Wednesday, December 8, 2010

Sous Vide @ Home - Part 1

So it's my birthday in 2 days and instead of studying thin films and quality control, I spent the day online, reading up on sous vide functional principles, looking for equipment online and getting in touch with my inner geek :) damn the things people do when they have too much time..

Because a thermomix is just too expensive right now ($2500, but if you think that's bad, a pacojet costs $6000), I decided that one last birthday treat was in order but couldn't decide on whether to get a high power food processor, an ice cream machine or a sousvide supreme. Getting any of them would contribute a lot in the kitchen and getting all would be swell but at an average price of $700 each, it wasn't going to happen.

The food processor was last on the wish list and while the sousvide supreme is a really nice toy, $800 for the set up, EXCLUDING the vacuum sealer, was just too much. So, I decided to buy the ice cream machine and a low end vacuum sealer and build my own sous vide rig. I am studying to be an engineer after all, may as well put all that tuition to some use :)

A sous vide set up requires 3 basic components. Something to hold the conducting liquid(water bath), something to heat the conducting liquid (incandescent light bulb or heating element from an old electric kettle) and most importantly something that makes sure the temperature stays the same by switching the heating element on and off (equipment with a feedback loop to control the heater). I studied the product reviews and as much online literature about sous vide machines and decided to use the sousvide supreme, convection circulated design as opposed to the open top, thermal immersion circulators I used at the brewery which cost abt US$1200 for just that small box (PID controller) at the top left of the picture below. Wish I had this now :(

Beer kept overnight at 19°C for daily testing

The convection design costs less because it requires less equipment, you depend on convection currents as opposed to pumps to circulate the water in your bath. My own rig will use an old school mechanical rice cooker as the bath and heating element. The heat coming from the bottom should also create good convection circulation of the conducting liquid. The rice cooker is well insulated and has a fitted cover that will help maintain the higher than ambient temperatures with less energy. On the flip side, cooling it down will also be harder.

I study materials science and not electrical engineering so all the circuit diagrams for the amplifiers and other feedback units we study are truncated to black boxes on our diagrams. In any case, Singapore doesn't have a 'radio shack' analogue and sourcing all the individual parts seemed like too much work to complete my home sous vide machine. Instead, I got a PID controller unit online and 2 sets of platinum resistive thermal device probes. I chose to use RTD probes because sous vide is a process that can take a long time, up to 72 hours in some recipes and this requires high long term accuracy. After a bit of research, I found the appropriate circuit diagrams and process codes here. Now all the GPA hammering shit from first and second year engineering, like C++ programming and electrical circuitry is reaping returns.

According to wikipedia, A proportional–integral–derivative controller (PID controller) is a generic control loop feedback mechanism (controller) widely used in industrial control systems – a PID is the most commonly used feedback controller. A PID controller calculates an "error" value as the difference between a measured process variable and a desired set point. The controller attempts to minimize the error by adjusting the process control inputs.

In essence, Proportional calculations set an initial or present platform to achieve the desired outcome. These are supplemented by the Integral calculations which using previous data, calculate feedback based on previous data and Derivative feedback are predictions worked out from current rates of change to ensure that the temperature stays constant to within 0.1°C with proper calibration. Compare this with a simple thermostat in an oven that switches on and off within a sinusoidal range of about 15°C peak amplitude.

I HATE math so I'm glad there're pre-written formulas from the circuit diagram site and that's why Man invented the computer, to do all that calculus :) $300, 4 hours and programming software courtesy of NTU later, I'm ready to rock and roll. This works out so much better than buying a sousvide supreme. By changing the size of the rice cooker, which costs less than $100, I can easily adjust the set up to sous vide items for 2 or 25 people in a snap. All this for less than half the price that the sousvide supreme is being sold at. I can already see myself cooking a 4kg ribeye sous vide and then searing the outside on the cooking plate I bought today.

Will update when I can, probably when the PID controller arrives. I hope this works! In the meantime, I will be reading up on this great free resource for sous vide cooking data.

1 comment:


yo Jeremy...that is some complicated shit right there...duh!!! engineering n chemistry...not my area of expertise...wat I do know is I cud use a sous vide machine in my kitchen n $2500 seems like alot of money for me right now. Any proposals?
Also..I didn't know your were a part time patissier? those macarons looks way pro man...and as u can see from all my previous menus...there never was any ice cream..reason being I dun have the gadget n also...I'd hate to fork out that kinda money for ben n jerries or haagen daz.
text me the flavor of the week pls.
muchos gracias

A is for Apple