Antioxidant Measurement and Applications - American Chemical Society

Chapter 26

Downloaded by PEPPERDINE UNIV on August 23, 2017 | http://pubs.acs.org Publication Date: March 12, 2007 | doi: 10.1021/bk-2007-0956.ch026

Control of Irradiation-Induced Lipid Oxidation and Volatile Sulfur Compounds Using Antioxidants in Raw Meat and Ready-to-Eat Meat Products Xuetong Fan Eastern Regional Research Center, Food Safety Intervention Technologies Research Unit, Agricultural Research Service, U.S. Department of Agriculture, 600 East Mermaid Lane, Wyndmoor, P A 19038

Ionizing radiation is a non-thermal processing technology used for extending shelf-life and disinfestation of fruits and vegetables, and for inactivating foodborne pathogens and spoilage microorganisms of various foods. However, ionizing radiation can promote lipid oxidation, particularly during postirradiation storage when exposed to oxygen, and induce development of an off-odor in meats. Free radicals, such as hydroxyl radicals and hydrated electrons, generated from radiolysis of water, attack food components (proteins, amino acids, lipids etc.), leading to an increased rate of lipid oxidation and production of volatile sulfur compounds. Most of the volatile sulfur compounds, such as hydrogen sulfide, methanethiol, methyl sulfide, dimethyl disulfide and dimethyl trisulfide have very low odor thresholds. Antioxidants applied either as additives, ingredients, or dietary supplementation inhibited lipid oxidation, but had a limited effect on production of volatile sulfur compounds, suggesting the mechanisms for irradiation-induced lipid oxidation and production of volatile sulfur compounds are different. Combination of antioxidants with packaging systems may be used to reduce both lipid oxidation and production of off-odor compounds.

U.S. government work. Published 2007 American Chemical Society.

Shahidi and Ho; Antioxidant Measurement and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

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Ionizing radiation is a non-thermal processing technology used for retarding fruit ripening, disinfecting fruits and vegetables, and for inactivating foodborne pathogens and spoilage microorganisms in many foods. Irradiation can kill, injure and inactivate foodborne pathogens and enhance food safety in meat and meat products. However, commercial use of the technology is limited partially due to concerns on adverse effects of irradiation on product quality. When foods are irradiated, particularly at high doses, an off-flavor can develop. Volatile sulfur compounds (VSCs) derived from proteins and sulfur-containing amino acids and compounds that are derived from lipid oxidation may contribute to the off-odor due to irradiation (7, 2).

Lipid Oxidation and Radiolysis of Lipids Lipid oxidation is a process involving oxidation of unsaturated fatty acids in the presence of oxygen. Lipid oxidation is important for raw and cooked meats because it causes quality deterioration such as change in color, loss of nutritional values, and production of off-odors/off-flavors (rancid, warmed-over flavor, etc.) due to production of aldehydes, ketones and many other compounds (3, 4). It is generally believed that three phases are involved in lipid oxidation: (1) initiation, the formation of free radicals; (2) propagation, the free-radical chain reactions; and (3) termination, the formation of stable products. The oxidation of fatty acids occurs via a free radical chain mechanism involving the abstraction of hydrogen atoms with subsequent attack by molecular oxygen, leading to formation of hydroperoxides. The hydroperoxides decompose to give up a variety of breakdown products including aldehydes, alcohols, ketones, hydrocarbons, etc. (3,4). In foods such as meats that contain mostly water, irradiation exerts its effects mainly via free radicals generated from radiolysis of water. The primary free radicals generated from radiolysis of water are hydrated electrons (e "), hydroxyl radicals ( O H ) and hydrogen atoms (H), which in turn attack food components, such as proteins, lipids, amino acids, and induce chemical changes in raw and cooked meats (5). Exposure of fatty acids and lipids to irradiation in the presence of oxygen accelerates the lipid oxidation process. This is probably because irradiation enhances the following three reactions: formation of free radicals which can combine with oxygen, breakdown of hydroperoxides and destruction of antioxidants (6). For example, it has been shown that irradiation (3 kGy) caused a 15% reduction of free β-tocopherol and a 30% reduction of free α-tocopherol in chicken breast muscle (7). Obviously, reduction in the concentrations of antioxidants may increase the rate of lipid oxidation of meats during storage. In the absence of oxygen, irradiation directly causes cleavages at certain locations in the lipid molecules, leading to the formation of radiolytic compounds which are mainly dependent on the fatty acid composition of the fat aq



403 (6). The radiolytic compounds in the absence of oxygen are largely hydrocarbons. Because lipid oxidation is a significant quality deterioration problem in meats, measurements of lipid oxidation have been used to indicate the stability and potential shelf-life of meats. Thiobarbituric acid reactive substances (TBARS) assay is a widely used method for the determination of lipid oxidation in meats and meat products (8). However, the T B A R S measurement is not specific and does not directly measure volatile compounds that contribute to the off-odors. Aldehydes are the major lipid oxidation products contributing to oxidation flavor and rancidity in meats. Hexanal is the predominant oxidative volatile adehyde found in many meats and meat products (3). A linear relationship between hexanal content and sensory scores in cooked ground pork (P), and a correlation between hexanal and T B A R S (10) have been found. There is an inconsistency in terms of whether irradiation induces lipid oxidation of raw meats measured immediately after irradiation. Many studies demonstrated that irradiation accelerated T B A R S values (11-13) while in some studies irradiation had no effect on T B A R S values (14,15). Generally speaking, the rate of irradiation-induced lipid oxidation was higher in aerobically packaged meats than in vacuum packaged samples. Most studies on raw meats have shown that irradiated meats had accelerated lipid oxidation during storage, particularly when stored in aerobic packages (12,16). When irradiated meats were cooked and stored, T B A R S values in the cooked meats increased more rapidly than the raw meats. In cooked ready-to-eat (RTE) meat products, T B A R S can be either increased or decreased by irradiation. Many studies showed that irradiation increased T B A R S values in R T E meats (17-19). Jo and others (20) found irradiation (4.5 kGy) increased T B A R S values of cooked pork sausages, but the difference disappeared during 7 days of storage at 4°C as T B A R S increased in all samples. On the other hand, T B A R S values can be reduced by irradiation in R T E meats and the reduction increased with higher irradiation doses (21,22). Furthermore, T B A R S values in some irradiated R T E meat products decreased during storage. For example, lipid oxidation in cooked beef exposed to air was inhibited by high dose (5-48 kGy) of irradiation and the T B A R S decreased during storage (23). As a result, the reduced T B A R S values by irradiation enhanced oxidative stability during storage (24). Lipid oxidation can result in formation of many aldehydes. It has been shown that irradiation increased production of hexanal and pentanal in a number of meat products even though the increases may not always be significant (11,25,26). Hexanal was found in emulsions prepared from arachidonic acid, not in those from linolenic acid during a post-irradiation storage period (27), suggesting that hexanal can be produced from arachidonic acid (27) and/or linoleic acid (28) in meats in the presence of oxygen. Without oxygen, only n-1 alkanes and n-2 alkenes were produced. C\ hydroperoxide is first produced by the reaction of arachidonic acid with hydroxyl radicals and oxygen (Figure 1). 5


404 Hexanal is then synthesized from the cleavage of C j - C i bond of the C hydroperoxide radical, similar to the proposed formation of hexanal from linoleic acid (28). Other mechanisms of hexanal production may exisit, such as rearrangement of conjugated double bonds after abstraction of H followed by reaction with oxygen to form hydroperoxide radical. Pentanal and hexanal were highly correlated with T B A R S values in irradiated meats (29,30). More often higher levels of aldehydes were produced in aerobically packaged meats than vacuum packaged samples. In addition, the production of aldehydes generally increased in aerobically packaged meats during storage, and the increase in irradiated meats was faster than non-irradiated ones (31-33). The levels of aldehydes in vacuum packaging had little change or decreased during storage (20,29,33).


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Contribution of Volatile Sulfur Compounds to the Off-Odor Earlier studies using sterilization dose (20-60 kGy) described the irradiation-induced off-odor as 'metallic', 'sulfide', 'wet dog', 'wet grain', 'goaty' or 'burnt' (34). The odor has been called 'irradiation odor'. More recent studies using low doses (

Antioxidant Measurement and Applications - American Chemical Society

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