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Two-dimensional visible/near-infrared correlation spectroscopy study of thawing behavior of frozen chicken meats without
exposure to air $
Yongliang Liu, Yud-Ren Chen *
Beltsville Agricultural Research Center, ARS, USDA, Beltsville, MD 20705-2350, USA
Received 19 May 2000; received in revised form 3 August 2000; accepted 3 August 2000
The thawing behavior of frozen chicken meats without exposure to air was investigated by generalized 2D Vis/NIR correlation spectroscopy. The synchronous 2D visible correlation analysis revealed that intensities of the 435 and 555 nm bands increase,
because of the relaxation of DeoxyMb and OxyMb components, whereas those of the 475 and 620 nm bands decrease as MetMb and SulfMb decompose into small molecules due to speci®c enzymes. The corresponding asynchronous spectra indicated that the decomposition of MetMb and SulfMb species precedes the recovery of DeoxyMb and OxyMb, and that the DeoxyMb species
recovers faster than the OxyMb. Further, the asynchronous 2D NIR spectra suggested that the melting of ice crystals and the relaxation and proteolysis of proteins occurs earlier, indicating a coordination process for hydrophilic OÿH and NÿH groups. Moreover, strong correlation peaks correlating the bands in the visible and NIR spectral regions were observed and discussed.
Published by Elsevier Science Ltd.
Keywords: Two-dimensional correlation analysis; Vis/NIR spectroscopy; Frozen meat; Chicken meat; Thawing
Both the importance of freezing of meats and the proper procedures for freezing have been considered widely (Kinsman, Kotula & Breidemstein, 1994; Lawrie, 1985; Price & Schweigert, 1987). However, the subject of thawing, the opposite process to freezing, has been largely ignored. Generally, thawing increases molecular mobility, resulting in the relaxation of meat components frozen in a rigid state. Such relaxation may sometimes allow the reabsorption of the ¯uids (soluble proteins, vitamins, and salts) by the meat, but with a risk of meat discoloration and lipid oxidation if the thawing time is too long (Kinsman et al., 1994).
Visible/near-infrared (Vis/NIR) spectroscopy has found considerable application in safety and quality control issues of chicken meat products (Chen, Hu- man, Park & Nguyen, 1996; Chen & Marks, 1997, 1998; Chen, Park, Human & Nguyen, 1998; McElhinney, Downey & Fearn, 1999; Rannou & Downey, 1997). Applications include the quantitative prediction of the physical characteristics of heat-treated chicken patties (Chen & Marks, 1997, 1998), the identi®cation of the chicken species from other meats (McElhinney et al., 1999; Rannou & Downey, 1997), and the classi®cation of chicken carcasses into wholesome and unwholesome classes at the slaughter plant (Chen et al., 1996, 1998). Moreover, generalized two-dimensional (2D) correla- tion analysis was recently applied to the Vis/NIR spec- tral region for the characterization of chicken meats with various treatments and conditions (Liu & Chen, 2000; Liu, Chen & Ozaki, 2000a,b). It has turned out that the 2D Vis/NIR approach can not only establish the spectral band assignments but also monitor the complex sequence of events arising from the changes in
0309-1740/00/$ - see front matter Published by Elsevier Science Ltd.
PI I : S0309-1740(00 )00106-6
Meat Science 57 (2001) 299±310
$ Mention of a product or speci®c equipment does not constitute a
guarantee or warranty by the US Department of Agriculture and does
not imply its approval to the exclusion of other products that may also
* Corresponding author. Tel.: +1-301-504-8450; fax: +1-301-504-
E-mail address: email@example.com (Y.-R. Chen).
meats during such processes as discoloration and tenderization. In recent publications (Liu & Chen, 2000; Liu et al.,
2000a), 2D Vis/NIR spectroscopic study was focused on both the cooking time- and the storage time-induced physical and chemical changes of chicken meats, in which the intervention of air was apparent. In the visi- ble region, the signi®cant spectral intensity reductions of the 445 and 560 nm bands revealed that both absor- bances could be related to the discoloration of meats. Hence, the 445 and 560 nm bands have been assigned to deoxymyoglobin (DeoxyMb) and oxymyoglobin (OxyMb) components, which respectively are purplish and bright red in appearance (Kinsman et al., 1994; Lawrie, 1985; Price & Schweigert, 1987). The results also suggested that DeoxyMb and OxyMb degrade into metmyoglobin (MetMb), sulfmyoglobin (SulfMb), and small molecules through oxidation and reduction reactions. In addition, they showed that DeoxyMb, MetMb, and OxyMb components exist in all wholesome and unwholesome meats, with a clear indication that wholesome meats have more variation in OxyMb and DeoxyMb and less varia- tion in MetMb than do diseased meats (Liu et al., 2000b). The analysis of spectral intensity variations of the
CÿH and OÿH/NÿH vibration modes in the NIR region provided a very interesting point. It was observed that the intensity change of the C-H fractions occurs before those of the OÿH/NÿH groups for the chicken meats during cooking, but occurs after those of the OÿH/NÿH groups for the cold stored meats conditions (Liu & Chen, 2000; Liu et al., 2000a). Obviously, the reverse sequence of the CÿH groups varying before/after the OÿH/NÿH groups suggested dierent mechanisms of chemical and biochemical reactions in the chicken meats corresponding to the external treatments. Possibly, the cooking of meats leads to the oxidization of lipids ®rst, whereas the meats in cold storage ®rst undergo dena- turation and proteolysis of the proteins. Our previous reports also indicated that intensity
reduction of the visible bands due to DeoxyMb and OxyMb occurs before those of the NIR bands ascribed to the CÿH and OÿH/NÿH vibrational modes, suggest- ing the possibility that the discoloration of meats pre- cedes the other developments, such as tenderization (Liu & Chen, 2000; Liu et al., 2000a). In the present study, the same strategy was applied to
unravel the thawing behavior of frozen chicken meats without exposure to air, complementary to the previous investigation (Liu & Chen, 2000; Liu et al., 2000a). Vis/ NIR spectra were measured over a time span of 0ÿ180 min after the beginning of thawing, during which phy- sical, chemical, and biochemical reactions could be induced by the melting of ice crystals and the increased mobility of meat components.
2. Materials and methods
2.1. Meat samples
Five wholesome fresh chicken carcasses were selected by a Food Safety and Inspection Service (FSIS) veter- inarian from the processing line at a poultry slaughter plant located on the Eastern Shore of Maryland (Cor- dova, MD, USA). The carcasses, selected in the morn- ing, were packaged into polyethylene bags which were then placed in a plastic cooler ®lled with ice, and trans- ported to USDA's Instrumentation and Sensing Laboratory in Beltsville, MD. In the afternoon, ®ve fresh breast meats (1 cm thick and 3.8 cm diameter) were cut from each of ®ve chicken carcass, sized to ®t into the spectrophotometer's quartz-windowed cylind- rical cup. Then ®ve cups with sliced meats inside were sealed in a polyethylene bag and stored in a freezer for 2ÿ7 days until Vis/NIR measurements were taken.
2.2. Spectroscopic measurement and 2D correlation analysis
All the Vis/NIR re¯ectance spectra were recorded on a scanning monochromator NIRSystems 6500 spectro- photometer (NIRSystems, Silver Spring, MD, USA) equipped with a rotating sample cup. Each spectrum was collected over the 400ÿ2500 nm wavelength range at 2 nm intervals, with 16 scans. The spectra of indivi- dual frozen meat samples were measured successively by a time increment of either 10 min (in the ®rst 60 min) or 15 min after the beginning of thawing at room tem- perature, eventually producing 15 spectra over a time span of 180 min. Because the meat was covered by the quartz-window tightly, the thawing occurred without exposure to air. Five spectra, collected for ®ve frozen meats with the
same thawing time, were averaged using Grams/32 software (Galactic Industrious Corp., Salem, NH, USA). Then the 15 averaged spectra, representing the thawing behavior of frozen meats, were loaded into the PLSPlus/IQ package in Grams/32 to perform principal component analysis (PCA). The PC scoresÿscores plot suggested two clusters of spectra: one consisted of seven spectra measured in the 0 to 60 min range and the other of eight spectra in the 75ÿ180 min range. This small time range was taken to accentuate the thawing-time induced change of the Vis/NIR spectra. The subsequent 2D correlation spectra of meats were
derived from these two sets of Vis/NIR spectra by using the generalized 2D correlation analysis (Liu & Chen, 2000; Liu et al., 2000a,b; Noda, 1993). The principle for analyzing the positive/negative cross-peaks in synchro- nous and asynchronous spectra has been described by Noda. In a synchronous spectrum, positive peaks (shown in solid lines) indicate that intensity changes
300 Y. Liu, Y.-R. Chen /Meat Science 57 (2001) 299±310
observed at two spectral coordinates are in the same direction, while negative peaks (shown in dashed lines) mean that intensity changes are in opposite directions. Asynchronous peaks are used for evaluating the sequence of spectral intensity changes. A positive cross- peak indicates that the spectral intensity change observed at l1 occurs earlier than that at l2