Beef Longissimus Lumborum Steak pH Affects External Beef Longissimus Lumborum Steak pH Affects External Bioelectrical Impedance Assessment Bioelectrical Impedance Assessment

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Introduction
Meat is a highly perishable commodity which naturally contains myoglobin, a protein responsible for the red color of fresh beef.Meat color is unstable, but is considered one of the major criteria for consumers when selecting meat purchases (Kropf, 1993).Bioelectrical impedance, a non-destructive analysis, was first documented for use by medical sciences in the early 1900s (Morse, 1925).Later, Swatland (1985) used electrical impedance to evaluate the relationship between the quality of pork carcasses and its electrical properties.Bioelectrical impedance analysis has been demonstrated to provide an accurate fat content determination in different pork and beef grinds; however, the smaller the grind size (1/8-inch plate), the more accurate bioelectrical impedance analysis was for predicting fat content (Marchello et al., 1999).In this study, external bioelectrical impedance analysis was used to assess postmortem chemical changes in normal-and high-pH beef longissimus lumborum steaks during simulated retail display.

Experimental Procedures
The experiment was designed as a split-plot with loin as the whole-plot and paired steaks as the sub-plot.Display day was treated as the sub-plot treatment.Meat-pH and display day were treated as fixed effects.Beef strip loins (n = 20; Institutional Meat Purchase Specifications #180) were obtained from a commercial processor (postmortem age = 14 days).Loins were sorted into two treatments, normal-pH (5.61-5.64;n = 11) and high-pH (6.2-7.0;n = 9), fabricated into five 1-inch thick steaks (n = 100), and randomly assigned to one of five display days: 1, 3, 5, 7, and 9. Steaks were packaged on Styrofoam trays with a moisture absorbent pad, overwrapped with polyvinyl chloride film, and displayed under fluorescent lighting (32 W Del-Warm White 3000° K; Philips Lighting Co., Somerset, NJ) at 32-40°F in coffin-style retail cases (model DMF 8; Tyler Refrigeration Corp., Niles, MI) in the Kansas State University Color Laboratory.External bioelectrical impedance values, oxygen consumption, metmyoglobin reducing ability, protein degradation, water holding capacity, and pH were assessed on each storage day.

Results and Discussion
There was no meat-pH × display day interaction (P > 0.05) for external bioelectrical impedance values; however, an effect on meat-pH and display day was found (P < 0.05).External bioelectrical impedance was 20% higher (P < 0.05; Figure 1) for high-pH meat than normal-pH meat.As seen in Figure 2, steaks on day 1 had lower external bioelectrical impedance values (P < 0.05) compared to days 5 and 7, but similar (P > 0.05) to days 3 and 9.
There was no meat-pH × display day interaction (P > 0.05) for metmyoglobin reducing ability values; however, an effect on meat-pH and display day was found (P < 0.05).The metmyoglobin reducing the ability of high-pH meat was increased by 12% (P < 0.05) in comparison with normal-pH meat.Metmyoglobin reducing ability increased (P < 0.05) over the 9 days of retail display.There was no meat-pH × display day interaction (P > 0.05) or display day effect for intact or degraded desmin, however, a meat-pH effect (P < 0.05) was found.Normal-pH meat had 33 and 43% higher (P < 0.05) amount of intact and degraded desmin, respectively, than high-pH meat.The meat-pH × display day interaction was marginally significant for troponin-t 40 and 30 KDa (P = 0.0601).In addition, no meat-pH × display day interaction (P > 0.05) was found for troponin-t 36, 34, and 30 KDa.

Implications
External bioelectrical impedance is a method that could be used to separate normal-and high-pH strip loins with potential for rapid, in-plant use to identify dark-cutting beef.

Figure 2 .
Figure 2. External bioelectrical impedance values of beef longissimus lumborum steaks displayed under fluorescent lights at 32-40°F for up to 9 days.ab Means with different superscripts differ (P < 0.05).

Table 1 .
Correlation coefficients between electrical measurements 1 of high-pH beef longissimus lumborum steaks and water holding capacity, pH, oxygen consumption, metmyoglobin reducing ability, desmin, troponin-t, and degraded portion