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Butter Oil and Ghee
Anhydrous Milk Fat/Butter Oil and Ghee B K Mortensen, Tikøb, Denmark ª 2011 Elsevier Ltd. All rights reserved.
Introduction Milk fat is an essential part of the nutritional value of milk, but in the form of milk, cream, or butter, it has a limited keeping quality owing to microbiological deterioration, lipolysis, or oxidation. Because most of these spoilage processes take place in the water phase or at the interface between the water and the fat phase, it is well known that removal of the water phase extends the keeping quality of milk fat considerably. Butter surplus in many dairy-producing countries, combined with production fluctuations during the year and the unequal allocation of milk production in the world, has encouraged production of more stable forms of milk fat products. This is by no means a modern innovation because such products have been known for several thousand years by various names in Asia, the Middle East, and in milkproducing areas of Africa. Considerable quantities of anhydrous milk fat products are still produced in these parts of the world, often by indigenous methods, mainly for use in cooking. During World War II, large quantities of anhydrous milk fat was produced and used as spread by the Allied troops, but it was not until the 1960s that the production of anhydrous milk fat increased rapidly, especially in areas where large surplus of butter made it an obviously good idea to convert milk fat into products with higher storage stability, making export to new markets feasible.
Specifications The Codex Alimentarius Commission under the Joint FAO/WHO Food Standards Programme has established a standard (CODEX STAN 280-1973) with later revisions and amendments, for milk fat products intended for further processing or culinary use. The standard applies to products designated anhydrous milk fat, milk fat, anhydrous butter oil, butter oil, and ghee, defined as fatty products derived exclusively from milk and/or products obtained from milk by means of processes that result in almost total removal of water and nonfat solids. Ghee is furthermore defined as a product with an especially developed flavor and physical structure. The main specifications in the standard concerning the composition of the products and the designations to be used are shown in Table 1. It is further stated in the
standard that many food additives acting as antioxidants can be added. These additives, and the quantities allowed to be used, are listed in Codex General Standard for Food Additives (CODEX STAN 192-1995) with the remark that the designation ‘anhydrous milk fat’ is reserved for products where no additives are used. Finally, it should be mentioned that CODEX STAN 280-1973 also contains a list of quality factors intended for voluntary application by commercial partners but not for application by governments. These factors are listed in Table 2.
Products It is difficult to estimate the production volume of the various types of milk fat products as no official statistics are available. Normally the figures for all product types are pooled under the names AMF or butter oil independent of their composition, and without consideration for the specifications established by the Codex Alimentarius Commission. Furthermore, production figures for anhydrous milk fat and ghee are often converted into butter equivalents and included in the butter data. In the following text the term ‘AMF’ is used for both anhydrous milk fat/anhydrous butter oil, milk fat, and butter oil because both the production technology and the application of these products are quite similar, and AMF is furthermore the term commonly used in both industry and international trade. However, ghee is discussed separately.
AMF Major producers of AMF are New Zealand and Australia, each with big annual production volumes, but many countries with an industrialized dairy industry produce minor quantities. AMF is mainly exported to countries lacking sufficient production of fresh milk, especially in Asia, the Middle East, the South and Central Americas, and the Pacific, for production of recombined milk and other dairy products. Another important market for AMF is the food industry in which it is used for production of ice cream, bakery products, confectionery, and such. Furthermore, AMF is often an important commodity in different food aid programs.
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516 Butter and Other Milk Fat Products | Anhydrous Milk Fat/Butter Oil and Ghee Table 1 Composition of milk fat products (CODEX STAN 280-1973)
Minimum fat (g 100 g 1) Maximum water (g 100 g 1)
Anhydrous milk fat/ Anhydrous butter oil
Milk fat/butter oil/ghee
99.8
99.6
0.1
Product characteristics
AMF consists of a mixture of triacylglycerols (glycerol esterified to three fatty acids) with different physical properties, for example, melting point, which means that AMF melts over a wide temperature range. Normally AMF is completely melted at temperatures higher than 40 C where it appears as a transparent liquid with a light yellow color caused by the natural content of -carotene. When AMF is cooled it starts to crystallize and transform into a pale whitish yellow solid with a soft, slightly grainy texture. Like most fats, milk fat crystallizes from the melt in different polymorphic forms, of which only one is stable, whereas the others are metastable. On rapid cooling, metastable -crystals with a rather simple and loose structure usually form first, but these crystals are later rearranged irreversibly into the more stable 9-form, and finally into the most thermodynamically stable, compact-structured -form. However, it seems that most of the milk fat crystals remain in the 9-form even after prolonged storage. The -form has the lowest melting point, and the -form normally has the highest. The 9-form is preferred as it provides a fine arrangement of the fat crystals. The -form is often not desirable because the large crystals of this arrangement may result in a coarse and sandy texture. Over the years, polymorphism of milk fat has been studied thoroughly, but it is still unclear whether polymorphism has any importance under practical conditions and most costumers are probably not very concerned about the crystal structure in AMF.
AMF has a relatively high-average melting point, and that means that the functionality of AMF is not optimal for all applications. That milk fat consists of a mixture of triacylglycerols with different melting points implies it can be split into fractions with different melting properties suitable for different applications. The main procedure is to split AMF into two fractions: (1) a highmelting part called the stearin fraction and (2) a lowmelting part called the olein fraction. Milk fat has a unique flavor mainly attributed to components like short-chain free fatty acids, aldehydes, ketones, and lactones, and this is the main advantage of using AMF instead of cheaper fats and oils. Unpleasant off-flavors, however, can occur in AMF, one of which is the rancid flavor originating from the milk or butter used as starting material for production (see Butter and Other Milk Fat Products: The Product and Its Manufacture). Rancid flavor is caused by lipolysis (enzymatic hydrolysis of the ester linkage in the triacylglycerols) resulting in release of free fatty acids (FFA); in particular, increased concentration of free short-chain fatty acids give a distinct off-flavor. It is possible to remove, or at least reduce, this off-flavor during manufacturing of AMF as described later. As stated in CODEX STAN 280-1973 the content of FFA should not be higher than 0.3 or 0.4 expressed as the percentage of oleic acid, depending on the designation of the product. Another typical off-flavor that limits the storage stability is metallic, fishy, or oily flavor caused by lipid oxidation, which involves complex reactions between lipids and oxygen promoted by catalysts such as copper and iron ions. As stated in CODEX STAN 280-1973 the extent of the oxidation should not be higher than 0.3 or 0.6 milliequivalents of oxygen per kilogram fat, depending on the designation of the product. Stored butter used for production of AMF could be the origin of this off-flavor but it can also arise during prolonged storage of AMF. The oxidative stability of AMF can be improved considerably if different precautions are taken during processing. Contamination with copper and iron ions, which could originate from the processing equipment or from metal containers used for packaging of the
Table 2 Recommended quality factors (CODEX STAN 280-1973)
Maximum free fatty acids (FFA) expressed as oleic acid (g 100 g 1) Maximum peroxide value (milli-equivalents of oxygen kg Maximum copper content (mg kg 1) Maximum iron content (mg kg 1) Taste and odor Texture
1
fat)
Anhydrous milk fat/anhydrous butter oil
Milk fat/butter oil/ ghee
0.3
0.4
0.3 0.6 0.05 0.05 0.2 0.2 Acceptable for market requirements after heating a sample to 40–45 C Smooth and fine granules to liquid, depending on temperature
Butter and Other Milk Fat Products | Anhydrous Milk Fat/Butter Oil and Ghee
product, should be avoided, dissolved oxygen in the product should be reduced during processing, and the product should be package in light- and airtight containers and stored at low temperature. Also, addition of antioxidants to delay the oxidation is a possibility, depending on the designation of the product (CODEX STAN 192-1995). AMF will normally have a shelf life of several months, if these precautions are taken even if stored at ambient temperature, which will often be the case.
Applications
An important application of AMF is production of recombined liquid milk. Low-heat- or medium-heat skim milk powder is mixed with water and AMF. The AMF is added at a temperature higher than 40 C and has to be properly dispersed and emulsified into the product. In large-scale production, the melted AMF is injected in-line in a continuous operation and emulsified in a high-shear mixer. The mixing could also take place in a batch operation, in which the melted AMF is added to a solution of milk powder in water followed by emulsification, for example, in a homogenizer or a mixing device. It might be necessary to add suitable emulsifiers to create a stable emulsion. Also cream, cultured milk products, cheese and butter could be manufactured in a recombination process if the fat content of the mix is adjusted accordantly. AMF can also be used in production of blends, low-fat dairy spreads and blended spreads and an obvious application for low-melting fractions of AMF is in butter and dairy fat spreads in order to make these products more spreadable at low temperatures (see Butter and Other Milk Fat Products: Milk Fat-Based Spreads). Addition of high-melting fractions could also be an option with the aim of making butter more stable at high temperatures, which could be desirable in tropical countries. One disadvantage of using fractions of AMF is the increased production costs. Considerable quantities of AMF are used for production of ice cream in which it contributes not only to the flavor of the product, but also to a smooth, full, rich and creamy mouthfeel. One of the advantages of using AMF instead of cream or butter is that AMF is easier to handle in an automatic process line for preparing the ice cream mix. The advantages of using milk fat in the bakery industry are the buttery flavor and the creamy mouthfeel it imparts in products such as puff pastry, croissants, and Danish pastry and the functional properties, especially that of the stearin fraction of AMF with a higher melting point and solid fat content, that are useful in such products. AMF, also used in butter cookies where the main concern is flavor, has shortening properties such as crunchiness. A serious problem in butter cookies is fat blooming, which are mold-resembling pale stains, on the surface of the cookies formed by crystals of high–melting
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point fat; this can be avoided using the olein fraction of AMF. Milk fat in different forms, often AMF, is also used as an important ingredient in a variety of confectionery products despite its relatively high cost compared with most other fats and oils. Milk fat is primarily used for its buttery flavor, but another important property of milk fat is its ability to form part of the continuous fat phase in chocolate as a result of its compatibility with cocoa butter at most levels of addition. In dark chocolates, small amount (2–3 g 100 g 1) of milk fat is used to soften the product and to control texture formation. In milk chocolates, higher addition (often up to 20–30 g 100 g 1) is used. Milk fat has also the ability to inhibit the formation of fat bloom in chocolates, which appears as a grayish powdery discoloration on the surface of the chocolate composed of large fat crystals formed by phase transition in cocoa butter crystals at low ambient temperature. Ghee Ghee is the name used in India for a variety of milk fat products mainly made from cows’ or buffaloes’ milk by methods that can be traced back to ancient times. Similar products are produced in the Middle East and Africa commonly from goat, sheep, or camel milk. In these areas, the products are mostly known as maslee or by some variant of the Arabic word samn. These products are discussed in the following paragraphs under the common name ghee. The products were traditionally manufactured in rural areas where cream or butter was heated over open fire until the water had boiled off. During this treatment, considerable browning of the nonfat milk solids occurs that develops a special taste and a strong antioxidative effect. Today, ghee is often factory made by more industrialized methods. India is the major producer of ghee, but the product is now also gaining popularity in the Western world and can be purchased in many ethnic shops. In the Codex Alimentarius Commission standard applying to milk fat products (CODEX STAN 280-1973) the only difference in the specifications between ghee and other AMF products is that ghee is characterized as having a special flavor and physical structure. This is a rather unspecific and not very useful definition, but the problem is of course that the specification tries to cover all the varieties of ghee made in different countries. However, the main point is that ghee is an almost anhydrous milk fat product obtained by a high-temperature process that leaves part of the nonfat milk solids in the product. Product characteristics
The flavor of ghee is very dependent on the manufacturing process. Much of the typical flavor is the result of a
518 Butter and Other Milk Fat Products | Anhydrous Milk Fat/Butter Oil and Ghee
very complex mixture of compounds including carbonyls, lactones, FFA, and esters generated during processing. The typical flavor of burnt nonfat milk solids combined with some component derived from oxidation of the milk fat is also important. Lactones, which have a coconutlike flavor, seem to be especially important for the characteristic flavor of ghee. The levels of these are increased by increasing temperature during the production, and it is often twice as high in ghee as in butter. The role of phospholipids in ghee has been much debated. Some studies have shown that high content of phospholipids results in poor keeping quality, whereas others have indicated that phospholipids dissolved in fat, as is the case in ghee, have distinct antioxidative properties. This needs further investigation. Ghee has an intense yellow color, unless produced from buffaloes’ milk, which contains much less -carotene than cows’ milk. Ghee should have a soft and granular texture, with a large proportion of crystalline fat suspended in sufficient liquid fat so the product can be poured at room temperature. Such a texture could be promoted by slow cooling under constant stirring, but textural changes might occur during storage depending on the temperature. Ghee normally has a shelf life of 6–8 months even at ambient temperature, but considerably longer storage stability has been experienced probably because of the antioxidative effect created during processing. However, ghee eventually deteriorates during storage; the limiting factor is normally the development of oxidized flavor.
AMF Technology AMF is produced from either cream or butter in a process where centrifugal separators are the essential equipment as shown in Figure 1. Manufacture of AMF from cream
The process for production of AMF directly from cream is shown in Figure 2. Cream is concentrated in a special separator (4) to a fat content of 70–80 g 100 g 1 after which the highly concentrated fat emulsion is homogenized (7), which results in disruption of the fat globule membranes causing a phase inversion. The mixture of butter oil and serum is then separated in a centrifugal separator or concentrator (9), where the fat content is concentrated to approximately 99.5 g 100 g 1. It might be necessary to wash the oil phase with water to obtain a clear transparent, bright shining product (Figure 1). This production step, called polishing, involves addition of 20–30 g water 100 g 1 to the oil. It is important that the water temperature remain the same as the oil temperature. After mixing and a short holding time, the water is removed again in a separator together with the water-soluble components. Polishing could be combined by neutralization if the content of FFA in the oil is too high. Neutralization implies that a sodium hydroxide solution is injected continuously into the oil stream coming from the concentrator (9) after which the mixture is held for some seconds in a holding cell to allow the reaction to take place followed by addition of the
Butter
Cream
Neutralization
Melting
Separation
Polishing
Concentration
Phase inversion
Separation
Heating
Applications
Ghee is primarily used for cooking and frying and as dressing or toppings for various foods. It is also used in the manufacture of snacks and sweets often mixed with vegetables, cereals, fruits, and nuts. In some parts of the world, ghee is considered as a sacred product and is used in religious rites. It could also be mentioned that ghee is used in Ayurveda, which is a system of traditional medicine developed in India several thousand years ago and now also practiced in other parts of the world as alternative medicine.
Vacuum
Manufacturing Technology The overall principle in the manufacturing technology for the different milk fat products is removal of water, leaving an almost pure fat phase. The industrialized process for manufacturing of AMF is described later, followed by a separate discussion of the manufacturing process for ghee.
Cooling
Packaging Figure 1 Block diagram showing principles of production of anhydrous milk fat (AMF) from cream or butter with possibilities for polishing and/or neutralization.
Butter and Other Milk Fat Products | Anhydrous Milk Fat/Butter Oil and Ghee
519
1 2
5
4
3
8
12
9
6
7
10
11 13
Cream Buttermilk Heating media Cooling media Vapor Figure 2 Production line for anhydrous milk fat (AMF) from cream. 1, balance tank; 2, plate heat exchanger; 3, balance tank; 4, preconcentrator; 5, separator (optional) for ‘buttermilk’ from the preconcentrator (4); 6, buffer tank; 7, homogenizer for phase inversion; 8, plate heat exchanger for cooling; 9, final concentrator; 10, balance tank; 11, plate heat exchanger for heating/cooling; 12, vacuum chamber; 13, storage tank. Reproduced with permission from Dairy Processing Handbook by permission of Tetra Pak A/B, Lund, Sweden.
polishing water. When the added water is removed again in a separator, the saponified FFA will be removed simultaneously. After the final concentration the butter oil is heated to 90–95 C in a plate heat exchanger (11) and pumped to a vacuum chamber (12), where the oil is introduced either as a thin film or as a spray so the increased surface area makes it possible to further remove water and dissolved air. The oil, now having a fat content of higher than 99.8 g 100 g 1, is finally pumped back to the plate heat exchanger, where it is cooled to approximately 40 C and pumped to a storage tank (13), where flushing with an inert gas and addition of antioxidants is a possibility before packaging. Manufacture of AMF from butter
If it has already been produced, butter is the starting material for the production of AMF; the process, which is shown in Figure 3, is initiated by melting of the butter. Both sweet cream and cultured butter as well as salted and unsalted butter can be used, often in the form of cold stored or frozen butter stored for some time. The melting process often starts by heating the butter in a
temperature-controlled room, but microwave thawing can also be used. After warming, further heating follows in a special melting equipment (1) by indirect heating with steam or hot water to a temperature well above the melting point of the fat. If the production of AMF takes place immediately after the butter manufacture without any intermediate storage, the butter is pumped directly to the melting equipment. After melting the now liquid blend of milk fat and butter serum is pumped to a holding tank (2), where it is held for about 30 min to ensure complete melting of the fat and aggregation of the proteins in the aqueous phase. From the holding tank, the blend is pumped to a special separator or concentrator (3). The separation is a critical stage in the process as it is very dependent on the type of butter used. The more or less undenaturated proteins in the serum phase of sweet cream butter will often result in a certain degree of emulsification of the fat, which will complicate the separation. This can be counteracted by lowering the pH-value of the serum phase to about 4.5 for example by adding citric acid during melting. This will denaturate the proteins and consequently reduces their emulsifying ability.
520 Butter and Other Milk Fat Products | Anhydrous Milk Fat/Butter Oil and Ghee
1
6
2
5
3 4
7
Cream Buttermilk Heating media Cooling media Vapor Figure 3 Production line for anhydrous milk fat (AMF) from butter. 1, melter and heater for butter; 2, holding tank; 3, concentrator; 4, balance tank; 5, plate heat exchanger for heating/cooling; 6, vacuum chamber; 7, storage tank. Reproduced with permission from Dairy Processing Handbook by permission of Tetra Pak A/B, Lund, Sweden.
After separation, the fat phase is further treated in a similar process as described when using cream as starting material.
AMF
Fractionation of AMF
As already mentioned, AMF is not suitable for all applications, but its functional properties can be altered by using a fractionation process where the milk fat is split into fractions with different melting points. Different methods of fractionation have been developed. One of the simplest and most efficient methods is crystallization from a solvent, such as acetone, but the use of organic solvents in food production and the consequent loss of butter flavor are problematic and have prevented commercial exploitation of this method. The most commonly used fractionation process (Figure 4) in commercial operation today is one in which liquid AMF is crystallized in a tank, where it is taken through a controlled temperature program. During this process triglycerides with high melting points will crystallize, and afterwards the slurry is divided into a liquid and a solid fraction by vacuum or pressure filtration, by centrifugation, or by a combination of these methods. The applied cooling program strongly influences the composition of the fat crystals, and the cooling rate influences the quantity of the crystallized fat and the size of the crystals. This so-called dry fractionation or crystallization from the melt is relatively cheap and is based on a physical process not involving solvents.
Melting
Controlled temperature program
Separation of liquid and solid fat
Stearin fraction
Olein fraction
Figure 4 Block diagram showing the principle of fractionation of anhydrous milk fat (AMF).
Furthermore, this process has the least effect on flavor and is generally accepted by legislation. The result of the fractionation is a high-melting fraction, the so-called stearin fraction, and a low-melting fraction, the so-called olein fraction. These two fractions
Butter and Other Milk Fat Products | Anhydrous Milk Fat/Butter Oil and Ghee
can be further fractionated by a multistep fractionation process into even harder and softer fractions, respectively. Packaging of AMF
An often used procedure in connection with packaging of AMF is to flush the containers with an inert gas before, during, or after filling and use airtight containers. The traditional way of packaging AMF is in large iron drums or barrels containing 190–200 kg and enameled on the inside or lined with a plastic foil, a so-called inner liner, to prevent direct contact between the metal and the AMF. A certain headspace in the barrels is required to control the drop of pressure arising from the reduction in volume caused by crystallization of the AMF. Nowadays a commonly used packaging is cardboard boxes with an inner liner of plastic foil, the so-called bag-in-box system. Different size of packaging is used, for example, 25 kg. Also consumer packaging in cans containing 0.5, 1, or 2 kg seems to be gaining popularity. For very big customers, shipping often takes place in road tankers or very large bulk tanks suitable for transport with containerships. Ghee Technology From ancient times, ghee has been prepared in households by heating fermented milk or butter in open pans causing the water to boil off. The fat is then separated through a strainer, and finally cooled slowly to give a coarse texture of crystallized fat and liquid oil. Nowadays ghee is often factory made from milk, cream, or butter using different technologies in which heat treatment is the essential process step. If milk is the starting material, it is normally separated into cream and skim milk, and the cream is then subsequently churned into butter, melted, and processed into ghee in a process where the butter is heated to about 100–104 C until the residual water content is lower than 1 g 100 g 1. After this the temperature is increased to 110–115 C, and the product is held at this temperature until the desired flavor has developed. The advantage of this method is that most of the water is removed by the skim milk and by the buttermilk, which is a much more energy-efficient method than removal by heating. In an even more efficient method the butter is heated to about 85 C, held for 30 min to allow the aqueous phase to form a bottom layer of residual buttermilk, which is then removed. Finally the fat phase is further heated to about 110 C. One of the industrialized methods used for production of ghee is a batch process in which butter or cream is boiled in a stainless steel vessel by indirect heating with steam. India has a large production of ghee, and because of the commercial importance of the production, considerable improvements in the manufacturing process have
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been developed over the years in that country. Thus a continuous ghee manufacturing process has been developed employing scraped surface heat exchangers and centrifugal separators and clarifiers. A method based on heating with microwave technology has also been developed. Ghee is normally packed in metal cans and containers of various sizes internally lined with a nontoxic lacquer. Cardboard boxes with an inner liner of plastic foil are also used.
Final Remarks AMF and similar products have many applications, and in some parts of the world, they are of vital importance for the economy of the dairy industry. An essential characteristic of the products is their prolonged keeping quality caused by the very low water content, which makes the products suitable for acting as a buffer during fluctuations in milk production over the year. It also makes the products suitable for shipment to faraway destinations to help leveling the unequal allocation of milk production in the world. See also: Butter and Other Milk Fat Products: Milk Fat Based Spreads; The Product and Its Manufacture. Enzymes Exogenous to Milk in Dairy Technology: Lipases. Enzymes Indigenous to Milk: Lipases and Esterases. Flavors and Off-Flavors in Dairy Foods. Milk Lipids: Fatty Acids; Lipid Oxidation; Lipolysis and Hydrolytic Rancidity.
Further Reading Anonymous (2003a) Anhydrous milk fat (AMF) and butteroil. In: Dairy Processing Handbook, 2nd edn., pp. 293–299. Lund: Tetra Pak Processing Systems AB. Anonymous (2003b) Butter and dairy spreads. In: Dairy Processing Handbook, 2nd edn., pp. 277–292. Lund: Tetra Pak Processing Systems AB. Deffense E and Hartel RW (2003) Fractionation of milk fat. In: Rossell B (ed.) Oils and Fats, Vol. 3: Dairy Fats, pp. 158–186. Leatherhead: Leatherhead Food International. Hartel RW (1996) Applications of milk fat fractions in confectionery products. Journal of the American Chemical Society 73: 945–953. Illingworth D, Patil GR, and Tamime AY (2009) Anhydrous milk fat manufacture and fractionation. In: Tamime AY (ed.) Dairy Fats and Related Products, pp. 108–166. Oxford: Wiley-Blackwell. International Dairy Federation (1991) Utilizations of milkfat. Bulletin No. 260/1991. Brussels: International Dairy Federation. Munro DS, Cant PAE, MacGibbon AKH, Illingworth D, and Nichola P (1998) Concentrated milkfat products. In: Early R (ed.) The Technology of Dairy Products, 2nd edn., pp. 198–227. Glasgow: Blackie Academic & Professional. Sserunjogi ML, Abrahamsen RK, and Narvhus J (1998) Current knowledge of ghee and related products. International Dairy Journal 8: 677–688.
Introduction Milk fat is an essential part of the nutritional value of milk, but in the form of milk, cream, or butter, it has a limited keeping quality owing to microbiological deterioration, lipolysis, or oxidation. Because most of these spoilage processes take place in the water phase or at the interface between the water and the fat phase, it is well known that removal of the water phase extends the keeping quality of milk fat considerably. Butter surplus in many dairy-producing countries, combined with production fluctuations during the year and the unequal allocation of milk production in the world, has encouraged production of more stable forms of milk fat products. This is by no means a modern innovation because such products have been known for several thousand years by various names in Asia, the Middle East, and in milkproducing areas of Africa. Considerable quantities of anhydrous milk fat products are still produced in these parts of the world, often by indigenous methods, mainly for use in cooking. During World War II, large quantities of anhydrous milk fat was produced and used as spread by the Allied troops, but it was not until the 1960s that the production of anhydrous milk fat increased rapidly, especially in areas where large surplus of butter made it an obviously good idea to convert milk fat into products with higher storage stability, making export to new markets feasible.
Specifications The Codex Alimentarius Commission under the Joint FAO/WHO Food Standards Programme has established a standard (CODEX STAN 280-1973) with later revisions and amendments, for milk fat products intended for further processing or culinary use. The standard applies to products designated anhydrous milk fat, milk fat, anhydrous butter oil, butter oil, and ghee, defined as fatty products derived exclusively from milk and/or products obtained from milk by means of processes that result in almost total removal of water and nonfat solids. Ghee is furthermore defined as a product with an especially developed flavor and physical structure. The main specifications in the standard concerning the composition of the products and the designations to be used are shown in Table 1. It is further stated in the
standard that many food additives acting as antioxidants can be added. These additives, and the quantities allowed to be used, are listed in Codex General Standard for Food Additives (CODEX STAN 192-1995) with the remark that the designation ‘anhydrous milk fat’ is reserved for products where no additives are used. Finally, it should be mentioned that CODEX STAN 280-1973 also contains a list of quality factors intended for voluntary application by commercial partners but not for application by governments. These factors are listed in Table 2.
Products It is difficult to estimate the production volume of the various types of milk fat products as no official statistics are available. Normally the figures for all product types are pooled under the names AMF or butter oil independent of their composition, and without consideration for the specifications established by the Codex Alimentarius Commission. Furthermore, production figures for anhydrous milk fat and ghee are often converted into butter equivalents and included in the butter data. In the following text the term ‘AMF’ is used for both anhydrous milk fat/anhydrous butter oil, milk fat, and butter oil because both the production technology and the application of these products are quite similar, and AMF is furthermore the term commonly used in both industry and international trade. However, ghee is discussed separately.
AMF Major producers of AMF are New Zealand and Australia, each with big annual production volumes, but many countries with an industrialized dairy industry produce minor quantities. AMF is mainly exported to countries lacking sufficient production of fresh milk, especially in Asia, the Middle East, the South and Central Americas, and the Pacific, for production of recombined milk and other dairy products. Another important market for AMF is the food industry in which it is used for production of ice cream, bakery products, confectionery, and such. Furthermore, AMF is often an important commodity in different food aid programs.
515
516 Butter and Other Milk Fat Products | Anhydrous Milk Fat/Butter Oil and Ghee Table 1 Composition of milk fat products (CODEX STAN 280-1973)
Minimum fat (g 100 g 1) Maximum water (g 100 g 1)
Anhydrous milk fat/ Anhydrous butter oil
Milk fat/butter oil/ghee
99.8
99.6
0.1
Product characteristics
AMF consists of a mixture of triacylglycerols (glycerol esterified to three fatty acids) with different physical properties, for example, melting point, which means that AMF melts over a wide temperature range. Normally AMF is completely melted at temperatures higher than 40 C where it appears as a transparent liquid with a light yellow color caused by the natural content of -carotene. When AMF is cooled it starts to crystallize and transform into a pale whitish yellow solid with a soft, slightly grainy texture. Like most fats, milk fat crystallizes from the melt in different polymorphic forms, of which only one is stable, whereas the others are metastable. On rapid cooling, metastable -crystals with a rather simple and loose structure usually form first, but these crystals are later rearranged irreversibly into the more stable 9-form, and finally into the most thermodynamically stable, compact-structured -form. However, it seems that most of the milk fat crystals remain in the 9-form even after prolonged storage. The -form has the lowest melting point, and the -form normally has the highest. The 9-form is preferred as it provides a fine arrangement of the fat crystals. The -form is often not desirable because the large crystals of this arrangement may result in a coarse and sandy texture. Over the years, polymorphism of milk fat has been studied thoroughly, but it is still unclear whether polymorphism has any importance under practical conditions and most costumers are probably not very concerned about the crystal structure in AMF.
AMF has a relatively high-average melting point, and that means that the functionality of AMF is not optimal for all applications. That milk fat consists of a mixture of triacylglycerols with different melting points implies it can be split into fractions with different melting properties suitable for different applications. The main procedure is to split AMF into two fractions: (1) a highmelting part called the stearin fraction and (2) a lowmelting part called the olein fraction. Milk fat has a unique flavor mainly attributed to components like short-chain free fatty acids, aldehydes, ketones, and lactones, and this is the main advantage of using AMF instead of cheaper fats and oils. Unpleasant off-flavors, however, can occur in AMF, one of which is the rancid flavor originating from the milk or butter used as starting material for production (see Butter and Other Milk Fat Products: The Product and Its Manufacture). Rancid flavor is caused by lipolysis (enzymatic hydrolysis of the ester linkage in the triacylglycerols) resulting in release of free fatty acids (FFA); in particular, increased concentration of free short-chain fatty acids give a distinct off-flavor. It is possible to remove, or at least reduce, this off-flavor during manufacturing of AMF as described later. As stated in CODEX STAN 280-1973 the content of FFA should not be higher than 0.3 or 0.4 expressed as the percentage of oleic acid, depending on the designation of the product. Another typical off-flavor that limits the storage stability is metallic, fishy, or oily flavor caused by lipid oxidation, which involves complex reactions between lipids and oxygen promoted by catalysts such as copper and iron ions. As stated in CODEX STAN 280-1973 the extent of the oxidation should not be higher than 0.3 or 0.6 milliequivalents of oxygen per kilogram fat, depending on the designation of the product. Stored butter used for production of AMF could be the origin of this off-flavor but it can also arise during prolonged storage of AMF. The oxidative stability of AMF can be improved considerably if different precautions are taken during processing. Contamination with copper and iron ions, which could originate from the processing equipment or from metal containers used for packaging of the
Table 2 Recommended quality factors (CODEX STAN 280-1973)
Maximum free fatty acids (FFA) expressed as oleic acid (g 100 g 1) Maximum peroxide value (milli-equivalents of oxygen kg Maximum copper content (mg kg 1) Maximum iron content (mg kg 1) Taste and odor Texture
1
fat)
Anhydrous milk fat/anhydrous butter oil
Milk fat/butter oil/ ghee
0.3
0.4
0.3 0.6 0.05 0.05 0.2 0.2 Acceptable for market requirements after heating a sample to 40–45 C Smooth and fine granules to liquid, depending on temperature
Butter and Other Milk Fat Products | Anhydrous Milk Fat/Butter Oil and Ghee
product, should be avoided, dissolved oxygen in the product should be reduced during processing, and the product should be package in light- and airtight containers and stored at low temperature. Also, addition of antioxidants to delay the oxidation is a possibility, depending on the designation of the product (CODEX STAN 192-1995). AMF will normally have a shelf life of several months, if these precautions are taken even if stored at ambient temperature, which will often be the case.
Applications
An important application of AMF is production of recombined liquid milk. Low-heat- or medium-heat skim milk powder is mixed with water and AMF. The AMF is added at a temperature higher than 40 C and has to be properly dispersed and emulsified into the product. In large-scale production, the melted AMF is injected in-line in a continuous operation and emulsified in a high-shear mixer. The mixing could also take place in a batch operation, in which the melted AMF is added to a solution of milk powder in water followed by emulsification, for example, in a homogenizer or a mixing device. It might be necessary to add suitable emulsifiers to create a stable emulsion. Also cream, cultured milk products, cheese and butter could be manufactured in a recombination process if the fat content of the mix is adjusted accordantly. AMF can also be used in production of blends, low-fat dairy spreads and blended spreads and an obvious application for low-melting fractions of AMF is in butter and dairy fat spreads in order to make these products more spreadable at low temperatures (see Butter and Other Milk Fat Products: Milk Fat-Based Spreads). Addition of high-melting fractions could also be an option with the aim of making butter more stable at high temperatures, which could be desirable in tropical countries. One disadvantage of using fractions of AMF is the increased production costs. Considerable quantities of AMF are used for production of ice cream in which it contributes not only to the flavor of the product, but also to a smooth, full, rich and creamy mouthfeel. One of the advantages of using AMF instead of cream or butter is that AMF is easier to handle in an automatic process line for preparing the ice cream mix. The advantages of using milk fat in the bakery industry are the buttery flavor and the creamy mouthfeel it imparts in products such as puff pastry, croissants, and Danish pastry and the functional properties, especially that of the stearin fraction of AMF with a higher melting point and solid fat content, that are useful in such products. AMF, also used in butter cookies where the main concern is flavor, has shortening properties such as crunchiness. A serious problem in butter cookies is fat blooming, which are mold-resembling pale stains, on the surface of the cookies formed by crystals of high–melting
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point fat; this can be avoided using the olein fraction of AMF. Milk fat in different forms, often AMF, is also used as an important ingredient in a variety of confectionery products despite its relatively high cost compared with most other fats and oils. Milk fat is primarily used for its buttery flavor, but another important property of milk fat is its ability to form part of the continuous fat phase in chocolate as a result of its compatibility with cocoa butter at most levels of addition. In dark chocolates, small amount (2–3 g 100 g 1) of milk fat is used to soften the product and to control texture formation. In milk chocolates, higher addition (often up to 20–30 g 100 g 1) is used. Milk fat has also the ability to inhibit the formation of fat bloom in chocolates, which appears as a grayish powdery discoloration on the surface of the chocolate composed of large fat crystals formed by phase transition in cocoa butter crystals at low ambient temperature. Ghee Ghee is the name used in India for a variety of milk fat products mainly made from cows’ or buffaloes’ milk by methods that can be traced back to ancient times. Similar products are produced in the Middle East and Africa commonly from goat, sheep, or camel milk. In these areas, the products are mostly known as maslee or by some variant of the Arabic word samn. These products are discussed in the following paragraphs under the common name ghee. The products were traditionally manufactured in rural areas where cream or butter was heated over open fire until the water had boiled off. During this treatment, considerable browning of the nonfat milk solids occurs that develops a special taste and a strong antioxidative effect. Today, ghee is often factory made by more industrialized methods. India is the major producer of ghee, but the product is now also gaining popularity in the Western world and can be purchased in many ethnic shops. In the Codex Alimentarius Commission standard applying to milk fat products (CODEX STAN 280-1973) the only difference in the specifications between ghee and other AMF products is that ghee is characterized as having a special flavor and physical structure. This is a rather unspecific and not very useful definition, but the problem is of course that the specification tries to cover all the varieties of ghee made in different countries. However, the main point is that ghee is an almost anhydrous milk fat product obtained by a high-temperature process that leaves part of the nonfat milk solids in the product. Product characteristics
The flavor of ghee is very dependent on the manufacturing process. Much of the typical flavor is the result of a
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very complex mixture of compounds including carbonyls, lactones, FFA, and esters generated during processing. The typical flavor of burnt nonfat milk solids combined with some component derived from oxidation of the milk fat is also important. Lactones, which have a coconutlike flavor, seem to be especially important for the characteristic flavor of ghee. The levels of these are increased by increasing temperature during the production, and it is often twice as high in ghee as in butter. The role of phospholipids in ghee has been much debated. Some studies have shown that high content of phospholipids results in poor keeping quality, whereas others have indicated that phospholipids dissolved in fat, as is the case in ghee, have distinct antioxidative properties. This needs further investigation. Ghee has an intense yellow color, unless produced from buffaloes’ milk, which contains much less -carotene than cows’ milk. Ghee should have a soft and granular texture, with a large proportion of crystalline fat suspended in sufficient liquid fat so the product can be poured at room temperature. Such a texture could be promoted by slow cooling under constant stirring, but textural changes might occur during storage depending on the temperature. Ghee normally has a shelf life of 6–8 months even at ambient temperature, but considerably longer storage stability has been experienced probably because of the antioxidative effect created during processing. However, ghee eventually deteriorates during storage; the limiting factor is normally the development of oxidized flavor.
AMF Technology AMF is produced from either cream or butter in a process where centrifugal separators are the essential equipment as shown in Figure 1. Manufacture of AMF from cream
The process for production of AMF directly from cream is shown in Figure 2. Cream is concentrated in a special separator (4) to a fat content of 70–80 g 100 g 1 after which the highly concentrated fat emulsion is homogenized (7), which results in disruption of the fat globule membranes causing a phase inversion. The mixture of butter oil and serum is then separated in a centrifugal separator or concentrator (9), where the fat content is concentrated to approximately 99.5 g 100 g 1. It might be necessary to wash the oil phase with water to obtain a clear transparent, bright shining product (Figure 1). This production step, called polishing, involves addition of 20–30 g water 100 g 1 to the oil. It is important that the water temperature remain the same as the oil temperature. After mixing and a short holding time, the water is removed again in a separator together with the water-soluble components. Polishing could be combined by neutralization if the content of FFA in the oil is too high. Neutralization implies that a sodium hydroxide solution is injected continuously into the oil stream coming from the concentrator (9) after which the mixture is held for some seconds in a holding cell to allow the reaction to take place followed by addition of the
Butter
Cream
Neutralization
Melting
Separation
Polishing
Concentration
Phase inversion
Separation
Heating
Applications
Ghee is primarily used for cooking and frying and as dressing or toppings for various foods. It is also used in the manufacture of snacks and sweets often mixed with vegetables, cereals, fruits, and nuts. In some parts of the world, ghee is considered as a sacred product and is used in religious rites. It could also be mentioned that ghee is used in Ayurveda, which is a system of traditional medicine developed in India several thousand years ago and now also practiced in other parts of the world as alternative medicine.
Vacuum
Manufacturing Technology The overall principle in the manufacturing technology for the different milk fat products is removal of water, leaving an almost pure fat phase. The industrialized process for manufacturing of AMF is described later, followed by a separate discussion of the manufacturing process for ghee.
Cooling
Packaging Figure 1 Block diagram showing principles of production of anhydrous milk fat (AMF) from cream or butter with possibilities for polishing and/or neutralization.
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1 2
5
4
3
8
12
9
6
7
10
11 13
Cream Buttermilk Heating media Cooling media Vapor Figure 2 Production line for anhydrous milk fat (AMF) from cream. 1, balance tank; 2, plate heat exchanger; 3, balance tank; 4, preconcentrator; 5, separator (optional) for ‘buttermilk’ from the preconcentrator (4); 6, buffer tank; 7, homogenizer for phase inversion; 8, plate heat exchanger for cooling; 9, final concentrator; 10, balance tank; 11, plate heat exchanger for heating/cooling; 12, vacuum chamber; 13, storage tank. Reproduced with permission from Dairy Processing Handbook by permission of Tetra Pak A/B, Lund, Sweden.
polishing water. When the added water is removed again in a separator, the saponified FFA will be removed simultaneously. After the final concentration the butter oil is heated to 90–95 C in a plate heat exchanger (11) and pumped to a vacuum chamber (12), where the oil is introduced either as a thin film or as a spray so the increased surface area makes it possible to further remove water and dissolved air. The oil, now having a fat content of higher than 99.8 g 100 g 1, is finally pumped back to the plate heat exchanger, where it is cooled to approximately 40 C and pumped to a storage tank (13), where flushing with an inert gas and addition of antioxidants is a possibility before packaging. Manufacture of AMF from butter
If it has already been produced, butter is the starting material for the production of AMF; the process, which is shown in Figure 3, is initiated by melting of the butter. Both sweet cream and cultured butter as well as salted and unsalted butter can be used, often in the form of cold stored or frozen butter stored for some time. The melting process often starts by heating the butter in a
temperature-controlled room, but microwave thawing can also be used. After warming, further heating follows in a special melting equipment (1) by indirect heating with steam or hot water to a temperature well above the melting point of the fat. If the production of AMF takes place immediately after the butter manufacture without any intermediate storage, the butter is pumped directly to the melting equipment. After melting the now liquid blend of milk fat and butter serum is pumped to a holding tank (2), where it is held for about 30 min to ensure complete melting of the fat and aggregation of the proteins in the aqueous phase. From the holding tank, the blend is pumped to a special separator or concentrator (3). The separation is a critical stage in the process as it is very dependent on the type of butter used. The more or less undenaturated proteins in the serum phase of sweet cream butter will often result in a certain degree of emulsification of the fat, which will complicate the separation. This can be counteracted by lowering the pH-value of the serum phase to about 4.5 for example by adding citric acid during melting. This will denaturate the proteins and consequently reduces their emulsifying ability.
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1
6
2
5
3 4
7
Cream Buttermilk Heating media Cooling media Vapor Figure 3 Production line for anhydrous milk fat (AMF) from butter. 1, melter and heater for butter; 2, holding tank; 3, concentrator; 4, balance tank; 5, plate heat exchanger for heating/cooling; 6, vacuum chamber; 7, storage tank. Reproduced with permission from Dairy Processing Handbook by permission of Tetra Pak A/B, Lund, Sweden.
After separation, the fat phase is further treated in a similar process as described when using cream as starting material.
AMF
Fractionation of AMF
As already mentioned, AMF is not suitable for all applications, but its functional properties can be altered by using a fractionation process where the milk fat is split into fractions with different melting points. Different methods of fractionation have been developed. One of the simplest and most efficient methods is crystallization from a solvent, such as acetone, but the use of organic solvents in food production and the consequent loss of butter flavor are problematic and have prevented commercial exploitation of this method. The most commonly used fractionation process (Figure 4) in commercial operation today is one in which liquid AMF is crystallized in a tank, where it is taken through a controlled temperature program. During this process triglycerides with high melting points will crystallize, and afterwards the slurry is divided into a liquid and a solid fraction by vacuum or pressure filtration, by centrifugation, or by a combination of these methods. The applied cooling program strongly influences the composition of the fat crystals, and the cooling rate influences the quantity of the crystallized fat and the size of the crystals. This so-called dry fractionation or crystallization from the melt is relatively cheap and is based on a physical process not involving solvents.
Melting
Controlled temperature program
Separation of liquid and solid fat
Stearin fraction
Olein fraction
Figure 4 Block diagram showing the principle of fractionation of anhydrous milk fat (AMF).
Furthermore, this process has the least effect on flavor and is generally accepted by legislation. The result of the fractionation is a high-melting fraction, the so-called stearin fraction, and a low-melting fraction, the so-called olein fraction. These two fractions
Butter and Other Milk Fat Products | Anhydrous Milk Fat/Butter Oil and Ghee
can be further fractionated by a multistep fractionation process into even harder and softer fractions, respectively. Packaging of AMF
An often used procedure in connection with packaging of AMF is to flush the containers with an inert gas before, during, or after filling and use airtight containers. The traditional way of packaging AMF is in large iron drums or barrels containing 190–200 kg and enameled on the inside or lined with a plastic foil, a so-called inner liner, to prevent direct contact between the metal and the AMF. A certain headspace in the barrels is required to control the drop of pressure arising from the reduction in volume caused by crystallization of the AMF. Nowadays a commonly used packaging is cardboard boxes with an inner liner of plastic foil, the so-called bag-in-box system. Different size of packaging is used, for example, 25 kg. Also consumer packaging in cans containing 0.5, 1, or 2 kg seems to be gaining popularity. For very big customers, shipping often takes place in road tankers or very large bulk tanks suitable for transport with containerships. Ghee Technology From ancient times, ghee has been prepared in households by heating fermented milk or butter in open pans causing the water to boil off. The fat is then separated through a strainer, and finally cooled slowly to give a coarse texture of crystallized fat and liquid oil. Nowadays ghee is often factory made from milk, cream, or butter using different technologies in which heat treatment is the essential process step. If milk is the starting material, it is normally separated into cream and skim milk, and the cream is then subsequently churned into butter, melted, and processed into ghee in a process where the butter is heated to about 100–104 C until the residual water content is lower than 1 g 100 g 1. After this the temperature is increased to 110–115 C, and the product is held at this temperature until the desired flavor has developed. The advantage of this method is that most of the water is removed by the skim milk and by the buttermilk, which is a much more energy-efficient method than removal by heating. In an even more efficient method the butter is heated to about 85 C, held for 30 min to allow the aqueous phase to form a bottom layer of residual buttermilk, which is then removed. Finally the fat phase is further heated to about 110 C. One of the industrialized methods used for production of ghee is a batch process in which butter or cream is boiled in a stainless steel vessel by indirect heating with steam. India has a large production of ghee, and because of the commercial importance of the production, considerable improvements in the manufacturing process have
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been developed over the years in that country. Thus a continuous ghee manufacturing process has been developed employing scraped surface heat exchangers and centrifugal separators and clarifiers. A method based on heating with microwave technology has also been developed. Ghee is normally packed in metal cans and containers of various sizes internally lined with a nontoxic lacquer. Cardboard boxes with an inner liner of plastic foil are also used.
Final Remarks AMF and similar products have many applications, and in some parts of the world, they are of vital importance for the economy of the dairy industry. An essential characteristic of the products is their prolonged keeping quality caused by the very low water content, which makes the products suitable for acting as a buffer during fluctuations in milk production over the year. It also makes the products suitable for shipment to faraway destinations to help leveling the unequal allocation of milk production in the world. See also: Butter and Other Milk Fat Products: Milk Fat Based Spreads; The Product and Its Manufacture. Enzymes Exogenous to Milk in Dairy Technology: Lipases. Enzymes Indigenous to Milk: Lipases and Esterases. Flavors and Off-Flavors in Dairy Foods. Milk Lipids: Fatty Acids; Lipid Oxidation; Lipolysis and Hydrolytic Rancidity.
Further Reading Anonymous (2003a) Anhydrous milk fat (AMF) and butteroil. In: Dairy Processing Handbook, 2nd edn., pp. 293–299. Lund: Tetra Pak Processing Systems AB. Anonymous (2003b) Butter and dairy spreads. In: Dairy Processing Handbook, 2nd edn., pp. 277–292. Lund: Tetra Pak Processing Systems AB. Deffense E and Hartel RW (2003) Fractionation of milk fat. In: Rossell B (ed.) Oils and Fats, Vol. 3: Dairy Fats, pp. 158–186. Leatherhead: Leatherhead Food International. Hartel RW (1996) Applications of milk fat fractions in confectionery products. Journal of the American Chemical Society 73: 945–953. Illingworth D, Patil GR, and Tamime AY (2009) Anhydrous milk fat manufacture and fractionation. In: Tamime AY (ed.) Dairy Fats and Related Products, pp. 108–166. Oxford: Wiley-Blackwell. International Dairy Federation (1991) Utilizations of milkfat. Bulletin No. 260/1991. Brussels: International Dairy Federation. Munro DS, Cant PAE, MacGibbon AKH, Illingworth D, and Nichola P (1998) Concentrated milkfat products. In: Early R (ed.) The Technology of Dairy Products, 2nd edn., pp. 198–227. Glasgow: Blackie Academic & Professional. Sserunjogi ML, Abrahamsen RK, and Narvhus J (1998) Current knowledge of ghee and related products. International Dairy Journal 8: 677–688.