Is Lard Still an Appropriate Media for Testing HGIs?

September 14, 2016

Why do we still use pork lard for testing and rating hydromechanical grease interceptors? There are so many different oils used in cooking today with varying densities and viscosities; isn’t it time to change from pork lard to something else? These questions come up from time to time and deserve an answer.

The issue at question is whether the results of tests that use lard would vary distinctly from the results of tests that use some other test media. There would likely be some difference, to be sure, but the question is whether the difference is statistically significant or not.

All grease interceptors work off of the same principle in fluid dynamics called gravity-differential separation. Fats, oils and grease (FOG) have densities less than water. Specific gravity is the ratio of the density of a liquid (substance) to the density of water. At room temperature water has a density of about 1.0.

The lard used in testing grease interceptors is required to have a specific gravity of 0.875 +/- 0.005. What you may not realize is that at room temperature lard has a density in the range of 0.91 - 0.92. At 160 deg. F the specific gravity of lard will be closer to 0.87 – 0.88, which is why this range is specified in the standards that govern the testing and rating of grease interceptors.

Now consider that the common cooking oils used today have specific gravity's that fall in a range from 0.91 to 0.93 at room temperature (60 deg. F):
Oil, Corn 0.924
Oil, Coconut 0.924
Oil, Olive 0.91-0.92
Oil, Peanut 0.92
Oil, Rapeseed 0.92
Oil, Sesame seed 0.923
Lard 0.91-0.92
Note: data obtained from, “Specific Gravity and Viscosity of Fluids”


According to a 1992 study titled, “Densities of Vegetable Oils and Fatty Acids,” by Hossein Noureddini, B.C. Teoh, and L. Davis Clemmons, all members of the Department of Chemical Engineering at the University of Nevada, the density of several common vegetable oils did in fact decrease as temperature increased. Here is a sample of the data contained in Table 3 of the report:

Density of Vegetable Oils      
oC (oF)  Rapeseed Corn Soybean Coconut
23.9 (75) 0.9073 0.9188 0.9193 0.9250
37.8 (100) 0.8987 0.9082 0.9082 0.9107
48.9 (120) 0.8908 0.9028 0.9023 0.9033
60.0 (140) 0.8839 0.8939 0.8939 0.8949
82.2 (160) 0.8686 0.8800 0.8795 0.8795


I think we can safely assume that in a worst-case-scenario the specific gravity's of today’s cooking oils will be similar to that of lard at 160 deg. F. That being the case, what benefit would there be to change from lard to another oil for testing and rating grease interceptors?

Consider also that the rise rate for any given liquid with a density of less than 1.0 will depend upon these factors:

  • density of the globule
  • size of the globule
  • viscosity of the media/globule
  • temperature of the media/globule
  • velocity of the globule (horizontal)
  • flow environment (turbulent or laminar)

Of these factors, only density and viscosity would be distinguishing characteristics between lard and other cooking oils. Viscosity is how easily a liquid pours. Water has very low viscosity while room temperature Honey is very viscous. The more viscous a liquid is the more resistance works against it, slowing its rise rate. It may stand to reason that a fat which is a solid at room temperature may be more viscous than an oil, which is not. At 130 deg. F lard will have a viscosity of 34.3 centistokes, while the previous list of oils will have the following values:

Oil, Corn 28.7
Oil, Coconut 14.7 - 15.7
Oil, Olive 24.1
Oil, Peanut 23.4
Oil, Rapeseed 31
Oil, Seasame seed 23
Lard 34.3

Note: data obtained from, “Specific Gravity and Viscosity of Fluids”

Notice that the viscosity of each of these oils is less than that of lard, which means that we should be able to safely conclude that in a worst-case-scenario these oils will have better rise rates than lard given all other parameters are equal.

The current tests under PDI and ASME adequately evaluate the factors that affect gravity-differential separation resulting in reliable reporting of an interceptor’s efficiency using lard. Lard has been used successfully as a test medium for grease interceptors for over 70 years, by hundreds of manufacturers and thousands of devices, which have millions of installations across the country. 

All one has to do to verify whether today's modern HGI is operating efficiently in the real world to to open one up and inspect its contents. If there is FOG collecting inside then you know the device works at least up to its rated capacity. 

Changing from Lard to say olive oil, would have a beneficial effect on reported efficiencies because even though their densities are very close, the viscosity of olive oil is much lower than lard. Olive oil would actually be more efficiently removed in a grease interceptor than lard.

I wonder if the question about using lard as a test media is the result of decades of installations of grease interceptors, which while working efficiently up to their rated capacities, are rarely cleaned out as often as they should be (i.e. daily or every couple of days). HGIs are commonly found serving only multi-compartment sinks and are being serviced on a monthly or bi-monthly or even quarterly basis. While it is true that the devices operate efficiently up to their rated capacity, they simply are not being maintained often enough to ensure their ongoing efficient operation. There is simply no test protocol, test media, or test apparatus that can ensure that an FSE will keep their interceptor properly maintained and in efficient operating condition.

Now that you have a better understanding of the role of density and viscosity of the media used for testing and rating grease interceptors, hopefully you will have more confidence in the test method employed by ASME A112.14.3 and PDI G101. Lard is an appropriate media and any alternatives would only improve test results but wouldn’t give anyone assurance that the interceptor would perform better in the field than on the test apparatus.


  • The Interceptor Whisperer

    Oct 04, 2016

    Good point Greg. Still, if your argument holds, then viscosity of the lard/oil/grease is not relevant and therefore Lard is an effective media today as it always has been. In other words, there is no need to change to another media simply because other oils are more commonly used in cooking today. The debate on temperature as regards to testing rages on and so long as the test requires 160 degree water/test media mixture the economic benefit of switching away from lard would appear to rest solely on the difference in cost to purchase, which favors lard over virtually any other cooking oil.

  • Greg Williams

    Oct 03, 2016

    Hi Ken, I do appreciate this discussion about interceptor performance but I am afraid that this time your article contains a critical error. Looking at the beloved Stokes’ Law, v = 2/9(d1 – d2)gr2/η, you will see that there is one term for viscosity at the very end, nu. (Sometimes it is shown as mu. One is kinematic viscosity, the other is dynamic viscosity, there is a simple conversion between the two.)

    This term is for the viscosity of water. The viscosity of oil does not enter into the equation so, as far as anyone knows the viscosity of oil is not relevant. I am sure the 160 F was chosen not to lower the density but to keep the lard molten. If lard started solidifying in the pipes is would essentially randomize the test and make is useless.

    Since hot water improves performance (again you can work this out with Stokes’ Law, which I have done in a post to the Pre-treatment coordinators’ Yahoo group) one could argue that using hot water and lard actually inflates performance over using cold water and oil. I have not done that calculation but it certainly throws into doubt the conservative nature of using lard.

    It is true that there is a large amount of historical data with lard testing and there is value in being able to compare new tests to old tests. Having said that, if we never took into account progress in knowledge we would still be using whiskey as an anaesthetic in amputations. Oil has the advantage of being liquid at room temperature so you could test with it at 160 F or 60 F, or anywhere in between, whatever makes sense. Also, if you are testing large capacity units the fact that you do not have to spend time and energy melting hundreds or thousands of pounds of lard reduces the cost and environmental impact of testing. I run a testing lab so I make more money using lard but I cannot promote a practice simply because that is how it was done 50 years ago. There needs to be a better reason than that.

  • The Interceptor Whisperer

    Sep 22, 2016

    Bill, the viscosity of cotton seed oil at 130 deg. F is 20.6 centistokes, though at room temperature it is much closer to lard, but still less.

  • Bill Deardurff

    Sep 22, 2016

    Great article. After reading just the title, the assumption is who uses lard any more, we should have a more realistic test. However, Lard appears to be a good media since it is generally the “worst case”. I have heard that one chain of fish restaurants uses cottonseed oil. Local stories imply it has a heavier density and they needed to add a second interceptor (for more time). I do not know how cottonseed oil’s viscosity compares to lard.

  • Chris Karner

    Sep 18, 2016

    This is a great article. Thanks for the information.

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