Antioxidants, primary

D. Burdick, S. Laermer, S. Young, and P. Zambetti, "A New Primary Antioxidant System for Polyolefins," presented 2l. ddditives 95 Clearwater,... 

Aromatic amines are the most effective primary antioxidants (Fig. 34) but they are discolouring and can only be used where the darker colours are acceptable (for instance in rubber adhesive formulations containing carbon blacks as fillers). The... 

The other class of primary antioxidants are the phenols (hindered phenols, hindered bisphenols, hindered thiobisphenols, polyhydroxy phenols) (Fig. 34). Phenolic antioxidants are generally used when the discolouration of the amine antioxidants cannot be tolerated. Phenols may produce coloured reaction products (yellow, tan or pink) but the discolouration is significantly less than produced with amines. 

Process 4, conversion of peroxy radicals to hydroperoxides can be interrupted by traditional primary antioxidants (see Fig. 16). The fastest reacting primary antioxidants are the aromatic amines (e.g. Naugard 445). However, these materials yellow upon exposure to UV light which restricts their applieations. More common in adhesives are the hindered phenol types of which numerous types are available, with Irganox 1010 the most common choice for adhesives. 

MAHGOUB s E o and HUDSON B J F (1985) Inhibition of the pro-oxidant activity of copper by primary antioxidants in lard, Food Chem, 16, 97-101. 

Wieboldt et al. [560] have described SFE-SFC-FTIR analysis of hindered phenol primary antioxidants and phosphite secondary antioxidants in PE. SFE is more selective for the lower-range low-MW polymer than Soxhlet-type extraction. This yields a chromatogram with less interference from low-MW polymer peaks in the region where the additive components elute. As a result, SFE appears to be a better choice than Soxhlet-type extraction for the selective removal of additives from flaked polymer. SFE and dissolution/precipitation methods were compared for a PVC/stabiliser system [366]. 

Antioxidants act so as to interrupt this chain reaction. Primary antioxidants, such as hindered phenol type antioxidants, function by reacting with free radical sites on the polymer chain. The free radical source is reduced because the reactive chain radical is eliminated and the antioxidant radical produced is stabilised by internal resonance. Secondary antioxidants decompose the hydroperoxide into harmless non-radical products. Where acidic decomposition products can themselves promote degradation, acid scavengers function by deactivating them. 

Unwanted degradation and oxidation processes can be avoided or at least suppressed for some time either by structural modiflcation of the polymer or by special additives. In practice, the addition of so-called antioxidants is particularly effective. Chemical substances that slow down oxidations and the following aging phenomena serve for this purpose. Antioxidants are sufficiently effective even in concentrations below 1 wt% and are added as early as possible to the polymer to be protected, e.g., already during the drying of powdery polymeric materials or during the preparation of granulates. Some of the most important so-called primary antioxidants are sterically hindered phenols and secondary aromatic amines secondary antioxidants are thioethers as well as phosphites and phosphonites. 

Antioxidant-Treated Beef Patties. The effect of several primary antioxidants, PG, TBHQ, Tenox 20 (which contains TBHQ and citric acid) and Tenox 4A (which contains BHA and BHT) were also evaluated by instrumental, chemical and sensory methods for their effectiveness in raw/stored and cooked/stored beef, see Tables 1-4. These particular antioxidants were chosen based on their antioxidant effects on MFD as determined previously (14) and because of their GRAS status. Experimental samples were prepared similarly to those of the 0-and 2-day controls, except the antioxidants, either dissolved or suspended in water, were mixed into the raw ground meat. 

When the antioxidants were used in the cooked/stored samples, data indicated that they were very effective in inhibiting lipid oxidation and MFD. The chemical and off-flavor indicators were reduced and the on-flavor notes were increased. Thus, phenolic-type primary antioxidants that function as free radical scavengers are very effective tools for preventing lipid oxidation and MFD in ground beef. It should also be noted that the intensity of the desirable flavor notes remained at very high levels, which meant that the patties retained their beefy tastes. Therefore, for an antioxidant to be highly effective, it should not only prevent lipid oxidation, but it should also retain the desirable flavor properties of the food commodity. 

Additional particulars should also be noted. First, vacuum when used in combination with EDTA, showed a very large decrease in lipid oxidation when compared to those EDTA-treated samples without vacuum (EVB vs ENB). This was not the case when the primary antioxidant, PG, was used with or without vacuum (EVA vs ENA). In this latter case, PG was effective with or without vacuum, although there were some intensity differences observed and by combining with vacuum, MFD and lipid oxidation was decreased. Next, vacuum alone lowered lipid oxidation (EVO vs ENO). Finally, by combining vacuum with EDTA a strong synergistic effect was observed. 

Effect of Sodium Ascorbate on Stored Ground Beef. The effectiveness of sodium ascorbate (SA) as an antioxidant was determined on both raw and cooked stored ground beef. The experimental models, i.e., standards, controls and experimental samples were similar to those described in the section on primary antioxidants. Data from the use of 150 and 200 ppm and 250 and 500 ppm are reported in Tables 7 and 8, respectively. At a concentration of 500 ppm, SA was the most effective inhibitor of MFD and lipid oxidation in both raw and cooked/stored ground beef when compared to the other three concentrations. Tlie markers of MFD and lipid oxidation were suppressed to the greatest extent and the desirable beefy flavor markers were maintained at their highest intensities when SA was added to the beef samples. SA was effective as an inhibitor at all levels tested. 

Role of Other Antioxidants A great deal has been written about the primary antioxidants, which may be used to inhibit development of WOF. Porter (36) has reviewed these compounds, their structures, and probable modes of action so they will not be discussed. The discussion will focus on the other natural antioxidants that may be useful in controlling WOF development. 

Processing Additive Stabilizers Primary antioxidants (sterically hindered phenols, seoaryl amines) hydroperoxide decomposers (organophosphites, thioesters), acid absorbers (lead salts, Ca/Ba-Ba/Cd-Ba/Sn salts, organotins, epoxidized oils)... 

Primary antioxidants, also termed as chain-breaking antioxidants, interfere with the chain reaction in Scheme 2.1, by trapping radicals or labile hydrogen atom donors. These are exemplified by hindered phenols and alkylarylamines. Scheme 2.8 schematically demonstrates the scavenging activity of a typical hindered phenol. 

Photoantioxidants are typified by the class of HAS which although were developed for photostabilization of polyolefins, also possess thermal antioxidant properties. They are generally assumed to function as primary antioxidants in that they scavenge radicals and in particular, peroxy radicals. 

The hindered phenolics are primary antioxidants and are normally used in conjunction with the thiodipropionate esters. These combinations are synergistic. They are of only limited value as heat stabilizers when used alone the thiodipropionate esters are secondary antioxidants, also of only limited value when used alone. 

Two principal classes of antioxidant are effective in thermal oxidation. Chainbreaking or primary antioxidants limit the rate of the chain propagation steps (Eqs. 3-2 and 3-3) by trapping carbon- or oxygen-centered free radicals. Hydroperoxide decomposing or secondary antioxidants prevent chain initiation by interfering with ROOH. Photoantioxidants protect plastics exposed to photo-oxidation. 

Primary antioxidants terminate free radical chains and function as electron donors. They include the phenolic antioxidants, butylated hydroxyanisole (BHA), butylated hydroxy-toluene (BHT), tertiary butyl hydroquinone (TBHQ), alkylgalates, usually propylgallate (PG), and natural and synthetic tocopherols and tocotrienols. 

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