How to Pit Cook a Pig

This article explains how the pigs are cooked in the deep at the historical Palomares Adobe site.  in the ground.  Pigs have been cooked in the deep pit and have turned out great every time.  The description is  from the perspective of an engineer (which I am) rather than from the culinary arts.  I am proud to say my most important cooking utensil is a shovel.

The pit was dug  at the Historic Palomares Adobe in 1998 to feed the several hundred people who come to the annual fiesta . The pit has now used about twenty times in the last twelve years with mouth watering success with both pork and beef every time.

Pit Details

The diameter of the initial hole was 4.5 feet across and 7 feet deep. A short shovel and bucket was used to dig the deep hole.  A metal sleeve was then placed in the hole. The sleeve was a welded steel sheet rolled and welded into a tube.  The metal tube was 6 foot deep, 4 feet in diameter and about 3/16 of an inch thick.  It weighs about 300 pounds.  Two U shaped eyes were welded on to allow the tube to be secured with ropes and was lowered into the hole with a wench. The bottom of the hole was filled with the ground level.  2 feet of rocks to bring the depth of  the pit to about 5 feet.  A 3/16 inch sheet of steel was used for a lid.  The top of the pit stopped at the ground level. If it was possible I would opt for having a layer of fire bricks or rocks around the outside of the metal sleeve to further store heat.  The metal sleeve stores heat and allows the pit to be reused  year after year with little maintenance effort.

The pit should be constructed 10 or more feet from to any tree or structure that might catch on fire.  Near by trees are sprayed just to make sure rising embers do not catch them on fire.  I suggest having a water hose handy during the wood burning phase.

How the pit works

The idea of pit cooking is to make the pit a medium in which  heat is stored for cooking.  A fire is built to heat the rocks in the bottom of the pit, the 300 pound steel sleeve, and ground around the pit.  The meat is then put in the pit and covered. The fire no longer burns but the residue heat cooks the meat.   It is like cooking in an oven that is heated up and then turned off when the food is put in the oven.
 
Temperature Vs. Time

This is a graph of pit temperatures measured in October 1999 with electrical recording type K thermocouple temperature probes. The probe wires were 6 feet long. The temperature recording devise was a "Bitlogger" made by Logic Beach Inc.  It recorded temperatures every minute.  Many different brands of temperature probes and loggers exist but this one worked nicely.

Two temperature values are important.

First the internal temperature of the pig needs to be greater than 180 degrees F to insure the meat is well cooked. The second important temperature is the air temperature in the pit which should not fall below 160 degrees before the meat is ready to serve.  This ensures the meat was kept so hot before serving that  it will not have bacterial problems.
Because the pig remained at high temperature for so many hours, the pig meat was so tender it fell off the bone.

The air temperature was measured with a thermocouple suspended in the pit air with 8 inches of lead wire exposed to the air.  The Pig internal temperature was measured with a thermocouple inserted in a leg close to the middle of a long leg bone.  The meat was about 4 inches thick above the thermocouple.  Use probe leads that will not melt.

Depending on the cook book slightly different temperatures may be found for what is considered for the pork to be cooked.


The 1999 Pig Roast

The fire was started 4.5 hours before the pig was place in the pit.  Only hard wood was used.   The pit was initially 5 feet deep to the top of the rock layer.  About a foot and a half of mostly smaller logs was put in the pit. News papers and some starting fluid had the fire burning strong in 20 minutes.  After the fire was burning for about an hour the wood level was maintained at a foot and a half below the pit's top. Wood was allowed to burn down to a depth of  2.5 feet depth but in the last 20 minuets a couple more logs were added.  This was probably too close to the start of cooking to be adding wood.  The wood was still flaming a little and the lid was put on to help snuff out the fire.   Meat was added but some flames were started in the process.   Data was recorded every minute from 5 PM (17:00) till 9:45 AM the next morning. Midnight was about half way through the chart. The pig was initially about 44 degrees F. The temperatures were manually checked every few hours through out the night. The pig was removed from the pit at 1 PM the second day.

It is a little scary when after 1.5 hours of cooking the pig temperature is only at 90 degrees and the pit air temperature and the pit temperature had fallen 175 degrees.

The total cooking time was 20 hours.  Resist urges to open the pit until ready to start serving the meat.

The meat fell off the largest bones with just a scrape.  People universally commented on how the meat was tender and wonderful.




The 2000 Pig Roast
The same set of thermocouples were used to monitor the pig and pit air temperatures for the third pig roast on October first, 2000.

Wood burnt until only glowing coals smaller than a inch and a half or so in diameter were seen.
In fact the start temperature of the 2000 pig roast was about the same temperature as the 1999 roast was 2 hours into the cooking.

When the fire is cooking and flames are roaring up, the wind can change the direction to put you into the stream of very hot air, so be careful.  Safety glasses are a good idea as they allow you to look into the pit without getting hot smoke and cinders in your eyes.  You can count on a bit of hair being singed.  Use a long metal pole to move the burning wood and coals around to get them burning  better and hotter.

This year 165 pounds of pig was cooked. I cook one pound of butchered pig per person. There will be left overs. The head is not counted in the weight or usually taken to be cooked.  The pig was placed in the pit some 2 hours after being taken from the meat locker.  The pig was covered in spices from the adobe's historical herb garden.  This included rosemary, thyme, lavender, limes, grape leaves, and oregano.  The meat was cut into smaller pieces with a sharp cleaver and placed in 4 large aluminum disposable heating pans.  The meat and pans were warped in aluminum foil. Each pan was then placed into burlap bags that had been soaking in water for over an hour.  Iron bailing wire was tied around the burlap bags lengthwise and widthwise.  A strong 2 inch diameter wire loop was made on the top of the bag to allow it to be lifted.  An existing 1/4 inch steel rod 3 feet long, with U bends at each end, was ran through the meats wire loop and the other end was put over a pole that two people could use to lower the meat into the hot hole. This keeps you safely away from the edges of the fire pit.

Before the meat the inserted the flames should be out with just red hot coals in the bottom of the pit.  Flames in part burn the burlap is used and is not good for most temperature sensors.

Cover the top of steel cover with at least an inch and a half of dirt to give thermal insulation and keep the pit temperature from dropping.  Cover the edges with dirt also, but know steam will escape out the sides.  The top of the dirt will feel pretty cool to the hand after 10 minutes..  The steel plate will be at the air temperature.  The humidity inside the pit stays high.  Putting the cover on the pit right after quickly putting in the meat is very good as it extinguishes the what flames there might be quickly. 

One of the meat trays has the thermocouple deep in meat and one thermocouple is used to monitor air temperature.  If the lead is two short it will be effected more by the heating of the wire from the sides.  If the wire is too long it will fall down and touch the top of a meet tray and not measure temperature too well.  If you do not have an electronic temperature recorder just take temperature with a remote probe (that will read high enough and not melt) every hour or two.

When the meat is cooked and it is serving time, use the hook tied to a long pole or handle so two people can safely remove the meat.

The meat is so tender it falls off the bone.  Have a trash can to put the bones in.  The meat is very quickly removed, with a set of tongs and cleaver, from the old cooking pan into a clean serving one.

In 1999 the burlap bags charred quite a bit.  In 2000 the burlap did not char at all even though set on the live coals.  They were very wet.

2000 Pig Rost Temperatures by Gallivan

One interesting physics feature shown is a pulsing change in temperature starting about 3:00 am, (when I was not watching).  The effect started just as the upper air temperature fell below boiling (212 deg F).  This oscillation effect could be seen for the next 5.5 hours. It could be that the four different containers of pig on the coals were starting to steam at that time and built up pressure and changed the humidity of the pits  inside.   When excess steam built up it would make a tunnel through the dirt and shoot out hot steam.  I do not think excessive juice was falling into the pit to cause the cooling.


1998 Pit Cooking
This was the year the pit was dug. In 1998 the pig was cooked by letting the fire burn for about 5 hours.  The pig was then cooked in the pit for about 7 hours.  The cooking was a rush job but the pig was definitely done. The whole pig was cooked and weighted only about 65 pounds.  A 45 pound pig was put on a spit, cooked some three hours more than the pit pig and was barely done enough. Cooking a whole pig on a spit was not easy.  In 1998 the temperatures were not measured or recorded electronically. When the pig is pulled from the ground and a mechanical thermometer is inserted it is probably too late to do further cooking for an event. The pig was smaller than done in subsequent years so it put less of a thermal load on the pit.  The pit  probably stayed hotter longer and heated up the pig faster.  Not having temperature data leads to hand weaving and guessing.

Wood
A cord of wood is 128 cubic feet of wood, bark and air gaps. This is equivalent to the volume of a stack 2x4x16 feet.   The volume of the pit is about 50 cubic feet, or 40% of a cord.   About 35% of a cord of hardwood was burned in the pit.  If the wood is not dry and seasoned, a lot of the heat is wasted boiling water out of the logs in the form of steam instead of heating up the pit.  When the coals are burning the temperature is supposed to be hotter then when flames are burning, but I do not think the coals are adding as much heat to the pit as when the logs are burning.

 Burning has three stages.  In the first stage, heat is absorbed by the fuel, and water in the wood is evaporated as steam. In the second stage, the volatile matter is released and burned. The volatile ignite, burn, and give off heat at about 1,000F. The third stage in combustion occurs when most of the volatile matter has been removed. The surface of the remaining residue (charcoal) reaches a glowing temperature and burns when oxygen from the air is brought in contact with it. This combustion exposes additional surface area until the entire mass is consumed.

The efficiency of converting potential heat into the heating of the open pit is very low.  A regular fireplace can be less than 10%. Most of the heat is in the hot rising air. The hot air rises in the middle of the pit  and pulls the cooler air down around the sides, that we are trying to heat.  The radiative heating has to be an important part making the pit's steel sides hot. Conduction will heat the bottom rock layer.

Look at the ends of logs to determine if the wood is dry. It should have been dried at least over a full summer season. Seasoning checks (open splits) can be seen on the ends of at least partially dried wood. These "checks" result from the natural shrinkage of wood as it dries. If the wood has been seasoned outside, it will be somewhat discolored. It should not smell sappy. The wood should be hard and sound. Decayed wood has a low fuel value and, when dried, will feel light and burn rapidly.


Some calculations
Newly cut hardwood has about 75% moisture content, seasoned air dried hardwood is about 20% moisture.  A cord of hardwood weighs about 4,400 pounds.  A "cord" of water would weigh 7,660 pounds. Thus the average density of a cord of hardwood is 0.57 grams/cc.

A cord of hard wood contains about 16 million BTU's (British thermal units) of energy.   A soft wood has only about 60% of this value.

To heat the pit only about 0.35 cord was used or 3.8 million BTU's.  To heat 300 pounds of steel to 300 degrees Fahrenheit above ambient takes 778,000 BTU's.  I assure the closed air temperature is a close indication of the sleeve's effective temperature.  Steel's specific heat is 0.111 BTU/(lb-deg F).   Not counting the rock bottom to the pit or the lid the heating efficiency is 20 %.  I am sure the rocks are getting pretty hot but I have not data on the temperature profile of the rocks. I imagine the pits heating efficiency is close to 25%.  Incidentally electrical engineers have historically used metric units.

After 14  hours of cooking the pig has risen 120 degrees to an equilibrium temperature, from 44 to 184 degrees. Assuming the pig has the specific heat of water, the pig has absorbed 165 * 140 = 23,100 BTU.  This is some 3% of the pits stored energy.


2012 Zorro  themed Together We Read program

In October of 2012 a pit roast was again performed.  About 14 roasts have been successfully accomplished to date.  One has even written up in the LA Time culinary section.

A half cord of very hot burning Eucalyptus wood was ordered from Pomona Feed and Fuel, although less than  a tenth of a cord was actually burned to heat the pit.  The fire was started about 8 on Saturday night.   The fire was initially quite hot but the last wood was put in about 10 PM.   An optical pyrometer indicated the pits steel wall's temperatures were up to 560 degrees and was confirmed with a thermometer attached to the pit's walls.   The coals burned until about 1:15 in the morning when the meat, with thermocouples deeply inserted, were put on the embers.   The pit was relatively cool compared to the flaming heating stage.  The side wall were optically measured with a far Infrared pyrometer to be 510 degrees F when the steel lid was put on the pit. The lid was covered with about 3 inches of dirt to insulate the pit.  At about 1:30 the pit cooking started and the pit air temperature was found to agree with the optical readings of 480 degrees F.  A graph is attached below that shows the temperature profile of the meats and the pit. The thermal timeconstant of the pit ( time to drop 63% of the temperature level) is about 8 hours.   Jumps in the outside air temperature were do to solar radiation on the uncovered thermocouple, as the peak air temperature was actually 92 degrees F. From the graph it can be told the meat was actually quite well cooked by normal standards in 4 hours.  Our slow cooking process lasted 15 hours. The meat took 20 hours to prepare counting getting the pit up to temperature.

Fifty pounds combined of boneless beef and pork were cooked in four seperate containers. The meat searved seventy people with leftovers.  Our initial estimate was half a pound of boneless meat per person. The meat was wrapped in aluminum foil, placed in aluminum covered pans and tied with bailing wire to keep the temperature probes in and allow a wire loop to be made to allow meat to be lowered carefully onto the coals.  The meat was again very tender and juicy. Local herbs were used for seasoning.  The meat is not sliced but rather pulled apart to be served. 

2012 Temperature profile

October 2012 Temperature profile graph of meat, Pit Air temperature and outside atmospheric air.

 


Horno Cooking

The pit cooking technique is the same as used for the "horno" or "primitive outdoor adobe oven". The Italians used the same technique for the very hot outdoor ovens used to cook pizzas.
Palomares Horno- 2001 Inside the Horno

The first Picture is of the Horno.  The second Horno Picture is a flash picture of the interior.  It shows some coals on the floor, a wooden stick to remove food and the fire brick interior.

The Palomares Adobe has an horno that has been used to bake great bread.  A fire is built in the oven to heat up the inside of the adobe dirt oven.  After the fire has been burning for hours the wood is removed and the inside is allowed to cool down to the proper cooking temperature. (This an old art by itself and uses such techniques as throwing corn husks in the horno and estimating the time it takes them to ignite). Several shifts of food are then cooked. The food needing the highest temperature is cooked first.

Disclaimer:
I advise anyone doing pit cooking to do their own research and always make sure safety is first and foremost. I have done fire eating and walking and can tell you do not want to be burned. I hope this  helps others understand and encourages them to do pit cooking, even if you use other styles of pits.

Jim Gallivan
"The Authority in Unrestricted Pig Roasting"

Revised July 9th, 2011.