Dairy Science & Technology · Dairy Processing & Product Technology · also Fat Rich Dairy Products, Technology of Fat-rich Products
Cream products, butter, anhydrous milk fat, butter oil and ghee: processes, yields, defects and quality.
In the exams: ICAR AICE subject 37 Unit 1 and subject 34 Unit 5; ICAR AIEEA-PG group 17 Unit I
Fat-rich dairy products — cream, butter, anhydrous milk fat, butter oil and ghee — turn the most valuable component of milk into products that can be stored, traded and used in cooking and manufacturing. Each is a different physical system. Cream is an oil-in-water emulsion; butter is a water-in-oil emulsion made by inverting cream; anhydrous milk fat and ghee are almost pure fat, one bland and the other cooked to develop flavour. Understanding them means understanding the milk fat globule, fat crystallisation, partial coalescence and phase inversion, and the oxidation and lipolysis that limit shelf life.
This manual covers the types of cream and their standards; cream processing by pasteurisation, neutralisation and ripening; batch and continuous butter making; overrun and yield calculations; cultured and salted butter and spreads; butter defects and grading; anhydrous milk fat and butter oil; the four main ghee methods; ghee flavour, granulation and keeping quality; fractionation and modification of milk fat; and the use of ghee residue and buttermilk. Cream separation and standardisation are covered in market-milk-processing and the chemistry of milk lipids in the dairy chemistry subjects; this manual refers to them rather than repeating them.
ICAR AIEEA-PG Dairy Technology covers the manufacture of dairy products and their defects; ICAR AICE-JRF/SRF Dairy Chemistry (subject 34, Unit 5) covers cream, churning, butter and ghee chemistry, ghee constants and adulteration, and Dairy Technology (subject 37, Unit 1) covers fat-rich products. Fat standards, permitted neutralisers and antioxidants, and analytical limits for ghee differ by country and region; values in this manual are textbook or named-standard examples and must be checked against the current regulation.
Milk fat exists as globules, mostly between about 1 and 10 micrometres in diameter in cow's milk (buffalo milk globules are larger on average), each surrounded by the milk fat globule membrane: a layer of phospholipids, proteins and glycoproteins derived from the secreting cell. The membrane keeps globules apart and protects the fat from lipase. Because fat is lighter than skim milk, globules rise; in raw cow's milk this is accelerated by cold agglutination, in which immunoglobulins cluster globules at low temperature so that a cream line forms within hours. Buffalo milk lacks this agglutinin activity, so despite larger globules it creams more slowly. Gravity creaming was the original way to obtain cream; centrifugal separation replaced it, and is described in market-milk-processing. Damage to the membrane, by excessive pumping, foaming or agitation of raw milk, exposes fat to lipase and causes rancidity, and makes free fat that affects cream stability and churning. The state of the globule and its membrane therefore governs every product in this subject: whipping and churning depend on controlled destabilisation of the globules, while cream liqueurs and UHT creams depend on keeping them stable.
Cream is the fat-rich portion of milk, normally obtained by centrifugal separation, in which fat globules are concentrated in a continuous phase of skim milk. Its fat content can range from about 10 per cent to over 50 per cent, and each fat range behaves differently: low-fat creams are pourable and used in coffee; medium-fat creams are used in cooking and desserts; whipping creams contain enough fat to form a stable foam; and very high-fat creams are spoonable or used as raw material for butter and ghee. Cream is sold fresh (pasteurised), ultra-pasteurised or UHT-treated, sterilised in cans, whipped in aerosol containers, cultured (sour cream, crème fraîche) or as clotted cream. The separation and standardisation of cream are covered in market-milk-processing; this chapter deals with the products.
Standards define cream classes by minimum fat, and the names and limits differ between countries. For example, in the United Kingdom single cream must contain at least 18 per cent fat, whipping cream at least 35 per cent, double cream at least 48 per cent and clotted cream at least 55 per cent. In the United States, half-and-half contains 10.5 to below 18 per cent fat, light cream 18 to below 30 per cent, light whipping cream 30 to 36 per cent and heavy cream at least 36 per cent. Codex has a standard for creams and prepared creams, and India's FSSAI defines its own classes. A cream must be labelled with the name its fat content entitles it to under the law of the market where it is sold.
Whipping cream forms a foam because of partial coalescence. When cream with partly crystallised fat is beaten, air bubbles are incorporated and fat globules collide; crystals poking out of one globule pierce the membrane of another, and the globules stick together into clusters without fully merging. These clusters adsorb at the air bubble surfaces and form a network that stabilises the foam and gives it stiffness. Whipping therefore needs enough fat (commonly at least about 30 per cent), enough solid fat at the whipping temperature (the cream must be well cooled and aged so that fat crystallises), and not too much homogenisation, which coats globules with protein and makes them resist coalescence. Over-whipping continues coalescence until the fat separates as butter grains: whipping and churning are the same process at different stages.
Overrun measures the air incorporated: overrun = (volume of whipped cream − volume of cream) ÷ volume of cream × 100. Aerosol whipped cream uses nitrous oxide, which dissolves in the fat and expands on release. Clotted cream, a traditional English product, is made by heating shallow pans of milk or cream slowly and cooling them so that a thick, crusted, high-fat layer forms and is skimmed off. Sterilised cream in cans, UHT cream and cream liqueurs (cream homogenised with alcohol, sugar and flavours) are further products with their own stability problems, chiefly creaming, feathering in hot coffee and age thickening.
Cream needs a stronger heat treatment than milk because fat protects micro-organisms from heat and because cream for butter must be free of enzymes that would cause defects. The U.S. Pasteurized Milk Ordinance, for example, requires products with 10 per cent or more fat or with added sweeteners to be heated 3 °C (5 °F) higher than milk, giving 68.3 °C for 30 minutes in vat pasteurisation or 74.4 °C for 15 seconds in HTST. Cream for butter making is commonly heated well above this, often to around 85–95 °C for a few seconds or more. Such temperatures inactivate milk lipase and most bacterial enzymes, and create sulphydryl compounds from denatured whey proteins that act as antioxidants in the butter. Too high a temperature gives a cooked flavour that is carried into the butter. Where cream carries feed or weed taints, vacuum treatment (vacreation, used notably in New Zealand) combines heating by direct steam with flash cooling under vacuum, which strips volatile taints and removes the added water.
In traditional and small-scale butter and ghee making, cream is often collected sour, because it has been held without refrigeration and lactic acid bacteria have grown. Acid cream cannot be pasteurised without curdling and coagulated protein would be lost as curd in the buttermilk or burnt particles in ghee; salted butter made from high-acid cream also develops fishy and oily flavours in storage, catalysed by acid and traces of metals. The acidity is therefore reduced before pasteurisation, a practice called neutralisation, using food-grade alkalis such as sodium bicarbonate or carbonate (soda neutralisers) or calcium or magnesium hydroxide or oxide (lime and magnesia neutralisers). The amount is calculated from the acidity to be removed and the neutralising value of the alkali: for example, 84 g of sodium bicarbonate neutralises 90 g of lactic acid, and 74 g of calcium hydroxide neutralises 180 g of lactic acid (one mole of hydroxide with two moles of acid). Because the acid is dissolved in the serum, not the fat, the acidity of cream is sometimes expressed on the serum: serum acidity = cream acidity ÷ (1 − fat fraction).
Neutralisation is a corrective for poor raw material. Over-neutralisation gives a soapy, alkaline flavour, and rules on whether neutralised cream may be used, and with what agents, differ between countries. The better solution is to collect cream sweet, cool it and process it quickly.
'Ripening' of cream has two meanings. Biological ripening is fermentation of pasteurised cream with a mesophilic aroma culture (acid-producing lactococci with citrate-fermenting Lactococcus lactis biovar diacetylactis and Leuconostoc), to make cultured (lactic) butter with a diacetyl aroma and some acid. Physical ripening (ageing) is the controlled cooling and holding of cream to crystallise the fat in the right proportion and crystal form before churning. Because milk fat composition varies with season and feed (summer fat, with more unsaturated fatty acids, is softer; winter fat harder), plants use temperature programmes to adjust butter consistency. A typical approach for hard winter fat is a cold-warm-cold sequence, which encourages the higher-melting fractions to crystallise into fewer, larger crystals and leaves more liquid fat, giving softer butter; soft summer fat is cooled quickly to a low temperature to form many small crystals and a firmer butter. These programmes are often described under the name of the Swedish Alnarp method. The aim is to produce butter that spreads well from the refrigerator without being greasy at room temperature.
Butter is a water-in-oil emulsion containing at least 80 per cent fat. It is made from cream, an oil-in-water emulsion, by phase inversion. During churning, cream is agitated with air at a temperature where part of the fat is solid (commonly around 8–14 °C depending on season and fat composition). Air bubbles form and collect fat globules at their surfaces; the globule membranes are damaged, liquid fat spreads at the air interface and acts as a glue, and globules cluster by partial coalescence. As the foam collapses, the clusters grow into visible butter grains, and the liquid phase, now poor in fat, separates as buttermilk. This foam-flotation account (associated with King and with Rahn) has replaced older explanations that churning is simply mechanical collision. Churning time and fat loss depend on temperature, fat content of the cream, the proportion of solid fat, churn speed and fill level, and the condition of the cream. Too warm a churning temperature gives soft grains and high fat loss; too cold gives long churning times.
In batch churning, aged cream (commonly around 35–40 per cent fat) fills a rotating stainless steel churn to about half its volume. Rotation throws the cream about and incorporates air; after some minutes the cream thickens, the foam breaks and butter grains of a few millimetres appear. The buttermilk is drained, and the grains may be washed with chilled water to remove buttermilk, although washing is often omitted in modern practice because it removes flavour and the water must be of potable quality. Salt, if used, is added as dry salt or brine. The butter is then worked, by continued slow rotation with internal working devices or between rollers, so that the water is dispersed into fine droplets and the salt dissolved and distributed. Water content is adjusted by adding water during working until the target composition is reached. Good working gives a close, dry, uniform body with no visible free water; under-worked butter is 'leaky', and over-worked butter is greasy and sticky.
Most butter today is made in continuous machines developed from the Fritz process. Cream enters a churning cylinder in which a fast-rotating beater produces butter grains within seconds. Grains and buttermilk pass to a separating section where the buttermilk drains; the butter is then squeezed and kneaded by screw conveyors through a series of working plates and a vacuum section that removes air, with salt slurry and water dosed and controlled continuously. Composition is controlled by in-line measurement of moisture and salt. Continuous machines need cream of consistent fat content (often higher than for batch churns) and steady temperature. An older alternative, the concentration and phase-inversion method, separates cream to a fat content close to that of butter and then inverts the emulsion by cooling and mechanical treatment without forming grains or buttermilk in the usual sense. The NIZO method makes cultured-flavour butter from sweet cream: sweet-cream butter is churned and then blended with a concentrated culture and a lactic acid concentrate, so that the buttermilk remains sweet and easy to use while the butter has a lactic flavour.
Codex sets compositional minimums and maximums for butter: at least 80 per cent milk fat, at most 16 per cent water and at most 2 per cent non-fat milk solids, with salt where used. National standards follow similar or stricter limits, and some countries have a separate class for table, cooking or desi butter. A typical salted butter might contain around 81–82 per cent fat, 15–16 per cent water, about 1–2 per cent salt and about 1 per cent non-fat solids; unsalted butter has more water or fat in place of salt. Butter makers aim to keep fat just above the legal minimum and water just below the maximum, because water sold at the price of butter is profit while fat given away is a loss. A plant must always keep a margin for variation between packs.
Overrun in butter making is the increase in mass of butter over the mass of fat used to make it, expressed as a percentage: overrun = (mass of butter − mass of fat in the cream) ÷ mass of fat in the cream × 100. It arises because butter contains water, salt and non-fat solids as well as fat, and it is reduced by fat losses in buttermilk and other streams. If all the fat were recovered, butter of 80 per cent fat would give a theoretical overrun of 25 per cent (100 ÷ 0.80 = 125 kg of butter per 100 kg of fat). In practice fat losses in buttermilk, wash water and handling reduce this, and overrun is a key measure of plant efficiency.
Worked example. Cream: 1,000 kg at 40 per cent fat, so 400 kg fat. Churning efficiency (fat recovered in butter) 99 per cent, so 396 kg fat in butter. Butter of 81 per cent fat: butter = 396 ÷ 0.81 = 488.9 kg. Overrun = (488.9 − 400) ÷ 400 × 100 = 22.2 per cent. Buttermilk (ignoring water and salt added) = 1,000 − 488.9 = 511.1 kg containing 4 kg fat, i.e. about 0.78 per cent fat.
Fat losses are measured by the fat content of buttermilk, which in well-run churning is a fraction of a per cent but rises with high churning temperature, unsuitable fat crystallisation, very fresh or very thin cream, high acidity, or overloaded continuous machines. Churning efficiency = fat in butter ÷ fat in cream × 100. Mass balances also give the cream needed for a target quantity of butter, the expected buttermilk, and the salt to add: if salt content of the final butter is to be S per cent, salt added must allow for any salt lost in buttermilk or brine. The salt dissolves in the water droplets, so its concentration in the aqueous phase, salt ÷ (salt + water) × 100, is what controls microbial growth in salted butter. Composition control is verified by testing samples for moisture, salt, fat (often by difference) and non-fat solids.
Sweet-cream butter is made from unfermented cream and has a clean, mild, creamy flavour; it is the usual type in English-speaking countries and South Asia's organised sector. Cultured (lactic, ripened-cream) butter is made from cream fermented with a mesophilic aroma culture, or flavoured by the NIZO method, and has a pronounced diacetyl aroma and a lower serum pH; it is traditional in much of continental Europe. Salted butter contains added salt, commonly around 1–2 per cent but up to about 3 per cent in some markets; salt adds flavour and, dissolved in the water droplets, inhibits microbial growth, so salted sweet-cream butter keeps well. Combining salt with high acidity, however, encourages oxidative defects in storage, so salted butter is usually made from sweet cream and cultured butter is usually unsalted or lightly salted. Desi butter (makkhan) in South Asia is churned from dahi rather than from cream and is mostly used for ghee making.
Spreads are products designed to spread straight from the refrigerator, or to contain less fat than butter. Reduced-fat and low-fat dairy spreads contain less than 80 per cent fat (fat classes such as about 60, 40 or 20 per cent are common, but legal names and limits differ by jurisdiction) with more water, stabilised by milk proteins, emulsifiers and thickeners. Because less fat must hold more water in fine droplets, these products are made in scraped-surface heat exchangers and pin-workers similar to margarine lines rather than by churning. Blends combine milk fat with vegetable oils to improve spreadability and reduce saturated fat; their naming is regulated, and in many countries the word 'butter' cannot be used for a product containing vegetable fat. Spreadable butter can also be made by using liquid fractions of milk fat (olein), by blending with oil, or by cream-ageing programmes that leave more liquid fat. Consistency is assessed by hardness and spreadability measurements and by solid fat content at refrigerator and room temperatures.
Butter flavour defects arise from the cream, the process or storage. Feed and weed flavours come through the milk from the cow's diet. Cooked flavour comes from excessive cream heat treatment. Acid or sour flavour comes from acid cream. Rancid (lipolytic) flavour comes from free fatty acids, particularly butyric acid, released by milk lipase (in milk that was agitated, foamed or homogenised before pasteurisation) or by bacterial lipases from psychrotrophs that survived as enzymes. Oxidised, tallowy or metallic flavours come from oxidation of unsaturated fat, catalysed by copper and iron, light and high storage temperature. Fishy flavour is an oxidative defect of salted butter made from acid cream, associated with trimethylamine formed from lecithin breakdown in the presence of acid, salt and metal ions. Cheesy, putrid or yeasty flavours indicate microbial growth from poor hygiene. Surface taint and discolouration on the block surface are caused by microbial growth and oxidation where the butter is exposed.
Body and texture defects include crumbly or brittle butter (hard fat, too fast cooling, wrong cream ageing), greasy or sticky butter (overworking, soft fat, high churning temperature), leaky butter with visible water droplets (underworking), gritty texture from undissolved salt, and mealy or grainy texture from overheated or poorly ripened cream. Colour defects include mottled butter (uneven salt distribution causing patches of different colour), streaky butter (incomplete mixing of batches), and a pale or uneven colour where annatto or carotene varies. Colour naturally varies with feed: butter from pasture-fed cows is yellower because of beta-carotene, while buffalo milk butter is white because buffaloes convert carotene to vitamin A.
Butter is graded by trained graders using a trier or by cutting the block, scoring flavour, body and texture, colour and salt, and package. In the United States, USDA grades are based on a score: U.S. Grade AA corresponds to a score of 93, Grade A to 92 and Grade B to 90, with flavour carrying the greatest weight. Other countries use similar point systems or grade descriptions; India and other countries also apply Agmark or national grades to butter and ghee. Grading is supported by chemical tests: moisture, salt and fat content, free fatty acids, peroxide value, and microbiological counts including coliforms, yeasts and moulds and lipolytic organisms. Keeping-quality tests hold samples at elevated temperature for a period and regrade them to predict storage stability.
Milk fat products with almost all water and non-fat solids removed are traded internationally because they store well and are used for recombination and in food manufacture. Codex defines several grades: anhydrous milk fat (also called anhydrous butter oil) with at least 99.8 per cent milk fat and at most 0.1 per cent water; milk fat (butter oil) with at least 99.6 per cent fat; and ghee, a traditional product, also defined by Codex with its own limits. Anhydrous milk fat (AMF) is made without the cooking step of ghee and has a bland flavour, which suits recombined milk, chocolate, confectionery, bakery and ice cream, where butter flavour is not wanted or where moisture would cause problems.
AMF is made either directly from cream or from butter. From cream: pasteurised cream of around 40 per cent fat is concentrated in a separator to about 70–75 per cent fat; this concentrated cream is then phase-inverted, usually in a homogeniser-like phase-inversion device or by mechanical treatment, so that the fat becomes the continuous phase; the oil is then concentrated in a second separator to around 99.5 per cent fat. The fat may be washed with hot water to remove remaining protein and salts, re-separated ('polished'), and finally dried in a vacuum dryer to reduce moisture to the specified level. From butter: unsalted butter (often stored butter) is melted, held to let the phases separate, and the oil concentrated in a separator, washed, polished and vacuum-dried as before. The serum separated in AMF manufacture (sometimes called beta serum) is rich in milk fat globule membrane material and phospholipids.
Before packing, AMF may be neutralised to reduce free fatty acids, deodorised, and protected from oxidation. It is packed hot or cooled into drums or containers flushed with nitrogen to exclude oxygen; properly made and packed AMF keeps for many months. Quality tests include moisture, free fatty acids, peroxide value, copper and iron, and flavour.
AMF is the main fat source for recombined milk, evaporated milk and sweetened condensed milk in countries that import dairy ingredients, because it ships and stores without refrigeration in drums under nitrogen. Before use it is melted gently, often in hot rooms or with drum heaters, avoiding local overheating that causes oxidised or cooked flavours. In chocolate it improves bloom resistance and softens texture; in bakery it gives flavour and shortening; and fractionated AMF is chosen where a particular melting profile is needed. Because AMF has almost no water or protein, it is not a medium for bacteria; its quality risks are oxidation and absorption of off-odours.
Ghee is clarified milk fat made by heating butter or cream to drive off almost all the water, at a temperature high enough to brown the non-fat solids slightly and develop the characteristic cooked, nutty flavour. It is the most important fat-rich dairy product of South Asia and is also made in the Middle East (samn) and parts of Africa. The clarification temperature, commonly around 110–120 °C at the end of boiling, is a key variable: higher temperatures give more flavour and colour and more natural antioxidants but risk a burnt taste; regional preferences differ. After clarification the non-fat solids settle as brown ghee residue, and the clear fat is filtered or centrifugally clarified and packed. Ghee contains very little moisture (Indian standards set a maximum, commonly cited as 0.5 per cent) and keeps for months at ambient temperature.
Desi (indigenous) method: whole milk is boiled, cooled and set to dahi; dahi is churned with a wooden churner to obtain desi butter (makkhan), which is heated in an open pan to make ghee. It gives a strongly flavoured ghee but low fat recovery, high labour and variable quality, and it is still the main household and village method.
Direct-cream method: cream (preferably of high fat, around 70–80 per cent, to reduce the water to be evaporated and fuel use) is heated directly in a jacketed ghee kettle with stirring until clarification. It avoids the butter stage, but the large amount of serum solids increases browning and residue and needs more heat when cream fat is low.
Creamery-butter method: unsalted creamery butter (or white butter) is melted at about 60 °C and heated in a ghee kettle to clarification temperature. This is the standard method in organised dairies; it gives good flavour, good fat recovery and uses butter, including stored butter, as a buffer between seasons.
Pre-stratification method: butter is melted at around 80–85 °C and held undisturbed for about half an hour, so that it separates into three layers: a top layer of denatured protein particles, a middle layer of fat, and a bottom layer of buttermilk serum. The bottom layer, which holds most of the water and much of the non-fat solids, is drained off before heating. The rest is heated to clarification temperature. Removing most of the water before boiling saves fuel and time and reduces acidity and residue; the flavour can be slightly milder because less serum is present.
Continuous systems use scraped-surface or film-type heat exchangers and centrifugal clarifiers, with flavour developed in a controlled final heating stage.
Ghee yield is calculated from the fat entering and the fat lost in residue and other streams: ghee = fat recovered ÷ fat fraction of ghee. The fuel needed is dominated by evaporating water: about 2,257 kJ per kg of water at atmospheric pressure, plus sensible heat to raise the material to boiling and to the final temperature. A butter of 82 per cent fat and 16 per cent water carries about 160 kg of water per tonne, and pre-stratification, by removing most of that water as liquid before heating, can cut the evaporation load substantially. This is why the method is popular in large plants.
Ghee flavour is formed during clarification by reactions between the fat, the proteins and lactose of the non-fat solids, and the heat. Compounds include lactones (from hydroxy fatty acids in milk fat), free fatty acids, carbonyl compounds such as aldehydes and methyl ketones, and Maillard reaction products. The method strongly affects flavour: desi ghee from fermented butter has a mildly acid, rich flavour; direct-cream ghee is milder; creamery-butter ghee depends on whether the butter was sweet or ripened; and higher clarification temperatures give stronger, more 'cooked' flavour. Ghee from cow milk is yellow because of beta-carotene, while buffalo ghee is white with a slight greenish tinge; consumers in different regions prefer different colours and flavours, so producers adjust milk blend and clarification temperature for their market.
Good ghee in many markets is expected to be granular (danedar): when it solidifies, it forms visible crystals of high-melting glycerides suspended in liquid fat, rather than a smooth, greasy mass. Granulation depends on fat composition (the proportion of high-melting triglycerides, higher in buffalo ghee) and on the cooling regime: slow cooling at a moderate temperature, sometimes with seeding with previously granulated ghee and without disturbance, encourages large crystals, while rapid chilling gives small crystals and a smooth texture. Producers who want granular ghee hold it at a suitable temperature for a period after filling.
Ghee keeps well because it contains little water, so microbial spoilage and hydrolytic rancidity are slow; its main deterioration is oxidative rancidity of the fat. Keeping quality improves with low moisture and low acidity, absence of copper and iron, protection from light and oxygen (filled to the brim or under nitrogen, in opaque containers), cool storage, and the presence of natural antioxidants. Phospholipids from the milk fat globule membrane and Maillard products formed during clarification are natural antioxidants; ghee made by methods that retain more phospholipids, or clarified at higher temperature, tends to keep longer. Some countries permit synthetic antioxidants such as butylated hydroxyanisole in ghee at specified levels; others do not.
Quality and authenticity are checked with analytical constants: the Butyro-refractometer (BR) reading, the Reichert-Meissl value (volatile water-soluble fatty acids, high in milk fat because of butyric acid), the Polenske value (volatile water-insoluble fatty acids), saponification and iodine values, free fatty acids (acid value), moisture, and tests for vegetable fat and animal body fat such as the phytosterol test and the Baudouin test for sesame oil where sesame oil marking of vanaspati is required. Natural values vary with species, region and season, so standards in India specify limits that differ for some cotton-tract regions where cottonseed feeding changes the fat.
Milk fat is a mixture of hundreds of triglycerides that melt over a wide range, from below 0 °C to around 40 °C. This breadth gives butter its plasticity but limits its use: at refrigerator temperature too much fat is solid for easy spreading, while for laminated pastry and some confectionery a higher melting fat is wanted. Fractionation separates milk fat into fractions with different melting ranges, typically a higher-melting stearin and a lower-melting olein, each with a different solid fat profile and use. Stearins are used in puff pastry, croissants and chocolate; oleins in spreadable butter, cold-spreading products and for flavour in confectionery. Because nothing is added, fractions remain pure milk fat.
Dry fractionation (melt crystallisation) is the main commercial method. Molten AMF is cooled slowly under controlled agitation to a chosen temperature, at which the higher-melting triglycerides crystallise; the crystals are then separated from the liquid by vacuum filtration or membrane filter presses. Repeating the process at lower temperatures on the olein gives further fractions. Solvent fractionation (crystallisation from acetone or hexane) gives sharper separation but is not commonly used for food milk fat because of solvent cost and acceptance. Detergent fractionation uses a surfactant solution to wet the crystals so they can be separated centrifugally. Supercritical carbon dioxide extraction separates triglycerides by molecular weight and can also remove cholesterol and flavour.
The yield of each fraction and its properties are linked by simple mixing rules: for example, the iodine value of the original fat equals the mass-weighted average of the iodine values of its fractions. Fraction yields depend on the crystallisation temperature and the season of the fat.
Milk fat can also be modified by blending with vegetable oils (giving blends, not butter), by interesterification (chemical or enzymatic rearrangement of fatty acids among the glycerol positions, which changes melting behaviour without changing fatty acid composition), by hydrogenation (rarely used for milk fat), and by cholesterol removal, for example with beta-cyclodextrin, which forms an insoluble complex with cholesterol that can be separated, or by steam stripping at high temperature and vacuum. Feeding cows on diets rich in unsaturated oils changes the fatty acid profile at source, producing softer fat with more unsaturated and conjugated linoleic acid, though with a greater tendency to oxidise. Any modified product must be labelled according to national standards, which usually restrict the term 'butter' to unmodified or only physically fractionated milk fat.
Ghee residue is the brown, moist, partly charred material of non-fat solids and entrained fat left after clarification. It contains a considerable amount of fat, protein (largely denatured casein), lactose and its browning products, minerals and phospholipids, in proportions that depend on the method: direct-cream ghee gives more residue than creamery-butter ghee. Much of the fat can be recovered by pressing or centrifuging the hot residue. The residue is rich in phospholipids and Maillard products and therefore has antioxidant properties; extracts of ghee residue added back to ghee have been shown to improve its keeping quality. Residue is used in traditional sweets and confectionery in South Asia, in bakery products, and as animal feed; its value is often underused, so recovering fat and finding food uses improve plant economics.
Sweet-cream buttermilk, the liquid from churning sweet cream, resembles skim milk in composition but contains more milk fat globule membrane material, which is released when globules are disrupted during churning. That membrane material is rich in phospholipids and membrane proteins, which have useful emulsifying properties and are studied for nutritional and health effects. Buttermilk is therefore used fresh or as buttermilk powder in bakery, confectionery, recombined dairy products, ice cream and dressings, where its emulsifying capacity is valuable. Buttermilk from cultured cream is acid and less versatile; it is sometimes used in feed or blended into acid products. Buttermilk can also be concentrated by ultrafiltration or microfiltration to make phospholipid-rich ingredients. The serum (beta serum) from AMF manufacture is an even richer source of these components.
In South Asia, the buttermilk left after churning dahi for desi butter (chhach) is a traditional drink in its own right, often salted and spiced. Fat loss into buttermilk is a measure of churning efficiency; buttermilk that is too rich in fat shows that the churning or the cream preparation needs correction.
A student uses this subject to explain churning and phase inversion, calculate overrun, butter yield, neutraliser dose and ghee yield, compare the four ghee methods and interpret ghee analytical constants. Butter and ghee practicals in the dairy plant and questions in AIEEA-PG and AICE Dairy Chemistry draw directly on it.
A butter and ghee section officer five years in sets cream ageing programmes by season, adjusts churning temperature and working to hit fat and moisture targets, monitors buttermilk fat and overrun, chooses between creamery-butter and pre-stratification ghee, controls granulation and keeping quality, and investigates rancid, oxidised or fishy complaints. This subject gives the reasoning and calculations behind each of those tasks.
A plant head decides whether to convert surplus fat into butter, AMF or ghee, which markets to sell to, whether to invest in continuous butter making, fractionation or AMF plant, and how to manage stored butter between flush and lean seasons. Overrun, yield, energy and quality data from this subject feed those decisions and pricing.
A professor or research director works on fat crystallisation, partial coalescence, milk fat globule membrane ingredients, ghee flavour chemistry and antioxidants, adulteration detection and novel fat fractions. They set AICE Dairy Chemistry questions on churning, ghee constants and deterioration and advise standards bodies on ghee and butter specifications.
This manual does not yet give: current FSSAI limits for cream classes, butter, ghee moisture and analytical constants by region; exact Codex limits for every milk fat product grade beyond those named; detailed equipment design for continuous butter machines and AMF plants; quantitative crystallisation kinetics; full methods for adulteration tests; and legal rules on neutralisers, colours and antioxidants, which differ by country. Temperatures, fat contents and compositions are representative values, not recipes.
Original practice questions written to the syllabus and pattern of the exams named in each question's examTags. They are not past papers and do not reproduce any board's questions.
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