Oxidation in Foods and Beverages and Antioxidant Applications (eBook)

Understanding Mechanisms of Oxidation and Antioxidant Activity

Eric A Decker, Ryan J Elias, D. Julian McClements (Herausgeber)

eBook Download: PDF | EPUB

2010 | 1. Auflage
432 Seiten
Elsevier Science (Verlag)
978-0-85709-044-7 (ISBN)

Infos

Oxidative rancidity is a major cause of food quality deterioration, leading to the formation of undesirable off-flavours as well as unhealthful compounds. Antioxidants are widely employed to inhibit oxidation, and with current consumer concerns about synthetic additives and natural antioxidants are of much interest. The two volumes of Oxidation in foods and beverages and antioxidant applications review food quality deterioration due to oxidation and methods for its control.The first volume focuses on oxidation mechanisms and antioxidant activity. Initial chapters in part one describe oxidation processes in foods, including the role of metals, heme proteins and lipoxygenase. The impact of oxidation on food flavour and the health aspects of oxidized fats are also covered. Final chapters in part one review the measurement of the extent of lipid oxidation and methods for food shelf-life determination. Part two discusses the ways in which antioxidants inhibit food oxidation, factors affecting antioxidant efficacy, methods to measure antioxidant activity and novel antioxidants.With its distinguished international team of editors and contributors, the two volumes of Oxidation in foods and beverages and antioxidant applications is standard references for R&D and QA professionals in the food industry, as well as academic researchers interested in food quality. - Describes oxidation processes in foods, including the role of metals, heme proteins and lipoxygenase - Reviews the impact of oxidation on food flavour and the health aspects of oxidized fats - Discusses the ways in which antioxidants inhibit food oxidation, factors affecting antioxidant efficacy and methods to measure antioxidant activity

Textauszug

1

Understanding oxidation processes in foods

L.H. Skibsted,     University of Copenhagen,Denmark

Abstract:

Oxidative deterioration in foods involves oxidation in both the aqueous phase (e.g., proteins) and the lipid phase (e.g., polyunsaturated lipids). Formation of free radicals is an early event that occurs prior to the progression of oxidation and is most often associated with the aqueous phase. Linear free energy relationships are found valuable for classification of such early events as electron transfer and hydrogen atom transfer. Inspiration for protection of processed foods against oxidative deterioration of their vulnerable constituents has been found in the antioxidant mechanisms appearing during evolution of aerobic life forms in an increasingly oxidizing atmosphere. A two-dimensional classification of antioxidants opens up for an understanding of the special role of carotenoids, and optimal protection seems to depend on a proper balance between antioxidants and antireductants.

Key words

food protection

protein oxidation

lipid oxidation

free radical kinetics

antioxidants

antireductants

1.1 Introduction

Other forms of life than the aerobic forms dominating now were previously characteristic of our planet. When, almost three billion years ago, blue-green algae (Cyanobacteria) developed photosynthesis, oxygen accumulated in terrestrial atmosphere concomitant with oxygenation of the shallower surface ocean (Frei et al., 2009). The oxygenation of the Earth’s atmosphere seems to have fluctuated and lagged behind the development of oxygenic photosynthetic organisms in the upper ocean because bacteria became deprived of nitrogen available for their growth and accordingly production of oxygen (Godfrey and Falkowski, 2009). Oxygen is a potent oxidant and changed the conditions for life dramatically initially in the oceans and in two spikes separated by a lower level period also in the atmosphere. The evolution of oxygenic photosynthesis caused the greatest selective pressure on primordial life (Benzie, 2003), since life dependent on one-electron Fe(III)/Fe(II) cycling and oxygen invariably will create reactive oxygen species (ROS), which need to be controlled by scavenging or through further reaction. During early evolution endogenous protection systems had accordingly to be developed for protection against oxidative stress together with development of chelators for tuning of Fe(III)/Fe(II) reduction potentials in electron transport systems and for prevention of precipitation of Fe(III) as hydroxide in the increasingly oxidizing atmosphere. It is the paradox of aerobic life, that oxidative damage occurs to key metabolic sites, and this continuing threat prompted the development of customized antioxidants concomitant with the appearance of various aerobic life forms.

1.2 Reactive oxygen and nitrogen species

Due to spin restrictions, atmospheric oxygen is rather unreactive, since ground state oxygen is a biradical and as such a triplet, whereas organic compounds are singlets. However, in a series of one-electron reductions as part of respiration, the reactive forms of oxygen such as superoxide, hydrogen peroxide, and the hydroxyl radical, have transient appearance during formation of water:

1.1

Especially the hydroxyl radical, OH, is highly reactive with rates of reaction with most organic compounds like lipids approaching the diffusion limit. Such high reactivity is evident, for example, for wine and other alcoholic beverages in which any hydroxyl radicals formed during maturation or oxidative deterioration are converted to the l-hydroxyethyl radical by reaction with ethanol prior to any further reaction (Elias et al., 2009). The spin restriction for the initial electron transfer to yield the superoxide radical as in the reaction sequence of Eq. 1.1, is revoked by reaction of oxygen with transition metal ions either in enzymes like lipoxygenases (iron-based) or in simple hydrated ions under some conditions:

1.2

Likewise, the superoxide radical may leak from the mitochondria during respiration. The spin restriction, i.e. singlet/triplet reactions are forbidden, is also circumvented through reaction of organic material with singlet oxygen, 1O2, which is electronically excited oxygen with spin pairing or with the allotropic form, ozone, O3. Singlet oxygen is formed in some highly exergonic reactions but more importantv in photosensitized reactions like:

1.3

1.4

1.5

in which riboflavin, vitamin B2, as present in tissue and many foods, absorbs light and through efficient intersystem crossing (ISC) yields the longer lived triplet-riboflavin subsequently reacting with ground state oxygen (or acting as an oxidant itself). Ozone is formed during electric discharge or by UV-light absorption by oxygen in the atmosphere:

1.6

Formation of reactive nitrogen species (RNS) is likewise linked to aerobic life manifestations in close interplay with formation and decay of ROS and with Fe(III)/Fe(II) redox cycling (Carlsen et al., 2005). Although less studied than ROS, RNS such as peroxynitrite, ONOO−, formed by reaction of the superoxide ion with nitric oxide, an important signalling molecule and also formed in nitrite-cured meat:

1.7

is involved in oxidative protein modifications like nitration of tyrosine residues (Ischiropoulus and Almehdi, 1995). Carbon dioxide present in the atmosphere throughout evolution may also be involved in oxidative stress through formation of free radicals (Goldstein and Czapski, 1998):

1.8

Transformations among ROS and RNS are often assisted by enzymes such as oxidoreductases depending on transition metal mediated electron transfer in the catalytic site. Myoglobin, traditionally considered solely as an oxygen storage protein, seems to have far wider functions as a chemical reactor for small molecules like O2 and NO in relation to the balance between aerobic and anaerobic metabolism (Frauenfelder et al., 2001). In Fig. 1.1 is shown as an example, the oxidation of nitrosylmyoglobin, which is also highly relevant for oxidative deterioration of cured meat. Both O2− and NO are radicals (doublets), but in animal tissue (muscle/meat) they act differently, when they are released from iron(II) heme pigments like myoglobin, since O2− is a prooxidant through conversion to H2O2 or OH, while NO is an antioxidant through scavenging of other radicals (Kanner et al., 1991):

Fig. 1.1 Heme pigments are often involved in interaction between reactive oxygen species and reactive nitrogen species as shown for oxidation of NO by oxygen in nitrosylmyoglobin with the heme cavity acting as a chemical reactor as suggested by Frauenfelder et al. (2001) to yield peroxynitrite/nitrate and metmyoglobin. Absorption of visible light opens up for a parallel reaction pathway initiated by photodissociation of nitric oxide.

1.9

1.10

HNO, nitrosyl, also involved in regulation of oxidative stress, is hampered in proton dissociation, since NO−, isoelectronic with O2, is in a triplet state. NO−, which otherwise would form the strong oxidant ONOO− by reaction with O2, cf. Eq. 1.7, is not produced under physiological conditions due to the spin restriction for the proton dissociation. HNO in contrast reacts as an oxidant rather specifically with thiols and may be involved in thiol depletion and accordingly protein modification (Paolocci et al., 2007). HNO is a unique molecule, as it may donate a hydrogen atom to a lipid peroxyl radical (LOO):

1.11

in a process typical for chain-breaking antioxidants, as discussed in Section 1.4.

NO formed, may further react as a radical scavenger and chain-breaker in lipid autoxidation:

1.12

The role of the unusual acid/base couple HNO/NO− in oxidative processes should be further explored.

1.3 Evolution of antioxidants

Various foods are known to have strongly varying oxidative stability and since food resistance to oxidative deterioration is linked to the antioxidants present in the actual food raw-material, it is of relevance to try to follow the evolution of antioxidants as challenged by the changing conditions for aerobic life. Compounds now considered as valuable antioxidants may have appeared through evolution with other functions or with other functions in addition to be protectors against oxidative stress. In Table 1.1 an attempt is made to classify antioxidative protection into eight groups also in relation to contemporary food technology.

Table 1.1

Evolution of protection against oxidative stress for aerobic life as seen as inspiration for food technology

1.3.1 Physical barriers

Organisms may obtain protection against...

Produkt-Details

• Erscheint lt. Verlag: 27.9.2010
• Sprache: englisch
• Themenwelt: Technik ► Lebensmitteltechnologie
• ISBN-10: 0-85709-044-5 / 0857090445
• ISBN-13: 978-0-85709-044-7 / 9780857090447