Intermezzo

Photo-oxidation and enzymatic oxidation. (Poly)unsaturated fatty acids may also undergo oxidative degradation in photoreactions (photo-oxidation) and enzymatic processes (enzymatic oxidation). The photo-oxidations that (poly)unsaturated fatty acids can undergo are of two types:

1.) Free-radical chain reactions, which start from the excited state of another molecule:

A = molecule of another food component, e.g., riboflavin (in milk), and A* = excited state of A.

A* may abstract an electron or a hydrogen atom from the substrate, RH: RH + A* ^ R • + AH •

These radicals can undergo further reactions in the presence of oxygen to form hydroperoxides:

2.) Singlet oxygen (:O2) reactions in which the absorption of photons by molecules of another food component is followed by energy transfer to ground-state oxygen, leading to the formation of singlet oxygen:

A = e.g., protoporphyrin (occurring in hemoglobin, myoglobin and most of the cyto-chromes).

Singlet oxygen can attack the double bonds in the unsaturated fatty acids (e.g., linoleic acid), yielding hydroperoxides:

As against autoxidation, tocopherols (e.g., vitamin E) can provide protection against sensitized photo-oxidation by acting as quenchers of singlet oxygen. Photodegradation of foods can be further prevented by using packaging materials that absorb the photochemi-cally active light, and by removing endogenous photosensitizers and oxygen from the food.

Oxidative degradation of fats and oils may also be enzyme-mediated. The oxidation of fats and oils of plant origin may be catalyzed by lipoxygenase. The lipoxygenase-mediated oxidation is a hydroperoxide-initiated free radical chain reaction. Enzymatic oxidation also leads to the formation of hydroperoxides. Lipoxygenases can be inactivated by heat treatment.

In the three routes of oxidative degradation described above — autoxidation, photo-oxidation and enzymatic oxidation — a large variety of products is formed from fats and oils (see Figures 6.3 and 6.4).

Autoxidation

Autoxidation

Photo - oxidation

13 - Hydroperoxyoctadeca - 9, 11 - dienoic acid

9 - Hydroperoxyoctadeca - 10, 12 - dienoic acid

12 - Hydroperoxyoctadeca - 9, 13 - dienoic acid

Photo - oxidation

Enzymatic oxidation

Enzymatic oxidation

10 - Hydroperoxyoctadeca - 8, 12 - dienoic acid

13 - L - Hydroperoxyoctadeca - cis, 9 - trans, 11 -dienoic acid

COOH

9 - D - Hydroperoxyoctadeca - trans, 10 - cis, 12 -dienoic acid

Figure 6.3 Hydroperoxide formation in autoxidation, photo-oxidation and enzymatic oxidation of linoleic acid.

COOH

COOH

COOH

COOH

COOH

COOH

The initially-formed lipid hydroperoxides are unstable. They degrade further in metal-ion catalyzed reactions to compounds such as alkanes (e.g., ethane and pentane) and (unsaturated) aldehydes (e.g., hydroxynoneal), and ketones (e.g., acetone):

Pentane

Unsaturated aldehydes, in particular a,0-unsaturated carbonyl compounds, may undergo toxic conjugations (Michael additions) with biologically essential nucleophiles such as sulfhydryl compounds and DNA bases. Hydroxynonenal is known to form adducts with DNA. Further, while the hydroperoxides are relatively unvolatile, tasteless, and odorless, the products formed in the secondary degradation reactions are volatile, and play a role in the development of off-flavors.

Hydroperoxides can also react with other nutritive components, such as amino acids and carotenoids. For example, methionine is oxidized to the sulfoxide, and lysine degrades to diaminopentane, aspartic acid, glycine, alanine, a-amino adipic acid and many other substances.

In all three of the oxidative degradations, termination of the chain reaction may result in dimerization of oxygen-centered radicals:

The formation of these peroxidic dimers may lead to polymerization reactions. This can be explained by both their instability and the large variety of functional groups they may contain. The dimers readily undergo homolysis, even at low temperatures. Further, homolysis may be followed by several rearrangements.

Another well-known end product of the peroxidation of polyunsaturated fatty acids such as linoleic acid and arachidonic acid is malondialdehyde:

Malondialdehyde is capable of cross-linking to primary amino groups, forming a conjugated Schiff base with the general structure:

where R may be free amino acids, proteins or nucleic acids.

Enzymes may be inactivated as a result of cross-linking, either directly by a reaction or indirectly by alteration of the membrane structure. Furthermore, malondialdehyde has been demonstrated to be carcinogenic in experimental animals and mutagenic in the Ames

Acyclic and cyclic compounds dimers

cyclic peroxides; hydroperoxy compounds

Acyclic and cyclic compounds

dimers

RO"

► keto, hydroxy and epoxy compounds cleavage cleavage

Aldehydes, ketones, hydrocarbons, furans, acids

► keto, hydroxy and epoxy compounds

Aldehydes cleavage

Alkyl radicals

O2 condensation Hydrocarbons O2

Hydrocarbons Alkyltrioxanes terminal shorter aldehydes and dioxolanes ROOH

Hydrocarbons aldehydes alcohol acids epoxides

Semi - aldehydes or oxo - esters

Figure 6.4 Diagrammatic representation of the degradation and polymerization reactions following autoxidation of unsaturated fatty acids.

test. The above data emphasize that peroxidation of food components may have negative effects on its nutritional value, sensoric quality and safety.

6.2.1.2.3 Effects of processing techniques on the oxidation of dietary fats and oils. The effects of various processing techniques used in food manufacture on the oxidation of dietary fats and oils can be largely predicted on the basis of tissue damage, exposure to oxygen, presence of metal ions, time, and temperature range involved.

Processing may already cause problems in the early stages of food manufacture. Rapid freezing of raw plant material may be accompanied by lipoxygenase-mediated oxidation. This phenomenon depends on the extent of tissue damage, and on storage temperature and time. Blanching and storage at low temperature may inhibit peroxidaton, but cannot prevent it.

During dehydration and freeze-drying, food lipids are extensively exposed to air as a thin film, thereby promoting autoxidation. The water content of a foodstuff is critical for the autoxidation rate. Excess water prevents extensive contact of lipids with oxygen. Further, storage temperature and time are important factors in determining the extent of oxidative deterioration. In the case of dehydration, the detection of oxidatively developed off-flavors may be facilitated, while the natural flavors and odors may disappear.

During baking lipids may be spread in thin films over large surfaces. Since baked products are usually consumed fairly soon after production, autoxidation will only occur to a limited extent.

Some types of fermentation are used for the production of substances that are undesirable in other products. Examples are the formation of short-chain acids and carbonyl compounds in cheeses and the high rancidity of a number of traditional Asian fermented fish and soy products.

Minor effects of the above processing techniques on product quality, shelf life, and vitamin content have been reported. Nutritional value and food safety do not appear to be much affected, not even under extreme conditions.

Deep-frying of foods in oils gives more rise to concern. As dealt with before, saturated and unsaturated fatty acids may undergo decomposition upon heating in the presence of oxygen. A diagrammatic summary of the thermolytic and oxidative mechanisms involved is shown in Figure 6.5.

fatty acids,esters and triacylglycerols saturated unsaturated thermolytic reactions

acids, hydrocarbons propanediol esters, acrolein, ketones

long-chain alkanes, aldehydes, ketones and lactones thermolytic reactions

acyclic and cyclic dimers

volatile and dimeric products of autoxidants

Figure 6.5 Diagrammatic representation of thermal and oxidative decompositions of lipids.

Frying of food under normal conditions may result in the formation of small amounts of stable peroxides. During industrial processing under vacuum, dimers, polymers, and cyclic products may be formed. The products that are formed when cooking oils are heated may be taken up by the fried food products. Meat, deep-fried in rapeseed oil, appeared to contain 0.63 to 1.1% of the nonvolatile oxidation products of the oil. French fried potatoes have been shown to contain secondary oxidation products of the cooking oil of high molecular mass. If oil is used in discontinuous batch-type operations, as in restaurants and at home, it is eventually discarded because of either high viscosity or excessive foaming. Discarded oils are found to contain approximately 25% polymers. Stable foams are formed if the polymeric oxidation product content is about 9%.

Generally, under normal frying conditions, oxidation of fats and oils has no harmful consequences. However, it should be noted that inappropriate heating and storage of fats and oils may lead to the formation of harmful substances at toxic levels. From the viewpoint of food safety, the conditions under which moderately rancidifying fats and oils are handled in the consumer's kitchen deserve particular attention.

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