Pork processing: Proteins influencing water holding capacity and drip loss

Dripping meat

When you buy a vacuum-packed joint of meat, there is often a red solution in the packaging which might be assumed to be blood. In fact, it is a result of leakage from the joint by a process known as drip loss and the liquid consists of a solution of soluble proteins in water from the breakdown of the meat.

Fresh meat from slaughtered animals contains about 70% water which is essential to its quality but it begins to leak away soon after death. Boning and cutting can result in losses of 1-2% and further long-term storage can lead to much greater losses of up to 12%. Drip loss on this scale represents a large reduction in the yield of meat leading to financial losses as well as affecting the appearance, nutritional value and palatability of the meat to the consumer.

Some animals retain water after death better than others and this water holding capacity (WHC) appears to be the key to drip loss, although the factors governing WHC are unclear. A number of factors are thought to be influential, including genetic predisposition and the meat handling protocols.

A team of scientists based in Ireland has explored possible molecular factors in a proteomics study of pork. Pig farming is important economically in Ireland, providing employment for a large number of people and a large export business, so understanding yield reduction due to drip loss will have major consequences.

Anne Maria Mullen, Alessio Di Luca, and Ruth Hamill from the Teagasc Food Research Centre, Ashtown, and Giuliano Elia from UCD Conway Institute of Biomolecular and Biomedical Research, Belfield, wanted to identify proteins and peptides associated with drip loss in order to try and gain a better understanding of the process.

Drip loss comparison

Muscle samples were taken from 31 pigs that were killed at a slaughterhouse and the drip loss exudates were collected by centrifugation. Portions taken one day postmortem were used to classify the meat into high, intermediate and low drip loss types, corresponding to losses of approximately 6, 4 and 2.5%, respectively.

The exudates collected on days 1, 3 and 7 postmortem were used for the proteome analysis. Proteins in meats from each class were compared in one experiment by 2D differential gel electrophoresis using fluorescent dyes. After image analysis of the gels, the protein spots which displayed different abundances across the phenotypes were selected for further investigation.

The samples were run again by 2D PAGE but this time on preparative gels and the protein spots were cut out for analysis by mass spectrometry. MALDI MS and MS/MS were carried out in the first instance and the peptide masses were searched against a porcine database. When spots could not be identified this way, the proteins were reanalysed by LC-tandem-MS.

Proteins related to stress, structure and metabolism

A total of 376 distinct protein spots were visualised and 89 were identified by mass spectrometry. They were used to build a 2DE database which is available as Porcine Database in the University College Dublin 2D PAGE database. Most of the proteins fell into one of three groups representing structural proteins, stress response proteins or metabolic enzymes and many were found in several spots, showing that isoforms must be present.

Some interesting proteins were detected within this large group. For instance, breakdown products of myofibril structural proteins that have previously been linked to meat tenderness were detected, so they might be useful as biomarkers of pork tenderness. Peroxiredoxin, a second protein associated with tenderness, was also identified.

Within these proteins, the abundances of 20 were found to vary significantly between the three phenotypes. The changes were visualised by principal components analysis which showed clear clustering of the phenotypes using two principal components.

When the high and low drip loss phenotypes were compared, triosephosphate isomerase, creatinine kinase M-type, serum albumin and transferrin were less abundant in the high group, while β-tropomyosin was more abundant.

Three stress-related proteins were also found to be differentially expressed. One of them, stress-induced phosphoprotein, was less abundant in the high and low drip loss samples than in the intermediate one, the first time that an association with WHC has been reported.

Some myofibrillar proteins also varied in abundance between phenotypes, including titin, tropomyosin α-1 chain and β-tropomyosin. Their changes indicate rapid degradation of the cell structure following animal death.

Although this is a preliminary investigation into the WHC of pork, the results will help to decipher the molecular mechanisms involved while the new protein database will be useful for other researchers in this area. In the long-term, it may be possible to control drip loss and maintain the yields of pork meat during storage.