Fats and Oils

Introduction

Fat is having a relatively high melting point and are solid at room temperature whereas that having lower melting points and are liquid at room temperature are called as oils. Fat is present naturally in many foods. This fat is often referred to as invisible fat. Examples of foods containing appreciable quantities of invisible fat include meat, poultry, fish, dairy products, eggs, nuts and seeds. Visible fats are made from these products. They are lard, cooking oils, salad oils, margarine and butter. Fats play a variety of roles in both food preparation and nutrition.

Nutritional Importance

1. The are concentrated sources of energy. Weight for weight, they furnish 2.25 times more energy than proteins and carbohydrates. All oils and fats except butter give 900 kilocalories per hundred grams.
2. They reduce bulk in the diet.
3. They are excellent sources of fat soluble vitamins A, D, E and K. Butter contains 15,000 I.U of vitamin A. refined vegetable oil and hydrogenated shortenings contain little or no vitamin A but vegetable oils are good sources of vitamin E.
4. They play a part in the biosynthesis of several long chain fatty acids.
5. They provide essential fatty acids, which are components of membranes of living cells.
6. Fats are slow in leaving the stomach and hence retard digestion.

Fatty Acid Composition of Oils

Percentage Fatty Acid Composition of Commonly Used Oils

When fats contain high proportion of saturated fatty acids such as palmitic and stearic acids. ,they have relatively high melting points whereas fats containing high proportion of unsaturated fatty acids such as the monounsaturated oleic acid and polyunsaturated linoleic acid, they have relatively low melting points and are oils at room temperature.

Plasticity

Most fats that appear to be solid at room temperature actually contain both solid fat crystals and liquid oil. The liquid part is held in a network of small crystals. Because of this unique combination of liquid and solid, the fat can be molded or pressed into various shapes without breaking. This property of fat is called plasticity.

Processing and refining of Fats

Fats and oils commonly used in food preparation are separated from various materials and refined. Many oils come from seeds or fruits, lard comes from pork tissue and butter comes from cream. Further processing produces fats such as margarine and hydrogenated shortening.

Refined Oils

Vegetable oils are removed from oil containing seed fruits or nuts by various pressing processes, by solvent extraction and by combination of these. After extraction, the crude oils are refined.

Specific Fats

• Lard

To make lard, fatty tissues of the hog are chopped into small pieces and heated, with or without the addition of water, to remove fat from the cells, a so-called rendering process. The quality of the lard depends on the location of the fatty tissue in the animal and on the method of heating. An antioxidant is added to delay the onset of rancidity. To improve its quality it may be hydrogenated bleached or emulsifier is added.

• Hydrogenation

The process or hydrogenation changes liquid oils into more solid plastic shortenings and to increase the stability of the oils to prevent spoilage from oxidation, which results in undesirable rancid flavour and odours. This chemical change makes the fatty acids more saturated. The melting point of the fat is thereby increased. With sufficient hydrogenation it becomes solid at room temperature. Soyabean oil, cotton seed and palm oil are used for hydrogenation.

• Winterisation

In this process, the temperature of the oil is lowered to a point at which the higher-melting triglycerides crystallise. Then the oil is filtered to remove these crystals. The remaining oil has a lower melting point and does not crystallise at refrigerator temperatures. It is referred to as salad oil.

• Butter

Butter is the fat of cream that is separated more or less completely from the other milk constituents by agitation or churning. Butter formation is an example of the breaking of an oil-in-water emulsion by agitation. The resulting emulsion that forms in butter itself is a water-in-oil emulsion with about 18% water being dispersed in about 80% fat and a small amount of protein acting as the emulsifier. Many volatile fatty acids and substances called diacetyl, formed from bacterial action is an important flavour component of butter. Butter is highly valued by many for its flavour.

• Margarine

Margarine is made from one or more optional fat ingredients churned with cultured pasteurised skim milk or whey. Other ingredients added to margarine are vitamins A and D for nutritive purposes; diacetyl as a flavour constituent, lecithin, monoglycerides and/or diglycerides of fat-forming fatty acids as emulsifying agents, artificial colours, salt, citric acid or certain citrates and sodium benzoate as preservative to the extent of 0.1%.

Flavour reversion

Flavour Reversion is defined as a change in edible fats that is characterized by the development, in the refined material, of an objectionable flavour prior to the onset of true rancidity. It may develop during the exposure of the fat to ultra violet or visible light or by heating. A small amount of oxygen seems to be necessary for the reaction that is catalysed by the presence of small amounts of metals such as iron and copper. Selective hydrogenation decreases the amount of linolenic acid and aids in preventing flavour reversion. Soyabean oil is most susceptible for flavour reversion.

Rancidity

Spoilage of fats may occur on storage, particularly if the fats are highly unsaturated and the conditions of storage are conducive to chemical change in the fats. Rancidity is of two types-hydrolytic and oxidative.

• Hydrolysis

Hydrolysis is brought about by enzymes that decompose fats into free fatty acids and glycerol. Heating thoroughly to destroy the lipase enzyme that catalyses the hydrolysis of triglycerides should prevent hydrolytic rancidity. Contaminating microorganisms may also produce lipase and these can similarly be destroyed with sufficient heating.

• Oxidation

Only unsaturated fats and foods which have lipoxygenase are susceptible to oxidative changes. Highly hydrogenated and saturated fatty acids are relatively resistant to oxidation.

• Prevention of rancidity

1. Storage at refrigerator temperature prevents rancidity.
2. Rays of light catalyse the oxidation of fats by the use of coloured glass containers that absorb the active rays, fats can be protected against spoilage. Certain shades of green bottles and wrappers and yellow transparent cellophane wrappers are effective in preventing rancidity.
3. Vacuum packaging also helps to retard the development of rancidity by excluding oxygen.
4. Antioxidants naturally present in the food such as vitamin C, beta carotene and vitamin E protect against rancidity.

Smoking Point

Smoke point of fat is the temperature at which smoke comes continuously from the surface of the fat. Development of free fatty acids by some hydrolysis of the fat during frying causes a decrease in the smoke point. Suspended matter such as flour or batter particles also lower the smoke point. And the greater the surface of the fat exposed, the lower is the smoke point.

Smoking temperatures of fats

Changes in Fat On Heating

There are four main ways in which the oil decomposes and all of these pathways lead to a diminution of smoke point and operating temperature. Cooking oil is chemically decomposed in the cooking process and this decomposition leads to a deterioration in cooking quality if the oil is kept in use for a long time.

• Pyrolysis (thermal breakdown)

The action of heating the oil, whether or not it is in contact with food, causes a breakdown in its molecular structure.

• Oxidation

Reaction of the hot oil surface with oxygen in the air causes molecular breakdown and ultimately the development of rancidity. This change will occur whenever the oil is heated.

• Hydrolysis

As water is boiled in large quantities from food during frying, some of this water will tend to decompose the oil.

• Reaction with food residues

Any food fragments left in the oil after cooking will undergo chemical reaction, particularly if the oil is heated again without the residues being removed. Straining the oil after use can help to prevent this type of decomposition.

I. Fat/Oil used as medium of cooking

Fat is used in shallow and deep fat frying. Pan-frying is used to cook dosas, chapathis, omelettes, cutlets and tikkis. Deep fat frying method is used in preparing pooris, vadas, cutlets, bajjis and pakodas.

II.Fat improves the texture of foods

Fat plays an important role in the proper development of texture in cakes, biscuits and cookies. Its function is particularly important in pastry where there is no sugar to contribute to tenderness.

1. Fats help in leavening

The role of fat in the leavening process is its ability to trap air during the mixing process. So the fat is indirectly responsible for the tenderness of the cake. Thus fat contribute to the grain and volume of baked products. Though all packed products use fat, amounts used are different.

2. Fat as a shortening agent

One of the most important function of fat is to shorten baked products which otherwise are solid masses firmly held together by strands of gluten. The usual fat used, if any, in yeast bread is butter or margarine. In biscuits, a hard fat must be used so that fat can be distributed in small pieces to give the desired flakiness to the biscuits. Shortened cakes are made using a plastic fat, which combines readily with the ingredients in the flour mixture. Butter is used in puff pastry.

3. Fats for smoothness

They limit the size of water crystals and help in maintaining smooth texture. They affect the smoothness of crystalline candies through the retardation of crystallisation and the gelatinisation of starch in starch thickened mixtures. Fat prevents lump formation and brings smoothness.

III.Fats improve palatability

Fat gives taste and flavour to the food some fats like ghee is used at the table to improve the flavour. Ghee and butter when used in the recipe improve the flavour. The ability of fats to take up or dissolve certain aromatic flavour substances is frequently used in food preparation. Onion, ginger, garlic, peppers and other flavourful foods are cooked in oil so initially flavour can be incorporation into other foods. Aromatic fruit and other flavours are also dissolved by fat. Colouring agents like turmeric, when added to oil helps in the distribution of colour through out the food.

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Technologies
(Food Technology)