MODELO DE RENTABILIDAD ECONÓMICA

Another form of spoilage occurs when food begins to break down by natural processes of decay. This process takes place when enzymes naturally present in foods interact with (usually) oxygen and/or water, breaking down the food’s carbohydrates, lipids, proteins, and other biochemical compounds into their component parts. This type of spoilage has a number of manifestations.

One such change is rancidity. When a fat or oil decomposes into its fundamental components, fatty acids and glycerol, it is said to have become rancid. Fats and oils, members of the chemical fam-ily of lipids, are esters of the trihydric alcohol known as glycerol (C₃H₅(OH)₃) and long saturated and/or unsaturated fatty acids that have the general formula CH₃(CH₂)_nCOOH, where n is of the order of 10, 12, 14, 16, or 18. The equation below shows the general changes that take place when an enzyme oxidizes or hydrolyzes (reacts with water) a lipid to form glycerol and fatty acids.

The fatty acids produced by such reactions typically have unpleas-ant odors that make some types of food inedible. For example, when butter begins to spoil it develops an “off” smell caused by the pres-ence of butyric and other organic acids. These organic acids form when the fatty acid esters that make up butter begin to hydrolyze and oxidize.

The oxidative and hydrolytic reactions by which fats and oils are converted to glycerol and fatty acids are complex, and are catalyzed by inorganic catalysts (metal ions), enzymes (lipoxidases) that occur naturally in food, and forms of energy, such as sunlight. In all cases,

Oxidation or hydrolysis of a lipid

the catalysts cause lipid free radicals to form at the double bonds in a lipid molecule. A lipid free radical is a portion of a lipid mol-ecule containing a single, unpaired electron. These highly reactive structures react readily with free oxygen molecules, forming other free radicals known as lipid peroxy radicals. Lipid peroxy radicals then react with other lipid molecules to form lipid hydroperoxides (the primary products of oxidative rancidity) and additional lipid free radicals. The process is, thus, self-perpetuating once it has been initiated. The sequence of events in this auto-oxidative process is outlined above.

To prevent the decomposition of fats and oils, then, chemists must fi nd substances that (1) react with lipid free radicals and/or (2) react with oxygen. Either reaction will arrest the auto-oxidative process just described. A rather large number of substances have been found that meet either or both of these criteria. The chemical structures of some of the most common of these substances are shown in the diagram on page 31. In the past, probably the most widely used of these free radical scavengers were BHA and BHT, used best in com-bination. More recently the use of naturally occurring substances,

Auto-oxidation of lipids

such as ascorbic acid (vitamin C) or one of the tocopherols (vita-min E), has gained favor wherever it is possible.

Another natural change that unfavorably affects the appearance, odor, and fl avor of food is browning. This term refers to the process that occurs when the surface of fruits, vegetables, and shellfi sh has been cut or bruised. Most people have observed this reaction in their

Chemical structures of some common free radical scavengers

own kitchen. An apple, potato, or banana that has been sliced gradu-ally begins to turn brown and, within half an hour, no longer looks good enough to eat. The two known forms of browning are enzy-matic and nonenzyenzy-matic (or Maillard) browning.

Enzymatic browning occurs when enzymes that occur naturally in plant materials react with phenolic compounds in the fruit or veg-etable. The enzymes most commonly involved are the polyphenol oxidases, or PPOs. PPOs are copper-containing proteins that react readily with phenol-based constituents of plant foods such as tyro-sine and catechol. They catalyze two reactions: The fi rst converts the phenol to a diphenol (as shown below) and the second converts the diphenol to a quinone. The quinones thus formed then begin to polymerize, forming a brownish pigment responsible for the discol-oration of bruised or cut fruits and vegetables.

One of the most effective additives for the prevention of brown-ing is sulfi te. Sulfi te reacts with quinone to prevent polymerization,

Enzymatic browning

and hence, it prevents a brown color from developing. Other addi-tives have been found to effectively inhibit the polymerization of quinone, among them ascorbic acid, citric acid, 4-hexylresorcinol (4HR), and ethylenediamine tetraacetic acid (EDTA).

Nonenzymatic browning occurs when sugars and proteins in foods begin to react with each other. The process is quite complex, involving about two dozen steps (the Maillard reaction). It is initiated when the carbonyl group (>C=O) of a sugar reacts with the amino group of a protein or amino acid, splitting out water and forming a compound known as a N-glycosylamine. The process ends with the formation of another brownish-colored polymer, melanoidin. In the early stages of the reaction, the chemical products give the food a light brown color and a sweet smell, characteristic of the carameliza-tion process. As the reaccarameliza-tion proceeds, the color becomes darker and the odors less pleasant.

Nonenzymatic browning occurs ubiquitously, in both raw and cooked foods. The reactions by which browning develops are strongly affected by heat, and many common cooking and baking techniques encourage their development. For example, the odors and colors that develop when sugar is caramelized, when bread is baked, and when fruits and vegetables are stored for long periods are all caused by Maillard reactions.

Sulfi tes are very effective in reducing nonenzymatic browning.

They combine with and inactivate sugars that would otherwise re-act with amino groups, halting the Maillard rere-action at its outset.

As a secondary effect, they act as bleaching agents; they remove the brown coloring that might otherwise be developed as a result of Maillard reactions. Today sulfi tes are essentially the only additives being used to deter nonenzymatic browning.

Food preservation has long been a part of human culture.

Without methods of preservation, people would be limited to eating only fresh foods at the times of the year when they are available.

More important, people would be subject to a number of food-borne diseases that develop in spoiled foods. Food chemists have devel-oped a host of chemicals that can be used in the preservation of foods. These preservatives extend the variety of foods available and provide protection against disease caused by spoiled foods.

Critics sometimes object to the use of so many chemicals in food preservation, claiming that such chemicals may cause allergic reac-tions, cancer, or other health conditions. The goal of a responsible program of food preservation is to conserve the values offered by preservatives without introducing new risks to human health from the preservatives themselves.