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Mechanisms of Volatile Production from Amino Acid Esters by Irradiation

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Mechanisms of Volatile Production from Amino Acid Esters by Irradiation Food Research International 81 (2016) 100–107 Contents lists available at ScienceDirect Food Research International journal homepage: www.elsevier.com/locate/foodres Mechanisms of volatile production from amino acid esters by irradiation Xi Feng a,EunJooLeeb, Kichang Nam c, Cheorun Jo d,KyungyukKoe,DongUkAhna,⁎ a Department of Animal Science, Iowa State University, Ames, IA 50010, United States b Department of Food and Nutrition, University of Wisconsin-Stout, Menomonie, WI 54751, United States c Department of Animal Science and Technology, Sunchon National University, Suncheon 540-742, Republic of Korea d Department of Agricultural Biotechnology, Center for Food and Bioconvergence and Research Institute for Agriculture and Life Science, Seoul National University, Seoul 151-921, Republic of Korea e Division of Food Additives and Packaging, Department of Food Safety Evaluation, Ministry of Food Drug Safety, Cheongwon-gun, Chungbuk 363-700, Republic of Korea article info abstract Article history: Proteins and its constituents (amino acids) are known as the major contributors to the off-odor in irradiated Received 2 December 2015 meat. However, radiolytic degradation of amino acids occurred not only at side chains but also at amino- and Received in revised form 4 January 2016 carboxyl-groups of the α-carbon. A model system with amino acid esters was used to elucidate the mechanisms Accepted 6 January 2016 of volatile production at side chains of amino acids by irradiation. The low-molecular weight aldehydes, which Available online 9 January 2016 contributed to the irradiation off-odor, were mainly from acidic, aliphatic and aliphatic hydroxyl group amino acid esters through the radiolysis of amino acid side chains or Strecker degradation. However, the contribution Chemical compounds studied in this article: Methionine (PubChem CID: 6137) from non-sulfur amino acids was minor compared with sulfur amino acids. Among the sulfur-containing Dimethyl disulfide (PubChem CID: 12232) amino acids, methionine made the greatest contribution to the irradiation off-odor not only through the direct Thiobis methane (PubChem CID: 1068) cleavage of the side chain. However, the chemical reactions of sulfur compounds with other compounds pro- duced by irradiation also played significant roles to the off-odor of irradiated meat. Keywords: © 2016 Elsevier Ltd. All rights reserved. Irradiation Volatiles Amino acid esters Sulfur amino acids Irradiation odor 1. Introduction 1995). These researches also indicated that radiolysis of amino acids played an important role in the production of off-odor volatiles in irradi- Irradiation is the most effective method to inactivate foodborne ated meats, because more than 70% of meat is water, the second major pathogens in meat (Delincée, 1998; Diehl, 2002). In addition to improv- components in meat are proteins and its constituents (amino acids), ing the meat safety, irradiation leaves no chemical residues, minimizes which are constituted by a variety of amino acid groups, including acidic, nutrient loss, eliminates further cross contamination after processing, amide, basic, aromatic, aliphatic, aliphatic hydroxyl and sulfur containing and prolongs shelf-life of meat (Farkas, 2006; Roberts, 2014). However, groups (Lawrie & Ledward, 2006). the use of irradiation on meat products is limited because of off-odor Ahn (2002) used amino acid homopolymers and Ahn et al. (2016a,b) production by irradiation (Brewer, 2009; Lee & Ahn, 2004). used amino acid monomers to elucidate the production mechanisms of All irradiated meat produces characteristic irradiation odor such as off-odor volatiles in meat by irradiation. However, the production “hot fat,”“burned oil”, “burned feathers”, “bloody and sweet”,or mechanisms of off-odor volatiles in irradiated meats cannot be fully ex- “barbecued corn-like” odor, regardless of degree of lipid oxidation plained by analyzing the volatiles from irradiated amino acid homopol- (Hashim, Resurreccion, & McWatters, 1995; Heath, Owens, Tesch, & ymers or monomers because they found that the radiolytic degradation Hannah, 1990; O'Bryan, Crandall, Ricke, & Olson, 2008). Several off- of amino acids occurred not only at side chains but also at amino and odor volatile compounds, including 2-methyl butanal, 3-methyl butanal, carboxyl groups of the α-carbon. In this study, amino acid esters were 1-heptene, 1-octene, 1-nonene, hydrogen sulfide, sulfur dioxide, used to determine the contribution of the side chains to the production mercaptomethane, dimethyl sulfide, methyl thioacetate, dimethyl disul- of volatiles from amino acids by irradiation. The amino acid ester sam- fide and dimethyl trisulfide, were newly generated or increased in meat ples were randomly divided into 2 groups and irradiated at 0 kGy (con- by irradiation (Arvanitoyannis, 2010; Fan, Sommers, Thayer, & Lehotay, trol group) or 5 kGy (treatment group) using a linear accelerator. The 2002; Lin et al., 2007; Panseri et al., 2015; Patterson & Stevenson, objective of this study were 1) to determine the volatile compounds newly produced from each amino acid ester by irradiation, 2) to eluci- ⁎ Corresponding author. date the production mechanisms of off-odor volatiles from amino acid E-mail address: [email protected] (D.U. Ahn). esters by irradiation, and 3) to characterize the odor and evaluate the http://dx.doi.org/10.1016/j.foodres.2016.01.006 0963-9969/© 2016 Elsevier Ltd. All rights reserved. X. Feng et al. / Food Research International 81 (2016) 100–107 101 contribution of volatiles from amino acids to the odor of irradiated 2.3. Odor characteristics model systems. Twelve trained sensory panelists characterized the odor of samples. 2. Materials and methods Panelists were selected based on interest, availability, and performance in screening tests conducted with samples similar to those to be tested. 2.1. Sample preparation During training, a lexicon of aroma terms to be used on the ballot was developed, and references were selected as anchors to identify the over- Twenty one amino acid esters, including aspartic acid di-methyl ester, all odor characteristics of samples. Each sample was placed in a glass vial aspartic acid β-methyl ester, glutamic acid γ-methyl ester, alanine meth- with a randomly selected 3-digit number, and the sample temperature yl ester, isoleucine methyl ester, leucine methyl ester, proline methyl was brought to 25 °C before samples are tested. During the tests, one ester, valine ethyl ester, serine ethyl ester, threonine methyl ester, aspar- treatment was presented to each panelist each time, and the order of agine t-butyl ester, glutamine t-butyl ester, phenylalanine ethyl ester, presentation was randomized. Panelists characterized overall odor tryptophan ethyl ester, tyrosine ethyl ester, arginine ethyl ester, histidine characteristics. All the sensory evaluation tests were done at 25 °C in a methyl ester, lysine ethyl ester, cysteine ethyl ester, cystine di-ethyl ester, sensory panel room equipped with white fluorescent lighting. or methionine methyl ester, were purchased from Sigma-Aldrich (St. Louis, MO, USA) and used to make the aqueous model system. Each 2.4. Statistical analysis amino acid ester (5 mg/L) was dissolved in a citrate–phosphate buffer (100 mM, pH 6.0), transferred to a 40 ml vial and irradiated at 0 kGy or Newly formed volatiles in irradiated amino acid esters after irradia- 5.0 kGy absorbed dose using an Electron Beam irradiator (Circe IIIR, tion (5 kGy) were reported as mean values with standard deviation. Thomson CSF Linac, St. Aubin, France). Alanine dosimeters were placed at the top and bottom of a vial and read using a 104 Electron Paramagnet- 3. Results and discussion ic Resonance Instrument (Bruker Instruments Inc., Billerica, MS) to check the absorbed dose. Four replications were prepared for each amino acid 3.1. Acidic, amide, basic and aromatic group amino acid esters ester. Immediately after irradiation, four 2 mL-portion amino acid ester solutions were taken from each replication, and they were individually Two different aldehydes, acetaldehyde and propanal, were produced transferred to four different sample vials, flushed with helium gas by irradiated acidic group amino acid esters (aspartic acid di-methyl (99.999% purity) for 5 s at 40 psi, and then capped. One portion of each ester, aspartic acid β-methyl ester, and glutamic acid γ-methyl ester) replication was used to analyze volatile profiles, and the other three (Table 1). Ahn et al. (2016a) indicated three possible pathways to produce were used to determine odor characteristics. Volatile profiles and odor aldehydes from aspartic acid and glutamic acid: 1) the side chain and – − characteristics of irradiated amino acid esters were studied. A purge- NH2 group cleavage from the α-carbon: generates an acetic acid or and-trap dynamic headspace/GC–MS was used to quantify and identify propionic acid, which is further converted to acetaldehyde through the volatile components, and trained sensory panelists evaluated the overall oxidation–reduction reactions (McMurry, 2004); 2) the Strecker degrada- − odor characteristics of the samples. tion removes –NH2 and –COOH from the α-carbonmoietyinaminoacid through the actions of ozone (produced by irradiation) and generates al- 2.2. Volatile compounds analysis dehyde (Yaylayan, 2003); and 3) the acetic acid formed from aspartic acid and glutamic acid can react with hydroxyl radical (•OH) and produce an A purge-and-trap apparatus (Precept II and Purge & Trap Concentra- ethen-1-ol (CH2CHOH), which
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