Thread by @galindozip

What’s the kuf of a coffee paper filter?

What’s the kuf or UF coefficient?

I’m an amateur in extracorporeal tec but I’m a self proclaimed expert in coffee process and preparation.

So lets answer this absolutely irrelevant question

#nephtwitter #coffee #tweetorial

When making coffee many processes are similar to dialysis

-diffusion and convection
-semipermeable membrane
-waste product

Poll
When using drip💧methods like v60, chemex, kalita, or conventional coffee machine, what is the mechanism that extracts coffee?

To answer this, first we have to identify the parts of a drip coffee system that are equivalent to a dialysis system:

The coffee +water at the upper compartment will be equivalent to blood. The weight of the water poured at the top will push the water through the membrane with extracted coffee.

So the answer is CONVECTION

The kuf, also known as:

-UF coefficient
-Membrane hydraulic permeability coeff.
-Water permeability coeff.

Is the amount of water that goes through a membrane in a particular time at a particular hydrostatic pressure

So the units are: ml/hr/mmHg

Kuf is one of the characteristics that classify hemofilters as low flux, high flux, high cutoff, protein leaking, etc

A high flux hemofilter will be defined as >12-20 ml/hr/mmHg

But what about the coffee filter?

To measure Kuf we need know that:

Kuf= UF rate/ trans membrane pressure (PTM)

So we have to know the UF rate and PTM of our system

PTM can be define as the difference in pressure of both compartments.

We are at 🇲🇽 city at very high altitude and low atmospheric pressure

Mexico City = 0.77 atm ~ 580mmHg

With out pouring water both compartments will have the same pressure so the PTM will be 0

According to “static fluid pressure” a column of any fluid will apply pressure depending on the column height & not in the shape, volume or surface area of the column

by knowing the height of our water column we can know the pressure in mmHg

1 cm H20 = 0.73 mmHg

when adding water to the system we obtain a new PTM

For a 10 cm water column we get an extra 7.3 mmHg in the upper compartment.

PTM= 7.3mmHg

For this experiment I used my daily laboratory 🔬 🧫 🧪 (my coffee shelf)
& the dripper system known as CLEVER since is the only one with a control valve, and uses #4 paper filters, the same as conventional office machines

EXPERIMENT
1. Marks on filter every 2 cm
2. With closed valve, poured water to the 10 cm mark
3. Opened the valve and poured water to mantain the level at 10 cm during 20 seconds
4. Close the valve, measure obtained water (UF)

Repeat for every level & repeat 3 times

RESULTS:

💥UF rate
10cm, 20 sec, 310ml = 15.5 ml/sec
8cm, 20 sec, 274 ml= 13.7 ml/sec
6cm, 20 sec, 226 ml = 11.3 ml/sec
4cm, 20 sec, 155 ml = 7.7ml/sec
2cm, 20 sec, 60 ml= 3ml/sec

💥PTM
10cm= 7.3mmHg
8cm= 5.84mmHg
6cm= 4.38mmHg
4cm= 2.9 mmHg
2cm=1.43 mmHg

Having PTM and UF rate, Kuf can be solved

💥Kuf
15.5/7.3= 2.1 ml/sec/mmHg
13.7/5.84=2.3 ml/sec mmHg
11.3/4.38= 2.5 ml/sec mmHg
7.7/2.9 = 2.9 ml/sec/mmHg
3/1.43= 2 ml/sec/mmHg

The variation was explained by 👨‍🔬 error in valve closing and pouring speed

⚡️Kuf = 2.3 ml/sec/mmHg

To this point we know that a #4 paper filter has a Kuf of:

2.3 ml/sec/mmHg or translated to conventional Kuf units: 8280 ml/hr/mmHg

But what would happen when we add coffee an repeat the experiment?

When adding coffee, big particles would not pass through the filter and depending on the amount of water and the time to precipitate, particles will accumulate at the membrane.

This effect is similar to hemo-concentration during UF and protein polarization of the membrane

EXPERIMENT 2
1. 18 grams of OAXACAs coffee medium roasted and 24 clicks in the comandante grinder
2. The same steps as experiment 1

RESULTS:
💥UF (ml/sec)
10cm =7.95
8cm=5
6cm=3
4cm= 1.1

💥Kuf (ml/sec/mmHg)
10cm= 1
8cm= 0.8
6cm= 0.6
4cm= 0.4

The Answer: KUF lowers

Since PTM is a fixed variable; it stays the same, while Kuf and UF both decrease

If we want a fixed UF no matter the Kuf: PTM would rise whith lower Kuf

ie. Fixed UF of 7.7ml/sec, Kuf 0.4

PTM= 7.7/0.4
=19.2 mmHg
=25 cm of💧are needed to mantain UF

Conclusions:

✅Dialysis and coffee brewing are similar processes
✅#4 paper coffee filters has a Kuf of 2.3 ml/sec/mmHg or 8280 ml/hr/mmHg
✅If you got up to here you have a completely useless fact and maybe you got the Kuf concept clearer

What’s Kuf of other filters?