When n-3 LC PUFA are incorporated into food systems the oxidation mechanisms and thereby the oxidation rates may change dramatically. This is due to the fact the oxidisability depends on the physical structure and form of the lipid. Moreover, food systems contain a wide range of different ingredients that may influence lipid oxidation, as will be discussed in the following.
Metals can catalyse oxidation by two different mechanisms:
1 by electron transfer:
M(n+1)+ + RH-~Mn+ + R’ + H+ 15.1
2 by catalysing the decomposition of hydroperoxides:
Mn + ROOH-~Mn+1 + RO’ + OH- 15.2
Mn+1 + ROOH-~Mn + ROO’ + H+ 15.3
Fe and Cu are the most active metals, but Mn, Zn, Co and Ni can also act as pro-oxidants. Ferrous iron (Fe2+) is more effective in decomposing peroxides than the ferric ion (Fe3+) and both ferrous and ferric iron are more effective than copper.
Trace metals are present in most foods. Even after refining and deodorisation most oils will contain trace levels of lipid hydroperoxides. Therefore, metal-catalysed decomposition of lipid hydroperoxides is probably the reaction responsible for the initiation of lipid oxidation in most foods. The reactions in equations 15.2 and 15.3 not only generate free radicals, which may initiate further oxidation reactions, but will also give rise to the formation of secondary volatile oxidation compounds as previously described. In fish oil-enriched mayonnaise, the iron present in the egg yolk, which is used as an emulsifier, was suggested to be the most important catalyst of oxidation. The mechanism by which iron promotes lipid oxidation in mayonnaise was suggested to be as follows: the low pH in mayonnaise (pH < 4.2) is responsible for releasing small amounts of iron ions from the oil±water interface where iron is bound to the egg yolk protein phosvitin. Subsequently, the released iron promotes the decomposition of pre-existing lipid hydroperoxides located at the oil±water interface and perhaps also in the aqueous phase. The effect of pH on iron release will be discussed further below. Recent results have indicated that metals from certain milk proteins are also important oxidation catalysts in fish oil-enriched milk.
Oxygen is required for oxidation to occur. Therefore, lipid oxidation may be reduced by reflushing the food product with nitrogen as observed in fish oil-enriched mayonnaise and by packaging in an air-tight container. The total amount of oxygen in the headspace above the product and the oxygen dissolved in the product will limit the extent of oxidation. However, usually oxidation can proceed for a relatively long time, because plenty of oxygen will be available even in a closed container, unless oxygen has been completely removed. Importantly, the oxygen is not required for the decomposition of lipid hydroperoxides. Therefore, off-flavour products may be formed even after all oxygen is consumed.
The large interfacial area in emulsions increases the potential contact area between the oil droplet and trace metals in the continuous, aqueous phase. The interfacial area is governed by the size of the droplets in emulsions. In the literature, contradicting reports are available on the effect of the droplet size on oxidation. In fish oil-enriched mayonnaise, lipid oxidation was faster in mayonnaises with small droplet sizes in the initial part of the storage period, whereas no effect of droplet size was observed in the later part of the storage period. The following mechanism was suggested to explain these findings: in the initial phase of the oxidation period a small droplet size, i.e. a large interfacial area, would increase the contact area between iron located in the aqueous phase and lipid hydroperoxides located at the interface and this would increase oxidation. In the later stage, oxidation proceeds inside the oil droplet and therefore the droplet size is less important. Further studies are required to elucidate this matter.
Lipid oxidation is significantly affected by the water activity in foods, especially in powders. Water may act as a solvent for metal ions, and metal salt hydrates may be formed. These hydrates are less lipid soluble and less active than the metal ions themselves. Lipid oxidation may decrease, owing to the formation of hydrogen bonds between water and lipid hydroperoxides, which prevent their decomposition into initiating free radicals Water may facilitate thenbreakdown of alkoxyl radicals formed by the decomposition of hydroperoxides. Moreover, a decrease in water activity will also decrease the so-called glass transition temperature, which is the temperature at which the food matrix changes from the glassy state to the rubbery state. It has been suggested that lipids are more susceptible to oxidation in the rubbery state, because they can react more readily with oxygen in this state. In the glassy state, the lipids are encapsulated because there is less free volume that is not taken by the macromolecules and therefore diffusion of oxygen is also limited. Thus, bringing the powder into the glassy state by optimising the recipe or by decreasing the storage temperature/water activity may reduce oxidation. It is important to take these phenomena into consideration in relation to the incorporation of fish oil into powders such as infant formula.
Temperature affects oxidation rates in an exponential manner. The mechanism of oxidation changes with temperature, especially above 60ëC, and the lipid hydroperoxides from different fatty acids decompose into secondary volatile oxidation products at different temperatures. Therefore, it is difficult to mathematically predict the effect of temperature on shelf-life and sensory properties of foods. Nevertheless, Presa-Owens et a1. attempted to predict the shelf-life of fish oil-enriched infant formula using an accelerated stability test (Rancimat). They reported that shelf-life predicted by long-term studies at 25 ëC and 60 ëC based on peroxide values and sensory evaluation were in accordance with the shelf-life predicted by repeated Rancimat measurements at temperatures ranging from 60 to 130ëC.
Mei et a1. showed that the effect of NaCl on oxidation depended on the charge of the emulsifier, the concentration of NaCl and the concentration of ferrous in corn and salmon oil emulsions. In traditional mayonnaise, salt was shown to increase anisidine values and the effect of salt was shown to depend on the salt concentration and salt type. In fish oil-enriched mayonnaise, NaCl did not, however, promote free radical formation, which indicated that NaCl was not a pro-oxidant. Taken together, these data suggest that the effect of NaCl on lipid oxidation should be investigated in each individual food system.
Metal ions are generally more soluble at low pH than at high pH. This may explain why lipid oxidation generally is slowest at high pH values. Furthermore, pH influences the emulsifier charge and this may significantly affect oxidation as will be discussed later. In fish oil-enriched mayonnaise, lipid oxidation increased with decreasing pH. The following hypothesis was suggested to explain this phenomenon: the egg yolk used as an emulsifier in mayonnaise contains large amounts of iron, which is bound to phosvitin. At the natural pH of egg yolk (pH 6.0) the iron also forms iron bridges between phosvitin and other components in egg yolk, namely LDL and lipovitellin. These components are located at the oil—water interface in mayonnaise. When pH is decreased to 4.0, which is the pH in mayonnaise, the iron bridges between the egg yolk components are broken and iron becomes dissociated from LDL and lipovitellin. Thereby, iron becomes more active as a catalyst of oxidation.
Proteins are commonly used as emulsifiers in foods to facilitate the formation and enhance the stability of oil-in-water emulsions. During homogenisation they are absorbed to the oil droplet surface where they lower surface tension and prevent coalescence of droplets by forming protective membranes around the droplets. Proteins also have a stabilising effect on the emulsion by providing the emulsion droplets with a positive or negative electrical charge at pH values below or above the pI of the proteins. It has been suggested that the electrical charge of the interfacial layer around the oil droplet significantly influences oxidation in emulsions in the presence of metal ions. Compared with a nonionic emulsifier, an anionic emulsifier was shown to increase oxidation in corn oil model oil-in-water emulsions whereas a cationic emulsifier decreased oxidation. These results were explained by the ability of the emulsifier to attract and repel metal ions to the oil—water interface, respectively. Therefore, the charge and thereby the type of emulsifier may affect the oxidative stability of the emulsion.
pH will affect the charge of the emulsifier and this may in turn affect the oxidative stability of emulsions. Recently, it was reported that oxidation increased with increasing pH in salmon oil-in-water emulsions stabilised by whey proteins. More specifically, lipid oxidation rates were significantly lower at pH values below the pI of the whey protein isolate. This was suggested to be due the fact that the proteins would be positively charged at pH values below pI and therefore they would repel metal ions near the oil—water interface. However, the surface charge of the emulsifier does not seem to be the only factor influencing lipid oxidation. Thus, Hu et al. also reported that the order of lipid oxidation rates in salmon oil-in-water emulsions stabilised by either whey protein isolate, sweet whey or two of the proteins present in whey protein, namely a-lactalbumin or ,3-lactoglobulin, did not equal the order of the positive charge of the emulsion droplets. In another study, in corn oil-in-water emulsions it was observed that casein resulted in lower oxidation rates than whey protein isolate and soy protein isolate, even though all emulsions were cationic at low pH.
Based on these findings it was proposed that other factors responsible for the differences in oxidative stability of protein stabilised oil-in-water emulsions could be differences in how the proteins influences the thickness or packing of the emulsion droplet interface. Increasing the thickness of the interfacial layer could make it more difficult for aqueous iron to interact with lipid hydroperoxides located near the interface. Another factor affecting the antioxidative effectiveness of proteins could be their amino acid composition. The sulphydryl group of cysteine has thus been reported to have antioxidant activity because of its ability to scavenge free radicals. Antioxidative effects of tyrosine, phenylalanine, tryptophan, proline, methionine, lysine and histidine have previously been reported in the literature.
Surfactants are small lipophilic and hydrophilic molecules that are used to form emulsions. Normally, surfactants will be present in excess in emulsions and surfactants not associated with the emulsion droplets will form micelles in the continuous phase. It has been suggested that surfactant micelles are able to reduce lipid oxidation by altering the physical location of lipid hydroperoxides and/or iron in emulsions.
More research is required to completely understand the role of emulsifiers in the lipid oxidation in complex food systems.
Previous studies have indicated that some carbohydrates in high concentrations are capable of scavenging free radicals and thereby act as antioxidants. Sucrose addition has been suggested to be able to decrease oxidation by decreasing the concentration of oxygen in the aqueous phase, and sucrose may also decrease the diffusion coefficient of oxygen via its increasing effect on the viscosity of the emulsion. Apart from reducing the diffusion of oxygen, a high viscosity of the emulsion may also reduce the diffusion of metals and other reactants and reaction products, and this may slow down oxidation rates.
Addition of antioxidants to foods may delay the onset of oxidation or slow down the rate at which it proceeds. Antioxidants are usually classified as either primary or secondary antioxidants. The former are also referred to as free radical scavengers as they are chain-breaking antioxidants that delay or inhibit the propagation stage by donating a hydrogen atom to the lipid radical, the peroxyl radical or the alkoxyl radical. Primary antioxidants are often phenolic compounds such as the synthetic antioxidants BHA, BHT, propyl gallate or as naturally occurring compounds, such as tocopherol, and plant polyphenols, such as carnosic acid. The secondary antioxidants act by a number of different mechanisms such as metal chelation, oxygen scavenging and replenishing hydrogen to primary antioxidants. The secondary antioxidants often exert synergistic effects together with primary antioxidants. EDTA, lactoferrin and citric acid are examples of metal chelators that have been shown to reduce lipid oxidation in fish oil emulsions and fish oil.Ascorbic acid and the glucose oxidase±catalase enzyme system are examples of oxygen scavengers.Ascorbic acid is also able to regenerate tocopherol by replenishing hydrogen.
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