The long-delayed tempeh incubator completed

I update this post from time to time as I learn more from experience. Most recent update June 13, 2021.

This post is about the incubator; here’s how I make tempeh. 

Let me say out that outset that a tempeh incubator is not needed if your oven has a “proofing” setting (used to incubate yeast in bread dough), since that works as well for tempeh (and probably for yogurt as well).  But my oven lacks that, and I found that my tempeh incubator v. 1.0 helped a lot in getting a good batch of tempeh — however, it was made of cardboard, and thus it fared ill from the humid environment it enclosed as the tempeh guys did their job. The heating mat and thermostat worked well, but I needed a better box. (I don’t put the tempeh directly on a mat, but on this raised rack after bagging it in large Ziploc fresh-produce bags, which work perfectly.)

The photo shows version 2.0 of the tempeh incubator, but from what I’ve learned in making that, I can describe version 3.0, also made of rigid 1″ foam insulation, but purchased as 24″ squares. The link is to Home Depot, but a craft store might also sell them. From those, you can cut all pieces required. Dimensions given here are v. 3.0 dimensions, to better accommodate a heating mat 20.75″ long by making the internal space 22″ long, which also provides the side benefit of eliminating the need for some cuts.

Base: 24″ x 12″ — ends are glued to the top of the base, sides glued to the side of the base, as shown in photo
2 Ends: 12″ x 8″ — a small hole cut in the bottom of one end for the electrical plug from the heat mat
2 Sides: 24″ x 9″
Lid: 24″ x 14″ — the lid covers the box, lying on top of the sides and ends.

I usually place a heavy book on the lid when the incubator is working. The lid can be placed at a slant so the incubator is partially open if the tempeh starts getting too hot as it matures. For a 3-cup batch, which generates a lot of heat, I first removed the lid altogether and finally removed the batch from the box, putting it on a raised rack on the countertop to finish. I now do this routinely with a 3-cup batch: start it in the incubator, then after 24 hours remove it to finish (usually 48 additional hours, for a total of 72 hours) on a raised rack on the countertop.

The interior dimensions are thus 22″ long, 12″ wide, 8″ high; the exterior dimensions 24″ long, 14″ wide, 10″ high.

With those dimensions only three 24″ squares are needed:

Square 1, from which the base and ends are cut 
Square 2, from which the sides are cut
Square 3, from which the top is cut

Important: When cutting, make sure the knife stays perpendicular to the surface of the board so that the cut edges are square and don’t slant. This ensures good contact for gluing.

Use galvanized 2″ nails around the edges of the sides (going through the side and into the base and ends) and across the ends of the base (going up into the end pieces of the box) as reinforcement. I left the weight on the box overnight to give time for the glue to fully set. Some nails are placed during glue-up; others are added later. See assembly steps below.

Once the glue has set, push the nails through the edge of the side that’s on top into the base and end pieces. Then turn the box to put the other side on top and repeat. You can push the nails in with your hand, though in a few places (where the nail encounters glue) you may need to tap the nail with a hammer. After the nails are in place, tap them gently with a hammer to seat the nails. They seem to be securely held.

I used Clear Gorilla Glue, though the original Gorilla Glue will work as well. Detailed assembly directions are below. The stack of books in the photo top serve as a weight to clamp the pieces together until the glue is fully set. I left it overnight.

I see in the photo that the glue dripped. So it goes. This project is not headed for the county fair or anything like that — it’s purely utilitarian, and it will work as well with or without glue drips. But I would have wiped it off with a damp paper towel if I had noticed it at the time.

Optional reinforcement: You can reinforce the box more by getting small right-angle braces and gluing them to sides/ends and base and to ends and sides. I didn’t do this because it didn’t seem necessary. I can always do it later if needed. — Update: Never needed. The box is plenty sturdy without braces.

Step-by-step assembly instructions

In thinking about my experience in assembling the box and the missteps I made, I realized exactly how I should have assembled it. This recommended assembly sequence will avoid the problems I encountered. First, cut the pieces to the sizes listed above. Then follow these steps:

1. Cut the mousehole for the power cord in the center of the bottom edge of one end (the bottom edge being the one that will rest on the base).
2. Secure the bottom of the ends to the base. Each end will sit atop the base, flush with the edges of the base. Put an end in place and then fix the end piece in place with two of the 2″ galvanized nails. Push each nail through the bottom of the base into the bottom of the end piece, placing the nail 1/2″ from the end of the base, and 1″ from each side of the end of the base — thus 4 nails are used, 2 for each end. This is not a particularly strong join, but it will keep the ends from slipping out of position as you do the next step. If you want, you can also glue the bottom of the ends to the base, but I don’t think that is actually required. Still, you can do that if you want.
3. The result of step 1 is a U-shaped thing, the base with the two upright ends attached by the nails. Place that on its side so that a side piece will go on top, flush with the outer edges of the U. Take one of the side pieces and run a bead of glue 1/2″ from the edge along the length of the side and across each end of the side. This will glue the side to the base and the end pieces. Place the side on the assembled portion, and push a nail through each corner of the side piece, 1/2″ in from each edge. Two of those nails will go into the end pieces (at the top) and two into the base piece (at the bottom of the box). This will keep the side in place as the glue dries. Look for any glue drips and wipe them off. Put some weight (heavy books, cast-iron skillet, whatever) on the side and leave it for a couple of hours until the glue has set reasonably well.
4. Remove the weight and turn the structure over, so that the second side piece can also be placed on top of the assembly so far. Again run the bead of glue, put the side in place, and push a nail through each corner of the side piece into the end pieces and base. Again, put weights on top, and this time leave it overnight.
5. Now push or tap a series of nails around three edges of the side, into the ends and the base, spacing the nails 2-3 inches apart. This will reinforce the structure. You could also push a series of nails through the bottom into the end pieces, but I don’t see that as necessary.

First batch in new incubator

Success! This is the first trial batch after about 40 hours. I’m going to let it go a little longer, but already it’s pretty solid. (The tempeh mycelium welds the beans together into a solid mass.) The dark spot is not a problem but rather a place where the mold is sporing — it’s perfectly edible. (I later figured out that sporing is triggered by too high a temperature after the tempeh mold has taken hold. If I remove the batch from the incubator once the mold has started and let it finish at room temperature, I don’t get the dark patches. See this post for more information on the incubation.)

I’m delighted with the success and that the (sturdy) box worked so well. 

I made just a small test batch (1 cup dried beans). My usual batch is larger (2 or 3 cups dried beans, or 2 cups dried beans and 1 cup intact whole grain — and my next batch I’m going to try 4 cups (measured before cooking and then cooked)). I get my tempeh starter culture from Cultures for Health, but when I need starter again I’m going to order a 200g packet from TempehStarter.com.

I think I’ll use some for tempeh breakfast sausage. The recipe at the link works well and produces tasty sausage. Tempeh bacon is also tasty, and in fact tempeh is good with a variety of marinades. Mostly, though, I dice tempeh and use it in stews, stir-fries, curries, and chili.

My basic post on how I make tempeh reflects the learning curve, and it begins with a summary of what I learned in the process. I highly recommend making your own tempeh — it’s considerably better than the store-bought I’ve had. Update: Basic steps for homemade tempeh summarizes succinctly the method I use; also, see my article in Medium.