UNIT - 3 FATS AND OILS

Introduction

• The basic use of fats and oils in cookery is to add richness and flavor to food and as a cooking medium to fry or cook food. They improve the texture of various preparations such as cakes, pastries and biscuits.
• Fats and oils are found in plants, animals and marine foods.
• They are organic compounds composed of C, H and O
• Collectively known as LIPIDS
• Immiscible in water but soluble in organic solvent. (Ether, Chloroform, Benzene and Acetone)
• Unlike carbohydrates – contains small proportion of O and larger proportion of H and C
• Provide more energy per gram than carbohydrates.

Classification based on origin

Classification based on degree of saturation

Difference between Fat and Oils

FAT                                                                             OIL                                                     

Remains solid at room temperature.                                    Remains liquid at room temperature.       
Relatively more saturated.                                                   Relatively unsaturated.
Relatively higher melting point.                                          Low melting point.                                   
More stable.                                                                         Less stable.                   

RANCIDITY

• Development of any undesirable odour and flavor in fats and oils causing spoilage.
• Observed when fats and oils are stored for some time.
• Rancidity develops in fats, oils and the fatty phases of foods such as pickles, fried snacks, cakes, cheese and salad dressings.
• Different fats and oils  show varying degree of resistance to spoilage.
• Vegetable oils deteriorate slow.
• Animal fats deteriorate fast.
• Marine oils having high proportion of unsaturated fatty acids deteriorate most rapidly.

Types of rancidity

HYDROLYTIC – by presence of moisture

OXIDATIVE – by presence of oxygen

HYDROLYTIC RANCIDITY

• Hydrolytic rancidity is brought about by hydrolysis of triglyceride molecule to glycerol and free fatty acids by the  presence of moisture in oils. The rate of hydrolysis is hastened by-
• The presence of enzymes e.g. lipase present in oils which have not been subjected to heat treatment.
• Microorganisms such as molds, yeasts and bacteria present in oils or contaminants during processing.

OXIDATIVE RANCIDITY or AUTO OXIDATION 

• The spontaneous uptake of oxygen by the unsaturated oils exposed to air is known as oxidative rancidity.
• It is the most common and important type of rancidity which results in the production of rancid or tallow flavours.
• Moisture and impurities do not have any effect on oxidative rancidity
• It is a chain reaction.
• Once it begins, it is continuous process.
• Occurs in two stages.

OXIDATIVE RANCIDITY or AUTO OXIDATION 

  First stage
Ø  Induction period fat and oil takes up oxygen from the air.
Ø  Oxygen from the air requires a free radical to combine with the fat.
Ø  Heat light and traces of metal help to form free radicals.
Ø  Free radical is formed to the carbon (by removal of 1 hydrogen molecule) adjacent to the carbon involved in double bond.
Ø  The free radical combines with oxygen (O2) forming a peroxide.
Ø  The new free radical combines with another hydrogen atom of another fatty acid to form hydro peroxide and a new free radical.
Ø  This new free radical again takes up two oxygen atoms.
Ø  The chain reaction continues till all unsaturated fatty acids are used up or all oxygen gets exhausted.
    Second stage 
Ø  The peroxide and hydro-peroxide formed rapidly break down into aldehydes and alcohols
Ø  Break down contributes to the undesirable flavor and odor in rancid fat.

REVERSION

• Many fats undergo a change in flavour before turning rancid.
• This change in flavor is very different  from rancid flavour and is called reversion.
• In rancidity the change in flavour is same for all fats.
• In reversion the flavor may be buttery, beany, grassy, painty and fishy.
• Reversion is seen in fish oils, linseed and soya bean oil.
• For reversion very small amount of oxygen is required as compared to oxidative rancidity.

Difference between rancidity and reversion

Factors leading to rancidity and reversion

Temperature

High storage temperature accelerates the development of odour and flavours in fats and oils.
 Moisture
Presence of moisture in butter and oils brings about hydrolytic rancidity. Clarified butter or Pure ghee does not turn rancid because the moisture is removed by heat.

 Air 
The amount of air in contact with the fat or oil is an important factor in determining its shelf life. Auto-oxidation occurs in the presence of oxygen and reversion occurs with very less amount of oxygen. Potato chips and salted nut turn rancid at a faster rate due to their large surface area.

Light
Light accelerates both reversion and rancidity.

Metals
The presence of metal in traces accelerates the development of both reversion and rancidity as they are active pro-oxidant. Metal contamination can occur from equipment used for extraction and refining of oil.

Degree of unsaturation
This is an important criterion for oxidative rancidity and reversion.
Oils containing high proportions of unsaturated fatty acids and shortening made from such oils show flavour reversion.

Absence of anti-oxidants
The natural presence of antioxidants or addition to oils prevents rancidity. Antioxidant takes up oxygen and gets oxidized thereby preventing rancidity.

Prevention of rancidity

• Store fat at low temperature in a cool, dark place.
• Use airtight container with minimum headspace.
• Keep away from strong smelly foods
• Use steel and aluminum container for storage. Copper and iron containers accelerate rancidity.
• Avoid undue exposure to light and air.
• Addition of anti-oxidants delay the rancidity
• Natural antioxidants present in oil are vitamin E and lecithin.

Synthetic Antioxidants

    I.          BHT – butylated hydroxyl toluene
   II.          BHA – butylated hydroxyl anisole
   III.         TBHQ – tertiary butyl hydroquinone
   IV.         EDTA – ethylene diamine tetra acetic acid
If fats and oils are stored for longer period of time they should be hydrogenated. It increases their shelf life and prevents rancidity

Effect of heat on fats and oils

During cooking or prolonged heating of fats and oils certain changes are seen:

• There is an increase in the free fatty acid content.
• Smoke point is lowered.
• Iodine number decreases.
• Melting point falls.
• Fat turns darker in colour.
• Fats get polymerized.

*The smoke point of an oil or fat is the temperature at which, under specific and defined conditions, an oil begins to produce a continuous bluish smoke that becomes clearly visible.

*All commonly used fats and particularly those high in polyunsaturated fatty acids tend to form larger molecules (known broadly as polymers) when heated under extreme conditions of temperature and time.

Polymerization

• This takes place because of the intense heat which the fat is subjected to during frying.
• Lipid breakdown takes place and free fatty acids are released.
• Fatty acids undergo further changes and form polymers.
• The polymers increase the viscosity of the hot fat.
• The colour darkens and quality deteriorates.
• Gum may be formed at the edge of the vessel.
• It is of utmost importance to avoid unnecessary heating of fats and oils and controlling frying temperature and time.

Care of Fats and Oils

• Fats and oils are used in many preparations and as a method of cooking food. If care is not taken while heating and storing fats, it may result in wastage of food as well as fat used for preparing it.
• Do not overheat fats, as they decompose at high temperature.
• Follow a time and temperature chart for frying food.
• Cover fats when left in the deep fryer and ensure that the temperature does not exceed 90 – 95 0 Celsius.
• Strain fat after use and used fat should be stored in closed containers in the refrigerator.
• When fat has to be reused for frying, replace with equal quantity of fresh fat.
• Do not use fats with a low smoke point for frying.
• To prevent fat from going rancid, it should be stored in an airtight container away from light.
• Fat should be stored in tall containers to keep minimum surface area exposed.
• Copper or rusted containers should not be used for storing fats.

Extraction of Fats and Oils

There are three methods for extraction of fats and oils from animal or vegetable tissues.

• Rendering
• Pressing
• Solvent Extraction

Rendering

This method is mainly used for extracting animal fat from fatty tissues. The tissue from which fat is to be extracted is carefully removed from the carcass and chopped or minced.
Rendering is of two types: Wet rendering and Dry rendering.

Wet Rendering: It is carried out in the presence of water. The chopped tissue is treated with very hot water or steam. Fat melts and forms  a layer on top, which is skimmed off. Fat obtained by this method has a bland flavor and complete extraction is not obtained. Antioxidants are added to prevent rancidity.

Dry Rendering: The chopped tissues are heated without addition of water. Lipids escape from the cells and melted fat is removed by draining and squeezing the fat out of the residue.

Pressing

In this method, oil is extracted by the application of high pressure to oilseeds or fruits rich in oil. The oil obtained is filtered to remove any unwanted matter. Oil obtained from the first pressing is called virgin oil and is particularly bland in taste.

In hot pressing, the oil bearing tissue is rolled, crushed or ground into flakes, and then heated by steam to 70 degree Celsius. The hot tissues are pressed to extract oil. Along with oil, gum and free fatty acids are also extracted.

Solvent Extraction

The crushed or flaked tissue is mixed along with the solvent to extract oil. This method is used to extract the fat remaining in the seedcake after pressing. The solvent is separated from the mixture by evaporation.

Refining of Oil

The oil extracted by rendering, pressing or solvent extraction is called crude oil. It may contain undesirable constituents such as gums, free fatty acids etc.
Crude oil needs several types of treatment to extend its shelf life and make it suitable and pure for use.

Steps in refining oil are as follows:

1.Settling – The crushed solid part is allowed to settle down and is removed by filtration.
2.Degumming and neutralization – The gum and free fatty acids are removed by steam distillation.
3.Bleaching – This step removes undesirable colouring and flavouring contaminants. Oil is filtered through activated charcoal till it becomes light in colour.
4.Steam deodourisation – Steam is injected into the hot oil under pressure to get rid of unwanted odour and then it is cooled rapidly.

Winterization of Oil

• After steam deodorization oils are chilled rapidly without stirring.
• Large filterable crystals are formed.
• These crystals are made of heavy triglycerides with high melting point.
• Separated by filtration and the cold viscous oil obtained is said to be winterized.
• Winterized oils do not solidify in refrigerator.
• Suitable to be used in food which require refrigeration e.g. salad dressings and mayonnaise which can be poured even when chilled.
• Olive oil is not winterized or deodorized as flavor is lost.

Hydrogenation of oil

• Liquid oils can be converted to solid fats by the process known as hydrogenation.
• In this process there is an addition of hydrogen to unsaturated fat thus converting oils into solid fats.
• Hydrogenation takes place in a reactor where hydrogen gas is bubbled through the liquid in the presence of nickel as a catalyst.  
• In this process some of the double bonds between the carbon atoms of the fatty acids portion of the triglyceride molecule are broken and hydrogen is added. This chemical change makes the fatty acid more saturated. The melting points of the fats are thereby increased.
• Hydrogenation increases the stability of oils and prevents it from spoilage due to oxidation which results in rancidity.
• Hydrogenation is utilized in the manufacture of a wide variety of fats such as vanaspati and margarine.
• Sometimes additives such as antioxidants, Vit. A, D are added to the fat.
• Air maybe whipped in, to impart a snow white color.
• Palm oil, palmolein, rice bran, cotton seed, sunflower, maize, soyabean, groundnut, and sesame oils are generally hydrogenated.

Shortenings

A shortening is defined as a fat, solid at room temperature, which can be used to give foods a crumbly and crisp texture such as pastry.  Examples of fat used as “shorteners” include butter, margarine, vegetable oils and lard.

• Oils and fats are used in a baked product to reduce the development of gluten giving the foods a crumbly texture.  The fats and oils break down the gluten into “shorter strands” hence the term shorteners.  Coating the flour in fat prevents the flour from absorbing water hindering the formation of gluten.  If too much gluten developed, the food would be stretchy and elastic.
• Shortening is used in most doughs and batters, to give the baked product a crisp and crumbly texture.   Rubbing the fat in causes the baked product to have a flaky texture, as the dough is separated into layers.  When fat is whisked with sugar, a process called creaming, the texture will be more like a cake, and be soft and springy.
•   Fat that covers the greatest surface area of the flour particle in a particular baked product is said to have the greatest shortening power

Factors affecting shortening power of fats

• Nature of fat – greatest unsaturation have greatest shortening power.
• Concentration – concentration of fat increases, shortening power  also increases.
• Temperature – fats are less plastic and oils are more viscous at low temp.
• Other ingredients – emulsified fat and oils have less shortening power
• Manipulation of fat – proper creaming and stirring of fat increases the shortening power

Popular fats and oils

• Oils – from different oil seeds are available refined or unrefined as a single type of oil or as a blend of two or more oils.
• Butter – available as salted and unsalted.
• Spreads – emulsions of oil and water. Available in various flavours. They are blends of hydrogenated oils, water, milk solids, flavouring and colouring. They are easy to spread as compared to butter and margarine. They provide less calories as the air and water content is more.
• Vanaspati –prepared by hydrogenation of oil.
• Margarine –  a substitute for butter which is fortified with vitamin A & D. 
• Suet – fat around the kidneys of animals.
• Dripping – obtained while roasting meat and used for shallow frying.
• Olive oil – used for salad dressings.
• Fresh cream – obtained by skimming whole milk. Synthetic cream is also available, which is prepared from vegetable oils, water, sugar, soy proteins and added flavor.

Commercial uses of fats and oils

• Fats and oils are used in the food industry because of their ability to
• Increase tenderness and make the product soft.
• Fry or cook food.
• Crispness of biscuits.
• Puff pastry.
• Soft and tender cakes with high volume.
• Get creamed and form foams.
• Impart flavor, colour and aroma to food.
• Softer bread.

Test your understanding of Lipids