A combination of postmortem ageing and sous vide cooking following by blowtorching and oven roasting for improving the eating quality and acceptance of low quality grade Hanwoo striploin

Objective It is well recognized that beef cuts from a low quality grade are usually associated with tougher, drier and less flavorful. Thus, the present study aimed at investigating the combined effects of postmortem ageing and sous vide (SV) cooking followed by oven roasting or blowtorching on the eating quality of low quality grade Hanwoo beef striploins. Methods Hanwoo beef striploins (quality grade 3) obtained from 36 month-old Hanwoo steers were used, and the samples were chiller aged for 0 and 14 d at 4°C. After ageing, the samples were prepared into 2.5-cm steaks which were then SV cooked at 55°C for 5 h and then raised to 60°C for 1 h, and thereafter the SV-cooked the steaks were further roasted in oven for 20 min (SV+OV) or blowtorched (SV+TC) for 2 min. The cooked samples were analyzed for microbiological quality, browning index, Wanrner-Bratzler shear force (WBSF), aroma flavor compounds and sensory properties. Results The SV cooking significantly reduced the WBSF values in beef samples (p<0.05). Blowtorching after SV cooking led to a browner surface of the beef steaks (p<0.05). The samples treated with SV+OV or SV+TC exhibited higher levels of Maillard reaction-derived aroma flavor compounds such as; pyrazines and sulfur-containing compounds compared to those just SV cooked. More especially, the SV+OV− or SV+TC− treated samples presented significantly higher flavor and overall acceptability scores compared to those just SV cooked (p<0.05). Ageing beef for 14 d significantly improved the tenderness by reducing the WBSF and increasing the tenderness scores. Conclusion Thus, the combination of postmortem ageing and SV cooking followed by additional treatments (blowtorching or oven roasting) could be used to improve the eating quality especially tenderness and flavor as well as overall acceptability of low grade Hanwoo beef.


INTRODUCTION
Hanwoo beef is one of the most popular cattle meat types in Korea due to its unique pal atability [1,2]. The most important factor making Hanwoo beef more palatable, juicier and flavorful compared to other imported beef breeds is its high marbling or intramuscular fat level [3]. Because, marbling has been proven to positively affect eating quality attributes such as tenderness, flavor and juiciness [4]. It is well recognized that eating quality attributes especially tenderness and flavor are extremely important factors determining purchasing decision by consumers for beef in many meat markets [5]. A recent study by O'Quinn et al [6] also reported that tenderness and flavor become the most important drivers of beef eating satisfaction. In Korea, studies regarding the con sumer's choice have shown that Korean consumers much prefer Hanwoo beef and they are willing to pay more money for the more abundantly marbledHanwoo beef because it is regarded as a premium product [7]. In the Korea meat industry, 24 h after slaughter, all cattle carcasses are evalu ated for meat quality grade by official graders using Korean carcass grading system (KAPE) [8], and five different quality grades such as; 1 ++ , 1 + , 1, 2, and 3 are classified, mainly de pending on the degree of marbling and other parameters such as meat and fat color [9]. As a result of Korean con sumers' preference, the average prices for the higher quality grades such as; grade 1 + and 1 ++ loin cuts are about 54 and 64 US dollars which are several times higher than the average prices (24 US dollars/kg) for those from the lower quality grade groups (e. g., grade 2 and 3) [8] The Hanwoo beef cuts from low quality grade group (e.g., grade 3) are not only cheaper in price but also of redundancy. Additionally, the beef cuts with low quality grade are often sold at a discount. This is, in part because the beef cuts with low quality grade are tougher and less flavorful [10]. In order to deal with the problem, solutions include; applying postmortem ageing methods or/using suitable methods of cooking. In meat in dustry, postmortem ageing is the most commonlyapplied method to enhance tenderness and flavor of beef [11].
Sous vide, initially developed in France since 1970s, is a method of cooking that involves sealing the food in plastic bag and placing it in a water bath at a specific temperature (50°C to 70°C) for an extended period of time (several hours to multiple days) [12]. This cooking method allows for greater control and consistency to achieve higher uniformity in degree of doneness [13]. In meat cooking, the sous vide technique has become a method to produce good textural properties (e.g., tender and juicier) and better sensory quality, and mini mizing shrinkage of meat [13,14]. Furthermore, sous vide is one of the cooking methods effectively increasing the low value of tough meat cuts by transforming into tender meat cuts [15,16]. Thanks to these advantages, perhaps, sous vide is widely known and extensively used in restaurants and by foodies over the world in recent decades [16,17] Apart from these advantageous points, however, the sous vide cooking method also has some drawbacks such as; the sous videcooked meat usually lacks browning on the surface and roasted aroma characteristics [16]. It is well known that the development of the roasted aroma and surface brown color in meat during cooking results from the Maillard reaction between amino acids and reducing sugar, and lipid oxidation [18]. Therefore, the lack of surface brown color and roasted aroma notes in the sous videcooked meat probably is due to the low temperatures (around 60°C to 80°C) and less dehy dration on the meat surface [18]. In order to overcome this problem, chefs frequently roast or fry the surface of sous vide cooked meats [17]. More recently, researchers have coupled the sous vide cooking with roasting (sous vide followed by roasting or roasting before sous vide) and reported that roast ing either before or after sous vide results in lamb meat with a browner surface and a more intense cooked meat flavor [16].
Searing with a blowtorch is one of the best way to enhance the brown and roasted flavor on the outside of a beef steak. This is due to the chemical reactions such as Maillard reac tion that takes place when amino acids and sugar are heated to a high temperature [19] Though the blowtorching and sous vide method has been widely used in cooking beef [15], with the best of our knowledge, no attention has been paid to investigating the combined effects of sous vide cooking with roasting/or blowtorching on the final eating quality of beef. We hypothesized that roasting and blowtorching with their extremely different heat levels may produce various ef fects on the Maillard reactions (e.g., browning and volatile aroma compounds) which subsequently affects the color and flavor characteristics of sous vide cooked meat. Therefore, the objective of this study was to investigate the combined effects of postmortem ageing and sous vide cooking followed by roasting/or blowtorching on the eating quality properties and volatile flavor compounds of low quality Hanwoo beef cut.

Beef cut samples
Beef striploins (n = 15) obtained from carcasses (lowquality grade 3) of Hanwoo steers (36 months old) after 24 h slaughter were used in the present investigation. Each beef striploin was vacuumpackaged individually in plastic bags, kept in cooler box and transported to the Laboratory of Meat Science, Chonbuk National University for analysis.

Postmortem ageing treatment
For chiller ageing treatment, each of the striploins were di vided into two equal portions (a total of 30 portions were made from the 15 striploins), and a half (n = 15; 7 from cra nial and 8 from caudal portions) were assigned to the 0day ageing while, the remaining portions (n = 15) were assigned to the 14day ageing treatment. The 0day aged samples were immediately used for analyses (on the sampling day) while, the samples assigned to the 14day ageing were individually vacuumpackaged in a nylon/PE vacuum bag, and then aged in an ageing room at 4°C.

Sous vide cooking
When the ageing was completed, all the 0 and 14 daged samples were subjected to sous vide cooking treatment. For the sous vide cooking, each aged sample was made into 2.5 cm thick steaks (30 steaks per ageing group) which were then 80% vacuumpackaged individually in plastic bags (nylon/ polyethylene pouches). The steaks were sous vide cooked at 55°C for 5 h and then raised to 60°C for an additional 1 h in a temperature controlled water bath as described in our pre vious study [20]. Once the cooking was completed, the cooked samples were removed from the water bath and cooled for 30 min in a circulatory water bath and then chilled overnight in a chilling room.

Roasting and blowtorch treatment of sous vide cookedsteaks
In order to achieve the sous videcooked beef steaks with brown color on their surfaces as well as attractive roasted flavors, these samples were subjected to additional cooking treatment such as roasting and blowtorching. Out of the 30 sous vide cooked steaks (in each ageing group), ten of those were considered as control, another ten were roasted at 250°C for 20 min in an oven with dry air (SV+OV) [16], and the rest of the steaks (n = 10) were blowtorched for 2 min on both sides of the steaks (1 min per side) (SV+TC). Thereafter, each 7 steaks from the sous vide cooking (SV), SV+OV, and SV+TC at each ageing period were used for volatile flavor compounds analysis and sensory evaluation, while the remaining 3 steaks were placed in plastic bags, sealed and stored overnight at 4°C for microbial analysis and instrumental color measure ment.

Wanrner-Bratzler shear force value measurement
In order to determine whether the ageing and sous vide cook ing enhances tenderness of the beef samples, WanrnerBratzler shear force (WBSF) was measured on the steaks cooked to a standard core temperature of 70°C (control) as recom mended by American Meat Science Association (AMSA) [21] and those cooked by the sous vide method. The WBSF was measured on cores (5 cores/steak) with an average di ameter of 0.5 inches removed parallel to the muscle fiber direction using a 0.5inch metal corer. The WBSF values were obtained by completely cutting the cores in an Instron Universal Testing Machine (Model 3342, Instron Corp, Nor wood, MA, USA) using a Vshaped WB blade at a crosshead speed of 200 mm/min and a 40 kg load cell.

Microbial analysis
Raw beef before and after sous vide cooking (after chilling overnight) was used for microbial analysis. Immediately af ter removing the plastic bags, microbiological samples were aseptically taken by swabbing on 10 cm 2 surface of sample, transferred to sterile plastic tubes containing 10 mL of pep tone water. After vortexing, serial dilutions were then made in the peptone water. Aerobic bacteria were determined by spreading 1mL of appropriate dilutions onto 3M Petrifilm Aerobic Count Plate (3M Health Care., Paul, MN, USA) while, lactic acid bacteria (LAB) were determined by spreading 0.1 mL of appropriate dilutions onto Man Rogosa Sharpe (MRS) agar plates. The plates were then incubated for 24 h at 37°C. Each sample was done in duplicates and total aerobic bacteria as well as LAB were expressed as log 10 numbers of colony forming units/gram (cfu/cm 2 ).

Sensory evaluation
The SV, or SV+OV and SV+TC striploin steaks were assessed by a 12member untrained panel using a descriptive analy sis method following the methods of Deda et al [22] and Meilgaard et al [23] with suitable modifications. The member panels were chosen from students, researchers and faculty at the university. Prior to the assessment, the samples were warmed up for 1 min in a microwave. Thereafter, each the steak was prepared into 6 slices (40 mm×40 mm×4 mm) which were then assessed by 6 panelists. The slices were dispensed on individual plates and served to the panelists. The samples were evaluated for flavor, juiciness, tenderness and overall acceptability using a 7point hedonic scale. In which: tenderness (7 = extremely tender, 6 = very tender, 5 = moderately tender, 4 = neither tender nor tough, 3 = mod erately tough, 2 = very tough, and 1 = extremely tough); juiciness (7 = extremely juicy, 6 = very juicy, 5 = moderately juicy, 4 = neither juicy nor dry, 3 = moderately dry, 2 = very dry and 1 = extremely dry); flavor and acceptability (7 = like extremely, 6 = very like, 5 = moderately like, 4 = neither like nor dislike, 3 = moderately dislike, 2 = very dislike, 1 = dislike extremely). Drinking distilled water and saltfree crackers were provided between samples to cleanse the palate. All sensory sessions were carried out in the sensory panel booth room equipped with white lighting.

Volatile flavor compound identification
Following the sensory evaluation, the remaining samples were immediately used for analysis of flavor compounds. The com ponent of volatile flavor compounds was analyzed by solid phase microextraction (SPME) following the procedure of Ba et al [24]. Briefly, about 1.0 g of sample was taken and placed into a 20mL headspace vial (Part No. 51882753, Agilent, Santa Clara, CA, USA) and 1.0 μL of internal stan dard (2methyl3heptanone, 816 mg/mL in methanol) was also added. The vial was then sealed with PTFEfaced silicone septum for extraction. A SPME device containing carboxenpolydimethylsiloxane (75 μm) fiber (Supelco, Bellefonte, PA, USA) was used for extraction of the compounds. All steps such as; extraction, absorption, desorption of the flavor com pounds were done using a fully automated SPME sample preparation instrument (Model: AOC5000 Plus) connected to gas chromatography (GC) (Model: 7890B GC) with mass spectrophotometry (MS) (Model: 5977B MSD, Agilent Tech nologies, USA). The extraction was carried out at 65°C and agitated at 250×rpm for 60 min. Volatiles were separated on a DB5MS capillary column (30 m×0.25 mm i.d.×0.25 μm film thickness, Agilent J & W Scientific, Model No.1225532, Folsom, CA, USA). The GC/MS conditions set were same as those by Ba et al [25]. Compounds were identified by com paring their mass spectra with those already present in the Wiley registry of mass spectral data (Agilent Technologies, USA) and by comparing their retention times with those of external standards. Approximated quantities of the volatile compounds were quantified by comparison of their peak areas with that of the internal standard obtained from the total ion chromatogram using a response factor of 1. Each steak sample was analyzed in duplicates.

Statistical analysis
The Statistic Analysis System (SAS) package (SAS Institute, Cary, NC, USA; 2007) was applied for analyzing the obtained data. Means and standard errors were calculated for the vari ables. The data were analyzed by using the general linear model procedure considering ageing and cooking as the main effects. Means were compared using Duncan's multiple range test. The correlations between the cooking treatments (SV, SV+OV, and SV+TC) with quality parameters, volatile flavor compounds and sensorial traits were determined using Pear son's linear correlation coefficient. The significance was defined at p<0.05. Table 1 shows the WBSF values of striploin steaks cooked to core temperature of 72°C (a standard core temperature used to determine shear force as recommended by American Meat Science Association [21]) and those cooked by the sous vide method, as well as the microbial count of steaks before and after sous vide cooking. Till now, WBSF is the most widely used instrumental method to evaluate meat tenderness, and its results are good predictions of tenderness ratings. Basically, the higher WBSF values obtained from meat are negatively related to consumer perception of tenderness and overall acceptance [26]. Results showed that the WBSF value (55.30 and 35.50 N after 0 and 14 d ageing, respectively) of striploin steaks cooked to the core temperature of 72°C (standard) were significantly higher than the values (42.26 and 23.43 N after 0 and 14 d ageing, respectively) of those cooked by sous vide method (p<0.05). It was estimated that after the sous vide cooking the shear force of steaks was reduced by nearly a half. Our results are in good agreement with the finding of Ismail et al [15]. Ageing significantly reduced the WBSF of the samples regardless of cooking method (70°C cooking or sous vide), however, it was observed that ageing for 14 d followed by sous vide cooking produced the steaks with the lowest shear force (23.43 N). Especially, when compared to the shear force values (2745 N) of semitendinosus muscle steaks (from same cattle breed and same quality grade) cooked with sous vide under different durations and temperatures [15], all the striploin steaks cooked by sous vide method had considerably lower value after 14 d ageing. This implies that the combination of postmortem ageing and sous vide cook ing could effectively improve tenderness of the low quality grade beefderived steaks. Researchers have found that meat toughness increases when cooked at temperature ranges of 40°C to 50°C and 60°C to 80°C whereas, it decreases between 50°C and 60°C [27]. In the present study, the steak samples were sous vide cooked at 55°C to 60°C, and the decrease in WBSF values may be due to the denaturation of perimy sium connective tissue and collagen shrinkage [15]. On the other hand, previous studies reported that sous videpro cessed products generally exhibit a highly microbial growth during storage [28]. In the present study (as displayed in Table 1), total aerobic plate count (APC) and LAB for the raw steaks (before sous vide cooking) at 0 and 14 d ageing were found at 2.39 and 1.16 (log 10 CFU/cm 2 ) and 4.71 and 2.30 (log 10 CFU/cm 2 ), respectively. Noticeably, no APC or LAB growth was detectable in the sous videcooked steaks, meaning that under the sous vide cooking conditions em ployed in this study, all the microbes were eliminated, which agrees with the finding of Jeong et al [29].

Effects of ageing and cooking treatment on color properties
The effects of sous vide cooking and/or following by oven roasting or blowtorching on the color parameters of striploin steaks are presented in It is recognized that cooking meats with sous vide gener ally has some drawbacks such as, the lack of surface brown color that is not attractive to consumers. In order to counter act this problem, in the present study, after sous vide cooking the steaks were subjected to high temperatures treatment such as roasting in oven or blowtorching. Results revealed that blowtorching after sous vide (SV+TC) significantly increased the BI values on the surface compared to the sous vide (SV) or sous vide followed by oven roasting (SV+OV) (p<0.05). Similar observation reported by Jorge et al [16] for lamb loin sous vide cooked for 12 h followed by roasting for 15 min at high temperature. Researchers have proven that the brown color on meat' s surface results from formation of colored com pounds named melanoidins through the Maillard reaction [32]. The mechanism underlying the increased BI values may be related to the more intense dehydration of the steak's sur face which favors for the formation of brown compounds. Interestingly, postmortem ageing was found to increase the BI values; after 14 d ageing all the sous vide cooked samples or those followed by roasting and blowtorching had signifi cantly higher BI values compared to those aged for 0 d (p< 0.05). This result could be explained due to the increased re lease of meat juice to the meat surface during postmortem ageing as a result of proteolytic activity, since this juice con tains a huge amount of flavor precursors (e.g., free amino acids and reducing sugars) which subsequently participate in the Maillard reaction (browning reaction) at high temperature (blowtorching) to produce the colored compound as well as aroma compounds. In accordance with the above described results, the result of Pearson's correlation (Table 3) also shows that the sous videcooked samples were positively correlated to lightness (r = 0.994) and negatively correlated to BI (r = -0.900). While, the sous videcooked samples followed by blowtorching (SV+TC) were positively correlated to BI (r = 0.969) and negatively correlated to lightness (r = 0.967).

Effects of ageing and cooking treatment on volatile flavor compounds
Volatile flavor compounds mainly responsible for the de velopment of aromas of cooked meat are formed from the precursors (e.g., amino acids, reducing sugars and fatty acids etc.) in meat during cooking/heating [33]. Till now, thousands of volatile compounds have been identified from various cooked meat types and many out of them have been reported to significantly contribute to flavor characteristics of cooked meat [34]. It was proven that the three main routines re sponsible for producing the volatile compounds in cooked meats are i) Maillard reaction (a chemical reaction between amino acids and reducing sugars), ii) lipid oxidation/deg radation, and iii) the interaction between the lipiddegraded products with Maillard reaction products [34]. Noticeably, the quality and quantity of the volatile compounds gener ated strongly depend upon the level of heat (vary among the cooking methods) applied on the meats for instance; the Maillard reaction is intensified at above 140°C (e.g., roast ing and grilling) while, the lipidsoxidized and degraded products are more predominant at lower cooking tempera ture such as in boiled and steamed mea products [33,34]. The effects of ageing and cooking treatments on the number and quantity of volatile flavor compounds are summarized in Table 4. The outcome of our analysis revealed that a total of 48 volatile compounds were tentatively detected and iden tified in the Hanwoo beef striploin steaks sous vide (SV) cooked or SV followed by oven roasting (SV+OV) and fol lowed by blowtorching (SV+TC). Based on their chemical families, these compounds were classified into the follow ing classes: aldehydes (17), alcohols (8), pyrazines (5), sulfur containing compounds (5), furans (2), and hydrocarbons (11). Aldehydes were the most predominant class found in this study, which agrees well with the finding of Kim et al [35] and Ba et al [25]. Previous studies have reported that alde hydes associated with green, fatty and fruity odor notes at very low odor detection threshold are important in the develop ment of flavor characteristics cooked meat [34,36]. Six out of 17 aldehydes were significantly affected by the cooking treatment (p<0.05). Amongst, 2methyl butanal and 3 methyl butanal are the Strecker degradation (a part of Maillard re action) products of leucine and isoleucine, respectively [37]. The amounts of these two compounds were higher in the SV+TCtreated samples compared to those treated with SV+OV (p<0.05) whereas, they were not detectable in the   0.01 ± 0.002 0.01 ± 0.001 0.01 ± 0.001 nd, not detectable. 1) IM, identification method: The compounds were identified by either mass spectra (MS) from library or authentic standards (STD).
2) SV, sous vide cooking at 55°C for 5 h and raised to 60°C for 1 h; SV+OV, sous vide cooking followed by oven roasting at 250°C for 20 min; SV+TC, sous vide cooking followed by blowtorching for 2 min. Means within a same row with different superscripts ( a-c ) differ significantly at (p < 0.05). Means within a same column in each parameter with different superscripts ( A,B ) differ significantly at (p < 0.05).
sous videcooked samples. This implies that the Maillard reaction of the amino acids was enhanced due to the in creased temperature during cooking treatment. Furthermore, benzaldehyde (the oxidized product of C18:3n3), and oc tanal (the oxidized product of C18:1n9) [38], whose amounts were also significantly higher in the samples treated with the SV+TC compared to those just sous vide cooked (p<0.05).
Octanal is an important compound that contributes to the fruityfatty and oily odor notes of cooked meat [39]. The post mortem ageing apparently showed a minor effect on the aldehydes; only nonanal decreased whereas, Enonenal in creased its amount with increased ageing time up to 14 d (p<0.05). Additionally, the cooking treatment (e.g., blow torching) considerably affected the total amount of aldehyde class ( Table 5). The total aldehydes were significantly higher in the samples treated with SV+TC compared to those just sous vide cooked (p<0.05) ( Table 5). These results could be explained due to the two steps of cooking treatment: the sous vide cooking leads to the formation of lipid oxidationderived aldehydes and free radicals which favor the rate of aldehydes production from the lipid oxidation reactions during further roasting and blowtorching treatment [16]. In line with these observations, the result of Pearson' s correlation analysis ( Table  3) also show that the samples treated with SV+TC were posi tively correlated to 2methyl butanal (r = 0.956), 2methyl butanal (r = 0.897), and Nonanal (r = 0.891) (p<0.05). This means that increasing the cooking temperature for instance; by using oven roasting at 250°C or blowtorching after the sous vide cooking favored the generation of these aldehydes which are associated with the pleasant odor notes as men tioned above. Alcohols have been reported to contribute to cooked meat flavors due to their low odordetection threshold [34]. The alcohols were the third most predominant class of volatiles, with 8 compounds were found in the samples at 0 and 14 d ageing. Most of these alcohols have been identified and re ported for cooked meats in literature [25,34,40]. Results showed that only 1octen3ol (the product of C18:2n6 oxidation) showed statistical difference among the treatments, with its higher amount being found in the samples treated with SV+ OV or SV+TC compared to those just sous vide cooked (p< 0.05). Two interesting exceptions were 1octanol and 1un decanol, which were not found in the sous videcooked samples, but were found in the samples roasted or blow torched after the sous vide cooking.
Pyrazines, the Maillard reactionderived products have been reported to be associated with pleasant odor notes (e.g., roasty) which significantly contribute to the cooked meat flavor [34,36]. It seems that the most interesting result obtained in the volatiles analysis was the pyrazine class. Particularly, only one compound (3ethyl2,3methyl pyrazine) was found in the SVcooked samples while, all the five identified pyr azines were found in the samples sous vide cooked followed by oven roasting (SV+OV) or blowtorching (SV+TC). Espe cially, the amounts of most of pyrazines such as 3ethyl2,5 dimethyl pyrazine and trimethyl pyrazine were significantly higher in the samples treated with SV+TC compared to those treated with SV+OV (p<0.05). Similarly, previous studies have reported that pyrazines are the major class of volatile compounds in meats grilled at high temperature [18,34]. The ageing led to an increase in level of 3ethyl2,3methyl pyr azine in the samples treated with SV+OV and SV+TC (p< 0.05). Furthermore, the total amount of pyrazines were sig nificantly higher the samples treated with SV+TC followed by the SV+OV and SV on both ageing days (p<0.05) ( Table  5). As far as the pyrazines are important, the amount of the pyrazines increased in the oven roasting or blowtorching after sous vide, such increases may be the consequence of more intense Maillard reaction due to the high heating treatment.
In contrast, only one pyrazine found in the SV cooked sam ples as mentioned above, probably is due to the low cooking temperature (55°C to 60°C) and higher moisture content of the surface that inhibits pyrazines production by the Maillard reaction. The result of Pearson's correlation analysis (Table 3) also revealed that the SV+TC treated samples were highly correlated to Methyl pyrazine (r = 0.896), 3ethyl2,5dimethyl pyrazine (r = 0.981), Trimethyl pyrazine (r = 0.936) and total pyrazines (r = 0.891) (p<0.05). Sulfurcontaining heterocyclic compounds are known to be the products derived from the Maillard reaction or the Strecker of sulfurcontaining amino acids. The sulfurcon taining compounds associated with roasty, meaty and onion odor notes play a key role in the cooked meat flavors [19,34]. Results showed that only three (carbon disulfide, 2thiophene methanol and 3phenyl thiophene) out of the five sulfur containing compounds were found in the SVcooked samples whereas, all of them were found in the samples treated with SV+OV and SV+TC. No statistical differences occurred among the SV, SV+OV and SV+TC for all the compounds (p>0.05). In general, most of these sulfurcontaining com pounds have been reported for cooked meats [25,40]. The ageing apparently had a minor effect on the sulfurcontaining compounds; 2thiophen methanol increased in its amount, and 3phenyl thiophene decreased in its amount in the sam ples treated with the SV+TC and SV+OV, respectively after 14 d ageing (p<0.05). The reason behind the increased amounts of these two compounds could be related to increased levels of flavor precursors (sulfurcontaining amino acids: cysteine and methionine) resulting from the proteolysis during the postmortem ageing.
Though the amount of individual sulfurcontaining com pounds showed no statistical difference as mentioned above, the total amount of this class was significantly higher in the samples treated with the SV+OV or SV+TC compared to those just SV cooked (p<0.05) ( Table 5). Thus, the oven roasting or blowtorching treatment after the sous vide partly stimu lated the generation of sulfurcontaining compounds, thus probably enhancing the intense flavor characteristics of sous vide -cooked beef.
Finally, furans and hydrocarbons are mainly derived from the oxidation of lipids such as C18:2n6 and C18:3n3 [34,38]. These volatile classes possess a high odor detection threshold, thus contributing less significantly to flavor development of cooked meat [34]. In the present study, hydrocarbons were the second most predominant class (11 compounds) found in the samples. However, only three compounds (3,4di methyl heptane, 2methyl heptane and hexadecane) showed statistical difference among the treatments (p<0.05). It may be said that the cooking treatment (e.g., roasting or blow torching after sous vide) and ageing had a minor effect on the components of the hydrocarbons class. This is in agree ment with result reported by Roldan et al [18] for sous vide cooked lamb loin.
Overall, it seems then clear that the sous vide cooking at low temperature (55°C to 60°C) resulting in a lack of surface dehydration leads to a smaller quantity of the volatile com pounds derived from Maillard reactions in the cooked beef steaks. However, this drawback could be overcome by com bining sous cooking with other more intense heat treatment such as roasting in oven or blowtorching. As a consequence, these thermal treatment processes boosted the Maillard re action on the surface, thus promoting the generation of Maillard reactionderived flavor compounds such as pyrazines and sulfurcontaining compounds which subsequently may en hance the roasty and meaty intensity of cooked beef.

Effects of ageing and cooking treatment on eating quality properties
Satisfaction is an important factor determining the quantity of meat that is purchased by consumers [5,6], and the most important aspect of meat quality is eating quality, usually defined as scores given by taste panelists. The effects of age ing and cooking treatment on the sensorial traits are presented in Table 6. As expected, the flavor scores given by the panelists were significantly higher in the samples treated with sous vide + blowtorching (SV+TC), followed by sous vide + oven roasting (SV+OV), and the lowest in the samples just sous vide cooked (p<0.05). The results indicating the higher flavor scores for the samples roasted or blowtorched after sous vide cooking may be explained due to the more intense cooked flavor caused by higher amount of distinct flavor compounds such as pyrazines and sulfurcontaining compounds (Tables  4, 5). On the other hand, the results of Pearson's correlation analysis also revealed that the flavor score was positively cor related to Methyl pyrazine (r = 0.985), 2,4dimethyl pyrazine (r = 0.916), 3ethyl2,5dimethyl pyrazine (r = 0.987), tri methyl pyrazine (r = 0.963) and 2methylthiophene (r = 0.867) (data not shown). This implies that the flavor inten sity of cooked beef increased with increased concentrations of these flavor compounds.
Regarding juiciness, blowtorching after sous vide cooking (SV+TC) did not show a negative effect on the juiciness com pared to the samples just sous vide cooked, as no statistical difference in juiciness scores occurred between the SV and SV+TC (p>0.05). Unexpectedly, the samples roasted in oven after sous vide cooking exhibited a drier appearance, indicated by a significantly lower juiciness score compared to those just sous vide cooked or blowtorched after sous vide (p<0.05). This result could be related to the excess dehydration or moisture loss caused by roasting in oven for a long duration (20 min). Our result is in agreement with finding reported by Jorge et al [16] for lamb loin. Additionally, the Pearson's correlation analysis also revealed that the samples roasted after sous vide cooking was negatively correlated to the juiciness score (r = -0.985).
A similar trend as observed on the juiciness was found for the tenderness in which the samples roasted in oven after the sous vide cooking (SV+OV) presented a significantly lower tenderness score compared to those just sous vide cooked or sous vide cooked followed by blowtorching (SV+TC) (p< 0.05). This result could be due to a higher level of moisture loss caused during roasting in oven, which made the samples become drier and tougher. Interesting, the panelists gave similar tenderness scores for the SV and SV+TC samples (p>0.05). Regarding the overall acceptance, the panelists gave the highest score for the samples treated with SV+TC, followed by the SV+OV and SV on both examining days (0 and 14 d). This could be explained due to association with the synergistic effect of their higher tenderness, juiciness and especially flavor scores. Noticeably, postmortem ageing also increased the overall acceptability scores of the sous videcooked and the SV+TCtreated samples.

CONCLUSION
The present study for the first time, evaluated the combined effects of postmortem ageing and cooking treatments (sous vide alone, sous vide followed by oven roasting or blowtorch ing) on the quality properties of low quality grade Hanwoo beef striploin. In general, the sous vide cooking of beef resulted in significantly lower WBSF and safety in term of micro biology. Blowtorching after sous vide cooking produced a browner surface of beef steaks. Particularly, roasting or blow torching after sous vide cooking resulted in higher quantities of important aroma flavor compounds such as pyrazines and sulfurcontaining compounds compared to those just sous vide cooked. Furthermore, roasting or blowtorching after sous vide cooking resulted in increased flavor scores of cooked beef steaks. However, compared to the roasting in oven the blowtorching after sous vide cooking seemed to produce more beneficial effects on the improvement of the technological quality (e.g., redness and BI) and eating quality (e.g., flavor intensity), and especially juiciness main tenance of cooked beef. Postmortem ageing beef for 14 d significantly improved the tenderness by reducing the WBSF values, and increasing the sensorial tenderness scores. Based on the results obtained in the present study, it may be concluded that the combination of postmortem ageing and sous vide cooking followed by blowtorching or oven roasting should be applied to improve the eating quality attributes such as tenderness and flavor as well as the overall acceptability of the low quality grade Hanwoo beef. Table 6. Sensorial mean score (7-point scale) of beef striploin steaks as affected by ageing and cooking treatment