Tenderness, ageing rate and meat quality of pork M. longissimus thoracis et lumborum after accelerated boning

Abstract

The impact of accelerated boning on tenderness, ageing rate and meat quality of pork longissimus muscle was investigated. Accelerated boning of eight Large White×Landrace pork carcasses at 30 min post-mortem followed by chilling at 0°C resulted in tougher pork with higher drip loss and a darker surface lightness relative to conventional boning at 24 h post-mortem. The increased toughness was still observed at 4 days post-mortem, a time by which 80% of ageing generally had occurred as seen in experiment 2. The increased toughness could be attributed to cold shortening and a reduction in proteolysis.

Introduction

Accelerated boning of pork carcasses is one approach used to reduce the costs of pork boning but attention must be paid to maintain meat quality. Accelerated boning of meat carcasses involves the removal of primals or cuts from the carcass pre-rigor (van Laack, 1989) while conventional boning involves the removal of primals from the carcass after rigor mortis has set in. Accelerated boning enables production costs to be reduced due to lower labour requirements, a reduction in chiller space and energy input and increased product turnover (6, 39). Accelerated boning of pork has been slow to be employed world wide, largely due to the logistics of changing to accelerated boning and also the potential detrimental impact on tenderness because of the rapid drop in muscle temperature that occurs when muscles are removed pre-rigor. To date the majority of accelerated boning studies have been carried out on beef; therefore, studies on pork are required to determine if the economic benefits of accelerated boning can be employed with pork without reducing quality, particularly, tenderness.

A major problem experienced with accelerated boned meat is cold toughening, which can result in up to 30% increase in toughness in pork cuts (Cross & Seideman, 1985) and an increase in drip loss (Honikel, Kim, Hamm, & Roncales, 1986). Cold toughening occurs if muscles are chilled rapidly before rigor sets in. Under conventional chilling and boning conditions of pork, cold toughening is minimised or prevented by slower cooling of the carcasses before rigor and by the muscles being restrained by attachment to bones and tendons within the carcass (Locker & Hagyard, 1963).

One of the causes of cold toughening is cold-shortening. Cold shortening occurs, when the muscle temperature goes below 15°C (20, 28) before rigor sets in. Moreover, as the temperature decreases below 15°C the potential for cold shortening increases which can lead to decreased tenderness and increased drip loss. In contrast, some research has shown that fast chilling rates can be used without inducing cold shortening. For example, Honikel et al. (1986) observed that when stored at 10°C minimal sarcomere length shortening with the M. cleidoccipitalis located in the neck region and Fischer, Honikel, and Hamm (1980) reported that even at 4°C there was minimal sarcomere length shortening and increase in drip loss with the M. longissimus thoracis et lumborum (LTL).

Another cause of toughening in rapidly chilled muscles is that the rapid chilling and resulting low muscle temperatures early post-mortem reduces the activity of proteolytic enzymes (Dutson & Pearson, 1985). This lower proteolytic activity reduces the proteolytic breakdown of the muscle proteins which is responsible for the increased tenderness that normally occurs in the pre-rigor period early post-mortem. Moreover, with rapidly chilled muscles that have cold shortened, proteolysis is postulated to be reduced during the post-rigor ageing period since the tightly packed structure of the cold shortened muscle is less susceptible to proteolytic breakdown.

Whichever is the cause of cold toughening, the rapid chilling of muscles early post-mortem has the potential for producing tough meat both initially and after post-mortem ageing.

In contrast to the negative impact of accelerated boning on tenderness, the use of accelerated boning for pork muscles has the potential to improve other meat quality traits. Accelerated boning of pork muscle has been shown to reduce drip loss (van Laack & Smulders, 1992) and the incidence of pale meat colour (van Laack, 1989). The darker colour seen in pork after accelerated boning has also been reported in beef (Shaw & Powell, 1995) and has been attributed to the lower pre-rigor temperature, slower pH decline rate and reduced protein denaturation.

However, there have also been reports of increased drip loss associated with accelerated boning. Studies by Honikel et al. (1986) and Iversen, Henckel, Larsen, Monllao, and Moller (1994) with pork muscle, and Shaw and Powell (1995) with beef muscle have shown that rapid chilling of pre-rigor muscle increases drip loss. In these studies, it has been suggested that the reduced water holding capacity of the muscle is due to cold shortening. The reduced sarcomere length of the muscle cell decreases the interfilament volume of the cell thereby decreasing the amount of water the muscle cell can hold and reducing the water-holding capacity of the muscle.

When assessing tenderness and other meat quality attributes, care must be taken to avoid naturally occurring variations within a muscle due to the different fibre types, collagen content and post-mortem changes that occur in different parts of the muscle. The studies carried out to-date on the effect of location within a muscle on Warner–Bratzler shear force (WBSF) have produced inconclusive results. A number of studies have shown significant variations in WBSF within a muscle (1, 15, 27, 30, 45, 51) while other studies have reported no differences (36, 52). To ensure locational effects of sampling did not bias results in experiment 3, an initial experiment was carried out to determine the effect of anatomical location within the LTL on WBSF.

Differences in the rate of ageing of pork with post mortem storage have been reported in the literature. For example, Buchter and Zeuthen (1971) and Harrison, Bowles, Anderson, Tuma, and Kropf (1970) observed an increase in tenderness up to the 6th and 8th days post-mortem, respectively, while Bennett, Bramblett, Aberle, and Harrington (1973) found that ageing more than 1–2 days did not improve the palatability of pork. Similarly, Feldhusen and Kuhne (1992) found that lowest WBSF values were attained at 2–3 days post-mortem and Dransfield, Jones, and MacFie (1980–1981) found on average 50% of the tenderisation of pork has been observed to occur in 2 days.

Because of variability in ageing rate in previous studies, a second preliminary experiment was carried out to determine the ageing rate of pork LTL by measuring the change in the WBSF over an extended ageing period. The results from this experiment were then used in the subsequent experiment on accelerated boning to ensure that ageing is essentially complete when samples are assessed for tenderness by WBSF.

As pointed out earlier, there are conflicting results in the literature on the effect of accelerated boning on tenderness and drip loss of pork (8, 24, 47). Due to these conflicting findings, the research described in this study was carried out to investigate the influence of accelerated boning on pork tenderness, ageing rate and meat quality and to determine the biochemical properties responsible for any changes that occur.