Problems related to meat pH
There are two main adverse conditions related to pH in meat quality:
1 pale, soft and exudative (PSE)
2 dark firm and dry (DFD).
These conditions can influence the quality of salted meat products. The following sections give some background information on these conditions.
Pale, soft, exudative meat
As its name suggests, PSE meat has a pale, lean, soft texture and a low water-binding capacity. Numerous studies have been carried out to understand the development of PSE meat condition.
The main reasons are:
- A pH and temperature phenomenon: an accelerated rate of glycolysis while carcass temperature is still high. This leads to a lower pH at high temperatures, causing greater protein denaturation than in 'normal' muscle resulting in elevated water loss through breakdown of the structural proteins (Bowker et al., 2000).
- Myosin in PSE meat has a low solubility; therefore this meat swells less than normal meat and water retention is reduced. This is because denatured proteins cannot hold or bind muscular water as well as fully native proteins, mainly because the length of the myosin filament is reduced by around 8-10% during this process of denaturalization, and the capacity of the meat itself to retain its own tissue water is greatly reduced as a result (Gregory, 1998; Feiner, 2006).
- PSE meat is pale because the sarcoplasmic proteins precipitate during high-temperature and low-pH conditions to form a reflective pale coating on the structural proteins of the meat.
As you saw above, glycolysis is one of the key metabolic pathways in the conversion of muscle to meat. The question is, what are the reasons for an abnormal post-mortem glycolysis? Various possible mechanisms have been proposed, which are not independent of each another, including:
- muscle which has predominantly glycolytic fibres (for example, 'whiter' muscles such as gluteus medius, longissimus, biceps femoris)
- elevated muscle glycogen levels
- pigs with porcine stress syndrome (PSS).
The last two are known to be associated with gene mutations. In the case of pigs with PSS, when stressed, these pigs show an increased glycogenolysis, resulting in rapid lactic acid accumulation before the meat has cooled (Bowker et al., 2000).
Dark firm, dry meat
DFD meat, on the other hand, has a high water-holding capacity. This condition occurs when pigs suffer chronic stress and use most of their carbohydrate energy reserves before they are slaughtered. The consequence of this is that not enough lactic acid can be produced to reach muscle pH values lower than 6.0 at 24 hours after slaughter.
Some factors that have been found to increase the risk of DFD are:
- pre-slaughter exercise
- winter season
- fasting for too long before slaughter
- low stocking density during transport
- increased lairage time at the slaughterhouse.
(Guardia et al., 2005).
Look at this list of factors and write down the mechanisms underlying the increased risk of DFD.
Effects of PSE and DFD on processed pig-meats
The addition of salt to ground or diced meat normally improves its water-holding capacity. However, as you have seen above it depends on the meat pH. If you need to remind yourself of the relationship between pH and water-holding capacity you should refer to the subtitle 'water holding capacity' in this section.
In the case of DFD meat, this has a pH higher than the isoelectric point and therefore water-holding capacity is higher than normal. Less salt is needed during curing to achieve a satisfactory water-holding capacity, yet the meat can be more prone to bacterial spoilage partly because water activity will be quite high (the reason for this will become clearer in the next section).
Conversely, in the case of PSE meat, salt helps to solubilise and extract the myosin in the meat, which will form a gel and bind the meat after cooking. Its suitability for processed meats depends on the extent to which myosin gel formation is needed for the particular product. However, PSE meat can produce excessive volumes of a fluid jelly, rendering the product unappealing. In addition, the salt has to get to the myosin to solubilise it and the precipitated proteins in PSE meat can prevent this, contributing also to poorer gel setting with this type of meat.
Summarize the overall effects on the quality of a salted meat product if DFD and PSE meat were included in it. Would the effects be good or bad? Explain your answer.