Ad-Hoc Yogurt Reactor
Edition: 2018-10-18
Introduction
Belay that Amazon gadget order.
Fermented foods made with various strains of microbes may become a staple of the
Undoctored and Wheat Belly programs. These foods are unlikely to be offered in the
retail market for some time, so we need to be able to make them at home. Further, the
specific requirements of the particular species and strains vary, and cannot be presumed to be
fully compatible with, say, typical retail yogurt appliances (due to possibly novel optimal
temperatures required). Sous vide devices may provide more flexibility, but that’s
still more expense than is really necessary.
For yogurt in particular, there are common problems with retail yogurt makers,
and yogurt settings on programmable pots. These include:
• fermentation temperature completely unspecified
• no fine adjustment for temperature provided
• regulation band unspecified
• run time too short (perhaps only 4-15 hours)
• no pasteurization cycle
These machines are often extremely disappointing.
They only have one job, and
outside of conventional yogurt using
conventional cultures, they do it poorly.
Assess Requirements
Configuring some thermal confinement, water bath, a fermentation container,
and a source of low heat is
often pretty easy, but it is necessary to know the temperature range desired.
For the test batch used in developing the reactor described here, I was shooting
for 97°F. We also have a programmable pot, but its undocumented,
non-adjustable yogurt cycle was
found to run between 105-115°F.
Cooler Caper
The rig at right worked on the first try, and was composed entirely
of material found around the home. You do not need to buy a $900 Igloo
and thermostatically-controlled warming plate. If you are going to
invest in anything, get a digital probe thermometer with a generous
lead length.
Insulated Housing
Some sort of insulated box is needed, ideally one that is low fire risk.
Depicted is a 52 quart insulated polyethylene camp cooler, but a Banker’s
Box nestled in bubble
wrap might do. The idea is to have a very low/slow heat exchange with the
room environment, which does also imply low/no air leakage.
Water Bath (may be optional)
We make ½ gallon at a time, in stainless steel pot, with a heavy
(probably aluminum core) base, and do not use a water bath as shown.
For small batches, or containers having negligible thermal mass (such
as Ball® jars), some additional thermal mass may be needed
(beyond just the fermenting mix). This provides
a thermal flywheel effect, tending to average-out any temperature swings,
and also isolating the batch from direct radiation from the heat source.
Pick a water-tight container that fits in the housing without touching the
top or sides. Place some sort of stand-off device(s) below the batch
container or bath, such as a trivet. Inverted sake cups were used here.
This decouples the batch from
the container floor, and room floor below it, and provides complete
internal air circulation around the bath.
Choice of water bath will to some extend be driven by…
Fermentation Jar(s)
Decide how much yogurt you want to make at once. More than 2 quarts/liter might
present some challenges in scaling the bath and housing.
For the test batch, a 1 quart Ball® jar was used,
(lid loose) in a 2 quart
sauce pan. This bath container does not really need to be metal.
Plastic, ceramic or glass is fine, as the temperature is not very
high (unless you plan to use it also for running a
pasteurization cycle on the stovetop).
Heat Source
Surprisingly little heat is needed for yogurt. Depicted here is an old plastic
shop light, with a retired 14W compact fluorescent bulb (CFL). That’s 14W consumed.
The amount of light (Watt-equivalent lumens) is irrelevant. The light and the
waste heat all end up as heat.
DO NOT let a bare bulb contact the insulated housing. Even CFL and LED
bulbs get too hot.
I had to test several old bulbs (starting with a 40W incandescent) before
finding one that hit and held temperature. A 13W was not quite enough — however,
using a heat source that is not quite enough may be OK. Just plan to drape a
towel or blanket over the closed housing. This provides as much as 10°
more heat rise, adjustable by layering.
If you use an incandescent bulb, or dimmable CFL/LED, you could use an external
in-line dimmer to control power level. It wasn’t necessary for my setup.
Thermometer
A generic digital probe thermometer was used here. It needs to be something
that’s reasonably accurate in the 90-120°F (32-49°C) range.
You can use a wireless household indoor-outdoor set, but won’t be able
to probe the water bath directly (and can’t use a metal housing for anything
wireless).
If the digital thermometer has an alert function, determine what temperature
to have have generate an alarm. For production batches, an alert at
105°F (41°C) or
110°F (43°C) is useful.
You can also use a mechanical (candy) thermometer, but you’ll need to open
the housing periodically to check, which usually results in a temporary
loss of a degree or two.
Test and Dial-in
|
CAUTION
Monitor the setup continuously while dialing-in the
the heat source. If the heat source is too great for the
application, a thermal run-away presents a real risk of fire. |
|
Close the lid. Do some trial runs with just the heat source in the housing.
Don’t bother with a simulated payload until the inside air temp alone
is roughly dialed-in.
Fill the fermenting jar(s) with plain water, at the target temp.
Place the jar(s) in the bath container and add water, again to target temp.
Place this pre-warmed test load in the insulated housing, close
lid, and turn it on.
Monitor the temperature for many hours, ideally overnight, particularly
if the whole rig is in some part of the home subject to temperature swings.
Developing your scheme on the basement floor might be ideal, or a closed
closet floor if you have no basement.
For the rig depicted, I found that it natively wanted to be at 98°F,
-1°F, +2°F, as measured at the bath water. Perfect for the
test batch.
Yes, you will need a timer. Your cell phone or an alarm clock suffices.
Yogurt recipes and instruction steps are not part of this article,
as they are independent of the equipment used.
Plan D: Range Warming Drawer
If you have a range with a warming drawer, and the drawer is deep enough for
fermentation containers, run it for an extended period with water-only,
and, for (if any) settings, characterize the average temperature, and
variation range. Ours turned out to run too hot at its lowest setting,
yet did not generate enough waste heat to warm the oven bay above it.
Even if this gambit works, you could be tying up your range for 36 hours
at a time, many times per month.
___________
Bob Niland [disclosures]
[topics]
[abbreviations]
Tags: