Table 2 shows the significances of one-factor and two-factor experimental measurements. There was no significant difference (P > 0.05) of moisture content, and fat content between nonES beef bottom round and ES fresh samples (Table 3).
However, ES fresh roast beef had significantly (P < 0.05) lower pH value when compared to nonES samples as would be expected. Stimulation significantly decreased pH of fresh meat because ES has been reported (Dutson et al., 1980; Ockerman and Szczawinski, 1983) to increase the pH decline. Yields of nonES and ES roast beef were not significantly different (P > 0.05). Basically, electrical stimulation did not change the composition of roast beef significantly.
The pH values of precooked roast beef had a significant two-way interaction (P < 0.01) between stimulation and days; that is, the effect of electrical stimulation on pH is dependent on storage days. NonES precooked roast beef maintained the same pH up to 4 days of refrigerated storage (P > 0.05); however, the pH of ES roast beef significantly increased (P < 0.05) during 4 days of storage (Table 4). At day 0, cooked ES roast beef had significantly (P < 0.05) lower pH than NonES as would be expected, but ES had higher pH at day 4 (P < 0.05).
For lipid oxidation, TBARS values of NonES and ES were significantly increased (P < 0.05) during 4 days of refrigerated storage, but there is no treatment effect (P > 0.05) due to stimulation. Even though stimulation could destroy cell membrane that should promote lipid oxidation in meat, ES tissue still had the same trend of lipid oxidation as NonES roast beef up to 4 days. The electrical stimulation could release catalyses to promote lipid oxidation by disrupting muscle structure. However, the cooking process also releases a great amount of catalyses by muscle denaturation and would promote chemical oxidation; therefore, the stability of lipid oxidation is relatively less influenced by stimulation when it is compared to the high temperature of roasting (71oC).
At day 0, only three panelists (averaged of triplication tests) discriminated treatments correctly by the triangle test. Therefore, there was no significant difference between the two treatments because a significance (a=0.05) only occurs when at least five out of a six panel members have the correct answers (Meilgaard et al., 1991). Comparisons of WOA, WOF, and tenderness scores indicated that there was no significant difference for NonES and ES. The WOA scores of both treatments did not significantly change, but WOF scores significantly increased (P < 0.05) and tenderness scores significantly decreased (P < 0.05) during storage. Aroma would be less sensitive to off-flavor development than flavor since flavor is a combination of taste and smell by sensory evaluation. This agrees with Jeremiah and Martin (1980) who reported that there was no odor difference between stimulated and non-stimulated rib steaks after 4 days of retail display.
The RBF was evaluated by the panel and there was a significant two-way interaction. It shows that this score is influenced by electrical stimulation and is dependent on storage time. From day 2 to day 4 in refrigerated storage at 4oC, precooked roast beef without ES did not significantly change; however, ES had a significantly (P < 0.05) decreasing score during the same period (6.06 dropped to 4.83). That is the reason; ES had significant (P < 0.05) lower RBF score than NonES at day 4 (Table 4). Sekikawa et al. (1999) indicated that ES increased the content of free amino acids due to protein degradation via proteases and other enzymes during this process. One amino acid such as alanine was decreased slightly during storage compared to the non-electrically stimulated treatment. However, small compounds, which break from hydroperoxides in lipid oxidation including pentanal, hexanal, 2,4-decadienal, octanal, nonanal, 2,3-octanedione, have been recognized as contributors to WOF (Vega and Brewer, 1994; Thongwong et al., 1999). Those compounds were identified from lipid oxidation, but there was no significant (P > 0.05) difference in this research due to stimulation as measured by TBARS and sensory test (WOF and WOA). But, electrical stimulation did cause a decrease of desirable roast beef flavor probably due to complicated reactions of other materials such as amino acids.
Shear values (Table 5) indicated that stimulation produced a significantly (P < 0.01) more tender precooked roast beef when compared to non-ES; however, the trained panel did not detect this difference in tenderness. It could be that the objective method is more sensitive than the subjective tenderness evaluation and also low voltage stimulation was utilized in this study which is often not as effective at increasing tenderness as high voltage stimulation.
Bacterial counts were numerically lower for electrical stimulation tissue, but did not significantly change the mesophile, thermophile, and psychrophile growth in precooked roast beef at day 7. From a microbiological standpoint, ES should reduce microbial growth due to the lower pH value of muscle from elimination of ATP and glycogen, release of proteolytic enzymes, and destruction of bacterial cells (Dutson et al., 1980; Mrigadat et al., 1980). Ockerman and Szczawinski (1983) reported a reduction of microflora by ES that became less significantly important in an inoculated beef tissue with storage time. Also, Kotula (1980) indicated that ES did not affect microbial growth on beef primals when they were tested by aerobic plate counts (APC) incubated at 5, 20, and 35oC (41, 68, and 95oF). On the other hand, the amount of microbial counts of precooked roast beef at day 7 was relative lower in this study when compared to other reported literature of fresh stimulated meat as would be expected due to cooking process.