15 Neutral fats I

Prof. M. N. Gupta

Objectives

•  To learn about the major oils obtained from plant sources and their fatty acid composition
•  To learn about the importance of free fatty acid content with ricebran as an example
•  To learn about fat splitting process to obtain fatty acids
•  To learn about the desirable properties of fats/oils

Concept Map

3. Description

Fats and oils are called neutral fats. However, as we mentioned in the last module, endogeneous lipases or chemicals result in slow hydrolysis of the ester bonds. As we know from school chemistry, ester bond is susceptible to both acids as well as alkalies. Hence, some free fatty acids (FFA) is invariably present in the fats and oils. Hence we have the curious situation of talking of acid value of the neutral fat or oil!

Biochemists look at the fat/oil as the source of concentrated energy. One gram fat on an average produces twice as much energy as one gram of carbohydrate or protein.

The source of this energy is hydrocarbon chain in the long chain fatty acids which esterify the glycerol.

Biotechnologists have always been interested in how to obtain oil in an environment friendly way. Lately focus has been on developing the biorefinery concept in which the rest of the plant (after oil extraction) is used for obtaining value added products.

Oils/fats are triglycerides. However, most of the oils/fats are not a single triglyceride. In these different triglycerides, nature of the fatty acid chains are different. So, when we talk of the composition of a fat/oil in terms of a fatty acid, we are merely referring to the sum total of the fatty acid present in different triglycerides which constitute a particular fat/oil.

In the last quarter of the last century, the production and consumption of fats/oils rose by average rate of 3-7% per year. In 2004-05, it was estimated to be 136 million tons!

The edible oils which are mostly obtained from plant seeds are evaluated on the basis of certain parameters. The standards for the quality control are given in AOCS official methods and recommended handbook or in the IUPAC standard methods for the analysis of oils, fat and derivatives.

Free Fatty Acid Content

One such important parameter is the amount of free fatty acid content is the oil. A major source of this free fatty acid (FFA) content is the oil/fat hydrolysis catalyzed by endogenous lipases. The FFA is determined by HPLC or GC.

Rice bran oil is a good example of this. Rice bran and polish is the source of this oil. Rice bran is about 5-8% of rough rice and polish is about 2-3% of the same. Some perspective about the importance of rice bran as an oil source can be gained by comparing its fat content (15-20%) to the corresponding figure in wheat bran (3-4%).

Rice bran oil production as a by-product of rice milling started in USA in the late 1950s. By 1980s, most of this production was stopped as it was not found profitable as a commercial activity. Two  developments created resurgence in its production. First was learning how to prevent excess FFA content of the oil. More important, the observation that rice bran oil contains oryzinol, a serum cholesterol lowering compound led to rice bran oil emerging as an important edible oil worldwide. So, technological developments often shape market forces.

At one time, rice bran was used as just animal feed or in aquaculture. Till recently, in India that was the prime use of the rice bran obtained by rice mills. Rice bran has a lipase which has been studied extensively.

During milling, ~60% of the lipase present in the testa cross layer of the grains gets mixed with oil present in the aleurone, subaleurone and germ. As much as 5-7% FFA content increase has been reported.’ FFA presence lends a soapy taste to the oil. Simultaneous presence of lipoxygenase catalyzes oxidative damage which produces more off flavours and some off-odors.

Treatment of rice bran by dry heat, wet heat and extrusion are some important methods for inactivating these enzymes. The process is called stabilization of rice bran.

Caustic Refining

Caustic refining is part of oil refining process in which FFA are removed from crude edible oils. It also removes at least partially, phospholipids and coloured impurities. In fact, degumming to remove gummy phospholipids is generally carried out before caustic refining. So, caustic refining completes the degumming process.

In case degumming is not a part of the operations, caustic refining is conventionally carried out as a first step in the oil processing. If caustic refining is not done at early stages, subsequent refining steps converts coloured materials with darker products and some other impurities may result in cloudy oil due to their precipitation.

Initially FFA content is measured and dilute NaOH is mixed in slight excess to ensure neutralization and removal of other impurities. This amount has to be decided very carefully as any excess will start alkali catalyzed hydrolysis of oil to form more FFA.

While FFA neutralization is very fast and is over nearly instantaneously, process is prolonged to ensure degumming and removal of other impurities. This process time differs from oil to oil. For soyabean oil, ≥ 5 min is required, less time is beneficial for corn oil.

The caustic refining is completed by heating to separate soap (whatever is formed) and other impurities.

While it causes significant amount of loss of oil, caustic refining remains a method of choice to reduce FFA content.

One of the common product category in oil processing plants are emulsifiers or biosurfactants. The range of such by-products includes monoglycerides and diglycerides, propylene glycerol esters, lecithin, sorbitan ester etc.

Monoglycerides (MG), Diglycerides (DG) are the most common emulsifier which are made by oil processing industries. While these are partial hydrolytic products of fats/oils during fat splitting to produce FFA (+glycerol), their industrial importance has led to several strategies being developed. MGs can be produced by:

•  Esterification of fatty acids with glycerol
•  Reacting salts of fatty acids with glycerol halohydrins
•  Transesterification of triglycerides with glycerol

The first two methods involve use of fatty acids or their alkali metal/ silver salts. The direct esterification offen results in a mixture of monogycerides, diglycerides and triglycerides.

The composition of this mixture varies with type of fatty acids, ratio of glycerol: FA, catalyst and other reaction conditions. Other, mono- and di-glycerides are predominant reaction products.

Due to acyl migration from position 2 (of glycerol backbone) to position 1 (or equivalent position 3), the diglycerides is generally 1,3 diglyceride.

As is discussed elsewhere, lipases can be used to obtain monoglycerides/diglycerides. What is more fatty acids can be esterified by variety of alcohols to produce diverse kind of compounds which find applications as flavouring agents, fragrances and biosurfactants.

Fat Splitting

Hydrolysis of Fat/oil to produce fatty acids and glycerol is industrially known as fat splitting. This process is the key to the manufacture of fatty acids, soaps, fatty alcohols and other oleochemicals. Industrial level process consists of introducing oil from the bottom of the reaction which is called splitter and it runs into the water stream coming from the top.

A mixture of diglycerides and monoglycerides is always present. At distillation temperature, these react with the desired product fatty acids reform triglycerides. This reversible reaction produces significant losses.

indicates % of free FA in the product stream. The target DOS in a successful fat splitting process is >98.5%.

The temperature in the splitter is kept at 230-265 oC so that water is able to mix with oil. Fats with short chain FA require less temperature to mix with water.

The water stream apart from acting as the reactant, also removes glycerol. Removal of this product drives the reaction towards fatty acids production and prevents reversible reaction.

The fatty acid production was started in early years of last century to produce stearic acid for candles. Stearic acid and oleic acid were the primary fatty acids produced by oleochemical industry.

Five vegetable oils lead the list of oils which are responsible for the increase in production level of oils: Soybean oil, palm oil, palm kernel oil and rapeseed/canola oil. Little less increase has resulted from oils from cottonseed, peanut, sesame, corn, olive, coconut, linseed and castor. The animal fats whose production/consumption have increased is butter, lard, tallow and fish oil.

For vegetable oils, increased area of cultivation and yields have both contributed to these enhancements. About 80% is used as food, rest is utilized either as feed or by oleochemical industries.

The food applications include their use as frying oils and in baking fats, spreads, salad oils and confectionary fats/ice cream. Some oil sources such as peanuts, beef, lamb, pork, chicken and fish eaten as such constitute another category of consumption of fats/oils.

Given their applications, it is logical that for each application, the fat/oil has a list of ideal properties, Crystallization and melting properties are two such key properties. For example, salad oils which crystallize during storage are not acceptable!

For incorporating in spreads, the solid fat content at 4 °C should be < 30-40 %, otherwise the spread cannot be used immediately after taking it out of the refrigerator. At the same time, enough solid content is required at ambient temperature as well.

In the mouth temperature, the fat should completely melt otherwise mouth feel quality will not be acceptable. The spread will taste like waxy material.

Fats form β’ and β kinds of crystals. Fats which form β’ type crystals are preferable in spreads since these remain small in size, entrap good amount of liquid oil and have desirable look on the surface (which is necessary in a spread!).

Oils with fatty acids with mixed chain lengths of C16 – C18 range tend to form β’ type crystals. On the other hand, oils with just C18 fatty acids is more likely to form β-form of crystals.

In fact textural properties of fats are important in the context of their use in wide range of products such as lipstick, chocolate, butter, whipped cream, icecream and margarine. For processed food, mouth feel is an important property.

  

Many properties collectively decide the texture of fat rich product. Structure of triglycerides, solid fat content, melting and crystallization properties are some of these important properties.

We have already discussed the importance of crystallinity in spreads. Many factors influence crystallinity and operate differently in bulk form and when fats are present as emulsions (which is the case with processed food in large number of cases).

Oxidative Stability

The presence of UFA and PUFA in triglycerides makes the latter prone to oxidation. In materials like paints, this results in deterioration of the films. In food, it leads to rancidity and becomes noticeable even at lower level.

The oxidation of fats/oils follows broadly two mechanisms. In autooxidation, the reaction occurs with triplet oxygen. In the second route, photooxidation, more reactive singlet oxygen is involved. The photooxidation, besides the obvious need of light also requires a photosensitizer.

Nevertheless, in both unsaturated hydroperoxides RCH=CHCH(OOH)R’ are formed. These are unstable and breakdown into some short chain molecules which include aldehydes. These are responsible for the undesirable odour and off-smell.

Oxidation is facilitated by heat, light, metal ions, initiators. All such factors are called prooxidants. Presence of antioxidants prevents development of these undesirable traits. Fats/oils, even in refined form are likely to contain both prooxidant as well as antioxidants.

Obviously, increasing amount of antioxidants would enhance oxidative stability of fats/oils. Primary antioxidants are radicals scavengers and can inhibit initiation and propagation steps of the free radical process. Molecules with extended unsaturated systems such as carotene can also act as primary antioxidant by forming an addition product.

RO₂˙ + B → ROOB˙

(Carotene)

Secondary antioxidants include metal chelators including amino acids. Metal ions promote initiation step by reacting with hydroperoxide. So, these metal chelators tie up with the metal ions. Use of appropriate surfactant is known to result in formation of emulsions in which metal ion-hydroperoxide contact is prevented.

Ascorbyl palmitate is the lipid soluble derivative of ascorbic acid and acts as an O2 scavenger. Presence of phospholipids is also known to prevent oxidation by ill understtod mechanisms. However, in refining the aim is to degum the oil by removing phospholipids.

Naturally occurring antioxidants like like tocopherols (Vit E). sesamol (in sesame oil), oryzanols (rice bran oil) also help in preventing oxidation in their respective oils.

Tocopherols and tocotrienols are widely present in oils of plant origin. Normally, this activity is supplemented by adding ascorbyl palmitate at 200-500 ppm level.

Many synthetic compounds, mostly phenols such as BHA (butylated hydroxyanisole with trade name of E320), BHT (butylated hydroxytoluene, E321), propyl gallate ( PG, E310) and TBHQ (t-butylhydroxyquinone) are widely used to stabilize oil/fat containing food.

Effect of the anti-oxidants vary from oil to oil as FA composition is a critical parameter. Temperature is another critical parameter. Some of the antioxidants are volatile, so storage temperature are important. Many antioxidants have a synergy in preventing oxidation.

The major oil already indicated constitute about 20% of all fat/oil production. That is not to say that fats/oils from other sources are not important. Cocoa butter is needed for chocolates and cannot be substituted directly by any other oil!

The cocoa bean (Theobroma cacao) is used for producing cocoa powder and cocoa butter. Both are used in chocolates. As with other fats/oils, its uniqueness originates in the fatty acid composition of the triglycerides which constitute it.

Cocoa butter has 26% palmitic acid, 34% stearic acid and 35% oleic acid. The major triglycerides are: POP (18-23%), POSt (36-41%) and StOSt (23-31%). Here P=palmitic acid, O= oleic acid and St= stearic acid. Thus, all triglycerides are of symmetrical nature with identical saturated FA at terminal C-atoms and oleic acid at position 2.

The result is a fat with a melting point marginally below average mouth temperature! Attempts to obtain other triglycerides with similar melting profiles have been made.

Legally, unless specifications are adhered to, products containing such triglycerides cannot be called chocolate, these are called confectionary. According to EU specifications, chocolate is allowed to have a small amount of palm mid-fraction or one or more of the other tropical fats listed above.

Table 4: Detailed Fatty acid composition of selected vegetable oils

Among some common vegetable oils, linseed oil has high level of linolenic acid. Some others also have high % content of oleic and/or linoleic acid. Among vegetable oils, cottonseed and ricebran oils have higher levels of palmitic acid.

Both sesame and rice bran oils because of their high oxidative stability are used as blends to increase the average oxidative stability. Sesame oil also contains sesamin, sesamolin and other lignans which are strong antioxidants. Olive oil has polyphenols as the antioxidants.

It may also be worthwhile to look at the composition of several important and the top 5 sources of oils and fats.

Increasingly, oleochemical industries looks for oils which are known to have health benefits. Hence, the increasing market penetration for oils like olive oil, rice bran oil and saffola.

Also, it makes economic sense to look for uses for the spent meal left after oil has been isolated from the seed/ fruit etc.

Summary

•  Oils from plants and their fatty acid composition
•  Importance of free fatty acid content and oil refining
•  Fat splitting at the industrial level
•  Desirable properties of fats/oils