Investigation of the rigor process of beef muscles and development of recommended aging times for key muscles
Kwan, Siou Yong
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This thesis is aimed to evaluate three electrical stimulation systems in beef processing. A new setting has replaced the traditional 15 Hz immobilisation setting in all processing plants. However, the impact of the new setting on various meat quality parameters – pH decline rate, tenderness, drip loss, colour, etc. – was of interest. That is the first objective of this study. The second objective of this study is to understand the behaviour of representative meat cuts in response to the three stimulation treatments. The output from this work will be a comprehensive muscle profile database of commercial value. 36 pasture-fed steers (male castrates), aged 18 to 28 months and weighing between 280 and 330 kg were singly selected for the study. The subcutaneous fat thickness was between 2 and 4 mm. The steers were electrically stunned across the head only, bled and dressed according to Ministry for Primary Industries (MPI) requirements. The time at this point of the experiment was designated Day 0. Three stimulation treatments were devised: the traditional 15 Hz treatment is Stimul 1 and the new setting, 15 Hz and 400 Hz. is Stimul 2. Stimul 3 is Stimul 2 paired with Smart Stimulation. The 36 animals were randomly assigned to one of them. Six pH and temperature measurements were taken from each carcass with a TESTO® 205 pH meter (Testo AG, Lenzkirk, Germany) at approximately hourly intervals from the start of chilling to about 8 hours post slaughter, then finally at 24 hours: the pHu. The pH meter was calibrated before use and at regular intervals using pH 4 and pH 7 buffers at room temperature (Thermofisher, New Zealand). The glass probe was inserted through the longissimus between the 12th and 13th ribs and into the core of the deeper M. semimembranosus on the hindquarter. One day after slaughter (Day 1) six single muscles: rectus femoris, longissimus dorsi, psoas major, gluteus medius, gluteobiceps and infraspinatus were excised from one side of the carcasses. Depending on the size and orientation of each muscle, 2 or 4 submuscles (100 mm x 50 mm x 50 mm) were excised, trimmed of visible fat, and labelled as Position 1, 2 etc. from a defined site depending on the muscle. In summary, a grand total of 648 samples were collected in this study. Each muscle subsample was vacuumed packed and randomly assigned to one of six different aging days, 1, 3, 5, 7, 14 or 21 days. At the prescribed day, the muscles were measured for drip loss, colour and tenderness. Although the pH-temperature data were unfortunately limited for Stimul 1, historical data strongly suggested that there would be a severe risk of heat shortening, particularly in the deeper semimembranous muscle. By contrast, the pH-temperature results in Stimul 2 and 3 showed little tendencies for heat shortening. The 15 Hz and 400 Hz combination in Stimul 2 reduces the incidence of burst tenderloins and ecchymosis. Stimul 3 did demonstrate an ability to control pH decline rate in the first few hours but only in longissimus dorsi and not extended to semimembranosus. In the two indicator muscles, Stimul 1 had the most variable pH values, particularly at early times after slaughter. Variability was reduced in Stimul 2 and – especially - Stimul 3 (Smart Stimulation). Thus, Stimul 3 appears more attractive. The stimulation treatments had no effect on drip loss, saturation, Hue angle and shear force, but the data reveal the rank order of drip loss amongst muscles is the same as saturation and Hue angle: Gluteus medius (RUMH, highest drip loss and most coloured) > Gluteobiceps (RUMC) > Psoas major (TEND) > Rectus femoris (KNUCK) > Longissimus dorsi (STRL)> Infraspinatus (BLD, lowest percent drip loss). The initial shear force values for infraspinatus (BLD), rectus femoris (KNUCK) gluteobiceps (RUMC), gluteus medius (RUMH) and psoas major (TEND) were below 10 kgF, and would provide an acceptable eating experience even one day after slaughter. The longissimus dorsi (STRL) showed a high variation of tenderness (between 5 kgF to 12 kgF). The tenderness variability of all muscles decreases with time and they would provide a good eating experience after 7 to 10 days after slaughter. The remarkably precise inverse relationship between the Japan colour score and L* showed that the colour tiles were as good, certainly cheaper and more reliable by virtue of their simplicity. This colour grading technique is currently in commercial practice by Silver Fern Farms.