3.1. Intrumental Color
Sub-primals in this experiment were wet aged for 21 days before being fabricated into steaks and displayed in multi-deck cases for 42 days. Limited research on extended storage (>60 days) of fresh beef is available in the literature; therefore, this was a novel opportunity to evaluate changes in fresh meat color over long storage periods. Anticipating large changes in myoglobin state in these long-stored meat products, instrumental color readings were used to measure the surface color changes between different pigment forms [
19]. There was an interactive impact (
p < 0.05) of the packaging method and storage period on the fresh surface color (
Table 2).
From day 0 to 42, steaks packaged in VPC packaging film were darker (
p < 0.05) than beef steaks packaged using VPA or VPB (
Table 2). Regardless of packaging treatment, L* values initially increased (
p < 0.05) through day 21. However, as the duration of storage increased, steaks in VPC became darker. Lightness is a characteristic of fresh meat as it blooms, and during lighted display and limited oxygen conditions, oxymyoglobin formation can be altered [
20]. An increase in L* using VPA and VPB films is likely the result of film thickness limiting oxygenation of myoglobin and mitochondria resulting in more light scattering on the surface of the steak. Similar changes in lightness were reported in previous studies using vacuum-packaged ground beef over a 14-day simulated display period [
20,
21]. However, previous literature on the storage of vacuum-packaged whole-muscle cuts after extended wet aging and subsequent fresh storage is limited.
An interaction between packaging film and storage day for objective redness values occurred (
Table 2). Steaks were redder (
p < 0.05) after day 35 of storage when using VPA and VPB consisting of greater barrier properties and a concentration of OMB on the surface of the steaks. However, there were some similarities (
p > 0.05) among packaging films for redness values from day 0 to 28 of the storage period. Steaks packaged in VPC were less red (
p < 0.05) and became more yellow (
p < 0.05) as storage time increased. Similar findings were reported when using vacuum packaging to store foal meat for 14 days in retail display cases [
22]. Additionally, the current results tend to agree with others that have evaluated retail color characteristics of vacuum-packaged
longissimus lumborum and noted an increase in redness (a*) and yellowness (b*) values over retail storage [
23].
As expected, hue angle values lacked considerable differences (
p > 0.05) among all packaging films through the first 21 days of storage (
Table 3). However, by day 28 until 42 of the study, steaks packaged in VPC were further (
p > 0.05) from the true red axis suggesting surface color deterioration was occurring. Surface vividness (C*) was greater (
p < 0.05) for steaks packaged in VPC than either in VPA or VPB. The changes in vividness suggest that the film thickness in VPA and VPB reduced the rate at which atmospheric gases, such as oxygen, could pass through the film to the surface of the steak and alter the percentage of OMB. It should be noted that current results are similar to previous work on vacuum-packaged beef loins, suggesting that hue angle and vividness stability values deteriorated after peaking during storage [
24].
Red-brown ratios (RTB) were calculated from objective measurements of spectral reflectance from 400 to 700 nm. An interaction (
p < 0.05) for packaging method × day of display is presented in
Table 3. Initially (day 0), regardless of the packaging film, RTB values did not differ (
p > 0.05). However, by day 35, steaks packaged in VPC had a browner surface color (
p < 0.05). Red-to-brown ratios for beef steaks using VPB packaging film showed a greater color shift (
p < 0.05) in contrast with the steaks packaged in VPA and VPC films. Furthermore, during the display period, red-to-brown values declined after day 28 (peak) as steaks shifted from a redder to browner surface color. Previous studies have reported similar color shifting of calculated values regardless of packaging method, and it is reasonable that the shift from red to brown is a function of greater metmyoglobin formation throughout the retail display period [
4]. It is not surprising that calculated spectral values for instrumental surface color in fresh beef meat are expected to shift from red to brown as the exposure time to atmospheric gases increases. Changes in RTB appear to be related to packaging thickness and the volume of oxygen exposure over time on the surface of the meat.
Calculated relative values of metmyoglobin (MMb) were lower (
p < 0.05) from day 21 to 42 when using VPA and VPB films (
Table 4), whereas steaks packaged in VPC appeared to have a greater (
p < 0.05) percentage of MMb measured objectively on the surface from day 7 to 42. Calculated relative values suggest the increase in metmyoglobin formation is associated with the oxygen transmission rate that occurred but was not measured throughout the storage period. Previous studies have mentioned that a cause of MMb formation can be accelerated by water loss and heme concentration, but fresh meat in a properly packaged condition should not discolor because of the purge [
5].
As the term suggests, deoxymyoglobin can be associated with muscle foods that are not exposed to oxygen, and this myoglobin form can be identified either in vacuum-packaged meat or within the interior of freshly cut meat [
24]. Similar to relative MMb values, DMb values in beef steaks packaged using VPA and VPB vacuum-packaged film were greater (
p < 0.05) than values calculated for steaks in VPC (
Table 4). As expected, when using relative values to calculate myoglobin forms of muscle foods, as one form increases (i.e., MMb or OMb) the other forms should decline. Current results tend to agree with results reported on beef steaks using polyvinyl chloride (PVC) overwrap exposed to 35 days in retail display, where the DMb formation was less overall but also increased over time [
25].
It is well known that gases, particularly oxygen, from within the atmosphere can react with meat pigments to form a bright red color in contrast to darker purple or brown colors that lack vividness. Calculated OMb values were greater (
p < 0.05) and declined throughout the entire storage period (
Table 4). However, from day 0 to 42, the greatest (
p < 0.05) decline in relative values of OMb occurred in steaks packaged in VPC. It is quite possible that the changes in relative myoglobin values, especially OMb, are associated with the oxygen transmission rate (OTR) of each packaging film. Oxygenation is a process that occurs when myoglobin is exposed to oxygen and the development of oxymyoglobin causes a cherry-red surface color—this process is commonly referred to as bloom [
25].
3.4. Cook Loss and Warner-Bratzler Shear Force
During cooking, meat can lose a large proportion of its mass, which can be attributed to moisture losses prior to and during the cooking process. There was no interaction between the packaging and storage period (
p < 0.05) for purge loss, cook loss, or WBSF. As expected, purge loss in packaged steaks increased (
p < 0.05) with increasing storage time (
Table 6), whereas cook loss was greater (
p < 0.05) in steaks after 28 days of storage (
Table 6). These shifts in moisture losses can be caused by the combination of storage time and temperature or cooking conditions, which ultimately can influence the objective tenderness values. Results in the current study agree with previous studies reporting that moisture loss in different retail beef cuts can be altered as storage time increases [
29]. Nonetheless, additional aging of meat has shown that lower cook loss can occur in beef cuts aged over 50 days [
30].
Changes in moisture during storage and cooking have been well documented to alter meat tenderness. Objective tenderness can be measured via Warner-Bratzler shear force (WBSF) and is often reported in newtons (N) of force. WBSF values were the greatest in steaks on day 21 (
p < 0.05), but steaks became more tender as storage duration increased (
Table 6). Countless studies have concluded that the tenderness and juiciness of meat are affected by heat exposure, and these sensory factors can influence customer satisfaction. Past studies using aging in beef loins around 42 days reported similar trends, where WBSF decreased linearly as the aging period increased [
27]. In addition, another study evaluated the tenderness properties of aging beef and concluded that the shear force would decrease when the storage time prior to cooking increased [
28].