Volatile compounds and odor traits of dry-cured ham (Prosciutto crudo) irradiated by electron beam and gamma rays

Radiation Physics and Chemistry 130 (2017) 265–272

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Volatile compounds and odor traits of dry-cured ham (Prosciutto crudo) irradiated by electron beam and gamma rays Qiulian Kong a, Weiqiang Yan a, Ling Yue a, Zhijun Chen a, Haihong Wang a, Wenyuan Qi a,nn, Xiaohua He b,n a b

Shanghai Shuneng Irradiation Technology Co., Ltd, Shanghai Academy of Agricultural Sciences, Shanghai 201403, China Western Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, Albany, CA 94710, USA

H I G H L I G H T S







The effects of gamma rays and electron beam irradiation on odor traits and volatile compounds of dry-cured hams were analyzed. Electron beam is better in maintaining ham odor than gamma rays at same doses. Both gamma rays and electron beam irradiation at dose of 6 kGy lead to the development of off-odors in hams. Gamma rays irradiation at dose of 6 kGy results in additional volatile compounds changes and worse odor.

art ic l e i nf o

a b s t r a c t

Article history: Received 14 January 2016 Received in revised form 14 August 2016 Accepted 4 September 2016 Available online 5 September 2016

Prosciutto crudo samples were irradiated at 0, 3 and 6 kGy by gamma rays (GR) and electron beam (EB), respectively. The odor scores and volatile compounds were examined after 7 days storage at 4 °C. Volatile compounds from samples without and with irradiation at 6 kGy were analyzed by GC–MS. Fifty-nine compounds were identified, including terpenes, aldehydes, alcohols, ketones, alkanes, esters, aromatic hydrocarbons and acids. Both GR and EB irradiation resulted in formation of (Z)-7-Hexadecenal, cis-9hexadecenal, tetradecane, E-9-tetradecen-1-ol formate, and losing of hexadecamethyl-heptasiloxane and decanoic acid-ethyl ester in hams. However, GR irradiation caused additional changes, such as formation of undecane and phthalic acid-2-cyclohexylethyl butyl ester, significantly higher level of 1-pentadecene, and losing of (E, E)-2,4-decadienal and octadecane. EB was shown to be better in maintaining ham's original odor than GR. Our results suggest that EB irradiation is a promising method for treatment of ready to eat hams as it exerts much less negative effect on the flavor of hams compared to GR irradiation. Published by Elsevier Ltd.

Keywords: Prosciutto crudo Irradiation Gamma rays Electron beam Odor Volatile compounds Chemical compounds studied in this article: (Z)-7-hexadecenal (PubChem CID: 5364438) E-9-tetradecen-1-ol formate (PubChem CID: 5364715) Undecane (PubChem CID: 14257) Phthalic acid, 2-cyclohexylethyl butyl ester (PubChem CID: 6423436) 1-heptadecene (PubChem CID: 23217) Heptasiloxane, hexadecamethyl- (PubChem CID: 10912) Decanoic acid, ethyl ester (PubChem CID: 8048) (E,E)-2,4-decadienal (PubChem CID: 5283349) Octadecane (PubChem CID: 11635)

n

Corresponding author. Corresponding author. E-mail addresses: [email protected] (W. Qi), [email protected] (X. He).

1. Introduction

nn

http://dx.doi.org/10.1016/j.radphyschem.2016.09.008 0969-806X/Published by Elsevier Ltd.

As a “Ready-To-Eat” (RTE) product, dry-cured ham is one of the most popular foods throughout the world. It is produced in many

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countries, and a large variety of types are available. Prosciutto crudo, one of the most typical meat products in Italy, has been made and acknowledged since Roman times (Ferrentino et al., 2013; Laureati et al., 2014; Storrustløkken et al., 2015). There are two famous types of Italian prosciutto crudo: prosciutto crudo di Parma and prosciutto crudo di San Daniele. In 2006, over 9 million thighs were processed for Parma ham, and over 2.5 million for San Daniele (Pugliese et al., 2010). The cured Prosciutto crudo is different from either bacon or pancetta. It is usually thinly sliced and eaten as is. Sometimes it is lightly cooked in order to bring out the aroma and merge flavors (Flores, 1997). The stages of producting Prosciutto includes salting with sea salt, resting, and dry-aging. The penetration of the salt, the evaporation of the water, and the variations in acidity take place during these processes, giving rise to a slow selection of the microbial flora that leads to proliferation of lactobacilli capable of determining the final sensory characteristics of the prosciutto. Salting is a crucial phase in the process because the amount of salt used has to guarantee both an adequate preservation by inactivating microbial growth and a pleasant flavor (Flores, 1997; Garcia-Gil et al., 2013). Numerous chemical reactions take place during curing due to microbial activities that gradually making the meat easy to digest. The whole curing process is very delicate because any excessive microbial proliferation could facilitates unwanted reactions, such as acid fermentation caused by acidifying microorganisms, which directly affects the shelf-life and quality of ham including meat texture, color, and floavor etc (Blesa et al., 2008; Martin et al., 2008). The typical pH of hams ranges from 5.77 to 6.85 (Ruiz-Ramirez et al., 2005; Serra et al., 2005), and the water activity ranges from 0.6 to 0.9 (Serra et al., 2005). People used to believe that hams under these conditions are safe even in an unstable, non-refrigerated and uninspected environment. But according to the Meat and Poultry Hazards Control Guide made by the U.S. Department of Agriculture's Food Safety and Inspection Service (FSIS), meat pH should be declined to 5.3 within an acceptable time and temperature combination (FSIS, 2005). Since these meat products do not require further treatment (such as cooking) before consumption, the absence of pathogenic microorganisms is paramount. Although the physico–chemical parameters are not propitious to the growth of the bacteria, prosciutto crudo particularly vulnerable to contamination with microorganism from environment, utensils or personnel during the manufacture process, such as cutting, slicing and packing (Galan et al., 2011). Listeria monocytogenes is considered as the major pathogen affecting safety of RTE meat, causing high mortality rate up to 20%. In USA, zero-tolerance policy for L. monocytogenes in RTE meat is adopted, which requires a Food Safety Objective (FSO) value below 0.04 CFU/g at the retailer stage. In Europe, the FSO value is 100 CFU/g for RTE meats, or the criterion “not detected in 25 g” is applied before the product leaves the production plant if the manufacturer cannot demonstrate the achievement of FSO value (Sara et al., 2014). L. monocytogenes is widely distributed in the environment, and has been often isolated from wet and cool food processing environment. It is a compulsory step to eliminate L. monocytogenes for RTE meat products before marketing. But L. monocytogenes is psychrotrophic and can survive during refrigeration and storage. It can grow at pH levels between 4.4 and 9.4 (FAO/WHO, 2004). Traditional heat treatment can inactivate L. monocytogenes, the D-values at 61 °C and 65 °C were 124 s and 16.2 s, respectively. The effectiveness of inactivation by pasteurization depends upon package size and characteristics of products (MeiJun et al., 2005). But heat treatment may involve undesirable changes of the quality characteristics of the product. Like any other RTE foods, prosciutto

crudo requires additional non-thermal treatment to control the microbiological quality without modifying the sensory quality. Irradiation is a safe and effective method among the existing technologies for meat preservation (Alfaia et al., 2007), and is proven to be one of the best technologies in eliminating pathogens from raw meat and extending shelf life (Bari et al., 2006; Trindade et al., 2010). When irradiation is applied for packaged foods, it also protects food from environmental recontamination. Food irradiation is a “cold treatment”, used at bactericidal levels which do not substantially raise the internal temperature, leaving the food closer to its original texture and nutritional state. According to IAEA NTR 2008 (IAEA, 2008), applications of irradiation doses may be grouped into three categories: low dose (up to 1 kGy) for sprout inhibition, delay ripening, and insect disinfestation; medium dose (1 kGy to 10 kGy) for reduction of spoilage and pathogenic microbes in meat, poultry, seafood, and spice; high dose (above 10 kGy) for sterilization of packaged food and improved rehydration. The US has approved using irradiation for reduction of bacterial contamination in foods, such as beef, poultry, pork, molluscan shellfish, eggs, fresh fruit and vegetables, spices and seasonings, lettuce and spinach, and seed sprouting. The maximum doses for poultry, refrigerated meat, and frozen meat are 3.0, 4.5 and 7.0 kGy, respectively (Kundu et al., 2014; Li et al., 2015). Irradiation has been used at as many as 40 different foods to control microbial contamination and prevent sprouting or insect pest disinfestations in about 60 countries. Currently, gamma sources, electron beam generators, and X-ray accelerators are most commonly used for food irradiation (Kundu et al., 2014; Li et al., 2015). Electron beam irradiation is a promising method with a potential to reduce the burden of foodborne illness, and could become an alternative when thermal treatment is not an option. Electron beam irradiation inspires greater consumer confidence because of its radioisotope-free nature. Electron beam also has higher dose rate (103–105 Gy/s) than gamma rays (0.01–1 Gy/s), thus less time is required for equivalent pasteurization treatment (Li et al., 2015). So far, fresh and dry-fermented meat products have been developed and prepared as RTE using electron beam irradiation (Galan et al., 2011). Meat products are rich in fat. Lipid oxidation, as a secondary effect of ionizing radiation, is a major cause of flavor development (Kundu et al., 2014; Trindade et al., 2010). The flavor of hams is produced by a complex mixture of volatile compounds, which are formed from chemical or enzymatic oxidation of unsaturated fatty acids and further interaction with proteins, peptides and free amino acids, or from spices added in the production process, such as garlic or pepper (Marusic et al., 2011; Marusic et al., 2014). Volatile compounds are markedly affected by production aspects (rearing, salting and curing), geographical origin and ripening process of raw meat (Pham et al., 2008). Fatty acids, esters and some sulfur and nitrogen containing compounds are potent odoractive compounds with a low odor threshold. For instance, propyl and amyl formates are the most abundant esters present in San Daniele hams whereas butanoates and hexanoates are the most important esters in Iberian and Parma hams (Del Pulgar et al., 2011). For meat irradiation, the major concerns of effects on nutritional value and sensory characteristics are lipid oxidation, meat color fading and off-odor production (Alfaia et al., 2007). Gamma rays irradiation of meat often led to the development of off-odors, color and rancid flavors in beef (Chen et al., 2007; Park et al., 2010). However, few studies have compared the influence of gamma rays to electron beam irradiation on the odor and volatile compounds of hams. Previous research from our laboratory demonstrated that electron beam irradiation was superior to the gamma irradiation in keeping sensory traits of meats. The purpose of this

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study was to investigate the effects of two different irradiation methods on the volatile components of dry-cured ham (prosciutto crudo), and to relate these effects to the sensory properties of the irradiated hams.

2. Materials and methods 2.1. Ready-to-eat dry-cured hams Prosciutto crudo Prosciutto crudo was purchased from a local company in Shanghai, China. The ham was imported from Italy, and vacuumpackaged in a food grade polypropylene plastic bag (70 g/bag) with thickness less than 1 cm. These hams were stored in cold before sold. The manufacturer-recommended shelf life was 9 months when stored at cold. The hams were suggested as ReadyTo-Eat and to keep cold for the leftovers. For this study, hams were obtained and transported in refrigerated boxes to the lab, stored under 4 °C before irradiation treatment. 2.2. Dose uniformity ratio (DUR) In order to keep the irradiation dose as uniform as possible throughout the experiment, the samples arranged in a single layer without any stacking between each other were immobilized by cello tape to avoid displacement and treated with targeted absorbdoses ( 710%). Irradiations were conducted at Shanghai Shuneng Irradiation Technology Co., Ltd. (Shanghai, China). An electron beam system with a 10-MeV and 12-kW linear accelerator (ESS-010-33, IHI, Japan) and a cobalt-60 irradiator with 1.2*105 Ci (the design capacity is 1.5*105 Ci) were used. Two dose levels (3 kGy and 6 kGy) were used. The accelerator uses a single beam (top only) which operates at a dose rate of 8.44 kGy/s and was set to deliver a constant dose of 3 kGy and 6 kGy by running the conveyor belt at 107.2 mm s  1 and 53.6 mm s  1 separately. The cobalt-60 irradiator is a stationary irradiation source and samples were irradiated at the points of dose rate at 1.89 kGy/h. In order to lower the dose uniformity ratio (DUR) and control the actual dose within the limits of 710% targeted dose, the treatment of gamma rays was processed carefully with low dose two-sided simultaneous irradiation. Four CTA FTR-125 (Fujifilm, Japan) Dosimeters were placed inside the hams bags described above in the areas where minimum and maximum absorbed dose were measured during extensive dose mapping. After irradiation, the dosimeters were read at 280 nm with a UV–Vis–NIR Spectroscopy UV-1800 (SHIMADZU, Japan) to determine the dose variation within each bag. Each treatment level was applied to three bags of hams, and repeated three times with nine bags of hams total. The CTA doses in the paper were calculated directly according to the instruction of the manufacturer, and the formula was as follows: D ¼[(ODɑ-OD0)/ K]
(0.125/ t)
f. OD0 and ODɑ were the absorbance before and after irradiation, respectively. “t” was the film thickness (0.125 mm). The dosimeters were measured within 2hrs after irradiation, and “f” value was 1.0. “k” value was 0.0063 for electron irradiation and 0.0081 for gamma ray irradiation. The routine dosimetry system of CTA was calibrated with transfer-standard dosimeter in a production irradiator. Each dose level had 6 CTAs and transfer-standard dosimeters respectively and repeated 3 times. The transfer-standard dosimeter was alanine dosimeter. The target doses were 2.0, 5.0, 8.0, 12.0, 20.0, 30.0, 50.0 and 70.0 kGy. The calibration was carried out according to ISO / ASTM 51261 (ISO/ASTM51261:2013E, 2013).

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2.3. Irradiation and storage Same irradiation method as "2.2. Dose uniformity ratio (DUR)" was conducted for this section samples. To keep samples cold (4 °C), single layers of samples were layered on the upper surface of ice-bags inside the thermal boxes during irradiation. The thermal box is made of soft plastic with less than 0.2 cm thickness and uses air as thermal barrier. The impact from the thermal boxes on the penetration of the electron beam was negligible. Cellulose Triacetate (CTA) Dosimeters FTR-125 (Fujifilm, Japan) and UV–Vis– NIR Spectroscopy UV-1800 (SHIMADZU, Japan) were used to measure the irradiation dose. The applied radiation dose levels were 0 (control), 3 kGy and 6 kGy. Each treatment level was applied to three bags of hams, and repeated three times (total nine bags of hams per dose level). After irradiation, the hams were stored in a 4 °C refrigerator for 7 days until analysis conducted. 2.4. Analysis of odor traits The sample odor was evaluated by a panel of 10 untrained sensory assessors, 5males and 5 females between 21 and 50 years old. Members were recruited from people who have participated in previous studies, and selected according to their familiarity with the dry-cured hams and had a certain sensitivity and ability to reproduce their evaluations. They were asked to rank the five coded samples according to the point of scales and judge the samples using a 10 point intensity line scale, where 0 ¼ not detected and 9 ¼ extremely strong (Marusic et al., 2014; Sara et al., 2014). Two traits related to odor characteristics of dry-cured hams were evaluated, which were grouped into positive odor and negative odor. The evaluations were conducted at midmorning before extraneous aromas and odors fill the air. A quiet room with sufficient space between the testers, adequate light and ventilation was arranged (Ferrentino et al., 2013). The testers were given three slices at room temperature (20–22 °C) from each sample, and served in a plastic tray with lid labeled with a random 3-digit code (Storrustløkken et al., 2015). Odor was evaluated after removing the lid by smelling. The attributes of each sample were given scores ranging from 0 to 9. Nine sensory sessions were carried out over three non-consecutive days with three sessions per day. Each panelist was presented with five samples (1. non-treated; 2. treated by 3 kGy γray; 3. treated by 6 kGy γ-ray; 4. treated by 3 kGy EB; 5. treated by 6 kGy EB) within a session in a randomized order (Gallardo-Escamilla et al., 2005; Ventanas et al., 2010). The variance of panelists and sessions were analyzed. The suitability and agreements of assessors were also assessed according to the standard ISO 8586 (ISO8586 2012E). 2.5. GC–MS analysis The volatile compound profile was obtained by SPME-GC–MS technique (Ahn et al., 2000). Ten gram of samples, finely cut with a knife, were subjected to a three-phase DVB/Carboxen/PDMS 75-μm SPME fibre (Supelco, Bellafonte, PA, USA). Samples were exposed in the head space of the vials at 50 °C for 30 min for volatile compound sampling after 5 min equilibration time. After extraction, the SPME fibre was immediately injected to a Finnigan TRACE GC–MS (Thermo Quest Finnigan Co., USA). The temperature of the injector, used in the splitless mode, was 250 °C and desorption time was 3 min The Finnigan TRACE GC–MS (Thermo Quest Finnigan Co., USA) equipped with a TRACE TR-5 capillary column (30 m long, I. D.0.25 mm, film thickness 0.25 mm, Thermo Fisher Scientific, Waltham, MA, USA) was used. Analysis was carried out by using

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helium as the carrier gas at 0.8 mL/min flow rate. Oven program was 40 degrees for 4 min, then 5 °C /min to 100 °C, and then 8 °C /min to 230 °C for 7 min The mass spectra were obtained at 70 eV with the m/z range of 33–500, and the ion source temperature was 200 °C. Mass spectra obtained were compared with those of known compounds in the Mainlib, Replib, Wiley and Nist library by using a computer. Determination of the percentage composition was based on peak area normalization (expressing the area of a given peak as a percentage of the sum of the areas of all the peaks) without the use of correction factors. Each sample was analyzed in three replicates. 2.6. Data analysis All results are presented as means 7 standard errors of triplicate measurements and analyzed by one-way analysis of variance. The statistical package SPSS (SPSS, Chicago, IL, USA) was used. The differences were considered significant at P r0.05.

3. Results and discussion 3.1. Dose uniformity ratio (DUR) Dose uniformity ratio (DUR) is the ratio of the maximum dose to the minimum dose in a product container. For research samples, the ratio should be close to unity (less than 1.05), where the objective is to correlate radiation effect in the sample to the dose (IAEA, 2005). But this ratio increases with the density of the product and the size of the container. For commercial operation, the DUR would be significantly larger for large containers, and the dose limit ratio is not lower than1.5 (IAEA, 2005). Previous research showed that 2–4 kGy irradiations were effective for reducing the level of pathogenic microorganisms (Galan et al., 2011). So two dose levels, 3 kGy and 6 kGy, were designed for prosciutto crudo irradiation. The DUR was shown in Table 1. For EB irradiation, the average DUR of 3.0 and 6.0 kGy were 1.05 7 0.02046 and 1.04 70.03210, respectively. For GR irradiation, the average DUR of 3.0 and 6.0 kGy were 1.03 70.00990 and 1.02 70.00820, respectively. The EB treatment was one-side irradiation by single beam (top only), while the GR treatment was two-sided simultaneous irradiation by low and rigidly controlled dose. This means EB irradiation can meet the DUR demand more easily than gamma ray irradiation. It must be pointed out that these results were obtained from a single layer of bags. It is much more complicated when irradiating a box of hams because of the large differences in densities inside of the boxes. When irradiating hams in bulk or commercial packages, a precise design of the irradiation process is needed to ensure uniform dose distribution. It is critical that the radiation dose be as uniform as possible throughout the hams being treated, which ensure the uniformity ratios of all irradiation treatments were within an acceptable level. Only single layer of bag of hams was treated in this study. 3.2. Analysis of odor traits It has been reported that ionizing radiation caused physical and chemical changes in packaging materials. During irradiation treatments, foods are normally processed in prepackaged form to prevent recontamination. Food packaging materials in the form of single- or multi-layered films or trays are also exposed to irradiation. As a result of such exposure, some chemical and physical properties of plastic packaging materials may undergo changes. Irradiation can sometimes lead to unwanted changes in polymer properties due to chain scission, cross-linking, free radical

Table 1 Dose uniformity ratio (DUR) of irradiated hams (Prosciutto crudo).

EB 3 kGy EB 6 kGy γ-ray 3 kGy γ-ray 6 Gy

Maximum thickness of bags cm

Maximum dose of bags kGy

Minimum dose of bags kGy

Dose uniformity ratio (DUR)

0.55 7 0.02464 0.56 7 0.05433 0.59 7 0.01554 0.577 0.02496

3.317 0.04001 6.317 0.01166 3.02 70.1801 5.87 70.2322

3.167 0.06518 6.05 7 0.09493 2.94 7 0.1627 5.737 0.1869

1.05 7 0.02046 1.047 0.03210 1.03 7 0.00990 1.02 7 0.00820

Data shown represent the mean 7 standard deviation (n ¼ 3). EB treatment is one-side irradiated using a single beam (top only). γ-ray treatment is two-sided irradiated using a stationary cobalt-60 irradiator.

formation, discoloration, etc. The types of changes are dependent on the plastic materials, additives, radiation dose and the conditions of irradiation. Polypropylene (PP) is one of the most sensitive polymer to irradiation and at higher irradiation dosage, these films become extremely brittle. However, irradiation up to 10 kGy usually induces no statistically significant changes of mechanical properties in the case of monolayer packaging materials and the PP-based multi-layer film (Goulas et al., 1995; Johnsy George, 2007). Therefore, interaction between irradiation treatments and the polypropylene bags used for containing the samples was not considered in this study. According to the standard ISO 8586 (ISO8586 2012E), the suitability and agreements of assessors were analyzed (Table 2). All ten assessors had significant variation between the treatments at level of α ¼0.01, which F value (4, 40) is 3.83 (ISO8586 2012E). This suggested that the scores given by the panel could describe the difference of the treatments. On the other hand, the interaction of assessors and samples was not significant, F values of positive scores were 0.85 and 0.46, and F values of negative scores were 0.88 and 0.65, while F0.05 value (40, 100) is 1.50 and F0.05 value (9, 100) is 1.96 (ISO8586 2012E). This suggested that the panelists agreed about the ranking of the samples and therefore were able to assess sample differences. In the odor traits test (Fig. 1), non-irradiated hams gave the highest positive scores and the lowest negative scores, while GRtreated ones at 6 kGy gave the lowest positive scores and the highest negative scores. Irradiation led to the development of offodors in a dose dependent manner. However, EB did not cause much off-odors compared to GR at the same dose level. The panel found no differences of odor between non-irradiated samples and samples EB-irradiated at 3 kGy. But for irradiation at 6 kGy dose, the positive odor scores of hams decreased significantly in sequence of control, EB and GR irradiation, while the negative odor scores increased significantly in the same sequence. So three treatments, control (0 kGy), 6 kGy by γ-ray and 6 kGy by EB were analyzed for volatile compounds. Our results also showed that not all panelists disliked the irradiated hams. Five assessors thought the hams treated by 3 kGy EB irradiation had much stronger positive odor than non-irradiated ones, and three assessors gave higher scores of positive odor to the hams treated by 3 kGy GR irradiation (Table 3). The assessors who preferred the irradiated hams thought they had more flavor than the non-irradiated hams. Similar results were obtained by M. L. BARI (Bari et al., 2006). 3.3. Analysis of volatile compounds Fifty-nine volatile compounds of prosciutto crudo were identified by gas chromatographic–mass spectrometry (GC–MS). These compounds were grouped into 8 chemical classes: 4 terpenes, 10

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Table 2 Analysis of suitability and agreements of assessors for odor scoring of irradiated hams (Prosciutto crudo). Number of assessors

1

2

3

4

5

6

7

8

9

10

F values of positive odor for different assessors Residual standard deviation of positive odor for different assessors F values of negative odor for different assessors Residual standard deviation of negative odor for different assessors

73.71n 0.940 14.73n 0.960

99.58n 0.730 13.47n 0.963

158.96n 0.753 71.63n 0.667

82.11n 0.767 45.61n 0.650

53.19n 0.869 25.93n 0.863

106.60n 0.792 23.74n 0.983

121.24n 0.683 14.43n 0.940

89.32n 0.789 29.52n 0.997

139.80n 0.820 9.42n 0.960

116.56n 0.847 6.16n 0.872

The analysis is processed according to the standard ISO 8586 (ISO8586 2012E). F0.01 (4, 40) value is 3.83 (ISO8586 2012E). n

Means result given by the assessor is significant at the level α ¼0.01.

esters and acids, and higher amounts of terpenes, alcohols, alkanes and aromatic hydrocarbons (Table 4). Ahn DU also reported that the amounts of aldehydes and ketones were not increased by irradiation. This suggests that, although most of the volatile compounds are originated from oxidation of unsaturated fatty acids, the major contributor of off-odor in irradiated hams is not lipid oxidation (Ahn et al., 2000). As the most abundant group of compounds, the amounts of alkanes of irradiated samples found in this study were higher than that of non-irradiated ones (Table 4). The result was consistent with the findings of Champaign and Nawar who found that hydrocarbons were the major radiolytic products in fat (Ahn et al., 2000). In addition, as the levels of most alkanes, the cyclo-compounds in irradiated samples were higher than that in non-irradiated samples. The same results were obtained from irradiated raw turkey breasts with aerobic package and cooked turkey breasts with vacuum package (Lee et al., 2004). In this study, irradiation resulted in losing of hexadecamethylheptasiloxane and decanoic acid-ethyl ester, and formation of (Z)7-Hexadecenal, cis-9-hexadecenal, tetradecane, E-9-tetradecen-1ol formate in hams. In addition, irradiated samples had significant lower amounts of z-8-Hexadecene, hexadecanal, octadecanal, 2-heptadecanone, 2-nonadecanone, n-nonylcyclohexane, hexadecanoic acid-methyl ester and N-(tert-butoxycarbonyl) glycine, and higher amounts of 8-heptadecene, 1-hexadecanol, and pentadecane (Table 4). Formation of heptadecene and pentadecene were also found in irradiated beef, veal, mutton, lamb and pork (Urbain, 1986). Although there were a lot of changes in varieties and levels of ham volatile compounds after irradiation, it is well accepted that only a limited number of volatile compounds contribute to the overall flavor. Decrease of positive odor score of irradiated samples in this study could be due to the loss of decanoic acid-ethyl ester and the lower level of hexadecanoic acid-methyl ester (Table 4). Decanoic acid-ethyl ester is an important flavor material, and had been reported in many kinds of hams and meat products (Diana et al., 2001; Huan et al., 2005; Pugliese et al., 2010). It was also reported that methyl-branched short-chain esters were positively

Fig. 1. Average Odor Scores of irradiated hams. The different letters indicate statistically significant difference at the level α¼ 0.05 between different treatments for positive odor or negative odor.

aldehydes, 2 alcohols, 4 ketones, 21 alkanes, 10 esters, 5 aromatic hydrocarbons and 3 acids. Alkanes were the most abundant group of compounds (Table 4). Different kinds of hams have their own main volatile compounds. Nives Marušić investigated the volatile compounds of Istrian dry-cured ham. The total of 92 and 50 volatile compounds were found in two separate researches, and aldehydes were the most abundant group in 92 compounds, while terpenes were the major groups representing of the total 50 compounds (Marusic et al., 2011; Marusic et al., 2014). Aldehydes were also reported as the most represented compounds in ‘Toscano’ ham, similar to results of Iberian lean tissue and Chinese Jinhua hams (Pugliese et al., 2010). Irradiation could change the components and proportions of the chemicals. Our results of volatile compounds analysis showed that, irradiated samples had lower amounts of aldehydes, ketones, Table 3 Statistical analysis of differences between assessors in odor scoring of irradiated hams. Number of assessors Average Average Average Average Average Average Average Average Average Average

positive Scores of control negative Scores of control positive Scores of EB-3 kGy negative Scores of EB-3 kGy positive Scores of γ- ray-3 kGy negative Scores of γ- ray-3 kGy positive Scores of EB-6 kGy negative Scores of EB-6 kGy positive Scores of γ- ray-6 kGy negative Scores of γ- ray-6 kGy

1

2 ab

8.22 0.33abc 8.67abde 0.44abcde 8.33ac 1.22abc 6.44af 3.22abe 2.22abc 7.89ab

3 ac

8.11 0.44b 8.56abde 0.22CE 7.67bde 1.78be 7.00abe 2.78bgh 2.56CE 7.44bf

4 d

9.00 0.00c 8.22BCE 0.78b 8.33ac 0.33cdf 5.11c 4.67c 1.44be 8.22ac

5 bce

8.44 0.00c 8.67abde 0.22CE 8.89c 0.11df 8.44d 0.44d 3.44fg 6.67de

6 de

8.78 0.22abc 8.56abde 0.33abcde 7.56de 0.67cf 6.67abe 2.33beh 3.56 f 6.78eg

7 de

8.89 0.11abc 7.78c 0.67abc 7.22ef 2.67e 5.56c 4.11acf 1.89cde 7.89ab

8 a

7.78 0.44b 8.89de 0.11d 8.44ac 1.00bf 7.33eg 2.56bgh 2.67cdfg 7.00def

9 bd

8.67 0.22abc 8.44abd 0.44abcde 6.89 f 4.22 g 5.78cf 3.56afg 2.56CE 7.22fg

10 de

8.89 0.00c 8.89de 0.22CE 7.33ef 2.22e 7.44eg 1.89hj 1.11e 8.22ac

8.56bcd 0.11abc 9.00e 0.00d 8.22abg 1.22abc 7.89dg 1.00jd 1.67be 8.11ac

Scores are given in a 10 point intensity line scale, where 0¼ not detected and 9¼ extremely strong. Data shown are average scores. Values in a row with different superscript letters indicate statistically significant difference at the level α ¼0.05.

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Table 4 Effects of irradiation on volatile compounds in hams (Prosciutto crudo). Volatile compound

Control 0 kGy

Gamma rays 6 kG

EB 6 kGy

terpenes z-8-Hexadecene 1-Pentadecene 3-Heptadecene, (Z)8-Heptadecene total

0.55 7 0.05251a 1.03 7 0.11131ac 0.577 0.11712a 0.50 7 0.10256a 2.65

0.47 70.0252b 1.29 70.01734b 0.50 7 0.05515a 2.46 7 0.05576b 4.72

0.45 70.02089b 1.09 7 0.09573c 0.497 0.04056a 1.99 7 0.19206c 4.02

aldehydes 2,4-Decadienal,(E, E)2-Undecenal 7-Hexadecenal, (Z)cis-9-Hexadecenal Dodecanal Hexadecanal Nonanal Octadecanal Octanal Tetradecanal total

0.217 0.36172a 0.81 7 0.12349a – – 0.53 7 0.07287a 15.39 7 3.04687a 0.75 7 0.07629a 1.19 70.03248a – 1.16 70.16179a 20.04

– 0.66 70.02520a 0.677 0.02003a 0.15 70.26017a 0.337 0.28972a 8.60 7 0.39336b 0.55 7 0.03517a 0.727 0.02003b – 0.88 7 0.06668a 12.56

0.26 70.22890a 0.89 70.26982a 0.55 70.03799b 0.40 70.04599a 0.46 70.07576a 9.917 2.02292b 1.047 0.25786a 0.737 0.09537b 0.487 0.08356 1.117 0.33317a 15.83

alcohols 1-Hexadecanol Cyclopentadecanol total

1.06 70.20519a 0.80 7 0.20420a 1.86

1.69 70.09305b 0.58 7 0.03219a 2.27

1.477 0.11309b 0.56 70.05527a 2.03

ketones 2-Heptadecanone 2-Nonadecanone 2-Pentadecanone Ethanone, 1,1′-(1,4-phenylene)bistotal

1.02 7 0.08168a 0.80 7 0.14678a 0.58 7 0.09057a 0.40 7 0.34555a 2.8

0.78 7 0.07516b 0.61 7 0.03219b 0.50 7 0.06668a 0.55 7 0.15970a 2.44

0.75 70.08028b 0.54 70.02089b 0.487 0.04014a 0.62 70.01533a 2.39

alkanes 2,2,4,4-Tetramethyloctane Cycloheptasiloxane, tetradecamethylCyclohexasiloxane, dodecamethylCyclononasiloxane, octadecamethylCyclooctasiloxane, hexadecamethylCyclopentasiloxane, decamethylCyclotetrasiloxane, octamethylCyclotrisiloxane, hexamethylDecane Dodecane Heptadecane Heptane, 2,2,4,6,6-pentamethylHeptasiloxane, hexadecamethylHexadecane n-Nonylcyclohexane Octadecane Pentadecane Tetradecane Tridecane Undecane 1-Monolinoleoylglycerol trimethylsilyl ether total

0.42 7 0.38119a 5.187 2.45007a 6.497 2.75688a 1.90 7 0.84527a 2.65 7 0.41668a 2.88 7 0.94028a 1.217 0.19490a 4.44 70.51865a 0.58 7 0.04631ab 1.077 0.18523a 1.55 7 0.24697a 4.98 7 0.93473ab 0.117 0.19253 1.81 7 0.34064a 0.58 7 0.05921a 0.86 7 0.75815a 2.44 70.19707a – 0.777 0.17425a – 1.68 70.05186a 41.6

0.80 7 0.17374a 4.93 7 1.07164a 7.647 0.83167a 1.667 0.57168a 2.54 7 0.28081a 4.42 7 1.89863a 1.677 0.16033b 3.80 7 0.36050a 0.63 7 0.04047a 1.337 0.07105a 1.477 0.20531a 6.68 71.61770a – 1.43 70.10163a 0.48 70.00578b – 4.50 7 0.20061b 2.65 7 0.06516a 1.02 70.12521a 0.28 7 0.24468 1.59 70.15557a 49.52

0.357 0.32983a 5.94 70.76780a 10.59 7 1.37928a 1.53 7 0.34388a 2.62 70.08111a 5.92 71.73582a 1.447 0.25512ab 4.18 71.07687a 0.497 0.9642b 1.03 7 0.16519a 1.477 0.01159a 3.69 71.48401b – 1.447 0.04374a 0.487 0.04374b 0.917 0.05794a 4.08 70.57910b 2.447 0.34766a 0.84 70.09883a – 1.23 7 0.11174b 50.67

esters 1,2-Benzenedicarboxylic acid, bis(2-methylpropyl) ester 2,6-Difluoro-3-methylbenzoic acid, hexadecyl ester Decanoic acid, ethyl ester Dichloroacetic acid, tridecyl ester Hexadecanoic acid, ethyl ester Hexadecanoic acid, methyl ester Phthalic acid, 2-cyclohexylethyl butyl ester Tetradecanoic acid, ethyl ester E-9-Tetradecen-1-ol formate Ethyl Oleate total

1.19 70.39621a 2.65 7 0.51381a 3.26 7 0.65382 0.59 7 0.15785a 0.32 7 0.28456a 2.017 0.57386a – 0.44 70.75844a – 0.117 0.19253a 10.57

0.737 0.63326a 3.25 7 0.21579a – 0.62 7 0.11947a 0.48 70.03611a 1.137 0.03517b 0.49 70.10278 1.05 70.09657a 1.707 0.08517a 0.28 7 0.24618a 9.73

1.40 7 0.17838a 2.677 0.11053a – 0.647 0.05880a – 1.107 0.05149b – – 1.28 7 0.11573b – 7.09

aromatic hydrocarbons Benzene, 1,3-bis(1,1-dimethylethyl)1,4-Benzenediol, 2,6-bis(1,1-dimethylethyl)Phenol, 2,6-bis(1,1-dimethylethyl)-4-(1-methylpropyl)Oxime-, methoxy-phenylIndan, 1-methyl-3-nonyltotal

4.047 2.20155a 1.55 7 0.27507a 1.71 70.25249a 0.63 7 0.17238a 0.58 7 0.10899a 8.51

5.26 7 0.40471a 1.43 70.06438a 1.41 70.08749ab 0.79 7 0.17717a 0.47 70.01734a 9.36

6.03 71.51911a 1.26 7 0.01738a 1.25 7 0.06104b 0.58 70.09485a 0.46 70.04178a 9.58

Q. Kong et al. / Radiation Physics and Chemistry 130 (2017) 265–272

271

Table 4 (continued ) Volatile compound

Control 0 kGy

Gamma rays 6 kG

EB 6 kGy

acid 3-Amino-2,3-dihydrobenzoic acid N-(tert-Butoxycarbonyl)glycine Glycine total

3.30 7 0.93587a 7.22 7 1.2584a 0.86 7 0.79184a 11.38

3.30 7 0.10455a 5.677 0.05299b 0.43 7 0.44077a 9.4

2.56 7 0.14622a 5.32 7 0.24354b – 7.88

packaging contaminants Dibutyl phthalate

0.62 7 0.05053a

–

0.50 7 0.11573a

Data shown are the average percentage of the total area of GC, standard errors, (n ¼3). Values in a row followed by a different superscript lowercase letters are significantly different (po 0.05).

related to dry-cured ham flavor of Italian type dry-cured ham (Maria et al., 1993). Although both irradiation treatments resulted in changes of volatile compounds in hams, our sensory study indicated that EB irradiation was better in maintaining ham's original odor than GR irradiation (Fig. 1). GR irradiation resulted in losing of (E, E)-2,4decadienal and octadecane, which are the most potent odorants of meats (Christlbauer and Schieberle, 2011; Resconi et al., 2013; Shahidi et al., 1986). However, these volatile compounds were not affected by EB irradiation (Table 4). In addition, GR irradiation resulted in formation of undecane and phthalic acid-2-cyclohexylethyl butyl ester, and significantly higher contents of 1-pentadecene, 8-heptadecene, (z)-7-hexadecenal and E-9-tetradecen-1ol formate. We suspect that the poor odor score of GR-treated samples is related to the changes in these volatile compounds caused by GR irradiation (Table 4).

4. Conclusions Irradiation is an effective way to eliminate pathogens present in foods, but it also produces changes in the aroma, color and flavor that could significantly affect consumer acceptance. The results presented here indicate that irradiation induce the dose-dependent odor changes of dry-cured ham (Prosciutto crudo). Significant differences in odor scores and volatile compounds were observed between control and two irradiation treatments at 6 kGy, and formation of (Z)-7-Hexadecenal, cis-9-hexadecenal, tetradecane, E-9-tetradecen-1-ol formate, and losing of hexadecamethyl-heptasiloxane and decanoic acid-ethyl ester were detected in irradiated hams. Recent advances in electron beam technology have made this mode of sterilization a worthy competitor to the traditional gamma processing. Our results indicate that electron beam irradiation was better in maintaining hams normal odor compared to gamma ray irradiation at the same dose level. Results of GC–MS showed that gamma rays irradiation caused additional changes, such as formation of undecane and phthalic acid-2-cyclohexylethyl butyl ester, losing of (E, E)-2,4-decadienal and octadecane, and significant higher level of 1-pentadecene. Therefore, EB irradiation is a better alternative non-thermal processing technology for drycured ham (Prosciutto crudo).

Acknowledgments This work was funded by the financial supports from China government (2011AA100804, 201103007, 2014BAA03B05). We are grateful to Chunbao Li, Nanjing Agricultural University, China, for the technical assistance.

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