Edible Oil Refining
Except for animal products and olives, plant and single-cell oils are obtained by refining after crushing, which purifies the TAGs from the crude mixture of lipids and aliphatic molecules. The crude oil is mixed with hot water to first degum the oil. Degumming removes the polysaccharide gums and other impurities, such as phospholipids and proteins, all found in cell wall fragments, using hot water or dilute acid. When mixed with hot water, the active surface and more polar molecules found in crude oil are solubilized into the aqueous phase, which is then centrifugated, separating the gum from the now-degummed oil. Next, neutralization removes free fatty acids from the degummed oil by adding caustic soda (NaOH). The alkalinity ensures the fatty acids are deprotonated, allowing them to form fatty acid soaps and be removed with water via washing. Neutralized oil is now free of gums and fatty acids. Neutralization improves the flavor and stability of the oil. The neutralized oil undergoes bleaching, which is the adsorptive purification of oil by passing it through a diatomaceous earth bed under a vacuum, allowing for deodorization to remove volatile compounds (aroma).
Diatomaceous earth is a naturally occurring, soft, sedimentary rock high in silica (used in chromatography) comprised of the fossilized remains of microalgae (diatoms). The high silica material easily crumbles into a fine powder with high porosity, thus effectively reducing the contents of chlorophyll, residual soap and gums, trace metals, and oxidation products, which improves flavor and color and increases shelf-stability. Bleaching efficacy relies on the processing variables temperature, humidity, contact time, gauge strength of the vacuum, oil quality and moisture, and the characteristics of the adsorbent bed. During this stage, fatty acids soaps are completely removed, iron and phosphorus are reduced to < 0.2 ppm, chlorophyll to < 0.05 ppm, and the peroxide value is <0.5 mEq/kg ( which is the milliequivalents (meq) of active oxygen per kilogram of oil).
Winterized oils undergo an additional step that cools the oil to crystallize high melting point TAGs so it can be stored at refrigerated temperatures without turning cloudy from forming small crystals, which is important for manufacturing salad dressing. Finished oils include store-bought and industrially- used corn, canola, sunflower, olive and vegetable oils. It is also important to note that extra virgin and virgin olive oils are the first and subsequent crushes of olives, and it is not purified further to separate the pigments, which is why olive oil has an as different color than other oils. After olives have been pressed, the remaining olive material will be solvent extracted to produce olive oil (without the designation ‘virgin’ or ‘extra-virgin’).
Milk Fat Separation-Centrifugation
Milk fat composition varies based on breed, genetic variation within the breed, animal health, environment, management practices, and diet; however, the prominence of the Holstein breed leads to a fairly consistent composition of 3.6 % fat, 3.2 % protein, and 4.7 % lactose. Milk has the highest fat content after birth in the colostrum, followed by a steady decline over the first two months of lactation, after which the fat content slowly increases through the rest of lactation. Through the first half of lactation, the short- and intermediate-chain fatty acids increase in concentration while long-chain fatty acids decrease, which remains constant through the latter half of lactation. In Canada, milk pricing is based not on the fluid weight but on a lipid and protein component basis. Therefore, all milk products in Canada undergo standardization, separating the lipid component from skim milk followed by blending the skim milk and cream to provide the various exact milk fat contents of 1, 2 and 4.25% whole milk and 18 and 35% creams. Milk fat is separated by centrifugation, where the centrifugal force is applied to the milk by spinning it rapidly in a centrifuge, causing the milk fat particles to separate from the more dense aqueous phase. A centrifuge has as many as 120 perforated, 45 to 60-degree-angle discs with 0.4 to 2.0 mm separation channels.
Prior to, or concurrently with, separating the fat, milk is clarified using a lower angular velocity to remove solid impurities, including cells and bacteria, from the milk. Separating fat from milk relies on creaming, characterized by Stoke’s law, due to the density difference between the lipid and water phases. Creaming is accelerated by increasing the gravitational force (F=mω2r) via centrifugation from 9.8 m/s2 to 1000 to 5000 x the force of gravity (g), or between 10,000 to 50,000 m/s². The centrifugal force is a function of the centrifuge’s radius (r) and angular velocity (ω). Milk is introduced at the inner edge of the disc stack with vertically aligned holes; the less dense fat globules move inwards through the separation channels toward the axis of rotation. The process efficiently removes fat from skim milk (0.1% fat); however, skim milk is needed to carry fat globules, and the cream content reaches a maximum of 35-40% milk fat. Additional processing is required to increase the milk fat concentration beyond 40%.
Milk Fat Separation-Churning
The cream is aged at 18 oC before churning to ensure fat crystals are present in the lipid droplets, while at the same time, they are not too solid to prevent coalescence. The milk fat concentration of cream is further increased by churning to form butter which is ~82% lipid, ~16% water, and the remaining 2% is protein, carbohydrate, and minerals; the typical fat composition is 65% saturated, 27% monounsaturated, 4% polyunsaturated, and 4% trans-unsaturated fatty acids. Butter is one of the most complex lipid sources, with over 100 fatty acids synthesized by the rumen microflora and incorporated into the milk fat. Butter churning uses an impeller designed to agitate cream, a high-shear process incorporating air. Air bubbles form and implode during this process, creating smaller fat droplets; as the droplet size decreases, insufficient milk fat globular membrane is present to cover the entire fat drop and coalescence of fat droplets occurs. After churning, the material enters a twin-screw impeller which adds further shear to increase fat coalescence; throughout the separation, buttermilk is removed from the butter and, done under vacuum, to remove air from the final product while preventing oxidation. After separation and the buttermilk is drained, working establishes a continuous fat phase containing a finely dispersed water phase (butter) by further shearing the material under a vacuum.
While there are more sophisticated methods to isolate and purify milk fat, the vast majority of fluid milk is centrifuged, standardized to kinds of milk and creams, and further fat isolation is done by churning cream into butter. Ghee further concentrates lipids from butter by heating butter to 105-115oC; as the mixture simmers, the remaining proteins float to the surface and are removed. Heat is applied until the mixture is clear with no remaining solid on the surface, and the clarified oil is decanted off the more dense solids that have precipitated to the bottom. The clarified butter is then heated to 130 oC, which triggers the Maillard reaction between lactose and residual proteins creating the light brown pigments while eliminating the carbohydrate and protein, during which the fat content to 98.9 % fat, with trace water and non-fat solids (products of the Maillard reaction).
Milk Standardization – Pasteurization
Health Canada prohibits the sale of unpasteurized milk; however, unpasteurized milk cheese, made from raw or unpasteurized milk, is sold in Canada as the fermentation process eliminates harmful bacteria in raw milk. Milk is pooled at the farm level from multiple cows, several farms’ milk is collected in a single tanker truck, and then several trucks are pooled into the raw tank receiving before homogenization and pasteurization. Pasteurization is intended to eliminate pathogenic microorganisms while subsequently reducing spoilage microorganisms. Pasteurization eliminates gram-negative organisms from milk, while some thermophilic and mesophilic spoilage microorganisms survive. Low-temperature long-time (LTLT) processing occurs at 63oC for 30 min but has limited applications in dairy processing due to thermolytic modifications arising from Maillard reactions between lactose and proteins, resulting in the formation of brown pigments. Long hold times denature milk proteins restricting the application in fluid milk processing. However, these changes, especially protein denaturation, may be desirable for small-scale ice cream, cheese and other dairy product manufacturing or producing soup broths where cooked flavors are desirable. LTLT is often done in small-scale batches using a heated vessel/kettle; the downfall to batch processing is the low surface area for heat transfer of the steam jacket kettle (Surface area = circumference x height), requiring a longer time to heat. The coldest spot in the kettle must reach 63 oC before the 30 min hold begins; the radius (meters, m) from the heat transfer surface to the coldest point is the characteristic dimension.
Juices, broths, fluid milk, and creams are processed continuously using high-temperature, short-time (HTST) pasteurization at 72°C for 15 seconds. HTST uses a plate heat exchanger, where the stainless steel plates are stacked together, and unpasteurized fluid travels between the plates in shallow channels. Channels are oriented to maximize turbulent flow, ensuring the constant exchange of molecules at the hot steel surface. The gaps (10-2, cm) between stacked plates alternate between raw product and steam, allowing heat transfer from both sides of the gap, creating a large surface area for heat exchange and a small characteristic dimension (half of the gap width (10-3, mm)).
HTST allows continuous operation, a large surface area for heat exchange, and a small characteristic dimension to the coldest point. This design strategy allows for rapid heating and cooling, which preserves the nutritional and sensory quality of the product. HTST-processed milk and milk products have extended shelf life under refrigerated conditions but are not commercially sterile, as pasteurization eliminates pathogens and reduces the spoilage of microorganisms. Once an HTST operates, a fraction of energy is recycled by initially passing gaps, one with pasteurized milk and the other with cold raw milk, reducing the steam used in operation.
Ultra-high-temperature (UHT) processes milk at 140°C for four seconds and is commercially sterilized rather than pasteurized allowing milk or other fluid, such as juice, to be stored for months without refrigeration. Commercial sterilization eliminates pathogenic and spoilage microorganisms capable of growing in sealed food containers. UHT processing atomizes fluid milk into small droplets as it enters the superheated pressurized heat exchanger. The atomized droplets are in direct contact with steam, drastically increasing the surface area for heat exchange while decreasing the characteristic dimension that heat must travel to the center of the droplet (10-6 m, αm), resulting in near-instantaneous heating. UHT processing does not have a conductive barrier between the droplet and heated chamber; therefore, steam and product are in direct contact with each other, and steam condenses, increasing the volume of the milk. After the hold time, the hot pasteurized product enters a cooled chamber under a vacuum, causing evaporative cooling and eliminating the added water from the condensed steam. The commercially sterile product is cooled instantly in a vacuum chamber and must be aseptically packaged in a pre-sterilized, airtight container.
Homogenization
Skim milk and cream are recombined to standardized milk fat concentrations; to ensure that creaming does not spoil the product prior to the best-before date. Homogenization passes recombined milk through a small orifice; early versions operated under low pressure, and the particle size reduction was limited to the opening size. Morden day seat-valve homogenizers use very high pressures (>2500 kPa) to pass the fluid through the small orifice, and the particle size reduction creates a monodispersed fine emulsion.
Stage one primarily reduces the lipid droplet size; as a consequence, there is an increase in the oil-water interfacial area. Stage two operates at lower pressure (~500 kPa) and redisperses lipid droplets that have flocculated, allowing time for the diffusion of other surface active molecules in milk to reach the oil-water interface. There are three primary mechanisms of particle size reduction occurring concurrently during homogenization. As the fluid enters the orifice, the volumetric flow rate must equal the flow rate before constriction. Bernoulli’s principle conserves energy (potential, kinetic & pressure) through the flow area, where a reduction in the cross-sectional area results in fluid acceleration through the orifice at V2. The acceleration of fluid alters the flow profile from laminar to turbulent flow, generating eddies that break droplets; second, fluid flows more slowly near the channel surface than the droplet in the center of the cross-section resulting in particle elongation. Droplet elongation increases the dispersed phase surface area and eventually leads to dissociation into smaller droplets. Finally, at V2, fluid flows much faster than at V1 due to the reduction in the cross-sectional area; the fluid acceleration coincides with a large pressure drop.
The pressure drop is sufficient to reduce the liquid boiling point to below the fluid temperature causing water vapor bubbles to form in the orifice. As the fluid exits the orifice and the flow channel expands, the linear velocity reduces, and the pressure returns to P1, reducing the boiling point below the temperature of the fluid. The pressure decrease upon exiting the orifice leads to cavitation, where the vapor bubble implodes, forming a micro jet that generates enormous shear forces, further reducing the droplet size. The stability of homogenized milk arises from the small, uniform, monodispersed lipid droplets and the increase in particle density arising from more surface active molecules adhering to the new droplet interfacial area. From Stokes’s law, a decrease in particle radius and an increase in particle density result in a lower terminal velocity, while the monodispersed droplets prevent sweeping flocculation as the uniform particles travel at the same rates. Particle size reduction is limited by the increasing Laplace pressure as particle size decreases.
Hydrogenation of Lipids
Fats and oils impart physical structure to a wide range of whole food (e.g., milk products, meat, and fruits such as olive and avocados) and ultra-processed formulated foods (e.g., plant-based meats, pizza, frozen dinners, cookies, cake, ice cream, chocolate, and pastries), most of which require solid fat (long-chain saturated fatty acids) to impart the desired sensory and organoleptic properties. Oils are liquid at room temperature with high concentrations of unsaturated (oilseeds, corn, olive) or medium-chain (coconut, palm kernel (fractionated)) fatty acids. Oil seed crops (soybean, sunflower, canola, flax, peanut, cotton) are 20-40% lipid with high concentrations of mono or polyunsaturated fatty acids, making them liquid and devoid of fat crystals. Foods formulated with refined oil include sauces, salad dressing, mayonnaise, and plant-based meats or added during frying (e.g., French fries, donuts, and potato chips). Solid fats are high in long-chain saturated fatty acids, which form solid colloidal fat crystal networks at room temperature and impart essential physical structure and organoleptic properties. Corn, peanut and Oilseeds are grown across Canada (edible oil exporter); however, domestic solid fat production (e.g., milk fat, lard (pork fat), and to a lesser extent, eggs) requires imports of solid fat, as the climate prohibits the cultivation of Elaeis guineensis (oil palm tree) and Theobroma cacao (cocoa bean). All commodities, high in saturated, solid fat, face significant challenges; animal welfare and environmental sources place pressure to move away from conventional animal sources, coupled with governments limiting the export of palm oil and cocoa beans to curb deforestation, leading to the need to create solid fat.
Hydrogenation adds molecular hydrogen to each carbon adjacent to a fatty acid unsaturated bond using a heated, pressurized hydrogen gas vesicle where the oil and catalyst, usually nickel but also palladium or platinum, blend is mixed, allowing hydrogen gas to pass through the slurry. The Horiuti–Polanyi mechanism sequentially describes hydrogenation, where molecular hydrogen is adsorbed onto the nickel catalytic surface (step 1) and dissociates into two hydrogen atoms (step 2). Concurrently, unsaturated fatty acids are also adsorbed onto this nickel surface by their double bond, which forms a half-hydrogenated intermediate (step 3).
If a second hydrogen atom (step 4) is added to this intermediate, the original double bond is saturated and no longer reactive; but since the first hydrogen addition (step 2) is reversible, the intermediate can also dissociate from the catalyst and reform an unsaturated bond. This process repeats for each point of unsaturation; thus, linolenic (18:3) transforms to linoleic (18:2), then to oleic (18:1) and ends with stearic (18:0) acid formation. The half-hydrogenated intermediate is reversible, allowing the unsaturated double bond to reform, which either adopts the cis or trans configuration and is the primary source of industrially produced trans eladic acid. Originally methods to attain the desired viscoelastic properties of the fat employed partial hydrogenation of the liquid oil, reducing the unsaturated fatty acids content while increasing both the saturated and trans unsaturated fatty acid concentrations transforming the liquid oil into solid fat. However, recent legislation prohibits foods containing industrially-produced trans fats banning partial hydrogenation. Hydrogenation is the only method to increase the solid fat content of liquid oils, transforming them into plastic fats with desirable functional and sensory properties. Oils are still hydrogenated; however, the reaction runs to completion to ensure that the liquid oil is fully hydrogenated, ensuring only saturated fatty acids remain while preventing trans fats from forming. The downfall to fully hydrogenated fats is their extremely high melting temperatures giving them a plastic organoleptic perception with limited functional properties.
Blending and Interesterification
Fully hydrogenated fats are not directly added to foods because they have poor physical properties such as high melting points (mp > 50 oC) and hard waxy structure, which are imparted by the fully-saturated, long-chain TAGs. When fully hydrogenated fats are blended with liquid oil (e.g., corn, oilseed, olive) and melted above their melting temperatures to form a solution, upon cooling, the two lipid sources do not co-crystallize, and the product quickly phase separates (crystals precipitate out of oil-crystal dispersion) as graining or coarsening occurs (Ostwald’s Ripening).
Liquid oils blended with fully-hydrogenated fats are made functional by the chemical processes of inter- and intra-esterification. Interesterification removes and then exchanges fatty acids between TAGs, while intra-esterification exchanges fatty acid position on a single TAG. Interesterification transforms an unfunctional blend of corn oil, high in monounsaturated TAGs, and fully hydrogenated corn oil, high in tristearin (glycerol with three stearic acid fatty acids) into a functional blend by randomizing fatty acid position between TAGs. Interesterification does not change the fatty acid composition; it only modifies their TAG position. The resultant inter-esterified fat now has a broad distribution of TAGs resulting in a broad melting profile. Blends of oils and careful control of the interesterification conditions are used to tailor the sensory and physical properties mimicking functional fats such as cocoa butter and milk fat. Numerous catalytic methods include enzymatic and chemical synthesis, which randomizes fatty acids differently, coupled with careful temperature control, directing the blend toward a composition with the desired melting profile.
Brominating Oils for Emulsion Stability
Emulsion destabilization arises due to divergent densities between the continuous and dispersed phases leading to the creaming of the lower-density phase. Brominated vegetable oil (BVO), typically around 8 ppm, has been employed since early 1930 because droplets containing BVO remain suspended in the water. Bromination lipids are applied to beverages containing citrus oil to prevent separation before expiry. Unsaturated oil is brominated to diminish the density differences between the continuous aqueous phase from the lipid-dispersed phase. Adding bromine to unsaturation points along the fatty acid chains increases lipid density; for example, lemon oil has a density of 0.88 g/cm3, while after bromination, the density increase to 1.33 g/cm3. The density of the continuous water phase is matched by blending three parts lemon oil with and one part brominated lemon oil. Eliminating the density difference between particles eliminates the buoyant force acting on the particle, imparting stability. BVO had its GRAS (generally regarded as safe) status revoked in 1970 and was replaced with food additive regulations, restricted to 15 ppm in the United States. The only allowable use of BVO in Canada currently in beverages containing citrus oils; BVO is completely banned across the European Union.