On days when we talk about pork in class there is often an acronym, PSE, used to describe low quality pork. The other day, while working on pork, we found at least 25% of the classes pork butts were very soft and mushy without much marbling. I stated that these were PSE and would not be a very high quality dining experience when eaten. PSE or Pale Soft and Exudative refers to the initial color and texture of the meat and is not caused by microbes or virus, it is more of a condition. In pork, fat and meat should be firm when chilled and the color can range from a light pink to a darker, almost red color and still be considered quality. Some breeds will have a lighter color and also the age and diet of the animal will have something to do with it. But PSE pork will lose some of its natural color and become a light blotchy pink. Its meat will be soft and jello-like and the fat will be very mushy also. If you place an opened cut of PSE pork in a pan it will ooze ( exudate) moisture and in a vacuum bag will exhibit excessive purge. When cooked PSE pork will dry out much faster when cooked and will not hold a brine or process well.
PSE is caused by a sudden drop in pH levels during chilling which causes cells to leak out moisture. http://www.asas.org/symposia/0601.pdf The fact that PSE pork has a low pH actually means it is preserved somewhat. The level of pH is part of what is considered when curing meat such as salami. Pork with a pH of around 6 is ideal where PSE pork is nearer to 5.
So what causes the sudden drop in pH? Hogs will sometimes get a condition known as PSS or Porcine Stress Sydrome. This is usually a heritable condition that has been passed on through the gene pool. Pigs that typically have PSS have what is known as the Halothane gene, and will be prone to the disease but will not always dsiplay it. Normal pigs that don't have the gene for PSS sometimes become super stressed when transported or placed in new pens before slaughter and will also turn PSE. Its a tricky thing. PSE is caused both by genetics and also environment. Weather also can be a contributor. Hot humid weather tends to trigger it too. Any dramatic weather changes can cause it. The incidence of PSE are higher in summer months than in others. Stunning and slaughter facilities can contribute to it too. If pigs are stressed at the time of slaughter they can show it at a higher rate. Pigs that are miss handled even for a short time right before slaughter can show signs.
Some farmers claim that hogs that are more sensitive to stress are those that have no entertainment. The hog in the wild is a scavenger that needs to root out its food and dig up earth. They bask in the sun when its not too hot and create wallows when it is. If they are stuck in concrete pens they get bored and understimulated leading to a stress prone pig. Hogs that are given more stimulation tend to be easier going when it comes to slaughter time.
Not all PSE is the same. There are varying levels of it. A hog that goes through slaughter may appear normal but may end up showing some PSE condition in its final cuts. One side of the pig may have it while the other not so much. Severe jello - like conditions are almost always from pigs that have the PSS gene whereas slight PSE will be in normal pigs that were handled poorly before slaughter. Also the chilling temperature can have an effect after the hog is slaughtered. If the temperature is not brought down quick enough, PSE like conditions can set in. There is also another condition known as RSE ( Red Soft and Exudative) where the pork is looking more like quality pork but still has the mushy texture. I have seen this quite a bit lately too.
So why were 25% of the pork butts at some level of PSE or RSE in my class? The pork came from the midwest and it could be the recent swings in weather had some effect. Maybe that particular load came from hogs that could have been jostled around in the truck. But there could be a systemic reason for it. In the late 1980s hogs that were considerably leaner were introduced into herds. These hogs tended to be carriers for the halothane gene, thinking they would be better producers of lean. This web site explains the entire problem quite well. http://www.omafra.gov.on.ca/english/livestock/swine/facts/04-053.htm The Landrace pig known for its fast growth and leaner muscle was found to be a carrier of the gene more often than other breeds. This breed still influences our herds today. This is not to say that all Landrace are going to have PSE or that any of the popular heirloom breeds can't have it. Any pig can have it but the trend towards leaner hogs has certainly increased the instance of stressed hogs. From my observations the small market heirloom breeds I've worked with lately simply don't have it. They are solid and the fat is super firm.
Another issue could be the use of hog food additives such as Paylean which are designed to increase muscle mass. Ractopamine is active ingredient that is used to create the larger hogs. In general Paylean makes pigs prone towards stress if handled poorly. If overused or used incorrectly it can create fatigued hogs that are no longer ambulatoryrequiring more aggressive methods to get them to slaughter. Paylean does not cause a pH drop or color change directly but if the hog is more prone towards stress because of it then indirectly it could have a negative PSS effect. The effect of super lean pork is a dry cooking product even if it isn't exudative.
Here is the website from Paylean and they state that it has no negative effects on pork color or tenderness and helps to improve yield scores. http://payleanpays.ca/pdfs/Paylean%20Technical%20Reference%20Guide.pdf My issues are with overly lean pork. Too lean and no flavor, that simple.
Another protection against PSE is to inject sodium bicarbonate into the carcass right after slaughter to adjust the pH level. This effectively stops the effects of PSE. Hormel Corporation is licensed to produce sodium bicarbonate treated pork for certain products. http://nationalhogfarmer.com/news/farming_pork_quality_2/
Whether you buy large name pork or pork from a local farmer PSE can be an issue. I've seen pork that had great marbling and solid fat in the same bag as jello pork. I wish large companies would isolate the higher quality non-PSE pork and sell it as such. I find that my local producers are raising a very high quality but very fatty pork and rarely see any PSE. My nephew raised hogs that we slaughtered and chilled. One had a slight exudation probably due to the chill time but its flavor was far superior to any pork I've eaten this year. They were very happy hogs indeed.