of carbonyl analysis to follow the main reactions involved in the process of deterioration of dehydrated dairy products: prediction of most favourable degree of dehydration. Lait 82, 423–439.
MILK AND MILK PRODUCTS j Microbiology of Dried Milk Products Masters, K., 2002. Spray Drying in Practice. SprayDryConsult International ApS, Charlottenlund. Muir, D., 1999. Milk and milk products j Microbiology and dried milk products. In: Robinson, R.K., Bath, C.A., Patel, P.D. (Eds.), Encyclopedia of Food Microbiology, first ed. Elsevier, Oxford, pp. 1441–1445. Pisecky, J., 1997. Handbook of Milk Powder Manufacture. Niro A/S, Copenhagen.
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Refstrup, E., 2003. Drying of milk. In: Roginsky, H. (Ed.), Encyclopedia of Dairy Sciences. Academic Press, London, pp. 860–871. Schuck, P., 2011. Dehydrated dairy products j milk powder: types and manufacture. In: Fuquay, J.W., Fox, P.F., McSweeney, P.L.H. (Eds.), Encyclopedia of Dairy Sciences, second ed., vol. 2. Academic Press, San Diego, pp. 108–116. Schuck, P., Dolivet, A., Jeantet, R., 2012. Analytical Methods for Food and Dairy Powders. Wiley-Blackwell, Oxford.