Summary
Background
Dark cutting meat usually has a very high pH which makes the meat appear dark. Because consumers prefer "bright-cherry" coloured meat, dark cutting carcases are often heavily discounted by up to $0.45/kg hot carcase weight. In Australia the incidence of dark cutting is almost 10% in beef. That equates to a potential loss for the industry of almost $36 million per year. In sheep and lambs the figures are even higher - an estimated 15% of sheep and lamb carcases are classified as high pH. Some classes of livestock tend to be more susceptible than others.
Dark cutting is more prevalent in pasture fed cattle than feedlot cattle, in milk fed vealers compared to weaned vealers and sucker lambs compared to carry over lambs. Heifers in season, bulls, and cattle treated with hormonal growth promotants and freshly shorn sheep in cold weather may also be especially susceptible to dark cutting. In addition to the unacceptable dark colour, high pH meat has the following features:
a coarse texture and higher water holding capacity, therefore the meat loses a lot of moisture during cooking.
reduced shelf life as bacteria grow more rapidly due to the higher pH and moisture.
it appears undercooked.
reduced tenderness at pH ranges 5.7 - 6.2.
stronger flavour in sheep.
high pH beef (above pH 5.7) is excluded from many meat brands, food service operations and markets such as the valuable Japanese trade.
Dark cutting meat is ultimately rejected by consumers at the retail level on the basis of its colour.
Measures can be taken to reduce or eliminate high pH by improving the handling and care in marketing livestock. There are other benefits such as:
reduced bruising
improved animal welfare
reduced carcase weight loss
Early Research
Project M.466 (1994) and M.628 (1996) - Development of a semi-automatic system for the early detection of dark cutting beef.
When rested animals are killed without stress after having been off-feed for less than 24-48 hours, their most economically important muscles have good levels of glucose stored in them. During the development of rigor mortis this glucose is converted to lactic acid, causing the pH to fall, until either it has all been converted, or until sufficient lactic acid has been produced that the reaction becomes pickled at around pH 5.5. When stressed animals are killed they have low levels of glucose in their muscles, less lactic acid is therefore produced and the pH of their muscles does not fall as far as in unstressed animals. Meat that has low pH is lighter and brighter in colour than high pH meat (or dark cutting meat). Low pH meat is generally more tender, juicy and has a better flavour than dark cutting meat.
The later project drew upon the findings from the early detection of dark cutting beef project M.466 and focussed on the development of a semi-automated system for the early detection of dark cutting beef. This report outlines the development and testing of a semi-automated system which comprised a simple tool, that is easy to sterilise, suitable for taking fat-free muscle samples and a machine into which the samples were placed. The system then processed the samples so that their ultimate pH could be automatically read and transmitted within 25 minutes and with re-configuration, within 15 minutes. Samples could be taken at intervals of around 15 seconds.
Abattoir trials indicated that the ultimate pH indicated for samples taken from the m. semitendonosis at the legging stand gave a sensitive and reliable indication of the ultimate pH of the m.semitendonosis and the m. longissimus dorsi. It was considered that the relationship is sufficiently close and reliable for the machine's pH reading to be used as a basis for identifying at the scales, suspect dark cutting carcases and those yielding low pH meat most suitable for vacuum packaging.
The report showed that the system caters both for conventional beef systems in enabling immediate sorting and marshalling of carcases for next-day dispatch, or for hot-boning systems where dark cutting carcases need to be culled at the scales.
This project explored strategies for the control of dark cutting, and looked at thermography as a means of assessing cattle for the likelihood of yielding dark cutting beef, prior to slaughter.
In 2000, project TR.001 focussed on reducing the incidence of dark cutting beef carcases in Southern Australia. The aim of this project was to provide new information on the factors affecting dark-cutting in beef carcasses. The project focused on glycogen depletion and repletion in the M semimembranosus (SM) and M semitendinosus (ST) as an indicator of dark-cutting. Factors which were investigated included nutritional manipulations on-farm and at the abattoir, and handling procedures including mixing, transport, lairage duration and pre-slaughter handling. The overall objective of the project was to demonstrate to industry that the incidence of darkcutting beef carcasses can be reduced by at least 50%.
In 2001, a later project PRTEC.005 looked at the world best industry practice in objective prediction and management of dark cutting in beef cattle in meat processing facilities.
