Neil, the volume is the area of the cylinder times the height. Area of a circle is the radius (half the diameter) multiplied by itself and then pi (3.14).
I don’t know what assumptions are made when a recipe calls for a specific pressure during pressing, but it seems like some assumption about the volume must be made and that is what is translated into the surface pressure. Interesting topic, I bet there is a doctoral thesis on it somewhere
Guys,
From what I have read, commercial cheese operations work with PSI while the home cheesemaker usuallyworks with force. This is something I had never really considered before, but I’m now convinced that force is going to be much different when working with different size wheels.
If I take 30 lbs. of direct pressing weight (force) and put it over a 4” wheel I am getting most (if not all) of that weight on the cheese. It’s weight, pushing directly down on the entire surface of the cheese. On the other hand if I use that same 30# on an 8” wheel I am putting that direct weight on the center of the wheel but I’m not sure that the weight is being distributed over the entire surface. I don’t really have any mathematical evidence that this is the case but it does seem logical to me.
For my next 8” wheel I’m going to start by doubling my weight. If the curds still do not seem to be knitting together as I’d like, I will increase the weight by half again.
I want to move to doing larger batches all the time but I have to figure this out before doing so exclusivly.
I have the pots and I have the molds for larger batches of cheese. I just don’t have the experience at this point.
In your press, the weight is transferred through the follower, which spreads it evenly - resulting in PSI. Force and PSI are two ways of describing the same thing, one from above the cheese and the other from within the cheese.
But Rich….
On my follower there are concentric circles in the center. These do not go all the way out to the edge of the follower. Since this is the case, it seems to me that there will be more force (PSI) over the center of the cheese.
That’s why I typed that force is different than PSI. In my opinion, force would be the pressure exterted over a given surface area.
From what I’ve seen on a commercial press, (horizontal, pneumatic style) they use large flat followers that are the exact same diameter as the molds. They will take several molds and line them up and then press with a given PSI via a pneumatic ram. In this case it seems that the force would be exerted over the entire surface area of the cheese whereas with my type of vertical press, I’m pressing heavily on the center but not necessarily on the outer parts of the wheel.
Please don’t get me wrong. I do respect your opinion and I do not have any type of engineering background. This is all new to me and like I said before, I have no mathematical proof for my opinion.
I guess the only way to know for sure that I’m thinking correctly is to give it a try. I just hate trying something (that takes 6 hours and $20.00 worth of supplies) only to find out that my thinking is flawed.
Yeah, I not only have perfectionist tendencies, I’m cheap too.
I have no problem with cheap, but frugal sounds much better. Dave, your follower must go to the edge of your wheel, because if it didn’t your curd would squish out around the follower. It sounds like it increases in size by increments. But the end result is the same - the pressure is exerted across the entire surface evenly. In fact, I’m thinking the purpose of the concentric circles you mentioned is to keep the lowest level of the follower from bowing in at the center. If the pressure were greater in the center than at the edges, you would end up with a concave upper surface. The follower does not absorb pressure. Being a solid it merely becomes a part of the pressing agent, evenly disbursing it across the surface.
As to the commercial type press, the principle is the same as applying direct weight. Its just a different method of applying force to the follower. I’d be willing to bet the curd inside the mold cannot distinguish one iota of difference between pneumatically induced vs direct weight induced pressure.
If I take 30 lbs. of direct pressing weight (force) and put it over a 4” wheel I am getting most (if not all) of that weight on the cheese. It’s weight, pushing directly down on the entire surface of the cheese. On the other hand if I use that same 30# on an 8” wheel I am putting that direct weight on the center of the wheel but I’m not sure that the weight is being distributed over the entire surface. I don’t really have any mathematical evidence that this is the case but it does seem logical to me.
If you put 30# of weight (or pressure, weight after all is just a measurement of the pull of gravity on a body on a scale ) on the center of a 4” follower, you are getting 30# evenly spread over approximately 12.56 square inches for a pressure of 2.4 pounds per square inch. If the round is 8” then the area becomes about 50.25 inches and the PSI is 0.6 pounds per square inch.
As long as your follower is reasonably dense and flat, the pressure will be distributed over the surface of the cheese evenly as long as the pressure on the follower is equally centered around the center of the follower. It may not be intuitive, but it is true. The follower does not “absorb” any of the pressure or distribute it unevenly unless the follower is made of compressible material and not perfectly flat and thick. (For nit-pickers, a follower may exhibit these characteristics at a microscopic level not relevant to the discussion.)
The follower should be exactly the same size as the mold so that no part of the cheese round is not under the pressure of the follower or you will end up with mis-shaped cheese with poor curd consistency at the edges. So, if you make your own followers, they should be made of a rigid material (non-compressible), as perfectly flat and thick as you can make it, must fit exactly in the mold, made of a food grade material, and white in color.
It is important to understand the fundamental difference between Force and Pressure. What we measure and call Weight is a certain type of Force, specifically, it is m*g where m is the mass of the object and g is the acceleration of gravity (often casually, but incorrectly called the “force of gravity”). Here in the U.S. we most commonly measure weight in pounds and ounces. Pressure is Force/Area, and we most commonly measure this in units of pounds per square inch, or psi. Rick’s examples above are spot on regarding the change in pressure when the surface area increases.
I believe it is correct that the weight required for pressing does increase proportional to the surface area of the cheese. There is a certain amont of force that is required to expel whey and knit the curds together for a particular cheese, and it is the distributed force, i.e., the pressure that is actually “felt” in the cheese (if you could imagine yourself inside the cheese.) It can be helpful to imagine an extreme case, for example a 4 in wheel vs. a 12 in wheel with a 50 lb weight applying pressure to the wheel. I suspect the thickness of the cheese may also be a factor, and I have a feeling this topic is probably covered in some of the more technical (and expensive) cheese making texts that I have not yet purchased.
If you are looking for a more personal experience, try the following: Take off your shoes, and stand on a solid floor. For most people, the pressure on the soles of your feet is somewhere in the range of 1-2 psi (it is your weight divided by the surface area of your feet.) Now find a small object, preferably between 1 and 2 inches in its largest dimension, and put it under one heel. If you lift up the other foot, and try to put all of your weight (FORCE) on that heel with the small object under it, I can almost guarantee it will hurt. Same FORCE, greater PRESSURE.
The reason the plastic followers that come with many cheese molds have the raised circle on the backside is to help distribute the force uniformly over the follower. The plastic follower, if pressed in the center, will flex more than a metal or wood follower which is stiffer. By pressing on the ring it helps distribute the force more evenly between the center and the edge. It also makes it easier to keep the follower level.
BTW, we are havin a NYE party tonight and I will be serving a variety of cheese I have made. I plan to take some pix to post in the next day or two.
Yeah, what he said.
Honestly, my thinking was along the same line as FineWino. I just did not have the words (or the technical knowledge) to write it all out and make my thoughts clear.
Thanks for the above post. I guess only a real life trial will tell the tale, but I’m happy that there seems to be some science to back up my thoughts.
Hello,
the reason Carter and I actually began to talk about pressure vs force as expressed in wieght, is because most recipies refer to a certain number of pounds of wieght.
I started with the std 4 inch moulds and have had thoughts of moving to a much larger 10kg mould. But in the meantime, I have scaled up to a mould like this:
This is two moulds, that are 6in in diameter.
So the question is, this.
If most recipes call for a 4in mould (12.5sqin), what do I do for my setup which presents 56.5sqin to the press.
So, If the recipe calls for 40lbs of wieght, that would be 3.2psi for the std 4” mould, my setup would recieve 4.5 times less pressure (.71psi).
So, Carter and I started to try and understand this.
The results are still out, and it appears that there is some play in the required psi. Certainly we do not want to crush the cheese, but we do need to recognize the press needs of varying sized wheels.
I am converting my recipes wieghts, to PSI measurements, which lets me standardize my cheese pressing whether I am using a mould that is 4” or 12” in diameter.
Wayne..
First of all, I’m glad you joined up.
I think your analytical method of cheesemaking will be a great addition to this forum.
I realize that all of the data is not in on this subject, but I am yet to have a failure since using the higher pressing weights.
If nothing else, between yourself and Carter, you have certainly shaken up the home cheesemaking world and have made a lot of us re-think our ideas concerning this subject.
All I can say is that I have had great results using your methodology.
I’m not changing a thing unlessl someone comes up with something that convinces me this is not correct.
Oh and Wayne…..
Somewhere on here, FineWino has posted a quick read pressure chart in Excel format.
It is somewhat like the one that Carter posted on the other forum, but this chart does include mold sizes from 4” to 8” with pretty much every imaginable size in between.
I’m sure if you look around a little you will find the post. I downloaded it right away and have been using it for all of my larger cheeses.
Again, welcome to the forum.
FYI, the spreadsheet Dave referred to can be found HERE. If you need to add other mould diameters, the weight required is a function of the square of the diameter. For different weights, it is a linear scale. If you know how to manipulate formulas in excel, you can expand the table as much as you want.
While small deviations won’t make a lot of differnce, I am quite confident that in general this is the correct way to scale the force (weight) used for pressing cheeses. Because cheese is slightly elastic (compressible) in the press (it is expelling whey and shrinking) one could get away with a slightly lower force for a thinner cheese but might need a slightly higher force for a thicker cheese. It is the pressure (force/area under the follower) that causes the whey to drain and curds to knit.
Thanks,
I was further thinking that for mould of a given diameter, there *may* be reasons to increase the force.
So, for example, given the same type of cheese:
A mould that has a curd depth of 3” will require a pressing protocol.
That same mould that has a curd depth of 20” might require a modification of the pressing protocol.
I know 20” depth is an extreme, But I wanted illustrate my point. Curds do have a certain sponginess, and drag at the sides of the moulds that both provide resistance to pressing. This resistance would increase with depth, and may require more psi for the same diameter mould.
I might be overthinking it, and i certainly won’t be making any 20” tall wheels of cheese, but I might make a wheel 10"X6”
