lipid insulators

[brainj]

I tested safflower oil, olive oil, lard, and vegetable shortening for their ability to insulate (in an ice bath). The lard took the longest to fall in temp with the crisco as a close second. Why did this happen? Does this have to do with saturated versus unsaturated fats? Or is is solid versus liquid? I thought the results were because saturated fats insulated best, but the crisco did very well and it is a plant fat...I need to finish my 8 pg literature review and my conclusion, but am not sure how to proceed. I know that if the valence molecular level is full, then insulation occurs. The valence level is full for the saturated lard, so I thought that would be the explanaton, until I realized the Crisco was so close to the lard (2 1/2 hrs. for lard and crisco as opposed to 20 mins for the oils). I don't have anything on solids and liquids in my Review of Literature, and that problem came about from not fully understanding Crisco vs. lard. Orginally, I thought that Crisco was a saturated animal fat like lard. I wrote about animals thinking that half my variables were from animal origin. Turns out that the Crisco is actually vegatable shortening. I don't how I missed this fact! Coukd you please help? My project is due very soon!

[kgudger]

Hello and welcome to the forum! I don't know exactly which experiment you are doing, but i do see one area of concern. You are surprised that Crisco and Lard have similar characteristics, and ask

Does this have to do with saturated versus unsaturated fats

? Crisco is essentially saturated vegetable fat. Please see http://en.wikipedia.org/wiki/Crisco and http://en.wikipedia.org/wiki/Hydrogenated_oil. This probably explains why they have the same insulation value. This should also help with your literature write up. With respect to solid vs. liquid, one hint is that Crisco is a solid at room temperature. In general, a saturated fat will be solid at room temperature. Partial hydrogenation (like margarines) is done to cause the oil to solidify. Please let us know how you are doing.

Keith

[Craig_Bridge]

Or is is solid versus liquid?

Thermal transfer via conduction and convection in solids and liquids differ significantly. Most liquids change density with temperature change which causes convection currents. Colder molecules within a liquid phase will flow down and warmer molecules will flow up. This stirring movement allows more efficient heat transfer within the liquid. Solids can't flow and move around so heat transfer within the solid is entirely conduction so the middle of the solid takes the longest to change temperature.

How did you control your comparisons? Did you control the starting temperature of the test material and ice bath? In the case of the solid test materials, did you let them sit at the starting temperature long enough for the entire mass to equalize in temperature? Did you control the weight of the ice bath? Did you use the same test container? Did you weigh out the same amount of each test material or did you use the same volume? The total amount of heat transfer is a function of the mass (or weight). If you used volume and the test materials are the same density, then their masses won't be the same. If your lard samples weighed more than your crisco samples then your difference result might be more to do weight differences than material thermal property differences.

[brainj]

Thank you for you quick response. I used the same size rubbermaid container (3L) for the ice bath and tried to my best ability to keep the temp of the ice bath constant checking with a thermometer and adding some ice occassionally. All of the lipids were tested in a smaller disposable Ziploc brand round container (473ml) placed in the ice bath. I drilled a small hole in the lid of the lipid container and used an automatic temp. probe recorder. I did not weigh the lipids, but rather kept the size the same filling the same container each time. Water was my control. Constants were the temperature probe used connected to our computer, the amount of lipid, size of ice bath. There will be some error in ambient temp. as it took me weeks to run all the trials. Also, big question, I thought when I looked up crisco it said it was only 3 g saturated fat and 6g polyunsaturated. I thought that meant I couldn't call it a saturated fat, because it is more polyunsaturated. That was where all my confusion started! My whole conclusion was that sat. fats insulated best, but if crisco is not saturated, I will need to explain why it did so well, and I don't really know why. I would be grateful for any clarity you could give me.

[Craig_Bridge]

If the Crisco you tested was a solid (and not the Crisco Oil), then my previous brief explaination of heat transfer in solids and liquids is probably a major part of the differences. Solids will typically retain heat longer than a similarly shaped volume of liquid simply because their molecules don't flow around like those in a liquid do.

I realize that you have done a lot of work to obtain your results; however, you really need to determine if the differences in weight between the different oils and the weight differences between lard and solid vegetable shortening to determine if there was enough weight difference to explain the differences in the heat retention times.

If you weigh equal volumes of each of the liquids, lard, and vegtable shortening, you can estimate the weight ratios of your various test samples without redoing all of your experiments. This is far from ideal and introduces additional concerns about errors. Without doing this or repeating your experiments with equal weight samples, I don't see any way you can draw any meaningful conclusions. The differences in basic heat transfer properties between sollids and liquids also prevent you from coming to any conclusions based on comparing the results with a liquid to a solid other than the differences attributable to the heat transfer differences in solids and liquids.

[ChrisG]

Craig raises a good point about the mass of each sample. If you don't have time & resources to make those measurements yourself, you can come up with a mass estimate based on the density of the substances, which is available online:
"The density of Crisco Pure Vegetable Oil. was 0.9182 g/ml at 20 °C"
http://linkinghub.elsevier.com/retrieve ... 4X03001689
http://hypertextbook.com/facts/2000/IngaDorfman.shtml
The density of those various lipids appears close enough that the relative mass of each trial might not be a major factor in your experiment.

To answer the question about solid versus liquid, you could easily repeat the experiment by using crisco and lard in a hot water bath that is warm enough to liquify those fats.
http://www.ochef.com/1157.htm

Because you are looking at processes of heating and cooling that are affected by liquid convection, you might also consider the viscosity of the oils, which strongly affects convection and heat transfer.
These values can be found in textbooks or scattered around on the web. For example:
http://hypertextbook.com/physics/matter/viscosity/
If you have trouble finding what you need, please let us know!

Chris