Question

how to Predict the properties of a lipid mixture based on its components (saturated vs unsat, long vs short chain, etc.), and how to explain the structural reasons for the properties ? please give examples

Answer 1

Lipids are one of the major constituents of foods. They are important in our diet for a number of reasons. They are a major source of energy and provide essential lipid nutrients. Nevertheless, over-consumption of certain lipid components can be detrimental to our health, e.g. cholesterol and saturated fats. In many foods the lipid component plays a major role in determining the overall physical characteristics, such as flavor, texture, mouthfeel and appearance. For this reason, it is difficult to develop low-fat alternatives of many foods, because once the fat is removed some of the most important physical characteristics are lost. Finally, many fats are prone to lipid oxidation, which leads to the formation of off-flavors and potentially harmful products. Some of the most important properties of concern to the food analyst are:

• Total lipid concentration
• Type of lipids present
• Physicochemical properties of lipids, e.g., crystallization, melting point, smoke point, density and color
• Structural organization of lipids within a food

Properties of Lipids

Lipids are usually defined as those components that are soluble in organic solvents (such as ether, hexane or chloroform), but are insoluble in water. This group of substances includes triacylglycercols,diacylglycercols, monoacylglycercols, free fatty acids, phospholipids, sterols, caretonoids and vitamins A and D. The lipid fraction of a fatty food therefore contains a complex mixture of different types of molecule. Even so, triacylglycercols are the major component of most foods, typically making up more than 95 to 99% of the total lipids present. Triacylglycerols are esters of three fatty acids and a glycerol molecule. The fatty acids normally found in foods vary in chain length, degree of unsaturation and position on the glycerol molecule. Consequently, the triacylglycerol fraction itself consists of a complex mixture of different types of molecules. Each type of fat has a different profile of lipids present which determines the precise nature of its nutritional and physiochemical properties. The terms fat, oil and lipid are often used interchangeably by food scientists. Although sometimes the term fat is used to describe those lipids that are solid at the specified temperature, whereas the term oil is used to describe those lipids that are liquid at the specified temperature.

The fat serves as efficient sources of energy when stored in an adipose tissue. It serves as an insulating material in the subcutaneous tissues and around certain organs. It provided building blocks for different high molecular weight substance, e.g., acetic acid and can be used for the synthesis of cholesterol and certain hormones. They produce metabolites through oxidation in the tissues which are used in the introversion of substances.

Chemical properties

Saponification Number

The saponification number is a measure of the average molecular weight of the triacylglycerols in a sample. Saponification is the process of breaking down a neutral fat into glycerol and fatty acids by treatment with alkali:

Triacylglycerol + 3 KOH Glycerol + 3 Fatty acid salts of potassium

The saponification number is defined as the mg of KOH required to saponify one gram of fat. The lipid is first extracted and then dissolved in an ethanol solution which contains a known excess of KOH. This solution is then heated so that the reaction goes to completion. The unreacted KOH is then determined by adding an indicator and titrating the sample with HCl. The saponification number is then calculated from a knowledge of the weight of sample and the amount of KOH which reacted. The smaller the saponification number the larger the average molecular weight of the triacylglycerols present.

Acid value

The acid value is a measure of the amount of free acids present in a given amount of fat. The lipids are extracted from the food sample and then dissolved in an ethanol solution containing an indicator. This solution is then titrated with alkali (KOH) until a pinkish color appears. The acid value is defined as the mg of KOH necessary to neutralize the fatty acids present in 1g of lipid. The acid value may be overestimated if other acid components are present in the system, e.g. amino acids or acid phosphates. The acid value is often a good measure of the break down of the triacylglycrols into free fatty acids, which has an adverse effect on the quality of many lipids.

Classification of fatty acid:

What Is Saturated Fat?

Saturated fats have no double bonds in their chemical structure. They are “saturated” with hydrogen atoms. Because of their chemical structure, they have a solid consistency at room temperature.

Examples ofSaturated fatty acid:

i. Acetic acid – CH₃COOH

ii. Propionic acid – C₂H₅COOH

iii. Butyric acid – C₃H₇COOH

iv. Caproic acid -C₃H₇COOH

v. Caprylic acid – C₇H₁₅COOH

vi. Laurie acid -C₁₁H₂₃COOH

vii. Myristic acid – C₁₃H₂₇COOH

viii. Palmitic acid – C₁₅H₃₁COOH

ix. Stearic acid – C₁₇H₃₅COOH

x. Arachidic acid – C₁₉9H₃₉C00H

xi. Behenic acid – C₂₁H₄₃COOH

Saturated fats can be found in a variety of foods, including:

• Animal meat including beef, poultry, pork
• Certain plant oils such as palm kernel or coconut oil
• Dairy products including cheese, butter, and milk
• Processed meats including bologna, sausages, hot dogs, and bacon
• Pre-packaged snacks including crackers, chips, cookies, and pastries

Why Limit Saturated Fats in Your Diet

The AHA recommends that less than 5 percent to 6 percent of your daily caloric intake consist of saturated fat.

Although the amount of LDL appears to be increased by consuming saturated fats, studies have shown that the type of LDL that is increased is actually the large, buoyant LDL. Larger LDL particles do not appear to increase your risk of heart disease. In contrast, small, dense LDL—the type that has been shown to promote the formation of atherosclerosis in studies—appears not to be affected. In a few cases, the risk was even reduced with saturated fat consumption.

Some studies also suggest that the type of saturated fat-containing foods can make a difference in your heart health. One large study suggested that consuming dairy products may actually lower your risk of cardiovascular disease. At the same time, including processed meats in your diet could increase your risk of cardiovascular disease.

What Is Unsaturated Fat?

Unsaturated fats are typically liquid at room temperature. They differ from saturated fats in that their chemical structure contains one or more double bonds. They can be further categorized as:

Examples of Unsaturated fatty acid:

i. Linoleic acid – C₁₇H₃₁COOH

ii. Linolenic acid – C₁₇H₂₉COOH

iii. Oleic acid – C₁₇H₃₃COOH

iv. Cervonic acid – C₂₁H₃₁COOH

• Monounsaturated fats: This type of unsaturated fat contains only one double bond in its structure. Monounsaturated fats are typically liquid at room temperature and include canola oil and olive oil.
• Polyunsaturated fats: This type of unsaturated fat contains two or more double bonds in their structure. They are liquid at room temperature. Polyunsaturated fats include safflower oil, sunflower oil, and corn oil.

Incorporating Unsaturated Fats in Your Diet

The AHA recommends that most of your daily fat intake should come from monounsaturated and polyunsaturated fats. Foods containing unsaturated fats include:

• Nuts
• Plant oils such as canola, vegetable, or plant oil
• Certain fish like salmon, tuna, and anchovy, which contain omega-3 unsaturated fatty acids
• Olives
• Avocados

  Chemical Tests for lipid:

  1. Salkowski Test:

  Sample dissolve in chloroform and add equal volume of concentrated H₂SO₄. To produce bluish-red to cherry to cherry-red colour.

  2. Liebermann- Burchard Test:

  Sample is dissolve in chloroform in a dry test tube. Add few drop of acetic anhydride and few drop of concentrated H₂SO₄. The solution becomes red, then blue and finally bluish- green in colour.