Radical reactions in foods are of importance, especially in relation to autoxidation of fats. Such reactions lead to quality loss, first of all because of the formation of undesired flavor compounds (oxidative rancidity), and second because of possible formation of hazardous compounds. Radical reactions are also of importance in the body because reactive oxygen species that are formed in all kinds of biochemical reactions may cause damage to DNA, proteins, and cell membranes. Antioxidants in foods could be considered as health-promoting compounds, because they may slow down radical reactions in the body if and when they are absorbed in the body. However, there is currently much debate in the literature whether or not antioxidants in foods can be considered as health promoting. Antioxidants are added also to foods to prevent autoxidation. In any case, radical reactions are of importance and so we spend some attention to the kinetics of radical reactions. However, we limit ourselves to the basics because radical reactions are very complex and it would take too much space to discuss all intricacies. Readers who are interested in more details are referred to some selected references suggested at the end of this chapter.
A radical is an atom or a group of atoms possessing one or more unpaired electrons, sometimes also called a free radical (the word free seems to be superfluous). A radical is usually indicated with a dot, e.g., Hor OH. Radicals can be formed from so-called homolytic scissions of covalent bonds:
A! B þC (4:125)
as opposed to heterolytic scissions that result in ions:
A! Bþþ C (4:126)
pH
log k
−8
−7
−6
−5
−4
−3
−2
−1 0
0 5 10
Specific acid-catalysis:
slope = −1
Specific base-catalysis:
slope = 1
FIGURE 4.26 pH–rate profile of the demethylation of aspartame as a function of pH. Specific acid catalysis of demethylation of aspartame leading toL-a-aspartyl-L-phenylalanine (^), specific base catalysis of formation of
L-a-aspartyl-L-phenylalanine (&), formation of 3-carboxymethyl-6-benzyl-2,5-diketopiperazine (~). Dataset in Appendix 4.1, Table A.4.10.
Homolytic scissions are more likely to occur in the absence of solvent. Radicals are unstable and they recombine with each other or attack other groups.
In general, the kinetics of radical reactions are quite intricate, and it is not well possible to give a general scheme. In most cases, derivations are made based on steady-state assumptions. This could result in a scheme such as
Initiation: A!ki B þC (4:127)
Propagation: A þ B !kp Bþ D (4:128)
D !k0p B þE (4:129)
Termination: B þB !kt B B (4:130)
C þC !k0t C C (4:131)
Overall reaction: A!k Bþ E þ B B þ C C (4:132) Suppose we are interested in the kinetics of formation of compound B:
d[B]
dt ¼ kp[A] [B] (4:133)
It is necessary tofind an alternative equation for this because we cannot easily determine the concen-trations of radicals. If we apply the steady-state approximation, which implies that we assume that d[radicals]=dt ¼ 0:
d[B]
dt ¼ ki[A] kp[A] [B] k0p[D] 2kt[B]2 0 (4:134) d[D]
dt ¼ kp[B] [A] k0p[D] 0 (4:135)
It then follows that k0p[D] ¼ kp[B] [A] and substituting this in Equation 4.134 results in:
ki[A] kp[A] [B] þ kp[B] [A] 2kt[B]2¼ ki[A] 2kt[B]2¼ 0 (4:136) Hence:
[B] ¼ ki
2kt
1=2
[A]1=2 (4:137)
We now have an expression for [B] which we can substitute in Equation 4.133:
d[B]
dt ¼ kp
ki
2kt
1=2
[A]3=2¼ k[A]3=2 (4:138)
A kinetic equation having a fractional order is a typical result for radical reactions. Instead of using the steady-state approximation, one can also resort to numerical solutions of the differential equations that describe each step in the proposed reaction mechanism.
Lipid peroxidation. For foods, lipid peroxidation is most important; it is an oxidative degradation of lipids such as unsaturated fatty acids, sterols, carotenoids, phospholipids, etc. It is called autoxidation because the reaction takes place with molecular oxidation via a self-catalytic mechanism. The classical scheme for lipid peroxidation is divided in three phases, namely initiation, propagation, and termination. A special role is played by oxygen. Normal oxygen is in the so-called triplet state (3O2), with two unpaired electrons in the 2pp orbital and this is not a very reactive oxygen species. However, oxygen can also exist in the singlet state (1O2), which has two paired electrons in the 2pp orbital at one atom and none at the other atom, and this is a very reactive, electrophilic reagent. Singlet oxygen can be formed in various reactions, such as electromagnetic radiation, photodecomposition (see Section 4.6), or irradiation with g-rays, enzymatic reactions involving such enzymes as lipoxygenase, peroxidase, xanthine-oxidase, and due to catalytically acting metals (especially iron and copper). If RH represents a lipid, and X a radical, this results in the following scheme:
Initiation: 3O2 !1O2 (formation of singlet oxygen) (4:139) RHþ1O2! ROOH (formation of hydroperoxides) (4:140) ROOH! ROO þH (formation of peroxyl radicals) (4:141)
Propagation: ROO þRH ! ROOH þ R (4:142)
R þ3O2! ROO (4:143)
Overall reaction: RH þ3O2! ROOH (4:144)
Termination: 2ROO ! ROOR þ O2 (formation of peroxides) (4:145)
R þROO ! ROOR (formation of peroxides) (4:146)
R þR ! RR (formation of peroxides) (4:147)
The products formed in the termination step are nonradicals. Once radicals are formed, also triplet oxygen may react in the propagation step (Equation 4.143). The hydroperoxides formed (ROOH) are very unstable and decompose into aldehydes and ketones, which are the end products and the cause of rancidflavor. Figure 4.27 shows a general, schematic profile for the course of lipid oxidation.
There are many empirical kinetic expressions available in literature describing the course of oxidation as a function of oxygen concentration, the type of lipids (the amount of unsaturated bonds and their location are particularly important), presence of metals, temperature, and notably water activity.
Oxygen consumption
Peroxides formation
End products formation
Time
Concentration
FIGURE 4.27 Schematic picture showing the course of lipid autoxidation.
Lipid oxidation is much faster at low water activity. The empirical expressions differ in the assumptions made to come to a kinetic expression. It is not well possible to list all derived expressions here. The interested reader is referred to references cited at the end of this chapter.
Antioxidants. Antioxidants are very important in foods as they can protect fat autoxidation to some extent. Antioxidants are present naturally in foods, such as tocopherols, vitamin C and vitamin E, polyphenols, carotenoids; also synthetic antioxidants are sometimes added, such as butylated hydroxytoluene (BHT). Primary antioxidants interfere directly in the radical reactions as radical scavengers, secondary antioxidants sequester trace metals or quench singlet oxygen, so that the initiation phase is inhibited. Antioxidants are obviously reducing agents, i.e., they donate electrons, and they act in combination with an oxidizing agent. Denoting an antioxidant as AH, a simple scheme for a primary antioxidant would be:
R þAH ! RH þ A (4:148)
A þR ! AR (4:149)
The radical A that is formed out of the antioxidant is considered less unstable because the unpaired electron is stabilized because of electron resonance in the resulting molecule structure. Figure 4.28 shows the effect of an antioxidant (a-tocopherol) on b-carotene oxidation. Once the antioxidant is consumed, autoxidation can still proceed, which is the reason that the stability of carotene shown in Figure 4.28 depends on the concentration ofa-tocopherol. To indicate the complexity of the reactions taking place, 17 differential equations formed the basis for a kinetic model that was able tofit the data shown in Figure 4.28 (the model is not shown here). In fact, this type of kinetic analyses is well suited for the multi-response technique (Chapter 8).
0.2
0 0.4 0.6 0.8
0 20 40 60 80 100
Time (h) [β-carotene] (mol L–1)
FIGURE 4.28 Oxidation of b-carotene at an oxygen composition of 40 mol% at 608C, without a-tocopherol (~), with 3.8mmol L1(&) and 7.5mmol L1a-tocopherol (
). Dataset in Appendix 4.1, Table A.4.11.Besides in lipid oxidation, radical reactions occur also in the Maillard reaction, during high tempera-ture treatments such as roasting, and during photochemical reactions. Also reactions associated with the heme group in the blood protein hemoglobin are radical reactions, of importance for meat products.