Introduction

It has been difficult to provide a definition for the class of substances called lipids. Early definitions were mainly based on whether the substance is soluble in organic solvents like ether, benzene, or chloroform and is not soluble in water. In addition, definitions usually emphasize the central character of the fatty acids—that is, whether lipids are actual or potential derivatives of fatty acids. Every definition proposed so far has some limitations. For example, monoglycerides of the short-chain fatty acids are undoubtedly lipids, but they would not fit the definition on the basis of solubility because they are more soluble in water than in organic solvents. Instead of trying to find a definition that would include all lipids, it is better to provide a scheme describing the lipids and their components, as Figure 2-1 shows. The basic components of lipids (also called derived lipids) are listed in the central column with the fatty acids occupying the prominent position. The left column lists the lipids known as phospholipids. The right column of the diagram includes the compounds most important from a quantitative standpoint in foods. These are mostly esters of fatty acids and glycerol. Up to 99 percent of the lipids in plant and animal material consist of such esters, known as fats and oils. Fats are solid at room temperature, and oils are liquid.

The fat content of foods can range from very low to very high in both vegetable and animal products, as indicated in Table 2-1. In nonmodified foods, such as meat, milk, cereals, and fish, the lipids are mixtures of many of the compounds listed in Figure 2-1, with triglycerides making up the major portion. The fats and oils used for making fabricated foods, such as margarine and shortening, are almost pure triglyceride mixtures. Fats are sometimes divided into visible and invisible fats. In the United States, about 60 percent of total fat and oil consumed consists of invisible fats—that is, those contained in dairy products (excluding butter), eggs, meat, poultry, fish, fruits, vegetables, and grain products. The visible fats, including lard, butter, margarine, shortening, and cooking oils, account for 40 percent of total fat intake. The interrelationship of most of the lipids is represented in Figure 2-1. A number of minor components, such as hydrocarbons, fat-soluble vitamins, and pigments are not included in this scheme.

Fats and oils may differ considerably in composition, depending on their origin. Both fatty acid and glyceride composition may

HEXOSES

HEXOSES

Figure 2-1 Interrelationship of the Lipids

result in different properties. Fats and oils can be classified broadly as of animal or vegetable origin. Animal fats can be further subdivided into mammal depot fat (lard and tallow) and milk fat (mostly ruminant) and marine oils (fish and whale oil). Vegetable oils and fats can be divided into seed oils (such as soybean, canola), fruit coat fats (palm and olive oils), and kernel oils (coconut and palm kernel).

The scientific name for esters of glycerol and fatty acids is acylglycerols. Triacylglyc-erols, diacylglycerols, and monoacylglycer-ols have three, two, or one fatty acid ester linkages. The common names for these compounds are glycerides, triglycerides, diglyc-erides, and monoglycerides. The scientific and common names are used interchangeably in the literature, and this practice is followed in this book.

Table 2-1 Fat Contents of Some Foods

Product

Fat (%)

Asparagus

0.25

Oats

4.4

Barley

1.9

Rice

1.4

Walnut

58

Coconut

34

Peanut

49

Soybean

17

Sunflower

28

Milk

3.5

Butter

80

Cheese

34

Hamburger

30

Beef cuts

10-30

Chicken

7

Ham

31

Cod

0.4

Haddock

0.1

Herring

12.5

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