VI. PROCESSING

A. General

Food fats and oils are derived from plant seeds, fruits and nuts (generally referred to as oilseeds) and animal sources. Animal fats are generally heat rendered from animal tissues to separate them from protein and other naturally occurring materials. Rendering may be accomplished with either dry heat or steam. Rendering and processing of meat fats is conducted in USDA inspected plants. Vegetable oils are obtained by the extraction or the expression of the oil from the oilseed source. Historically, cold or hot expression methods were used. These methods have largely been replaced with solvent extraction or pre-press/solvent extraction method which gives a better oil yield. In this process the oil is extracted from the oilseed by hexane (a light petroleum fraction) and the hexane is then separated from the oil, recovered, and reused. Because of its high volatility, hexane does not remain in the finished oil after processing.

The fats and oils obtained directly from rendering or from the extraction of the oilseeds are termed “crude” fats and oils. Crude fats and oils contain varying but relatively small amounts of naturally occurring non-glyceride materials that are removed through a series of processing steps. For example, crude soybean oil may contain small amounts of protein, free fatty acids, and phosphatides which must be removed through subsequent processing to produce the desired shortening and oil products. Similarly, meat fats may contain some free fatty acids, water, and protein which must be removed.

It should be pointed out, however, that not all of the nonglyceride materials are undesirable elements. Tocopherols, for example, perform the important function of protecting the oils from oxidation and provide vitamin E. Processing is carried out in such a way as to control retention of these substances.

B. Degumming

Crude oils having relatively high levels of phosphatides (e.g., soybean oil) may be degummed prior to refining to remove the majority of those phospholipid compounds. The process generally involves treating the crude oil with a limited amount of water to hydrate the phosphatides and make them separable by centrifugation. Soybean oil is the most common oil to be degummed; the phospholipids are often recovered and further processed to yield a variety of lecithin products.

A relatively new process in the United States is enzymatic degumming. An enzyme, phospholipase, converts phospholipids, present in crude oil, into lysophospholipids that can be removed by centrifugation. Crude oil, pre-treated with a combination of sodium hydroxide and citric acid, is mixed with water and enzymes (phospholipase) by a high shear mixer, creating a very stable emulsion. The emulsion allows the enzyme to react with the phospholipids, transforming them into water-soluble lysophospholipids. This emulsion is broken by centrifugation, separating the gums and phospholipids from the oil. This process generates a better oil yield than traditional degumming/refining. Enzymatic degumming is currently not widely commercialized.

C. Refining/Neutralization

There are two general types of refining alkali refining and physical refining. Alkali refining (i.e. treatment of the fat or oil with an alkali solution) is the most widespread method and is performed to reduce the free fatty acid content and to remove other impurities such as phosphatides, proteinaceous, and mucilaginous substances. This process results in a large reduction of free fatty acids through their conversion into high specific gravity soaps. Most phosphatides and mucilaginous substances are soluble in the oil only in an anhydrous form and upon hydration with the caustic or other refining solution are readily separated. After alkali refining, the fat or oil is water-washed to remove residual soap.

Oils low in phosphatide content (palm and coconut) may be physically refined (i.e. steam stripped) to remove free fatty acids. In physical refining, free fatty acids in crude or degummed oil are removed by evaporation rather than being neutralized and removed as soap in an alkaline refining process.

D. Bleaching

The term “bleaching” refers to the process for removing color producing substances and for further purifying the fat or oil. Normally, bleaching is accomplished after the oil has been refined.

The usual method of bleaching is by adsorption of the color producing substances on an adsorbent material. Acid-activated bleaching earth or clay, sometimes called bentonite, is the adsorbent material that has been used most extensively. This substance consists primarily of hydrated aluminum silicate. Anhydrous silica gel and activated carbon also are used as bleaching adsorbents to a limited extent.

E. Deodorization

Deodorization is a vacuum steam distillation process for the purpose of removing trace constituents that give rise to undesirable flavors, colors and odors in fats and oils. Normally this process is accomplished after refining and bleaching.

The deodorization of fats and oils is simply a removal of the relatively volatile components from the fat or oil using steam. This is feasible because of the great differences in volatility between the substances that give flavors, colors and odors to fats and oils and the triglycerides. Deodorization is carried out under vacuum to facilitate the removal of the volatile substances, to avoid undue hydrolysis of the fat, and to make the most efficient use of the steam. In the case of vegetable oils, sufficient tocopherols remain in the finished oils after deodorization to provide stability.

Deodorization does not have any significant effect upon the fatty acid composition of most fats or oils. Depending upon the degree of unsaturation of the oil being deodorized, small amounts of trans fatty acids may be formed by isomerization.

F. Fractionation (Including Winterization)

Fractionation is the process of separating the triglycerides in fats and oils by difference in melt points, solubility or volatility. It is most commonly used to separate fats that are solid at room temperature but is also used to separate triglycerides found in liquid oils.

Fats that are solid at room temperature usually contain a mixture of many individual triglycerides, all of which have different melting points. These components can be separated from one another by the fractionation process.

The result of fractionation is the production of two components, called fractions that typically differ significantly from each other in their physical properties. The fractions can be fractionated again (“double” fractionation) to produce additional fractions, which will have unique physical properties. The process was originally developed to fractionate animal fats such as beef tallow.

There are two types of fractionation techniques: dry and wet. Dry fractionation refers to a process that does not use a solvent to assist in the separation of the fat components. The fat is first melted, and then cooled slowly to generate large, high melting point fat crystals. The slurry of crystals suspended in liquid oil is transferred to a high pressure filter press where the liquid (olein) fraction is squeezed out and the hard (stearin) fat is retained on the filter. This process is widely applied to palm oil and palm kernel oil to generate several unique products from a single natural source, without the need for chemical processing. Fractions produced in this way can be blended together or mixed with liquid vegetable oils to make a wide variety of functional products for many food applications.

Figure 4

In the case of wet fractionation, the fat is first dissolved in a solvent, then cooled slowly and separated in a high pressure filter press. The resulting specialty fractions can be used in value added applications such as confectionery or as a substitute for cocoa butter. Traditionally, fractionation using solvents resulted in fractions of greater purity compared to dry fractionation. However, the use of solvent fractionation is declining due to improvements in dry fractionation filter press technology that can generate fractions of similar quality.

Winterization is another form of fractionation. In this case the starting oil is usually a liquid at room temperature that contains a small amount of dissolved solid fat. At lower temperatures solid fat crystals can form in the oil, giving it an undesirable cloudy appearance. The winterization process cools the oil and the crystals that form are removed by filtration. Cottonseed, and to a diminishing degree partially hydrogenated oils are sometimes winterized.

A similar process called de-waxing is applied to some oils to remove waxes and other high melting point components. Oils that are typically dewaxed include canola, sunflower, rice bran oil and corn. Dewaxing is generally conducted on oils to improve physical appearance (e.g. to make them clear for retail bottling) not for functionality.

G. Hydrogenation

Hydrogenation is the process by which hydrogen is added to points of unsaturation in the fatty acids. Hydrogenation was developed as a result of the need to (1) convert liquid oils to the semi-solid form for greater utility in certain food uses and (2) increase the oxidative and thermal stability of the fat or oil.

In the process of hydrogenation, hydrogen gas reacts with oil at elevated temperature and pressure in the presence of a catalyst. The catalyst most widely used is nickel which is removed from the fat after the hydrogenation processing is completed. Under these conditions, the gaseous hydrogen reacts with the double bonds of the unsaturated fatty acids as illustrated below:

The hydrogenation process is easily controlled and can be stopped at any desired point. As hydrogenation progresses, there is generally a gradual increase in the melting point of the fat or oil. If the hydrogenation of cottonseed or soybean oil, for example, is stopped after only a small amount of hydrogenation has taken place, the oils remain liquid. These partially hydrogenated oils can be used to produce institutional cooking oils, liquid shortenings and liquid margarines. Further hydrogenation can produce soft but solid appearing fats which still contain appreciable amounts of unsaturated fatty acids and are used in solid shortenings and margarines. When oils are more fully hydrogenated, many of the carbon to carbon double bonds are converted to single bonds increasing the level of saturation. If oil is hydrogenated completely, the carbon to carbon double bonds are practically eliminated.

The hydrogenation conditions can be varied by the manufacturer to meet certain physical and chemical characteristics desired in the finished product. This is achieved through selection of the proper temperature, pressure, time, catalyst, and starting oils. Both positional and geometric (trans) isomers are formed to some extent during hydrogenation, the amounts depending on the conditions employed.

See Figure 5 for characterization of trans isomer formation as related to increase in saturated fat during hydrogenation.
Biological hydrogenation of polyunsaturated fatty acids occurs in some animal organisms, particularly in ruminants. This accounts for the presence of some trans isomers that occur in the tissues and milk of ruminants.

Figure 5*

Hydro Chart

* Source of Chart: Cargill Dressings, Sauces and Oils

H. Interesterification

Another process used by oil processors is interesterification which causes a redistribution of the fatty acids on the glycerol fragment of the molecule. This rearrangement process does not change the composition of the fatty acids from the starting materials. Interesterification may be accomplished by chemical or enzymatic processes.

Chemical interesterification is a process by which fatty acids are randomly distributed across the glycerol backbone of the triglyceride. This process is carried out by blending the desired oils, drying them, and adding a catalyst such as sodium methoxide. When the reaction is complete, the catalyst is neutralized and the rearranged product is washed, bleached, and deodorized to give a final oil product with different characteristics than the original oil blends.

Enzymatic interesterification is another means by which oils and fats can be interesterified. This process uses immobilized lipases to rearrange the fatty acids on the glycerol backbone of the triglyceride. These immobilized lipases can target fatty acids at specific positions on the glycerol backbone, therefore, the rearrangement of fatty acids during enzymatic interesterification is less random than with chemical interesterification. After interesterification, the oil is deodorized to make finished oil products. Enzymatic interesterification has gained popularity over the last decade due to the flexibility of the process and reduced capital process input.

The predominant commercial application for interesterification in the US is the production of specialty fats such as shortenings, table spreads, confectionary fats (see also Chapter VIII D.) and nutritional lipids. These processes permit further tailoring of triglyceride properties to achieve the required melting curves.

Figure 6*


* Source of Chart: Bunge Oils

I. Esterification

Fatty acids are usually present in nature in the form of esters and are consumed as such. Triglycerides, the predominant constituents of fats and oils, are examples of esters. When consumed and digested, fats are hydrolyzed initially to diglycerides and monoglycerides which are also esters. Carried to completion, these esters are hydrolyzed to glycerol and fatty acids. In the reverse process, esterification, an alcohol such as glycerol is reacted with an acid such as a fatty acid to form an ester such as mono-, di-, and triglycerides. In an alternative esterification process, called alcoholysis, an alcohol such as glycerol is reacted with fat or oil to produce esters such as mono- and diglycerides. Using the foregoing esterification processes, edible acids, fats, and oils can be reacted with edible alcohols to produce useful food ingredients that include many of the emulsifiers listed in Section K.

J. Additives and Processing Aids

Manufacturers may add low levels of approved food additives to fats and oils to protect their quality in processing, storage, handling, and shipping of finished products. This insures quality maintenance from time of production to time of consumption. When their addition provides a technical effect in the end-use product, the material added is considered a direct food additive. Such usage must comply with FDA and USDA regulations governing levels, and product labeling. Typical examples of industry practice are listed in Table V.

When additives are included to achieve a technical effect during processing, shipping, or storage and followed by removal or reduction to an insignificant level, the material added is considered to be a processing aid. Typical examples of processing aids and provided effects are listed in Table VI. Use of processing aids also must comply with federal regulations which specify good manufacturing practices and acceptable residual levels.

TABLE V

Some Direct Food Additives Used in Fats and Oils


Additive Effect Provided
Butylated hydroxyanisole (BHA)
Butylated hydroxytoluene (BHT)
Tertiary butylhydroquinone (TBHQ)
Propyl Gallate (PG)
Antioxidant, retards oxidative rancidity



Carotene (pro-vitamin A)
Annatto
Color additive, enhances color of finished foods
Dimethylpolysiloxane (Methyl Silicone) Inhibits oxidation tendency and foaming of fats and oils during frying
Vegetable oil tocopherols
Rosemary Extracts
Antioxidant, retards oxidative rancidity


Lecithin Water scavenger to prevent lipolytic rancidity, emulsifier
Citric acid
Phosphoric acid
Metal chelating agents, inhibit metal-catalyzed oxidative breakdown
Polyglycerol esters Crystallization modifier and inhibitor

 

TABLE VI

Some Processing Aids Used in Manufacturing Edible Fats and Oils


Aid Effect Mode of Removal
Sodium hydroxide Refining aid Water wash, Acid neutralization
Carbon/clay (diatomaceous earth) Bleaching aid Filtration
Nickel Hydrogenation catalyst Filtration
Sodium methoxide Chemical interesterification catalyst Water wash, acid neutralization,
Phosphoric acid
Citric acid>
Refining aid, metal chelators Neutralization with base, bleaching, water washing
Acetone
Hexane
Isopropanol
Extraction solvent, fractionation media Solvent stripping and deodorization
Nitrogen Inert gas to prevent oxidation.  
Silica hydrogel Adsorbent Filtration
Enzymes Degumming, interesterification, structured triglycerides Filtration and immobilization

K. Emulsifiers

Many foods are processed and/or consumed as emulsions, which are dispersions of immiscible liquids such as water and oil, e.g., milk, mayonnaise, ice cream, icings, and sauces. Emulsifiers, either present naturally in one or more of the ingredients or added separately, provide emulsion stability. If emulsions lack stability, separation of the oil and water phases can occur. Some emulsifiers also provide valuable functional attributes in addition to emulsification, including aeration, starch and protein complexing, hydration, crystal modification, solubilization, and dispersion. Typical examples of emulsifiers and the characteristics they impart to food are listed in Table VII.

L. Blending

Blending of oils and fats is a common approach to produce a wide array of edible products such as baking and frying shortenings, margarine oils, specialty products and even salad and cooking oils. The basestock used for blending may be composed of liquid oils and/or modified oils produced through hydrogenation, fractionation, interesterification, or trait enhanced vegetable oils.1 These basestock oils and fats can be blended in multiple combinations to satisfy various needs related to cost, nutrition, functionality and oil availability. Blends are often formulated for use in liquid applications such as for frying or spray oils; and in semi solid applications such as baking shortenings.

Liquid blending can entail combining two or more oils to target a flavor or nutritional profile in a frying application. For the application where an improved oxidative stability is desired, commodity vegetable oils such as canola or soybean oil can be blended with oils that contain low levels of linolenic acid or linoleic acid (e.g. corn oil and cottonseed oil). Another approach for increasing the oxidative stability is to blend less stable oils such as canola or soybean with one of the significantly more stable trait enhanced vegetable oils.

Blending is also used to achieve a certain solid fat content in shortenings and to create a nutritional profile in food products. Typically, palm oil or palm stearin is blended with the liquid vegetable oils, including trait enhanced oils, to target varying baking or frying applications.

Shortening blends incorporating liquid oils mixed with interesterified hard stocks (solid fats) can also yield a desired functional shortening for use in baking applications.  These interesterified hard stocks can be produced from palm/palm kernel oil blends or soybean/fully hydrogenated oil blends. A different approach to shortenings is to mix liquid oils with fully hydrogenated vegetable oil targeting a specific functional shortening.

TABLE VII

Emulsifiers and Their Functional Characteristics in Processed Foods


Emulsifier Characteristic Processed Food
Mono-diglycerides Emulsification of water in oil
Anti-staling or softening
Prevention of oil separation
Margarine
Bread and rolls
Peanut butter
Lecithin Viscosity control and wetting
Anti-spattering and anti-sticking
Chocolate
Margarine
Lactylated mono-diglycerides Aeration
Gloss enhancement
Batters (cake)
Confectionery coating
Polyglycerol esters Crystallization promoter
Aeration
Emulsification
Sugar syrup
Icings and cake batters
Sucrose fatty acid esters Emulsification Bakery products
Sodium steroyl lactylate (SSL)
Calcium steroyl lactylate (CSL)
Aeration, dough conditioner, stabilizer Bread and rolls
Sorbitan Esters Crystallization modifier Creams, water in oil emulsions
Propylene glycol esters Emulsification Margarine, Baking, Ice Cream



1Trait enhanced vegetable oils produced from oilseeds that have been modified through breeding or biotechnology to have a higher content of more stable fatty acids such as oleic acid or a lower content of less stable linolenic and linoleic acids.

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