FOOD CHEMISTRY ; LIPIDS
Definition of lipids
- general term for biomolecules that are water-insoluble but soluble in organic media (in biological systems this usually means membrane-bound)
- categories of lipids:
Lipids include fats, oils, waxes, cholesterol, other sterols and most steroids.
- fatty acids – incl. long chain carboxylic acids (C12 - C20)
- fats and oils - triesters of fatty acids and glycerol (triglycerides)
(monoglycerides are monoesters, diglyecrides are diesters)
(function as energy storage materials)
- phosphatides - phosphate esters of glycerol (+ fatty acids)
(membrane constituents)
- steroids - polycyclic structure with specific substituents
(hormones, special functions)
- prostaglandins - cyclic structure with long chains
(hormones, special functions)
Fats and Oils
- fats are solids, oils are liquids (same basic triglyceride structure)
- serve as insoluble deposits that can be retrieved for energy
Fatty Acids
- all natural fatty acids have an even number of carbon atoms, since they are synthesized from acetate (C2) subunits
- natural unsaturated fatty acids have cis double bonds
cis-double bonds lead to a lower melting point
unsaturated and polyunsaturated fats are also more easily digestible - unsaturated fatty acids can not pack tightly as fatty acids with no double bonds.
- Fatty acids do not accumulate in cells, They are quickly converted to glycerides.
Phospholipids
- With a polar phosphate head group
Steroids
- cholesterol is the precursor to most other steroids: vitamin D, bile salts, hormones
- cholesterol is present in body tissues and is found in some foods esp. eggs.
The role of visible and invisible fat in foods
The major fat structures within the fats and oils, are dominantly triglycerides; however, the other categories/ types of lipids are important. Certainly, the monoglycerides and diglycerides are well-established emulsifiers in a variety of food products. Other lipids’ importance may be more subtle. For example, the lipoprotein complex in gluten has a major role in the elasticity and strength of gluten; however, it is generally not extensively discussed as gliadin and glutenin are so important. The role of the lipid in the plastid of green and yellow vegetables is rarely mentioned; however, it functions to solubilize the pigment. The sterols in the wax of shiny apples is another compound lipids.
The use of fat in foods continues to expand as they become more healthy and as the industry learns to modify the natural product. The role of fat in a food product can be as varied as the product itself. In shortened cake it serves to tenderize, incorporate air, and possibly add flavor. In salad dressing it is part of the structure, the small droplets in a second liquid. These roles and other roles can be listed as:
textural qualities
emulsions
shortening or tenderizers
medium for transferring heat
aeration and leavening
spray oils
These uses are impacted by the functionality of the particular fat or oil. These functionality's or roles are: gives satiety; heat transfer; flavor; texture: body, mouthfeel;
tenderizes: gluten, starch; decreases temperature shock in frozen desserts; "solubilizes" flavors and colors;
dispersal; foaming; incorporation of air.
The differing roles of fat and oil can be seen in the following recipes:
RecipeRole or Purpose
MayonnaiseThe oil primarily serves the function of the dispersing phase.
FrostingThe butter fat to allow the creation of a foam.
Surfactant is a short term for surface active agents. Polar lipids like lecithin in soy bean oil serve as specialized surfactants known as emulsifiers. By interacting with water on one end of the molecule and repelling it on the other end, emulsifiers keep fat globules dispersed in water, or water droplets dispersed in fat. These chemical properties of lecithin are used in the food industry to prevent fat from separating out of chocolate, mayonnaise, peanut butter and salad dressings.
Fats and oils make up to 95% of food lipids and phospholipids, and sterols make up the other 5%. Traditionally fats were considered to be solid at room temperature and oils liquid at room temperature. However this is often used to distinguish fat and oils from animals and plants respectively.
There are several classes of lipids. In food preparation, we are most concerned with simple lipids. These are the triglyceride lipids, the major component of fat, butter, shortening, oil, etc.
They have a simple formulas composed of glycerol and a variety of fatty acids. Fatty acids have a basic formula consisting of hydrocarbons and carboxyl group (COOH). They have long straight chains. The "R" group refers to the many possibilities.
The composition of the "R" group makes the fatty acid either saturated, e.g stearic fatty acid, or unsaturated,eg. oleic fatty acid. The presence of a double bond is responsible for the liquid properties of native vegetable oils. The hydrogen atoms around the double bond can either be on opposite sides (trans) or on the same side (cis). The ‘cis’ double bonds are ‘kinked’ they disrupt the physical interactions between the fatty acid molecules preventing them from packing together tightly to form crystals. This disruption keeps the fatty acid molecules from associating with each other, resulting in a liquid structure. If the double bonds are removed by adding hydrogen, the kinks are removed allowing the FA molecules to more easily associate with each other. The result is crystalisation (solid fat) at room temperature.
Glycerole is the backbone of a glycerol lipid. Since triglycerols have three fatty acids, you can get mixed triacylglycerols in which there are different FA on each of the glycerol bonds eg soy bean oil with saturated, mon-and polyunsaturated FA. The image below gives an example of glycerol plus three fatty acids to form a triglyceride. The bond between the FA and the glycerol is called an ester bond. Water is split off from the hydroxyl group of the glycerol molecule and the carboxyl group of the fatty acid.
fill in
Relationship of structure to melting point, crystallinity and plasticity of fat.
In order to have crystalline fat, one must have the fat be a solid. For that reason, the following melting points are pertinent. Plasticity occurs due to a mixture of solid fat crystals and liquid oil. Plastic fats are soft and can be spread, but they can not flow. Plasticity of a fat is due to a mixture of a number of different triglycerides with each having its own melting point. When
a large number of these triglycerides are below their melting points the mixture is solid and consists of a network of minute crystals surrounded by a smaller quantity of liquid triglycerides. Though solid, the network is not rigid hence the crystals can slide over each other giving rise to the plasticity of the fat. On raising temperature, a large propotion of the fat melts, the solid network breaks, plasticity increases until it becomes liquid
Fatty Acids # of CarbonAtomsFormulaMelting Point0C
Saturated Fatty Acids
Butyric 4 -7.9
Caproic 4 -3.9
Caprylic 8 16.3
Capric 10 31.3
Lauric 12 44.0
Myristic 14 54.4
Palmitic 16 62.8
Stearic 18 69.6
Arachidic 20 75.4
Behenic 22 80.0
Lignoceric 24 84.2
Unsaturated Fatty acids
Palmitoleic 16 -0.5 to 0.5
Oleic 18 13
Linoleic 18 -12 to -5
nolenic 18 -14.5
Arachidonic 20 -49.5
A number of generalizations can be made in view of the fatty acids and the melting points. It is apparent that the following factors affect the melting point.
longer chain increases melting point
number of double bonds (more double bonds the lower the melting point)
cis conformation (lower melting point than trans)
arrangement on the glycerol molecules affect crystallization
shorter chain or more double bonds more ability to emulsify.
It is not enough to know the characteristics of the fatty acids. Their arrangement of these fatty acids upon the glycerol backbone will make a difference. If one reviews the characteristics of different fats and oils one will note that fats differ in melting point, flash point and smoke point. That is because of the heterogeneity fatty acids and triglycerides. Fats exist in several crystalline forms, ie. they are polymorphic. Each crystalline form has its own melting point. When oils are cooled, mixtures of different crystalline forms and melting points can be obtained depending on how the cooling was carried out. The way oil is cooled affects the texture and consistency of the product formed. Hence this is an important factor in fat manufacture.
Smoke, flash, and fire points of oils(0C)
OIL/FAT SMOKEFLASHFIRE
Castor, Refined 200298335
Corn, Crude 178294346
Corn, Refined 227326359
Olive, Virgin 175-199321361
The degree of crystallization of a triglyceride determines whether it is a solid or a liquid. The more crystalline the fat the more likely it will be a solid. There are a number of factors which will effect the crystal type and characteristics. The most important is fatty acid composition. Generally, the more saturated and/or the longer the chain length the more likely it will be a solid. The arrangement of fatty acids on the glycerol backbone will also make a difference.
Depending on how the various FA chains associate, the crystalline solid fat can have different appearances such as, a smooth shiny solid, or a rough puffy solid. These crystalline forms also have different light reflection characteristics and physical hardness. This difference in physical properties is used when making shortening, which is crystallized into a very white, soft crystalline form at the factory. However, upon melting and re-solidification it becomes more transluscent and grayer, due to the formation of a different crystal structure.
In addition to the solidity or melting point of each individual triglyceride, we are also concerned with the combination of triglycerides throughout the fat mixture. This impacts the plasticity and the melting point range. There are four main types of lipid crystals.
Types of Lipid Crystals
alpha crystals: small
beta prime crystals: stable and fine
intermediate crystals
beta crystals: course
In crystals that are polymorphic, the chemical formula is the same. They form different crystals depending upon the temperatures and rate of cooling.
Fats and oils are extracted from either plants or animals. Extraction methods vary. For example, the adipose tissue of the pig is heated, melts the fat and it is further processed. Butter is made by reversing the oil in water emulsion of cream into a water in oil emulsion. Plant extraction procedures involve a variety of different extraction methods.
Processing of fats
Fats and oils are obtained by rendering, pressure expelling and solvent extraction after cleaning milling and separation of seeds. The method employed depends on oil content.
Rendering – The meats are heated in steam or water to melt the fat. The melted fat then rises and is separated by skimming or centrifugation.
Pressing or expelling – Seeds may be cooked, ground, cracked or flaked to partially breakdown cell structure (gem or whole seed). The cake is then pressed to obtain oil.
Solvent extraction – Seeds are cracked and hexane used for extraction then the solvent is distilled for reuse. A combination of pressing and solvent extraction leads to higher yields.
After removal of the plant fat from the seed, pod or grain, it is further refined as follows.
Degumming
The first step in the refining process of many oils is degumming. Oils are mixed with water to hydrate phosphatides, which are removed by centrifuging. Phosphoric or citric acid or silica gel are added to enhance the process. Degumming removes valuable emulsifiers such as lecithin. Cottonseed oils are not degummed, but degumming is necessary for such oils as soybean and canola.
Alkali Refining The degummed oil is then treated with an alkali to remove free fatty acids, glycerol, carbohydrates, resins, metals, phosphatides and protein meal. The oil and alkali are mixed allowing free fatty acids and alkali to form a soap. The resulting soap stock is removed through centrifuging. Any residual soap is removed with hot water washings. Cottonseed oil is also refined using a process call miscella through this process oil is refined in the miscella stage prior to removal of the solvent. The oil produced using this method has higher yields and has what some consider as lighter, more desirable color.
Bleaching Trace metals, color bodies such as chlorophyll, soaps and oxidation products are removed using bleaching clays which adsorb the impurities. Bleached oils are nearly colorless and have a peroxide value of near zero.
Depending on the desired finished product, oils are then subjected to one or more processes of the following processes.
Winterization (Fractionation) Oils such as salad oils, or oils that are to be stored in cool places undergo a process called winterization so that they will not become cloudy when chilled. The refined, deodorized oils are chilled with gentle agitation, which causes higher melting fractions to precipitate. The fraction, which settles out, is called stearin. Soybean oil does not require winterization, but canola, corn, cottonseed, sunflower, safflower and peanut oils do.
Hydrogenization
Treatment of fats and oils with hydrogen gas in the presence of a catalyst ( eg. nickel) results in the addition of hydrogen to carbon-carbon double bond. The catalyst reduces the amount of energy required for a reaction to proceed. Nickel therefore provides surface upon which the reaction can take place. Hydrogenation produces oil with mouth feel, stability, melting point and lubricating qualities necessary to meet the needs of many manufacturers. It is important to note that hydrogenation is a selective process that can be controlled to produce various levels of hardening.
Deodorization
Deodorization is a steam distillation process carried out in a vacuum, removing volatile compound from the oil. This may be a batch or continuous process. The end product is a bland oil with a low level of free fatty acids and a zero peroxide value. This step also removes any residual pesticides or metabolites that might be present. Some manufacturers favor the use of cottonseed oil because it can be deodorized at lower temperatures, which results in more tocopherols (natural antioxidants) being retained. Deodorization produces some of the purest food products available to consumers. Few products are as thoroughly clean as refined, bleached, and deodorized oil.
Lard interester
This process allows fatty acids to be rearranged or redistributed on the glycerol backbone. This is most often accomplished by catalytic methods at low temperatures. The oil is heated, agitated and mixed with the catalyst at 90oC. There are enzymatic systems which may be used for interesterification. It does not change the degree of saturation or isomeric state of the fatty acids, but improves the functional properties of the oil.
Tempering
Modification/ Conditioning by chilling and agitation to influence crystallization rate and crystal form.
Rancidity
When discussing the stability of a fat or oil, generally, the emphasis is on oxidation or hydrolysis of the triglyceride molecule. However, if one uses the definition that rancidity is the development of any disagreeable flavor or odor, one might list the following forms of rancidity.
absorption of odors
action of microorganisms
action of enzymes (hydrolytic rancidity)
atmospheric oxidation
- common oxidative rancidity
- flavor reversion
- enzymatic oxidations
- oxidized flavors in milk and milk products
An understanding of the two major forms of rancidity, hydrolytic and oxidative, is critical. Of the two, oxidative rancidity is considerably more complex. There are a number of additives which not only impact rancidity but also the flavor and color of the fat or oil. Some of these are listed:
Hydrolytic rancidity
Deterioration of fat due to hydrolysis occurs primarily in dairy products. Hydrolytic rancidity is hydrolysis of triglyceride into its component fatty acids and glycerol. The reason it causes an odor and flavor deterioration is because we taste individual fatty acids more than the total triglyeride. Since lipase naturally occurs in dairy products, it happens that short chain fatty acids are a major component. These short chain fatty acids like butyric acid are particularly able to be perceived by the tongue sensory buds. The rate of hydrolysis is hastened by the presence of water, enzymes and microorganisms. Fats that have not been heat treated may contain lypases which catalyse hydrolysis. They may also contain moulds, yeasts and bacteria naturally present in the oil or contaminated during processing.Since hydrolytic rancidity occurs naturally, the best defense is to keep butter in the refrigerator.
Oxidative Rancidity
This is the most common type that produces tallowy flavors. It is caused by the reaction of unsaturated oils with oxygen and its occurrence does not depend on the impurities or moisture in oil. If one has oil or fat with some degree of unsaturation, it is unavoidable that oxidation will take place. It must be minimized through care taken in processing and care taken after purchase. During processing, one may refine, bleach, hydrogenate, deodorize, and give additives [antioxidants] to minimize oxidation. There are mechanisms for inhibiting lipid oxidation. After processing, the consumer wants to minimize the availability of oxygen and decrease the speed of the reaction. This may be done easily by keeping covered and refrigerating.
Processing mechanisms for inhibiting lipid oxidation
Hydrogen donation by the antioxidant
Electron donation by the antioxidant
Addition of lipid to aromatic ring of the antioxidant
Formation of a complex between the lipid and the aromatic ring of the antioxidant
Antioxidants Used To Decrease Oxidative Rancidity of Fat or Oil
butylated hydroxyanisole (BHA) improves oxidative stability, antioxidants
butylated hydroxytoluene (BHT) improves oxidative stability, antioxidants