Mark P. Richards1, Birol Kilic2, Rong Li1
1Department of Animal Sciences - Food Sciences
University of Wisconsin - Madison
Madison WI, U.S.A.
2Department of Food Engineering
Ataturk University
Erzurum, Turkey
Introduction
Consumers are finding less time to prepare meals. The food industry is responding to this by increasing the availability of pre-cooked meats (1). A major problem with pre-cooked meats is the development of an objectionable warmed over flavor via lipid oxidation during refrigerated and frozen storage (2). The susceptibility of meat and meat products to lipid oxidation depends on a number of factors, one being the level of polyunsaturated fatty acids present in the particular meat system (3).
Biological membranes contain phospholipids, which are rich in polyunsaturated fatty acids and are the sites where oxidative changes are initiated in meat and meat products. Other factors are pre-slaughter events such as stress, post-slaughter events such as carcass temperature, carcass pH, disruption of the integrity of muscle membranes by grinding, mechanical deboning, restructuring, and cooking (4). Catalysis of lipid oxidation can occur by the action of low molecular weight metals and iron containing heme proteins such as hemoglobin and myoglobin.
Mechanically separated turkey (MST) is a low-cost material currently used in cured products such as hot dogs and bologna at levels approaching 50% of the product weight. Mechanically separated turkey is typically prepared from necks, backs or frames that are passed through a sieve under high pressure (5). Burnt flavors and residual bone fragments in MST have been reduced to negligible levels through the use of temperature control and improved sieving during recovery of mechanically separated turkey from necks and frames.
It would be desirable to utilize MST in uncured poultry products as well as cured products. However, mechanically separated turkey is highly susceptible to lipid oxidation in the absence of curing agents. Cured meats contain nitrite in the formulation. The primary way that nitrite is believed to exert its antioxidant effect is by conversion of nitrite to nitric oxide that binds to the ferrous (Fe2+) iron atom in the heme ring of hemoglobin and myoglobin. The NO-ligand prevents interaction of the heme group with lipids and release of iron from the heme group during cooking and storage. Nitrite can also act as an antioxidant by chelating metals and scavenging free radicals.
Sage (1.5% of product weight) decreased lipid oxidation values 49% in uncured, pre-cooked meatballs containing 20% mechanically separated turkey after 4 days of refrigerated storage (6). This level of antioxidant might be considered excessive both in terms of costs and spice flavor impact. Rosemary oleoresin (200 ppm) was as effective as a commercial antioxidant mixture (BHA/BHT and citric acid) in dehydrated but not frozen cooked poultry containing 18% mechanically separated turkey (7). Exploring other antioxidant strategies is needed.
The purpose of this work was to further examine the utility of MST in pre-cooked, uncured poultry products and modes of inhibiting lipid oxidation during storage.
Materials and methods
Materials:
Fresh skinless, boneless chicken breast meat was purchased from the University of Wisconsin-Provision. Muscles were trimmed of visible fat and connective tissue and stored at 4°C for no more than 24 hours before use. Mechanically separated turkey (MST) was provided by Oscar Mayer Foods (Madison, WI). MST was used within 48 hours of manufacture. Sodium ascorbate, tetraethoxypropane, cumene hydroperoxide, ammonium thiocyanate, iron sulfate and barium chloride were obtained from Sigma Chemical Co. (St. Louis, MO). HPLC grade chloroform containing ethanol preservative was obtained from Fisher Scientific (Pittsburgh, PA). All other chemicals were reagent grade.
Preparation of samples:
Each breast was cut into five mm thick slices by a meat slicer Model No. 5402 (Toledo Scale Corp., Toledo, OH). Four treatments were investigated. Each treatment contained chicken breast muscle with the following separate additions:
i) 1% salt (control);
ii) 1% salt + 10 % MST;
iii) 1% salt + 0.1 % sodium ascorbate;
iv) 1 % salt + 0.1 % sodium ascorbate + 10 % MST.
Meat slices were immersed in the marinating mixture for 12 hours at 4°C. Meat blocks were prepared by impaling the marinated pieces onto a central spike. Each side of a 2 kg meat block was cooked for 4 minutes in an open gas oven. The meat block was then rotated and while cooked surface was cut into 4-6 mm thickness, the other side was cooked for four minutes. This procedure was repeated continuously until whole meat block was cooked. Following cooking and slicing, cooked slices from each treatment were stored at 4°C or –20°C.
Lipid oxidation measurements:
Thiobarbituric acid reactive substances (TBARS) were determined in triplicate from each group using the muscle extraction procedure of Lemon (8). For determination of lipid peroxides, lipid was extracted in chloroform/methanol (1:1). An appropriate volume of 0.5 % NaCl was used to separate the mixture into two phases. After addition of chloroform/methanol to an aliquot of the chloroform layer, ammonium thiocyanate and ferrous chloride were added to assay for lipid peroxides (9).
Statistical analysis:
Analysis of variance with a MIXED split-plot procedure of the SAS system was used to evaluate data from storage studies. Means were separated using differences of least squares (10).
Results and discussion
Control samples are defined as those containing pre-cooked chicken breast muscle that was immersed in 1% salt marinade prior to cooking. TBARS values of samples also containing ascorbate in the marinade were significantly lower than that of control samples during 4°C and –20°C storage (Table 1 and 2).
Ascorbate can inhibit lipid oxidation due to its hydrogen donating properties (11) which allow various components in muscle foods to be reduced. For example, ascorbate reduces tocopheroxyl radicals back to tocopherol in tissue membranes, which increases the ability of tocopherol to inhibit lipid oxidation (12). Hydroxyl, peroxyl and superoxide radicals can also be scavenged by ascorbate to impede the lipid oxidation process (13).
Thiobarbituric acid reactive substances (TBARS) values were much higher in samples containing 10% MST than control samples whether the storage condition was refrigerated or frozen temperatures (Table 1 and 2). Mechanically separated turkey is a finely comminuted product. The mechanical separation will rupture blood vessels, erythrocytes and small bones, releasing hemoglobin, which likely contributed to the rapid TBARS development in the samples containing MST.
Takama et al. (14) suggested that minced flesh of trout was susceptible to rancidity due to the dispersed blood pigments in the flesh caused by the mechanical destruction of the tissue. Lee et al. (15) proposed that the heme proteins, hemoglobin and myoglobin, were the predominant catalysts of lipid oxidation in mechanically separated chicken. This was based on poor inhibition of lipid oxidation by iron chelators, and the inhibitory efficacy of thiol compounds.
The hemoglobin content determined in mechanically separated chicken was 50.4 µmol hemoglobin/kg tissue (15). Neumann and Beutling (16) reported 2.8 µmol hemoglobin/kg chicken breast muscle in well-bled broilers. This suggests that incorporating MST into our samples provided a substantial amount of hemoglobin. Cooking accelerates denaturation of the heme protein. Denaturation has been proposed to expose the heme moiety to lipid and stimulate lipid oxidation (17). Hemoglobin is also a possible source of low molecular weight (LMW) iron. Cooking apparently facilitates the release of iron from the heme protein (18). These low molecular weight iron complexes in the ferrous (Fe2+) and ferric (Fe3+) oxidation state can react with lipid hydroperoxides to form alkoxyl and peroxyl radicals, respectively, that are capable of propagating lipid peroxidation (11). Ferritin is another source of low molecular weight iron in poultry tissues (19).
Mechanically separated poultry is not only a rich source of hemoglobin and non-heme iron but also lipid hydroperoxides. We found that freshly prepared MST contained 29.2 ± 5.3 µmol of lipid peroxides/kg of tissue. This was at least 29 times greater than the amount found in minced chicken breast muscle that was freshly prepared (data not shown). Since lipid hydroperoxides are key reactants in the pro-oxidativity of hemoglobin and non-heme iron, it is likely that the excess amounts of peroxides and iron in samples containing mechanically separated turkey can explain the rapid rates of lipid oxidation that were observed compared to samples without MST.
Thiobarbituric acid reactive substances (TBARS) values of samples containing MST with added ascorbate were significantly higher than that of samples containing only mechanically separated turkey during 4°C storage (Table 1).
This indicates that a pro-oxidant effect of ascorbate was evident when the system contained MST during refrigerated storage. A possible explanation for the pro-oxidant effect of ascorbate is that ascorbate effectively maintained low molecular weight iron in its reduced state (Fe2+). Fe2+ is more active in the homolytic decomposition of lipid hydroperoxides than Fe3+ and thus a better catalyst of lipid oxidation (20). Ascorbate can chelate Fe3+ in the presence of oxygen to generate Fe2+ and HOO• (21). These HOO• radicals have a favorable electron-reduction potential to initiate lipid oxidation (12). There likely was a large pool of low molecular weight iron in MST-containing samples considering that MST is seeded with high levels of lipid hydroperoxides and hemoglobin combined with the fact that cooking temperatures and excess peroxides accelerate the release of iron from hemoglobin (18; 22).
It has also been found that ascorbate accelerated the decomposition of lipid hydroperoxides in a β-carotene-linoleate model system, which can produce lipid radicals capable of stimulating lipid oxidation (11). These authors found that the ability of ascorbate to accelerate lipid hydroperoxide decomposition was accelerated in the presence of metal ions. Thus, the anti-oxidative effect of ascorbate in the absence of mechanically separated turkey that was converted to a pro-oxidative effect in the presence of MST may indicate that the excess iron and lipid hydroperoxides from the MST were activated as lipid oxidation catalysts by ascorbate which overwhelmed the ability of ascorbate to inhibit lipid oxidation at lower metal and peroxide concentrations as in control samples.
The pro-oxidant effect of ascorbate in the presence of MST that occurred at 4°C (Table 1) did not occur at –20°C (Table 2). This may be due to the immobilization of reactants during frozen storage that will not occur at unfrozen temperatures (23). Frozen storage could physically decrease the interaction of pro-oxidants in the system (e.g non-heme iron, heme iron, ascorbate and peroxides), thereby lowering the pool of free radical species capable of propagating lipid oxidation processes. Future work should continue the search for natural antioxidants that inhibit lipid oxidation at low concentrations in pre-cooked products, especially those containing mechanically separated poultry.
Acknowledgments
This work was supported by the College of Agricultural and Life Sciences, University of Wisconsin-Madison, HATCH project WIS04512. Thanks to Oscar Mayer for supplying mechanically separated turkey.
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From Proceedings of the "Midwest Poultry Federation Convention", St. Paul, Minnesota, U.S.A.
